28 January 2009
Distribution:
International participants of the first SOLAR-C science definition meeting
Prof. David Southwood, Director of Science and Robotic Exploration, ESA
Dr. Richard Fisher, Director, Heliophysics Division, NASA Headquarters
CC:
Prof. Hajime Inoue, Executive Director, ISAS/JAXA
Prof. Junichiro Kawaguchi, Program Director, JSPEC/JAXA
Prof. Tadayuki Takahashi, Director of External Affairs, Office for Space Science, ISAS/JAXA
Prof. Masato Nakamura, chair, space science steering committee, ISAS/JAXA
In this letter, we report the current SOLAR-C status and propose a near-term
development plan for your consideration.
The SOLAR-C Working Group (WG) has been continuing its activity since it wasestablished in the ISAS/JAXA Space Sciences Steering Committee in December 2007. Witha series of successful missions, Hinotori, Yohkoh and Hinode, space solar physics in Japan
has broadened its observational frontier from flares and coronal activity, towards magnetic
phenomena on the photosphere, and has provided many leading scientific results as well as
new insights into the physics of the Sun with significant contributions from our international
colleagues. In particular, Hinode has so far revealed that (1) the polar regions of the Sun,
which had been considered weak in magnetic field strength and low in X-ray activity, show
patches of magnetic field concentration exceeding 1 kG as well as frequent X-ray jets, andthat (2) plasmas in the chromosphere, which is the interface layer between the photosphereand corona, show a rich variety of jet and wave phenomena, which may be a manifestationof yet-unknown but essential processes for energy transfer in the solar atmosphere and forchromospheric and coronal heating.
Based on these key Hinode results, the WG has been studying in parallel two possible
mission plans for post-Hinode space solar physics, namely, polar-observing mission ("PlanA") and high resolution spectro-polarimetry mission whose observation target centered on
the photosphere-chromosphere-transition region-corona coupling ("Plan B"). Since Plan A isto observe the polar region with an out-of-ecliptic orbit, a feasibility study on the spacecraftsystem is a crucial issue in addition to the value of the mission science. In this regard, withthe help of JAXA engineers, a very preliminary study on the spacecraft system wasconducted, particularly in the area of orbit, ion engine, communications and power supplysystems. Further study is necessary to identify the best approach to achieve the required
spacecraft orbit.. We are now at the stage of initiating an in-depth study for assessing thefeasibility of Plan A. As for Plan B, in addition to a study on science cases, a very preliminary
study on the communication system, spacecraft orbit, data processing, and science
operation has been conducted by WG members. Note that the data rate from the Plan B
spacecraft is estimated to be an order of magnitude higher than that of Hinode. A feasibility
study on (a) continuous observation of the Sun with continuous telemetry/commanding links
from a geo-stationary orbit and (b) continuous observation with a massive data downlink
from sun-synchronous orbit is in progress.
In addition to the above studies, the WG has made several SOLAR-C presentations andhad discussions with participants in research meetings both inside and outside Japan. The
WG participated in the following meetings held abroad in JFY (Japan Fiscal Year) 2008:
1.

NASA Heliophysics Division town hall meeting (May; in Maryland, U.S.A.)

2.

American Geophysical Union (AGU) spring meeting (May; in Miami, U.S.A.)3.

AGU fall meeting (December; in San Francisco, U.S.A.)

4.

European Solar Physics meeting (September; in Freiburg, Germany)5.

Visits to space and astronomy institutions in Europe

Through these presentations and discussions we believe that there has been a significant
growth of interest in SOLAR-C over the past year, both in the U.S. and in Europe.
The WG held a SOLAR-C Science Definition Meeting for four days, between 18-21
November 2008 at ISAS/JAXA. In the meeting, the science cases for two alternativemissions, called Plan A and Plan B, were extensively reviewed with distinguishedinternational researchers invited by the WG (22 from the U.S. and 12 from Europe). Strong
support was expressed by the international meeting participants for the science objectivesof both Plan A and Plan B, with each considered to have a strong scientific rationale to bepursued as a post-Hinode solar physics mission. Also, a common understanding wasestablished among the participants that both plans needed to be further developed in orderto clarify their scientific potential.
The WG came to the conclusion that we should not hastily choose either Plan A or Plan B

at this stage of the activity, but that we should further pursue the scientific and technicalfeasibilities of the two plans, hopefully, under an appropriate international framework. The
WG will continue to study both plans in parallel for a time interval as described below. Thisconclusion arises from the following considerations:1.

A strong international consensus was formed in the SOLAR-C Science Definition
Meeting that it is highly desirable to continue the study of both options.

2.

Solar missions that are expected to have high scientific synergy with SOLAR-C,
namely ESA Solar Orbiter and NASA Solar Probe Plus, have not entered theimplementation phase.

3.

Any decision to be made for SOLAR-C would have profound influence for theJapanese and international solar communities and related disciplines over a timescale of a decade.

As a part of a continuing investigation of the two plans, the WG will establish a set ofinternational sub-Working Groups to carry out extensive studies for the next six months. Thesub-WGs are formed for areas which are identified by the WG as being critically importantfrom the scientific and technical viewpoints but where further investigation is needed toestablish the technical feasibility of meeting the scientific requirements. There will be a totalof five sub-WGs. They are:(a) Engineering Investigation of Plan A Spacecraft
Chair: H. Hara (NAOJ)
Purpose: Investigate engineering feasibility of the Plan A spacecraft jointly withJSPEC (JAXA Space Exploration Center) as a part of a new "Out-of-Ecliptic Solar
Explorer WG" at JSPEC (see below). This sub-WG will be a J-side-internal one.
(b) Plan A: Helioseismology and the SolarDynamo
Chair: T. Sekii (NAOJ) and Co-chair: T. Appourchaux (IAS)
Purpose: Establish science cases for the Plan-A mission in terms of seismology andthe advancing our understanding of the solar dynamo through helioseismology
measurements from out of the ecliptic.
(c) Plan B Measurement of the Chromospheric Magnetic Field
Chair: Y. Katsukawa (NAOJ) and Co-chair: B. Lites (HAO)
Purpose: Establish science cases for plan-B mission especially in terms ofchromospheric and coronal magnetometry/extrapolation. This includes investigationof thescientific feasibility of magnetic field measurement in the chromosphere andin the corona, including requirements and specifications of the relevant instruments.

(d) UV/EUV high-throughput spectroscopy
Chair: T. Shimizu (ISAS/JAXA) and Co-chair: TBD
Purpose: Establish science cases for the UV/EUV range in terms of the plan-B andthe plan-A mission purpose and develop plans for a model payload forhigh-resolution, high-throughput, and high-cadence spectroscopy.
(e) Next generation X-ray telescope
Chair: T. Sakao (ISAS/JAXA) and Co-chair E. DeLuca (SAO)
Purpose: Establish science cases and investigate technical feasibility for
imaging observations withX-ray

SOLAR-C, in particular with photon-counting

spectroscopy.
The main charge to each sub-WG is to complete a mission proposal document for its
relevant field in the next 6-8 months that will form the basis of a SOLAR-C mission proposal
in a year from now. Our long-term and short-term roadmaps are shown in Figure 1 andFigure 2, respectively.
The action plans for the WG are, (1) extend the term for investigating both Plan A and B inthe JFY2009, and prepare a mission proposal for both the plans with international sub-WGs,(2) select and propose a single SOLAR-C mission plan in JFY 2010 hopefully with trilateralcoordination among ESA, NASA and JAXA, and (3) proceed to the initial review (MDR) inlate JFY 2010 or in early JFY 2011. The target launch date is February 2017 (JFY 2016).
This new strategy, including the new relationship with JSPEC/JAXA to be described below,
was reported to and approved by the ISAS Space Sciences Steering Committee on 22January 2009.
For the Plan A spacecraft, there are two major orbit options.

The first is to perform theorbit maneuver with large ion engines while the second uses Jupiter and Earth swing-bys.
The former requires the engineering development of a large propulsion and power supply
system, while the second requires detailed engineering study on navigation and guidance,
thermal control including hibernation, and radiation protection. Common engineering issues
include high-rate data transfer and spacecraft system design for long term operability. TheEngineering aspects of Plan A have strong relevance to ongoing JSPEC projects. There islunar and planetary exploration heritage at JSPEC from the Hayabusa mission whose orbit
maneuver was performed with ion engines, and also from the development of the large
lunar orbiter Kaguya. The Jovian mission study performed at JSPEC is also relevant to theSOLAR-C mission. Furthermore, the Plan A spacecraft has engineering relevance for thestrategic roadmap of JAXA’s lunar/planetary exploration under JSPEC initiatives.
Considering the suitability and effectiveness of the development, establishing a WG
preparation team was proposed to the JAXA’s Space Exploration Committee at JSPEC on
23rd of this January leading to the "Out-of-Ecliptic Solar Explorer WG". This new WG isstudying only the engineering aspects for Plan A. The proposal was approved and thepreparation team is to start its activities including fundamental but preliminary design for thespacecraft. For both Plan A and Plan B, studies on science cases and model payloads areto be performed chiefly by solar physicists at both ISAS and NAOJ, with active participationfrom universities and research organizations in Japan. Table 1 summarizes task sharingamong the three organizations, ISAS, JSPEC and NAOJ (including universities) for Plan A
and Plan B.
This letter is intended to demonstrate our strong desire to proceed with SOLAR-C and topropose concrete steps to do so for your understanding and support. The JAXA SOLAR-C
working groups are offering US and European scientists and their corresponding space
agencies to participate in the JAXA SOLAR-C program, in order to continue the remarkablescientific accomplishments we have made together over the past 25 years.
Sincerely yours,Saku Tsuneta
Chair, ISAS/JAXA SOLAR-C WG
Chair, preparation team for JAXA/JSPEC Out-of-Ecliptic Solar Explorer WG

Table 1:

Task sharing between ISAS, JSPEC and NAOJ (including universities andresearch organizations in Japan) for Plan A and Plan B.

Figure 1:

Figure 2:

SOLAR-C schedule.

Roadmap for starting SOLAR-C.