NGSPM-SOT Report

July 2017

Next Generation Solar Physics Mission
Science Objectives Team(NGSPM-SOT)

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Executive Summary
In June 2016, the United States National Aeronautics and Space Administration (NASA), Japan
Aerospace Exploration Agency (JAXA), and European Space Agency (ESA) chartered a Next
Generation Solar Physics Mission (NGSPM) Science Objectives Team (SOT) to study andreport on a multilateral solar physics mission concept. The SOT was formed as a means ofimproving international coordination in solar physics, and in particular developing an NGSPM
concept for the next decade. This is likely to be realized as a Japan-led mission launched after
2024, with substantial contributions from the United States and Europe. The primary role of
the SOT was to develop and document scientific objectives and priorities for an NGSPM,
within the resources and framework specified by the agencies. The guiding philosophy wasthat a next-generation effort should bring closure to science objectives by exploring andexploiting new windows on, e.g., wavelength, continuity, spatial resolution, temporal
resolution, spectroscopy and polarimetry, vantage point and orbit, with due consideration given
to readiness of technology and analysis methods.
The Charter of the SOT stated that the team was intended to “represent the broad interests of
the heliophysics research community”. SOT members conducted more than 6 sessions atscientific meetings in the US, UK and Japan to inform the community of our work and receive
feedback. A public call for white papers in the fall of 2016 resulted in 34 submissions
describing science objectives, instruments and/or mission concepts, covering a wide variety of
topics. The most common topics involve coronal heating, magnetic geometry/topology/
reconnection and heating processes in flares and CMEs. Most of the measurements requested
fall into three categories, spectroscopic, magnetic or multi-platform observations.
Drawing on the science objectives of the white papers, the 2015 Solar-C proposal, anddiscussions within the SOT, the following set of top-level science objectives was adopted.
•••

Formation mechanisms of the hot and dynamic outer solar atmosphere
Mechanisms of large-scale solar eruptions and foundations for predictions
Mechanisms driving the solar cycle and irradiance variation

SOT members wrote specific sub-objectives (17 in total) under these headings and listed theessential observational tasks (56 in total) and measurements required to address each of them.
That list of required measurements implied a set of notional instruments (15 in total) to satisfy
all, and some critical specifications for those were compiled (spatial and temporal resolution,
field-of-view, possible wavelength range). Lengthy discussions took place evaluating thescientific sub-objectives according to criteria such as impact to solar physics, need for spaceobservations, and technical feasibility (see Section 3.1 for the complete list of criteria). This“bottoms-up” review of the current state of heliophysics showed that there are two broad
avenues, both with distinct merits, for future research: physical mechanisms on elemental(small) scales, versus global processes affecting/involving large fractions of the solar interior
and/or atmosphere. Transformative progress would certainly be made possible by sustained,
global views from multiple view points, as is discussed in Section 3.3.1. This provides a strong
motivation for international coordination over the upcoming decades to fill critical gaps invantage points in order to obtain a truly global understanding of the Sun. However,comprehensive implementation schemes likely exceed the resources available for a NGSPM
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on the timescale of the next decade. Therefore, the SOT chose to focus its recommendations
on the other avenue, i.e., the study of fundamental physical processes at high spatial andtemporal resolution through all temperature regimes of the solar atmosphere.
There is every reason to believe that a high-resolution focus for NGSPM will be richscientifically. Game-changing discoveries may be found on these elemental scales, such as thenature of magnetic stresses resulting from braiding of magnetic field lines; the magnetic
topology at the footpoints of spicules, jets, and prominences; the transport of energy via Alfvén
waves in flares; and the development and effects of turbulence in all levels of the solar
atmosphere. Given current constraints, probing the Sun at highest resolution is both feasible
and well-suited to NGSPM near-term opportunities. It will be important for NGSPM to observein conjunction with ground-based observatories, in particular DKIST (and later EST) in order
to combine the unique strengths of space- and ground-based resources.
Considering a minimum set of instruments with which NGSPM can address the greatest
number of high-priority Tasks consistent with the objective of small length- and time-scale
activity, we find that a majority of the key required measurements can be met with thefollowing suite of instruments:
1.
2.
3.

0.3" coronal/TR spectrograph0.2"- 0.6" coronal imager
0.1"- 0.3" chromospheric/photospheric magnetograph/spectrograph

While other combinations of candidate instruments would likely also produce significant
results, the list above represents the smallest set of instruments addressing as many as 32 of the
56 essential observational Tasks listed in Section 3.1; it is the list of instruments that the SOT
feels should receive the highest priority for selection. The great majority of the Tasks require
some combination of instrumentation for closure or significant progress. Thus the strawman
suite of instruments considered above has been optimized to maximize the scientific returnfrom the NGSPM.
The Charter of the NGSPM-SOT assigns the task of suggesting possible mission profiles that
could satisfy the science objectives identified by the SOT, within the likely available
resources. The three instruments outlined above could be realized in one single large mission.
This large mission is smaller, less complex, and less expensive than the Solar-C proposed in2015. The largest telescope has decreased in size, its focal plane package is less complex, andthe launch cost has been reduced significantly. The second mission concept is to form a
constellation of small or middle-class satellites for realizing the three instruments, by utilizing
spacecraft of JAXA, NASA, and/or ESA (including ESA member states). The merit of thismission concept is to increase the possibility that some of the instruments are launched as earlyas possible in the mid-2020’s, but a risk is that the scientific synergy among the three
instruments will be limited unless there is significant overlap in observing time of the missions.
The SOT finds that the suite of instruments listed above are the highest priority foradvancing the science objectives mentioned above within the next decade. We recommendthat the NGSPM consist of the instruments listed above operating simultaneously, in fullSun orbit(s), with sufficient telemetry coverage.
We recommend that NGSPM be realized with a single platform, as a JAXA Strategic Large
mission with contributions from NASA (SMEX-level), ESA (MoO), and ESA member
4states. If the single-platform approach is not possible or available, a combination of twoor three spacecraft can achieve many of the NGSPM objectives, with some loss ofcapability and at increased risk.
We recommend that the agencies form a unified Science Definition Team for NGSPM assoon as possible to define the agencies’ respective contributions in more detail.
Some more specific recommendations to the three agencies are also included, in the complete
list in Section 4.8.

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Table of Contents

Executive Summary ........................................................................................................... 3
Acronyms .......................................................................................................................... 9
Chapter One: Introduction............................................................................................... 11
Chapter Two: Community Input ...................................................................................... 13
2.1. Town Hall meetings ........................................................................................................... 13
2.2. White papers ..................................................................................................................... 14

Chapter Three: Science Objectives and Tasks ................................................................... 23
3.1. Description of Objectives ................................................................................................... 23
3.2. Required measurements and Notional Instruments ........................................................... 27
3.3. Choosing a path for NGSPM ............................................................................................... 29
3.4. Team Conclusion: An optimal instrumentation suite for elemental science........................ 32

Chapter Four: Assessing possible mission designs ............................................................ 35
4.1. Top-level priority of instruments ....................................................................................... 35
4.2. Mission concepts ............................................................................................................... 35
4.3. Description of the Instruments .......................................................................................... 36
4.4. Modification of Instruments for constellation of small/mid-class missions ........................ 39
4.5. Uniqueness and synergy with other missions..................................................................... 40
4.6. International coordination ................................................................................................. 42
4.7. Possible additional instrument ......................................................................................... 46
4.8. Recommendations ............................................................................................................. 46

Appendix A Charter for NGSPM-SOT .............................................................................. 49
Appendix B Science objectives for NGSPM ..................................................................... 51
Appendix C Existing facilities .......................................................................................... 73
Appendix D SOT members .............................................................................................. 75
Appendix E Town Hall Meetings and Community Q&A Discussions ................................. 77
Appendix F Epsilon mission system configuration ........................................................... 797

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A c ro n y m s
AGU
AIA
AO
AR
ASJ
AU
CH
CLASP
CME
CMOS
CSHKP

CZ
DKIST
EIS
ENA
EPIC
ESA
EST
EUV
EW
FIP
FOV
FOXSI
FWHM
GBO
GOES
Hi-C
HMI
HSO
HXR
ID
IRIS
ISAS
JAXA
JSPC
KE
MDI
MHD
MIDEX
MK
MoO
Mx
NAOJ
NASA
NEO
NGSPM
NIR

American Geophysical Union
Atmospheric Imaging Assembly
Announcement of Opportunity
active region
Astronomical Society of Japan
Astronomical Unit
coronal hole
Chromospheric Lyman-Alpha Spectro Polarimeter
Coronal Mass Ejection
Complementary MOS (image sensor)
Carmichael-Sturrock-Hirayama-Kopp-Pneumanconvection zone
Daniel K. Inouye Solar Telescope
Extreme UV Imaging Spectrometerenergetic neutral atom
European Participation in Solar-C
European Space Agency
European Solar Telescope
Extreme ultra violet
east-west
first ionization potential
field of view
Focusing Optics X-ray Solar Imager
full width at half maximum
Ground-based Observatories
Geostationary Operational Environmental Satellite
High Resolution Coronal Imager
Helioseismic and Magnetic Imager
Heliophysics System Observatory
Hard X-ray
identification
Interface Region Imaging SpectrographInstitute of Space and Astronautical Science
Japan Aerospace Exploration Agency
Japan Solar Physics Community
kinetic energy
Michelson Doppler Imager
Magnetohydrodynamics
Medium-Class Explorers
Million Kelvin
Missions of Opportunity
MaxwellNational Astronomical Observatory of Japan
National Aeronautics and Space Administrationnear earth orbit
Next Generation Solar Physics Mission
Near Infrared

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NLFFF
NS
NST
NUV
PIL
PROBA
PSP
RHESSI
Q&A
QS
SAD
SDO
SMEX
SO
SOHO
SOT
SOT
SSI
SST
SUMER
SUVI
SWAP
SXI
SXR
SXT
TES
TR
TRACE
TSI
UK
US
UV
WG
XRT

nonlinear force-free fieldnorth-south
New Solar Telescope
Near ultra violet
polarity inversion line
Project for On-Board Autonomy
Parker Solar Probe
Reuven Ramaty High Energy Solar Spectroscopic Imager
Question & Answer
quiet Sun
Supra-arcade downflow
Solar Dynamics Observatory
Small Explorers
Solar Orbiter
Solar and Heliospheric Observatory
Science Objectives Team (of NGSPM)
Solar Optical Telescope (on Hinode)
Solar Spectral Irradiance
Swedish Solar Telescope
Solar Ultraviolet Measurements of Emitted Radiation
Solar Ultraviolet Imager
Sun Watcher using Active Pixel System Detector and Image Processing
Solar X-ray Imager
soft X-ray
Soft X-ray Telescope
Transition Edge Sensor
transition region
Transition Region and Coronal Explorer
Total Solar Irradiance
United Kingdom
United States (of America)
Ultra violet
Working Group
X-Ray Telescope10

C h a p te r O n e : In tro d u c tio n
In June 2016, the United States National Aeronautics and Space Administration(NASA), Japan Aerospace Exploration Agency (JAXA), and European Space Agency (ESA)
chartered a Next Generation Solar Physics Mission (NGSPM) Science Objectives Team (SOT)
to study and report on a multilateral solar physics mission concept (Appendix A). This effort
builds upon the highly successful collaborations between NASA, JAXA, and ESA, including
Yohkoh (Solar-A), Hinode (Solar-B), Geotail, Chromospheric Lyman-Alpha Spectro
Polarimeter (CLASP) (sounding rocket), and the Solar and Heliospheric Observatory (SOHO).
The SOT was formed as a means of improving international coordination in solar
physics, and in particular developing an NGSPM concept for the next decade. This is likely tobe realized as a Japan-led mission launched after 2024, with substantial contributions from theUnited States and Europe (subject to the normal proposal and peer review cycles of all thepartners). The SOT was encouraged by the agencies to consider mission design options on
different scales to be ready for future opportunities should they arise.
The primary role of the SOT was to develop and document scientific objectives andpriorities for an NGSPM, within the resources and framework specified by the agencies.
Appendix B describes these science objectives, and the outstanding tasks and key observations
required to achieve them. Changes in landscape due to major new capabilities offered by, e.g.,
the Daniel K. Inouye Solar Telescope (DKIST) were part of our considerations. Prioritization
thus emphasized science enabled by space observations and not accessible to existing or nearterm assets, either space or ground-based (see Appendix C). The guiding philosophy was thata next-generation effort should bring closure to science objectives by exploring and exploiting
new windows on, e.g., wavelength, continuity, seeing/resolution, vantage point and orbit, withdue consideration given to readiness of technology and analysis methods.

Figure 1-1: SOT activities and community input
The membership of the SOT was selected jointly by the Institute of Space andAstronautical Science (ISAS)/JAXA, NASA, and ESA, and included four members from Japan,

11

four from the US and four from ESA member states. Two helioseismology experts were added
later to the membership for assessing helioseismology-related objectives. The SOT
membership was chosen to represent the full range of solar physics sub-disciplines (Appendix
D). SOT members acted as points of contact with the worldwide community, from whom input
was sought in the form of short ‘white papers’. These served two main purposes; to widen thescope of the discussion beyond the expertise and interest of the SOT members, and -- groupedroughly by objective/type of observation/platform -- to generate a top level view of the
momentum behind different ideas, the level of community interest and backing, and the levelof technological readiness. The team met regularly over the course of a year, with several
telecons and 4 face-to-face meetings. In addition to the white papers, community input wassought by organizing town-hall or discussion sessions at 8 meetings in Japan, the US and the
UK. The timeline for SOT activities and community input is shown in Figure 1-1.
The report is structured as follows, with the intention of closely following the charter
given to the SOT by the agency partners (see Appendix A). In Chapter 2 we give details of the
input gathered by the SOT from the community. In Chapter 3 we present prioritized scientificobjectives and their required measurements, taking into consideration alignment with agency
goals and likely resources. In Chapter 4 we present a set of recommended mission design
strategies, and discuss international coordination necessary to realize the NGSPM.

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C h a p te r T w o : C o m m u n ity In p u t
2.1. Town Hall meetings

The Charter of the SOT makes plain that the Team is intended to “represent the broadinterests of the heliophysics research community”. The Team thus felt a strong obligation totake input from the community to ensure that our discussions were not too narrowly focused,
and to provide information back to the community regarding our progress and methodology.
To this end, we conducted Question & Answer (Q&A) sessions at major conferences to interact
with attendees. In parallel, the membership list of the SOT was published so that community
members could freely communicate with us informally at any time. In the following
paragraphs, we briefly summarize these Q&A sessions; more detailed descriptions are given
in Appendix E.
2.1.1 Q&A at Hinode-10
The first “Town Hall”-type meeting was conducted during the Solar-C session at the
Hinode-10 meeting in Nagoya, Japan, primarily for the purpose of introducing the SOT to thecommunity and starting a dialogue. After presentations of the SOT charter and somepreliminary studies of instrument concepts in Japan, an extensive Q&A with the full audience
was conducted, addressing the goals and objectives of the SOT and the NGSPM.
2.1.2 Town Hall at AGU
The second major Q&A session was a Town Hall meeting at the AGU in San Francisco.
Being in December, this occurred approximately half-way through the SOT’s deliberations.
The SOT members were joined by agency representatives from NASA and JAXA, who
presented the rationale for chartering the SOT. The reports were followed by an extensive Q&A,
including discussion about the agencies’ expected budget, timeframe, and mission scope(small/medium/large).
2.1.3 UK Community Meeting
During the UK Solar Physics Community's annual missions forum, attended by around
50 community members, the purpose, membership and progress of the SOT were described,
based on the presentations given at the AGU Town Hall. The outcome of the call for white
papers was also summarised.
2.1.4 Japanese Community Meetings
Community meetings were organized multiple times by JSPC (Japan Solar Physics
Community) to discuss science objectives and future directions of solar physics in Japan. The
first symposium (3-4 October 2016, ISAS/JAXA) was an opportunity to discuss key scienceobjectives based on the NGSPM-SOT study at the early phase with the community. Four subgroups formed in the JAXA Solar-C WG presented science objectives and mission concepts
based on Epsilon opportunity and identified scientific issues to be addressed in the next meeting,which was the JSPC annual meeting on 20-22 February 2017 at ISAS/JAXA. Then, someschedule updates on the announcement of opportunity in JAXA were reported to thecommunity during the ASJ (Astronomical Society of Japan) meeting (Fukuoka) on 17 March
13

2017. The JSPC also held a one-day meeting on 13 July 2017 at NAOJ for having community
consensus toward the coming Epsilon AO, with inputs from NGSPM-SOT report as well as
study updates from the JAXA Solar-C WG.
2.1.5 Q&A at Hinode-11
At the conclusion of the Hinode/IRIS joint science meeting in May-June 2017, inSeattle, Washington, a 75-minute session was allocated for a presentation and Q&A regarding
the Draft Report of the NGSPM-SOT. Following a PowerPoint presentation of the main pointsof the Draft Report, a Q&A with the audience discussed initial reactions of the community to
the findings and preliminary recommendations.
2.2. White papers

2.2.1. Science objectives
Table 2-1 (at the end of this Chapter) provides a brief summary (the lead author, title,
primary science theme, closest sub-objectives, and instrument discussed) of all the 34 white
papers received. The 34 papers in total cover a large variety of science objectives and ideas.
Table 2-2 shows the number of white papers categorized according to science theme. This tableshows that coronal heating, magnetic geometry/topology, and heating in flares and CMEs
remain very high priority topics for large fractions of the scientific community. Coronal heating
is one of the fundamental questions in solar and stellar physics, and many solar physicists
consider spectroscopic and magnetic-field (spectro-polarimetric) observations at higher
resolution and higher accuracy to be of utmost importance. Magnetic geometry and topology
are fundamental to understanding the formation mechanism of the hot and dynamic outer
atmosphere. There is also a sense that our knowledge of heating processes involved in flaresand CMEs is incomplete.
Table 2-2: Ranked topics based on the submitted white papers
#
1
2
3
4
5
6
7
8
9
10

TopicsCoronal Heating
Magnetic geometry/topology/reconnection
Heating processes in flares/CMEs
Magnetic energy buildup and instabilities
leading to eruptions
Probing flare processes via particle
acceleration
Solar dynamo and magnetic dynamics
Solar wind acceleration
Solar internal structureSpace weather monitoring
Irradiance# of WPs
15
1195

Related sub-objectives
I-1, I-2, I-3, I-4, I-5
I-1, I-4, I-5, I-6, II-1, II-2, II-4, II-5
II-3, II-4, II-6
I-6, II-1, II-2, II-5

7

II-4, II-6

6
6
4
5
3

II-5, III-2, III-3
I-5
III-1, III-2
II-1, II-2, II-3
III-4

Some white papers discuss the importance of other aspects of solar flares, e.g., magnetic
energy buildup and instabilities leading to eruptions, and particle acceleration processes. The
causes of long-term variations of solar magnetic activity are particularly poorly understood,requiring considerable progress in understanding the workings of the solar dynamo. For thistask, observations relating to the solar dynamo and probing internal structure may give uniqueconstraints on theory and models; we received eight white papers on these topics. Three white14

papers discussed total and solar irradiance variability, in particular as a function of solar latitude.
There were also six white papers that addressed science associated with solar wind acceleration.
Having these white papers was valuable to make sure that no topics of interest to thebroader community were missed in our team’s discussions (Chapter 3).
2.2.2. Instrument concepts
The proposed concepts of observations are classified in a) spectroscopic observations(Figure 2-1), b) magnetic-field observations (Figure 2-2), and c) multi-platform observations(Figure 2-3). Note that a few white papers that cannot be classified in these three categories areshown in a) spectroscopic observations (in particular, in-situ observations). In Chapter 3,
science objectives of NGSPM will be classified into three categories:
I. Formation mechanisms of the hot and dynamic outer atmosphere;
II. Mechanisms of large-scale solar eruptions and foundations for predictions;III. Mechanisms driving the solar cycle and irradiance variation.
The upper panel of Figures 2-1, 2-2, and 2-3 represent a 3D map defined by the threescience objectives, in which is given the instrument concept (type of instrument, key
performance, and scientific target) proposed by each white paper for addressing its primaryscience objective. The area covered by the instruments proposed for NGSPM (in Chapter 3
below) is shown by a hatched region around the origin. The lower panels in Figures 2-1 and
2-2 give instrument concepts of the white papers as a function of wavelength in the horizontal
axis and atmospheric height (plasma temperature) in the vertical axis.
a) Spectroscopic observations
Eight white papers (Young, Peter, Ugarte-Urra, Imada, Klimchuk, Del Zanna, Warren,
Reep) proposed a high-throughput EUV spectroscopy instrument, although the key
performance stressed in each paper is different, including a wide temperature (0.01K-10MK)
coverage for tracing energy flow (Young) and for magnetic connectivity of the atmosphere(Peter), high cadence for dynamic diagnostics in loop structures (Ugarte-Urra) and for
diagnosing non-equilibrium plasma (Imada), high temperature (5-10MK) lines forinvestigating nanoflares (Klimchuk, Del Zanna), and energy deposition and waves in flares(Warren, Reep). For nanoflare investigations, some white papers discuss the importance ofline selection (Del Zanna, Klimchuk) and also propose spectroscopy in soft X-ray and lowenergy hard X-ray ranges (Narukage, Christe, Caspi). New technologies are proposed for soft
X-ray spectroscopy, i.e., photon-counting with high speed readout CMOS detectors (Narukage),
and TES X-ray microcalorimeter arrays (Christe). Low-cost and low-resource instruments thathave already been flown on sounding rockets and CubeSats are also proposed (Caspi). Soft Xray spectroscopy is proposed for diagnosing superhot (>30 MK) plasma, and for investigating
non-thermal distributions and energy transport in flare plasma (Matthews). Proposals were also
made to investigate particle acceleration in flares by hard X-ray spectroscopy for electrons(Christe, Reep, Warren) and by gamma-ray and neutron monitoring for ions (Shih).
UV imagers for spectral lines are suggested with super-high spatial resolution for
topological studies of small-scale dynamics (Matthews, Sterling), and with a moderate spatial
resolution for linking small-scale dynamics to large-scale magnetic structures (Parenti).

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Spectroscopy (+α) observations
In-situ

γ-ray, neutron

Krasnosselskikh
Vourlidas, Erdelyi

ShihMatthews

Caspi
Narukage
Christe

Chromosphere 0.01MK

Klimchuk

0.1MK

TR

Del Zanna
Reep
Imada Warren

1MK

UV spectroscopy
Young Ugarte-Urra
Peter

Morton10MK

HXR spectroscopy

2R

SXR spectroscopy

3R

CoronaUV spectro-imager
Parenti SterlingCoronagraphic
spectrometer

Height(non thermal signature)

NGSPM
Photosphere
1R
Convection
Zone

100

1000
NIR

Visible

FUV

10

1
1

10
0.1

EUV

SXR

HXR

(keV)
(nm)
γ

In-situ

Wavelength

Figure 2-1: Mapping of white papers for spectroscopic observations (upper) in three scienceobjective categories and (lower) in wavelength and atmospheric height16

Investigations from the chromosphere through the outer corona and into the heliospheremay be important for understanding the acceleration of the solar wind. White papers exploring
solar wind origins argue for Doppler velocity measurements covering coronal hole boundaries
as well as active regions (Parenti), instruments capable of measuring polarization in hard Xray sources (Berrilli), and imaging-spectrometer coronagraph for tracing waves propagating up
to 2 solar radii (Morton). Others suggest in situ measurements by missions approaching very
close to the solar surface (Krasnosselskikh), at high latitudes (Vourlidas), or as part of acomprehensive solar observatory at L1 (Erdelyi).
b) Magnetic field observations
Optical telescopes with a variety of aperture sizes, all aiming at measuring the magnetic
field, were proposed, including 50cm-class visible-light/NIR telescopes for diagnosing
chromospheric magnetic fields relating to flares (Ichimoto) and high-cadence, long-duration
observations of target regions in the photosphere and chromosphere (Berrilli), a 1m-classvisible-light/NIR telescope for investigating magnetic structures of sunspots above and below
the visible surface (Zhao), and a 3m-class ultra-high resolution telescope covering wavelengths
from UV to NIR for studying magneto-convection and dynamo processes (Collet). 50-cm
optical telescopes with imaging spectropolarimeters (Orozco) and a Visible/IR coronagraphwith spectropolarimeter (DeLuca) were also proposed to target prominence/filament magnetic
structure and evolution. Optical magnetographs with Doppler velocity measurement
capabilities were also a standard requirements on all the dynamo-oriented white papers(Appourchaux, Vourlidas, Judge, Erdelyi).
New capabilities for directly measuring the magnetic fields in the upper atmospherebeyond the chromosphere have also been proposed. This was outlined in 2 white papers by
expanding spectro-polarimetry in the UV (Ishikawa, Casini) from the upper chromosphere to
the lower transition region. Furthermore, spectro-polarimetry with a coronagraph (Lin) is
described as a pathfinder for tomographic measurements of the coronal magnetic field.

17

3R

Lin
Plowman

2R
10MK

Corona1MK

CasiniIshikawa

0.1MK

TR

Collet3 m class

NGSPM

1 m class

Convection
Zone

Visiblespectro-polarimetry

1R

Ichimoto Judge Zhao
Orozco 50cm class
DeLucaChromosphere 0.01MK
Photosphere

UV spectro-polarimetry

Coronagraph spectropolarimetry

HeightMag. field observations

100

1000
NIR

VisibleAppourchaux Vourlidas
Judge Erdelyi

FUV

10

1
1

10
0.1

EUV

SXR

HXR

(keV)
(nm)
γ

In-situ

Wavelength

Figure 2-2: Mapping of white papers for magnetic field observations (upper) in three scienceobjective categories and (lower) in wavelength and atmospheric height18c) Multi-platform observations
An alternative to a single-platform observatory-type mission is proposed (De Luca), in
which NASA, ESA, and JAXA each define a small or mid-class mission and the set of threesmall missions forms a solar observatory on orbit. Proposals for stereoscopic measurements
obtained by complementing a NEO 50cm-class telescope with another 50cm telescope located
far from the Earth (e.g. L5) were made (Orozco, Judge). The idea is to improve magnetic fieldmeasurements and monitoring of the magnetic field at the chromosphere and photosphere fordeveloping space weather forecast capabilities. Finally, a dual spacecraft mission is proposed(Krasnosselskikh) where one spacecraft carrying in situ instruments goes very close to the sun,as it is monitored by a second spacecraft carrying remote-sensing instruments at a significantlyhigher orbit.
Missions leaving Earth orbit may be important for obtaining new understanding of theSun from different vantage points. The Lagrangian point L1 provides a stable environment inwhich a large formation flying mission is proposed for achieving ultra-high resolution
observations (Erdelyi). Missions on out-of-ecliptic orbits have been advanced for polarinvestigations (Appourchaux, Vourlidas), while a spacecraft at the Lagrangian point L5 has
been described, whose aim is to provide the synoptic observations needed to determine the 3D
structures of coronal mass ejections (Plowman). Multi-vantage-point, multi-mission
observations of the Sun (Brun) capture a global view of the Sun in all longitudes and latitudes
for understanding the global dynamical state of solar magnetism.

Figure 2-3: Mapping of white papers for multi-platform observations in three scienceobjective categories

19

Table 2-1: List of white papers
Lead author

Title

Science Theme

Closest subobjective

Instrument Category

Ugarte-Urra

Science Objective:
Understanding Coronal
Loop Plasma Dynamics
with 2D time-resolved
spectroscopy

Coronal heating

I-1, I-2, I-3, I-4, I-5(E)UV spectroscopy, spatiallyresolved

Caspi

Diagnosing Coronal
Heating Processes with
Spectrally Resolved Soft
X-ray Measurements

Coronal heating

I-2, I-5, II-6

X-ray spectroscopy, spatiallyresolved (soft)

Young

Energy Transfer from the
Chromosphere to theCorona Using Oxygen as aTrace Element
Coronal heating

I-1, I-3(E)UV and X-ray spectroscopy,
spatially resolved; UV and EUV
imaging for context; highresolution magnetic
measurements.

Imada

Clarify the Energy
Transfer from the
Photosphere to theCorona and Diagnose theEnergy Dissipation
Region: UV/EUV HighThroughput
Coronal heating; Heating processes
Spectroscopic Telescope in flares/CMEs
I-2, II-4(E)UV spectroscopy, spatiallyresolved

Klimchuk

The Case forSpectroscopic
Observations of Very Hot Coronal heating; Heating processes(5-10 MK) Plasma
in flares/CMEs
II-4(E)UV and X-ray spectroscopy,
spatially resolved

Del Zanna

The quest for the hot (510 MK) plasma in thesolar corona(E)UV and X-ray spectroscopy,
spatially resolved

ChristeSolving the Coronal
Heating Problem using X- Coronal heating; Heating processes
ray Microcalorimetersin flares/CMEs
I-2

X-ray spectroscopy, spatiallyresolved (soft)

Narukage

White paper of the "soft
X-ray imaging
spectroscopy"

Coronal heating; Heating processes
in flares/CMEs; Probing flareprocesses via particle acceleration I-1, I-2, II-4, II-6

X-ray spectroscopy, spatiallyresolved

Berrilli

ADAHELI PLUS: Near
Infrared
spectropolarimetric
imaging of 3d
atmosphere and X-ray
polarimetricmeasurements

Coronal heating; Solar dynamo andmagnetic dynamics; Solar wind
I-1, I-3, I-4, I-5, II-6,
acceleration
III-3

Vis/NIR imaging &spectropolarimetry; X-ray
polarimetry

10 Morton

Exploring Coronal
Dynamics: A Next
Generation Solar Physics
Mission White Paper

Coronal heating; Solar wind
acceleration

Coronagraphic spectrometer

11 Reep

Science Objective:
Understanding Energy
Transport by Alfvenic
Waves in Solar Flares

Heating processes in flares/CMEs;
Probing flare processes via particle
acceleration
II-6(E)UV spectroscopy, spatiallyresolved; HXR spectrometer

12 Warren

Science Objective:
Understanding Energy
Deposition During Solar
Flares with Very High
Cadence Spectroscopy

Heating processes in flares/CMEs;
Probing flare processes via particle
acceleration
II-4, II-6

(E)UV spectroscopy, spatiallyresolved; HXR observations
and high-spatial-resolution
EUV imaging.

#

1

2

3

4

5

6

7

8

9

Coronal heating; Heating processes
in flares/CMEs
I-2, II-4, II-6

I-3, I-5

20

13 Zhao

Simultaneous MultiWavelength Observations
to Study Sunspots'
Subphotosphere andAbove-Photosphere
Dynamics
Internal structure

14 Plowman

White Paper: Coronal
Sentinel

Magnetic energy buildup leading
to eruptions; Space weather
monitoring

I-3, II-5

Magnetograph, highresolution

Magnetograph, stereoscopic;
EUV imager; Coronagraphic
I-5, I-6, II-1, II-2, II-3 spectropolarimeter

15 DeLucaNGSPM Energy Storage in Magnetic energy buildup leading
the Solar Atmosphereto eruptions

I-6, II-1

Magnetograph/spectropolarim
eter (including chromospheric)and imager; UV+EUV+SXR
spectroscopy and images

16 Ichimoto

Advanced Solar Optical
Telescope (ASOT): A
space-borne
magnetograph for studyof solar eruptions

Magnetic energy buildup leading
to eruptions

II-1, II-2, II-3

Magnetograph/spectropolarim
eter and imager

17 Parenti

Linking the small scales
dynamics to the large
scales variation of thesolar atmosphere

Magnetic energy buildup leading
to eruptions; Coronal heating;Solar wind acceleration

I-1, I-2, I-4, I-5, II-1,
II-2, II-4

UV spectroscopy, spatiallyresolved; Magnetograph and
EUV imager (full disk)

18 Orozco Suarez

Magnetic energy buildup leading
MACHROS: The MAgnetic to eruptions; Magnetic
CHROmosphere Sentinel geometry/topology/reconnection

Magnetograph/spectropolarim
I-1, I-3, I-5, I-6 , II-2, eter and imager(chromosphere)
II-3, II-4, II-6

19 Casini

Stereoscopic
Magnetometry of the
Chromosphere

Magnetic geometry/topology -chromosphere

Magnetograph/spectropolarim
I-1, I-5, I-6, II-1, II-2, eter, stereoscopic (UV
II-4chromosphere)

20 LinA Space Coronal
Magnetometry Mission

Magnetic
geometry/topology/reconnection - I-5, I-6, II-1, II-2, II- large scale
3, II-4

Coronagraphic
spectropolarimeter,stereoscopic

21 Sterling

Solar Explosions Imager(SEIM): A NextGeneration HighResolution and HighCadence EUV Telescopefor Unraveling Eruptive
Solar Features

Magnetic
geometry/topology/reconnection - small scale
I-1, II-2, II-3

UV/EUV imaging

22 Ishikawa

High-Precision
Spectropolarimetry in
Ultra Violet (UV)

Magnetic
geometry/topology/reconnection;
Coronal heating
I-1, I-3, I-4, I-5

Spectropolarimeter andimager (UV chromosphere)

23 Peter

Magnetic
geometry/topology/reconnection;
Understanding the Upper Coronal heating; Heating processes
Solar Atmospherein flares/CMEs
I-1, I-2, I-3, I-4, II-4

(E)UV spectroscopy, spatiallyresolved;
Magnetograph/spectropolarim
eter; coronal imager andspectropolarimeter

24 DeLuca

Magnetic
geometry/topology/reconnection;
Heating processes in flares/CMEs;
NGSPM Energetic Events Probing flare processes via particle
in the Solar Atmosphere acceleration
I-2, II-4, II-6

Coronagraphic
spectropolarimeter;
EUV+SXR+HXR imaging andspectroscopy

25 Matthews

Non-thermal distributions
and energy transport in Probing flare processes via particle
II-6
the solar flares
acceleration

UV and X-ray spectroscopy,
spatially resolved;
Magnetograph/spectropolarim
eter and imager

26 ShihIon Acceleration in Solar
Eruptive Events

Gamma rays, ENAs, neutrons

Probing flare processes via particle
acceleration
II-6

21

27 Christe

Exploring impulsive solar
magnetic energy releaseand particle accleration
Probing flare processes via particle
with focused hard X-ray acceleration; Magnetic
imaging spectroscopy
geometry/topology/reconnection II-4, II-6

X-ray spectroscopy, spatiallyresolved (hard)

28 AppourchauxSolar dynamo and magnetic
Exploring the poles of the dynamics; internal structure;
Sun: POLAR Investigation irradiance; space weatherof the Sun (POLARIS+)
monitoring

II-1, II-2, II-3, III-1,
III-2, III-4

Magnetograph, White-lightcoronagraph; EUV imager; UV
spectrograph; TSI monitor -out-of-ecliptic

29 Vourlidas

Solar Polar Diamond
Explorer (SPDEx):
Understanding theOrigins of Solar Activity
Using a New Perspective

Solar dynamo and magnetic
dynamics; internal structure;magnetic topology/geometry/reconnection - large scale; solar
wind acceleration; Irradiance;
Space weather monitoring

I-5 , II-3, II-6, III-1,
III-2, III-4

Magnetograph, Coronagraph,
EUV imager, Heliospheric
imager, TSI monitor, UV
spectrograph, in situ
measurements -- , out-ofecliptic

30 Judge

The need to move far
from Earth

Solar dynamo and magnetic
dynamics; Space weather
monitoring

I-4, I-5, II-1, II-3

Magnetograph, coronagraph;out-of-ecliptic or stereoscopic

31 Collet

Lower solar atmosphereand magnetism at ultrahigh spatial resolution

Solar dynamo and magnetic
dynamics; Coronal heating

Magnetograph/spectropolarim
I-1, I-2, I-3, II-1, II-2, eter and imager (3-meter
II-5, III-1, III-2, III-3 telescope)

HiRISE: High Resolution
Imaging andSpectroscopy Explorer(E)UV and X-ray spectroscopy,
spatially resolved;
Magnetograph/spectropolarim
eter and imager;
Coronagraphic
Solar dynamo and magnetic
dynamics; Magnetic
I-1, I-2, I-3,1-4, 1-5, spectropolarimeter,
heliospheric imager, in-situ
geometry/topology/reconnection; II-1, II-2, II-3, II-5,
measurements, TSI monitor
solar wind acceleration; Irradiance III-1, III-2, III-4

32 Erdelyi

Coronal heating; Internal structure;
ICARUS: Investigation of Solar wind acceleration; Magnetic
Coronal AcceleRation and geometry/topology/reconnection - large scale
33 Krasnosselskikh heating Up to the Sun
I-5, III-1, III-2, III-3

34 BrunAll Around the Sun:advocating for
coordinated multivantage views multispace missions
observations of our star

Space weather monitoringin situ particles, fields, andwaves (ICARUS-1); UV
spectrometer, coronal and
HXR photometer,
magnetograph (ICARUS-2)

Concept, rather
than scienceobjective: 4pi
steradians
continuous
observations ofessentially all kinds All kinds of instruments,of solar output
stereoscopic22

C h a p te r T h re e : S c ie n c e O b je c tiv e s a n d T a s k s
3.1 Description of Objectives

To identify the science objectives of the NGSPM, the SOT used various sources. These
included discussions within the Team, community input (in the form of white papers) and theSolar-C proposal of 2015. This set of objectives formed the basis for the selection of the mostcompelling scientific problems to be addressed by the NGSPM.
All candidate objectives were subjected to a thorough evaluation to assess their interest,
timeliness, and scientific/technical feasibility. The following specific criteria were consideredby the SOT:
1. Relevance to NASA/JAXA/ESA objectives
2. Scientific impact on solar physics
3. Scientific impact on other disciplines and research fields
4. Inability of current/planned missions and ground-based facilities to accomplish theobjective
5. Need for space observations
6. Maturity of technology -- measurements can actually be made
7. Maturity of methodology -- data can be inverted to get findings
8. Widespread interest within the solar physics community
The SOT verified that the objectives are aligned with NASA/JAXA/ESA interests(criterion 1), as set forth in the corresponding agency programs or relevant documents (e.g.,
US 2013-2022 Decadal Survey in Solar and Space Physics, ESA's Cosmic Vision Programme,
JAXA's long-term strategy for space science in Japan). With this requirement cleared, the main
criteria leading to the final selection of objectives for the NGSPM were their scientific meritand expected impact (2), the inability of current/planned missions and ground-based facilities
to address the relevant scientific problem (4), and the need for space-based measurements (5).
Slightly lower weight was given to the maturity of technology and methodology (6, 7).
However, no objective was adopted unless the feasibility of the measurements and theirinterpretation was established. The remaining criteria were regarded as of somewhat lower
importance (3, 8).
Special attention was paid to assessing whether the scientific objectives could beachieved with existing or planned facilities, either on the ground or in space. The SOT
acknowledges that some of the questions may partly be solved using ground-based
observations, but those measurements are very often complementary rather than overlapping.
For truly transformative results, the benefits of space - in particular access to UV, EUV and Xray spectral windows, stability, large fields of view and long-duration measurements - turn out
to be essential in nearly all cases. To be specific, the next generation of 4-m class ground-basedsolar telescopes will provide observations of the photosphere and chromosphere at ultra-highspatial resolution, but this will be over relatively small fields of view and with incomplete
coverage in time. A space-based platform makes it possible to observe, monitor and track larger
fields of view for long periods of time, as well as providing simultaneous access to thetransition region and the corona for seamless observations of the entire solar atmosphere. These
capabilities are particularly important for studying the long-term evolution of both the quiet
Sun and active regions. For example, assessing coronal heating by braiding necessitates23following individual magnetic elements for long periods of time over large fields of view tocover several supergranules, which can only be achieved from space. Understanding theevolution of, and energy build-up in, a flaring active region requires continuous tracking of thewhole region from emergence - typically taking 1-2 days - through the onset and developmentof strong flaring activities once the majority of flux has emerged. Synergies with Solar Orbiter,
the Parker Solar Probe, the Daniel K. Inouye Solar Telescope or other ground-based facilities
were considered an asset in almost all cases.
Only those objectives that fulfilled a majority of criteria to a significant degree wereretained. The evaluation process led to the selection of three primary scientific objectives for
the NGSPM: (I) Formation mechanisms of the hot and dynamic outer atmosphere, (II)Mechanisms of large-scale solar eruptions and foundations for predictions and (III)Mechanisms driving the solar cycle and irradiance variation -- each with multiple subobjectives.
The full list of objectives and sub-objectives is presented in Table 3-1 below, along with
key observational tasks that would enable a forthcoming solar physics mission to makeprogress on the scientific objectives. The tasks were identified through Team discussions andalso via input from the community. A brief summary of each sub-objective was drafted by theTeam to delineate the current state of knowledge within solar physics and the main open
questions. These summaries are collected in Appendix B, where the tasks and associatedmeasurements are also given.
Table 3-1: Science Objectives, sub-objectives, and tasks
Sub-objectives

Tasks

I: Formation mechanisms of the hot and dynamic outer solar atmosphere.
I-1

understand the formation mechanism ofchromospheric fine scale structures and theirinfluence on the corona

I-1-1: magnetic topology and dynamics at
footpoints of jets
I-1-2: MHD waves, role in driving jets and heating
chromosphereI-1-3: coronal response to jets

I-2

test the nanoflare heating hypothesis

I-2-1: occurrence spectrum at better sensitivity
I-2-2: very hot component and non-thermal
motionI-2-3: thermal response to braiding

I-3

test the wave heating hypothesis

I-2-4: formation of braiding, relation tomagnetoconvection
I-3-1: energy flux, propagation and mode
conversion of MHD waves
I-3-2: excitation of MHD waves
I-3-3: location and mechanism of wave dissipation

I-4

understand role of flux emergence inatmospheric heating

I-4-1: topology of emerging flux and interaction
with surroundings

24

I-4-2: flux emergence/cancellation rates

I-5

identify sources and driving mechanisms of thesolar windI-4-3: transfer and release of energy carried into
atmosphere by emerging flux
I-5-1: magnetic field geometry at the base of thesolar windI-5-2: coronal magnetic field configuration
I-5-3: wind mass and energy supply due to
spicules in different regions
I-5-4: source regions of solar wind using massfractionation
I-5-5. acceleration profile
I-5-6: wave energy in the corona

I-6

understand the formation mechanism ofprominences

I-5-7: resonant heating; anisotropy, FIP
dependence
I-5-8: plasma parameters in the large-scalecoronaI-6-1: magnetic field structure
I-6-2: prominence mass supply/circulation
I-6-3: magnetic conditions of formation anddestabilization

II: Mechanisms of large-scale solar eruptions and foundations for predictions
II-1

measure energy build-up in flaring and CME
regions

II-1-1: large scale field configuration by
extrapolation from surface magnetic field
II-1-2: Poynting flux through the photosphereII-1-3: dark filament structure and development
prior to eruption
II-1-4: coronal magnetic field in eruptive regions

II-2

identify the trigger mechanism of flares andCMEs

II-2-1:chromospheric fine structure evolution
II-2-2: reconfiguration of coronal magnetic field
II-2-3: change of electric current system

II-3

understand the evolution and propagation of
CME and their effect on surrounding corona

II-3-1: kinematics of CME including rotation,
acceleration, and interaction
II-3-2: reconnection during the eruptionII-3-3: shocks and other waves associated with
CME

II-4

understand the processes of fast magnetic
reconnection

II-4-1: discontinuities in the magnetic field
II-4-2: key parameters determining reconnectionrate
II-4-3: temperature, density, velocity structuresassociated with reconnection

25

II-5

understand the formation of (delta) sunspots

II-5-1: origin and properties of subsurface field
II-5-2: coupling of convection and surface fields

II-6

understand particle acceleration and flareenergy transport

II-5-3: sheared polarity inversion lines and related
coronal structures
II-6-1: electron and ion distributions in coronaand chromosphereII-6-2: electrons in flight through solar
atmosphereII-6-3: dynamic response of the lower atmosphereII-6-4: evidence of Alfven waves transporting
energyIII: Mechanisms driving the solar cycle and irradiance variation
III-1

III-2

measure flow structures in the solar convectionzone

III-1-1: internal flows at high latitudes

locate and trace the global magnetic flux in theSun

III-2-1: acoustic anomaly in the deep convectionzone
III-2-2: flows in flux tubes near tachocline

III-1-2: meridional flows in the deep convectionzone
III-1-3: global convection features

III-2-3: origin of polar magnetic fields
III-3

III-4

quantify the turbulence in the dynamo
mechanism

III-3-1: small-scale helicities in the photosphere

understand the mechanism of solar irradiancevariations

III-4-1: brightness and elemental structures in UV
sources

III-3-2: kinetic/magnetic energy in turbulent
convection

III-4-2: Model construction of TSI and SSI

III-5

explore the deep internal structure of the Sun

III-4-3: long-term variability of photometricintensity
III-5-1: Detection of g-mode for understanding its
excitation and travel of waves in the Sun (and forinvestigating the solar core)

Nearly half of the sub-objectives identified for the NGSPM are new compared with theoriginal Solar-C proposal (8 out of 17). These include I-4, II-3, II-5, II-6, III-1, III-2, III-3, andIII-5. The rest of the sub-objectives were present in the Solar-C proposal, and remaincompelling. However, some of these have been substantially updated to take into accountrecent developments in solar physics (I-5, II-1, II-2, II-4, III-4).

26

3.2 Required measurements and Notional Instruments

Each of the tasks listed in Table 3-1 implies certain requirements for instrumental
capabilities, such as spatial resolution, wavelength range, temperature sensitivity, etc. The
measurements are to be carried out by science instruments, the details of which are normallydefined in mission proposals. Each sub-objective in Table 3-1 has multiple key observations,and several sub-objectives in the list share the same key observations, so that the Team has
identified notional ‘strawman’ instruments to execute each observation. As a result, it becomes
clear how many instruments are required for each sub-objective. Table 3-2 provides the list of
key observations (OBS-##) and notional instruments (T-##). These notional instruments
include several possible implementations that will need to be defined in mission proposals.
One example is instrument T-01, 0.1” Photospheric magnetograph, that could be either a filter
based instrument or a scanning spectro-polarimeter or a combination of both types of
instruments. In some cases, utilization of key technologies and thoughtful selection of someobservational parameters such as observing wavelengths can enable multiple requirements tobe met by a single instrument. Independent multiple instruments may be realized as a single
payload package in a proposal responding to a future AO. For example, T-01, T-04 and T-05
may be combined into a single instrument, and T-09 might be upgraded to cover the capabilities
of T-05 by extending the wavelength range to the UV where chromospheric spectral lines areformed.
Note that the “possible wavelength range” in Table 3-2 is neither exclusive (instruments
in neighboring wavelength ranges may be just as successful), or necessary (i.e. instruments
need not operate in all the listed wavelength bands).
Table 3-2: Notional instruments for required measurements, motivated by the scienceobjectives (see Appendix B).
Required Measurements
ID

OBS-1

OBS-2

Description
Imaging observationsof the photosphericradiance at highresolution of 0.1 arcsec
Imaging observationsof the photosphericvelocity and magnetic
fields at high resolutionof 0.1 arcsec
Spectro-polarimetricobservations of thephotospheric velocity
and magnetic fields athigh resolution of 0.1
arcsecImaging observationsof the photosphereand chromosphere at
resolution of 0.2 arcsec

Notional Instrument

Spatialresolution

Temporalresolution

FOV

Possible
Wavelength
range

ID

Instrument
Type

T-01

0.1"
photosphericmagnetograph

0.1"

10 s

>200"

NUV, visible,
IR

T-02

0.2"-0.5”
photospheric &chromospheric0.2"-0.5”

10 s

>300"

NUV, visible,IR

27

OBS-3

OBS-4

OBS-5

OBS-6

OBS-7

OBS-8

OBS-9

Imaging observationsof the photospheric(0.3”) andchromospheric (0.5”)
velocity and magnetic
fieldsFull-disk observationsof the photosphericvelocity and magnetic
fields at resolution of 1
or a few arcsec
Imaging observationsof the chromosphereat high resolution of
0.1 arcsec

Imaging observationsof the chromosphericvelocity and magnetic
fields at high resolutionof 0.3 arcsec or better

Spectroscopic
observations of thechromosphere at highresolution of 0.1 arcsec
Full-disk observationsof the chromosphericradiance at resolutionof 1 arcsec
Imaging observationsof the warm corona athigh resolution of 0.2
arcsecImaging observationsof the hot corona athigh resolution of 0.6
arcsecImaging observationsof full-disk coronal
activity
Spectroscopic
observations of thecorona and transition
region at highresolution of 0.3 arcsec

imagingmagnetographFull-disk
photosphericmagnetograph

~1"

1 min

full
disk

VisibleT-04

0.1"--0.3"
chromospheric
imager andmagnetograph

0.1"(imaging,
velocity)
0.3" (highprecision
magnetic
field)

10 s

>200"

NUV, UV,
visible, IR

T-05

0.1"
chromosphericspectrograph

0.1"

10 s

>300"

UV

T-06

Full-disk
chromospheric
imager

~1"

10 s

full
disk

UV

T-07

0.2"--0.6"
coronal imager

0.2" (EUV),
0.6" (SXR)

10 s

>300"

EUV, SXR

T-08

Full-disk
coronal imager

0.3"

10 s

full
disk

EUV, SXR

T-09

0.3" coronal/TR
spectrograph

0.3"

1 sec

>300"

EUV, SXR

T-03

28

OBS-10

OBS-11

OBS-12

OBS-13

OBS-14

OBS-15Spectroscopic imaging
of superhot andnonthermal coronalemission produced insolar flares with a low
background
Imaging observationsof coronal density
structures between~1.1 and ~15 solar radii
above the limb
Synoptic coronal vector
magnetic field andtopology (1”-2”),higher time resolution
CME precursor
evolution (10”).
Spectroscopic
observations of thecoronal velocity in theacceleration site ofsolar winds (2Rs < R <5Rs)
Imaging observationsof the extended coronaand heliosphere
Photometricobservations of the Sun
as a star

T-10

High-energy
spectroscopic
imager2"-5"100 msec

T-11

Large-angle
coronagraph~2"

1~5 min

T-12

Coronagraphic
polarimetersynoptic(1-2”) ;CME
precursor~
10”

synoptic1-5
minute ;CMEprecursor~ 1 hrglobalVisible, IR,
UV

T-13

Coronagraphic
spectrograph

~1"

~1 hr

>300"

UV, visible

T-14

Heliospheric
imager~1 deg~1 hr

globalVisibleT-15

Total Solar
Irradiance (TSI)and Solar
Spectral
Irradiance (SSI)
monitors~1 deg

1min

full
disk

All for TSI;UV for SSI

>300"

HXR

Visible3.3. Choosing a path for NGSPM

Each of the SOT members assessed the objectives and sub-objectives based on the eight
criteria listed in Section 3.1 above and the results were discussed in detail within theTeam. Overall the evaluations of each team member were in remarkable agreement. Some
criteria displaying larger discrepancies were subject to specific scrutiny. Most of thediscrepancies were found to be caused by the different assumptions and considerations madeby the members, and thus could be removed easily by agreeing upon the terms of evaluation.
The SOT acknowledges that, although the full range of science objectives in Table 3-1
are compelling, supported by the community, and technically feasible within the next decade,no single mission could achieve all the proposed objectives. Thus, the Team found it useful toconsider conceptual groupings of the objectives to clarify how the most science could beachieved within expected constraints and resources.
The SOT’s review of the current state of heliophysics, including recent discoveries anddevelopments, indicates that the discipline’s scientific objectives can be roughly divided into
two avenues: physical mechanisms on elemental (small) scales, versus global processes
29affecting/involving large fractions of the solar interior and/or atmosphere, and propagating into
the solar wind. This division is imperfect and subjective, as the processes are certainly notdecoupled. However, for the purposes of planning a mission to carry a suite of instruments, itis necessary to consider the physical scales of the observables, and this ‘bimodal’ division isuseful. Such a viewpoint is supported by the input from the white papers. It is the sense of the
SOT that the better near-term opportunity for breakthrough discoveries by the Next Generation
Solar Physics Mission will be achieved through the former avenue, probing beyond the stateof our present spatial and temporal resolution. Global, multi-vantage observations are
scientifically compelling, but also more technically challenging.
Nevertheless, we acknowledge that both avenues, “global” and “elemental”, havedistinct merits. We now briefly review these merits before presenting a prioritized, notional
instrumentation suite for the elemental avenue in section 3.4.
Table 3-3: Global-scale science questions addressable with multi-vantage observations
Scientific motivation

Orbit/vantage

Instruments required

Constrain models of the solar
interior, including solar-cycle
variation

Sustained away from Sun-Earthline -- either in-ecliptic or
polar/high latitude

Doppler-magnetograph (T-03)

Constrain magnetic fields andflows at poles, including solarcycle variation

Sustained polar/high latitude

Doppler-magnetograph (T-03)

Full spherical magnetic boundary
evolution

Sustained away from Sun-Earthline -- either in-ecliptic, or
polar/high latitude in bothhemispheres

Doppler-magnetograph (T-03)

Earth-directed CME and solar
wind analysis and monitoring(monitoring places requirement
for sustained measurements)(Sustained) away from Sun-Earthline -- either in-ecliptic or
polar/high latitude. Highlatitude view enables direct Bzmeasurement by polarimetriccoronagraph.

White light coronagraph/heliospheric Imager (T-11),
EUV/SXR imager (T-08),
polarimetric coronagraph (T-12)
coronagraphic spectrograph (T-13)
heliospheric imager (T-14)

Constrain models of 3D magnetic
fields, e.g., throughstereoscopy/tomography, vector
ambiguity resolution (sustained
measurements enable synoptic
models)(Sustained) away from Sun-Earthline -- either in or out of ecliptic

Chromosphericspectropolarimeters (T-02),
polarimetric coronagraph (T-12),
EUV imager (T-08)

Latitudinal variation of solar
irradiance over solar-cycle timescalesSustained polar/high latitudeTotal solar irradiance monitor (T15), UV spectrograph (T-05)

30

3.3.1 Global, multi-vantage observations
About a quarter of the white papers (9/341) argued for the large benefit of vantage points
away from the Earth-Sun line, including both in or near-ecliptic stereo views or out-of-theecliptic high-latitude views. The science motivating these observations is compelling. Table 33 highlights two major gaps in our current observational capabilities that could be addressedby such observations. First, although the STEREO and Ulysses missions moved away from
Earth-Sun line, neither observed the solar magnetic fields and flows needed for progress on
several key science topics enabled by these vantages. Solar Orbiter will for the first time obtainremote sensing observations out of the ecliptic. However, like STEREO, Ulysses, and theParker Solar Probe, it will not maintain a sustained vantage point -- a second requirement on
several science goals. Sustained polar/high-latitude views (which, depending on the sciencegoal, may range from multiple months to the solar cycle) can be achieved by a constellation of
spacecraft or by a single spacecraft on a carefully selected orbit.
Transformative progress would certainly be made possible by sustained, multi-vantage
views. Through the white papers, the community suggested a variety of implementation
options, addressing required technologies and orbits, and emphasizing the need forconstellations of spacecraft and potentially the use of a communication hub (mother ship). Suchcomprehensive implementation schemes likely exceed the resources available for a NGSPM
on the timescale of the next decade. However, international coordination over the upcoming
decades is desirable to enable the development of an optimized armada of spacecraft that would
fill critical vantage gaps, and thus obtain a truly global understanding of the Sun.
3.3.2 Elemental-scale activity
Solar activity on the smallest length and time scales illuminates fundamental physicalprocesses, often with less complex interactions and configurations than larger-scale analogues.
For one example, active region microflares can be considered as scaled-down versions of solar
flares, with much simpler magnetic topologies; and the time profiles of their brightenings, andtheir energy budgets, can be analyzed and reproduced with models much more simply thanflares that encompass whole active regions. Similarly, recent observations demonstrate thatwaves can be observed propagating along the bodies of individual spicules, such that timevarying wave amplitudes and phase speeds can be monitored; such quantitative precision islost in studies of unresolved ensembles of strands in coronal loops, and is possible only withadequately high spatial and temporal resolution. To empirically quantify the fluxes of energy
and mass, and to accurately characterize the dynamics and morphology relevant to fundamental
physical mechanisms which lie at the heart of the larger events, it is necessary to resolve thescales on which such mechanisms occur.
Game-changing discoveries may be found on these elemental scales, such as the natureof magnetic stresses resulting from braiding of magnetic field lines; the magnetic topology at
the footpoints of spicules, jets, and prominences; the transport of energy via Alfvén waves inflares; and the development and effects of turbulence in all levels of the solar atmosphere.
Specific examples of sub-objectives for which progress will substantially benefit fromobservations with increased resolution include “Processes of fast magnetic reconnection” (II4), “Identify[ing] the trigger mechanism” (II-2), and nearly all of the sub-objectives relating to
Objective I “Formation mechanism of the hot and dynamic outer solar atmosphere”. Most of
1

Appourchaux; Brun; Casini; Judge; Krasnosselskikh; Lin; Orozco Suárez; Plowman; Vourlidas

31the Tasks for these science topics make use of wavelengths that are not accessible from theground or require long-duration observations in a stable, seeing-free environment,
necessitating observations from a space platform.
There is every reason to believe that a higher-resolution focus for NGSPM will be richscientifically. Recent missions such as the Sunrise balloon-borne observatory (0.1” resolution),
Hi-C sounding rocket (0.2” resolution EUV imager), IRIS (0.3” resolution UV spectrograph)and Hinode (0.3” resolution photospheric spectropolarimeter) have yielded a disproportionate
number of results and publications. Given current constraints, continuing to increase resolution
is both feasible and well-suited to NGSPM near-term opportunities. In particular, we note thata great number of high-priority tasks in Table 3-1 can be achieved with a specific, modular
suite, as we now describe.
3.4 Team Conclusion: An optimal instrumentation suite for elemental science

In consideration of a minimum set of instruments with which NGSPM can address thegreatest number of high-priority Tasks consistent with the objective of small length- and timescale activity, we find that a majority of the key required measurements can be met with a suiteof instruments including:
1.
2.
3.
4.
5.

0.3" coronal/TR spectrograph (T-09)
0.2"- 0.6" coronal imager (T-07)
0.1"- 0.3" chromospheric imager and magnetograph (T-04)
0.1" photospheric magnetograph (T-01)
0.1" chromospheric spectrograph (T-05)

While other combinations of candidate instruments would likely also produce
significant results, the list above represents the smallest set of instruments addressing as manyas 32 of the 56 listed Tasks; it is the list of instruments that the SOT feels should receive thehighest priority for selection. For completeness, Table 3-4 indicates all the instruments thatwere deemed by the SOT to be needed for each of the Tasks.
Table 3-4 also serves to emphasize the necessity and value of combinations of
instruments: certain instruments like the coronal/TR spectrograph need to be operated intandem with an imager for context and pointing knowledge. Indeed, the great majority of theTasks in Table 3-4 require some combination of instrumentation for closure or significant
progress. Thus the strawman suite of instruments considered above has been optimized tomaximize the scientific return from the NGSPM. Asterisks in Table 3-4 denote the Tasks thatare fully addressed by the strawman suite of instruments. Obviously some of the Tasks are
addressed incompletely (e.g., II-3-3), and some not at all (e.g., I-5-5), due to the lack of someneeded capability, for instance a full-disk imager. It is anticipated that the NGSPM will build
upon synergies with other missions that will have such complementary capabilities (e.g., SDO
or its follow-on) and with ground-based telescopes to accomplish additional measurements.
Conversely, it is anticipated that the NGSPM will complement these facilities by granting
access to the upper solar atmosphere. Indeed, the NGSPM is timely in view of the next
generation of large solar telescopes coming online in the next decade (DKIST will see firstlight in 2020, and EST first light is planned for 2026). The next section of this report (Section4) considers specific potential mission architectures based on the suite of instruments listed
above.

32

Table 3-4: Instruments contributing to each Task. Asterisks denote the tasks that are fully
addressed by the strawman suite of instruments. Note that some tasks require multivantage observations as described in Table 3-3.
Task
I-1-1*: magnetic topology at footpoints of jets
I-1-2*: shock waves as driver of jets
I-1-3*: coronal response to jets
I-2-1*: occurrence spectrum at better sensitivity
I-2-2*: very hot component and non-thermal motion
I-2-3*: relation to braiding
I-2-4*: formation of braiding
I-3-1*: propagation; energy flux, mode conversion
I-3-2*: excitation;
I-3-3*: dissipation
1-4-1*: topology of emerging/cancelling flux
I-4-2*: emergence rate
I-4-3*: transfer and release of energy in atmosphereI-5-1: magnetic field geometry at the baseI-5-2: coronal magnetic field
I-5-3*: mass and energy supply from spicules
I-5-4*: source region of windI-5-5. acceleration profile
I-5-6*: wave energy in the corona
I-5-7: resonance heating; anisotropy, FIP dependence
I-5-8: plasma properties in the large-scale corona
I-6-1*: magnetic structure
I-6-2*: mass supply
I-6-3*: condition of formation
II-1-1: extrapolation from surface magnetic field
II-1-2*: Poynting flux through the photosphereII-1-3*: dark filamentII-1-4: coronal magnetic field
II-2-1*: change in chromosphereII-2-2*: coronal reconfiguration
II-2-3*: change of electric current system
II-3-1: kinematics including rotation, acceleration of CME
II-3-2: reconnection during the eruptionII-3-3: shocks and other waves associated with CME
II-4-1*: discontinuity of magnetic fields
II-4-2*: key parameters determining reconnection rate
II-4-3*: temperature, density, velocity associated withreconnection
II-5-1: subsurface field
II-5-2*: convection and surface field
II-5-3: sheared polarity inversion lines and related coronal
structures
II-6-1: evolution of electron and ion distributions
II-6-2: electrons in flight through solar atmosphereII-6-3: dynamic response of the lower atmosphere

Candidate Instruments Needed
T-01, T-04
T-01, T-04, T-05, T-07, T-09
T-05, T-07, T-09
T-07, T-09
T-07, T-09, T-10
T-07, T-09
T-01, T-04, T-07
T-04, T-05, T-07, T-09
T-01, T-04, T-07, T-09
T-04, T-05, T-07, T-09
T-01, T-04, T-05, T-07, T-09
T-01, T-04
T-01, T-04, T-05, T-07, T-09
T-01, T-03, T-04
T-12
T-07, T-09
T-09
T-11, T-13
T-07, T-09
T-09, T-13
T-11, T-12, T-13
T-01 or T-02, T-04
T-04, T-05, T-07, T-09
T-01 or T-02, T-04
T-01 or T-02, T-03, T-04
T-01 or T-02
T-01 or T-02, T-04, T-05
T-12, T-08
T-04, T-05
T-07 or T-08, T-09, T-12
T-01 or T-02, T-04, T-07 or T-08, T-12
T-05, T-07, T-08, T-11, T-13, T-14
T-07, T-08, T-09, T-10, T-11, T-13
T-07, T-08, T-09, T-11, T-13, T-14
T-04
T-04, T-07, T-09
T-07, T-09
T-03
T-01 or T-02, T-04
T-01, T-03, T-04, T-05, T-07, T-09
T-07, T-10
T-07, T-10
T-04, T-05, T-09, T-15

33

II-6-4*: evidence of Alfven waves transporting energy
III-1-1: Internal flows at high latitudes
III-1-2: meridional flows in deep convection zone
III-1-3: Search for global convection features
III-2-1: acoustic anomaly in the deep convection zone
III-2-2: Detection of a flow in flux tube near tachocline
III-2-3: Origin of polar magnetic fields
III-3-1*: small-scale helicities in the photosphereIII-3-2*: kinetic/magnetic energy in turbulent convection
III-4-1*: brightness and elemental structures in UV sources
III-4-2: Model construction of TSI and SSI
III-4-3: long-term variability of photometric intensity
III-5-1: Detection of g-mode for understanding its
excitation and travel of waves in the Sun (and forinvestigating the solar core)

T-04, T-05, T-07, T-09
T-03
T-03
T-03
T-03
T-03
T-01, T-03
T-01
T-01
T-01
T-03, T-06, T-15
T-03, T-06, T-15
T-03

34

C h a p t e r F o u r : A s s e s s i n g p o s s i b l e m i s s i o n d e s ig n s
4.1. Top-level priority of instruments

The Charter of the NGSPM-SOT assigns the task of suggesting possible mission
profiles that could satisfy the science objectives identified by the SOT, within the likely
available resources. In this Chapter, we consider several possible mission profiles, appropriate
to either a single large mission, or a constellation of small- or mid-class spacecraft. Although
the principles that the SOT used to suggest instruments for each of these mission profiles could
be applied to any set of instruments, in this chapter, we focus on permutations of the fivesuggested notional instruments discussed in Section 3.4, namely
1.
2.
3.
4.
5.

0.3" coronal/TR spectrograph (T-09)
0.2"-0.6" coronal imager (T-07)
0.1"-0.3" chromospheric imager and magnetograph (T-04)
0.1" photospheric magnetograph (T-01)
0.1" chromospheric spectrograph (T-05)

Of these 5 instruments, T-04, T-01, and T-05 can potentially be merged into a single
telescope with focal plane instruments, when suitable wavelengths in near IR, visible and near
UV are chosen in the design. Therefore, in the subsequent sections, the SOT has consideredsome mission concepts for accommodating the following three instruments.
1. 0.3” coronal/TR spectrograph (T-09)
2. 0.2"-0.6” coronal imager (T-07)
3. 0.1” chromospheric/photospheric magnetograph/spectrograph (T-01/04/05)

4.2. Mission concepts

The three instruments outlined above could be realized in one single large mission. If
not, they could be realized by forming a constellation of small/mid-class missions, as described
below.
Mission Concept 1: Large mission design
This design concept is to have T-09, T-07, and T-01/04/05 instruments on a single
platform. Significant contributions from all the agencies would be required for this mission.
The preferred orbit for this mission is geosynchronous, for uninterrupted solar observing (muchof the year) and high telemetry. There are significant scientific and operational advantages tothe large mission design, including the following:
1. All instruments are mounted on a single bus and hence launched and operated
simultaneously.
2. Instruments are designed from the start to be complementary, with tradeoffs optimized
for obtaining the required measurements.
3. Mission operations are integrated, ensuring coordinated scientific observing and costsaving by avoiding duplication of ground systems.

35

4. Use of a single spacecraft bus, launcher, and project management offices may reduce
total costs.
Mission Concept 2: Constellation of small/mid-class missions
This mission concept is to form a constellation of small or middle-class satellites forrealizing the three instruments, by utilizing spacecraft of JAXA, NASA, and/or ESA (including
ESA member states). Section 4.6 describes the programs of the agencies for missions of thisclass, all of which allow for international contributions to a mission led by one agency.
The merit of this mission concept is to increase the possibility that some of the
instruments are launched as early as possible in the mid-2020’s, but a risk is that the scientificsynergy among the three instruments will be limited unless there is significant overlap in
observing time of the missions. Moreover, another key point is how well the ideal performancerequired for the three instruments can be realized with the limited resource of small and middleclass missions. Orbits for these spacecraft could be low-Earth Sun-synchronous similar to
Hinode and IRIS. However, significantly more telemetry will be required than for either of
those missions.
4.3. Description of the Instruments

Before discussing possible mission design concepts and international coordination, thissection describes strawman designs for the three instruments and their approximate scale.
4.3.1. Design example: T-09 (0.3 arcsec coronal/TR spectrograph, Table 4-1)
About a quarter of the community white papers (8 of 34, Young; Peter: Ugarte-Urra; Imada;Warren; Reep; Klimchuk; Del Zanna) argue the importance of high-throughput EUV
spectroscopy and many of them suggest that an FUV/EUV spectroscopic telescope is ideal forthe proposed science topics, not only in coronal heating, but also in flare dynamics. Key
instrument performance requirements from the science objectives are: 1) high throughput forachieving much improved high temporal resolution, 2) a wide and seamless coverage of plasma
temperature from 0.1 MK to 10 MK, and 3) high spatial resolution (0.3") for distinguishingelementary magnetic structures in the upper atmosphere.
The strawman design concept is a 30cm single mirror telescope with a slit and grating
system, covering EUV and FUV wavelength bands where spectral lines are available fordiagnosing a wide range of plasma temperature from 100 kK to about 20 MK. With appropriate
wavelength choices in EUV and FUV, T-09 could be merged with T-05 to cover from the upper
chromosphere (15 kK) through the TR and to the corona in a single instrument. If the
instrument is more focused on higher temperature plasma for nanoflare heating and flarediagnostics, SXR is also possible. The instrument throughput is required to be about one orderof magnitude higher than previous EUV spectrometers such as Hinode/EIS, SoHO/SUMER.
The requirement for the high spatial resolution (0.3") makes the instrument long, of order 4meters.

36

Table 4-1: Key performance of Strawman T-09 (0.3 arcsec coronal/TR spectrograph)
performance
Temperature
coveragerationale

104–107.3 K

From upper chromosphereand TR to flaring corona

Spatial resolution

0.3”

Coronal loop fine structureTime resolution~1.0 sec ~ 10min

Flare dynamics
MHD time scale of finestructures, evolution of AR

Field of view
[EW×NS]>240”×240”

Large active region andsurroundings

Continuous timecoverage

5 hours (high cadence observation)
2 weeks (active region evolution)

Multi-hour wave studies athigh cadence. Maximum
uninterrupted timescale for
AR evolution is 2 weeks.

PhotometricaccuracydI/I < 0.1 (corona)

4.3.2. Design example: T-07 (0.2 - 0.6" Coronal Imager, Table 4-2)
A coronal imager instrument design has been studied extensively in the US. The AIA
and SUVI instruments now flying are full disk examples, and the Hi-C rocket payload
demonstrated that a normal incidence telescope can achieve spatial resolution of 0.2 arcseconds
or better. The value of such a capability goes far beyond providing context for a coronal
spectrometer, as the science objectives show: studies of loop braiding, waves, hot plasma,coronal connectivity, flare kernels and ribbons, etc. all require high resolution coronal images.
Exposure times of a few seconds (less in flares or bright active regions) are sufficient, so highcadence, multi-wavelength observations will be possible if the mission has enough telemetry(or high-rate data storage onboard). There is considerable freedom in normal-incidence EUV
design to choose the number of telescopes and/or the segmentation of the telescope mirrors to
observe multiple wavelength bands sampling coronal temperatures of 0.7 - 20 MK; optional
additions might be transition region and photospheric or low chromospheric images in the UV.
Image stabilization and sensitive cameras suitable for this instrument have already been
demonstrated in SDO/AIA, Hi-C, IRIS, and Proba-2/SWAP, so there are no technologicalbarriers to overcome. Primary mirror diameter(s) in the 20-25 cm range and instrument
length(s) of 2-3 m can be expected. A complementary approach to such an instrument (without
replacing the T-09 spectrometer) might be a multi-slit spectrometer with rapid scanningcapability. This would not only produce images but also spectra in a few lines for Doppler
shifts and line profiles at each pixel, with a cadence of ten(s) of exposures.
An alternate approach to coronal imaging is a grazing-incidence soft X-ray telescope,with a number of selectable focal-plane filters for analysis of plasmas over a range oftemperatures. Previous coronal imagers of this type include Yohkoh/SXT, Hinode/XRT, andGOES/SXI. As with the EUV imager considered above, sensitive cameras for soft X-ray
telescopes are already available. Sub-arcsecond spatial resolutions are significantly more37

challenging for soft X-rays, however, largely due to the very tight tolerances on smoothness ofthe grazing-incidence mirrors and the long focal lengths that would be required. Achieving
angular resolutions of 0.3 arcseconds/pixel with a typical X-ray telescope (usually the Wolter
Type-I scope) necessitates either enormously long telescope structures (~6 meters), or
significant advances in sensor design (3-micron pixels).
Table 4-2: Key performance of T-07 (0.2 - 0.6 arcsec coronal imager)
performance
5.5

7.3

rationale

Temperature
coverage10 –10K

From hot TR to flaring
coronaSpatialresolution

0.2” (EUV) - 0.6” (X-ray)

Coronal loop finestructure, wave motion
amplitudes

Time resolution~1 sec per image, ~10 sec for all wavelengths

Flare dynamics,
MHD time scale of finestructures, evolution of AR

Field of view
[EW×NS]>240”×240”

Large active region andsurroundings

5 hours (high cadence observation)
2 weeks (active region evolution)

Time series of waves, finestructure evolution; ARevolution

Continuous timecoverage
PhotometricaccuracydI/I < 0.1 (corona)

4.3.3. Design example: 0.1” chromospheric/photospheric magnetograph/spectrograph (T01/04/05, Table 4-3)
In this NGSPM instrument concept, a 1m aperture telescope achieves spatial resolutions of~0.1” for photospheric and chromospheric imaging and spectroscopy, and 0.13~0.5” forphotospheric and chromospheric magnetic field observations. A diameter of 1 meter isnecessary to achieve the required accuracy of polarization measurements in chromosphericlines (3x10-4) at the required spatial sampling (0.25”) and temporal resolution (~10sec). These
spatial resolutions will resolve most elementary structures in the photosphere (magnetic flux
tubes) and chromosphere (fibrils and jets). With its large field of view (>270”), uniform data
quality, continuous time coverage, and high precision spectro-polarimetry at high spatial
resolution, the strawman telescope T-01/04/05 will be a highly synergistic instrument with theforthcoming ground-based large solar telescopes (e.g. DKIST) that will achieve an ultra-high(<0.1”) spatial resolution with a limited FOV and shorter time-spans. The focal plane package
consists of a filtergraph imager and a spectrograph that conduct imaging and spectropolarimetric observations in dedicated spectral lines in wavelengths from 380nm (optionally
280nm) to 1100nm.

38

Table 4-3: Key performance of T-01/04/05(0.1” chromospheric/photospheric magnetograph/spectrograph)
performance
Spatial resolution

imagingpolarimetry

rationale0.1” (Photosphere, chromosphere)

Faculae point, ~ photon
mean free path inphotosphere0.13” (Photosphere)
0.25” (chromosphere)

flux tube diameter
Chromospheric fibril

Time resolution~1.0 sec (imaging)~10 sec (narrow FOV(~40²×40²) magnetograph,dopplergram)~10min (large FOV magnetograph)

Flare dynamics
MHD time scale of finestructures
evolution timescale of AR

Field of view
[EW×NS]~90”×90” (photosphere/chromosphere at 0.1”
resolution),~270”×270” (photosphere/chromosphere at 0.3”
resolution)

Super granulation
Active region, prominence>10”×10” (high cadence chromospheric Dopp. mag.
Obs)

Size of spiculesContinuous timecoverage

5 hours (high cadence observation)
2 weeks (AR evolution)

PhotometricaccuracydI/I < 0.03 (Photo./chrom. Imaging)10^(-3) photospheric magnetic field,
3 x 10^(-4) chromospheric mag. fielddBL =2 [G], dBT =70 [G]dBL =10 [G], dBT =100 [G]

4.4. Modification of Instruments for constellation of small/mid-class missions

If a constellation of small/mid-class missions is formed to realize the three instruments(T-09, T-07, and T-01/04/05), one (or two) of the instruments will be on board one satellite.
Note that the strawman system configuration of a JAXA Epsilon mission is described inAppendix F. Considering the resources of small/mid-class missions available, some of the
instruments described in the previous section may require a descope of instrument capabilities
with some impacts on their ability to achieve the science objectives. This section discusses
descope options.
4.4.1. Design example: T-02 (0.2”-0.5” photo/chromospheric imaging magnetograph)
The T-01/04/05 instrument for achieving 0.1" spatial resolution (0.1" chromospheric/photospheric magnetograph/spectrograph) is difficult to design under the system condition
given in Appendix F because a 1m telescope cannot be accommodated in the Epsilon envelope.
Thus, the instrument is required to become smaller to fit the system. Here we describe a 0.5mclass telescope (T-02), for the case of a constellation of smaller/medium platforms. Able to fit
into the size and weight constraints of the JAXA Epsilon vehicle, T-02 will be capable of39

photospheric and chromospheric imaging (0.2”) and chromospheric spectropolarimetry (0.5”)
in visible and near infrared wavelengths (and potentially UV). Compared with the T-01/04/05
instrument, T-02 has a spatial resolution reduced by a factor of 2 and a photon throughput
reduced by a factor of 4, and thus will not fully resolve the elementary structures in thephotosphere and chromosphere, and takes four times longer integration time to achieve therequired photometric accuracy at the same spatial resolution. These disadvantages are partly
offset by its larger field of view (~400 arcsec). Due to the above constraints, T-02 will focus
on a subset of the scientific objectives that are covered by the T-01/04/05 instrument, i.e.,
objectives that require a large field of view and continuous long time coverage, such as I-6, II1, II-2, and II-5.
4.4.2. Design Example: T-09 (0.3 arcsec coronal/TR spectrograph) for Epsilon
The length of the strawman T-09 instrument as described in section 4.3.1 is too long to fit
the Epsilon fairing volume with the vertically oriented concept given in Appendix F. Possible
solutions for optimization are 1) to shorten the instrument length to about 3 meters and mountit vertically on the spacecraft bus (an example is given in Figure E-1); and 2) to keep theoriginal dimensions and mount it obliquely on the side of the spacecraft bus. In solution (1),the spatial resolution may be slightly reduced to 0.35”-0.4” with a slight degradation in thethroughput.
4.5. Uniqueness and synergy with other missions

There are a number of solar missions currently operational. These include science
missions SOHO, Hinode, RHESSI, SDO, PROBA2, IRIS, as well as the GOES monitoring
mission (see Appendix C). None of these (except GOES) is currently funded until the firstlaunch opportunity for the Next Generation Solar Physics Mission, although it is reasonable toexpect the operation of at least some of these to be extended until there is an overlap. In addition,
there are a number of ground-based observatories that may also have potential scientificoverlap with NGSPM. Therefore, we briefly consider uniqueness and synergy between thesemissions, observatories and a putative NGSPM.
SOHO: Only a part of the instruments on SOHO are still providing data. None of those thatare still active has any overlap with the instruments on the NGSPM. Therefore, NGSPM will
provide unique science compared with SOHO, whose remaining instruments address mainly
objectives pertaining to the full Sun, complementing the high-resolution science that NGSPM
will carry out.
Hinode: Hinode has a similar profile as NGSPM, but carries smaller instruments withsignificantly lower spatial resolution and a much more limited coverage of wavelengths andheights in the atmosphere. Thus, the Hinode instruments do not adequately cover thechromosphere and transition region and hence do not meet the requirements posed in Table 32, so that they cannot be used to achieve the science goals listed in Table 3-1
RHESSI: This high energy mission (few keV X-rays to a few MeV gamma-rays) has no overlap
in wavelength with the NGSPM, though there is some overlap in temperature between the low
energy end of RHESSI and the top of the temperature range potentially covered by T-07 andT-09. Used as an imager, RHESSI can achieve spatial resolutions of around 2” and time40

resolutions of 4 seconds. It is limited to addressing objectives related to particle accelerationand magnetic reconnection, to the extent allowed by its limited spatial resolution. Here thesynergistic effects are expected to dominate.
SDO: SDO is a mission concentrating on the continuous coverage of the full solar disk at a
spatial resolution of ~1 arc sec. Therefore, it cannot fulfil the science goals identified for
NGSPM, making NGSPM unique relative to SDO. Conversely, SDO is complementary to thehigh resolution instruments proposed for the NGSPM. SDO will provide the context
images/magnetograms in which the high resolution data from the various NGSPM instruments
will be embedded. There is a very high potential for synergy.
Proba 2: With considerable similarity to SDO as far as coronal imaging is concerned (althoughwith lower cadence and restricted passbands), Proba 2 will also not to be able to reach thescience goals of NGSPM, but will be able to work synergistically with it.
GOES: The US national space weather spacecraft have coronal imagers showing the full diskin 6 EUV wavelength bands, with modest resolution. The GOES-16 instrument has just had
first light in 2017, and multiple spacecraft will provide this type of data into the 2030’s. The
spatial resolution of GOES is far too low (around 5 arc sec) to address any of the science goalsof NGSPM, but GOES can provide context data.
IRIS: IRIS is a spectrometer and imager, primarily observing the chromosphere and transition
region, and therefore it has some overlap with T-04, T-05 and T-09. The NGSPM combination
T-04/05 provides significantly higher spatial resolution in the chromosphere, as well as
magnetic field measurements (which IRIS is not capable of). Also, IRIS has very little
coverage of lines formed in the hot transition region or corona, and thus cannot provide thesimultaneous coverage of all temperature ranges of the outer solar atmosphere that T-09 does.
The next solar and/or heliospheric missions due for launch in this decade are Parker
Solar Probe, Solar Orbiter, and PROBA-3 with launch dates in this order.
Parker Solar Probe (PSP): PSP will make in-situ measurements of the inner solar wind andouter corona by flying to within 9 solar radii of the solar surface. The only optical instrument
it will carry is a heliospheric imager. Hence, it will rely on other spacecraft to provide thecontext information from remote sensing instruments looking directly at the Sun, including
high resolution instruments, as on NGSPM. PSP will probe the microphysics of the outercorona and solar wind in a unique manner, but will miss the connections between different
parts of the solar atmosphere that NGSPM will provide.
Solar Orbiter (SO): By orbiting the Sun and leaving the ecliptic, Solar Orbiter will provide
solar images, magnetograms and spectra from different vantage points than the NGSPM. This
will lead to strong synergies as SO and NGSPM will observe the same regions on the Sun fromdifferent directions, allowing them to be probed in a novel manner. SO will also host in situ
instruments to sample the inner heliosphere. Unlike NGSPM, SO will be missing all
spectroscopic information on the chromosphere, critical for achieving many of the NGSPM’sscience objectives. Also, the highly elliptical orbit (which allows the highest resolution to bereached only at perihelion) and very limited telemetry available to Solar Orbiter (orders ofmagnitude less than NGSPM, due to its great distance from Earth) mean that only very limited
high resolution data will be downlinked from SO, not sufficient to reach the NGSPM scienceobjectives.
41

Proba-3: The coronagraph that is at the heart of Proba-3, an ESA formation flying technology
demonstration mission, will allow observations of the corona in visible light to very close to
the solar surface, greatly expanding the reach of space coronagraphs. This instrument willaddress largely different science objectives than NGSPM, and will provide unique data, but atvastly lower spatial resolution and outside the solar disk only.
Ground-based Observatories (GBO): There are a number of ground-based observatories
around the world that study the Sun. The largest operational solar telescopes (in the optical andinfrared) are the NST at the Big Bear Observatory, Gregor on Tenerife, the SST on La Palmaand the Chinese 1-m telescope at the Fuxian Lake Observatory. The 4m diameter DKIST isunder construction and scheduled to start observing around 2019-2020, with the equally large
EST likely following in the 2025-2030 timeframe. In general, ground-based observatories
probe only the photosphere and chromosphere (with the exception of DKIST, which also hascoronagraphic capabilities). Any overlap will therefore be restricted to the instruments T-01/T04/T-05 and T-02. In particular, DKIST and EST will address some of the science objectives
also being addressed by NGSPM. The higher spatial resolution that DKIST and EST will beable to reach, will give them an edge over T-01/T-04/T-05 and T-02, but at the cost of a more
limited FOV (60”x60”), variable spatial resolution (due to seeing) and a much lower duty cycle.
As clearly demonstrated by MDI on SOHO, HMI on SDO and SOT on Hinode, even a smaller
space-based instrument has unique advantages compared with ground-based observatories.
These include the very high duty cycle for nearly uninterrupted observations and long timeseries, and the possibility of getting seeing-free observations over a large field-of-view. Both,NGSPM and DKIST will be very powerful tools that are strongly complementary to each other.
It will therefore be important for NGSPM to observe in conjunction with ground-based
observatories, in particular DKIST (and later EST) in order to combine the unique strengths ofspace- and ground-based resources.
4.6. International coordination

The NGSPM is envisioned as a multi-lateral mission, and as described in the SOT’scharter, “is likely to be a Japan-led mission expected to include substantial contributions fromthe United States and Europe.” One of the purposes of this SOT report is to assist the agencies
in planning the international coordination needed to realize the NGSPM. In this section wereview space programs which have historically been used by the respective agencies to support
missions comparable to NGSPM. After summarizing the three agencies’ mechanisms, we
discuss some possible configurations for a collaborative mission as an exercise for imagining
a framework for NGSPM.
4.6.1. JAXA opportunities
JAXA has space programs of three different mission sizes: (1) strategic large missions(300M$ class) for JAXA-led flagship science missions, launched by the H-IIA or the H-III
rocket currently in development; (2) medium-sized focused missions (<150 M$ class),launched by the Epsilon rocket; and (3) Missions of Opportunity for contributions to a space
mission led by a foreign agency. All are competitive among any science and engineering fields,
and international collaborations are recommended. The opportunities for the first and second
categories are going to appear roughly three and five times in 10 years, respectively. Possiblenear-future opportunities for the NGSPM are the strategic large mission for a launch in ~202742and the competitively-chosen medium-sized mission for a launch in ~2024 and subsequently
in two year intervals. A mission proposal to each satellite mission can be submitted by the
JAXA working groups that have been established under the JAXA Science/Engineering
Advisory Committee.
4.6.2. NASA opportunities
The NASA Heliophysics Division supports flight missions in several classes that may
be used for solar physics research, although all are also open to the heliospheric,magnetospheric and upper atmospheric disciplines. Strategic missions are large missions in
two lines, the Living With a Star Program and the Solar Terrestrial Probes. The Explorer
program has mid-sized missions (MIDEX) and Small Explorers (SMEX), which are highly
competitive among all the disciplines of heliophysics. Finally there are Missions ofOpportunity (MoO), which can be contributions to missions led by other agencies of the US or
foreign countries. The Heliophysics Division recognizes that our science requires many typesof observations in many locations, and so a constellation of spacecraft, known collectively as
the Heliophysics System Observatory (HSO), is the primary asset for which it is responsible.
Strategic missions are usually defined by the Decadal Survey and the Roadmaps
sponsored by the Heliophysics Division. The most recent Decadal Survey (published in 2012)
mentioned Solar-C favorably as a worthwhile example of foreign collaborations, which NASAwas encouraged in a general way to pursue. However, it did not list Solar-C either as a strategic
mission or a specific priority, and so there was no clear mandate for NASA to participate in it
or in the EPIC proposal to ESA, despite the widespread recognition that the scientific goals
were timely and compelling.
A MIDEX mission can be expected to have a budget in the vicinity of $250 M, basedon the Astrophysics MIDEX opportunity of 2016; contributions from other countries oragencies will be permitted as long as they are a modest fraction of the total. The Astrophysics
call for proposals listed several available launch vehicles with large capacities in mass andvolume to a sun-synchronous low earth orbit or geosynchronous orbit. With a cost cap similar
to the 2016 call, a MIDEX might provide two or more of the recommended instruments of thisreport.
The Decadal Survey also recommended that a Small Explorer mission be started every
2 or 3 years. These are typically missions led by a Principal Investigator, who is responsiblefor developing all phases of the mission, including spacecraft, instrument(s), ground system,
mission operations and data analysis. The cost cap for 2016 proposals was $165M, including
a nominal $50M for launch. With this cost cap, a SMEX could provide one of therecommended instruments of this report, perhaps with some reduced capability.
Finally, NASA periodically announces calls for Heliophysics Mission of Opportunity
proposals, typically up to $55M for “partner MoOs” that contribute to another agency’s mission.
This cost cap, which must include the total cost including post-launch mission operations anddata analysis, could support a sizable contribution to an instrument for a foreign agency or asmall complete instrument. This cost cap for an MoO has been used commonly in the past;but, as far as the SOT is aware, the NASA Heliophysics Division could allow a higher cost cap
and/or suggest that it be used for a specific mission such as NGSPM.

43

4.6.3. ESA opportunities
There are two possible ways to contribute to an international mission within the ESA
system. The first is via a proposal to a mission call, either a Medium (M) class mission or a
Small (S) class mission call. The missions are selected in competition between all of
astrophysics, solar system science, and fundamental physics. Previous M missions have had abudget of up to 550 M€, and are normally ESA led. At the time of writing, it is unlikely thatthe mission selected from the next M-mission call will be launched before 2030. The mostrecent S mission (S2) is a bilateral mission with a budget of up to 106 M€ with a maximum of
53 M€ from ESA.
The second way is to propose a contribution to a mission led by another agency via a
Mission of Opportunity, which does not have any particular announcement date and can beproposed whenever the opportunity arises. Missions of Opportunity have a cost cap of 50 M€.
In the European system, ESA typically provides support for spacecraft, launch,subsystems, telescopes, operations, but rarely for post-focus instruments. These are funded bynational agencies or national funding programs. This means that the total amount of funding
available from European sources can, in principle, be larger than the maximum that ESA canprovide.
4.6.4. Possible collaborative configurations
A variety of configurations are possible for accommodating the instruments of NGSPM,
depending on (among other things) the contributions of the partnering Agencies, the size/scopeof the selected instruments, and the decision whether to accomplish the NGSPM mission witha single platform or multiple (presumably smaller) platforms on separate spacecraft. Here we
describe some possible collaborative configurations; this is intended not as an exhaustive list,
but as only a few examples to illustrate avenues that the SOT has considered as potential
recommendations for the agencies.
In Figure 4-1, two possible collaborative configurations are represented. In the first(top row), NGSPM is realized as a single space platform by a JAXA Strategic Large mission,
with contributions for instruments, operations, or data processing provided from NASA and
ESA. The NASA contribution would likely have to be of a scale similar to a SMEX. In thesecond configuration (bottom row of Fig. 4-1), the NGSPM instruments are distributed among
a constellation to comprise a JAXA Epsilon, a NASA SMEX or MIDEX, and/or an ESA Sclass mission. Depending on the resources and priorities of the respective agencies, thisconfiguration could take the form of only two spacecraft, possibly with support from the thirdagency via a MoO. Regardless of which agencies provide the spacecraft for this latter
configurations, at least two space platforms are required to achieve the science objectives ofNGSPM.

44

Figure 4-1: Possible configurations for collaborative NGSPM mission
A single-platform configuration of the type represented in the top row of Fig. 4-1 is the
SOT’s highest priority recommendation, for the following reasons. Organizationally andlogistically, the multi-spacecraft configurations are the most challenging: delays in selectionof instrument proposals, in construction/integration/ testing, and in launch of the spacecraft
could easily result in the separate platforms being launched out of sync. Moreover, we findthat the 1m diameter of the T-01/04/05 telescope can only be accommodated on a platform aslarge as the Strategic Large platform, so that any configuration which places thephotospheric/chromospheric instruments on a smaller platform will necessitate T-02 instead of
T-01/04/05.
In terms of the required combination of coronal/TR spectroscopy and imaging, andlower-atmospheric magnetometry, this large mission has evident similarities to the previouslyproposed Solar-C mission. However, the largest telescope (T-04) has decreased in size, its focal
plane package is less complex, and the launch cost has been reduced significantly. We canenvision the international contributions being simpler and with cleaner interfaces.
If a single-platform configuration is not possible or available, another configuration
which might be plausible is a constellation comprised of a NASA MIDEX (e.g., with T-07 andT-09) and a JAXA Epsilon (e.g., with T-02), with MoO contributions from ESA for portionsof all three instruments. Alternatively, we can envision T-09 launched on a JAXA Epsilon,with contributions from the other agencies. Since T-09 alone is insufficient for the scienceobjectives of NGSPM, this would have to be paired with either T-01/04/05 and T-07 on a
JAXA Strategic Large, or T-02 and T-07 each to be launched on a SMEX/MIDEX or Epsilon.
In this case, dedicated AOs would be strongly recommended in order to preserve the ability toinclude all the high-priority instruments in such a “distributed” NGSPM mission. Also, it willbe crucial for the operational phases of the constellation spacecraft to overlap by at least two
years. We stress that other possible configurations exist, and the agencies should pursue thearrangement that best fits within the resource constraints while preserving the multi-instrument
combination (T-09, T-07 and T-01/04/05) that addresses the priority science objectives.

45

4.7. Possible additional instrument

As the next-highest priority instrument for elemental-scale science objectives, theaddition of T-10 to our existing top 5 priority instruments would add significantly to capacities
in coronal heating, reconnection, and flare science. T-10 is a high-energy (~0.5--50 keV)
spectroscopic imager, emphasizing superhot and non-thermal bremsstrahlung emission
produced in solar flares. Energy resolution of 100 eV at around 5 keV also allows diagnostics
based on SXR emission lines in this range. To make significant advances it must have a low
background - i.e. little scattered light - to allow simultaneous detection of bright chromosphericand faint coronal X-ray sources. This requirement pushes us towards precision grazingincidence focussing X-ray optics (Wolter Type-I), which currently implies a FWHM of around5”. Sub-second time resolution is required to capture the impulsive evolution of flare heating
and acceleration, and a large field-of-view encompasses a large active region and its overlying
coronal structures (~300”). Different channels and detectors, corresponding to different
focussing optics modules, are employed for low and high energy ranges.
4.8. Recommendations•

We recommend that the science focus of NGSPM be the investigation of physicalmechanisms on the smallest resolvable time- and length-scales relevant to the energeticor flux transfer processes, at all temperature domains in the solar atmosphere, as describedin Section 3.3.2.

•

We find that instruments with the capabilities represented by T-09, T-07, T-04, T-01, andT-05 are the highest priority for advancing the science objectives mentioned above within
the next decade.

•We recommend that the NGSPM consist of the instruments listed above operating
simultaneously, in full-Sun orbit(s), with sufficient telemetry coverage.

•

We recommend that NGSPM be realized with a single platform, as a JAXA Strategic
Large mission with contributions from NASA (SMEX-level), ESA (MoO), and ESA
member states. If the single-platform approach is not possible or available, a combinationof two or three spacecraft can achieve many of the NGSPM objectives, with some loss ofcapability and at increased risk.

•

We recommend that the agencies form a unified Science Definition Team for NGSPM assoon as possible to define the agencies’ respective contributions in more detail.

•Given the current timing of upcoming proposal opportunities, we recommend that JAXA
allow for the possibility of moving an instrument proposed for an Epsilon mission into a
Strategic Large mission at a later time.

•

Given the current timescale for M and L-class missions, we recommend that ESA support
European contributions to NGSPM in the form of a Mission of Opportunity and/or an ‘S’mission, depending on the approach taken to implement NGSPM by the agencies.

46

•We recommend that NASA support contributions by US scientists to all instruments of
NGSPM. The NASA contribution to a JAXA Strategic Large mission should be at least
the size of a SMEX mission. For a constellation configuration, the NASA contribution
should be as large as a MIDEX, with possible contributions from other agencies,
depending on the approach taken to implement NGSPM by the agencies.

•

We recommend that NASA have a proposal opportunity dedicated to NGSPM, in either
the single-platform or multi-spacecraft configuration, so that all 3 agencies can proceedwith the mission in a coordinated and timely fashion.

•

We recommend that in the longer term the international community coordinates to fillcritical gaps in available vantage points, in order to obtain a global understanding of the
Sun.

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A p p e n d ix A
Charter for the “Joint Next Generation Solar Physics Mission” Science Objectives TeamThe joint Next Generation Solar Physics mission (NGSP) Science Objectives Team (SOT) isan advisory group for the study of a possible multilateral United States National Aeronautics
and Space Administration (NASA), Japan Aerospace Exploration Agency (JAXA), andEuropean Space Agency (ESA) solar physics mission concept. NGSP will be a next-generation
solar physics satellite concept to capitalize on the highly successful collaborations between
NASA, JAXA, and ESA, including Yohkoh (Solar-A), Hinode (Solar-B), Geotail,
Chromospheric Lyman-Alpha Spectro Polarimeter (CLASP) (sounding rocket), and Solar andHeliospheric Observatory (SOHO). The membership of this multi-lateral team is selectedjointly by Institute of Space and Astronautical Science (ISAS)/JAXA, NASA, and ESA. The
team reports their results to responsible personnel of all agencies.
The SOT is composed of scientific specialists who have been selected to represent the broadinterests of the heliophysics research community. The primary role of the SOT is to develop
and document scientific goals and priorities of a potential multilateral NGSP mission within
the resources to be specified by the Agencies. Such mission would be subject to the normal
proposal and peer review cycles of all the partners.
NGSP is likely to be a Japan-led mission expected to include substantial contributions fromthe United States and Europe. While a previous series of studies for a next generation solar
physics investigation have been conducted over the past 5 years for a mission to launch in the
2019-2020 timeframe, the SOT study will investigate mission science requirements for apotential mission to be launched after 2024. The earlier studies will inform the SOT study.
Progress in solar physics research and instrument technology, as well as changes in priorities
and resources within the Agencies, are expected to change the scientific goals for the mission
being studied for the new time frame. The SOT will identify and assess the science
opportunities of the NGSP mission.
Specifically, the SOT will work closely with JAXA, the National Astronomical Observatoryof Japan (NAOJ), ESA, and NASA to:● Develop the scientific goals for NGSP, recognizing general limits on resources likely
to be available for this activity.
● Assess how these scientific goals are aligned with JAXA, ESA and NASA agency
priorities.
● Assess the required measurements necessary to meet the science goals.
● Assess the top-level observational (mission design) strategy for the NGSP mission toaccomplish the scientific goals.
● Identify the minimum performance for the mission systems that is necessary andsufficient to justify the international investment into the merged project● Deliver a science report that supports the generation of documents suitable for input
into any future joint Announcement of Opportunity or Call for Missions. The draft ofthis report will be delivered to the participating agencies 9 months after the first meetingof the SOT, and the final report 12 months after the first meeting.
49

Once the SOT has delivered the final report, the team will be disbanded.

50

A p p e n d ix B
Science objectives for NGSPM
I: Formation mechanisms of the hot and dynamic outer solar atmosphereI-1: Understand the formation mechanism of chromospheric fine scale dynamic structures andtheir influence in the corona
Background
The Sun is very dynamic as a result of the interaction of the solar magnetic field with the plasmaof the solar atmosphere, from the photosphere to the corona. A crucial region for this interaction is thesolar chromosphere, which is where the transition occurs from the plasma dominated lower atmosphereto the magnetic field dominated upper atmosphere. Understanding the structuring and dynamics of thechromosphere is important in itself but also in constraining the mass and energy loading processes thatdefine the heating of the corona and the acceleration and composition of the solar wind.
Chromospheric structuring is intrinsically small-scale. There is a plethora of small and rapid
chromospheric phenomena: fibrils, spicules, dark and bright mottles, mini- and micro-jets, Ellerman
bombs and moustaches, UV-bursts, mini-flares and mini-CMEs, and more with similarly evocative
names. With the theoretical understanding made possible by increasingly realistic, large-scale
simulations, time is ripe to replace such traditional phenomenology by physical understanding in a
coherent framework. Observationally, high spatial resolution is needed with full maps of both thevelocity field and vector magnetic field at high temporal cadence.
Tasks•
I-1-1: Observe magnetic topology and dynamics at the foot point of chromospheric fine scale jets
(eg. spicules) to see the discontinuity and shears of magnetic structure, and observe the interactionof magnetic field and convection by which these topologies are formed.
•
I-1-2: Observe propagation of slow mode MHD waves and/or torsional Alfvén waves along thejets, and identify driving mechanism of jets and evaluate the heating in the chromosphere•I-1-3: Observe the response of the corona above jets and identify the supplied mass andthermalization process
Key observations•
High spatial resolution (0.1"-0.3") of magnetic topology and dynamics for the photosphere andchromosphere with high cadence (10s or less) for a small FOV (>10") (I-1-1, I-1-2)•
Simultaneous imaging and spectroscopic measurements of transition-region and corona(0.01~5MK) with a similar spatial resolution (0.3") and cadence for a moderate FOV (>50") (I-13)
I-2: Test the nanoflare-heating hypothesis
Background
Coronal loops are seen as the basic structuring element of the solar outer atmosphere. In
addition to warm (1MK) loops, active regions have hot (3 – 4 MK) loops, which are spatially unresolved
by Hinode/XRT and SDO/AIA, probably because they are densely packed. Spectroscopic

51

measurements in the EUV have suggested that only 10% of the volume of hot coronal loops is filledwith plasma, when they are observed with a spatial resolution of 2ʺ − 3ʺ (Warren et al. 2008). Thus,
measuring plasma parameters within the overall loop envelope and understanding the sub-structures is
crucial for testing the nanoflare heating hypothesis.
A possible scenario is that these loops are heated by small-scale energy releases, often called
nanoflares, occurring in the corona. Nanoflares may be a result of braiding and subsequent reconnectionof magnetic field lines driven by magneto-convective forcing (Parker 1983). One important issue is toobservationally identify the occurrence rate and energy size of heating events. If nanoflares occur in aloop with high enough frequency, the plasma properties will resemble that of steady heating, producing
substantially isothermal emission at 2 – 3 MK. On the other hand, low-frequency nanoflares allow theloop to cool down before being reheated, which would result in a multi-temperature structure where
faint emission at extremely high temperatures (≈ 7 – 10 MK) is also produced. This tenuous and very
hot component has been inferred from Hinode/XRT data by various authors (e.g., Reale et al. 2009,
Schmelz et al. 2009) but it is not confirmed by EIS spectroscopic data (e.g., Warren et al. 2011), thecombined XRT and EIS observations (Testa et al., 2011; Winebarger et al. 2012), and FOXSI sounding
rocket hard X-ray measurement (Ishikawa et al. 2014). Spectral lines such as the relatively strong andunblended Fe XVIII 974.86 Å line (7.1 MK) in UV/EUV (Teriaca et al. 2012) and the X-ray
wavelengths for detecting plasma formed at even higher temperatures need to be used to quantitatively
explore this hot component.
The frequency of the energy deposition is also expected to affect the time scales of flows andbrightenings observed at the footpoints of the hot loops, in the so-called moss regions. The intensity ofmoss is very sensitive to changes in heating and the temporal evolution of the moss mimics the heatingof the coronal loop. Up to now, spectral and imaging studies of moss have found a relatively small
temporal variability that has often been interpreted as an indication of quasi-steady heating of the hotcore loops (e.g., Brooks & Warren 2009). However, very high spatial resolution (~0.25”) observations
by the Hi-C sounding rocket experiment showed a couple of braided field lines and the resulting
reconnection and heating of plasma to very high temperatures (Cirtain et al. 2013) and revealed a higher
variability on shorter time scales that seems consistent with impulsive heating (Testa et al. 2013).
The spatial distribution of the moss can be also used to infer how the heating depends on local
magnetic field strength (Winebarger et al. 2008). Coronal imaging observations will be able to detect
braiding in the corona, and UV spectroscopy with wide temperature coverage will allow full tracing of
the thermal evolution of the coronal loops in response to any kind of braiding. A crucial measurement
to establish whether magneto-convective forcing creates the braiding and subsequent reconnection of
the magnetic fields in the corona is to relate heating phenomena to the magnetic field structures andtheir activities at the photosphere and chromosphere.
High sensitivity of the instruments will allow us to determine the frequency of energy releaseand distinguish between heating that is effectively steady and heating that is fully impulsive. In addition
to making specific predictions of emission measure distributions, nanoflare models also predict line
profile asymmetries and shifts requiring a high spectral resolution spectrograph for detection of thegeneration of high speed plasma expected from the magnetic reconnection. This is truly a discovery
space, which likely holds the key to a wide variety of solar phenomena that involve the sudden releaseof magnetic energy. Besides its time scale, also the location of the heating along loops is a matter of
debate as different spatial distributions are expected to give different observational signatures. Heating
at the footpoints, for instance, is expected to lead to thermal instabilities that produce cool down-flowing
plasma due to condensation. Recent observations show cool plasma blobs with widths ≥ 150 km falling
along magnetic field lines over large part of active regions (Antolin et al. 2012). Spectroscopy with 20052

km sampling over a wide range of temperatures will allow following the temporal evolution of thethermodynamical state of the great majority of these blobs providing insights on the heating mechanism.
Tasks•
I-2-1: Observationally identify the occurrence rate and energy scale of elementary heatingprocesses with a sensitivity of 1022 erg•
I-2-2: Observe 107 K temperature plasmas and high velocity plasma motions in resolved coronalloops•
I-2-3: Full tracing of thermal evolution in coronal loops in response to any kind of braiding•
I-2-4: Establish whether magneto-convective forcing creates the braiding and subsequent
reconnection of magnetic fields in the corona
Key observations•
High spatial resolution for measuring plasma parameters and identifying magnetic sub-structures
within the overall loop envelope (I-2-1, I-2-2, I-2-3)•
Relatively strong and unblended UV spectral lines and/or X-ray measurements for exploring thehot (>5MK) plasma component (I-2-1, I-2-2)•
High cadence and high spatial resolution imaging for the temporal evolution of the moss at thecoronal footpoints (I-2-1, I-2-2)•
High sensitivity, high cadence, and high resolution UV spectroscopy for wide temperature
coverage (I-2-3)•
Simultaneous measurements of magnetic field structures and their activities at the lower
atmosphere (I-2-4)
I-3: Test the Wave-heating hypothesis
Background
Despite significant progress in the past decade, the detailed nature of the processes that power
the corona and solar wind remain poorly known. Waves, currents and reconnection may carry or release
substantial energy, but it remains unclear how important each is for the local energy balance, how thisdepends on solar region, and how the conversion of non-thermal to thermal energy works in detail.
Recent observations have confirmed the presence of many different types of waves throughout
the corona that may carry a substantial amount of energy [De Moortel & Nakariakov 2012].
Incompressible waves, e.g., Alfvénic waves, are suspected to be important in heating the corona[Matsumoto 2016] and driving the solar wind [McIntosh et al 2011], but it remains unclear what role
they play, as the limited spatio-temporal resolution of current observations leaves wave heating models
poorly constrained. The exact wave energy content is uncertain and direct observations of wave
dissipation are elusive [McIntosh & De Pontieu 2012; De Moortel & Pascoe 2012; Hahn & Savin 2013].
Measuring the properties of both transverse and longitudinal waves in coronal holes can sample thefootpoint regions of the solar wind and test recent models of wave dissipation [van Ballegooijen &Asgari-Targhi 2016]. These measurements will provide constraints to models of the origin of
turbulence in the heliosphere. Off-limb line broadening will also constrain wave damping in the source
regions of the fast solar wind.
Velocity and line broadening oscillations occur across coronal loops on spatial scales of 500
km and with the amplitudes (20-30 km/s) and periods (of 30-60 s) that are predicted by numericalmodels in which Alfvén waves generated in the photosphere propagate into the corona and dissipate tosmaller scales through a turbulent cascade [van Ballegooijen et al 2011]. Current instrumentation (e.g.,53EIS) cannot resolve these predicted signatures and cannot properly constrain the models. Recent IRIS
and SOT observations and modeling of prominence oscillations [Okamoto et al 2015, Antolin et al2015] may have revealed resonant absorption for the first time. While dissipation occurs on scales thatare smaller than can be directly observed, models suggest that the instability that leads to the cascadeto smaller scales leaves tell-tale signatures that can be detected through phase relations between Doppler
and line width and transverse motions of loops for various temperatures.
Tasks•
I-3-1: Determine the energy density of MHD waves as a function of height and frequency at
chromospheric, transition region and coronal temperatures, in various solar environments
including active region loops, coronal holes, quiet sun and prominences. Estimate the reflection
at the TR and transmission to the corona of the various waves and shocks traveling upwards
through the chromosphere. Determine the type of wave modes present in these structures and thenature of mode conversion that takes place.
•I-3-2: Identify the excitation mechanism of waves by observing the interaction of flux tubes andconvection, magnetic reconnection in the lower atmosphere.
•I-3-3: Observe discontinuous structures of physical quantities in the chromosphere and corona asa signature of non-linearization processes (shocks). Identify locations and mechanisms of
dissipation (by comparison with models), along with direct evidence for heating and estimates ofthe energy made available for thermalization. Compare the energy dissipation with heating
requirements in the different environments. Observe the dynamical effects (e.g., acceleration ofsteady flows or jets) that may result.
Key observations•
Measure time-series of line profiles (with enough spectral resolution to estimate line widths andasymmetries) at different heights in the photosphere/chromosphere/ corona, with sub-arcsec
resolution in different regions (AR, QS, CH, prominences); spatial coverage must be large enoughto allow simultaneous measurements over entire AR loops, prominences, plumes, etc. (I-3-1, I3-2, I-3-3)•
Simultaneously collect images with similar or higher spatial resolution for measuring motions in
the plane of the sky in the same temperature ranges (I-3-1, I-3-2, I-3-3)•
Measure phase relations between the observables as functions of frequency and height (I-3-1, I3-2)•
Measure correlations between incident wave amplitudes and heating or dynamical events (I-3-3)
I-4: Understand the role of magnetic flux emergence in the heating of the chromosphere,transition region and corona
Background
Magnetic flux emergence is a ubiquitous process in the solar surface. It occurs on all spatial
scales accessible to current instrumentation, from the tens of thousand km of large active regions down
to the 100 km of the tiniest magnetic flux concentrations observed in the quiet Sun. High spatial
resolution and high sensitivity polarimetric measurements have revealed enormous flux appearance
rates, particularly in the solar internetwork. While active regions bring flux at a rate of about 0.1 Mx
cm-2 day-1 during the maximum of the solar cycle (Schrijver & Harvey 1994), magnetic flux appears in
the internetwork at rates that are three to four orders of magnitude larger, independently of the solar
cycle (e.g., Zirin 1987; Thornton & Parnell 2010; Gosic et al. 2016; Anusha et al. 2016). Thus, theinternetwork supplies nearly all of the flux emerging on the solar surface. Also, internetwork regions

54

are present everywhere, from the poles to the equator. This has led to the idea that flux emergence inthe quiet Sun may be a crucial mechanism for chromospheric and coronal heating.
Support for this scenario is provided by the observation that about one quarter of the bipolar
flux structures emerging in the internetwork rise to the chromosphere, carrying magnetic energy withthem (Martinez González & Bellot Rubio 2009; Gömöry et al. 2010; Martínez González et al. 2010).
The resulting energy flow turns out to be of order 106-107 erg cm-2 s-1 (Ishikawa & Tsuneta 2009;
Martínez González et al. 2010), which is nearly sufficient to balance radiative losses in thechromosphere (Anderson & Athay 1989). The actual energy flow could be even larger, due to thelimited sensitivity and short time sequences that can be achieved with current instruments.
However, we still do not know how the energy carried by the loops is released in thechromosphere and perhaps also in the transition region and corona. The most obvious candidate ismagnetic reconnection. Reconnection can happen during the ascent of the loops and throughcancellation with pre-existing magnetic patches of opposite polarity, but only glimpses of those
processes and their effects on the chromosphere, transition region, and corona have been observed sofar (e.g. Guglielmino et al. 2009; Ortiz et al. 2010; Ortiz et al. 2016). Rising loops may also generate
waves and channel photospheric oscillations, whose energy would then be dissipated in higher layers.
These mechanisms provide additional ways to heat the solar atmosphere, but need to be confirmed
observationally.
To assess the role of emerging flux in chromospheric and coronal heating, it is necessary to
determine the exact amount of magnetic flux that is actually emerging in the solar photosphere on all
spatial scales (both in active and quiet regions), to characterize the processes whereby they transport
magnetic energy and magnetic helicity to the chromosphere and above, and to understand how theirmagnetic energy is released there. This requires high-sensitivity observations covering all the layers of
the solar atmosphere at similar spatial and temporal resolution for long periods of time. In order todemonstrate the existence of magnetic reconnection, it is important to measure magnetic fields not onlyin the photosphere but also in the chromosphere and transition region. Obtaining long duration
sequences of such measurements is not possible from the ground, due to the image degradation induced
by atmospheric seeing and the complete absorption of UV and EUV radiation in the Earth's atmosphere.
This investigation will also help solve an intriguing aspect of solar magnetism, namely how
magnetic flux disappears from the internetwork. So far, three mechanisms have been identified: fading,flux transfer to the network, and flux cancellation (Gosic et al. 2016). Interactions between mature
network and internetwork elements are likely to result in increased cancellation of opposite polarity
patches and magnetic reconnection, providing additional opportunities for atmospheric heating. Thus,
another important goal is to determine accurate flux cancellation rates in the quiet Sun and study theirinfluence in all layers of the solar atmosphere.
Tasks•
I-4-1: Determine the topology of emerging/cancelling magnetic flux in the photosphere andchromosphere on small and large spatial scales•
I-4-2: Determine flux emergence/cancellation rates as a function of flux content in the quiet Sun
and active regions, improving the current detection limit by at least one order of magnitude•
I-4-3: Characterize processes through which the energy carried by emerging magnetic flux istransferred to and released in the chromosphere, transition region, and corona. Determine thefraction of total magnetic energy dissipated in the upper atmosphere
Key observations

55

•
•
•
•
•
•
•
•

Simultaneous observations in visible, UV and EUV to study ascent of emerging flux throughout
the solar atmosphere with matching spatial resolution and cadence (I-4-3)
Spectropolarimetry in photospheric and chromospheric lines to infer magnetic topology ofemerging flux, preexisting fields, and interactions between them (I-4-1, I-4-2, I-4-3)
High polarimetric sensitivity (10-3-10-4) to detect the weakest quiet Sun magnetic features (I-4-1,
I-4-2, I-4-3)
Spectroscopic and imaging observations in transition region and corona to study consequences offlux emergence in upper atmosphere (I-4-3)
High spatial resolution to detect small internetwork flux concentrations and magnetic
reconnection (I-4-1, I-4-2, I-4-3)
Large field of view to cover several supergranular cells and study evolution of flux emerging in
the cell interiors and cancellations with network elements (I-4-2, I-4-3)
High cadence (~30 s) to study emergence and cancellation events (I-4-1, I-4-2, I-4-3)
Long, uninterrupted time sequences (2-5 days) to cover lifetime of supergranular cells andformation/decay of active regions (I-4-2)

I-5: The Sources and Driving Mechanisms of the Solar Wind
BackgroundThe solar wind carries a kinetic energy larger than the energy X-ray radiation of the solar corona,and it directly affects the Earth. It is intimately related to the solar dynamo, since it varies with thesolar cycle, it is driven by magnetic forces, and it may play an active role in shedding magnetic helicity.
The solar wind is divided into fast and slow streams. Not only do the speeds differ by nearly a factorof 2, but the temperatures, ionization states and elemental compositions are also different.
Three fundamental questions about the wind are 1) to what extent is it heated and driven by
MHD waves versus magnetic reconnection, 2) what role do jets and spicules play in injecting mass andenergy into the wind, and 3) how does the geometry of the magnetic field affect the wind parameters?
To answer these questions we need to determine the sources of the wind streams and to measuresignatures of the basic physical processes.
The sources can be found by comparing the ionization temperatures and elemental
compositions measured in situ with those observed remotely above the solar surface and in the region
where the wind accelerates. To first order, the high ionization temperatures and strong First Ionization
Potential (FIP) effect in the slow wind match those seen near streamers, while the low Ti and weak FIP
bias of the fast wind match coronal holes. It should be possible to trace specific parcels of gas back totheir solar origins by tracing the flow backwards in global MHD, though in practice this is tricky,
especially near solar maximum. More detailed study shows short time scale dropouts of heavy elements
matching the low abundances seen in the closed field regions of quiescent streamers, and models showthat the ionization state, which is given the product of density and flow time at the coronal temperaturein the MHD models, does not match that measured at 1 AU. Other outstanding problems include theroles of the S-web of open quasi-separatrix field lines and injection of plasma confined in loops byreconnection between open and closed field lines.
To answer those questions we need better models of the magnetic field structure and plasma
parameters on small scales along with sensitive UV spectroscopy and high cadence EUV images todetermine abundances and ionization states between the solar surface and about 5 RSUN, along withwhite light coronagraph images for the overall morphology. The geometry of magnetic flux tubes also
controls the adiabatic cooling of flows and determines the final speed of solar winds.

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By examining the temporal variation of Alfvénic Poynting flux and hydrodynamical acoustic
flux with height we can estimate the heating rate by the dissipation of Alfvénic waves and by shockwaves. The dissipation of Alfvénic waves is led by cascading MHD turbulence and nonlinear generationof compressive waves: the latter process eventually triggers the heating by shock dissipation. The
heating process can also be studied by measuring the preferential heating of heavy ions as a function of
charge-to-mass ratio. The preferential heating is usually interpreted as a signature of resonant heating
by Alfvénic waves from turbulent cascade, but shock waves have also been suggested. UV
spectroscopy with a substantial wavelength range and the sensitivity to cover the lines of many elements
and to obtain high temporal resolution is the key requirement, along with EUV and white light images
and high resolution magnetograph data to specify the lower boundary conditions. Instrument
parameters depend on the region being observed, but ~ 30 km/s resolution, ~ 30 second cadence and alarge portion of the 500 - 1500Å range are desirable.
Tasks•
I-5-1: Observe magnetic fields in the photosphere and chromosphere in coronal holes that initiate
the solar wind and determine the geometry of flux tubes connecting to the corona.
•
I-5-2: Determine the large scale configuration of coronal magnetic field•
I-5-3: Determine the mass and energy fluxes injected by spicules (low heights) and jets (larger
heights) in coronal holes, quiet regions and active regions from intensity changes in lines coveringa broad range of temperature•
I-5-4: Determine the mass fractionation in coronal base and identify the source regions of the solar
wind measured in interplanetary space by comparing the abundances of different FIP elements•
I-5-5: Determine the radial profiles of solar wind acceleration and the spatial distribution of thedriving force•
I-5-6: Determine the properties of compressive waves from intensity changes and of Alfvén
waves from velocity shifts as functions of height and measure their energy fluxes as a function ofheight, and identify wave reflection, cascade and turbulence•I-5-7: Identify the evidence of resonant heating by determining the preferential heating at larger
heights where the plasma is collisionless for several ions with a range of charge to mass ratiosfrom line widths with higher sensitivity (spatial and temporal resolution)•
I-5-8: Provide context observations of plasma parameters in the solar wind acceleration region
up to 5 solar radii to enhance the in situ measurements of particle distributions and wave properties
by Solar Orbiter and Solar Probe Plus beyond 10 solar radii. Spectra must be obtained off limb
below the trajectory of the in situ probes.
Key observations•
High-sensitivity spectropolarimetry of photospheric, chromospheric, and coronal lines (I-5-1, 15-2)•
Stereoscopic imaging and spectroscopy (I-5-2, 1-5-5, I-5-7)•
Sensitive UV spectroscopy (suggested range: 500-1650A) with 30 km/s resolution and highcadence (~30 sec) for examining temporal variability, line widths and line intensities as signaturesof driving mechanisms. (I-5-3, I-5-6)•
High-sensitivity coronal spectroscopy for Lyman lines and lines of different FIP elements (I-5-4,
1-5-5)•
Sensitive white light coronal and heliospheric images to obtain morphology and average electron
density. (I-5-2, I-5-5, I-5-6)•
Observe coronal holes, quiet regions and active regions off-limb to obtain plasma parameters asfunctions of height. (I-5-1, I-5-4, I-5-8)57

•In-situ measurements for comparison to source regions, provided by Parker Solar Probe and Solar
Orbiter. Measure the degree of preferential heating of elements of different charge-to-mass ratiosfrom the widths of lines of several ions, including He+. (I-5-4, I-5-7,1-5-8)

I-6: Formation mechanism of prominence
BackgroundSolar prominences (dark filaments as seen on the solar disk) are relatively cool (T ~ 104 K) anddense (n ~ 1011~12 cm-3) plasma suspended in surrounding hot corona above magnetic polarity inversion
lines on the solar surface. They are one of the most conspicuous ingredients of the solar atmospherewith the size of up to the solar diameter and the life time longer than weeks. Prominences occasionally
suffer a sudden destabilization followed by a dynamic eruption, which is often associated with a flareand CME that yield a significant impact on the space environments of the Earth. Recent high resolution
imaging observations have revealed that prominences consist of sub-arcsecond fine scale elongatedstructures and they are highly dynamic exhibiting oscillations (Okamoto et al. 2007), convections(Berger et al. 2010), persistent flows and turbulences (Hillier 2016). The prominence is thus aparticularly interesting object in regard to the fundamental MHD processes of partially ionized plasma
and the mechanism of the solar explosions. Major questions on the prominence are as follows:
1. What is the magnetic field structure that supports the prominence plasma?
The magnetic field configuration of prominences is still under debate especially for quiescent typeprominences, which is dominated by vertically elongated structures showing convective motions.
Erupting prominences often reveal a hint of helical structures, and direct measurements of magnetic
helicity or electric currents in prominences is of a particular importance to evaluate the stored magnetic
energy and to understand the mechanism of the eruptions and flares.
2. How is the prominence plasma supplied?
Two major scenarios are under debate, i.e., direct injection of chromospheric material through magnetic
reconnections or flux emergence, and condensation of the coronal hot plasma by thermal instability. To
understand the origin of prominence material in view of the mass circulation in the solar atmosphere is
one of the most important parts of understanding the formation mechanism of the hot and dynamic outersolar atmosphere.
3. What causes the sudden destabilization and eruption of the prominence?
The mechanism that initiates the instability of the prominence structure is still unclear. Two scenarios
are considered, i.e., self-destabilization, in which the magnetic structure of the prominence loses thestable condition according to gradual change of the magnetic boundary condition due to photosphericplasma flows, and externally triggered destabilization caused by magnetic reconnections between a partof the prominence and surroundings such as the emerging flux, adjacent flux loop and overlaying
magnetic fields. This question is described in topics II-2 and II-3.
Tasks
Understanding of the prominence is thus closely related to the understanding of the flare, CME and thecoronal heating mechanism. The following observational tasks are crucial to solve these problems;•I-6-1: Measure the magnetic field structure that supports prominences•
I-6-2: Detect mass circulation among chromosphere – prominence – corona•
I-6-3: Track the time evolution of photospheric and chromospheric magnetic fields near neutrallines, and clarify the condition of formation and destabilization of prominences.

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Key observations•
High precision spectro-polarimetry (accuracy 10-4) of prominence lines with spatial resolution 0.4”
for FOV > 200” (I-6-1, I-6-3)•
High resolution spectro-polarimetry (accuracy 10-4) of photospheric lines with spatial resolution
0.2” for FOV > 200” (I-6-1, I-6-3)•
High resolution images and velocity and magnetic fields in wide range of temperature (6x103 ~ 2
x 106K) with high spatial resolution (0.3” or higher) for FOV > 300”, and for continuous
observations over several days (I-6-2, 1-6-3)

II：Mechanisms of large-scale solar eruptions and foundations for prediction
II-1: Measure the energy build-up processes in flaring and CME regions
BackgroundSolar flares and coronal mass ejections (CMEs) originate from regions with complex, dynamic
magnetic field configurations. These fields exhibit pronounced non-potentiality of the coronal fieldreflecting the presence of free energy, i.e., energy in excess of the potential field energy that sets a
maximum to the energy available to power one or more flares or CMEs from the region. The energy
buildup occurs either below the surface, which is then transported to the solar atmosphere via interactionof emerging flux with the pre-existing field, or within the solar atmosphere itself. Quantifying thedegree of non-potentiality of, or measuring the amount of free energy contained within, a region that
subsequently erupts is a key towards comprehensive understanding of flares/CMEs and is indispensable
for space weather forecast research.
corona.

There are basically the following two ways to quantify the free magnetic energy in the solar

1. The free energy can be quantified by a three-dimensional magnetic field model reconstructed from
vector magnetic field observed on the solar surface and solving the boundary-value problem of MHD
equilibrium. The nonlinear force-free field (NLFFF) extrapolation from the vector magnetic fieldmeasured on the photosphere is then applied. However, a problem of this method is that thephotospheric magnetic field is usually not consistent with a force-free (magnetically-dominated) field(Metcalf et al. 1995). Several methods to overcome the problem have been proposed. One is to measure
the magnetic field vector at the upper chromospheric boundary where the plasma beta (the ratio ofplasma pressure to magnetic pressure) is relatively low and the magnetic field is more force-free thanthe photosphere. Another approach is to develop new MHD equilibrium models that explicitly takeplasma pressure and gravity into account. Observations of coronal loops or prominence morphologies,
or spectropolarimetric observations of prominence/corona magnetic fields (e.g., Lin et al., 2000; 2004;
Bak-Steslicka et al. 2013; Rachmeler et al. 2014; Lopez-Ariste, 2015) can provide further constraints.
These may be directly incorporated into boundary-value coronal models (e.g., Savcheva & vanBallegooijen, 2009; Malanushenko et al., 2014; Dalmasse et al., 2016), or be used along with theboundary condition in reconstructing the coronal magnetic field through stereoscopic/tomographic
methods (e.g. Kramar et al. 2014).
2. For active regions (ARs) the variation of magnetic free energy can be inferred by the measurementof Poynting flux on the photosphere. In order to do it, we need to know the velocity vector as well as
the magnetic field vector at the photosphere as a function of position and time. Local correlation
tracking and the inversion of the induction equation can be used to derive the velocity on the59photosphere from the evolution of magnetic field. However, the reliability of this method for themeasurement of magnetic free energy is not yet confirmed quantitatively.
Tasks•
II-1-1: Determine the magnetic field configuration and magnetic free energy stored in the coronausing models based on measurements of the photospheric and chromospheric magnetic boundary,and study its time evolution.
•
II-1-2: Measure the Poynting flux on the photosphere and identify the mechanism that carries themagnetic free energy into the corona.
•II-1-3: Measure the development of the magnetic structure of dark filaments (prominences on thelimb), in the time leading up to eruption.
•
II-1-4: Use intensity and polarimetry measurements to constrain coronal magnetic field models of
eruptive regions.
Key observations•
Stable and continuous measurement of vector magnetic fields in photosphere and chromosphere
for more than several days (II-1-1, II-1-2)•
Stereoscopic magnetograph measurements for resolving ambiguity (II-1-1, II-1-2, II--I-3, II-1-4)•
Velocity measurements in the photosphere (II-1-2)•
Vector magnetic fields and spectroscopy of dark filament (II-1-3)•
Coronal intensity and polarimetric measurements (II-1-4)•
Wide FOV covering AR or eruptive prominence (II-1-1, II-1-2, II-1-3, II-1-4)•
Data storage of large number of ARs (II-1-1, II-1-2)
II-2: Identify the trigger mechanism of solar flares and CMEs and distinguish between the
many CME models
Background
In order to accurately predict when, where, and how space weather events are initiated, we must
understand the interrelationship between small-scale processes, such as photospheric flux emergenceand magnetic reconnection, and large-scale magnetic topologies, such as quadrupolar fields in the
breakout model, or a magnetic flux rope structure. For example, flux emergence may drive changes inmagnetic topologies that result in flare and CME initiation. Observations indicate that many flares are
related to newly emerging flux and its interaction with pre-existing fields, and the higher the complexity
and shear of the field the greater the chance of flares occurring. Some numerical simulations suggestthat small magnetic flux emergences near magnetic polarity inversion lines may trigger the onset ofsolar eruptions. Alternatively, reconfigurations of the coronal magnetic field on global scales may leadto reconnections that trigger “sympathetic” eruptions. Measurements of magnetic fields and mass flows
in the solar atmosphere (including chromosphere and corona) will provide new observational insights
into flare and CME initiation.
Tasks•
II-2-1: High-cadence observation of plasma motions and fine magnetic structures interacting withsurrounding fields in the chromosphere before flare occurrence•II-2-2: Observe sudden reconfigurations of coronal magnetic field structures just before flareoccurrence•II-2-3: Observe dynamical changes of electric current system during eruptions.

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Key observations•
Vector magnetic and velocity fields in photosphere and chromosphere at high-cadence (II-2-1, II2-3)•
Spectroscopy of chromospheric/TR/coronal structures (II-2-1, II-2-2, II-2-3)•
Coronal imaging and polarimetry (II-2-2, II-2-3)•
FOV covering AR (II-2-1, II-2-3)•
Data storage of large number of events (II-2-1)
II-3: Understand the evolution and propagation of CMEs and their effect on the surrounding
coronaBackgroundThe driving mechanism of solar eruptions may be understood as a nonlinear feedback cycle
between magnetic reconnection and MHD instabilities. Because the time scale of the impulsive phaseof solar eruptions is as short as the Alfven time-scale, some kind of ideal MHD instability is thought tobe involved. In particular, the kink and torus instability modes have both been suggested as being
fundamental to the eruption. However, which mode of instabilities are responsible for the impulsive
and eruptive phases, and what determines the acceleration profile of the CME, is still controversial.
It is generally agreed that what leaves the corona and travels through interplanetary space isoften a magnetic flux rope (Vourlidas et al., 2013; and references therein). However, observations andmodels indicate that the magnetic structure that leaves the corona may bear little resemblance to theoriginal erupting structure because it may reconnect with surrounding fields and/or rotate during
eruption (Gibson and Fan, 2008; Shiota et al., 2010). In addition, deflection, distortion, and shock
formation are all likely contributers to the evolution of the CME as it moves through the corona andinner heliosphere (e.g., Demoulin, 2008; Savani et al 2010; 2011; Isavnin et al 2014).
In order to determine how the CME evolves and propagates during its eruption, we have tocarefully measure the evolution of the three-dimensional structure of the flux rope. Interactions withthe ambient corona as manifested in dimmings, EUV waves, and shocks may provide clues to thisevolution. In addition, if we can reconstruct a reliable three-dimensional coronal magnetic field in the
pre-eruptive state (see II-1), a stability analysis might help us to infer the structure of growing mode.
Tasks•
II-3-1: Observe kinematics including rotation, acceleration, and interactions of CMEs•
II-3-2: Observe reconnection during the eruption•II-3-3: Identify shocks and other waves associated with CMEs
Key observations•
EUV, white light coronagraph images with global FOV. (II-3-1: stereo; II-3-2, II-3-3)•
Heliospheric images (II-3-1 stereo; II-3-3)•
UV spectro coronagraph Doppler and line broadening (II-3-1; II-3-3)•
High resolution/high time cadence images in EUV, SXR, with large enough FOV to capture
eruption interactions, reconnections, shocks, waves (III-3-1, III-3-2, III-3-3)•
HXR observations (II-3-2)•
Doppler observations of erupting prominence (II-3-1)

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II-4: Understand the Processes of Fast Magnetic Reconnection
Background
It is generally accepted that magnetic reconnection is responsible for the initiation and the
dynamical progression of coronal mass ejections (CMEs) and eruptive flares, both of which are primary
drivers of heliospheric disturbances. As the central component in the two-dimensional framework due
to Carmichael (1964), Sturrock (1968), Hirayama (1974), and Kopp and Pneuman (1976), often called
the CSHKP model, reconnection forms the organizing element of much CME/flare research.
In a commonly invoked version of the CSHKP framework, a closed flux rope or plasmoid
moves upward as its magnetic connections to the Sun are eliminated by reconnection at a magnetic Xpoint. Inflow sweeps magnetic field from either side into the X-point; outflow ejects newly closed field
upward into the plasmoid and downward into a coronal arcade. As the reconnection progresses the Xpoint moves higher and its footpoints appear to spread apart. Images of post-eruption arcades and the
spreading “ribbons” of the footpoints corroborate the model’s overall geometry, including the cusp-likeappearance of the most recently closed flux (Tsuneta, 1996) whose apex would lie somewhat below the
X-point. Though the X-point itself is not directly observable, due to a lack of sufficient emission
measure, the reconnection process is inferred by a wealth of detectable signatures across theelectromagnetic spectrum (McKenzie, 2002).
However, the fundamental problem with scenarios invoking reconnection for, e.g., solar flaresis that the expected rates of magnetic diffusion through the highly ionized coronal plasma are much too
slow to account for the observed energy release. For although observations imply reconnection
occurring on timescales of seconds or minutes, the electrical conductivity of coronal plasma, and thelength scales of typical coronal features, tend to predict timescales that are several orders of magnitude
longer. Thus the focus of this science objective is understanding, “What allows reconnection toproceed? How does it get accelerated?”
The localized conditions and dynamic processes within the plasma – especially in the current
sheet defined by oppositely directed magnetic field – are critical for initiating, accelerating, andprolonging the reconnection; and yet despite the key role the local conditions play for reconnection,much is unknown about the dynamic physical processes in this region due to the small length- and timescales pertaining to the variations, in addition to the aforementioned poor visibility of the X-point.
To understand the role played by reconnection in driving coronal activity we must quantify its
effects and understand how they are related. Magnetic reconnection changes the topology of magnetic
field lines, while converting magnetic energy into other forms of energy such as heat and bulk kinetic
energy. Flows of magnetic field into and out of the reconnection site, and the conversion of magnetic
energy to heat and kinetic energy, have not been quantitatively measured with adequate precision. Evenas the X-point moves up the newly closed field lines should move downwards to dipolarize afterreconnecting. Supra-arcade downflows (SADs) are downward-moving features observed in the plasma
sheet, and are interpreted as direct evidence of the sunward outflows from highly localized (“patchy”)
reconnection within the current sheet. Analysis of the characteristics of these flows has yielded awindfall of information about current sheet reconnection, knowledge which may not be achievable byother means: e.g., no other direct signatures of, say, the spatial distribution of outflows are known. And
while the presence of shocks in the reconnecting structures has been anticipated (Hara et al., 2011;
Takasao et al. 2012), there is a lack of unambiguous observations. In non-flaring times – the prevailingcondition most of the time – flux transfer between neighboring domains within an active region, or
between distinct active regions, provides a crucial laboratory in which to identify and precisely measurereconnection episodes (e.g., Longcope et al. 2005).
Fundamental questions concerning magnetic reconnection in the corona include the following.
How does the rate of reconnection compare to the rate of its driving from the photosphere? (II-4-2)

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Does the relationship between topological change and energy dissipation depend on the amount of
current accumulated during the pre-reconnection phase, as theoretical models predict? (II-4-2) What isthe distribution of flux tube diameters, and fluxes, created by the reconnection process? What is thelength scale and spatial distribution of diffusion zones within current sheets? (II-4-1, II-4-2) Do these
distributions and length scales vary between different events or is it a fundamental characteristic ofcoronal reconnection? Is the reconnection patchy & discrete (cf. Longcope et al. 2005) or smooth andcontinuous (cf. Guo et al. 2014)? Are there shocks in the reconnecting structures, and if so then what
are the properties of the shocks? (II-4-3)
Tasks•
II-4-1: Observe discontinuity of chromospheric magnetic fields (current sheets)•
II-4-2: Clarify key parameters that determine the reconnection rate; length scale of current sheet,strength of the guide field, forcing by ambient gas flows, plasmoid ejections•
II-4-3: Observe structures in density, temperature and velocity associated with coronal magnetic
reconnection and verify relations of shock waves and plasma heating
Key observations•
Active region evolution: Detect all new connections in relation to flux emergence or in relation to
current buildup within/between domains. Measure amount of flux transferred by reconnection;
size (diameter) and flux density in each new flux tube; amount of heat energy deposited in corona;temperature & density within each new flux tube•
Flare current sheets: height and especially thickness of current sheet; spatial variation of density/
temperature/thickness/nonthermal broadening within current sheet; inflow/ outflow fromdiffusion sites•
High-res and high-sensitivity coronal imagery, EUV and/or SXR, 0.5”/pix or better, 60s cadence
uninterrupted.
•
Coronal spectroscopy to match, EUV+SXR, also uninterrupted, for energetics, Doppler,
nonthermal broadening•
Magnetic fields in chromosphere. (Being more nearly force-free, the chromospheric field provides
a more reliable boundary condition for extrapolations into the corona.) 30min cadence•
Uninterrupted for ~3 days at a time (duration of AR emergence, or buildup time to flare)
II-5: Understand the formation mechanism of sunspots, in particular delta sunspots
Background

Formation and evolution of sunspots is one of the most fascinating phenomena on the Sun.
Sunspots are the sites of energy-releasing events of various scales such as flares, coronal mass ejections(CMEs), jets, and brightenings in many different wavelengths. It has long been known that the mostcomplex sunspot groups called delta spots, in which umbrae of positive and negative polarities share acommon penumbra (Kuenzel 1960), produce larger flares (Sammis et al. 2000) and even solar particle
events (Warwick 1966). Therefore, not only from the viewpoint of solar physics but also from that ofthe practical space-weather research, understanding of the formation of sunspots, especially of the mostcomplex ones, is crucially important.
It is widely believed that sunspots are created through the emergence of magnetic flux fromthe convection zone (Parker 1955). Because direct optical observation of the interior is almost
impossible, numerical approaches have been adopted to investigate the flux emergence (see reviews by
Fan 2009, Cheung & Isobe 2014). In the present understanding, the emergence of magnetic fields iscaused by the coupling of magnetic buoyancy and convective action (Nelson et al. 2011), a strong

63

enough field is needed to successfully rise to the surface (Jouve & Brun 2009, Weber et al. 2011), andthe observed asymmetries between leading and following polarities are the result of Coriolis force andlarge-scale convection (Fan et al. 1993). The combination of numerical simulations of rising magnetic
flux and observations of surface flows places constraints on the subsurface rise speed of omega loops(Birch et al. 2016).
It has been proposed that helioseismology may be used to directly detect the subsurface
magnetic field and associated flows. However this has proved very difficult for individual active regions.
The way forward is to develop more reliable methods of helioseismic inference (to reduce biases), in
combination with statistical studies of many hundreds of emerging active regions (to beat down random
noise).
Various complex, fine-scale structures exist in sunspot regions (see reviews by Solanki
2003, Rempel & Schlichenmaier 2011, Borrero & Ichimoto 2011). Most of them are coupled with local
convection and some are accompanied by dynamic activity events. For example, Hinode/SOT has
discovered small-scale jet-like features in sunspot penumbrae (1000-4000 km; Katsukawa et al. 2007)
and repeated bursty ejections from light bridges (1500-3000 km; Shimizu et al. 2009). Coordinatedobservations with IRIS and ground-based high-resolution telescopes (~0.06” for SST) make it possible
to obtain thermal and velocity diagnostics in the chromosphere and transition region above the sites ofsuch events and their detailed photospheric morphologies (e.g., Toriumi et al. 2015a,b, Vissers et al.
2015). Chromopsheric (vector) magnetic measurements may help understand not only the abovementioned features but also the formation mechanism of the sunspot penumbrae (Shimizu et al. 2012).
One of the most prominent characteristics in the evolution of delta sunspots is theformation of strong-field, high-gradient, highly-sheared polarity inversion lines (PILs; e.g., Schrijver
2007). They indicate the existence of strong electric currents that can store free magnetic energy in thecorona. Many authors have pointed out the importance of sunspot motions in the formation of such
structures (Zirin & Liggett 1987, Tanaka 1991), which includes the emergence of a twisted flux tube
and a resultant sunspot rotation (Leka et al. 1996, Kubo et al. 2007), interaction between a newly
emerging bipole and a pre-existing spot (Kleint et al. 2015), and collision of two emerging bipoles(Chintzoglou & Zhang 2013). Therefore, a long-term (days to weeks) monitoring of photospheric vector
magnetic fields in a developing delta sunspot region with a wide enough field-of-view (a few times100”) is necessary for understanding the formation process of sheared PIL, the possible location of
future flare eruptions, and the large-scale supply of Poynting flux and current helicity. The obtained
vector magnetograms may also be utilized in reproducing the coronal magnetic fields with non-linear
force-free field extrapolation (NLFFF) techniques (see reviews by Wiegelmann & Sakurai 2012, Inoue2016) or compared with numerical modeling of delta sunspots (e.g., Toriumi et al. 2014, Fang & Fan
2015, Takasao et al. 2015). Stereoscopic observations, which would be realized by Solar Orbiter, Paker
Solar Probe, and L5-missions, may provide excellent opportunities to continuously monitor the deltasunspots over the whole life cycle.
Above the sheared PIL in delta sunspots, arcade fields gradually produce a twisted flux
rope, which is possibly due to the footpoint motion in the photosphere (van Ballegooijen & Martens
1989). This should be investigated by the simultaneous photospheric and chromospheric magnetic
measurements. In addition, surrounding and overlying coronal fields, which determines whether theflare becomes CME-eruptive or not (Sun et al. 2015), should be investigated by EUV and X-ray
observations.
Tasks•
II-5-1: Observationally reveal the origin and subsurface evolution of sunspot magnetic fields

64

•
•

II-5-2: Investigate the coupling of small-scale magnetic fields and local convection and theresultant energy-releasing events
II-5-3: Trace the formation process of sheared PILs and current-carrying coronal fields above

Key observations•
Long-term and wide-field-of-view monitoring of photospheric vector magnetic fields (II-5-2, II5-3)•
Simultaneous high-resolution imaging spectropolarimetry in the photosphere and chromosphere(II-5-2, II-5-3)•
Local helioseismology and reconstruction of three-dimensional subsurface velocity and magnetic
fields (II-5-1)•
Multi-wavelength (EUV and X-ray) monitoring of coronal magnetic fields (II-5-3)
II-6: Understand particle acceleration and flare energy transport
BackgroundThe energy for a solar flare is generally understood to be stored in stressed magnetic fields inthe solar corona. The flare energy, on the other hand, is mostly radiated by the chromosphere, with only
a relatively small fraction radiated by the corona. Therefore, a substantial fraction of the flare energy
must be transported from the corona to the chromosphere. Aspects of the transport process are
intimately related to the problem of particle acceleration (electrons, protons, alphas and other heavyions), since one possible mode of energy transport is by charged particle beams. However, at different
phases of the flare, energy transport by (M)HD waves, or by thermal conduction may be relevant.
Observationally, it is clear that non-thermal particles have a crucial role in a flare, with up to 50% of
the stored magnetic energy estimated to be converted to the KE of non-thermal electrons and ions,present in both the chromosphere and the corona.
The most widely-accepted model for solar flare energy transport links the energy transport
directly to the acceleration of electrons, in as much as the stored magnetic energy in the corona isconverted in situ to the kinetic energy of non-thermal charged particles which then stream along closed
magnetic field to the lower atmosphere where they deposit their energy collisionally, or along open
field to the heliosphere where they are responsible for the generation of radio emission. The top-level
questions to answer here are:
- How and where is magnetic energy converted into particle non-thermal kinetic energy (i.e. how
are particles accelerated, what is the relationship to field reconfiguration including in thereconnection region)?
- How do those accelerated particles propagate through the magnetised solar plasma?
Magnetic energy can also be transported through the atmosphere by magnetic disturbances. For
example an impulsive reconfiguration of the magnetic field, e.g. by magnetic reconnection, can launcha pulse of Alfven waves which are strongly ducted along the magnetic field to remote locations, where
the wave energy is dissipated and converted into heat, particle KE and radiation. Relevant top-level
questions here are:
- How are propagating magnetic disturbances generated in a flare reconfiguration?
- What happens to such disturbances as they propagate through the corona towards thechromosphere and into space?
- How and where is wave energy converted into particle non-thermal kinetic energy and/or
heating?

65

While the flare impulsive phase is characterised by rapid timescales and non-thermal particles,
these are not found at all sites in the chromosphere where flare excitation takes place, and are alsoabsent (or present at a much lower intensity) in the flare gradual phase. Particularly in the gradual phaseof some long-duration events there is evidence for long-lasting coronal heating, and energy transport tothe chromosphere to generate gradual-phase flare ribbons may be by thermal conduction from a strongly
heated corona. In this scenario the main question is the origin of the high-temperature coronal plasma
in the late phase of a flare.
The long-standing electron beam model is currently facing some observational challenges, inparticular high resolution imaging observations force the required beam flux of electrons to ever higher
number densities, with attendant uncertainties about the physics of the coronal beam propagation.
Additionally, limb flares show that the HXR sources are much lower in the atmosphere than can
comfortably be explained in the beam model, given what else is known about the characteristics of theatmosphere and the emitting electrons. Crucial therefore is to directly detect signatures of the electrons
in flight through the corona: at present only (occasional) coronal and chromospheric sources are seen,
with the link between the two still speculative.
Tasks•
II-6-1: Observe, at high cadence, the evolution of electron and ion distributions from thermal tonon-thermal, with high dynamic range and spatial resolution in the corona and chromosphere.
•
II-6-2: Detect signatures of electrons in flight through solar atmosphere.
•
II-6-3: Observe the dynamic response of the lower atmosphere to identify the energy transportmechanism.
•II-6-4: Identify the evidence of Alfven waves that carry the energy from the reconnection site to
the chromosphere.
Key observations•
High dynamic range (>100) HXR imaging spectroscopy capable of detecting and mapping
spatially the bremsstrahlung emission from electrons in transit through the corona simultaneouslywith footpoint emission and looptop emission if present;•high temporal resolution (<0.1s) direct imaging observations of the chromospheric emission freeof atmospheric seeing effects, to allow a detailed timing analysis of footpoint emission, including
conjugate footpoints;•
UV or EUV imaging spectroscopy of the corona and chromosphere, with excellent spatial andspectral resolution, and very well-characterised line shapes, to study the development of plasma
heating, turbulence, and ion and electron distributions (inc. non-Maxwellian) in the flare
atmosphere;•High spectral resolution Ly-a imaging spectroscopy/spectropolarimetry to look for the spectral
signature of charge-exchange interactions between a proton beam and ambient hydrogen, and thepolarization signatures of beam excitation.
•
Gamma-ray imaging / imaging spectroscopy with very large area detectors to properly located andcharacterise locations and temporal evolution of accelerated ions, of which we have only had
glimpses so far.

66

III: Mechanisms driving the solar cycle and irradiance variation
III-1: Measure flow structures in the solar convection zone that drive the regeneration of solar
magnetic fields
Background
Flows in the solar convection zone play essential roles in driving the regeneration of solar
magnetic fields. The essential ingredients are the differential rotation, meridional circulation, andturbulent convection. The differential rotation acts as the so-called omega effect in solar dynamo models.
The meridional flow may play a role transporting the magnetic flux in the latitudinal direction. The
convection plays an important role in dispersing the magnetic flux on the surface.
The solar differential rotation on the surface has directly been measured by the Doppler velocity
or by tracking some features, and the internal rotation over the solar convection zone has been estimatedfrom helioseismic approaches by measuring the modes of 5 min oscillation. The surface meridional
flow has been measured by the photospheric feature tracking and local helioseismology. The meridional
flow in deeper layers is not known (various researchers reach different conclusions). The global
dynamics of the solar convection zone is expected to be controlled by anisotropic stresses due to rotating
turbulent convection (Hanasoge et al. 2016). A better understanding of the large-scale flows could
therefore emerge from measurements of subsurface convection. Puzzling helioseismic results by
Hanasoge et al. (2012) indicate that convective velocity amplitudes are much lower than expected fromnumerical simulations. In this respect it is essential to better understand the influence of rotation on thelarge scales of convection (Featherstone & Hindman 2016).
Missing observations at present include the characterization of differential rotation, themeridional flow, and convection features at high latitudes and/or in the deep convection zone. The flow
structures at high latitudes will partially be characterized by Solar Orbiter. For measurement of flows
in the deep convection zone, long-term continuous observations from an out-of-the-ecliptic viewing
angle may be required: this is an opportunity for a future mission.
Tasks
The following tasks are defined for understanding the flow structures in the solar convection zone:•
III-1-1: Measure meridional flows, differential rotation, and convective flows at high latitudes.
•
III-1-2: Measure meridional flows in the deep convection zone.
•
III-1-3: Characterize large-scale convective flows in the solar interior.
Key Observations•
Measure full-disk photospheric intensity and Doppler velocity for helioseismology from out-ofecliptic vantage point to assess the flows at high latitudes. [III-1-1, III-1-2]•
Measure full-disk photospheric magnetic fields [III-1-1, III-1-3]•
Instrument requirements: ∆t (temporal cadence) < 1min, t > 3 years, ∆x (spatial sampling) ~ 1
arcsec, field of view = whole sun [III-1-1, III-1-2, III-1-3]
III-2: Locate and trace signatures of the global magnetic flux in the Sun
BackgroundFor understanding the origin of the solar magnetic fields, it is essentially important to identify
the formation depth of the magnetic flux that is responsible for the global solar magnetic fields or todetect the deepest locations of the rising flux tubes. The formation of the polar fields or the mechanism67

of the polar field reversal also needs to be understood for the strength of the solar activity of the next
cycle.
It is widely believed from theoretical considerations that the global solar magnetic fields areformed near the base of the convection zone (CZ). The signature from observations on the other handis rather limited; among the reported potential evidence for deep-seated magnetic flux near the base of
CZ are the variation of the rotation speed with 1.3 yr period (Howe et al. 2000) and the variation of thesound speed (Baldner and Basu 2008) from global helioseismology, and the variation of meridional
flow speed (Liang and Chou 2015) from local helioseismology. The effort will continue to solidify
these results. It is also desired that the theoretical suggestion (Rempel, Schüssler and Tóth 2000) that
there is jet-like flows inside the toroidal flux tubes near the base of CZ be also tested observationally.
Tasks
In the following, tasks for detection of the global magnetic flux in the deep convection zone are listed:•III-2-1: Search for solar-cycle related acoustic anomalies in the deep convection zone.
•
III-2-2: Search for flows in deep-seated flux tubes near the tachocline.
•
III-2-3: Determine how the polar-field patchy structures are formed by horizontal flux transport
for understanding the origin of polar magnetic fields.
Key Observations•
Measure full-disk intensity and Doppler velocity for helioseismology by two spacecraft with alarge-angle viewing angles (e.g. one from the Earth and the other from L5) [III-2-1, III-2-2]•
Measure full-disk photospheric magnetic fields for monitoring the convection zone before the riseof active regions [III-2-1]•
Measure high-resolution polar magnetic fields [III-2-3]•
Instrument requirements: ∆t (temporal cadence) < 1min, t > 3 years, ∆x (spatial sampling)~ 1
arcsec for full disk [III-2-1, III-2-2], but ∆x 0.1 arcsec for polar fields [III-2-3]
III-3: Quantify the role of turbulence in the solar dynamo
BackgroundAll solar dynamo models share a common premise, i.e., that differential rotation creates toroidal
fields from poloidal fields. Models differ, however, in the mechanism proposed to close the loop by
generating poloidal fields from toroidal fields. The two dominant dynamo paradigms are 1) the
Babcock-Leighton dynamo model, in which large-scale poloidal fields are generated by thedestabilization, rise, emergence, and dispersal of magnetic flux tubes from the interior, and 2) the
convective dynamo model, first proposed by Parker. The convective dynamo model basically argues
that kinetic helicity generated by rotation and stratification produces magnetic helicity, which then istransferred from small to large scales resulting in magnetic self-organization. In particular, the turbulent
alpha effect allows small-scale eddies to ultimately generate large-scale poloidal fields.
The convective dynamo relies on the Coriolis force (solar rotation) playing a significant role ingenerating kinetic helicity. At the solar photosphere, however, convective time scales are much shorter
than the solar rotation period, at least on solar granulation spatial scales (supergranules exhibit
sensitivity to the Coriolis force, e.g., Langfellner et al., 2015). Thus, the convective dynamo paradigm
is likely to depend upon kinetic helicity being generated below the solar surface. Closer to the surface,chaotic turbulent flows may still lead to the generation of magnetic fields in a local dynamo process,but this takes the form of a small-scale magnetic carpet, and cannot explain a cyclical solar cycle. It
may be, however, that that the local dynamo is somehow coupled to the global dynamo, or indeed that68

the small-scale local dynamo can help to maintain the magnetic self-organization that is the large-scale
dynamo (Hotta et al. 2016).
Tasks•
III-3-1: Observe small-scale kinetic and current helicities in the photosphere for determining
potential mechanisms for an alpha effect.
High-resolution measurements of the spectral transfer of magnetic helicity from small to large
scales and its transport through the solar atmosphere would provide evidence for a turbulent
alpha effect. Local helioseismic measurements and granulation- tracking measurements probe
kinetic helicity, at least on supergranule scales which are sufficiently large and slow to besensitive to the Coriolis force (Duvall & Gizon 2000, Hathaway 2013, Langfellner et al. 2014,2015)•

III-3-2: Measure kinetic and magnetic energy spectrum in small-scale turbulent convection forunderstanding local dynamo near the surface and its impact on the global dynamo.
High temporal/spatial resolution measurements of solar magnetic fields and flows can shedlight on turbulent processes associated with a local dynamo. Since the local dynamo does not
generate helicity, any evidence of magnetic helicity in the quiet sun photosphere indicates acoupling of the global and local dynamo consistent with a turbulent alpha effect. In general,comparing active and quiet solar regions will provide information about local-global dynamo
coupling.

Key observations•
Ultra-high resolution and sensitivity vector magnetic fields and flows at the photosphere (III-3-1;
III-3-2)
Note: DKIST will do this; space-based would allow uniform-quality data over time without
atmospheric effects.
•
Helioseismology to probe subsurface kinetic helicity. (III-3-1)•
UV observations could help quantify magnetic helicity transfer through the solar atmosphere. (III3-2)
III-4: Understand the mechanism of solar irradiance variations
BackgroundThe comparison of solar and climate data sets over long time scales displays correlations thatpoint to a solar inﬂuence on climate variability, at least in the pre-industrial age. Although the detailed
mechanism by which solar activity affects the climate is still a topic of intense study, the variation in
the Sun’s radiative output, quantiﬁed in terms of spectral solar irradiance (SSI) and total solar irradiance(TSI), are thought to play a key role (Gray et al. 2010).
The brightness of the Sun changes as a function of the level of activity on the solar disc. On
timescales from hours to months the evolution of active regions and their rotation onto and off thevisible disc cause variations in TSI as well as in radiation from X-rays to IR wavelengths. Over the 11year solar cycle, the rise and fall of solar magnetic ﬂux and activity leads to signiﬁcant variation of the
SSI at short wavelengths. Whereas in the visible most of the radiation reaches the Earth’s surface andheats it directly, the shorter wavelength radiation controls the chemistry, dynamics, and ionization state69

of the terrestrial upper and middle atmosphere, which are coupled to the troposphere and hence to the
Earth’s climate (Haigh 2007; Domingo et al. 2009; Solanki et al. 2013).
There is a qualitative disagreement between measurements and models of SSI, in particular in
the near- and middle-UV and the visible spectral ranges. It is not clear if the problem lies in the models,
due to missing knowledge of the brightness of small-scale magnetic features in the UV, or if it lies in
the instruments, due to degradation of the sensitivity of SSI measurements. The discrepancy is most
severe in the range from 250 nm to 400 nm. Recent measurements suggest a factor of three to six
stronger variability than previous observations and models. This is only possible if compensated byanti-phase changes in the visible (400 nm to 700 nm), for which no physical mechanism has been
identiﬁed, however (Ermolli et al. 2013).
The modelled and measured UV irradiance records introduce highly contrasting variability inthe stratosphere with rather different impacts on climate. Most critical are the relative amplitudes of theirradiance changes below and above 242 nm, responsible for ozone production and destruction,
respectively. High resolution imaging of magnetic elements in the UV below and above 242 nm and in
the visible are needed to provide key observations to determine the cause of the discrepancy between
various irradiance data sets and unravel the role of solar UV irradiance changes in the ozone balance inthe Earth’s atmosphere. Only a space mission with high resolution UV imaging capabilities can obtainstatistically signiﬁcant amounts of the necessary data. No mission has so far had these capabilities.
By entering these brightness values into state-of-the-art models, the SSI at the sampled
wavelengths can be computed. It is equally important to measure the total irradiance as well as spectral
irradiance in multiple wavelength bands in parallel. The combination of high-resolution and irradiancemeasurements along with appropriate modelling will resolve this important controversy and finallyprovide the climate community with the correct variation of SSI at different time-scales. The twoparallel approaches of together using, on the one hand, high-resolution data and modelling and, on theother hand, direct TSI and SSI measurements, will also provide irradiance data of unprecedented
stability, as the measured and modelled TSI and SSI can be used to calibrate and correct each other forjumps and degradations.
The photometric variability of the Sun over its activity cycle is noticeably lower than that ofother Sun-like stars with a similar level of chromospheric activity (Lockwood et al. 2007). Although anumber of possible reasons for this have been proposed, no clear answer has emerged so far. Is the Sun
unusual? By determining the brightness of magnetic features in exactly the same wavelength bands asused to observe other stars, it will be possible to make reliable models of solar/stellar irradiancevariations at these wavelengths. SSI measurements at the same wavelengths, when taken together withthe models, can be used to compare with stellar measurements far more consistently than done so far.
Tasks•
III-4-1: Measure brightness of elemental structures in multiple UV (200–400 nm) and visual
wavelengths in magnetized and non-magnetized regions with the aim of understanding themechanism of UV irradiance.
•
III-4-2: Construct solar irradiance model based on full disk magnetic fields for understanding of
the mechanism of UV irradiance obtained in III-4-1.
•
III-4-3: Understand low photometric variability of the Sun relative to other Sun-like stars.
Key observations

70

•

•
•

•
•
•
•

•

High-resolution (Δx ≤ 0.1”) UV images at a range of wavelengths including Lyα, 170 nm, 200242 nm, 242-300 nm, 320-360 nm, CN bandhead, G-band, the Strömgren b and y bands and the
Kepler and PLATO wavelength ranges (~420-900 nm, 500-1000 nm) [III-4-1 and III-4-3]
Photospheric vector magnetic fields at high resolution (Δx ≤ 0.1”) [III-4-1 and III-4-3]
High resolution imaging and magnetic field measurements to be repeated regularly in regions withdifferent magnetic flux (quiet Sun and active regions) and at different parts of disk (μ=0.1…1)[III-4-1]
Photospheric vector magnetic fields over the full solar disk (Δx ≤ 1”) [III-4-2 and III-4-3]
Chromospheric image: Lyα, Mg II h & k, Ca II H & K (III-4-1)
Observations of TSI with a stability of 0.001% per year, corresponding to a stability of better than
0.01 W/m2 per decade, and an absolute accuracy of 0.01% [III-4-1 and III-4-2]
Observations of SSI at a range of wavelengths starting at 115 nm, including the ozone and oxygen
absorption bands in the UV, the Strömgren b and y bands and the Kepler/PLATO bands in thevisible [III-4-1 and III-4-2 and III-4-3]
Long-term photometry (t > 5 year, half solar cycle) in photometric bands used in stellar surveys,
e.g. Strömgren b and y bands over full solar disk [III-4-3]

III-5: Explore the deep internal structure of the Sun
Background
While the acoustic p-mode oscillations have been used for understanding inside the Sun throughestimating the flow speed in the convection zone, the g-mode oscillations, which are believed to be
trapped in the radiative interior, have not been detected despite extensive efforts including the spaceobservations. The g-modes are more suited for investigating the core and radiative region of the Sunthan the p-modes. Due to the evanescent nature of the g-modes in the solar convection zone, theexpected velocity amplitudes on the solar photosphere from theoretical estimates (Gough 1985; Kumar
1996; Belkacem et al. 2009) is 10-4–10-1 cm/s in the frequency range of 10–200 µHz are much smaller
than the observed p-mode amplitudes of ~10 cm/s over a few mmHz range. The 10-year long-term
observations by SOHO have given the upper limits of 0.1–1 cm/s in the g-mode amplitudes over thefrequency of 30–300 µHz. A further high-precision measurement is required for the detection.
Tasks•
III-5-1: Detection of g-mode for understanding its excitation and travel of waves in the Sun (andfor investigating the solar core)
Key observations•
A high-precision Doppler measurement for detecting the velocity amplitude of 0.01 cm/s

71

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A p p e n d ix C
Existing facilities
Facility

Description

CURRENT (not exhaustive, ground-based high resolution facilities
restricted to ≥ 1m)
HinodeIRISNuSTAR
Proba-2
RHESSI
SDO
SOHO

Optical spectropolarimetry, EUV spectroscopy, SXR imaging
UV spectroscopy and imaging
Astronomical X-ray satellite with solar capability
EUV imager, radiometers
Hard X-ray spectroscopy and imaging
UV/EUV imaging, EUV irradiance, helioseismic & magnetic imager
Coronagraphs, radiometer, heliospheric imager and in situ instruments remain
Two-spacecraft mission, EUV imagers, coronagraphs, heliospheric imagers,
STEREO
particles
GOES
X-ray spectral irradiance, particle data, full-disk EUV imaging
NST
1.6m ground-based optical/NIR, imaging and spectropolarimetrySST
1m ground-based optical telescope, imaging and spectropolarimetryNVST
1m ground-based solar optical telescope, imaging and spectroscopy
GREGOR
1.5 m ground-based solar optical telescope, imaging and spectropolarimetry
BiSON
Network of optical telescopes for low-degree helioseismology
MLSO
Ground-based coronagraphs and polarimeter, full-disk imaging
NSO-Synoptic Chromospheric and photospheric magnetograms, helioseismic measurements
ALMA
mm-wave array with solar capabilities
LOFAR
Low-frequency radio astronomy array with solar capabilities
Nobeyama
Radio interferometer, 17 GHz and 34 GHz
IN CONSTRUCTION
Out of the ecliptic, solar encounter mission. EUV imaging & spectroscopy,
Solar Orbiter visible spectropolarimetry, coronagraphs and in-situ instruments. Launch
February 2019
Parker Solar Solar encounter mission, in-situ instruments, heliospheric imager. Launch
Probe
August 2018
Proba-3
Formation flying coronagraph, launch late 2018
DKIST
4m ground-based optical/NIR. First light 2020
EOVSAFrequency-agile 15-element radio interferometer, 1-18 GHz
CHSRFrequency-agile 100-element radio interferometer, 0.4-15 GHz
FUTURE POSSIBILITIES
EST
4m ground-based optical/NIR
FASR
Frequency-agile ~200-element radio interferometer, 0.5 MHz - 21 GHz
73

COSMO LC

1.5m ground-based visible/NIR coronagraph/polarimeter74

A p p e n d ix D
SOT members and their areas of expertise (as a table)
Luis Bellot Rubio

Instituto de Astrofísica deAndalucía, CSIC, Spain

Solar magnetic fields

Mats Carlsson

University of Oslo, Norway

Chromospheric physics

Lyndsay Fletcher

University of Glasgow, UK

Solar flares

Sarah Gibson

National Center for AtmosphericResearch, High Altitude
Observatory, USA

Solar corona and sun-earth
connections

Laurent Gizon

MPS, Germany

Helioseismology

Hirohisa Hara

NAOJ, Japan

Solar activity

Kiyoshi Ichimoto

Kyoto University, Japan

Solar observation and instrumentation

Kanya Kusano

Nagoya University, Japan

Solar flares and coronal massejectionsDavid McKenzie

NASA-MSFC, USA

Solar flares and magnetic
reconnection

John Raymond

Harvard Smithsonian CFA, USA

Solar wind

Takashi Sekii

NAOJ, Japan

Helioseismology

Toshifumi Shimizu ISAS/JAXA, Japan

Magnetic activities and heating inphotosphere-chromosphere-corona

Sami Solanki

MPS, Germany

Solar magnetism and space climate

Ted Tarbell

LMSAL, USA

Magnetic fields, space instrument
development & operation

75

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A p p e n d ix E
Town Hall Meetings and Community Q&A Discussions
E.1 Q&A at Hinode-10
The first “Town Hall”-type meeting was conducted during the Solar-C session at the
Hinode-10 meeting in Nagoya, Japan, primarily for the purpose of introducing the SOT to thecommunity and starting a dialogue. After presentations of the SOT charter and somepreliminary studies of instrument concepts in Japan, an extensive Q&A with the full audience
was conducted, addressing the goals and objectives of the SOT and the NGSPM. Discussion
during this Q&A touched upon the scope of science disciplines to be considered for NGSPM(solar physics, as distinct from heliospheric physics and space weather operational missions).
We introduced the concept of a Town Hall at the American Geophysical Union meeting in San
Francisco for further dialogue with the community, and began tentative plans for such a Town
Hall.
E.2 Town Hall at AGU
The second major Q&A session was a Town Hall meeting at the AGU in San Francisco.
Being in December, this occurred approximately half-way through the SOT’s deliberations.
The SOT members were joined by agency representatives from NASA and JAXA, who
presented the rationale for chartering the SOT. As the purpose of this Town Hall was to reporton our progress, and take additional input from the community, we described the two phasesof our work plan (define objectives, and then recommend mission concepts), and described ourprogress during the first phase. We then summarized for the attendees the white paper exercise(see Section 2.2 below) and how that input was enhancing the SOT’s discussions. These
reports were followed by an extensive Q&A, including discussion about the agencies’ expectedbudget, timeframe, and mission scope (small/medium/large).
E.3 UK Community Meeting
During the UK Solar Physics Community's annual missions forum, attended by around
50 community members, the purpose, membership and progress of the SOT was described,
based on the presentations given at the AGU Town Hall. The outcome of the call for white
papers was also summarised. Each UK author or co-author of a white paper was invited tomake a 1-slide presentation and poster of his or her concept or idea. This was then followed bya discussion about the alignment of the NGSPM-SOT, objectives, process and possible
outcomes with the UK's own priorities in solar physics.
E.4 Japanese Community Meetings
Community meetings were organized multiple times by JSPC (Japan Solar Physics
Community) to discuss science objectives and future directions of solar physics in Japan. The
first symposium (3-4 October 2016, ISAS/JAXA) was an opportunity to discuss key scienceobjectives based on the NGSPM-SOT study at the early phase with the community. Four subgroups formed in the JAXA Solar-C WG presented science objectives and mission concepts
based on Epsilon opportunity and identified scientific issues to be addressed in the next meeting,77

which was the JSPC annual meeting on 20-22 February 2017 at ISAS/JAXA. In the meeting,
the Japanese NGSPM-SOT members gave a series of reports on science objectives of solar
physics based on the NGSPM-SOT studies, followed by updates of Solar-C mission as well as
Epsilon-class mission concepts for primarily addressing scientific issues. At the same time, thefuture plan for ground-based facilities was also discussed associated with the future directionof space observations. Then, some schedule updates on the announcement of opportunity in
JAXA were reported to the community during the ASJ (Astronomical Society of Japan)
meeting (Fukuoka) on 17 March 2017. The JSPC also held a one-day meeting on 13 July 2017at NAOJ for having community consensus toward the coming Epsilon AO, with inputs fromNGSPM-SOT report as well as study updates from the JAXA Solar-C WG.
E.5 Q&A at Hinode-11
At the conclusion of the Hinode/IRIS joint science meeting in May-June 2017, inSeattle, Washington, a 75-minute session was allocated for a presentation and Q&A regarding
the Draft Report of the NGSPM-SOT. Following a PowerPoint presentation of the main pointsof the Draft Report, a Q&A with the audience discussed initial reactions of the community to
the findings and preliminary recommendations.

78

A p p e n d ix F
Epsilon mission system configuration
Epsilon rockets are the JAXA’s solid-fuel 3-stages vehicle and have successfullylaunched Hisaki in 2013 and Arase (ERG) in 2016. The epsilon rocket is used forcompetitively chosen medium-sized focused missions (<150M dollars class), which one
launch is expected every 2 years in the 2020’s. The Epsilon can install a 500 kg satellite intoa sun-synchronous polar orbit at an altitude of 570 km. If the weight of the satellite can be
reduced to 450 kg, the altitude can be increased to 600 km. In the case of a launch at the solar
maximum (2024), a thruster system is needed onboard to keep the orbit if the sunsynchronous condition is required for a period longer than one year. The payload volume
available in the rocket fairing has a height of about 5.4 m with a diameter of 1.9 m, which
tapers toward the top edge (0.236 m in diameter at the top edge) above 2.8 m height level.

Figure F-1: Spacecraft structural baseline configuration79

A spacecraft bus, jointly developed by ISAS/JAXA and an industrial contractor, hasbeen used in previous Epsilon rocket launches (Hisaki, Arase), which has about 1.1 m height.
The following descriptions are based on this spacecraft bus, although a different spacecraft bus
can be applicable. When the instrument is equipped in vertical configuration on the spacecraft
bus, the maximum length of the instrument on the bus is 4.3 m, so that it requires a tapered
structure at the top edge (0.236 m in diameter). If the instrument dimension is a rectangularshape with the size of 0.4 x 0.7 m at the top edge, the maximum length of the instrument isreduced to about 3.3 m (Figure F-1). The mission truss structure is used to mount the instrument.
Table F-1 summarizes a design of the spacecraft based on this spacecraft bus with thisdimension. By adapting the Ultra Fine Sun Sensor (UFSS) and high resolution gyroscope (IRU)
to the attitude control system, the short-term (10 s) pointing stability may be on the order of
0.2 arcsec at 3 sigma. For the mission concept for high spatial resolution instruments, it isimportant to consider an image stabilization system inside the instrument for guaranteeing theperformance at intermediate frequencies and the micro-vibration control for the high frequency
range. A high-speed telemetry channel (X-band 8 Mbps available in Japan, a higher telemetry
system to be developed in Japan, or a higher telemetry system to be provided by international
collaboration) shall be added to the system in addition to the standard S-band 2Mbps telemetry
channel.
Table F-1: Epsilon spacecraft design example with baseline configuration
Items

Specification

External
dimensions

Bus: 1.0 m x 1.0 m x 1.1 m (excluding the solar array paddles)
The instrument: 0.4 x 0.7 x 3.3 m (See Figure X)

Mass

451 kg (dry weight), 481kg (at liftoff), if the instrument is assumed 155 kg

Orbit

Sun synchronous polar orbit
Altitude 570 km (for 500 kg), 600 km (for 450 kg)
A thruster system to maintain the orbit

Power

Maximum total power allowed: 1,000 W

Communications

Commands: S-band
House-keeping telemetry: S-band 2Mbps
Mission telemetry: X-band (QPSK) 8Mbps or a higher telemetry system

Mission Data
handling

Standard Bus recorder: 2 Gbytes, write/read speed 15 Mbps
Mission data recorder: TBD Gbytes in the instrument

Attitude control

3-axis body control, Pointing accuracy much improved with Ultra Fine Sun
Sensor (UFSS) and high resolution Gyroscope (IRU), Tracking the solar
rotation

Lifetime

Longer than 3 years (desired)80