Interim Report:

SOLAR-C Plan-B
Toward understanding of the elementary structures& fundamental physical principles that govern
manetized plasma

ISAS/JAXA SOLAR-C Working Groupi

ii

Contents
Executive Summary..............................................................................................................1
Chapter 1: Science Goals and Requirements ................................................................... 1-1
1.1.

Understanding Small-scale Elementary Magnetic Structures .......................... 1-1

1.1.1. Elementary Structures Connecting from Photosphere to Corona .................. 1-1
1.1.2. Elementary Structures of Coronal Magnetic Loops ........................................ 1-2
1.1.3. Elementary Structures in Dynamical Chromosphere ..................................... 1-3
1.1.4. Elementary Structures in Photosphere ........................................................... 1-5
1.2.

Understanding Small-scale Elementary Magnetic Structures .......................... 1-7

1.2.1. Connecting the Chromosphere to the Corona ................................................. 1-7
1.2.2. Magnetohydrodynamic (MHD) Waves in the Solar Atmosphere.................... 1-8
1.2.3. Signatures of Energy Dissipations in the Corona ......................................... 1-13
1.2.4. Behaviors of Magnetic Fields in Corona Explored through Seismology....... 1-14
1.3.

Magnetic Reconnection...................................................................................... 1-15

1.3.1. The Solar Atmosphere as a Laboratory for Magnetic Reconnection ............ 1-15
1.3.2. Magnetic reconnection occurring in a wide variety of plasma conditions .... 1-16
1.3.3. Magnetic reconnection in weakly ionized plasma (solar chromospheres) .... 1-17
1.3.4. Magnetic reconnection in collisonless plasma (corona and transition region).................................................................................................................................. 1-18
1.3.5. Particle Acceleration coupled with Reconnection Dynamics ........................ 1-21
1.3.6. What is the role of 3-dimensionality in magnetic reconnection? .................. 1-23
1.4.

Small-scale Physics Initiating Large-Scale Phenomena .................................. 1-24

1.4.1. Flares and CMEs ........................................................................................... 1-24
1.4.2. Solar Winds .................................................................................................... 1-28
Chapter 2: Solar Ultra-violet Visible and IR Telescope ................................................... 2-1
2.1.

Introduction ......................................................................................................... 2-1

2.1.1. Solar UV-Visible-IR telescope in SOLAR-C Plan-B ........................................ 2-1

iii

2.1.2. Science targets with the instrument ............................................................... 2-2
2.2.

Basic requirements to the instrument ................................................................ 2-4

2.3.

Spectral lines compilation ................................................................................... 2-6

2.3.1. Spectral lines for diagnostics of chromospheric magnetic fields ................... 2-9
2.3.2. Spectral lines for diagnostics of chromospheric dynamics .......................... 2-10
2.4.

Configuration of the instrument ....................................................................... 2-10

2.5.

Optical telescope assembly ................................................................................ 2-12

2.5.1. Optical design ................................................................................................ 2-12
2.5.2. Mirror coating and photon budgets ............................................................... 2-14
2.5.3. Structure design............................................................................................. 2-16
2.5.4. Thermal design .............................................................................................. 2-18
2.6.

Focal plane instruments.................................................................................... 2-21

2.6.1. Interface with the telescope ........................................................................... 2-21
2.6.2. Spectro-polarimeter ....................................................................................... 2-22
2.6.3. Broad-band filtergraph .................................................................................. 2-24
2.6.4. Narrow-band filtergraph ............................................................................... 2-25
2.7.

Image stabilization system ............................................................................... 2-27

2.8.

Contamination control ...................................................................................... 2-28

2.9.

Integration and test plan .................................................................................. 2-28

2.9.1. Integration and initial M1-M2 alignment ..................................................... 2-28
2.9.2. Zero-gravity optical performance test ........................................................... 2-29
2.9.3. Opto-thermal test........................................................................................... 2-29
Appendix A. Diagnostics of dynamics and magnetic fields in the chromosphere ..........a1A.1. Limitations in the Zeeman diagnostics................................................................a1A.2. Prospects of the Hanle diagnostics ......................................................................a1
A.3. Inversion of chromospheric lines .........................................................................a2
A.3.1. Obtaining velocities and plasma parameters ...................................................a3
A.3.2. btaining magnetic fields ....................................................................................a3iv

A.4. Numerical modeling of the chromosphere ...........................................................a5
Chapter 3: EUV/FUV High-Throughput Spectroscopic Telescope .................................. 3-1
3.1.

Performance requirements for the spectrometer ................................................ 3-1

3.1.1. Spatial resolution requirement ....................................................................... 3-2
3.1.2. Temporal resolution requirement.................................................................... 3-3
3.1.3. Broad temperature coverage ........................................................................... 3-4
3.1.4. Necessity of low scattering optics .................................................................... 3-5
3.1.5. Substantial jump expected from the NASA IRIS mission .............................. 3-5
3.2.

Technical feasibility of each key element ........................................................... 3-6

3.2.1. Two element optical layout .............................................................................. 3-6
3.2.2. The telescope mirror and mirror coating......................................................... 3-6
3.2.3. Grating and grating coating ............................................................................ 3-7
3.2.4. Solar blind detectors for EUVS ....................................................................... 3-7
3.2.5. Removal of metal filters and thermal control concept .................................... 3-7
3.3.

Strawman Instrument ........................................................................................ 3-8

3.3.1. Optical Design.................................................................................................. 3-8
3.3.2. Effective Area ................................................................................................... 3-8
3.3.3. Selection of wavelength bands......................................................................... 3-9
3.3.4. Expected count rates and Temperature Coverage .......................................... 3-9
3.3.5. Improvements over previous instrumentation.............................................. 3-11
3.4.

Strawman design details ................................................................................... 3-12

3.5.

Further optimization for better instrument ..................................................... 3-13

Appendix 3-A. EUVS count rates for the brightest lines ........................................... 3-14
Chapter 4:

X-ray Imaging (Spectroscopic) Telescope .................................................... 4-1

4.1.

X-ray Telescope for the Overall Solar-C/Plan-B Science .................................... 4-1

4.2.

Photon-Counting Imaging Spectroscopy X-ray Telescope .................................. 4-2

4.2.1. Scientific Objectives ......................................................................................... 4-2
4.2.2. Required Specification for the Instrument...................................................... 4-6v

4.2.3. Baseline Instrumentation ................................................................................ 4-7
4.2.4. Expected Capability ....................................................................................... 4-13
4.3.

Ultra-High Spatial Resolution Normal Incidence EUV Telescope................... 4-20

4.3.1. Scientific Requirements for the Instrument ................................................. 4-21
4.3.2. Strawman Instrument Concept ..................................................................... 4-22
4.3.3. Strawman design details ............................................................................... 4-24
4.3.4. Technical feasibility for realizing the ultra-high spatial resolution ............. 4-25
4.3.5. Further optimization for a better instrument ............................................... 4-25
Appendix 4.A Event Processing .................................................................................. 4-26
Chapter 5: Spacecraft and Mission Design ...................................................................... 5-1
5.1.

Spacecraft Layout ................................................................................................ 5-1

5.2.

Science Telemetry ............................................................................................... 5-5

5.2.1. Data rate estimate ........................................................................................... 5-5
5.2.2. Data handling on board and data recorder ..................................................... 5-5
5.2.3. Science Telemetry System ............................................................................... 5-6
5.3.

Orbit..................................................................................................................... 5-8

5.4.

Pointing Stability and Attitude Control ........................................................... 5-10

5.4.1. Requirements on the pointing stability ......................................................... 5-10
5.4.2. Attitude control system ................................................................................. 5-13
5.4.3. Flexibility on the role attitude....................................................................... 5-14
5.5.

Spacecraft thermal design to dump heat load from 1.5m telescope ................. 5-14

5.6.

Spacecraft power system ................................................................................... 5-15

5.7.

Science operations ............................................................................................. 5-16

Appendix 5.A Data rate estimate................................................................................ 5-17
References ......................................................................................................................... R-1
Acronyms......................................................................................................................... R-10

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Executive Summary

The Solar-C Plan B mission is proposed to study energy transport and dissipation of themagnetic energy governing the dynamic solar atmosphere through elementary magnetic
structures. For this purpose, the plan B mission has three large state-of-art advanced
telescopes that can perform for the first time high spatial resolution, high throughput, highcadence spectroscopic and polarimetric observations seamlessly covering the entire
atmosphere from photosphere and chromosphere to transition region and corona.
Top-Level Science Goals

At the core of all the questions related to how the Sun floods the heliosphere with hot
plasma, radiation, particles, and magnetic fields is the need to understand the elementary
structures and fundamental physical principles that govern a magnetized plasma.
Theory and laboratory experiments provide many useful insights, but the insights have beenmostly triggered by new observations of the Sun’s atmosphere, which is the only
astrophysical plasma accessible with spatially resolving magnetic structures and dynamics.
By performing high spatial resolution imaging observations under extremely stablecondition never realized so far, “Hinode” opened a new frontier in solar physics researches.
For example, series of high resolution Ca II H images allow us for the first time to identify
waves existing along magnetic fields, which can be considered as a key part in heating of thecorona. Magnetic reconnection, which was firmly settled in solar physics with flareobservations by “Yohkoh”, is now used in explaining many aspects of dynamics observed in
the solar atmosphere. However, details of fundamental physical processes governing theheating and dynamics in the solar atmosphere have not yet been described well with physicalterms. This is because of the lack of spectroscopic measurements, which are most useful todiagnose plasma conditions. With Hinode observations, we also recognize that thechromosphere, which is the interface layer between the photosphere and corona, is the most
key atmospheric layer in understanding the heating and dissipation in the magnetic
atmosphere.
The outer atmosphere of the Sun (or stars) is a complicated system consisting of thephotosphere, chromosphere, and the corona. The energy is built up in the magnetic
atmosphere. The energy is transported via various methods, especially waves along magnetic
fields. The dissipation of the magnetic energy is the central engine for causing variability inthe emissions from the Sun. The energy build up, transport, and dissipation are mixed andcomplicated in the magnetic atmosphere, thus making it difficult to understand the governing
physical processes without spectroscopic and polarimetric measurements over the entire
atmosphere. Thus, the Solar-C Plan B mission is designed to make spectroscopic andpolarimetric measurements over the entire atmosphere with necessary spatial and temporal
resolution.
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Scientific objectives of the Solar-C Plan B mission are summarized in Table 1. As a path
to exploring the fundamental physical principles, we seek to understand:How elementary structures of the magnetic atmosphere responsible for heating anddynamics are created and evolve over each temperature domain of the atmosphere;How the energy that sustains the atmospheric structure is transported through small
elementary structures into large-scale corona and energizes the solar wind; and

How the magnetic energy is dissipated in the astrophysical plasma.

The solar atmosphere consists of elementary structures, one of which is magnetic flux tubes.
The elementary structures shall be understood with their relationship with the magnetic fieldat all relevant temperatures. It is essential to quantitatively trace how they are created andevolve in association with magnetic fields and plasma flows over wide temperature domain.
Magnetic field strength and direction in the chromosphere and corona, which is far above thephotosphere and where the plasma is in low plasma (β<1) condition, is the crucial information
to identify the magnetic structures and their roles for causing heating and dynamical nature in
the atmosphere. Coronal loops are well known magnetic structures in the corona, but EUV
density diagnostics suggests that coronal loops contain elemental structures that are about
10% volume of the coronal loops. The unresolved elementary structures would reflect thesource of energy inputs responsible for creating the hot temperature corona.
The energy for solar activity and heating is transported from the photosphere into thecorona associated with magnetic fields. The origin of the energy is the kinetic energy of thegas convection in and below the photosphere. The magnetic fields rooted on the photosphereplay key roles in transporting the energy upward the chrosmosphere and corona, but we havenot identified the mechanisms because of the complexity of fine structures, although somecandidates have been proposed, such as high-speed upward ejections of plasma, propagationof MHD waves, and winding and braiding of magnetic fields for forming field discontinuities
in the corona. It is extremely important to quantitatively evaluate how much energy is carriedby waves and how the magnetic energy is dissipated in the stratified atmosphere both forheating the chromosphere and the corona and for energizing the solar wind.
A complex system of magnetic fields in the solar atmosphere triggers various types ofdynamics and heating at different timescales and at different temperatures. They require theconversion of magnetic energy to heat plasma, drive flows, and accelerate particle to highenergies. This conversion mechanism is “magnetic reconnection,” which may be the central
engine in many solar phenomena, including the explosion of solar flares, the heating of thesolar corona, and the initiation of coronal mass ejections. The solar atmosphere has a wide
variety of plasma conditions; the inner corona consists of fully ionized, collisionless plasma,
whereas the chromosphere is in weakly ionized plasma condition. We seek to address threeimportant questions about magnetic reconnection. Quantitatively evaluation of fastreconnection in the weakly ionized chromosphere would be helpful to understand what
plasma condition controls the energy release rate. What controls the energy distribution
among thermal, kinetic, and non-thermal energies in early stage of flares? This would help tounderstand a wide variety of solar flares. How can reconnections in small scale develop to alarge-scale flare structure?
Exploring energy transfer and dissipation governing the solar phenomena is directly linked
to the space weather researches. Better understanding and predicting the space weather events
are the basis on human related activities in the space. One of the important goals of solar
physics research is to improve ability to predict geo-effective solar phenomena. High spatial
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Table 1. Solar-C scientific objectives and required instruments
Required Instruments
Section

Scientific Objectives

Specific questions to be answered

SUVIT

EUVS

XIT
PC

NI

A. Elementary
structures of themagnetic atmosphereresponsible forheating anddynamics

1.

What generates dynamical
chromosphere, such as jets?

1.1.3

◎

○

2.

What are the tunnel structures
for energy transport fromchromosphere to corona?

1.1.1
1.1.2
1.2.1

◎

◎

3.

How are waves excited insidemagnetic flux tubes?

1.1.4

◎

○

B. Energy transportand dissipation
through small-scale
magnetic structures1.

How do the observed waves
behave in stratified atmosphereand contribute to thechromospheric heating?

1.2.2

◎

◎

2.

How much energy is carried by
the waves that penetrate to thecorona?

1.2.2

◎

◎

3.

What are signatures of energy
dissipation in coronal loops?

1.2.3

◎

◎

1.

What determines energy
distribution in the energy
conversion by reconnection?

1.3.4
1.3.5

◎

◎

2.

How can reconnections in small
scale develop to a large-scale flarestructure?

1.3.6

○

◎

○

3.

Does fast reconnection reallyoccur in the chromosphere?

1.3.3
1.3.4

◎

○

1.

What are the magnetic fieldstructures during filament
eruptions?

1.4.1

◎

○

○

○

2.

How do magnetic flux and massenergy transport through theatmosphere and initiate flares and
CMEs?

1.4.1

◎

◎

○

○

3.

What types of magnetic
structures at the low atmosphere
are origins of solar winds?

1.4.2
1.1.4
1.2.2

○

◎

C. Magnetic energy
dissipation: Unveil
the mask ofreconnection region

D. Small-scale
physical processes
initiating large-scale
phenomena
regulating space
weather◎○

Note: [Origin] [Transport] [Dissipation]: Color indicates main physical process involvedin each specific question.
Note: SUVIT = Solar UV-Visible-IR Telescope (Section 2)
EUVS = EUV/FUV high-throughput spectrometer (Section 3)
XIT = X-ray Imaging Telescope, PC= Photon counting, NI= Normal incident
3and temporal resolution spectroscopic observations will much improve our knowledge about
the initiation of space weather phenomena. Small-scale physical processes in the emergenceof magnetic flux may play key roles in initiating the phenomena. Also, direct measurement ofmagnetic fields in the prominence or filaments tells how the magnetic field structures ofprominences are evolved and developed to eruptions. Thus, as a goal toward the space
weather researches, we seek to understand:How small-scale physical processes initiate large-scale dynamic phenomena regulating
space weather.
The four science goals defined above are discussed in more details in section 1.

Scientific Measurement Requirements and Proposed Model Payloads

The following scientific measurements is required for the set of mission instruments. All ofthem have not yet realized in any currently operating and planned missions, and they are
unique.
Precise polarimetric (S/N~10-4) and spectroscopic measurements for probing
nature of magnetic fields both in the chromosphere and photosphere.



High throughput spectroscopic measurements for probing nature of dynamics
with high time resolution (0.5-1sec).

Seamless and simultaneous coverage of spectroscopic measurements over eachtemperature domain of the solar atmosphere from chromosphere to the coronaand flares for understanding the entire picture of energy transport and dissipation.

High spatial resolution measurements for resolving elementary structures.

These measurement requirements are realized by the set of the following three science
instruments:
1) Solar UV-Visible-IR telescope (SUVIT)
2) EUV/FUV high throughput spectroscopic telescope (EUVS)
3) X-ray imaging (spectroscopic) telescope (XIT).
EUVS will provide the crucial link between the photospheric and chromospheric magnetic
field and plasma characteristics obtained by the SUVIT and the high temporal and spatial
resolution diagnostics of the corona provided by the XIT. Two possibilities are considered for
XIT: photon counting imaging spectroscopy telescope and ultra-high spatial resolution
telescope.

Solar UV-Visible-IR Telescope
The primary purpose of the UV-Visible-IR telescope (SUVIT hereafter, Table 2) is tophysically capture nature of gas dynamics and magnetic fields in the lower atmosphere, i.e.,from the photosphere through the upper chromosphere, and to understand dynamical
evolution of elementary structures of magnetized atmosphere. The telescope is a4diffraction-limited telescope with a 1.5 m aperture in diameter (the size may be optimizedin the future). It has potential to resolve structures with 0.1 arcsec or better for the firsttime from the space observations (Figure 1), which can reveal dynamical behaviors of the low
atmosphere through elementary magnetic structures and key physics responsible for energy
transfer and dissipation in the photosphere and the chromosphere.
Table 2. Key characteristics of UV-Visible-IR telescope
Item
Telescope
Aperture diameter
Focal plane instruments

Wavelength coverage
Spectral lines(spectroscopic observations)Spectral lines(imaging observations)

Spatial resolution
Exposure time
Spectral resolution
Field-of-view

Description
Aplanatic Gregorian telescope equipped with an image
stabilization system
ϕ1.5 mSpecto-polarimeter (SP)Broadband filtergraph (BF)
Narrowband filtergraph (NF)
280 nm to 1.1 μm(polarimetry in the chromosphere)
He I 1083 nm, Ca II 854 nm(polarimetry in the photosphere)
Si I 1082.7 nm, Fe I 853.8 nm(dynamics in the chromosphere)
Mg II h/k 280 nm(chromosphere)
Mg II h/k 280 nm, Ca II 854 nm, H I α 656 nm,
Na I D 589 nm, He I D 587 nm(photosphere)
Fe I TBD nm<0.1” at UV and visible<0.2” at near-IR(intensity observations) 0.1 - 1 sec(polarimetric observations) 1 – 20 sec~120,000 with the spectro-polarimeter~50,000 with the narrowband filtergraph180” x 180”

SUVIT will take the first attempt to measure magnetic fields in the chromosphere fromthe space. The Hinode Solar Optical Telescope measures magnetic field vectors at the thinphotospheric layer with spectro-polarimetric capability with polarimetric sensitivity of ~10-3,
and scientists have recognized that measuring magnetic fields in the chromosphere,
1,000-2,000 km above the photosphere, is essentially important to explore the stratified
magnetic atmosphere. SUVIT can diagnose magnetic fields both at the chromosphere andphotosphere (Figure 1).
Polarimetric sensitivity of ~10-4 is needed for diagnosing the chromospheric magnetic
fields with combination of the Zeeman effect and the Hanle (scattering polarization) effect.
Comparing with the Hinode Solar Optical Telescope (50cm in diameter), SUVIT canaccumulate roughly one order of magnitude larger number of photons in the same spatialsampling and in the same exposure duration. The polarimetric sensitivity of 10-4 is achieved
with 20 s exposure times for 0.2” pixels.
SUVIT covers a wide wavelength region from UV (~2800 Å) to near infrared (1.1 μm),in which there are several spectral lines best suitable for diagnosing dynamics andmagnetic fields in the chromosphere as well as the photosphere. He I 10830 Å and Ca II
8542 Å spectral lines are identified as best lines for diagnosing magnetic fields and Mg II h/klines at 2800 Å are for diagnosing dynamics in the chromosphere.
5

As the focal plane instruments, two instrument capabilities are equipped to the SUVIT:
Precise spectroscopic and polarimetric measurements allow us to determine physicalparameters such as temperatures, velocities, and magnetic fields. Imaging of intensities andmagnetic fields with high temporal cadence is another capability to capture dynamical
behaviors of magnetic fields within an observing field of view. Narrow-band imager is used
for polarimetric and Doppler measurements at limited number of positions in spectral lines.
Broad-band imager is for best possible high spatial monochromatic images of thechromosphere and photosphere.

Figure 1. Spatial resolution and polarimetric sensitivity achieved with Solar-C UV-Visible-IR
telescope, compared to other space-borne and ground-based large telescope.

EUV/FUV High-Throughput Spectroscopic Telescope (EUVS)The EUV/FUV high throughput spectrometer (EUVS) is a telescope for imaging
spectroscopy in the FUV-EUV region with a resolution and effective area an order ofmagnitude higher than currently available instruments for solar studies (Table 3, Figure 2).
The spatial resolution of EUVS is ~0.3 arcsec (plate scale: 0.16 arcsec/pixel), which is muchhigher than currently operating SUMER and EIS (2-3 arcsec, 1 arcsec/pixel). This higher
spatial resolution is required to spatially resolve elementary structures responsible for energy
transport in the solar magnetic atmosphere. The volumetric filling factor of coronal loops
derived from density sensitive line analysis suggests the existence of fine magnetic structuresin order of 0.3 arcsec or ~200km. Magnetic structures in the chromosphere are also of thissize, as seen as spicules. Thus, this performance allows us for the first time to trace flows of
the energy from its origin (the photosphere) through the chromosphere and transition region
and into the corona. Since the structures are small and dynamic, a high time cadence isnecessary and consequently a high-throughput instrument is required. EUVS will have 1-5 sec
exposure for intense lines with 0.3 arcsec spatial sampling, and 0.5-1 sec or shorter exposurefor 1 arcsec sampling.
Simultaneous spectroscopic measurements sampling all temperature ranges of the solar
atmosphere are essential to achieving the science goals of the Solar-C mission. EUVS is6designed to seamlessly acquire spectral lines emerging from a wide temperature in 104-107
K. Since lower temperature lines are mostly observed in the FUV region and higher
temperature lines are in the EUV region, EUVS is designed to cover a wide wavelength range
in FUV-EUV and it is something like the instrument combining EIS and SUMER.
The strawman spectrometer consists of an off-axis primary mirror and grating mirror. This
optical layout minimizes the number of optical elements, resulting in high throughput
performance. The spectrometer measures emission lines and continua in some numbers ofwavelength windows between ~100 Å and ~1200Å. Low-scattering optics should be used in
the spectrometer to allow us to diagnose low emission measure pixels. Many importantscience discoveries may be hidden in low emission measure regions, such as, reconnection
outflow regions that are much fainter than flare loops, and coronal holes where solar wind
acceleration and shock formation may be observed in the high corona.
Table 3. EUVS scientific characteristic
Item
Instrument design

Plate scale
Simultaneous field of view
Maximum raster width
Wavelength bands
Exposure times
Velocity resolution
Temperature coverage

Description- Φ 30-40 cm, off-axis paraboloid mirror- Single grating spectrograph- Two detectors, one for EUV band and the other for longwavelength bands- Image stabilization, slit-jaw camera~0.16"/pixel
Slit: 0.16", 0.32", 0.96"; Slot: 10", 40", 200"
Slit/slot length: 200" nominal, >300" extended+/- 75" nominal, +/- 200" extended
Four channels (Å):130-250; 510-630; 690-810; 940-1060
1-5 s with 0.33" spatial sampling
0.5-1 s in active regions with 1.0" spatial sampling
Doppler (centroid) shift measurement accuracy <3km/s
Turbulent velocity (width) <10km/s
104 – 107 in active regions with Δ(logT) <0.3

Figure 2. EUVS count rates for active region plasma, compared with currently operating
telescopes (SUMER and EIS) and planned IRIS. At least 100-1200 counts are needed forspectral analysis.
7

X-ray Imaging (Spectroscopic) Telescope
The X-ray imaging telescope images, with spectroscopic information, the uppermost layerof the Sun, the corona, in soft X-rays or EUV to achieve comprehensive understanding on
magneto-hydrodynamic processes across the entire layers of the solar atmosphere jointly with
SUVIT and EUVS. The telescope should aim to (a) understand forms and mechanisms ofstorage and dissipation of energy transferred upwards through the photosphere and thechromosphere, (b) quantitatively understand physics of magnetic reconnection with its
possible role for particle acceleration, and possibly, (c) perform diagnostics on wave
phenomena in the corona. In order to pursue such scientific objectives, the telescope isrequired to have 1) wide coverage on coronal temperatures to observe the entire portion of
single coronal loops, particularly, for the core portion of active regions, 2) high spatial
resolution consistent with that for EUVS, and 3) advanced spectral diagnostic capability forquantitatively investigating physical processes in the corona.
Table 4. Outline of the photon-counting X-ray telescope
Item

Description

Overall description

Perform imaging investigation of the X-ray corona
with the following two modes. Two modes selectable(not simultaneously).
(1) Photon-counting mode: Imaging spectroscopy for alimited region of interest.
(2) Photon-integration mode: Imagery with the entire,or part of, the imaging array.
Walter Type I (-like) grazing-incidence telescope witha segment mirror. A CMOS-APS detector to be used as
the focal-plane imaging array.

Envelope dimensionsof the telescope~40cm × 40cm × 4.5m (TBD)

Mirror focal length

4m

Angular resolution &Temporal resolution
for photon counting

For photon-integration mode:~1" (~0.5"/pixel)
For photon-counting mode:
For active regions and flares, generate an X-ray
spectrum for spatial area whose size*~< 2"×2" (goal) / ~4"×4" (min.)within every< 10 s (goal) / 30 s (min.)For photon-integration mode*:~13'×13'
For photon-counting mode**:>~3'×3' (goal)~80"×80" (nom.)

Field of view

Energy range

Remarks

Telescope plus the focal-planecamera. Includes housings.

~0.5-5 keV* (baseline)~0.5-~10 keV* (under study)

Data rate* Spatial area size in which a
spectrum with good counting
statistics can be synthesizedwith reasonable integrationtime.

* Off-axis image blur not taken
into account.
** Area with which photoncounting analysis can be
performed.
* Energy range above 2 keVavailable during flares, withphoton-counting mode.

max. 110 Mbps for 512×512 photon-counting area
max. 6.9 Mbps for 128×128 photon-counting area

A photon-counting imaging spectroscopy X-ray telescope (Table 4) is proposed, with aultra-high spatial resolution normal incidence EUV telescope (Table 5) as another option. The
photon-counting X-ray telescope will provide first-ever opportunity to perform
imaging-spectroscopic investigation of the corona in soft X-ray wavelength range, by
measuring energy and location of each incident X-ray photon onto the detector. In addition to8

the photon-counting capability, the telescope is designed to have photon-integration capability
for imaging observations (just like past solar soft X-ray telescopes) with one of the two modes
to be selected for a certain period of observation. The main observational targets for thephoton-counting X-ray telescope is to 1) investigate quantitatively, with spectroscopic
information, structure and dynamics of magnetized plasmas around the site of magnetic
reconnection, and 2) investigate temperature structure of active region corona, in particularthat for high temperatures (above ~5MK). It should be noted that the photon-counting X-ray
telescope provides a unique chance of investigating electron temperatures of thermal plasmas
and even detecting non-thermal emission in soft X-ray wavelengths, which is mostly not
visible with emission lines but should manifest itself as continuum component(s) in X-ray
spectra.
The other option, ultra-high spatial resolution normal incidence EUV telescope, will focus
on understanding connection between the transition region and the lower corona incollaboration with EUVS. The normal incidence telescope serves as delivering
complementary observables with EUVS, providing context imagery information for EUVS
with higher spatial resolution (0.2"-0.3") and wider field-of-view, and with overlapping
temperature coverage. Thus, the main target of this telescope is to resolve fine, hopefully
elemental, structures in the corona, study energy flows and magneto-acoustic waves as well as
small-scale heatings in coronal temperature range.
Table 5. Outline of the ultra-high resolution EUV telescope (Preliminary)
Item

Description

Remarks

Overall description

Consists of two telescopes each with Cassegrain optics.
Each telescope equipped with 3-sector mirrors fordifferent wavelengths.
Image stabilization system necessary.

Optical properties

32 cm aperture diameter. TBD focal length.

Angular resolution0.2"-0.3" with 0.1"/pixel

Field of view~400" × 400"

Wavelength Bands

171 Å, 195 Å, 211 Å, 335 Å, 94 Å and broad-UV band

Exposure time

1 s for an active region, 0.1 s for a flare

Cadence< 10 sData rate

TBD

TBD

Spacecraft and Mission Design
The spacecraft designed to meet required scientific performance is summarized in Table 6,
and the conceptual layout is shown in Figure 3. The spacecraft is designed mainly withconsidering the following items.
High pointing stability is required to realize the high spatial resolution and precise
polarimetric performance. The spatial resolution of the telescopes is twice to five times
higher than that of telescopes onboard Hinode. For the telescopes with angular resolution
better than about 0.4 arcsec, tip-tilt mirror system is equipped inside the telescopes to9stabilize solar images on their focal plane detectors.
High rate science telemetry is required to acquire spectroscopic and polarimetric datawith high cadence and spatial resolution. The minimum requirement is that the threetelescopes can acquire the data continuously with the post-compression data rate of about
8 Mbps in total. We will use an X-band telemetry system (16-QAM, 16Mbps) with
12-hour or longer duration of contacts at ground stations or a Ka-bad telemetry system(QPSK, ~80Mbps) with shorter duration of contacts at a ground station.
Table 6. Solar-C plan B mission overall characteristics
Spacecraft layout
WeightSize
Launch vehicle
Orbit
Power
Command & Telemetry

Attitude control

Science operations

Telescopes mounted on the optical bench, which is on the Bus
module. See Figure 1.x
4930 kg (at liftoff), 2330 kg (dry weight)
3.2 m x 3.2 m x 7.4 m, excluding solar array paddles
JAXA H-IIA (type 202)
An inclined geo-synchronous orbit (Baseline)
A sun-synchronous polar orbit (optional)~3,500 W (maximum)
Uplink and housekeeping downlink: S-band
Downlink: X-band 16-QAM (16Mbps)
Ka-band QPSK (80Mbps, optional)
3-axis body pointing with high accuracy
Image stabilization system in some telescopes
A function to change the pointing around Z-axis for matching
spectrograph slit direction to the observing target.
Hinode operation scheme, with a final target selection around 10
to 15 minutes before the uplink in cases where this is essential forthe science.

Figure 3. Solar-C plan B mission conceptual layout
An inclined geo-synchronous orbit (GSO, altitude 36,000km, inclination < 30 deg,
period 1 day) is considered as the baseline, with a sun-synchronous polar orbit as the
backup. The GSO orbit provides almost uninterrupted observations of the Sun and muchstable thermal environment, compared with Hinode, for minimizing the thermal
deformation of the structure. It also provides long duration viewing from a groundstation, allowing us to use X-band for high data rate, although the spacecraft weight andsize become bigger.
10

Another advantage of having a long-period downlink is that we can respond to
dynamical changes on the Sun much faster, by monitoring solar images in real time. The
optical and EUV spectrometers have relatively small fields of view and it is important tobe able to choose the observing region as close to real-time as possible.



The spacecraft design should utilize technical heritage from Hinode, especially the
structural design and pointing related technologies for the high pointing stability andtelescope alignment.

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