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Toward the inner Heliospheric system science exploration in the late 2020s, ISAS/JAXA is currently operating the Arase, BepiColombo/Mio, Hinode, and Akatsuki satellites, and Solar-C EUVST is scheduled for launch in the near future. These missions will be linked together with other satellite missions such as Solar-Orbiter, Solar Parker Probe, Cluster, THEMIS, MMS etc. to realize exploration of the inner Heliosphere with unprecedented scale.In the early 2030s, Japanese Solar Terrestrial Physics group is considering the FACTORS formation-flight satellite mission in order to reveal the energy coupling mechanisms and mass transport between the space and Earth’s atmosphere. In the late 2030s, another formation-flight magnetospheric satellite mission the science target of which includes understanding the cross-scale / cross-region coupling is also under consideration hopefully on orbit at the same time with European future mission Plasma Observatory. These future missions will closely collaborate with NASA’s future GDC and Magnetospheric Constellation missions.The future Heliospheric system science exploration will be conducted by multiple satellite missions further expanding their observation area while improving the quality of each individual satellite mission. Japanese Solar Terrestrial Physics group will conduct in-situ observation of space plasmas with MMX (Martian Moons Exploration) and MIM(Mars Ice Mapper) in the Martian system and with JUICE (Jupiter Icy Moons Explorer) in the Jovian system. Collaboration between Japanese Solar Physics and Solar Terrestrial Physics groups for considering the future out-of-ecliptic-plane mission is also about to start.In order to realize the future Heliospheric system science exploration, significant technological development is mandatory. Current status of the technological development in Japan for enabling the future Heliospheric system science exploration will also be presented.

The Dalton minimum is considered to be one of the unique solar activity periods that have been captured in direct sunspot observations since 1610. Specifically, the solar magnetic field in this period is of great interest. Derfflinger and Prantner’s sunspot observations of 1802–1824 and 1800–1844 are the most important references for this period. To understand the solar magnetic activity in the Dalton minimum, it is important to estimate the latitude/longitude distribution of the sunspots and the sunspot areas for that duration. In this study, we analyze Derfflinger and Prantner’s sunspot drawings to determine the sunspot parameters, particularly the sunspot area. We find that the sunspot areas obtained from Derfflinger’s drawings are overemphasized by a factor of eight relative to those derived from modern observations. We also analyze Prantner’s sunspot drawings to validate our analysis of Derfflinger’s drawings. Further, we generate solar magnetograms from Derfflinger’s sunspot drawings using a deep-learning model based on conditional generative adversarial networks. Our analysis of these sunspot areas will provide important information for restoring the magnetograms during the Dalton minimum.

Corona is the outermost layer of the Sun, which extends to several thousand kilometers from the visible photosphere. It is made up of very tenuous plasma but is very hot. The sudden increase of temperature in the coronal layer from the underlying chromosphere and photosphere makes it very interesting. The average temperature of a corona is measured about 2 MK, and it may show a large variation in temperature with respect to different surface features. Studying the variation of the temperature of the full-disk corona and of individual feature’s temperature along with the solar cycle will be interesting and important to understand the Physics of the Sun. Although an attempt has been made earlier to measure the temperature of coronal XBPs for a short period with high cadence of observations in localized regions, but the variation in temperature of the full-disk image and of individual features over the solar cycle is not measured yet. Since Hinode/XRT allows capturing images of the Sun in 8 different filters, it has got a unique feature that, using 2 different filters, the temperature map of the Sun can be generated. In order to study the temperature variations, we have used Al mesh and Ti poly filters of full-disk composite, level-2, X-ray images of the Sun obtained from Hin- ode/XRT. We developed a sophisticated python algorithm to segment different coronal features (ARs, CHs, BGs, & XBPs), derived the integrated intensity of all the features in both the filters, and generated the temperature maps of the corona using the filter ratio method. Because of the XRT straylight issue and unavailability of a good pair of images we have restricted our analysis for the period of 4 years (February 01, 2008 - May 08, 2012, covering the starting part of the solar cycle 24). In this paper, the first analysis in using direct energy values of the coronal features from segmented solar disk and its relation to solar activity is presented. We have discussed the variations of the temperature of a full-disk corona, and of all the features (ARs, CHs, BGs & XBPs).We found from the time series plots of the average temperature of the full-disk and of all the features show temperature fluctuations and they vary in phase with the sunspot numbers (solar activity). Although the temperature of all the features varies but the mean temperature estimated for the whole observed period of the full-disk is around 1.39 ± 0.28 MK and active regions (ARs) will be around 1.98 ± 0.44 MK, whereas BGs, CHs & XBPs are 1.37 ± 0.26 MK, 1.34 ± 0.33 MK, and 1.52± 0.34 MK respectively. In addition, we found that the mean temperature contribution estimated of the background regions (BG) will be around 91 %, whereas ARs, CHs, & XBPs are 5 %, 2 % and 2 % respectively to the average coronal temperature of the full-disk for the period: 2008-2012. The temperature values and their variations of all the features suggest that the features show a high variability in their temperature and that the heating rate of the emission features may be highly variable on solar cycle timescales. It is clear from the analysis that the filter ratio method can be directly used for temperature analysis of coronal features and to study their surface temperature variability as a function of solar magnetic activity.

The solar activity during the Maunder Minimum (MM; 1645 -- 1715) has been considered significantly different from the one captured in modern observations, in terms of sunspot group number and sunspot positions, whereas its actual amplitudes and distributions is still under active discussions. In its core period (1650/1660 -- 1700), Martin Fogelius and Henrich Siverus have formed significant long-term series in the existing databases with numerous spotless days, as the 13th and 7th most active observers before the end of the MM. In this study, we have analysed their original archival records, revised their data, have removed significant contaminations of the apparent "spotless days" in the existing databases, and cast caveats on the potential underestimation of the solar-cycle amplitude in the core MM. Still, they reported at best one sunspot group throughout their observational period and confirm the significant suppressed the solar cycles during the MM, which is also supported from the contemporary observations of Hook and Willoughby. Based on the revised data, we have also derived positions of notable sunspot groups, which Siverus recorded in 1671 (~ N7.5{\deg} +/- 2.5{\deg}), in comparison with those of Cassini's drawings (~ N10{\deg} +/- 1{\deg}). Their coincidence in position and chronology in corrected dates indicates these sunspot groups were probably the same recurrent active region (AR) and its significantly long lifespan (>~ 35 days) even during the MM., Comment: 18 pages, 5 figures, and 1 table; accepted for publication in the Monthly Notices of the Royal Astronomical Society; Figures 1 and 2 are reserved for the record version

The Maunder Minimum (1645 – 1715) was unique in terms of solar-cycle amplitudes and sunspot-position distributions registered in the last four centuries; however, little is known for its recovery and transition to the regular solar cycles until 1749 and the existing reconstructions vary from one to another here. This article presents a snapshot of Solar Cycle −3 including sunspot observations by Johann Christoph Muller (hereafter, JCM) in 1719 – 1720. We identified his sunspot drawings in the manuscript department of the National Library of Russia in St. Petersburg and compiled his biographical profile and observational expertise. Subsequently, we analysed his sunspot drawings and derived the group number and positions of the observed sunspots. The results and comparative analyses with contemporary observations revealed that JCM reported up to five sunspot groups, corresponding well with Sebastian Alischer’s records but contrasting with Johann Rost’s records in the existing databases. These comparisons indicated that Rost’s extremely large values recorded in 1719 – 1720 probably represented individual sunspot numbers instead of sunspot group numbers, unlike the understanding in the existing databases. Accordingly, JCM’s group number forms a robust reference for representing the solar activity in 1719 – 1720 and exhibits relatively moderate solar cycle amplitude in the immediate aftermath of the Maunder Minimum. Moreover, JCM’s sunspot drawings provide significantly detailed information on sunspot positions. Our analyses could locate the reported sunspot groups in both solar hemispheres, unlike those in the Maunder Minimum, which support the suggested transition between Solar Cycles −4 and −3.

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.

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.

We attempted to reproduce the total electron content (TEC) variation in the Earth's atmosphere from the temporal variation of the solar flare spectrum of the X9.3 flare on September 6, 2017. The flare spectrum from the Flare Irradiance Spectral Model (FISM), and the flare spectrum from the 1D hydrodynamic model, which considers the physics of plasma in the flare loop, are used in the GAIA model, which is a simulation model of the Earth's whole atmosphere and ionosphere, to calculate the TEC difference. We then compared these results with the observed TEC. When we used the FISM flare spectrum, the difference in TEC from the background was in a good agreement with the observation. However, when the flare spectrum of the 1D-hydrodynamic model was used, the result varied depending on the presence or absence of the background. This difference depending on the models is considered to represent which extreme ultraviolet (EUV) radiation is primarily responsible for increasing TEC. From the flare spectrum obtained from these models and the calculation result of TEC fluctuation using GAIA, it is considered that the enhancement in EUV emission by approximately 15–35 nm mainly contributes in increasing TEC rather than that of X-ray emission, which is thought to be mainly responsible for sudden ionospheric disturbance. In addition, from the altitude/wavelength distribution of the ionization rate of Earth's atmosphere by GAIA (Ground-to-topside Atmosphere and Ionosphere model for Aeronomy), it was found that EUV radiation of approximately 15–35 nm affects a wide altitude range of 120–300 km, and TEC enhancement is mainly caused by the ionization of nitrogen molecules.

The transition region is a thin layer of the solar atmosphere that controls the energy loss from the solar corona. Large numbers of grid points are required to resolve this thin transition region fully in numerical modeling. In this study, we propose a new numerical treatment, called LTRAC, which can be easily extended to the multi-dimensional domains. We have tested the proposed method using a one-dimensional hydrodynamic model of a coronal loop in an active region. The LTRAC method enables modeling of the transition region with the numerical grid size of 50--100 km, which is about 1000 times larger than the physically required value. We used the velocity differential emission measure to evaluate the possible effects on the optically thin emission. Lower temperature emissions were better reproduced by the LTRAC method than by previous methods. Doppler shift and non-thermal width of the synthesized line emission agree with those from a high-resolution reference simulation within an error of several km/s above the formation temperature of $10^5$ K., Comment: 17 pages, 10 figures, accepted for publication in ApJ

The power-law index of an occurrence frequency distribution of flares as a function of energy is one of the most important indicators to evaluate the contribution of small-scale flares to coronal heating. For a few decades, many studies tried to derive the power-law index using various instruments and methods. However, these results are various and the cause of this uncertainty is unknown due to the variety of observation conditions. Therefore, we investigated the dependence of the index on the solar activity, coronal features, released energy range, and active region properties such as magnetic flux, twist, and size. Our findings are (1) annual power-law index derived from time series of total solar irradiance (Sun-as-a-star observation analysis) has a negative correlation with sunspot number; (2) power-law index in active region is smaller than that of the quiet Sun and coronal holes; (3) power-law index is almost constant in the energy range of 1025 ≲ E ≲ 1030 erg; and (4) active regions that have more magnetic free energy density, unsigned magnetic flux, and shear angle tend to have smaller power-law indices. Based on the results and energy-scaling law of Petschek-type reconnection, we suggest that the power-law index of sunspot-scale events is smaller than that of granule-scale events. Moreover, we indicated that sunspot-scale events follow CSHKP flare model whereas granule-scale events follow Parker’s nanoflare model.

The EUV high-throughput spectroscopic telescope (EUVST) onboard the Solar-C mission has the high spatial (0.4′′) resolution over a wide wavelength range in the vacuum ultraviolet. To achieve high spatial resolution under a design constraint given by the JAXA Epsilon launch vehicle, we further update the optical design to secure margins needed to realize 0.4′′ spatial resolution over a field of view of 100′′×100′′. To estimate the error budgets of spatial and spectral resolutions due to installation and fabrication errors, we perform a sensitivity analysis for the position and orientation of each optical element and for the grating parameters by ray tracing with the Zemax software. We obtain point spread functions (PSF) for rays from 9 fields and at 9 wavelengths on each detector by changing each parameter slightly. A full width at half maximum (FWHM) of the PSF is derived at each field and wavelength position as a function of the perturbation of each optical parameter. Assuming a mount system of each optical element and an error of each optical parameter, we estimate spatial and spectral resolutions by taking installation and fabrication errors into account. The results of the sensitivity analysis suggest that budgets of the total of optical design and the assembly errors account for 15% and 5.8% of our budgets of the spatial resolution in the long wavelength and short wavelength bands, respectively. On the other hand, the grating fabrication errors give a large degradation of spatial and spectral resolutions, and investigations of compensators are needed to relax the fabrication tolerance of the grating surface parameters.

The EUV High-Throughput Spectroscopic Telescope (EUVST) of Solar-C mission 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 60 W of sunlight is absorbed by the multilayer coating on the mirror. We report a thermal design of telescope in which the temperature of the primary mirror bonding part and underlying tip-tilt and slit-scanning mechanisms is well lower than a glass transition temperature of adhesive (about 60°C) and thermal deformation of the primary mirror is small, although it is non-negligibly small.

We studied temporal variation of the differential rotation and poleward meridional circulation during solar cycle 24 using the magnetic element feature tracking technique. We used line-of-sight magnetograms obtained using the helioseismic and magnetic imager aboard the Solar Dynamics Observatory from May 01, 2010 to March 26, 2020 (for almost the entire period of solar cycle 24, Carrington rotation from 2096 to 2229) and tracked the magnetic element features every 1 h. We also estimated the differential rotation and poleward meridional flow velocity profiles. The observed profiles are consistent with those of previous studies on different cycles. Typical properties resulting from torsional oscillations can also be observed from solar cycle 24. The amplitude of the variation was approximately ±10 m s$$^{-1}$$ - 1 . Interestingly, we found that the average meridional flow observed in solar cycle 24 is faster than that observed in solar cycle 23. In particular, during the declining phase of the cycle, the meridional flow of the middle latitude is accelerated from 10 to 17 m s$$^{-1}$$ - 1 , which is almost half of the meridional flow itself. The faster meridional flow in solar cycle 24 might be the result of the weakest cycle during the last 100 years.

We statistically studied the transport of magnetic flux in and around sunspots using a magnetic-element tracking technique to investigate whether sunspot-decay processes are isotropic. Using this method, we detected moving magnetic features (MMFs). The observed radius of an MMFs region was approximately 1.7 times the sunspot radius; furthermore, the average apparent velocity of MMFs was statistically estimated to be approximately $350~\mbox{m}\,\mbox{s}^{-1}$ . We determined that the leading sunspots transport approximately 5% more magnetic flux to the Equator side than to the Pole side of the sunspots. In addition, the leading sunspots transport approximately 3% more magnetic flux to the back (East) than to the front (West) of the sunspots. On the other hand, the following sunspots do not show the magnetic-flux transport asymmetry. The statistics might not be sufficient for the analysis of the following sunspots. These asymmetries of magnetic flux transport might contribute to the cross-equatorial transport of net magnetic flux, which is an important physical quantity of polar magnetic-field reversal.

X-rays and extreme ultraviolet (EUV) emissions from solar flares rapidly change the physical composition of the Earth’s thermosphere and ionosphere, thereby causing space weather phenomena such as communication failures. To predict the effects of flare emissions on the Earth’s upper atmosphere, numerous empirical and physical models have been developed. We verify the extent of reproducing the flare emission spectra using a one-dimensional hydrodynamic calculation and the CHIANTI atomic database. To verify the proposed model, we use the observed EUV spectra obtained by the extreme ultraviolet variability (EVE) on board the Solar Dynamics Observatory (SDO). We examined the “EUV flare time-integrated irradiance” and “EUV flare line rise time” of the EUV emissions for 21 events by comparing the calculation results of the proposed model and observed EUV spectral data. The proposed model succeeded in reproducing the EUV flare time-integrated irradiance of the Fe VIII 131 Å, Fe XVIII 94 Å, and Fe XX 133 Å, as well as the 55 to 355 Å and 55 to 135 Å bands. For the EUV flare line rise time, there was acceptable correlation between the proposed model estimations and observations for all Fe flare emission lines. These results demonstrate that the proposed model can reproduce the EUV flare emission spectra from the emitting plasma with relatively high formation temperature. This indicates that the physics-based model is effective for the accurate reproduction of EUV spectral flux.

We attempted to reproduce the total electron content (TEC) variation in the Earth's atmosphere from the temporal variation of the solar flare spectrum of the X9.3 flare on September 6, 2017. The flare spectrum from the Flare Irradiance Spectral Model (FISM), and the flare spectrum from the 1D hydrodynamic model, which considers the physics of plasma in the flare loop, are used in the GAIA model, which is a simulation model of the Earth's whole atmosphere and ionosphere, to calculate the TEC difference. We then compared these results with the observed TEC. When we used the FISM flare spectrum, the difference in TEC from the background was in a good agreement with the observation. However, when the flare spectrum of the 1D-hydrodynamic model was used, the result varied depending on the presence or absence of the background. This difference depending on the models is considered to represent which extreme ultraviolet (EUV) radiation is primarily responsible for increasing TEC. From the flare spectrum obtained from these models and the calculation result of TEC fluctuation using GAIA, it is considered that the enhancement in EUV emission by approximately 15–35 nm mainly contributes in increasing TEC rather than that of X-ray emission, which is thought to be mainly responsible for sudden ionospheric disturbance. In addition, from the altitude/wavelength distribution of the ionization rate of Earth's atmosphere by GAIA (Ground-to-topside atmosphere and ionosphere model for aeronomy), it was found that EUV radiation of approximately 15–35 nm affects a wide altitude range of 120–300 km, and TEC enhancement is mainly caused by the ionization of nitrogen molecules. (265 words)

We studied temporal variation of the differential rotation and poleward meridional circulation during solar cycle 24 using the magnetic element feature tracking technique. We used line-of-sight magnetograms obtained using the Helioseismic and Magnetic Imager aboard the Solar Dynamics Observatory from May 01, 2010 to March 26, 2020 (for almost the entire period of solar cycle 24, Carrington Rotation from 2096 to 2229) and tracked the magnetic element features every 1 hour. We also estimated the differential rotation and poleward meridional flow velocity profiles. The observed profiles are consistent with those of previous studies on different cycles. Typical properties resulting from torsional oscillations can also be observed from solar cycle 24. The amplitude of the variation was approximately $\pm$10 m s$^{-1}$. Interestingly, we found that the average meridional flow observed in solar cycle 24 is faster than that observed in solar cycle 23. In particular, during the declining phase of the cycle, the meridional flow of the middle latitude is accelerated from 10 to 17 m s$^{-1}$, which is almost half of the meridional flow itself. The faster meridional flow in solar cycle 24 might be the result of the weakest cycle during the last 100 years.

The largest X9.3 solar flare in solar cycle 24 and the preceding X2.2 flare occurred on September 6, 2017, in the solar active region NOAA 12673. This study aims to understand the onset mechanism of these flares via analysis of multiple observational datasets from the Hinode and Solar Dynamics Observatory and results from a non-linear force-free field extrapolation. The most noticeable feature is the intrusion of a major negative-polarity region, appearing similar to a peninsula, oriented northwest into a neighboring opposite-polarity region. We also observe proxies of magnetic reconnection caused by related to the intrusion of the negative peninsula: rapid changes of the magnetic field around the intruding negative peninsula; precursor brightening at the tip of the negative peninsula, including a cusp-shaped brightening that shows a transient but significant downflow (~100 km/s) at a leg of the cusp; a dark tube-like structure that appears to be a magnetic flux rope that erupted with the X9.3 flare; and coronal brightening along the dark tube-like structure that appears to represent the electric current generated under the flux rope. Based on these observational features, we propose that (1) the intrusion of the negative peninsula was critical in promoting the push-mode magnetic reconnection that forms and grows a twisted magnetic flux rope that erupted with the X2.2 flare, (2) the continuing intrusion progressing even beyond the X2.2 flare is further promoted to disrupt the equilibrium that leads the reinforcement of the magnetic flux rope that erupted with the X9.3 flare., 26 pages, 10 figures, accepted by the Astrophysical Journal. There are animation files that are corresponding to Figures 2 and 7 but it is not available here. Please see the ApJ publication

Solar-C (EUVST) is the next Japanese solar physics mission to be developed with significant contributions from US and European countries. The mission carries an EUV imaging spectrometer with slit-jaw imaging system called EUVST (EUV High-Throughput Spectroscopic Telescope) as the mission payload, to take a fundamental step towards answering how the plasma universe is created and evolves and how the Sun influences the Earth and other planets in our solar system. In April 2020, ISAS (Institute of Space and Astronautical Science) of JAXA (Japan Aerospace Exploration Agency) has made the final down-selection for this mission as the 4th in the series of competitively chosen M-class mission to be launched with an Epsilon launch vehicle in mid 2020s. NASA (National Aeronautics and Space Administration) has selected this mission concept for Phase A concept study in September 2019 and is in the process leading to final selection. For European countries, the team has (or is in the process of confirming) confirmed endorsement for hardware contributions to the EUVST from the national agencies. A recent update to the mission instrumentation is to add a UV spectral irradiance monitor capability for EUVST calibration and scientific purpose. This presentation provides the latest status of the mission with an overall description of the mission concept emphasizing on key roles of the mission in heliophysics research from mid 2020s.

To understand the mechanism of coronal heating, it is crucial to derive the contribution of small-scale flares, the so-called nanoflares, to the heating up of the solar corona. To date, several studies have tried to derive the occurrence frequency distribution of flares as a function of energy to reveal the contribution of small-scale flares. However, there are no studies that derive the distribution with considering the following conditions: (1) evolution of the coronal loop plasma heated by small-scale flares, (2) loops smaller than the spatial resolution of the observed image, and (3) multiwavelength observation. To take into account these conditions, we introduce a new method to analyze small-scale flares statistically based on a one-dimensional loop simulation and a machine learning technique, that is, genetic algorithm. First, we obtain six channels of SDO/AIA light curves of the active-region coronal loops. Second, we carry out many coronal loop simulations and obtain the SDO/AIA light curves for each simulation in a pseudo-manner. Third, using the genetic algorithm, we estimate the best combination of simulated light curves that reproduce the observation. Consequently, the observed coronal loops are heated by small-scale flares with energy flux larger than that typically required to heat up an active region intermittently. Moreover, we derive the occurrence frequency distribution which have various power-law indices in the range from 1 to 3, which partially supports the nanoflare heating model. In contrast, we find that $90\%$ of the coronal heating is done by flares that have energy larger than $10^{25}~\mathrm{erg}$., Comment: 15 pages, 16 figures

In addition to regular Schwabe cycles (~ 11 years), solar activity also shows longer periods of enhanced or reduced activity. Of these, reconstructions of the Dalton Minimum provide controversial sunspot group numbers and limited sunspot positions, partially due to limited source record accessibility. We analysed Stephan Prantner's sunspot observations from 1804--1844, the values of which had only been known through estimates despite their notable chronological coverage during the Dalton Minimum. We identified his original manuscript in Stiftsarchiv Wilten, near Innsbruck, Austria. We reviewed his biography (1782--1873) and located his observational sites at Wilten and Waidring, which housed the principal telescopes for his early and late observations: a 3.5-inch astronomical telescope and a Reichenbach 4-feet achromatic erecting telescope, respectively. We identified 215 days of datable sunspot observations, which are twice as much data as his estimated data in the existing database (= 115 days). Prantner counted up to 7--9 sunspot groups per day and measured sunspot positions, which show their distributions in both solar hemispheres. These results strikingly emphasise the difference between the Dalton Minimum and the Maunder Minimum as well as the similarity between the Dalton Minimum and the modern solar cycles., Comment: 18 pages, 5 figures, accepted for publication in the ApJ. Figure 2 is available only in the record version

著者人数: 35名 (所属. 宇宙航空研究開発機構宇宙科学研究所(JAXA)(ISAS): 坂尾, 太郎; 清水, 敏文), Number of authors: 35 (Affiliation. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency(JAXA)(ISAS): Sakao, Taro; Shimizu, Toshifumi), Accepted: 2019-08-01, 資料番号: SA1190166000

Solar-C EUVST (EUV High-Throughput Spectroscopic Telescope) is a solar physics mission concept that was selected as a candidate for JAXA competitive M-class missions in July 2018. The onboard science instrument, EUVST, is an EUV spectrometer with slit-jaw imaging system that will simultaneously observe the solar atmosphere from the photosphere/chromosphere up to the corona with seamless temperature coverage, high spatial resolution, and high throughput for the first time. The mission is designed to provide a conclusive answer to the most fundamental questions in solar physics: how fundamental processes lead to the formation of the solar atmosphere and the solar wind, and how the solar atmosphere becomes unstable, releasing the energy that drives solar flares and eruptions. The entire instrument structure and the primary mirror assembly with scanning and tip-tilt fine pointing capability for the EUVST are being developed in Japan, with spectrograph and slit-jaw imaging hardware and science contributions from US and European countries. The mission will be launched and installed in a sun-synchronous polar orbit by a JAXA Epsilon vehicle in 2025. ISAS/JAXA coordinates the conceptual study activities during the current mission definition phase in collaboration with NAOJ and other universities. The team is currently working towards the JAXA final down-selection expected at the end of 2019, with strong support from US and European colleagues. The paper provides an overall description of the mission concept, key technologies, and the latest status.

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.

The Solar-C_EUVST is a mission designed to provide high-quality solar spectroscopic data covering a wide temperature range of the chromosphere to flaring corona. To fulfill a high throughput requirement, the instrument consists of only two optical components; a 28-cm primary mirror and a segmented toroidal grating which have high reflective coatings in EUV-UV range. We present a mission payload structural design which accommodates long focal length optical components and a launcher condition/launch environment (JAXA Epsilon). We also present a mechanical design of primary mirror assembly which enables slit-scan observations, 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.

Large sunspots can be observed with the unaided eye under suitable atmospheric seeing conditions. Such observations are of particular value because the frequency of their appearance provides an approximate indication of the prevailing level of solar activity. Unaided-eye sunspot (UES) observations can be traced back well before the start of telescopic observations of the Sun, especially in the East Asian historical records. It is therefore important to compare more modern, UES observations with the results of telescopic sunspot observations, to gain a better understanding of the nature of the UES records. A previous comparison of Chinese UES records and Greenwich photo-heliographic results between 1874 and 1918 indicated that a few of the UES were apparently not supported by direct photographic evidence of at least one sunspot with a large area. This article reveals that one of the Chinese unaided-eye observations had possibly captured the transit of Venus on 9 December 1874. The Chinese sunspot records on this date are compared with Western sunspot observations on the same day. It is concluded that sunspots on the solar disk were quite small and the transit of Venus was probably misinterpreted as a sunspot by the Chinese local scholars. This case indicates that sunspots or comparable "obscuring" objects with an area as large as 1000 millionths of the solar disk could easily have been seen with the unaided eye under suitable seeing conditions. It also confirms the visibility of sunspots near the solar limb with the unaided eye. This study provides an explanation of the apparent discrepancy between the Chinese UES observation on 9 December 1874 and the Western sunspot observations using telescopes, as well as a basis for further discussion on the negative pairs in 1900 and 1911, apparently without sufficiently large area., main text 14 pages, appendices 4 pages, and references 5 pages, and 4 figures: accepted for publication in Solar Physics

We revisit the sunspot drawings made by the Japanese astronomer Kunitomo Toubei during 1835-1836 and recount the sunspot group number for each image. There are two series of drawings, preliminary (P, containing 17 days with observations) and summary (S, covering 156 days with observations), all made using brush and ink. S is a compilation of drawings for the period from February 1835, to March 1836. Presently, the P drawings are available only for one month, September 1835; those of other periods have presumably been lost. Another drawing (I) lets us recover the raw group count (RGC) for 25 September 1836, on which the RGC has not been registered in the existing catalogs. We also revise the RGCs from P and S using the Zurich classification and determine that Kunitomo's results tend to yield smaller RGCs than those of other contemporary observers. In addition, we find that Kunitomo's RGCs and spot areas have a correlation (0.71) that is not very different from the contemporary observer Schwabe (0.82). Although Kunitomo's spot areas are much larger than those determined by Schwabe due to skill and instrument limitations, Kunitomo at least captured the growing trend of the spot activity in the early phase of the Solar Cycle 8. We also determine the solar rotation axis to estimate the accurate position (latitude and longitude) of the sunspot groups in Kunitomo's drawings., Comment: Main text 22 pages, reference 6 pages, 2 tables, and 12 figures. Figures with the original sunspot drawings are available only in the record version

The Maunder Minimum (1645–1715; hereafter MM) is generally considered as the only grand minimum in the chronological coverage of telescopic sunspot observations. Characterized by scarce sunspot occurrences and their asymmetric concentrations in the southern solar hemisphere, the MM has frequently been associated with a special state of solar dynamo activity. As such, it is important to analyze contemporary observational records and improve our understanding of this peculiar interval, whereas the original records are frequently preserved in historical archives and can be difficult to access. In this study, we consult historical archives in the National Library of Russia, St. Petersburg, and analyze a series of sunspot observations conducted at the Eimmart Observatory from 1681 to 1709, which is the second-richest sunspot data set produced during the MM, following La Hire's series, among existing data sets. We have further extended our analyses to neighboring observations to extend our investigations up to 1718. We first analyze source documents and descriptions of observational instruments. Our analyses have significantly revised the existing data set, removed contaminations, and updated and labeled them as Eimmart Observatory (78 days), Altdorf Observatory (4 days), Hoffmann (22 days), and Wideburg (25 days). The revisions have updated the temporal coverage of the contemporary sunspot observations from 73.4% to 66.9% from 1677 to 1709. We have also derived the positions of the observed sunspot groups in comparison with contemporary observations. Our results indicate hemispheric asymmetry in the MM and recovery of sunspot groups in both hemispheres after 1716, supporting the common paradigm of the MM.

We performed statistical analysis on the flare emission data to examine parameters related to the flare extreme-ultraviolet (EUV) spectra. This study used the data from the Geostationary Operational Environmental Satellite X-ray Sensors to determine the fundamental flare parameters. The relationship between soft X-ray data and EUV emission data observed by the Extreme Ultraviolet Variability Experiment on board the Solar Dynamics Observatory (SDO) MEGS-A was investigated for 50 events. The results showed the hotter Fe line emissions have strong correlation with soft X-ray data in many cases. However, our statistical study revealed that EUV flare peak flux of Fe xv, Fe xvi and He ii lines have weak correlation with soft X-ray peak flux. In EUV line light curves, there was time difference in peak time, however the tendency to reach the peak in order from the hotter line to cooler line was not so clear. These results indicate that the temporal evolution of EUV emission can be roughly explained by soft X-ray data. However, the time changes of temperature and density distributions in the flare loop must be needed for accurate reproduction. Moreover, we compared the geometrical features of solar flares observed by the Atmospheric Imaging Assembly on board the SDO with the fundamental flare parameters for 32 events. The ribbon distance strongly correlated with both soft X-ray flare rise and decay times. This results indicate that the geometrical feature is essential parameter for predicting flare emission duration.

In addition to X-rays, extreme ultraviolet (EUV) rays radiated from solar flares can cause serious problems, such as communication failures and satellite drag. Therefore, methods for forecasting EUV dynamic spectra during flares are urgently required. Recently, however, owing to the lack of instruments, EUV dynamic spectra have rarely been observed. Hence, we develop a new method that converts the soft X-ray light curve observed during large flare events into an EUV dynamic spectrum by using the Solar Dynamics Observatory/Atmospheric Imaging Assembly images, a numerical simulation, and atomic database. The simulation provides the solution for a coronal loop that is heated by a strong flare, and the atomic database calculates its dynamic spectrum, including X-ray and EUV irradiances. The coefficients needed for the conversion can be calculated by comparing the observed soft X-ray light curve with that of the simulation. We apply our new method to three flares that occurred in the active region 12673 on September 06, 2017. The results show similarities to those of the Flare Irradiance Spectral Model, and reconstruct some of the EUV peaks observed by the EUV Variability Experiment onboard the Solar Dynamics Observatory.

We have investigated an M1.3 limb flare, which develops as a magnetic loop/arch that fans out from an X-ray jet. Using Hinode/EIS, we found that the temperature increases with height to a value of over 10$^{7}$ K at the loop-top during the flare. The measured Doppler velocity (redshifts of 100$-$500 km s$^{-1}$) and the non-thermal velocity ($\geq$100 km s$^{-1}$) from Fe XXIV also increase with loop height. The electron density increases from $0.3\times10^{9}$ cm$^{-3}$ early in the flare rise to $1.3\times10^{9}$ cm$^{-3}$ after the flare peak. The 3-D structure of the loop derived with STEREO/EUVI indicates that the strong redshift in the loop-top region is due to upflowing plasma originating from the jet. Both hard X-ray and soft X-ray emission from RHESSI were only seen as footpoint brightenings during the impulsive phase of the flare, then, soft X-ray emission moves to the loop-top in the decay phase. Based on the temperature and density measurements and theoretical cooling models, the temperature evolution of the flare arch is consistent with impulsive heating during the jet eruption followed by conductive cooling via evaporation and minor prolonged heating in the top of the fan loop. Investigating the magnetic field topology and squashing factor map from SDO/HMI, we conclude that the observed magnetic-fan flaring arch is mostly heated from low atmospheric reconnection accompanying the jet ejection, instead of from reconnection above the arch as expected in the standard flare model., 55 pages, 21 Figures, accepted for publication in ApJ

As we are heading towards the next solar cycle, presumably with a relatively small amplitude, it is of significant interest to reconstruct and describe the past grand minima on the basis of actual observations of the time. The Dalton Minimum is often considered one of the grand minima captured in the coverage of telescopic observations. Nevertheless, the reconstructions of the sunspot group number vary significantly, and the existing butterfly diagrams have a large data gap during the period. This is partially because most long-term observations have remained unexplored in historical archives. Therefore, to improve our understanding on the Dalton Minimum, we have located two series of Thadd\"aus Derfflinger's observational records (a summary manuscript and logbooks) as well as his Brander's 5.5-feet azimuthal-quadrant preserved in the Kremsm\"unster Observatory. We have revised the existing Derfflinger's sunspot group number with Waldmeier classification and eliminated all the existing 'spotless days' to remove contaminations from solar meridian observations. We have reconstructed the butterfly diagram on the basis of his observations and illustrated sunspot distributions in both solar hemispheres. Our article aims to revise the trend of Derfflinger's sunspot group number and to bridge a data gap of the existing butterfly diagrams around the Dalton Minimum. Our results confirm that the Dalton Minimum is significantly different from the Maunder Minimum, both in terms of cycle amplitudes and sunspot distributions. Therefore, the Dalton Minimum is more likely a secular minimum in the long-term solar activity, while further investigations for the observations at that time are required., Comment: main text 17 pages, references 7 pages, and 8 figures; accepted for publication in the ApJ

The Ninth Symposium on Polar Science/Ordinary sessions: [OS] Space and upper-atmosphere sciences, Wed. 5 Dec. / 3F Seminar room D304, Institute of Statistics and Mathematics

We studied the solar surface flows (differential rotation and meridional circulation) using a magnetic element feature tracking technique by which the surface velocity is obtained using magnetic field data. We used the line-of-sight magnetograms obtained by the Helioseismic and Magnetic Imager aboard the Solar Dynamics Observatory from 01 May 2010 to 16 August 2017 (Carrington rotations 2096 to 2193) and tracked the magnetic element features every hour. Using our method, we estimated the differential rotation velocity profile. We found rotation velocities of $\sim$ 30 and -170 m s$^{-1}$ at latitudes of 0$^{\circ}$ and 60$^{\circ}$ in the Carrington rotation frame, respectively. Our results are consistent with previous results obtained by other methods, such as direct Doppler, time-distance helioseismology, or cross correlation analyses. We also estimated the meridional circulation velocity profile and found that it peaked at $\sim$12 m s$^{-1}$ at a latitude of 45$^{\circ}$, which is also consistent with previous results. The dependence of the surface flow velocity on the magnetic field strength was also studied. In our analysis, the magnetic elements having stronger and weaker magnetic fields largely represent the characteristics of the active region remnants and solar magnetic networks, respectively. We found that magnetic elements having a strong (weak) magnetic field show faster (slower) rotation speed. On the other hand, magnetic elements having a strong (weak) magnetic field show slower (faster) meridional circulation velocity. These results might be related to the Sun's internal dynamics., 12 pages, 4 figures, accepted to ApJ Letters. arXiv admin note: text overlap with arXiv:1010.1242 by other authors

Sunspot observations with telescopes in 18th century were carried out in Japan as well. One of these sunspot observations is recorded in an account called Sansaizusetsu narabini Kansei irai Jissoku Zusetsu (Charts of Three Worlds and Diagrams of Actual Observations since Kansei Era). We analyze manuscripts of this account to show in total 15 sunspot drawings in 1749-1750. These observations were carried out by contemporary official astronomers in Japan, with telescopes covered by zongurasus (< zonglas in Dutch, corresponding to "sunglass" in English). We count their group number of sunspots to locate them in long-term solar activity and show that their observations were situated around the solar maximum in 1749 or 1750. We also computed their locations and areas, while we have to admit the difference of variant manuscripts with one another. These observational records show the spread of sunspot observations not only in Europe but also in Japan and hence may contribute to crosscheck or possibly improve the known sunspot indices., 2018/4/12 accepted for publication in PASJ

Particles are accelerated to very high, non-thermal energies in solar and space plasma environments. While energy spectra of accelerated electrons often exhibit a power law, it remains unclear how electrons are accelerated to high energies and what processes determine the power-law index $\delta$. Here, we review previous observations of the power-law index $\delta$ in a variety of different plasma environments with a particular focus on sub-relativistic electrons. It appears that in regions more closely related to magnetic reconnection (such as the `above-the-looptop' solar hard X-ray source and the plasma sheet in Earth's magnetotail), the spectra are typically soft ($\delta \gtrsim$ 4). This is in contrast to the typically hard spectra ($\delta \lesssim$ 4) that are observed in coincidence with shocks. The difference implies that shocks are more efficient in producing a larger non-thermal fraction of electron energies when compared to magnetic reconnection. A caveat is that during active times in Earth's magnetotail, $\delta$ values seem spatially uniform in the plasma sheet, while power-law distributions still exist even in quiet times. The role of magnetotail reconnection in the electron power-law formation could therefore be confounded with these background conditions. Because different regions have been studied with different instrumentations and methodologies, we point out a need for more systematic and coordinated studies of power-law distributions for a better understanding of possible scaling laws in particle acceleration as well as their universality., Comment: 67 pages, 15 figures; submitted to Space Science Reviews; comments welcome

Plasma in the solar corona is believed to be in thermal equilibrium because of the occurrence of weak Coulomb collisions. To date, many studies have discussed the plasma dynamics in the solar corona assuming thermal equilibrium. Most phenomena observed in the solar corona can be explained under this assumption because the available temporal resolution is not sufficient to resolve non-equilibrium conditions. After Hinode was launched, a very high temporal resolution became available, especially for spectroscopic observation. Now, we can discuss plasma heating or acceleration in the solar corona using extreme-ultraviolet spectroscopic observations with high time resolution. Further, we can also observe the solar corona at multiple wavelengths with high spectral resolution. Owing to Hinode observations, we can now discuss plasma dynamics under thermal non-equilibrium conditions, such as non-equilibrium ionization. Recently, thermal non-equilibrium plasma has received attention not only in the field of solar physics but also in other fields such as X-ray astronomy.