arXiv subjects

Ryohko Ishikawa

Publications and source records attributed to Ryohko Ishikawa.

15 recordsLinked to original sources

arXiv2026

Science operation, data handling, and ground support system of the SOLAR-C mission

SOLAR-C is an international solar-observing satellite mission led by Japan Aerospace Exploration Agency (JAXA). It aims to elucidate mass and energy transport in solar atmospheres through extreme ultraviolet (EUV) spectroscopy. The mission carries the EUV High-throughput Spectroscopic Telescope (EUVST) and the Solar Spectral Irradiance Monitor (SoSpIM), enabling comprehensive observations across a wide temperature range (10^4 K to 10^7 K) with minimum temperature gaps and high spatial and temporal resolution. To achieve its science objectives regarding atmospheric heating and solar flare eruptions, SOLAR-C implements a flexible and responsive operational procedure and a data processing system, while building on the heritage of the Hinode and IRIS satellites. The Chief Observer creates observation timelines that include core observation plans and approved proposed plans while taking into account the solar activity levels. A SpaceWire communication architecture is employed onboard the spacecraft, and a method is implemented in which the mission instrument temporarily acts as the network master during data transfer to support the high data rate requirements. Telemetry is downlinked at ground stations worldwide and gathered at the Institute of Space and Astronautical Science, JAXA. The EUVST data are calibrated at the SOLAR-C Science Center at Nagoya University, while the SoSpIM data are calibrated at the Processing and Archiving Facility before being integrated into the science data products. The data will be made publicly available immediately. The integrated operational scheme for this mission is expected to advance our understanding of solar atmospheric heating and flare processes.

astro-ph.SR↗

arXiv2026

Filter and baffle design for the short-wavelength camera on SOLAR-C EUVST

SOLAR-C, which is scheduled to launch in the late 2020s with the Extreme Ultraviolet High-Throughput Spectroscopic Telescope (EUVST), will deliver EUV spectroscopy of the solar atmosphere with unprecedented temperature coverage and resolution. We present the optical design and mechanical impacts for the short-wavelength camera, and how our design choices affect scientific performance using complete forward modelling. We discuss the design and optical impact of the thin-film aluminium filter and supporting mesh, including diffraction and shadowing effects in spectral and spatial dimensions. We analyse this using our own Fourier optics method and analytical modelling, and quantify the scientific impact using forward modelling driven by a simulated solar atmosphere. We address the vulnerability of thin-film filters during launch depressurisation using vacuum chamber testing. We describe the stray-light baffle design and low-scatter coatings which suppress visible stray light.

astro-ph.IM↗

arXiv2026

Polarization measurements of the $2s$--$2p_{3/2}$ VUV transition in N$^{4+}$ excited by electron collisions

We present the linear polarization of the $2s$--$2p_{3/2}$ transition in Li-like N$^{4+}$ excited by electron collisions, measured at electron energies between 85 and 1000~eV. The measured polarizations are compared with theoretical values calculated using the Flexible Atomic Code (FAC). Although the overall trend that the degree of polarization decreases with increasing electron energy is reproduced, the theoretical values are systematically larger in magnitude than the experimental values. After examining possible experimental sources of depolarization, we suggest that the discrepancy likely originates from an overestimation in the theoretical model. Further theoretical and experimental investigations along the isoelectronic sequence are needed to resolve this discrepancy.

physics.atom-ph↗

arXiv2026

A vacuum-ultraviolet spectropolarimeter for an electron beam ion trap

We have developed a vacuum-ultraviolet spectropolarimeter for an electron beam ion trap (EBIT) to measure the linear polarization of emission lines from multiply charged ions around the Lyman-$α$ wavelength. The main components for polarimetry are a rotatable MgF$_2$ waveplate and a SiO$_2$/MgF$_2$ multilayer-coated fused silica plate that functions as a reflective polarizer. A grazing-incidence grating is mounted between them to provide wavelength dispersion. The polarization is determined from the intensity modulation of the spectral line as the waveplate is rotated. The performance of the spectropolarimeter was demonstrated by measuring the polarization of the $2s$--$2p_{3/2}$ transition in Li-like N$^{4+}$ (124~nm) excited by a 1000~eV electron beam in an EBIT. Clear modulation of the line intensity was observed as a function of the waveplate rotation angle. From the measured modulation amplitude, the degree of linear polarization was determined to be $P=-(0.178^{+0.014}_{-0.005})$, with the negative sign indicating that the emission is polarized predominantly perpendicular to the electron beam. This result demonstrates the capability of the present spectropolarimeter to determine polarizations with an absolute uncertainty $ΔP$ on the order of $0.01$. This instrument provides a useful tool for benchmarking magnetic-sublevel-resolved collision theories through polarization measurements of multiply charged ions excited by a unidirectional electron beam.

physics.atom-ph↗

arXiv2026

Pre-flare and active region plasma flows and structure seen by the short wavelength camera on SOLAR-C/EUVST

The mechanisms triggering solar flares and driving coronal heating occur across wide temperature ranges on small spatial scales and short timescales, making them difficult to observe with current instrumentation. The upcoming SOLAR-C mission, launching in the late 2020s, will provide unprecedented plasma diagnostic capability with its high-throughput extreme-ultraviolet (EUV) spectroscopic telescope (EUVST), capable of ~0.2 arcsec/pix spatial sampling (~0.4 arcsec resolution), continuous temperature coverage from 0.02-15 MK, and exposure times down to 0.5 seconds. We present forward modelling of the spectrograph's short wavelength camera (170-210 Å; SOLAR-C/EUVST-SW) and its response to log T~6.2 coronal plasma in a three-dimensional MHD-simulated pre-flare active region. We compare this performance to that of the previous-generation EUV Imaging Spectrometer (EIS) on Hinode (SOLAR-B). Our results demonstrate that SOLAR-C/EUVST can distinguish individual flux tubes in simulated active region loops which Hinode/EIS cannot resolve. In simulated pre-flare plasma, SOLAR-C/EUVST captures sharp velocity gradients between adjacent upflowing and downflowing plasma which Hinode/EIS is unable to resolve. Doppler velocity measurement accuracy will reach better than 1 km/s in active regions. We show that this next-generation spectrograph can be expected to directly observe processes potentially related to flare triggering, such as plasma flows from low-altitude reconnection linked to emerging flux, and determine whether active region loops consist of a small number of strands or the hundreds predicted by magnetic reconnection-induced nanoflare heating models.

astro-ph.SR↗

arXiv2026

Gender equality activities in Astronomical Society of Japan

The female ratio in science field, including astronomy, is still quite low in Japan. We, the Astronomical Society of Japan, are making efforts to equalise the gender balance. In this paper, we summarise our statistics, member's thinking shown in our questionnaire, the history and accomplishments of the day-care system during annual meetings, and other activities.

astro-ph.IM↗

arXiv2025

Determining the magnetic field of active region plages using the whole CLASP2/2.1 spectral window

The Chromospheric LAyer SpectroPolarimeter missions, CLASP2 and CLASP2.1, demonstrated that the near-UV spectral region between 279.30 and 280.68 nm is suitable for studying the magnetism of the solar chromosphere. In particular, the spectropolarimetric observations in the Mg II h and k resonant doublet, Mn I 279.91 and 280.19 nm resonant lines, and Fe II 279.79 and 280.66 nm lines acquired by these suborbital space experiments have been proven useful for inferring the magnetic field stratification in the solar chromosphere. However, several lines of the CLASP2/2.1 spectral region with significant circular polarization signals had remained unexplored. After identifying two Ni I (279.95 and 280.59 nm), one Mn II (280.62 nm), and one Fe I (280.53 nm) lines, here we apply the Weak Field Approximation (WFA) to the spectropolarimetric observations of active region plages by CLASP2 and CLASP2.1. By comparing the results with previous studies, we are able to estimate the formation heights of these CLASP2/2.1 additional spectral lines and to demonstrate their suitability to determine the magnetic field stratification from the photosphere to the upper chromosphere.

astro-ph.SR↗

arXiv2025

Determining the Magnetic Field in the Atmosphere of a Solar Active Region Observed by the CLASP2.1 Sounding Rocket Experiment

We determine magnetic fields from the photosphere to the upper chromosphere combining data from the Hinode satellite and the CLASP2.1 sounding rocket experiment. CLASP2.1 provided polarization profiles of the Mg~{|sc ii} $h$ and $k$ lines, as well as of the Mn~{|sc i} lines around 2800~{|AA}, across various magnetic structures in an active region, containing a plage, a pore, and the edges of a sunspot penumbra. By applying the Weak-Field Approximation (WFA) to the circular polarization profiles of these spectral lines, we obtain a longitudinal magnetic field map at three different heights in the chromosphere (lower, middle, and upper). This is complemented by data from Hinode (photospheric magnetic field), IRIS, and SDO (high-spatial-resolution observations of the chromosphere and corona). We quantify the height expansion of the plage magnetic fields and find that the magnetic fields expand significantly in the middle chromosphere, shaping the moss observed above in the transition region and corona. We identified an area with polarity reversal at the upper chromosphere around the edge of the pore, suggesting the presence of a magnetic discontinuity in the upper chromosphere. Transient and recurrent jet-like events are observed in this region, likely driven by magnetic reconnection. Around the penumbral edge, we find large-scale magnetic fields corresponding to the superpenumbral fibrils seen in the upper chromosphere. In the superpenumbral fibrils, we find Zeeman-induced linear polarization signals, suggesting the presence of a significantly inclined magnetic field, as strong as 1000~G in the upper chromosphere.

astro-ph.SR↗

arXiv2024

Mapping the longitudinal magnetic field in the atmosphere of an active region plage from the inversion of the near-ultraviolet CLASP2.1 spectropolarimetric data

We apply the HanleRT Tenerife Inversion Code to the spectro-polarimetric observations obtained by the Chromospheric LAyer SpectroPolarimeter. This suborbital space experiment measured the variation with wavelength of the four Stokes parameters in the near-ultraviolet spectral region of the Mg II h & k lines over a solar disk area containing part of an active region plage and the edge of a sunspot penumbra. We infer the stratification of the temperature, the electron density, the line of-sight velocity, the micro-turbulent velocity, and the longitudinal component of the magnetic field from the observed intensity and circular polarization profiles. The inferred model atmosphere shows larger temperature and electron density in the plage and the superpenumbra regions than in the quiet regions. The shape of the plage region in terms of its brightness is similar to the pattern of the inferred longitudinal component of the magnetic field in the chromosphere, as well as to that of the overlying moss observed by AIA in the 171 A band, which suggests a similar magnetic origin for the heating in both the plage and the moss region. Moreover, this heating is particularly significant in the regions with larger inferred magnetic flux. In contrast, in the superpenumbra, the regions with larger electron density and temperature are usually found in between these regions with larger magnetic flux, suggesting that the details of the heating mechanism in the chromosphere of the superpenumbra may be different to those in the plage, but with the magnetic field still playing a key role.

astro-ph.SR↗

arXiv2023

Evidence for the Operation of the Hanle and Magneto-Optical Effects in the Scattering Polarization Signals Observed by CLASP2 Across the Mg II h and k Lines

Radiative transfer investigations of the solar Mg II h and k resonance lines around 280~nm showed that, while their circular polarization (Stokes V) signals arise from the Zeeman effect, the linear polarization profiles (Stokes Q and U) are dominated by the scattering of anisotropic radiation and the Hanle and magneto-optical (MO) effects. Using the unprecedented observations of the Mg II and Mn I resonance lines obtained by the Chromospheric LAyer Spectro-Polarimeter (CLASP2), here we investigate how the linear polarization signals at different wavelengths (i.e., at the center, and at the near and far wings of the k line) vary with the longitudinal component of the magnetic field ($B_{L}$) at their approximate height of formation. The $B_{L}$ is estimated from the V signals in the aforementioned spectral lines. Particular attention is given to the following quantities that are expected to be influenced by the presence of magnetic fields through the Hanle and MO effects: the sign of the U signals, the total linear polarization amplitude ($LP$) and its direction ($χ$) with respect to a reference direction. We find that at the center and near wings of the $k$ line, the behavior of these quantities is significantly different in the observed quiet and plage regions, and that both $LP$ and $χ$ seem to depend on $B_{L}$. These observational results are indicative of the operation of the Hanle effect

astro-ph.SR↗

arXiv2021

Mapping Solar Magnetic Fields from the Photosphere to the Base of the Corona

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 II $h$ & $k$ and Mn I) and visible (Fe I) 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 gauss, strongly correlated with the Mg II $k$ 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.

astro-ph.SR↗

arXiv2015

Development of a Precise Polarization Modulator for UV Spectropolarimetry

We developed a polarization modulation unit (PMU) to rotate a waveplate continuously in order to observe solar magnetic fields by spectropolarimetry. The non-uniformity of the PMU rotation may cause errors in the measurement of the degree of linear polarization (scale error) and its angle (crosstalk between Stokes-Q and -U), although it does not cause an artificial linear polarization signal (spurious polarization). We rotated a waveplate with the PMU to obtain a polarization modulation curve and estimated the scale error and crosstalk caused by the rotation non-uniformity. The estimated scale error and crosstalk were <0.01 % for both. This PMU will be used as a waveplate motor for the Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) rocket experiment. We confirmed that the PMU has the sufficient performance and function for CLASP.

astro-ph.IM↗

arXiv2011

Spatial and temporal distributions of transient horizontal magnetic fields with deep exposure

We obtained a long exposure vector magnetogram of the quiet Sun photosphere at the disk center with wide FOV of $51" \times 82"$. The observation was performed at Fe I 525.0 nm with the shutter-less mode of the Narrow Band Filter Imager of the Solar Optical Telescope (SOT) on board Hinode satellite. We summed the linear polarization ($LP$) maps taken with time cadence of 60 seconds for 2 hours to obtain a map with as long an exposure as possible. The polarization sensitivity would be more than 4.6 (21.2 in exposure time) times the standard observation with the SOT spectro-polarimeter. The $LP$ map shows a cellular structure with a typical scale of $5" - 10"$. We find that the enhanced $LP$ signals essentially consist of the isolated sporadic transient horizontal magnetic fields (THMFs) with life time of 1-10 min, and are not contributed by long-duration weak horizontal magnetic fields. The cellular structure coincides in position with the negative divergence of the horizontal flow field, i.e., mesogranular boundaries with downflows. Azimuth distribution appears to be random for the scale size of the mesogranules. Some pixels have two separate appearances of THMFs, and the measured time intervals are consistent with the random appearance. THMFs tend to appear at the mesogranular boundaries, but appear randomly in time. We discuss the origin of THMFs based on these observations.

astro-ph.SR↗

arXiv2011

Relationship between vertical and horizontal magnetic fields in the quiet Sun

Vertical magnetic fields have been known to exist in the internetwork region for decades, while the properties of horizontal magnetic fields have recently been extensively investigated with \textit{Hinode}. Vertical and horizontal magnetic fields in the internetwork region are considered to be separate entities and have thus far not been investigated in a unified way. We discover clear positional association between the vertical and horizontal magnetic fields in the internetwork region with \textit{Hinode}. Essentially all of the horizontal magnetic patches are associated with the vertical magnetic patches. Alternatively, half of the vertical magnetic patches accommodate the horizontal magnetic patches. These horizontal patches are located around the borders of the vertical patches. The intrinsic magnetic field strength as obtained with the Stokes $V$ line ratio inside the horizontal patches is weak, and is in sub-equipartition field regime ($B<700$ G), while the field strength outside the horizontal patches ranges from weak to strong (kG) fields. Vertical magnetic patches are known to be concentrated on mesogranular and supergranular boundaries, while the horizontal magnetic patches are found only on the mesogranular boundaries. These observations provide us with new information on the origin of the vertical and horizontal internetwork magnetic fields, in a unified way. We conjecture that internetwork magnetic fields are provided by emergence of small-scale flux tubes with bipolar footpoints, and the vertical magnetic fields of the footpoints are intensified to kG fields due to convective collapse. Resultant strong vertical fields are advected by the supergranular flow, and eventually form the network fields.

astro-ph.SR↗

arXiv2008

Comparison of transient horizontal magnetic fields in a plage region and in the quiet Sun

Properties of transient horizontal magnetic fields (THMFs) in both plage and quiet Sun regions are obtained and compared. Spectro-polarimetric observations with the Solar Optical Telescope (SOT) on the Hinode satellite were carried out with a cadence of about 30 seconds for both plage and quiet regions located near disk center. We select THMFs that have net linear polarization (LP) higher than 0.22%, and an area larger than or equal to 3 pixels, and compare their occurrence rates and distribution of magnetic field azimuth. We obtain probability density functions (PDFs) of magnetic field strength and inclination for both regions.The occurrence rate in the plage region is the same as for the quiet Sun. The vertical magnetic flux in the plage region is ~8 times larger than in the quiet Sun. There is essentially no preferred orientation for the THMFs in either region. However, THMFs in the plage region with higher LP have a preferred direction consistent with that of the plage-region's large-scale vertical field pattern. PDFs show that there is no difference in the distribution of field strength of horizontal fields between the quiet Sun and the plage regions when we avoid the persistent large vertical flux concentrations for the plage region. The similarity of the PDFs and of the occurrence rates in plage and quiet regions suggests that a local dynamo process due to the granular motion may generate THMFs all over the sun. The preferred orientation for higher LP in the plage indicates that the THMFs are somewhat influenced by the larger-scale magnetic field pattern of the plage.

astro-ph↗