清水 敏文

SOLAR-C is a Japanese solar physics mission with contributions from the United States and European countries. It features the EUV High-Throughput Spectroscopic Telescope (EUVST) for EUV spectroscopy in a wide temperature range (104-107 K). The optical system's innovation omits an aperture filter, using only a 28 cm off-axis parabolic primary mirror and a diffraction grating assembly composed of two different gratings, resulting in an effective area ten times larger than the Extreme-ultraviolet Imaging Spectrometer (EIS) aboard Hinode. This design offers exceptionally high spatial (0.4 arcseconds) and temporal (1 second) resolutions for EUV-UV imaging spectroscopy across a broad wavelength range (170-212 Å, 464-522 Å, 558-610 Å, 719-847 Å, 928-1043 Å, 1115-1221 Å) within a 100×100 arcsecond field of view. A trade-off study focusing on manufacturability successfully eased the specifications of one grating. This presentation reports the latest optical design, optical alignment policy, sensitivity analysis result, and the current spatial resolution error budget plan.

Abstract Magnetic helicity is a physical parameter used to quantify the complexity of magnetic fields, providing an indication of the energy state in the coronal magnetic structure. We investigate the temporal evolution of magnetic helicity and its relationship to the occurrence of a variety of flares in the solar active region NOAA 12297, which was well observed using the Solar Dynamics Observatory/Helioseismic and Magnetic Imager in 2015 March. The active region produced many M-class flares and an X-class flare in two distinctive areas, both of which had a similar magnetic evolution, i.e., the opposite polarity of an emerging flux developed beside a preexisting sunspot, but exhibited flares with different magnitudes and frequencies. We derived the spatiotemporal evolution of the magnetic helicity injections and evaluated how spinning and braiding helicity injections evolved with time in the two areas. In one area, we observed a remarkable evolution, in which a negative spinning helicity injection in the preexisting sunspot increased in a positive helicity system, followed by the occurrence of the X-class flare. The negative helicity injection was clearly caused by the flux emergence that developed along the outer edge of the preexisting sunspot. The other area showed positive braiding helicity injections, with spinning helicity injections fluctuating concurrently with flux emergence, changing their signs several times, i.e., variable energy, and helicity input. The observed temporal behaviors of the helicity injections may explain different types of flare occurrences in the regions.

We have developed an advanced UV spectropolarimeter called Chromospheric LAyer SpectroPolarimeter (CLASP2), aimed at achieving very high accuracy measurements (<0.1% at 3 sigma) of the linear (Q/I and U/I) and circular (V/I) polarizations of the Mg II h and k lines (280 nm). CLASP2 was launched on board a NASA sounding rocket on April 11, 2019. It successfully detected the full Stokes vector in an active-region plage and in the quiet Sun near the limb across the Mg II h and k lines for the first time. To verify the polarization characteristics of CLASP2, the response matrix is estimated by combining the results obtained from the preflight calibration on the ground, with the results of the inflight calibration acquired at the solar-disk center. We find that the response matrix of CLASP2 in the Mg II h and k lines is notably close to an ideal response matrix, i.e., the scale factor and the crosstalk terms are close to 1 and 0, respectively. Moreover, the uncertainty of each Stokes parameter estimated by the repeatability of the measurements is verified to be within the required tolerance. Based on our investigation, we conclude that CLASP2 achieves 0.1% polarization accuracy at a 3 sigma level.

We developed a scan mirror mechanism (SMM) that enable a slit-based spectrometer or spectropolarimeter to precisely and quickly map an astronomical object. The SMM, designed to be installed in the optical path preceding the entrance slit, tilts a folding mirror and then moves the reflected image laterally on the slit plane, thereby feeding a different one-dimensional image to be dispersed by the spectroscopic equipment. In general, the SMM is required to scan quickly and broadly while precisely placing the slit position across the field-of-view (FOV). These performances are in high demand for near-future observations, such as studies on the magnetohydrodynamics of the photosphere and the chromosphere. Our SMM implements a closed-loop control system by installing electromagnetic actuators and gap-based capacitance sensors. Our optical test measurements confirmed that the SMM fulfills the following performance criteria: i) supreme scan-step uniformity (linearity of 0.08%) across the wide scan range (± 1005 ″), ii) high stability (3 σ= 0.1 ″), where the angles are expressed in mechanical angle, and iii) fast stepping speed (26 ms). The excellent capability of the SMM will be demonstrated soon in actual use by installing the mechanism for a near-infrared spectropolarimeter onboard the balloon-borne solar observatory for the third launch, Sunrise III.

Abstract The CLASP2 (Chromospheric LAyer Spectro-Polarimeter 2) sounding rocket mission was launched on 2019 April 11. CLASP2 measured the four Stokes parameters of the Mg iih and k spectral region around 2800 Å along a 200″ slit at three locations on the solar disk, achieving the first spatially and spectrally resolved observations of the solar polarization in this near-ultraviolet region. The focus of the work presented here is the center-to-limb variation of the linear polarization across these resonance lines, which is produced by the scattering of anisotropic radiation in the solar atmosphere. The linear polarization signals of the Mg iih and k lines are sensitive to the magnetic field from the low to the upper chromosphere through the Hanle and magneto-optical effects. We compare the observations to theoretical predictions from radiative transfer calculations in unmagnetized semiempirical models, arguing that magnetic fields and horizontal inhomogeneities are needed to explain the observed polarization signals and spatial variations. This comparison is an important step in both validating and refining our understanding of the physical origin of these polarization signatures, and also in paving the way toward future space telescopes for probing the magnetic fields of the solar upper atmosphere via ultraviolet spectropolarimetry.

Nanoflares and the shock formation of magnetohydrodynamic waves in the solar chromosphere have been considered as key physical mechanisms of the heating of the chromosphere and corona. To investigate candidates of their signature in the mm-wavelength, a tiny active region located on the solar disk was observed with the Atacama Large millimeter and sub-millimeter Array (ALMA) at 3 mm, coordinated with observatories on orbit including Hinode SOT spectro-polarimeter in the Cycle 4 solar campaign (19 March 2017). ALMA’s spatial resolution was moderate, far from the best performance, but it provided stable conditions that are suitable to investigate temporal variations in the mm-wavelength. We determined that the noise level is less than 20 K (σ) over 1 hour in the 20-s cadence time series of synthesized ALMA images. The time variations with amplitudes above the noise level were observed throughout the field of view, but variations exceeding 200 K, corresponding to energy input to the chromosphere on the order of 10^20-22 erg, were localized in two locations. One location was on the polarity inversion line, where tiny concentrated magnetic patches exist in weak field and a tiny magnetic flux may be emergent. The other location was at the outer edge of a bipolar magnetic region, which was under development with a successive series of magnetic flux emergence. This observation suggests that nanoflare-class energy inputs in the chromosphere can occur associated with emerging flux activities.

The Sunrise Chromospheric Infrared spectroPolarimeter (SCIP) is an instrument for the third flight of the Sunrise balloon-borne solar observatory planned for 2022. To verify the high spatial and spectral resolutions required in the balloon flight, the SCIP optical unit was subjected to a thermal-vacuum test in which the SCIP optical unit was installed in a vacuum chamber and was exposed to the thermal vacuum environment expected in the balloon flight. We verified the heater control performance and the temperature distribution in the SCIP optical unit in hot and cold conditions created by the shrouds in the vacuum chamber. We confirmed the optical performance, such as spatial and spectral resolution, and an air-to-vacuum difference of the optics by injecting the laser and white lights through a vacuum window.

We have developed a near-infrared spectropolarimeter onboard the SUNRISE balloon-borne solar observatory launched in 2022. In this paper, we present the development of the optical system from optical design to optical analysis, opto-mechanical analysis, and performance tests.

<title>Abstract</title>Although solar activity may significantly impact the global environment and socioeconomic systems, the mechanisms for solar eruptions and the subsequent processes have not yet been fully understood. Thus, modern society supported by advanced information systems is at risk from severe space weather disturbances. Project for solar–terrestrial environment prediction (PSTEP) was launched to improve this situation through synergy between basic science research and operational forecast. The PSTEP is a nationwide research collaboration in Japan and was conducted from April 2015 to March 2020, supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan. By this project, we sought to answer the fundamental questions concerning the solar–terrestrial environment and aimed to build a next-generation space weather forecast system to prepare for severe space weather disasters. The PSTEP consists of four research groups and proposal-based research units. It has made a significant progress in space weather research and operational forecasts, publishing over 500 refereed journal papers and organizing four international symposiums, various workshops and seminars, and summer school for graduate students at Rikubetsu in 2017. This paper is a summary report of the PSTEP and describes the major research achievements it produced.

Routine ultraviolet imaging of the Sun’s upper atmosphere shows the spectacular manifestation of solar activity; yet, we remain blind to its main driver, the magnetic field. Here, we report unprecedented spectropolarimetric observations of an active region plage and its surrounding enhanced network, showing circular polarization in ultraviolet (Mg <sc>ii</sc><italic>h</italic> &amp; <italic>k</italic> and Mn <sc>i</sc>) and visible (Fe <sc>i</sc>) lines. We infer the longitudinal magnetic field from the photosphere to the very upper chromosphere. At the top of the plage chromosphere, the field strengths reach more than 300 G, strongly correlated with the Mg <sc>ii</sc><italic>k</italic> line core intensity and the electron pressure. This unique mapping shows how the magnetic field couples the different atmospheric layers and reveals the magnetic origin of the heating in the plage chromosphere.

The EUV High-Throughput Spectroscopic Telescope (EUVST) of Solar-C mission is a revolutionary spectrometer that is designed to provide high-quality and high cadence spectroscopic data covering a wide temperature range of the chromosphere to flaring corona to investigate the energetics and dynamics of the solar atmosphere. The EUVST consists of only two imaging optical components a 28-cm clear aperture off-axis parabolic primary mirror and a two-split ellipsoidal grating without a blocking filter for visible light before the primary mirror to achieve unprecedented high spatial and temporal resolution in EUV-UV imaging spectroscopic observations. For this reason, about 53 W of sunlight is absorbed by the multilayer coating on the mirror. We present an instrumental design of the telescope, particularly, primary mirror assembly which enables slit-scan observations for imaging spectroscopy, an image stabilizing tip-tilt control, and a focus adjustment on orbit, together with an optomechanical design of the primary mirror and its supporting system which gives optically tolerant wavefront error against a large temperature increase due to an absorption of visible and IR lights.

The main characteristics of Solar-C_EUVST are the high temporal and high spatial resolutions over a wide temperature coverage. In order to realize the instrument for meeting these scientific requirements under size constraints given by the JAXA Epsilon vehicle, we examined four-dimensional optical parameter space of possible solutions of geometrical optical parameters such as mirror diameter, focal length, grating magnification, and so on. As a result, we have identified the solution space that meets the EUVST science objectives and rocket envelope requirements. A single solution was selected and used to define the initial optical parameters for the concept study of the baseline architecture for defining the mission concept. For this solution, we optimized the grating and geometrical parameters by ray tracing of the Zemax software. Consequently, we found an optics system that fulfills the requirement for a 0.4" angular resolution over a field of view of 100" (including margins) covering spectral ranges of 170-215, 463-542, 557-637, 690-850, 925-1085, and 1115-1275 A. This design achieves an effective area 10 times larger than the Extreme-ultraviolet Imaging Spectrometer onboard the Hinode satellite, and will provide seamless observations of 4.2-7.2 log(K) plasmas for the first time. Tolerance analyses were performed based on the optical design, and the moving range and step resolution of focus mechanisms were identified. In the presentation, we describe the derivation of the solution space, optimization of the optical parameters, and show the results of ray tracing and tolerance analyses.

We present an overview of high-resolution quiet Sun observations, from disk center to the limb, obtained with the Atacama Large millimeter and sub-millimeter Array (ALMA) at 3 mm. Seven quiet-Sun regions were observed at a resolution of up to 2.5″ by 4.5″. We produced both average and snapshot images by self-calibrating the ALMA visibilities and combining the interferometric images with full-disk solar images. The images show well the chromospheric network, which, based on the unique segregation method we used, is brighter than the average over the fields of view of the observed regions by ∼305 K while the intranetwork is less bright by ∼280 K, with a slight decrease of the network/intranetwork contrast toward the limb. At 3 mm the network is very similar to the 1600 Å images, with somewhat larger size. We detect, for the first time, spicular structures, rising up to 15″ above the limb with a width down to the image resolution and brightness temperature of ∼1800 K above the local background. No trace of spicules, either in emission or absorption, is found on the disk. Our results highlight the potential of ALMA for the study of the quiet chromosphere.

The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a suborbital rocket experiment that on 2015 September 3 measured the linear polarization produced by scattering processes in the hydrogen Lya line of the solar disk radiation. The line-center photons of this spectral line radiation mostly stem from the chromospherecorona transition region (TR). These unprecedented spectropolarimetric observations revealed an interesting surprise, namely that there is practically no center-to-limb variation (CLV) in the Q/I line-center signals. Using an analytical model, we first show that the geometric complexity of the corrugated surface that delineates the TR has a crucial impact on the CLV of the Q/I and U/I line-center signals. Second, we introduce a statistical description of the solar atmosphere based on a 3D model derived from a state-of-the-art radiation magnetohydrodynamic simulation. Each realization of the statistical ensemble is a 3D model characterized by a given degree of magnetization and corrugation of the TR, and for each such realization we solve the full 3D radiative transfer problem taking into account the impact of the CLASP instrument degradation on the calculated polarization signals. Finally, we apply the statistical inference method presented in a previous paper to show that the TR of the 3D model that produces the best agreement with the CLASP observations has a relatively weak magnetic field and a relatively high degree of corrugation. We emphasize that a suitable way to validate or refute numerical models of the upper solar chromosphere is by confronting calculations and observations of the scattering polarization in ultraviolet lines sensitive to the Hanle effect.

On 2015 September 3, the Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) successfully measured the linear polarization produced by scattering processes in the hydrogen Ly alpha line of the solar disk radiation, revealing conspicuous spatial variations in the Q/I and U/I signals. Via the Hanle effect, the line-center Q/I and U/I amplitudes encode information on the magnetic field of the chromosphere-corona transition region, but they are also sensitive to the three-dimensional structure of this corrugated interface region. With the help of a simple line-formation model, here we propose a statistical inference method for interpreting the Ly alpha line-center polarization observed by CLASP.