Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://star.arm.ac.uk/preprints/2015/686.pdf Äàòà èçìåíåíèÿ: Fri Jan 15 14:19:40 2016 Äàòà èíäåêñèðîâàíèÿ: Sun Apr 10 02:38:01 2016 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: images

ADAHELI+: exploring the fast, dynamic Sun in the x-ray, optical, and near-infrared
Francesco Berrilli Paolo Soffitta Marco Velli Paolo Sabatini Alberto Bigazzi Ronaldo Bellazzini Luis Ramon Bellot Rubio Alessandro Brez Vincenzo Carbone Gianna Cauzzi Fabio Cavallini Giuseppe Consolini Fabio Curti Dario Del Moro Anna Maria Di Giorgio Ilaria Ermolli Sergio Fabiani Marianne Faurobert Alex Feller Klaus Galsgaard Szymon Gburek Fabio Giannattasio Luca Giovannelli Johann Hirzberger Stuart M. Jefferies Maria S. Madjarska Fabio Manni Alessandro Mazzoni Fabio Muleri Valentina Penza Giovanni Peres Roberto Piazzesi Francesca Pieralli Ermanno Pietropaolo Valentin Martinez Pillet Michele Pinchera Fabio Reale Paolo Romano Andrea Romoli Marco Romoli Alda Rubini Pawel Rudawy Paolo Sandri Stefano Scardigli Gloria Spandre Sami K. Solanki Marco Stangalini Antonio Vecchio Francesca Zuccarello

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Journal of Astronomical Telescopes, Instruments, and Systems 1(4), 044006 (Oct­Dec 2015)

ADAHELI+: exploring the fast, dynamic Sun in the x-ray, optical, and near-infrared
Francesco Berrilli,a Paolo Soffitta,b Marco Velli,c,d Paolo Sabatini,e Alberto Bigazzi,f Ronaldo Bellazzini,g Luis Ramon Bellot Rubio,h Alessandro Brez,g Vincenzo Carbone,i Gianna Cauzzi,j Fabio Cavallini,j Giuseppe Consolini,b Fabio Curti,k Dario Del Moro,a,* Anna Maria Di Giorgio,b Ilaria Ermolli,l Sergio Fabiani,b,m Marianne Faurobert,n Alex Feller,o Klaus Galsgaard,p Szymon Gburek,q Fabio Giannattasio,a,b Luca Giovannelli,a Johann Hirzberger,o Stuart M. Jefferies,r Maria S. Madjarska,s Fabio Manni,t Alessandro Mazzoni,u Fabio Muleri,b Valentina Penza,a Giovanni Peres,v Roberto Piazzesi,a Francesca Pieralli,u Ermanno Pietropaolo,w Valentin Martinez Pillet,x,y Michele Pinchera,g Fabio Reale,v Paolo Romano,z Andrea Romoli,u Marco Romoli,c Alda Rubini,b Pawel Rudawy,q Paolo Sandri,u Stefano Scardigli,a Gloria Spandre,g Sami K. Solanki,o,aa Marco Stangalini,a,l Antonio Vecchio,i and Francesca Zuccarellobb
a b

University of Rome "Tor Vergata", Department of Physics, Via Della Ricerca Scientifica 1, 00133 Roma, Italy INAF--Institute for Space Astrophysics and Planetology, Via del Fosso del Cavaliere 100, 00133 Roma, Italy c University of Florence, Department of Physics and Astronomy, Via Sansone 1, 50019 Sesto Fiorentino (FI), Italy d University of California, Los Angeles, Department of Earth, Planetary, and Space Sciences, 595 Charles Young Drive East, Los Angeles, California 90095, United States e OHB-CGS, Via Gallarate 150, 20151 Milano, Italy f SERCO S.p.A., Via Galileo Galilei 00044 Frascati (Rome), Italy g INFN--Pisa Section, Largo Bruno Pontecorvo 3, 56127 Pisa, Italy h Instituto de Astrofisica de Andalucia, Glorieta de la AstronomÌa, s/n, 18008 Granada, Spain i University of Calabria, Department of Physics, Via P.Bucci, Cubo 31C, 87036 Arcavacata di Rende (CS), Italy j INAF--Arcetri Astrophysical Observatory, Largo Enrico Fermi 5, 50125 Firenze, Italy k University of Rome "Sapienza," Department of Aeronautical, Electrical and Energetic Engineering, Via Eudossiana 18, 00184 Roma, Italy l INAF--Rome Astronomical Observatory, Via Frascati 33, 00078, Monte Porzio Catone (RM), Italy m INFN--Trieste Section, Via Padriciano 99, 34149 Trieste, Italy n UniversitÈ de Nice Sophia Antipolis, Avenue Valrose 28, 06103 Nice, France o Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 GÆttingen, Germany p Niels Bohr Institute, Blegdamsvej 17, 2100 KÜbenhavn, Denmark q Space Research Centre, Polish Academy of Sciences, Bartycka 18A, 00-716 Warsaw, Poland r University of Hawaii, Institute of Astronomy, 2680 Woodlawn Drive, Honolulu, Hawaii 96822-1839, United States s Armagh Observatory, Armagh BT61 9DG, Northern Ireland, United Kingdom t SRS-Engineering Design S.r.l., Vicolo delle Palle 25-25/B, 00186 Roma, Italy u ANTARES s.c.a.r.l., Via Appia 1, 82018 S. Giorgio del Sannio (BN), Italy v University of Palermo, Department of Physics and Chemistry, Viale delle Scienze, Ed. 17, 90128 Palermo, Italy w University of L'Aquila, Department of Physics and Chemistry Sciences, Via Vetoio, 67100 Coppito (AQ), Italy x Instituto de Astrofisica de Canarias, C/ VÌa LÀctea, s/n, 38205 San CristÑbal de La Laguna, Santa Cruz de Tenerife, Spain y National Solar Observatory, 3004 Telescope Loop, Sunspot, New Mexico 88349, United States z INAF--Catania Astronomical Observatory, Via Santa Sofia 78, Gravina di Catania (CT), Italy aa Kyung Hee University, School of Space Research, Yongin, Gyeonggi-Do 446-701, Republic of Korea bb University of Catania, Department of Physics and Astronomy, Via Santa Sofia, 64, 95123 Catania (CT), Italy

Abstract. Advanced Astronomy for Heliophysics Plus (ADAHELI+) is a project concept for a small solar and space weather mission with a budget compatible with an European Space Agency (ESA) S-class mission, including launch, and a fast development cycle. ADAHELI+ was submitted to the European Space Agency by a European-wide consortium of solar physics research institutes in response to the "Call for a small mission opportunity for a launch in 2017," of March 9, 2012. The ADAHELI+ project builds on the heritage of the former ADAHELI mission, which had successfully completed its phase-A study under the Italian Space Agency 2007 Small Mission Programme, thus proving the soundness and feasibility of its innovative low-budget design. ADAHELI+ is a solar space mission with two main instruments: ISODY+: an imager, based on Fabry­PÈrot interferometers, whose design is optimized to the acquisition of highest cadence, long-duration, multiline spectropolarimetric images in the visible/near-infrared region of the solar spectrum. XSPO: an x-ray polarimeter for solar flares in x-rays with energies in the 15 to 35 keV range. ADAHELI+ is capable of performing observations that cannot be addressed by other currently planned solar space missions, due to their limited telemetry, or by ground-based facilities, due to the problematic effect of the terrestrial atmosphere. © The Authors. Published by SPIE under
a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. [DOI: 10.1117/1.JATIS.1.4.044006]

Keywords: Sun; satellites; infrared spectroscopy; Fabry­PÈrot; x-rays; polarimetry. Paper 15065 received Jul. 27, 2015; accepted for publication Nov. 6, 2015; published online Dec. 11, 2015.

*Address all correspondence to: Dario Del Moro, E-mail: delmoro@roma2.infn .it

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Berrilli et al.: ADAHELI+: exploring the fast, dynamic Sun in the x-ray, optical, and near-infrared

1

Introduction

The understanding of solar magnetic field generation and evolution is one of the most outstanding problems in solar and stellar physics. The solar magnetic field is continuously generated and destroyed on timescales ranging from fractions of a second to years, and fills the heliosphere, a volume of space that extends to at least 10 â 109 km from the Sun. Also, the solar magnetic field drives solar activity on all timescales from second to centuries, abruptly enhancing the emission of particles and x-ray from our star, but also leading to gradual long-term changes in its radiative output. Consequently, these processes dictate space weather, and how the solar plasma, radiation, and magnetic field interact with planetary environments; changes in the magnetic field drive solar activity and eventually define the space weather conditions. Solar activity forecasting is therefore a major issue of ESA Space Situational Awareness activities for the security of space assets. In addition, an investigation of the structure and dynamics of the magnetic field in the photosphere and in the chromosphere are fundamental to solve the mystery of the Sun's superhot corona or resolving fundamental problems in solar physics, such as the origin and acceleration of the fast solar wind. Advanced Astronomy for Heliophysics Plus (ADAHELI+) is the first and only solar space mission specifically designed to provide: (i) multiline spectropolarimetric imaging at high cadence (5 fps) and exceptionally long-duration observations of target regions in the photosphere and chromosphere; (ii) x-ray polarimetry of solar flares in the 15- to 35-keV energy. ADAHELI+ is capable of performing observations that cannot be addressed by other currently planned solar space missions, due to their limited telemetry, or by ground-based facilities, due to the problematic effect of the terrestrial atmosphere. ADAHELI+ is a solar space mission with two main instruments:
· ISODY+: an imager whose design is optimized to the

acquisition of the highest cadence, long-duration, multiline spectropolarimetric images in the visible/near-infrared (VIS­NIR) region of the solar spectrum.
· XSPO: an x-ray polarimeter which will, for the first time,

perform polarimetry of solar flares in x-rays (15 to 35 keV). ADAHELI+ builds on the heritage of the ADAHELI mission1­6 and the expertise of its industrial team with building and operating a small satellite mission. ADAHELI successfully completed its phase-A study under the Italian Space Agency (ASI) 2007 Small Mission Programme, proving the feasibility of its innovative low-budget design. ADAHELI+ was one of the 26 proposals submitted to the European Space Agency in response to the call for a small mission ESA D/SRE/FF/og28226 issued on March 9, 2012, but was not selected.

2

Key Technologies and Design Solutions

ADAHELI+ has been designed to match the requirements of its high-performance VIS­NIR telescope, a diffraction-limited 500-mm Gregorian solar telescope of 0.25 arc sec angular resolution (corresponding to 175 km on the Sun), capable of acquiring a quasisimultaneous series of fully two-dimensional (2-D) spectral images of the solar lower atmosphere at a rate of up to 5 framess, to reconstruct the three-dimensional (3-D) structure of the Sun's magnetic fields.
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A number of innovative solutions have been devised in order to meet the requirements within the limits of a small mission. The fast imaging and spectral scanning capability is made possible by the use of a Fabry­PÈrot interferometer (FPI), flying for the first time in this class of satellites. FPIs have a much higher transmittance (by a factor 30) with respect to long slit spectrograph of the same spectral resolution.7 FPI-based instruments are capable of acquiring a 2-D image at high spectral resolving power (better than 200,000, and have a very fast wavelength tuning, of the order of tens of milliseconds, resulting in the possibility of building stacks of 2-D tomographic images. FPIs have been selected, for their high performances, to be used in future large missions such as the James Webb Space Telescope, BepiColombo, and Solar Orbiter.8­10 A wide operational spectral range is achieved due to the capacitance-stabilized piezoelectric control of the FPI11 (which is, in that case, often referred to as CSE­FPI: capacitance-stabilized etalon­FPI) and suitable cameras. A great effort has been dedicated to redesign the ISODY+ layout, with respect to that of its precursor ISODY,3,4 to adopt an all-reflective design, minimizing both aberrations and mechanical complexity. The high resolution of the telescope needs high-precision tracking of the region of interest (ROI), to less than 0.1 arc sec during the acquisition. This is achieved by the combined action of the satellite attitude and orbit control system (AOCS) and the correlation tracker (CRT) correction system in ISODY+. For the thermal control of the VIS­NIR telescope, particular attention has been given to ensure a drastic reduction of the heat power reflected from the primary mirror. A dedicated heat rejecter has therefore been designed. X-ray solar polarimeter (XSPO) exploits the capability of the gas pixel detector (GPD)12,13 to reconstruct the emission direction of the photons absorbed via photoelectric effect in gas to derive the polarization of detected radiation. Also, a Peltier cooler takes care of keeping the GPD operative temperature in the required [5 to 20°C] range, with optimal working temperature of 10°C and a stability of ô2o C. A compact design has been adopted with the payload and the platform integrated in the same structure. All the internal units are then accommodated around the main telescope. In order to address the launch costs, the overall satellite envelope has been designed to be compatible with a dual launch through the ESA small launcher VEGA. The ground segment has been designed to cost and makes use of the ASI Acquisition Station and Satellite Control Center in Matera and Fucino (Italy), respectively. These stations are located at midlatitudes and the visibility time of the ground stations to the satellite is about 50 min day. However, the constraints in attitude pointing during observations, imposed by the telescope, and the intrinsically limited coverage of an X-band antenna, reduce the available time for data download to 25 min day. This requires a high data rate in downlink, of at least 150 Mbps, achieved by a special development of the X-band antenna. Science operations are managed by science personnel for long-term payload operations planning, quick-look analysis, processing, data distribution, and quality assessment. Processing and archiving infrastructures are hosted and managed through the ASI Scientific Data Center.

3

Science Goals

Understanding our own star is one of the major scientific challenges recognized by all space programs, see, e.g., NASA's
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Living with a Star Program and ESA's Cosmic Vision 2015 to 2025. ADAHELI+ is designed to specifically target a set of fundamental questions in heliophysics:
· What determines the Sun's superhot corona?

Table 1 Chromosphere fast dynamics: observational requirements.

Objective

Chromospheric and photospheric vector magnetic field and line of sight velocity field maps, large FOV, long-duration Simultaneous sampling of photospheric and chromospheric lines HeI 1083.0 nm line (chromosphere) SiI 1082.7 nm line (photosphere) CaII 854.2 nm line (chromosphere)

­ ­

How does the magnetic field couple the solar atmosphere from the photosphere to the high chromosphere? What is the role of waves in the Sun's corona?

Method

Spectral lines

· How is the magnetic field generated and destroyed on

timescales ranging from fractions of a second to years? ­ What is the nature of the coupling of magnetic field and photospheric plasma, in particular, in the polar regions?
FeI 617.3 nm line (photosphere) FOV 100 arc sec â100 arc sec to properly encompass the extent of a typical AR 0.5 arc sec at 1 m 150,000

· What is the nature of the polarization of hard x-ray (HXR)

sources in the solar atmosphere? ­ How is the fast solar wind generated and accelerated?

Spatial resolution Spectral resolving power Photometry Polarimetric precision Data set duration Full spectral scan cadence

3.1
3.1.1

Heating the Corona: Chromospheric Fields and Dynamics
Chromosphere fast dynamics

108 photons S/N 104 >2 h <1 min

The chromosphere is the "interface layer" between the photospheric plasma dominated by turbulent convective motions, and the tenuous corona where most of the structure is determined by the magnetic field. This atmospheric region still presents crucial observational and theoretical challenges. In the chromosphere, compressible waves excited at the photospheric roots of the granular convection, or through p-mode conversion, steepen and form shocks, accounting for large dynamical excursions and very short timescales, of a few minutes at most,14­16 requiring a fully time-dependent analysis. Understanding the mechanisms that sustain the chromosphere departure from radiative equilibrium will pave the way to understanding the formation of the super-hot corona and the origin of the solar wind. ADAHELI+ provides high-rate, long-duration observations to cover a large interval in the VIS­NIR range. Table 1 reports the observational requirements related to the study of the dynamics of the chromosphere.
3.1.2 Waves and heating of the solar upper atmosphere

The temperature at the top of the Sun's chromosphere (10;000 K) is higher than at the bottom (4300 K). Different mechanisms have been proposed to explain this rise in temperature, from the dissipation of upward-propagating waves,17 to resistive dissipation of fine-scale electric currents,18 to magnetic field reconnection.19 These waves, usually considered evanescent in a nonmagnetic atmosphere, propagate through so-called magnetoacoustic portals that are generated where the magnetic field is significantly inclined. Such conditions are ubiquitous on the Sun, both in active regions (ARs) and at the boundaries of convection cells.20 In addition to these, magneto-hydrodynamic (MHD) waves in magnetic structures can also significantly participate in the energy budget of the upper layers of the Sun's atmosphere. In particular, small-scale magnetic elements cover a significant fraction of the solar photosphere21,22 and harbor different kinds of waves that connect different layers of the
Journal of Astronomical Telescopes, Instruments, and Systems

solar atmosphere, eventually depositing a significant amount of energy in the upper chromosphere (see, e.g., Refs. 23 and 24 for a complete treatment of the topic). Among the many different kinds of MHD waves that small flux tubes can support (compressive and noncompressive), kink waves are probably the most promising due to their ability to travel long distances before being dissipated.15,16,25,26 Very recently, observations have revealed the propagation of kink waves in small magnetic elements to the solar chromosphere, with velocity of the order of 6 kms.27 However, although these authors have shown that this propagation is highly nonlinear, and thus subject to dissipation, no signature of energy losses was found between the photosphere and the chromosphere. For these reasons, a new fundamental question arises as to which are the main mechanisms responsible for the dissipation of the energy contained in MHD waves and operating at different heights of the solar atmosphere. To this regard, multiheight high spatial resolution spectropolarimetric observations are needed to study in detail the propagation mechanisms of different kinds of MHD waves and to assess the main dissipation mechanisms involved. Table 2 reports the observational requirements set by the investigation of waves in the solar atmosphere.

3.2
3.2.1

Magnetic Flux Emergence, the Solar Wind, and the Dynamo
Prominences, reconnection, and magnetic flux emergence; origin and acceleration of the fast solar wind

Prominences are chromospheric features rooted in the photosphere and extending out of either sides of a primary spine. Magnetic reconnection may create plasma upflows in filaments:
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Berrilli et al.: ADAHELI+: exploring the fast, dynamic Sun in the x-ray, optical, and near-infrared Table 2 Waves in the solar atmosphere: observational requirements. Table 3 Prominences: observational requirements.

Objective

Multiheight vector magnetic field and line of sight velocity field maps Simultaneous full-Stokes sampling of photospheric and chromospheric lines

Objective

Method

Vector magnetic field maps, plasma motions in barbs, localization of plasma motions, and brightenings as signatures of magnetic reconnection, large FOV, long-duration Simultaneous full-Stokes sampling of photospheric and chromospheric lines HeI 1083.0 nm line (chromosphere) SiI 1082.7 nm line (photosphere) CaII 854.2 nm line (chromosphere) FeI 617.3 nm line (photosphere)

Method Spectral lines HeI 1083.0 nm line (chromosphere) SiI 1082.7 nm line (photosphere) CaII 854.2 nm line (chromosphere) FeI 617.3 nm line (photosphere) Data set duration Full scan cadence >30 min 10 to 30 s FOV Spatial resolution Spectral lines

100 arc sec â100 arc sec 0.5 arc sec 150,000

as reconnection proceeds, the lower reconnected loops submerge below the solar surface, while the upper ones move upward and carry photospheric plasma with them. Presently, chromospheric jet-like events such as spicules/mottles (in the quiet Sun)28,29 and fibrils (in ARs)30 are investigated both through the latest generation of ground-based instruments,31,32 together with the space-based observatories Hinode and Solar Dynamics Observatory.33,34 Hinode observations of high-velocity spicules have recently revived the discussion on the contribution of these phenomena to coronal heating and solar wind generation.32,34 However, the short observation times typically allocated to this type of observations on space observatories, due to the shared telemetry among different instruments, and the intrinsic limitations of ground-based observations, hamper further advances on this subject. Experimental confirmation of magnetic reconnection models requires high cadence observations of photospheric and chromospheric lines searching for cancelation of magnetic flux and plasma motions along the line of sight. Uninterrupted space observations with long-time surveys of coronal holes in equatorial and polar regions, available through a dedicated mission such as ADAHELI+, will advance our understanding of where and how the fast solar wind is generated and accelerated.35 Table 3 reports the observational requirements associated with the analysis of chromospheric features.
3.2.2 Magnetic flux emergence and quiet Sun magnetism

Spectral resolving power Data set duration Full scan cadence

>2 h 10 s

kilo-Gauss fluxes45 and the exploration of vortex flow motions.46 This new information suggests that the quiet Sun magnetic fields are not magnetic debris from decaying ARs but have rather to be generated through some other small-scale mechanisms.47 Large fields of view [(FOV) supergranular scale], time-extended observations (>24 h), high spatial resolution, and fast time cadence are needed to investigate long-lived structures and see how the intermittent magnetic field appears and disappears. Table 4 reports the observational requirements set by the investigation of quiet Sun magnetism.

3.3
3.3.1

Solar Flares
X-ray polarimetry of solar flares

Magnetic flux emergence is a complex process involving a wide range of time and spatial scales: from the large ARs present during solar maxima (with flux content up to 1023 Mx) that host the most violent phenomena associated with energy release (flares, eruptive prominences, CMEs), to small bipolar flux concentrations (ephemeral regions and granular bipoles, with fluxes from 1016 to 1019 Mx), which populate the quiet Sun at all times during the solar cycle. Recent observations of the solar magnetic surface with space- and ground-based instruments have shown how the quiet Sun magnetic fields can no longer be regarded as sheets of unipolar magnetic flux stretching along the boundaries of large convection cells (i.e., supergranules). Ubiquitous transverse magnetic fields (see, e.g., Refs. 36­38) and fine intranetwork mixed polarity fields (see, e.g., Refs. 39 and 40) have been discovered using Hinode instruments.41 Moreover, the launch of the IMaX spectropolarimeter42 on-board the Sunrise balloonborne solar observatory,43,44 allowed the resolution of individual
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Due to energetic events such as solar flares, the Sun is an astrophysical source with an intense emission of x-rays. Their characterization will advance our understanding of the dynamics of the magnetic fields in the ARs of our star. Magnetic reconnection is the cause of the sudden release of energy in flares and it is responsible for the acceleration of particles,48­50 including the downward beaming and the upward solar wind. The nonthermal HXR emission, dominating at energies 15 keV, is generated by electrons slowing-down in the plasma of the solar atmosphere. Particles radiate via bremsstrahlung and heat the ambient plasma. This emission component is expected to be highly polarized,51­56 with a polarization degree as high as 40% at 20 keV.57 Polarization measurements are directly correlated to the study of the particles' acceleration directivity and therefore to the understanding of the plasma environment. There have been recent attempts to measure the x-ray polarization with the RHESSI satellite and with the Thomson-scattering polarimeter SPR-N on board of the CORONAS-F satellite.58­60 Both instruments had a small effective area for polarization measurements and were also heavily affected by the background. Furthermore, the RHESSI spectrometer (not specifically designed to operate as a polarimeter61) has a high energy threshold
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Berrilli et al.: ADAHELI+: exploring the fast, dynamic Sun in the x-ray, optical, and near-infrared Table 4 Quiet Sun magnetism: observational requirements. Table 6 VIS-NIR telescope main characteristics.

Objective

Vector magnetic field maps, small magnetic elements polarimetric observation, plasma motions in emerging and submerging regions, localization of brightenings, large FOV, very long-duration Full-Stokes sampling of photospheric lines SiI 1082.7 nm line (photosphere) FeI 617.3 nm line (photosphere)

Characteristic Wavelength range FOV M1 mirror diameter Mirror type

Value From 400 to 1100 nm 108 â 108 arc sec 500 mm Concave prolate ellipsoid (almost parabolic) 164 mm Concave prolate ellipsoid 1152 mm 3465 mm 500 mm 172 mm 30 mm Diffraction limited

Method

Spectral lines

M2 mirror diameter Mirror type M1­M2 distance EFL telescope Entrance pupil Obscuration Exit pupil Resolution

FOV

100 arc sec â100 arc sec to encompass several supergranular structures 0.5 arc sec 110,000 24 h 5s

Spatial resolution Spectral resolving power Data set duration Full scan cadence

for polarimetric measurements (100 keV), thus encountering the problem of the fast decrease of the x-ray flare flux. Neither instruments, therefore, reached the sensitivity needed to achieve significant results. Future missions, e.g., solar orbiter, will not address xray polarimetry among their science topics. The photoelectric polarimeter XSPO on board ADAHELI+ is a unique opportunity to complement all the other efforts for the understanding of solar flares. This instrument will perform sensitive x-ray polarimetry in the 15- to 35-keV energy band, in a spectral region where the highly polarized HXR emission starts to overwhelm the SXR component. Additional science topics related to fundamental physics can be addressed with the XSPO: a long-term accumulation of angular resolved data is suitable to reveal the presence of an x-ray-emitting region from the solar disc center, produced by the interaction of axions particles with local magnetic field and also the ARs may prove a good probe for searching for the axionic xrays.62,63 Table 5 reports the observational requirements set by the investigation of X-ray polarimetry of solar flares.

4
4.1

Observation Goals and Strategy
VIS­NIR Polarimetry for Chromospheric and Photospheric Diagnostic

achieved through observations of quasisimultaneous chromospheric (CaII 854.2 nm or HeI triplet 1083 nm) lines, producing maps of intensity, full magnetic vector, and Doppler velocity. Both lines are sensitive to Hanle and Zeeman effects, making them unique diagnostic tools for chromospheric magnetic fields covering a wide range of strengths. While the detailed physics of formation is complicated by many factors, spectral line inversion codes to analyze the emergent spectral line radiation in a variety of physical scenarios do exist and are becoming available to the scientific community (see, e.g., Refs. 64­67). In particular, observations and theory show that the HeI triplet is better suited for ARs studies, while the caII line is to be employed for quieter conditions, and for cross-calibration with ground-based instrumentation. Moreover, a photospheric, magnetically sensitive line (SiI 1082.7 nm), is available in the immediate spectral surroundings of the He triplet: this is crucial in order to obtain quasisimultaneous maps of photospheric magnetic fields that will aid the

Table 7 ISODY+ narrowband channel parameters.

The best diagnostics for chromospheric polarimetry lie in the NIR range. Science mission goals, as set in Sec. 3, will be
Table 5 X-ray polarimetry of solar flares: observational requirements.

Characteristic Spectral lines

Value HeI triplet at 1083 nm CaII line at 854.2 nm

Objective

Particle acceleration and plasma conditions during flares Polarization measure of x-rays 15 to 35 keV 70 arc min (fully coded) 1 arc min Spectral resolving power Wavelength stability Polarimetric accuracy

FeI line at 617.3 nm SiI line at 1082.7 nm > 150;000 Maximum drift 10 ms in 10 h 102 to 104

Method Energy band FOV Angular resolution

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interpretation of the polarimetric data in the chromosphere and the extrapolation to higher atmospheric levels. Finally, the much used and relatively well-known FeI 617.3 nm line can be used for photospheric diagnostic purposes (e.g., magnetic field, temperature, and line of sight velocity) as well as for cross-calibration with ground-based instrumentation. Similarly to ADAHELI+, Hinode "was designed to address the fundamental question of how magnetic fields interact with the ionized atmosphere"68 and is the modern touchstone for high-resolution