Space Solar Missions by Japan and Future Roadmap
To date Japan has launched the Hinotori, Yohkoh, and Hinodehow reversal of polarity occurs in the polar magnetic field, important forsatellites, achieving highly significant scientific results. SOLAR-C,

understanding the 11-year solar cycle. From the standpoints of continuity

the fourth space solar mission, is under study with a launch target ofwith Hinode data and conducting comparative studies with previous

fiscal year 2018. Currently the sun is showing signs of entering a period

activity cycles, it will be especially important to capture the transitionof unusual activity not seen in recent years, with the next few cycles

from the next solar minimum to solar maximum, making it essential topresenting an excellent opportunity to understand the links between therealize the SOLAR-C mission in the latter part of this decade. Moreover,sun and the earth’s environment. The high-spatial-resolution magnetic

for realizing the SOLAR-D mission early in the 2020s, the Destiny

field observations by Hinode at last have allowed us to investigate themission (to be proposed to ISAS Small Scientific Satellite #3) will play anlong-term magnetic field fluctuations across the entire sun, including

important role in demonstrating the applicability of the large ion engine

quiet regions and polar regions; and we are in the process of learningand the ultra lightweight solar array, etc.

Solar maximum

Solar maximum

Solar maximum

Solar maximum (expected) Solar maximum (expected)

1980

1990

2000

2010

2020

2030

SOLAR-C SOLAR-D
LaunchLaunchDESTINY
LaunchHinotori(hard X-ray)

Yohkoh(soft X-ray, hard X-ray)

SOLAR

Hinode(optical, extreme ultraviolet, soft X-ray)

SOLAR-C WG Activities
The SOLAR-C Working Group defined two mission plans, Plan A

Japan could enhance its own instrument development capability in thisfor exploring the polar regions of the sun away from the ecliptic plane,

opportunity by providing one major instrument. The decision reached byand Plan B for high-resolution spectroscopic observation of the sun

the committee was that Plan B would be implemented as SOLAR-C andseamlessly from the photosphere to the corona. It then formed a sub-Plan A, involving development elements for interplanetary flight, would

working group to study these plans, consisting mainly of the Japanese

be implemented as SOLAR-D. As an extension of the sub-working

solar physics community with cooperation from scientists in the Unitedgroup activities, a European group in December 2010 proposed theStates and Europe. The studies were summarized and made public in

SOLAR-C EUV/FUV high-throughput spectroscopic telescope (EUVS)

an interim report (http://hinode.nao.ac.jp/SOLAR-C/Documents/

to ESA’s Cosmic Vision, giving it the name LEMUR. In order to raise theInterim_report2011.html). Together with NASA a Joint Solar-C Science

technological readiness level, taking advantage of strategic development

Assessment Committee was formed to choose between the two plans,

funding by the Institute of Space and Astronautical Science (ISAS),

based on the interim report, from such standpoints as (1) the sciencestudies and development are being carried out toward realization of large-

merit, (2) the technical readiness of the spacecraft and the strawman

scale telescopes and focal plane instruments, a highly reliable rotational

science instruments, (3) initial estimates of the total costs to JAXA, (4)drive mechanism capable of high-frequency operation, a photon-counting

the broad participation of overseas space agencies, and (5) whether

X-ray telescope, and satellite systems for achieving image stability.

Contacts:

ISAS/JAXA SOLAR-C Working Group
3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan
SOLAR-C Preparatory Office, National Astronomical Observatory of Japan
2-21-1 Osawa, Mitaka, Tokyo 181-8588, JapanThe Next Space Solar Mission

SOLAR

SOLAR-C website:

http://hinode.nao.ac.jp/SOLAR-C/

Background image: Chromospheric jets at the limb of the sun as captured by Hinode. Fine
detailed structure can be seen in the erupting jets. In the chromosphere andcorona, extremely dynamic phenomena like these take place all the time,
influenced by magnetic fields.

SOLAR

From Hinode to SOLAR-C

SOLAR

Revealing the sun’s secret in the magnetic activity by magnetic field observationsof the chromosphere and high-resolution observations of the corona
The sun’s surface (photosphere) has a temperature of 6,000 K. Above that is theMagnetic elements

chromosphere at around 10,000 K, and farther out still is the corona at more than a(concentrated magnetic
field region)

million K; the origin of these atmospheric layers is still not well understood. In the vicinityof sunspots, huge eruptions called flares spontaneously release enormous amounts ofenergy, with their effects extending to interplanetary space. On a smaller scale, there are

2000kmprocesses at work heating the sun’s atmosphere everywhere, which are believed to be

Photosphere (6,000 K)

involved in formation of the chromosphere and corona. The source of this activity is themagnetic field energy created by the solar convection zone, but just how this results information of an active magnetic atmosphere is not well understood.
In order to understand the activity observed in the solar atmosphere, it is necessary to

Chromosphere (10,000 K)

view the photosphere, chromosphere, and corona as one system coupled by magnetic
fields. To do this it is necessary to resolve the size scale of the fundamental physicalprocesses connecting these atmospheric layers. From results obtained by the HinodeNew findings and New Questions by Hinodesatellite, the needed resolution is estimated to be 0.1 to 0.5 arcseconds. Since thechromosphere links the photosphere and the corona, it is especially important to study
properties and magnetic structure in the chromosphere. And in order to understand inCorona (1 million K)
Duration：~ 150 sec

detail the transition between the chromosphere and the corona, high-resolution coronalobservations are essential.
Using high-resolution spectropolarimetry, SOLAR-C will measure magnetic fields of boththe photosphere and the chromosphere; up to now measuring the chromospheric field
has proved extremely difficult and elusive. Moreover, the three-dimensional magnetic fieldstructure of the corona will be sought by indirect means based on these measurements.

(A) Chromospheric jet

In addition to chromospheric magnetic field measurements, SOLAR-C will observe theCoronacorona at resolutions far higher than with Hinode, and will attempt to capture the fine-Transition region(C) Swaying spicule(B) Waves propagating in a prominence (yellow arrow)
2,500 kmscale dynamics of the chromosphere and corona by means of spectroscopic observations.
structures involved with the magnetic field as well as their changes, while at the same

20,000 km

By determining the diversity of elementary processes (fine-scale structures) and global

Chromospheretime observing the corona at unprecedentedly high resolution, the mission will clarify
the processes that create the high-temperature atmosphere of the sun, and also theatmospheres of stars in general. Overall, the mission will yield a complete picture of the
sun’s vigorously changing magnetic activity.

Outstanding science questions of solar physics- H-ow are the chromosphere and corona heated?
- What drives the solar wind?
- How are the diverse magnetic structures observed in the chromosphere formed?
- How do solar flares and other such eruptive phenomena occur?
- What causes the solar magnetic cycle?

1

Photosphere
Convection zone(D) Transient horizontal magnetic fields
in the photosphere (shown in yellow)

Above the sun’s “surface” (photosphere) are layers of hotter plasma calledthat these events may be responsible for heating the chromospherethe chromosphere, transition region and the corona. While the photosphereand corona. In addition, Hinode observations of the photosphere showis ~ 6000 K temperature, the chromosphere and the corona are ~ 10,000

numerous transient horizontal magnetic field structures which may drive

K and over 1 million K, respectively. The heating mechanism of these highthe dynamics of the chromosphere (Figure D). Magnetic fields dominate

temperature layers is one of the outstanding questions of solar physics.

the dynamics in the chromosphere, transition region and the corona, andWhile the chromosphere is cooler than the corona, its higher density

likely play a central role in the heating mechanisms. For understandingmeans the amount of heating needed to maintain the chromosphere is 10

the heating mechanisms it is essential to carry out high accuracy and highto 100 times larger than that of the corona. Observations by the Hinodespatial-resolution observations of magnetic fields from the photosphere tosatellite discovered a variety of dynamic events in the chromosphere suchthe corona, including the chromosphere.

as various types of jets and wave phenomena (Figures A to C), indicating

2

SOLAR

Science Objectives of SOLAR-C
Is the solar corona heated by nanoflares, having only one billionth the energy

phenomena at the sun will require both (1) high resolution for capturing fine-of giant flares? Or are magnetohydrodynamic (MHD) waves responsible

scale structures and (2) high sensitivity for tracking rapid changes. For this,both for heating the corona and at the same time causing the solar wind totelescopes larger than those on Hinode are needed.

accelerate to supersonic speeds? High-resolution observations of magnetic
fields by Hinode revealed the presence of many kinds of magnetic structures

The high resolution of the three telescopes on SOLAR-C will enable

in the photosphere, differing greatly in size and in the scale of their changes

observations of the magnetic coupling from the photosphere, to theover time. But the structure of the magnetic field higher up in the solar

chromosphere and the corona. Observations with SUVIT of photospheric andatmosphere, in the chromosphere and corona, is still not well understood. Is

chromospheric magnetic fields will then make it possible to infer the three-

the coronal magnetic field made up of a multitude of magnetic sub-structures,

dimensional magnetic structure extending from the photosphere through to thecomplexly intertwined reflecting photospheric dynamics and readily conducive

corona. This in turn will help identify the kinds of magnetic structures that lend

to the generation of nanoflares? Or does the nature of the magnetic structures

themselves to magnetic reconnection, and clarify how waves are propagated,facilitate the transporting of energy to the corona by MHD waves?

reflected, and dissipated. Phenomena indicative of or byproducts of magnetic
reconnection, such as inflow and outflow, turbulence, shock waves and other

The SOLAR-C satellite, equipped with the Solar UV-Visible-IR Telescope

wave motions, will be captured by SUVIT and by spectroscopic observations(SUVIT), the EUV/FUV High Throughput Spectroscopic Telescope (EUVS/

using EUVS/LEMUR, while XIT will observe the rapid changes in

LEMUR), and the X-ray Imaging Telescope (XIT) all working together totemperature distribution of plasma heated by shock waves.

make closely coordinated observations, is designed to answer such questions.
Various phenomena spanning from the solar photosphere to the heliosphere

A few science questions to be tackled by SOLAR-C are described here.

are believed to be the result of magnetic reconnection and wave phenomena,

The success criterion is whether SOLAR-C observations give significant

which in turn give rise to physical phenomena on a smaller scale such asadvancement of our understanding to the major questions in solar physics.

Science question 1: Determining three-dimensional magnetic structuresin the solar atmosphere2000 km

As indicated above, determining the three-dimensional structure of
the solar magnetic field from the photosphere to the corona will benecessary for clarifying the many different phenomena associated withthose magnetic fields; and this itself is an extremely important sciencechallenge for SOLAR-C. SUVIT will resolve the magnetic elements

3

Science question 2: Can we understand the chromosphere, corona,and solar wind by MHD waves?

From Hinode results, we know that MHD waves are constantly being generated through the shufflingand twisting of magnetic flux tubes by convective motions in the photosphere. A theory attracting
attention today is that this wave motion, as it propagates outward while being reflected and refracted
in the chromosphere and chromosphere-corona transition zone, causes the heating of the corona
and acceleration of the solar wind. Needle-like spicules (figure on the right) extending from thechromosphere to the corona are thought to supply mass to the corona, while wave motion is believedto play an important role in the creation of dynamic phenomena such as these spicules. It has further
been suggested that dynamic phenomena in the chromosphere observed by Hinode (cover photo)
may be responsible for generating strong waves that propagate into the corona. One of the importantscience objectives of SOLAR-C will be to clarify the nature of these waves from the photosphereand chromosphere, and determine whether they are responsible for coronal heating and solar wind
acceleration. SUVIT and EUVS/LEMUR on SOLAR-C will for the first time identify waves fromobservations of small fluctuations in chromospheric magnetic fields. By observing the photosphere andchromosphere, as well as the MHD waves of the corona, it should be possible to estimate the wave
energy flux at each layer and gain an understanding of the amount energy in, and the mechanism ofenergy dissipation in, the chromosphere and corona.

Right figure: Spicules extending from the chromosphere to the corona at the limb of the sun (as observed from Hinode)shock waves and turbulence. Observing magnetic reconnection and wave

in the photosphere (believed to be around 100 km), and for the firsttime ever will directly observe the chromospheric magnetic field fromspace. By observing the magnetic fields in the magnetically-dominatedupper chromosphere (plasma <1), it will also be possible to infer
the coronal magnetic field, using the chromospheric observations
as a boundary condition. Using the three-dimensional magnetic fieldstructure information, it will be possible to advance our understandingof the role of magnetic reconnection and magnetohydrodynamic(MHD) waves in phenomena such as heating of the chromosphere andcorona, acceleration of the solar wind, and the triggering mechanismof solar flares. Hinode found that magnetic fields on a small scale of afew hundred km or less appear all over the sun’s surface with surface
convective motions, and their total magnetic flux is greater than that ofsunspots. A challenge for SOLAR-C is to find out how the occurrenceof these ubiquitous small-scale and transitory magnetic fields
affects the formation and evolution of the overall three-dimensional
photospheric-chromospheric-coronal magnetic fields.

SOLAR

Upper chromosphere

0
00

80

km

Lower chromosphere

Photospheric magnetic

160000 km

field

The three-dimensional structure of the solar magnetic field. The green lines are magnetic
field lines extrapolated from the photospheric magnetic field. In the upper chromosphere,
fine dark filamentary structures can be seen following the magnetic field lines.

Science question 3: Understanding magnetic reconnection on the sun

and its role to chromospheric and coronal heating

In the sun, phenomena such as solar flares, jets (cover photo) in the chromosphere and corona, andother eruptive activities occur over broad spatial and temporal scales. The results of observations from
Yohkoh and Hinode show that these phenomena are strongly connected to magnetic reconnection.
Magnetic reconnection is a phenomenon in which nearly-antiparallel magnetic field lines interact andreconfigure, quickly releasing magnetic energy, and accelerating and heating plasma. Jets caused bymagnetic reconnection might supply mass from the lower to the upper solar atmosphere. Moreover,the sudden deformation of magnetic structures in the magnetic reconnection process generates
MHD waves, and it is possible that these waves heat the corona. Another plausible theory is thatnumerous tiny flares are what heat the solar corona. An understanding of magnetic reconnection itself,
including the role of shock waves involved with magnetic reconnection, is essential to clarifying these
phenomena. There is however a lack of observational data regarding the temperatures, velocities, andmagnetic fields in the areas around the occurrence of magnetic reconnection. Up to now it has beendifficult to obtain such data since the plasma immediately after acceleration and heating by magnetic
reconnection is fainter than the background plasma, and because the properties of the region aroundthe reconnection undergo rapid changes. High-resolution spectroscopic imaging observations over abroad wavelength range by EUVS/LEMUR and XIT on board SOLAR-C will make it possible forthe first time to make seamless observations across the solar atmosphere, at various temperatures, ofreconnection-induced plasma flows, MHD waves, and shock waves. It also may even be possible todirectly observe changes accompanying reconnection in upper-chromospheric (plasma <1) magnetic
structures.
Top: Chromospheric jets at the limb of the sun (as observed from Hinode).
Bottom: Jets occurring due to magnetic reconnection (MHD simulation).

4

SOLAR-C’s Three Large Telescopes: From imaging to spectroscopic observation
Spacecraft system supporting
the observation instruments

On solar-observing satellite missions up to now, imaging observations were highly beneficial
for quickly obtaining a wide range of information. There are limits, however, to the physicalinformation that can be gained from imaging observations alone. In order to probe into the

EUV/FUV High Throughput
Spectroscopic Telescope

The spacecraft system must be able to accomplish thefollowing:

physical processes responsible for various solar phenomena, it is important to obtain highly
precise spectroscopic measurements of the light emitted by the solar atmosphere. Such

Flexibility similar to that of ground-based telescopes
to rapidly modify observation targets and programs in
response to changes in solar activity.

observations will allow us to obtain basic physical information, such as temperature, velocity,and magnetic energy, of plasma composing the solar atmosphere.

Continuous observations 24 hours a day, with nosatellite-night interruptions.

SOLAR-C is equipped with three telescopes, each sensitive to a different wavelength
regime, for seamlessly observing the entire range of temperatures in the solar atmosphere,

Ability to accurately align images taken with different
instruments.

from the photosphere to the corona.

Obtain and transmit 20 times more data than Hinode.

EUVS/
LEMUR

Spatial resolution

0.28˝

Temporal resolution<10 seconds (0.28˝ sampling)<1 second (1˝ sampling)

Field of view

280˝ x 300˝

Velocity resolution

2 km/s or lessObserved wavelengths 17-21 nm, 46 - 128 nm

Solar UV-Visible-IR Telescope (SUVIT)
for measuring magnetic fields in the chromosphere and photosphere

Spectroscopic emission line intensity (active region)

EUV/FUV High Throughput Spectroscopic Telescope(EUVS/LEMUR)
for high-resolution spectroscopic observations of the chromosphere and corona

for spectroscopic imaging observations of high-temperature plasma in thecorona and solar flares

10 2

Solar UV-Visible-IR Telescope

5

SOHO SUMER(1”/pxiel)

10 0

SUVIT

Hinode EIS(1”/pixel)

10 -2
10 4

10 5

(For determining physical quantities,
100-200 counts are needed.)

Temperature (K)

10 6

10 7

SOLAR

By performing spectral analysis of emission lines, the
EUVS/LEMUR telescope will measure the velocity,temperature, and density of solar plasmas. It will needmore than 5 times the spatial resolution and 10 times
the throughput* of the currently operational ultraviolet
spectrometers. It must be able to observe over a wide
wavelength range (17 to 128nm) in the ultraviolet,
seamlessly across the sharp-temperature-change transition
region from the chromosphere (around 10,000 degrees) tothe corona (5 million degrees). It will have high throughput
performance, enabling spectroscopic observations withshort exposure times of around 1 second, so that it canreliably capture the rapid temporal fluctuations of magnetic
structures and waves resulting from magnetic reconnection,
jets, and photospheric motions.
Up to now the spatial resolution of coronal observations has
been only a fraction of that of complementary photosphericobservations. SOLAR-C will to the extent possible employ
the similar spatial resolution from the photosphere to thecorona, enabling it to obtain decisive information as towhere and how heating takes place in the magnetic fieldstructures observed by SUVIT.
*When using observation modes optimized for spatial and temporal
resolutions, respectively.

Spatial resolution

0.1˝ - 0.2˝

Temporal resolution

0.1 - 1s (imaging)1 - 20 s (spectropolarimetry)

Field of view

XIT

180˝ x 180˝

Observed wavelengths 280 - 1100 nm

Active region / sunspot

High
Polarimetric sensitivity
for magnetic field measurements

Obtaining accurate magnetic field measurements of the chromosphere
will make it possible to calculate magnetic fields of the corona, andfrom this together with the photospheric data we can then expect toderive the three-dimensional solar magnetic field structure from thephotosphere to the corona. This should in turn bring us closer to anunderstanding of magnetic reconnection and wave phenomena thatare keys to heating and dynamics of the chromosphere and corona. To
achieve these goals, not only high spatial resolution but also seamlessobservations over a wide field of view and long time span are essential.
These requirements can be met only by observations from space.

(0.14”/pixel)

Count rates (counts/sec/arcsec)

X-ray Imaging Telescope (XIT)

SUVIT will take the first ever space-based measurements ofchromospheric magnetic fields; up to now chromospheric magnetic
field measurements have proved extremely difficult. Hinode wasable to achieve the major accomplishment of mapping photosphericmagnetic fields with high accuracy, by using the Zeeman effect. SUVIT
in addition to the Zeeman effect will use the Hanle effect to measuremagnetic fields in the chromosphere, which are weaker than thoseof the photosphere. The polarization of the chromosphere due to the
Hanle effect is very weak, being only about one-tenth the intensity of thephotospheric polarization. Since the strength of the polarization signal
from a point-like magnetic flux-tube element varies in proportion to thesquare of the observing-telescope’s aperture diameter, a telescopewith an aperture of roughly 1.5m, three times that of Hinode, isnecessary. Such a large telescope can achieve high resolution of 0.1 to
0.2 arcseconds, by which high-precision magnetic field measurementsof the photosphere and chromosphere can be carried out.

SOLAR-C
EUVS/LEMUR

10 4

SOLAR

0.0001Low

0.001

Photon Counting Imaging Spectroscopy Soft X-ray Telescope (XIT-PC):
Chromospheric jets
spiculesFlux tube / MHD wave

SOLAR-C SUVIT

Sensitivity
for measurementsof chromospheric fields

HINODE Solar Optical Telescope

Sensitivity
for measurementsof photospheric fields

0.01

X-ray Imaging
Telescope
XIT-PCXIT-NI

Spatial resolution

1˝ - 2˝

0.2˝ - 0.3˝

Temporal resolution

10 s

<10 s

Field of view

80˝ x 400˝

400˝ x 400˝

Observed wavelengths 0.5 - 5 keV 9 - 21 nm
Full disk observation（SDO HMI)
1000

100
10
1
Spatial scale (arcseconds)

0.1

Combining grazing incidence optics with a photon-counting CMOS detector, the telescope
will perform the world’s first X-ray imaging spectroscopic observations of the solar corona andflares. Boasting the highest spatial resolution to date of a grazing incidence optical system, it will
be able to obtain an overview of coronal structures and features across various temperatures.
With the ability to obtain an X-ray spectrum for each pixel, it will be able to observe the behaviorof plasmas after heating but before reaching thermal equilibrium, leading to breakthroughs in
research on coronal heating, flares, and shock waves accompanying magnetic reconnection.

Ultra High Spatial Resolution Normal Incidence EUV Telescope (XIT-NI):
This multilayer X-ray telescope will perform large-field-of-view, high-time-resolution direct
imaging of the corona at an unprecedentedly high spatial resolution of 0.2 to 0.3 arcseconds. It
will provide high-quality image information to complement the detailed spectroscopic informationfrom EUVS/LEMUR. In addition, in combination with the photospheric and chromosphericmagnetic field observations from SUVIT, it will shed light on the mechanisms of coronal heating
and solar wind acceleration.

6

Advances in Technologies for High-Resolution Space Observations in SOLAR-C

SOLAR

The space telescope technologies developed for the Hinode satellite, launched in 2006, are to be further advanced with SOLAR-C, notably in the area of high-resolution satellite-based observations as well as in leading-edge technologies of the spacecraft system for high-resolution observations.

Space telescope achieving diffraction-limited resolution

High pointing stability essential to high-resolution observations

The SUVIT telescope to be carried by SOLAR-C will be designed with an aperture nearlyTo achieve high-resolution imaging observations and high-precision

3 times larger than that of the solar optical telescope (SOT) on Hinode while being only

polarimetry with Hinode, a satellite attitude control system with high pointing

about twice as long. This compact design requires tight positional tolerance on its opticalstability was developed, as well as an image stabilization system using acomponents. The techniques resulting from development of the optical telescope on Hinode,

tip-tilt mirror to eliminate jitter that could not be suppressed by the attitude

including high-precision telescope structural and thermal design, assembly and alignment,

control system alone. In addition, a technique was introduced for controllingand optical interferometry techniques, will be applied and advanced further. The main

micro-vibrations in the high-frequency range. To maintain co-alignment

structure of the Hinode solar optical telescope used ultra-low-thermal-expansion carbon

among different telescopes, an optical bench with a low thermal expansion

fiber reinforced plastic (CFRP), with a thermal expansion ratio of 0.1ppm or less, to achieve

was developed for mounting the components. As a result, the highest

diffraction-limited performance for a 50cm aperture with no adjustment mechanism in orbit.

pointing stability in Japanese space missions was achieved over a wide

The heritage of the precision telescope structure developed in Hinode will be applied to afrequency range. In order to achieve the high resolution and polarimetry
Hinode solar optical telescope (SOT)

1.5m-class aperture telescope for SOLAR-C. Development is also being carried out in Japan

Pointing stability [arcsec, 0-p]

102

three times better than that of Hinode. For further reducing high-frequency

focal plane instrument.

micro-vibrations, techniques are being introduced to reduce the transmission

Heat dump mirror at the primary focus of
the Hinode optical telescope. By applying
a silver reflective coating to an aluminum
mirror, a solar light absorption ratio ofaround 5% was achieved. By means ofcoating durability tests and contamination
control of the mirror surface, the heat dump
mirror maintains a temperature of 50°C
or below even when subjected to a light
concentration 1500 times that of the sun.

A rotational drive mechanism is being
developed in Japan, which is a key
technology for development of the
focal plane instrument. The prototype
is a thin large-aperture-bearing drive
mechanism, and can be used as acontinuously-rotating waveplate needed
for a high-precision polarimetry.

The lifetime of a space solar telescope is largely influenced by degradation in
1.0

reflectivity due to contamination on the mirror surface and its blackening by

668 nmHinode optical telescope, the materials used in the telescope structure andadhesives were selected based on outgas measurements, and were subjected tobaking based on a mathematical model for contamination prediction, under the
careful contamination control. Since the SUVIT telescope aboard SOLAR-C will

Relative intensity

ultraviolet radiation, as well as resulting mirror thermal deformation. For the
0.8

430 nm
396 nm0.6

observe in the ultraviolet range, it will require even more stringent contamination
control than did in Hinode.
Simulated sensitivity deterioration of Hinode solar optical telescope
based on a mathematical model for contamination prediction

7

0.4

0

1

2

3
Elapsed years

4

5

10-2

SUVIT

10-3

EUVS/LEMUR
XIT-PCXIT-NI
10-5

10-4

10-3

10-2
10-1
Frequency [Hz]

100

101

102

103

The pointing stability achieved with Hinode and that required in SOLAR-C.
SOLAR-C will need to have pointing stability two to three times better than
Hinode. The keys are improving performance of the satellite attitude control
system in the mid-frequency range, and also reducing micro-vibrations in the highfrequency range.

system.

0.6

CT jitter (arcsec)

Tip-tilt mirror employed in the image
stabilization system of the Hinode solar
optical telescope. It tracks featureson the sun’s surface by driving threepiezoelectric actuators of the tip-tilt mirror.

0.4

Image stabilization sys. (Off)Image stabilization sys. (ON)

0.2
0.0
-0.2

On-orbit performance of the image stabilization system of
the Hinode solar optical telescope. Pointing jitters as large -0.4
as 0.2 arcseconds are reduced to as low as 0.03 arcseconds-0.6(3 ) by operating the image stabilization system.
0

Leading-edge technologies
to be newly developed
Grazing incidence X-ray mirror using
nanotechnology

Contamination control for the space optics and its mathematical modeling

SOLAR-C
Requirements

10-5
10-6

The aimed-for resolution of the X-ray telescopefor SOL AR- C is 1 arcsecond or better at X-ray
wavelengths of 0.5 - 10keV. This will necessitate
development of a grazing incidence X-ray mirror
with high surface accuracy. It is becoming feasible
to manufacture a high-precision grazing incidence
X-ray mirror in Japan by applying nanotechnology
measurement techniques. The X-ray focal plane isscanned with a nanometer-precision knife edge or cube,
and the light diffracted from it is measured. In this way,a point spread function can be measured precisely. The
results are used to estimate the surface figuring error,
which is fed back into processing of the mirror surface
to achieve an X-ray mirror of the required precision.

100
Reflectivity (%)

Wavefront error distribution of the
Hinod e solar optical telescope.
Wavefront error of 18nm (RMS) was
achieved. High-precision interferometry
with an error of 5nm (RMS) enabled
to eliminate alignment errors of opticalcomponents so that the telescoped elivered the dif frac tion - limited
performance at visible wavelengths.

100

10-4

of vibrations as well as to broaden the bandwidth of the image stabilization
The 50cm-aperture lightweight ULE-glass
primary mirror and 3-point static support
mechanism used in the Hinode solar
optical telescope. SOLAR-C will havea nearly 90% light-weighted mirror with astress-free support mechanism.

HINODE performance
Attitude control sys.
Image stabilization sys.
(Off)
(On)

101

10-1

precision desired for SOLAR-C, its pointing stability will have to be two toof a high-precision, long-life movement mechanism for space use, which is necessary for the

SOLAR

Mg II h&k
280 nm100

G-band430 nmNa I D589 nm200
300
Time (sec)HIα656 nm

400

Ca II IR854 nm500

He I
1083 nm90
Ag coating80

Al coating

70
60
50
200

Highly enhanced coating

400

600
800
Wavelength (nm)1000

Mirror coating with highly enhanced reflectivity
For achieving high optical performance in a solar telescope, the mirror coating must have a low solar
light absorption to ward off heat from unwanted sunlight and to prevent thermal deformation, alongwith a high throughput enabling high-sensitivity polarimetry. Since the Hinode optical telescope
carried out observations only in the visible wavelength range, a silver coating was used, which
required fewer development steps. For SUVIT, a highly enhanced coating is being developed
beyond the Hinode heritage, aiming at achieving a reflectivity of greater than 90% across a wide
wavelength range from ultraviolet to near infrared, and a solar light absorption of 3% or less.

8

The Breadth of SOLAR-C Science From SOLAR-C to SOLAR-D

SOLAR

Exploring the hitherto-unexplored sun’s polar regions

PulsarAGN

Laboratory plasma

108
Solar corona
Earth’s magnetoshpere
106

Solar photosphere/chromosphere

102

10

1015
Number density (/m3)

1030

Solar-terrestrial environment and space weather forecasting
Space weather forecasting attempts to predict in advance short-term changes
in the space environment, such as solar flares and associated Coronal Mass
Ejections, with accompanying energetic particles contributing to auroras andgeomagnetic storms. Predicting space weather requires both an understandingof the physical processes and the devising of forecasting algorithms based oncomputer simulations using data observed on the sun's surface as input. One
role of SOLAR-C will be to contribute to the development of flare forecastingalgorithms. Because of the extreme difficulty of directly observing magnetic
fields in the corona where flares occur, simulations have been carried out
using surface (photospheric) magnetic field observation data as boundary
conditions. SOLAR-C will observe magnetic fields of the chromosphere,
near the corona. The accuracy of simulations based on the resulting data will
be improved greatly. This should make it possible to achieve space weather
forecasting extending to the occurrence of flares themselves, which up to now
has been elusive.

SOLAR-C to clarify abnormalities in solar activity

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Solar Observatory, NAOJ

Meridional flows

The key to understanding global dynamo processes is the dynamic coupling
between magnetic fields and overall flows in the solar interior, such as differential
rotation and meridional circulation. In particular, since it is believed that themagnetic fields in the polar regions are the seeds of the next solar cycle, knowing
the relation between changes in high-latitude magnetic fields and flows inside thesun is essential to learning the mechanism giving birth to the magnetic fields of thenext cycle. Since the interior of the sun cannot be viewed directly, the only meansof diagnosing what is going on inside the sun is solar seismology, known as
helioseismology, using Doppler observations of oscillations on the solar surface.
In the ecliptic plane observations carried out up to now, however, the solar polar
regions could not be observed clearly, making it impossible to learn about the flow
inside the sun at its poles. To solve this problem SOLAR-D will observe the solar
surface magnetic fields and Doppler velocity fields from places outside the ecliptic
plane (maximum orbit inclination of ≤40°). By simultaneously observing changes
in and movements of the overall flows and magnetic fields across the entire sun
including polar regions, the mission will attempt to find out the origin of the solar
magnetic activity cycle.

Solar surface

Convection
Toroidal field

Radiation zone

P-modewave

Convection
zone

Observation of solar polar region
SOLAR-D
Ecliptic plane- Total irradiance monitor

Sun

Will measure the latitudinal dependence of the solar irradiance, and try to determine
why its variation is only around a third that of other solar-type stars having similar levelsof magnetic activity.
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Year 2005

2010

Earth

- X-ray/EUV imaging and spectroscopic telescopes
Will try to determine the three-dimensional structure of the polar solar wind, etc., basedon differences in appearance from different orbit inclinations.

orb
e

lan

it p

- Optional equipment(interplanetary imager, in-situ instruments, etc.)-D

Changes in sunspot numbers in the most recent cycle (red line) and in the past six
cycles (black lines). The current cycle of 12.6 years is longer than the usual 11-year
cycle; moreover, the next solar maximum period, which should arrive in the latter
half of 2013, is expected to be weak.

Differential rotation

Proposed payload:
Using helioseismology, will infer flows inside the sun,and will measure photospheric magnetic fields.

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sunspots
A toroidal fieldcreated from a
poloidal fieldby differential
rotation- Visible light imager for observing full-sun magnetic fields
and Doppler velocity

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Illustration of solar interior

Why explore the sun’s polar regions?

R
LA

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SO

The sun goes through repeated activity cycles of 11 years, but several times
in the past it has entered quiet periods during which sunspots vanished foryears or even decades. Such cessations in solar activity may affect the earth’s
weather, as seen in quiet periods since 1600 known as the Maunder Minimum
and the Dalton Minimum, when the earth did in fact cool off. During the period
from around 2007 to 2009, solar activity declined and the solar cycle was
12.6 years, which was longer than usual cycle. The last time the solar cycle

became longer was around 1800 right before the Dalton Minimum, more than
two centuries ago. The number of sunspots is an important index for tracking
solar activity; but for understanding the mechanism by which that activity
declines it is necessary to obtain an overall grasp of the changes in magnetic
fields across the entire sun, including sunspots, polar regions, and quiet
regions. By observing changes in the fine surface magnetic fields that may
cause global changes in the sun’s overall magnetic atmosphere, SOLAR-C
will monitor whether any long-term decline in solar activity occurs and help
determine how abnormalities in solar activity patterns affect the earth’s
climate.

Sunspot Relative Number

Devising algorithms for space weather forecasting60

Earth’s Ionosphere

104

Star and planet froming regions

SOLAR-C, by making accurate measurements of physical quantities ofsolar plasmas from the photosphere to the corona, such as magnetic fields,

For SOLAR-C the most important target is gaining an overall picture of thevarious magnetic activities seen in the solar atmosphere. SOLAR-D will take up
the challenge of understanding the “dynamo processes” by which the magnetic
atmosphere is formed. A notable example of a global dynamo process is the
11-year solar cycle. The greatest mystery of solar physics is why the numberof sunspots, activity, and polarity of the sun go through 11-year fluctuations.
Following up on Hinode, SOLAR-C will be able to obtain information about
surface magnetic fields and surface velocity fields in the sun’s polar regions.
SOLAR-D will add knowledge about rotation and flows in the solar interior, where
dynamo processes take place.

The 11-year solar cycle (Solar Observatory, NAOJ)

Latitude [degree]

1010

Explaining solar dynamo processes

SOLAR

Relative sunspot number(12 month runing mean)

The biggest scientific result from the Yohkoh mission was demonstratingthat solar flares occur because of magnetic reconnection. Now that it has
been established that magnetic reconnection is an essential process inmagnetic plasma dissipation and energy conversion, it has today become
an indispensable concept for understanding a wide range of phenomena
in astronomy, geophysics, and plasma physics. The transient horizontal
magnetic fields observed for the first time by Hinode indicate that the local
dynamo processes driven by convective motions generate more magnetic
energy flux than that appearing in sunspots alone. For astrophysics, thisrepresents an important discovery of elementary processes concerningmagnetic field creation. In addition Alfvén waves, whose existence had been
postulated earlier, were observed for the first time. Alfvén waves are efficientcarriers of energy in space, and their possible relation to the solar wind
acceleration mechanism is being studied. In such ways the sun plays the roleof a laboratory for investigating the behavior of space plasmas.

Temperature (K)The sun as a plasma laboratory

velocity, and temperature, will address unresolved issues in solar physics.
From comparative studies with plasmas of other astronomical objects, inthe geomagnetosphere, and in laboratories on earth, we will find universal
properties of plasmas and magnetic fields that exist throughout space.

Planned orbit of SOLAR-D

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