2. Scientific Requirements
2.1 Scientific Requirements
2.1.1 Requirements for Helioseismology Observables
The success of global helioseismology, which is centered at precise measurement of the solar
eigenfrequencies and inversion of these frequencies, is well documented. In particular, mapping thesolar differential rotation (Figure 7) and meridional flow (Figure 8) throughout the low- to midlatitude convection zone had a great impact on our understanding of the solar dynamo.
One inherent limitation of global helioseismology comes from inverse analysis based on the linear
perturbation theory, and the symmetry of the global eigenfunctions; the structure inversion is
primarily about estimating the spherically symmetric component of the thermal structure of the sun(even inversions for aspherical components of the thermal structure of the sun is accessing only
those components that are axisymmetric and north-south symmetric), and the rotation inversion isabout measuring the axisymmetric, north-south symmetric component of the solar internal flow.
Moreover, the sensitivity kernels for rotation inversion, derived from the eigenfunctions, do nothave large amplitude in higher latitude region.
Local helioseismology, based on local measurement of wave propagation, particularly wave travel
times, is better suited for revealing local structure free of any kind of symmetry that are mentioned
in the above. Local helioseismology has already been successful in measuring near-surface
component of meridional flow (Figure 8), which is the key ingredient in flux-transport dynamo, up
to around 60 latitude (Giles et al. 1997). Extending the measurement to higher latitudes is one of
the main scientific targets of SOLAR-C Plan-A.
Thus there are two main objectives of heliseismology observations from an inclined orbit: 1. Highlatitude local helioseismology: measure local wavefield in high-latitude region better than fromwithin the ecliptic place, for local-helioseismology analyses of various flows in the near-surface(and if possible, deeper) layers, such as meridional flow. 2. Multi vantage-point helioseismology: in
combination with observing facilities in the ecliptic plane, observe waves with large skip angles
better, for helioseismology analyses of deep interior, such as mid-latitude tachocline regions. The
first objective is more promising in producing important results, whereas the second objective ismore challenging.
Helioseismology requires long continuous observing periods at a high time cadence in order toresolve the various spatial and temporal scales associated with processes of generation anddissipation of solar magnetic fields. These scale range from supergranulation, which defines themagnetic network and is a primary source for magnetic field diffusion and has a typical spatial
scale of 30 Mm and lifetime of about 1 day, and larger-scale dynamics of active regions occupying
50-100 Mm and evolving on the scale of a week, to the global meridional and zonal flows
occupying the whole convection zone 200 Mm deep and varying with the solar cycle on the timescale of a year.
These three fundamental scales of solar dynamics and activity define three basic observing patterns
that are needed to achieve the primary objective above: relatively short (8 hour long) but highresolution (512512 pixels) time series to investigate the structure and dynamics on the32supergranular and magnetic network scales; longer (7-14 days) and high-resolution (512512
pixels) series to follow the evolution of individual active regions (particularly, on the far side of the
Sun in conjunction with Earth-side observations), and long time period (36-72 day) low resolution(256256 pixels) observations to investigate the global dynamics and circulation of the convection
zone down to the tachocline by both local and global helioseismology techniques.
All these observing programs require a 1 minute cadence for the Dopplergrams. However, a strict
temporal continuity requirement (95% coverage) is required for 8-hour chunks of these programs.
The longer observing programs may have gaps. However, the total coverage should be at least 7080%, in particular, for the global helioseismology 36-72-day runs in order to prevent the aliasingproblems in the measurements of mode frequencies and frequency splitting. The precision of thefrequency measurements is inverse proportional to the square root of the observing time. Currently,
72-day runs are used in the SOHO/MDI Structure program that is similar to the low-resolution(256256 pixels) SOLAR-C program. These runs provide inversion results for solar structure,asphericity and rotation through the whole convection zone, tachocline and outer radiative zone.
Shorter, 36-day runs will still provide new results for the convection zone sufficient for studying
the torsional oscillations and other global variations. However, the ability to resolve the tachocline
structure and dynamics will be greatly reduced. The investigation of the solar-cycle variation in thetachocline region may require 12-month long averaging of the frequency measurements. The
minimum science requirement for exploring the solar-cycle variations in the polar regions is tohave two or three 36-day runs per year.
Table 2. Helioseismology Observing Requirements
Measurement Objective(s)Length of

Image size

Imageobservation(pixels)

cadence

Global

36-72 days

256256

1 min

Local

8 hrs

512512

1 min

Meridional circulation, latitudinal andlongitudinal structures, secondary
cells,
relationshiptoactivelongitudes, magnetic flux transport

Local

8 hrs

512512

1 min

Supergranulation and large-scaleconvection patterns in polar regions(super-rotation, wave-like behavior,
network, flux transport, relationshipto coronal holes)

Local

8 hrs

512512

1 min

Structure and dynamics of the highlatitude tachocline (oblateness, flows)

Local

36 days256256

1 min

Tomography of the deep interior:

Local
36 days256256

1 min

Differential rotation and torsional
oscillations at high latitudes, subsurface polar jets

stereoscopic observations (SOLARC-SDO, SOLAR-C-GONG)

Technique(time-distance,
holography)

33

The different measurement objectives, the observing technique and the observing requirements arelisted in Table 2. Global helioseismology, which assumes symmetry about the rotation axis of theSun, requires a very accurate determination of the frequencies of the Sun’s acoustic modes of todetermine the longitudinal rotation as a function of depth; the longest observation times (36-72
days of continuous, 70-80% coverage data) are needed to determine the frequency and frequency
splitting of solar oscillation p-modes of low angular order, m. These modes, in particular, zonal
m=1 modes, propagate into the polar regions and provide the information about the polar structureand dynamics. The current ecliptic-viewpoint observations do not provide measurements
sufficiently accurate for helioseismic inversion of the solar structure and rotation in the polar
regions. It is important to complement the SOLAR-C observations with ecliptic-viewpoint data inorder to infer the structure and dynamics of the whole Sun, from the equator to the poles.
It can be seen from the table that some of these measurements require high latitude viewing formany days at a time. This was the driving requirement for choosing an orbit inclination with thehighest possible inclination. The high-latitude viewing will provide a unique opportunity to study
the dynamics of meridional flows and rotation in the polar regions and search for deep longitudinal
structures in the tachocline by local helioseismology. In addition, the high-latitude orbit will allow
us to obtain better coverage of the deep polar regions using the observing scheme with two vantage
points, SOLAR-C and ecliptic viewpoint helioseismology instrument.
The required telemetry for the helioseismic observations is case dependent. However, it is obviousthat the reduction of the data by on-board processing is mandatory. For a measurement of Doppler
velocity map with a simultaneous measurement of line-of-sight magnetic fields, two polarization
states of I +V and I-V need to be observed at four wavelength positions for a good estimate of theline-of-sight magnetic fields. This methodology was used in Hinode filtergram observations. The
eight images obtained at eight observing times can be reduced to two index maps by simple
arithmetic operations, each can be converted to Doppler velocity and line-of-sight magnetic field bya known non-linear function. If we need Doppler velocity maps of 512×512 imaging points every 1
min, ~13 kbit/s data rate is required for the data compression efficiency of 3 bit/s (512 ×512 pixel ×
3 bit/pixel /60 s =13,107 bit/s) that is achieved in Hinode. For the case of local helioseismology that
requires both a large field of view and high-spatial resolution, 1024×1024 imaging points may benecessary, and the required data rate becomes ~51 kbps ( 1024×1024 pixel × 3 bit/pixel /60 s=52,429 bit/s).

2.1.2 Requirements for Magnetic Fields Observables
Along with the Dopplergrams, full-disk line-of-sight magnetograms of 1024x1024 pixels will betaken every 5 (TBD) minutes to observe the response of the magnetic field to the surface, subsurface and interior flows. Together, these observations will be used to study phenomena such as
the evolution of active regions, flux transport and the solar cycle field reversal. SOLAR-C,
complemented by near-Earth magnetograph observations, will enable us to follow the evolution ofactive regions continuously for much longer than the half-solar rotation now possible. Full-sun
vector magnetic field observations shall be done every 60 min (TBD) or high-cadence observationsof vector magnetic fields shall be possible for a 4×4 (TBD) arcmin field of view.
The required data rate for line-of-sight magnetograms of 1024x1024 pixels of 5 minutes cadence isabout 10 kbps (1024 ×1024 pixel × 3 bit/pixel /300 s =10,486 bit/s). When four Stokes polarization34

states of I, Q, U, V are obtained at four wavelength positions, 16 imaging data need to be
downloaded to the ground. 2048×2048 pixel vector magnetograms every 2 hours requires ~27 kbps
(2048 ×2048 pixel × 3 bit/pixel × 16 images/7200 s =27,962 bit/s). For the active region monitor
by vector magnetic fields of 256×256 pixel imaging points, the data rate of 51 kbps (256×256 pixel× 3 bit/pixel × 16 images/60 s =52,429 bit/s) is required for cadence of 1 minutes.

2.1.3 Requirements for TSI Observations
The fastest approach to reach the science goals for TSI measurements is to compare a latitudinalscan of the solar irradiance to contemporaneous measurements from in-ecliptic spacecraft from thelong-term TSI monitoring programs, such as SORCE/TIM and PICARD/PREMOS or theirsuccessors. A slightly more demanding approach consists in a self-contained statistical analysis ofseveral latitudinal scans to detect latitudinal trends in irradiance. Both approaches would benefit
form observations at higher heliographic latitudes but extrapolation of trends in the SOHO/VIRGO
measurements, which cover ±7°, suggests that 30° is sufficient to answer the above questions.
The SOLAR-C TSI instrument produces the on-source and off-source irradiance data by theperiodic operation of a shutter located in front of one of cavities. The average and peak data
producing rates are ~2 kbits/s and ~4 kbits/s after the data compression. The full FOV is about 5
and there is a 15 arcmin pointing tolerance. This instrument is assumed to be continuously operated
in the SOLAR-C out-of-ecliptic orbit after the nominal operation. There may be a request of a
special operation to do an intentional offset pointing for instrument calibration.

2.1.4 Requirements for EUV imaging and Spectroscopy Observables
For the imaging of transition-region (TR) dynamics, a band at 304 Å is suitable. Polar plumes arevisible in a band at 171 Å and 195 Å. Hotter coronal plasma components in coronal bright points
and active regions are observed in a band at 335 Å. These are observed by a telescope consisting ofmultilayer mirrors that has a spatial resolution of ~2 arcsec by ~1 arcsec spatial sampling. The fieldof view covers the whole Sun when the spacecraft points to the disk center. One of these bands isobserved at a time. Wide slit observations of EUV scanning spectrometer may supplement theimaging of the TR and corona, but the images obtained with the wide slit are not sharp due to theblurring in the spectral direction. The wide slit observation cannot be used for the stereoscopic
observations with a similar telescope in other line-of-sight directions. In the case of selecting a
grazing-incidence telescope as a SOLAR-C coronal imager, observations of polar plumes may be
performed in a low-contrast condition and the TR observations become impossible. A nominal
operation is a combination of the full-disk observation at ~1 hr interval low cadence and smaller
field-of-view observations with a higher cadence of few min. A fast observation consuming all
allocated telemetry in a short time needs to be supported. ~20 kbps data recording rate is necessary
when a data compression of 1.5 bits/s is considered.
Emission line spectroscopy with focusing optics is required to measure flows of solar-wind source
region and to detect a signature of Alfvén waves that is a promising candidate to accelerate fastsolar wind. A candidate of wavelength is in EUV. Although EUV instruments are relatively heavy,
the SOLAR-C EUV scanning spectrometer needs to be achieved in a lighter weight condition35

because the total payload mass is limited. It is possible to enhance the instrument sensitivity of asmall EUV spectrometer that is better than that of SOHO/SUMER (Hinode/EIS) by reducing thenumber of reflections (or reducing the filter components that shut the visible light). A higher
sensitivity by more than a factor of 5 increase from the current space instruments is required. In
addition to measurement of emission-line intensity, a spectral resolution similar to EIS andSUMER is needed to evaluate the dynamics of hot plasmas. The FOV center needs to be changed
by moving an internal optics like SUMER, which could change the FOV in the north-south
direction to investigate the outer region of polar coronal holes. A nominal image size of 256x256
pixel data points with 10 sec cadence produces ~20 kbps data when the data are compressed in a 3
bit/pixel lossy data compression like JPEG2000.

2.1.5 Observables of inner heliospheric imaging (Optional)
The imaging data of the inner heliosphere in the visible-light range are obtained by periodic
exposures that produce a huge data volume. The exposure duration of ~1 min is a typical exposure,and the on-board summation of 60 frames, so that the image producing rate is one imager per 60
min. The final image size after the binning is 1024x1024 with 16 bit depth at each image point.
Assuming 40 percent data compression, the final data producing rate is about 2 kbps. The FOV size
is 60 to cover a large inner heliosphere and the FOV center point to a point above the solar southpole with an offset angle of ~45 (TBD) from the disk center. This setting of the instrument gives
an overhead view of the Sun-Earth space in the ecliptic plane when the spacecraft is in the northern
heliosphere. The spacecraft needs to rotate by 180 around the spacecraft Z axis if the Sun-Earthspace in the ecliptic plane should also be observed when the spacecraft is in the southern
heliosphere.
The following objects are contained in the observed data: solar wind structures and CME (viascattered light by electrons in the heliosphere), diffuse Zodiacal light (via scattered light by dust
particles in the heliosphere), stars, planets of our solar system, comets, and asteroids. The brightest
object is planets as a point source and scattered light by dust particles as a diffuse source. To
determine a dust distribution from the measurement of Zodiacal-light brightness, a special
observation in which the spacecraft Z axis is largely offset to the disk center may be required. Since
it is strongly related to the safety operation of spacecraft, a risky attitude maneuver shall be avoided.

2.1.6 Observables of in-situ measurements (Optional)
Not studied in detail. The measurement of solar wind velocity for main elemental species will bemandatory. The in-situ magnetic field will also be a basic parameter, but the level of ~1 nT
magnetic field at 1 AU may not be measured by the strong magnetic fields from permanent
magnets used in ion engines when the SOLAR-C orbit is formed with help of solar electric
propulsion system.

36

2.1.7 Summary of Required Data
Table 3 summarizes the data required for SOLAR-C Plan-A.
Table 3. Data required for SOLAR-C Plan-A
Measurement

Wavelength(Å)

1Photospheric line-of-sight
Doppler velocity:

Visible(Pixels)

Number ofimagestransmitted

Cadence

512×512

1

1 min

*13

1024 ×1024

1

1 min

*51

1

10 minSpatialsampling

Image Size

4.0 arcsec

Data rate(kbps)

Global Helioseismology

2

Photospheric line-of-sight
Doppler velocity: Local
Helioseismology

Visible3

Photospheric line-of-sightmagnetic field

Visible4

Photospheric vector
magnetic field

1.0 / 2.0
arcsec
1.0 arcsec

2048×2048

2.0 arcsec

1024 ×1024
2048×2048

Visible1.0 arcsec

1.0 arcsec

2048×2048
2048×2048

256×256*10*5

8 hours*7

10 min*5

1 hour

#1 hour

#

5 min

#

1

10 sec*19

16

5

Imaging of Chromosphere

Visible(TBD)

6Imaging of TR and Corona

EUV/Xray~1.2 arcsec

7

Imaging spectroscopy of
Chromosphere, TR, and Corona

EUV~1.0 arcsec

8

Monitoring TSI

-

-

-

-

1 min

2

visible0.03

1024 ×1024

1

1 hour

*1

-

-

-

-

-

~1

9

Heliospheric imaging[option]10 In-situ (TBD)~2.4 arcsec

1024×1024
spatial×spectral
256×256*(#)Data compression down to 3 (1.5) bit/pixel is assumed

37

1

2

3
3

20

2.2 Science Payload

Table 4 shows candidate payloads for SOLAR-C. The description of each payload is shown in thefollowing subsections.
Table 4. SOLAR-C Plan-A Model Payload
HAI (Helioseismic Activity Imager)
Weight65 kg
Aperture: 12.8 cm
Detector: APS or Backside illuminated CCD
Sampling: 0.5 arcsec/pixel (TBD)
Detector format: 4K4K (TBD)
Wavelength: 5000-8700 Å
LIM (Luminosity and Irradiance Monitor)
Weight5.0 kgNumber of cavities
3
EAI (EUV Activity Imager)
Weight
12 kg
Ritchey-Chretien telescope with Mo/Si mirrors
Detector: APS or Backside illuminated CCD
Sampling: 1.2 arcsec/pixel
Detector format: 2K2K (TBD)
Wavebands: 171, 195, 304, 335 (all TBD) Å
ESS (EUV Scanning Spectrometer)
Weight30 kg
Two reflection system: offset parabola + concave grating
Spectrometer with a factor of 4 (TBD) magnification
Detector: intensified APS or intensified CCD detector
Sampling 0.75 arcsec/pixel (TBD)
Detector format: 4K(spectral)1K(spatial) TBD
Waveband (TBD):
IHI (Inner Heliospheric Imager; Optional)
Weight8.0 kgDetector: APS or Backside illuminated CCD
Sampling: 0.03 deg/pixel
Detector format: 2K2K (TBD)
Others (In-situ Measurements etc.)
Weight kg
Mission data processor if required
WeightTotal weight (130 kg tentatively allocated for payload)

MDI+electronics 57 kg
HMI+electronics+ harness 73.5 kg

2 PMOD+electronics in
VIRGO: ~5 kg
TIM(4 cavities): 10 kg
STEREO EUVI
telescope alone: 7.7 kg

Use of MCP
148cm focal length
STEREO HI-1+HI-2+Electronics: 13.8kg

 kg
120++ kg

2.2.1 Helioseismic and Activity Imager
This instrument observes the Sun in the visible light for imaging the Sun in the continuum and atabsorption lines, for acquisition of the helioseismic information from Doppler velocity observations,and for mapping the photospheric magnetic fields. It consists of a refractive telescope, apolarization modulator, a tip-tilt mirror, a narrow band tunable filter, and a mechanical shutter, anda 2D imaging camera. The narrow bandpass filter has a bandpass of 100-200 mÅ in full width at38

half maximum (FWHM), and its central wavelength can be tuned in 5000-8700 (TBD) Å. A similar
type filter with a wider bandpass of 1 Å FWHM was operated for imaging the green-line (5303)
corona for 10 years at a ground-based observatory in Japan.
The baseline detector is a large-format CCD camera with 4K×4K (TBD) pixels. When a scientificgrade CMOS detector is available, it is better to be adopted from the viewpoint of radiation
hardness in the space environment.
The whole-sun area has to be observed in most of the time because the Doppler velocity
observations for helioseismology will mostly be running except for some special operations.
The baseline data recording rate of this instrument is 50 kbps. When one of data is compressed to 3
bits/pixel average, three types of data of 512×512 pixels, magnetograms, Dopplergrams, andcontinuum image, can be obtained every 1 min.
Figure 17 shows the schematic optical diagram of the instrument. One of programmatic difficulties
in developing this instrument is to realize a narrow band tunable Lyot filter that is being developed
to raise TRL. When it is difficult to be developed , the use of Michelson interferometer with awider band Lyot filter like SOHO/MDI and SDO/HMI observing a single absorption line or a
Fabry-Perot filter may become a fallback plan.
The selected line for the Doppler and magnetic field measurement is a TBD line at TBD Å. Ni I
6768 Å (Fe I 6173Å) was selected for SOHO/MDI (SDO/HMI).

Figure 17. Schematic optical diagram of HAI conceptual design.

2.2.2 EUV & X-ray Spectroscopic Imaging Telescopes
A fast cadence imager is needed for tracking the evolution of dynamic events in the outer solar
atmosphere. The SOLAR-C Plan-A is not a mission targeting to resolve elementary structures. It
will be sufficient to cover the whole Sun corona with a range of extended corona above the limb of
the Sun by an imaging system that resolves a few arcsec structures. Observations of solar polar
region from an out-of-ecliptic view are new issues and a telescope to observe the structures near thepolar region is required. A whole Sun imager of ~1 arcsec spatial sampling is proposed here. The
whole Sun with extended corona can be covered by a 2D imaging device of 2K × 2K pixels at the
distance of 1 AU from the Sun. An EUV imaging telescope like STEREO EUVI may be more
39

suitable than the X-ray telescope to observe polar plumes and inter-plume regions in polar coronalholes. In the late 2010’s and early 2020’s whole Sun EUV imagers are planned in Solar Orbiter and
GOES spacecrafts and the presence of these imagers needs to be considered for the selection of thetelescope bandpass. The SOLAR-C Plan-A EUV Activity Imager (Figure 18) is a Ritchey-Chretien
telescope consisting of two hyperboloid mirrors. Each mirror is coated with Mo/Si multi-layers toenhance the reflectance of EUV light. The full width at half maximum of the reflectance as afunction of wavelength is 10–15 Å around a central wavelength of each bandpass. Currently
selected wavelength bands are 171, 195, 304, and 335 Å for their central wavelengths. A
stereoscopic observation with SO will be possible at 171, 304, and 335 Å bands because the same
EUV band with SO is selected. The CMOS APS is a primary candidate for the detector and the fallback plan is a back-illuminated CCD that was used in Hinode, STEREO, and SDO imagers. The
imager on STEREO with a different selection of wavelength band can be a model payload.

Figure 18. (left): SOLAR-C EUV Activity Imager
Figure 19. (right): SOLAR-C EUV Spectrometer

The imaging spectroscopy is a key instrument to detect the source of fast solar wind flows frompolar coronal holes. The following performance is required: (1) the higher-throughput than SOHOSUMER and Hinode EIS for both quiet sun (QS) and active regions (AR), (2) 1 arcsec or slightly
better spatial sampling, (3) a scanning rage of ±4 arcmin in the scanning direction, and (4)simultaneous observations of emission lines formed in the chromosphere, transition region, andcorona. The SOLAR-C Plan-A science payloads have to be set at the side of the Sun, that is thespacecraft +Z direction, to the bus module because the surface of side bus panels located in thespacecraft ±X and ±Y directions are all used as radiators. Hence, the length of this imaging
spectrometer must not be long like Hinode EIS, which is longer than 3.2 m in the Z direction. In
order to realize a short instrument length without adding another reflection, the spectrometer needs
to have a magnification using a single concave grating. The design of this type of spectrometer was
first found by Roger Thomas (Thomas 2003). The instrument, SOLAR-C EUV Spectrometer(Figure 19), consists of a single off-axis parabola as a primary mirror of the telescope, a slit/slot
selector, a toroidal (or ellipsoidal) concave grating with varied line space as a disperser, and solarblind intensified CMOS detectors. Three wavelength bands are chosen to observe thechromosphere, transition region, and corona simultaneously: one is 310-340 Å (TBD), and 760-800
Å (TBD), and 1000-1100 Å (TBD).
The main body of these telescopes and imaging spectrometer are made of CFRP because of theweight reduction. In addition to the stringent contamination control of used materials including
sufficient material bakeout, one point of note is that temperatures of all optical components shouldslightly be higher than that of the main body by the thermal design for a long-term stable
instrument performance.
40

In the imaging spectroscopy with a grating spectrometer, there are two specific directions in theplane of the sky, that is, a slit direction and a scanning direction perpendicular to the slit direction.
The direction along the slit will be in the north-south direction in the imaging spectrometer. It will
be good to rotate the spacecraft around its Z axis for a specific observation to set the slit for any
direction.

2.2.3 Solar Irradiance Monitor
The SOLAR-C total solar irradiance monitor is an absolute radiometer with cone-shape cavities. In
front of each cavity there is a mechanical shutter to control the radiation input from the Sun. Eachcavity has a heater and a thermal sensor, so that each can be a reference cavity. One of cavities isperiodically exposed to the Sun. During the exposure, one of remaining cavities unexposed to the
Sun is heated by the heater attached until the electrical heat input balances the solar input. For thisoperation there needs two cavities in this instrument at least. The cadence of periodic exposure is 1
min (TBD). There is degradation of the cavity surface by being exposing to the space environment,
mainly due to solar UV illumination. To calibrate the degradation of the cavity surface, aninfrequent exposure of a reference cavity to the Sun is needed. Probably, there will be another TSI
instrument in low-Earth orbit at that time, so the SOLAR-C TSI monitor will also be calibrated tothe other monitor when the SOLAR-C spacecraft is located in the ecliptic plane. There are a few
groups to have heritage on developing the space TSI monitor, PMOD, JPL, LASP, and so on. A
cross calibration to a cryogenic absolute radiometer is required before the flight.

2.2.4 Inner Heliospheric Imager (Optional)
A slightly-modified heliospheric imager on STEREO can be an optional candidate payload. The
lens unit consisting of multiple optical lens has a large field of view (FOV) of about 60 and the
FOV center to the Sun is offset by 45 around the spacecraft X-axis looking at space above thesolar south pole when the spacecraft is in the ecliptic plane (Figure 20). When the spacecraft islocated at the northern heliosphere, the heliospheric imager can look at the interplanetary space
between the Sun and Earth. The spacecraft needs to rotate 180 around the spacecraft Z axis when
the same interplanetary space is observed in the southern heliosphere.

Figure 20. Field of View of Inner Heliospheric Imager

The scatted light level is reduced by the multiple baffles located in front of the wide-field lens unit.
A 2D imaging device of 2K × 2K pixels that is cooled down to -60 is placed at the focal plane.
41

The brightest component is from planets in the solar system as a point source and the zodiacal light
as diffused background. The bandpass is 5000-8000 Å (TBD) in the visible range. In order to avoid
the blooming due to a bright object in FOV, a CMOS device will be preferable. Each image is
obtained by ~1 min exposure duration. A multiple set of exposures is summed together by an onboard processing to enhance the component originating from electron scattering. The processeddata are reformed into a telemetry data.
If the FOV of this instrument can point to the opposite side of the ecliptic plane by the spacecraft
maneuver, a basic data set for determining the interplanetary dust distribution may be obtained forthe first time.

2.2.5 In-Situ Instruments (Optional)
The role of in-situ instruments has not been discussed in detail in the SOLAR-C Plan-A working
group. The out-of-ecliptic in-situ measurements were performed in the Ulysses mission. The orbit
inclination is about 80, which mostly covers the whole heliocentric latitude. The orbit is highly
elliptical, so that the there is a period of a fast and a slow latitudinal scan for the heliosphere withdifferent distances from the Sun. One major point of the SOLAR-C Plan-A orbit is that the
spacecraft periodically travels between northern and southern maximum latitudes in a half year atnearly 1AU distance from the Sun. It is possible to do a research on the heliospheric physics thatstrongly depends on the solar cycle. A transport of cosmic rays in a solar cycle may become atarget of SOLAR-C Plan-A regarding the production of cosmogenic isotope like 14C and 10Be,
which are used as indices of the past long-term solar activity.

2.2.6 Other Instruments (Optional)
Since the SOLAR-C Plan-A spacecraft travels in a unique orbit, some instrumental proposals may
arise from non-solar physics community. Although it may be important to consider them here to
maximize a rare opportunity, it is beyond the scope of this proposal.

422.3 Spacecraft Requirements given from the Science Objectives
2.3.1 Attitude Control Requirements
On the SOLAR-C Plan-A there are imaging instruments that require periodic exposures in a long
period of time, so that the spacecraft +Z axis is basically pointed to the Sun by a three-axis
stabilized attitude control system (ACS). Since one of main payloads, which has marginally cover
the whole Sun in the field of view, points to the Sun center for a day, a week, a month or a few
months for the purpose of helioseismic and magnetic field observations, the nominal pointing
direction is the center of the solar disk, so that the pointing coordinates of the other instrumentswith narrower field of view is restricted by the condition. Thus, when the other payload needs tolook at a different location on the Sun outside its narrow field of view, the payload has to have amechanism to change its own field of view. The tracking of the differential rotation of the Sun by
the spacecraft attitude control is not required for observations with a small field of view.
Yohkoh and Hinode also have three-axis stabilized ACS and the spacecraft Y-axis is stabilized topoint to the solar north. The functionality to rotate the spacecraft around the Z axis at any angle isto be requested, and provides a unique opportunity to observe scientific targets for the EUV
scanning spectrometer that has a specific orientation in the plane of the sky. It is the slit direction. If
the inner heliospheric imaging is adopted, the spacecraft rotation of 180 around the Z axis
becomes mandatory and a step-by-step rotation of the spacecraft may sample the scattered photons
from dusts in the solar system.
Table 5. Requirements for Attitude Control Stability (TBD)

Payload- Imager- Spectrometer

Duration

ΔX, ΔY (arcsec)

1 sec

0.6

3σ

10 sec

0.8

3σ

5 min
1 hr

1.6

0-p

24 hr

*2.0

ΔZ (arcsec)

400

Unit

0-p

*: will be better than Hinode due to no external disturbance except for solar radiation pressure in
the interplanetary orbit. An exceptional period happens during the operation unloading theangular momentum of reaction wheels.
The spacecraft pointing stability achieved in Hinode completely satisfies the SOLAR-C Plan-A
requirement. The long-term stability for an order of hour, a few arcsec per hour, will automaticallybe improved in SOLAR-C Plan-A, because the disturbance due to the thermal distortion disappears
in the interplanetary space. Table 5 shows requirements for the attitude control system.

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2.3.2 Time management
Except for the helioseismic instrument, the ISAS time tag system that was used in Hinode isapplicable to SOLAR-C Plan-A. An additional requirement is given from the helioseismic
instrument that requests a regular interval of exposure sequences. Table 6 summarizes therequirement for the time tag. The information of the time tag is distributed from the spacecraft data
handling unit to the instrument controller of the helioseismic payload.
Table 6. Requirement of Spacecraft Time Tag (TBD)

Short term stability (1 – 100 sec)

± 0.5 ppm TBD

Requirement in SOHOLong term stability (1 year)
Duty cycle± 1.0 ppm TBD
0.5±0.05 sec

Requirement in SOHO
Requirement in SOHO

2.3.3 Data Compression and Data Storage
Since the telemetry should effectively be used, all the data need to be compressed on-board thespacecraft. The 12-bit lossless DPCM and lossy JPEG image compression were adopted in Hinodeand a similar type of image compression is required in SOLAR-C Plan-A mission. It is efficient tocompress the data in one box that is connected to all payloads, but the alternative is that eachpayload produces the compressed data to the spacecraft. The selection of the scheme is TBD.
The spacecraft is accessible from the Earth every day and the downlink will be periodically
performed. But the total telemetry rate at the most distant location is not high and it is the bottle
neck in the high-cadence observations. The following design philosophy will be applicable.
- Each payload can send the data to the spacecraft at any time.
- The data storage area is divided into small number of volume and each payload can send the databy a specified data storage volume.
- When the data are sent to the ground, one of data storage volume is dumped according to aplanned schedule.
Some payload can do a high-cadence observation easily under this system, and the issue is thelargest data size of a data storage volume that is related to the longest duration of the high-cadenceof observation.
When the science data are produced in the rate of 100 kbps at the most distant part in the orbit
during the science operation phase, total data volume per day is 8.2 Gbits (100 kbps × 1024 bit/kbit
× 86400 s = 8.85×109 bits). However, the data rate can be higher when the distance to thespacecraft is shorter or when the Ka-band link condition is more stable than expected. When the
100 kbps data rate is possible at spacecraft position of 40 from the solar equatorial plane, the data
rate can be 2.0 (3.3) times larger at 30 (25). The capacity of the data recorder should be selected
for an observing case of the maximum data rate condition. The tentative number may be 32 Gbits(TBD) including the spacecraft housekeeping data.
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It will be useful from the operational point of view if the data recorder storage is divided into
several independent volumes or partitions and if each payload can send the data by selecting one ofvolumes.

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