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We developed an automated method for sunspot detection using digital white-light solar images to achieve a performance similar to that of visual drawing observations in sunspot counting. To identify down to small, isolated spots correctly, we pay special attention to the accurate derivation of the quiet-disk component of the Sun, which is used as a reference to identify sunspots using a threshold. This threshold is determined using an adaptive method to process images obtained under various conditions. To eliminate the seeing effect, our method can process multiple images taken within a short time. We applied the developed method to digital images captured at three sites and compared the detection results with those of visual observations. We conclude that the proposed sunspot detection method has a similar performance to that of visual observation. This method can be widely used by public observatories and amateurs as well as professional observatories as an alternative to hand-drawn visual observation for sunspot counting., "Solar Physics", accepted. 24 pages, 10 figures

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.

We carried out polarimetric observations of the white-light corona during the total solar eclipses that occurred on 2017 August 21 and 2019 July 2, and successfully obtained data at two different sites for both eclipses. After eliminating the sky background, we derived the brightness, polarization brightness, and degree of the polarization of the K+F corona from just above the limb to approximately 4 $R_\odot$. Furthermore, we isolated the K- and F-corona with plausible degree of polarization of the K-corona. The field of view covering up to approximately 4 $R_\odot$ enabled us to compare the derived brightness and polarization with a wide range of other observations. The results of the comparison show significant scatter; while some of the observations present very good coincidence with our results, the other ones exhibit systematic discrepancy., Comment: 25 pages, 11 figures

This study presents the results of a quantitative analysis of IRmag (infrared Stokes spectro-polarimeter, National Astronomical Observatory of Japan, (NAOJ)/Mitaka) observations in three spectral lines Fe i 1564.8 nm, Si i 1082.72 nm, He i 1083.0 nm, and comparison between IRmag magnetograms and ones from the Solar Dynamic Observatory/Helioseismic and Magnetic Imager (SDO/HMI), Global Oscillation Network Group (GONG), Wilcox Solar Observatory (WSO), and Solar Telescope for Operative Predictions (STOP). It is shown that there is a good correlation between observations in all three IRmag spectral lines, but with significant differences in the measured field strengths caused by different depths of formation and, probably, by calibration issues. The correlation of IRmag magnetograms with ones from other observatories, and between different observatories among each other, is as good as 0.70 – 0.95, depending on the combination, but nevertheless there are significant systematic differences (linear-regression coefficients deviating from unity) between different data sets (up to a factor of three or even more).

Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited., Context. Studies of long-term solar activity and variability require knowledge of the past evolution of the solar surface magnetism. The archives of full-disc Ca II K observations that have been performed more or less regularly at various sites since 1892 can serve as an important source of such information. Aims. We derive the plage area evolution over the last 12 solar cycles by employing data from all Ca II K archives that are publicly available in digital form, including several as-yet-unexplored Ca II K archives. Methods. We analysed more than 290 000 full-disc Ca II K observations from 43 datasets spanning the period between 1892-2019. All images were consistently processed with an automatic procedure that performs the photometric calibration (if needed) and the limb-darkening compensation. The processing also accounts for artefacts affecting many of the images, including some very specific artefacts, such as bright arcs found in Kyoto and Yerkes data. Our employed methods have previously been tested and evaluated on synthetic data and found to be more accurate than other methods used in the literature to treat a subset of the data analysed here. Results. We produced a plage area time-series from each analysed dataset. We found that the differences between the plage areas derived from individual archives are mainly due to the differences in the central wavelength and the bandpass used to acquire the data at the various sites. We empirically cross-calibrated and combined the results obtained from each dataset to produce a composite series of plage areas. The 'backbone' approach was used to bridge the series together. We have also shown that the selection of the backbone series has little effect on the final composite of the plage area. We quantified the uncertainty of determining the plage areas with our processing due to shifts in the central wavelength and found it to be less than 0.01 in fraction of the solar disc for the average conditions found on historical data. We also found the variable seeing conditions during the observations to slightly increase the plage areas during the activity maxima. Conclusions. We provide the most complete so far time series of plage areas based on corrected and calibrated historical and modern Ca II K images. Consistent plage areas are now available on 88% of all days from 1892 onwards and on 98% from 1907 onwards. © T. Chatzistergos et al. 2020., The authors thank the observers at the Arcetri, Baikal, Big Bear, Brussels, Calern, Catania, Coimbra, Kanzelhohe, Kharkiv, Kenwood, Kislovodsk, Kodaikanal, Kyoto, Manila, Mauna Loa, McMathHulbert, Mees, Meudon, Mitaka, European Union's Horizon 2020 research and innovation program Mt Wilson, Pic du Midi, Rome, Sacramento Peak, San Fernando, Schauinsland, Teide, Upice, Valasske Mezi.ri.ci, Wendelstein, and Yerkes sites for all their work in carrying out the observing programs. We thank Isabelle Buale for all her e fforts to digitise the Meudon archive. We thank Je ff Kuhn and Cindy Maui for locating and sharing with us the Mees Ca II K data. We thank Satoru Ueno and Reizaburo Kitai for providing the Kyoto observations. We thank the anonymous referee for the constructive comments that improved this manuscript. We thank Ester Antonucci, Alexi Baker, Angie Cookson, Martina Exnerova, Bernhard Fleck, Detlef Groote, Laurent Koechlin, Libor Lenza, Mustapha Meftah, Werner Potzi, Roger Ulrich, John Varsik, and Hubertus Wohl for providing information about various Ca II K data. This work was supported by FP7 SOLID, and by the BK21 plus program through the National Research Foundation (NRF) funded by the Ministry of Education of Korea. This research has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 824135 (SOLARNET). The Coimbra researchers thank the project ReNATURE (CENTRO-01-0145-FEDER-000007-BPD16). We acknowledge the "Observateurs associes" for their commitment to image acquisition and processing; IRAP for the instrumental and database management; Universite de Toulouse, CNRS, and Fiducial for the funding. We acknowledge Paris Observatory for the use of spectroheliograms and the Royal Observatory of Belgium, Brussels for USET data. The Kanzelhohe Ca II K data were provided by the Kanzelhohe Observatory, University of Graz, Austria. ChroTel is operated by the Kiepenheuer-Institute for Solar Physics in Freiburg, Germany, at the Spanish Observatorio del Teide, Tenerife, Canary Islands. The ChroTel filtergraph has been developed by the Kiepenheuer-Institute in cooperation with the High Altitude Observatory in Boulder, CO, USA. We acknowledge www.observethesun.com and www.solarstation.ru for storing the Kislovodsk data. This work used data provided by the MEDOC data and operations centre (CNES/CNRS/Univ. Paris-Sud). The Kenwood observations used here are from lantern slides in the Division of History of Science and Technology at Yale University's Peabody Museum of Natural History (objects YPM HST.340744, HST.340745, HST.340747, and HST.340752). The Yerkes observations are courtesy of the University of Chicago Photographic Archive, Special Collections Research Center, University of Chicago Library. This research has made use of NASA's Astrophysics Data System.

The solar group at the National Astronomical Observatory of Japan is conducting synoptic solar observations with the Solar Flare Telescope. While it is a part of a long-term solar monitoring, contributing to the study of solar dynamo governing solar activity cycles, it is also an attempt at contributing to space weather research. The observations include imaging with filters for H$\alpha$, Ca K, G-band, and continuum, and spectropolarimetry at the wavelength bands including the He I 1083.0 nm / Si I 1082.7 nm and the Fe I 1564.8 nm lines. Data for the brightness, Doppler signal, and magnetic field information of the photosphere and the chromosphere are obtained. In addition to monitoring dynamic phenomena like flares and filament eruptions, we can track the evolution of the magnetic fields that drive them on the basis of these data. Furthermore, the magnetic field in solar filaments, which develops into a part of the interplanetary magnetic cloud after their eruption and occasionally hits the Earth, can be inferred in its pre-eruption configuration. Such observations beyond mere classical monitoring of the Sun will hereafter become crucially important from the viewpoint of the prediction of space weather phenomena. The current synoptic observations with the Solar Flare Telescope is considered to be a pioneering one for future synoptic observations of the Sun with advanced instruments., Comment: 18 pages, 6 figures

Total solar eclipses are popular targets for amateur astronomers. At the same time, the eclipses are still scientifically important to observe the solar corona. Therefore, the eclipses are a good chance for amateurs to participate in scientific observations. In fact, some of amateur astronomers in Japan have been carried out scientific observations at the total solar eclipses collaborating with professional solar scientists over more than ten years. Some scientific results have been produced from the collaboration. We present here our collaborative activities as a practical example of citizen science.

We have carried out a statistical study of the average orientation of the magnetic field in solar filaments with respect to their axes for more than 400 samples, based on data taken with daily full-Sun, full-Stokes spectropolarimetric observations using the He I 1083.0 nm line. The major part of the samples are the filaments in the quiet areas, but those in the active areas are included as well. The average orientation of the magnetic field in filaments shows a systematic property depending on the hemisphere; the direction of the magnetic field in filaments in the northern (southern) hemisphere mostly deviates clockwise (counterclockwise) from their axes, which run along the magnetic polarity inversion line. The deviation angles of the magnetic field from the axes, are concentrated between 10--30 deg. This hemispheric pattern is consistent with that revealed for chirality of filament barbs, filament channels and for other solar features found to possess chirality. For some filaments it was confirmed that their magnetic field direction is locally parallel to their structure seen in Halpha images. Our results for the first time confirmed this hemispheric pattern with the direct observation of the magnetic field in filaments. Interestingly, the filaments which show the opposite magnetic field deviation to the hemispheric pattern, are in many cases found above the polarity inversion line whose ambient photospheric magnetic field has the polarity alignment being opposite to that of active regions following the Hale-Nicholson law.

Section 1 of the FM14 focus on bridging the astronomy research and outreach communities - recent highlights, emerging collaborations, best practices and support structures. This paper also contains supplementary materials that point to contributed talks and poster presentations that can be found online.

We performed statistical and event studies of linear polarization in the H$\alpha$ line during solar flares. The statistical study revealed that, among 71 H$\alpha$ flares analyzed, including 64 GOES flares, only one event shows significant linear polarization signals. Such an infrequent occurrence of significant linear polarization in solar flares is consistent with the result by Bianda et al. (2005), who studied 30 flares and found no polarization signals. In the event showing the significant polarization, the maximum degree of linear polarization was 1.16$\pm$0.06\%, and the average direction of the polarization deviated by -142.5$\pm$6.0 degrees from the solar north. The observed polarization degrees and the directions are consistent with the preceding reports (e.g. Henoux et al. 1990; Emslie et al. 2000; Hanaoka 2003). These strong linear polarization signals did not appear at major flare ribbons, nor did they correlate with either hard or soft X-ray emissions temporally or spatially. Instead they appeared at a minor flare kernel, which corresponds to one of the footpoints of a coronal loop. The active region caused coronal dimming after the soft X-ray peak. The observed flare show no direct evidence that the linear polarization is produced by high energy particles, which are often considered to generate the polarization. On the other hand, our study suggests the possibility that coronal mass ejections, which have been often observed in flares showing linear polarization signals, play an important role for exciting linear polarization at H$\alpha$ flare kernels., Comment: 12 pages, 6 Figures, and 1 Table. Accepted by ApJ

We investigate the eruption of a quiescent filament located close to an active region. Large-scale activation was observed in only half of the filament in the form of pre-eruption oscillations. Consequently only this half erupted nearly 30 hr after the oscillations commenced. Time-slice diagrams of 171 \AA\ images from Atmospheric Imaging Assembly were used to study the oscillations. The oscillations were observed in several thin and long features connecting the filament spine to the chromosphere below. This study traces the origin of such features and proposes their possible interpretation. Small-scale magnetic flux cancellation accompanied by a brightening was observed at the footpoint of the features shortly before their appearance, in images recorded by the Helioseismic and Magnetic Imager. Slow rise of the filament was detected in addition to the oscillations, indicating a gradual loss of equilibrium. Our analysis indicates that a change in magnetic field connectivity between two neighbouring active regions and the quiescent filament resulted in weakening of the overlying arcade of the filament leading to its eruption. It is also suggested that the oscillating features are filament barbs, and the oscillations are a manifestation during the pre-eruption phase of filaments., Comment: in press Astrophys. J

An adaptive optics (AO) system is developed for the 60cm domeless solar telescope of the Hida Observatory, Japan. Its performances are analyzed by the computer simulations, and improved by replacing the Zernike polynomials by Karhunen-Loeve functions. Also, a tomographic wavefront sensor is developed for a ground-layer AO system. From test data acquired at the Hida observatory, wavefront-phase maps both in the ground-layer and in an upper layer are successfully derived.

Differential Image Motion Monitors (DIMMs) are used not only for stellar observations but also for solar observations with the limb as the target to evaluate the seeing. In stellar observations, the differential image motions along and perpendicular to the DIMM baseline are consistently employed to evaluate Fried’s parameter. However this is not the case for solar-limb observations, in which we can measure the image motion only perpendicular to the limb. Therefore the validity of the methods so far proposed to calculate Fried’s parameter from a DIMM using the solar limb is still open to examination. We have focused on this problem and carried out DIMM observations of the solar limb using a Shack–Hartmann wavefront sensor of the Domeless Solar Telescope at Hida Observatory. Pairs of apertures in the Shack–Hartmann sensor act as multiple DIMMs that have different baselines in distance and orientation. We calculated Fried’s parameters based on three evaluating methods (those of Sarazin & Roddier, Sasiela and Conan et al.) and found that the method of Sarazin & Roddier gives a consistent value of Fried’s parameter over the set of multiple DIMMs.

Coronal jets, which extend from the solar surface to beyond 2 $R_\odot$, were observed in the polar coronal hole regions during the total solar eclipse on 2017 August 21. In a time-series of white-light images of the corona spanning 70 minutes taken with our multi-site observations of this eclipse, six jets were found as narrow structures upwardly ejected with the apparent speed of about 450 km s$^{-1}$ in polar plumes. On the other hand, extreme-ultraviolet (EUV) images taken with the Atmospheric Image Assembly of the Solar Dynamics Observatory show that all of the eclipse jets were preceded by EUV jets. Conversely, all the EUV jets whose brightness is comparable to ordinary soft X-ray jets and which occurred in the polar regions near the eclipse period were observed as eclipse jets. These results suggest that ordinary polar jets generally reach high altitudes and escape from the Sun as part of the solar wind., Accepted in ApJ, 14 pages, 6 figures, 2 tables

A solar adaptive optics system has been improved by using a high-speed electromagnetic deformable mirror and adopting a modified sum-of-absolute-differences algorithm in wavefront sensing. Results of laboratory experiments clearly show that the use of the mirror raises the temporal performance of the system. In solar observations, wavefront compensation using solar granules as a target is realized.

We developed a spectropolarimetry system with a ferroelectric liquid crystal polarization modulator for the 60-cm Domeless Solar Telescope of Hida Observatory. The telescope is equipped with two oblique-reflection mirrors, that alter the incident polarization before it reaches the polarimeter. Furthermore, it is impossible to calibrate the polarimeter using unpolarized light because of the instrumental polarization. Here, we develop a procedure to obtain the parameters of both the instrumental polarization and for the calibration of the polarimeter from the same calibration data sets. We apply the new procedure to carry out high-sensitivity spectropolarimetry. It is now possible to measure the polarizations of various photospheric and chromospheric lines with the Domeless Solar Telescope.

We developed a high-sensitivity imaging polarimeter using ferroelectric liquid crystals, and installed it in the Solar Flare Telescope to measure the polarization in the H α line and the Fe I 6303 line simultaneously. Now we are operating regular polarimetry observations for both the photosphere and the chromosphere with this advanced system. It will enable us to study the three-dimensional evolution of the vector magnetic field of active regions, which is essentially important to study active phenomena on the Sun. A brief description of the instrument and an example of the polarimetry data taken in the H α and the Fe I 6303 lines simultaneously are presented in this paper.

After a little more than forty years of work related to the interplanetary plasma and the heliosphere the IAU's Commission 49 was formally discontinued in 2015. The commission started its work when the first spacecraft were launched to measure the solar wind in–situ away from Earth orbit, both inward and outward from 1 AU. It now hands over its activities to a new commission during an era of space research when Voyager 1 measures in–situ the parameters of the local interstellar medium at the edge of the heliosphere. The commission will be succeeded by C.E3 with a similar area of responsibility but with more focused specific tasks that the community intends to address during the coming several years. This report includes a short description of the motivation for this commission and of the historical context. It then describes work from 2012 to 2015 during the present solar cycle 24 that has been the weakest in the space era so far. It gave rise to a large number of studies on solar energetic particles and cosmic rays. Other studies addressed e.g. the variation of the solar wind structure and energetic particle fluxes on long time scales, the detection of dust in the solar wind and the Voyager measurements at the edge of the heliosphere. The research is based on measurements from spacecraft that are at present operational and motivated by the upcoming Solar Probe + and Solar Orbiter missions to explore the vicinity of the Sun. We also report here the progress on new and planned radio instruments and their importance for heliospheric studies. Contributors to this report are Carine Briand, Yoichiro Hanaoka, Eduard Kontar, David Lario, Ingrid Mann, John D. Richardson.

We present a study of 10 microflares observed in 4–30 keV by SOXS mission simultaneously with Hα observations made at NAOJ, Japan during the interval between February and August 2004. The X-ray and Hα light curves showed that the lifetime of microflares varies between 4 and 25 min. We found that the X-ray emission in all microflares under study in the dynamic energy range of 4–30 keV can be fitted by thermal plus non-thermal components. The thermal spectrum appeared to start from almost 4 keV, low level discriminator (LLD) of both Si and CZT detectors, however it ends below 8 keV. We also observed the Fe line complex features at 6.7 keV in some microflares and attempted to fit this line by isothermal temperature assumption. The temperature of isothermal plasma of microflares varies in the range between 8.6 and 10.1 MK while emission measure between 0.5 and 2x1049 cm-3. Non-thermal (NT) emission appeared in the energy range 7–15 keV with exponent -6.8 ≤γ≤-4.8. Our study of microflares that had occurred on 25 February 2004 showed that sometimes a given active region produces recurrent microflare activity of a similar nature. We concluded from X-ray and simultaneous Hα observations that the microflares are perhaps the result of the interaction of low lying loops. It appears that the electrons that accelerated during reconnection heat the ambient coronal plasma as well as interact with material while moving down along the loops and thereby produce Hα bright kernels.

We carried out an imaging polarimetry observation of active region NOAA 10596 at 9–11 wavelengths around the Hα line with a ferroelectric liquid-crystal polarimeter during 2004 April 20–26. The latitude of the region was S8 ◦ , and it passed the central meridian on April 23. On the basis of this observation, the Stokes V/ Ifeatures of the active region in Hα, which are expected to indicate the chromospheric longitudinal magnetic field, were revealed as follows: (1) There is persistent weakening (sometimes reversal) of the Stokes V/ I signal at the center of the umbra. This is not due to the actual weakening of the magnetic field strength, but probably due to the peculiarity of the atmospheric structure of the umbral chromosphere. (2) The peak wavelengths of the Stokes V/ Iprofile of the plages and that of the sunspot are systematically different from each other and, therefore, it is difficult to apply the weak-field approximation to estimate the chromospheric magnetic field strength. Both of these results mean that an estimation of the chromospheric magnetic field on the basis of the Hα polarimetry is not straightforward, and that an inversion considering the atmospheric structure in the chromosphere is essentially important.

We developed a polarimeter with ferroelectric liquid crystals (FLCs) to observe polarization of flare kernels in the Hα line. Polarization is one of the important diagnostics of the high-energy particles in solar flares, and high-cadence imaging polarimetry with the precision of the order of 0.1% is required to observe the polarization of flare kernels. However, to achieve such high precision is difficult mainly due to the seeing-induced polarization error, which particularly appears around the flare kernels, because the brightness gradient is steep there. To reduce the seeing-induced error, a high modulation frequency is required, and our new polarimeter based on the combination of a high-speed CCD camera and FLCs realized high-frequency polarization modulation nearly 250 Hz. We evaluated the polarization error, and confirmed that the error was significantly reduced with the new polarimeter. We concluded that the polarimeter with FLCs meets the requirement of solar flare polarimetry.

The Hα observation is a powerful tool to study the high-energy aspect of solar flares. Spiky brightenings of flare kernels at the Hα center reflect the rapid fluctuation in particle acceleration; linear polarization of Hα emission might be evidence of accelerated protons; red-shifts of the Hα line are caused by the chromospheric evaporation. To study the spiky brightenings of flare kernels with high-cadence imaging at the Hα center, a high-speed Hα camera for the Solar Flare Telescope at Mitaka, NAOJ, had been developed and it started the regular observation in 2001 July. However, the polarimetry and the Dopplermetry are also important and they are required to be carried out in parallel with the high-cadence imaging at the Hα center. Then, we upgraded the original high-speed Hα camera to a new Hα camera system for the multi-aspect Hα observations, which performs all of the high-cadence imaging, the linear polarization measurements, and the off-band imaging for velocity measurements. The new system started the observation in 2002 July. In this paper, the multi-aspect Hα imaging system is described and sample Hα images are presented.

We are developing a new adaptive optics (AO) system for the 60cm domeless solar telescope of the Hida Observatory, Japan. The system has a deformable mirror with 97 piezo-actuators, a Shack-Hartmann wavefront sensor with a 10×10-microlens array and standard personal computers. We conducted solar observations in September, 2013, and confirmed that our AO system cancelled image-shifts so that the deviations were within the resolution of the telescope. We report the detailed performances of our new AO system.

Observationally derived three-dimensional magnetic loop structures of solar flares tell us what type of magnetic configurations is actually responsible for the reconnection/energy release in solar flares. So far, the loop configuration has been discussed on a rather schematic basis, because of insufficient data coverage. However, now we have data obtained with various advanced instruments, which are essentially important to define the three-dimensional magnetic configuration of flare loops. In this paper, we briefly present how the loop configuration of the solar flares can be determined based on recent advanced observations. We focus on the flares caused by emerging flux activity in region NOAA 8100, which appeared in October–November 1997. We present here the presumed quadruple structure of the magnetic loops of the analyzed flares in three dimensions, and suggest that the emergence of small loops can give rise to large-scale phenomena like jets and CMEs.

We present observations of a coronal mass ejection (CME) associated with an eruptive prominence during 1998 June 21-22 by LASCO (Large Angle Spectroscopic Coronagraph) aboard SOHO (Solar and Heliospheric Observatory). Various features in the three-part structured, white-light CME as observed by LASCO-C2 and C3 coronagraphs were compared with features in the other wavelengths, for example, in Fe XIV and Fe X emission lines obtained from LASCO C1, in Hα from Helio-Research and at 17 GHz obtained from Nobeyama Radioheliograph. We have investigated conditions in several data sets to understand the eruptive and the pre-eruptive scenario of the CME. The CME and the eruptive prominence accelerate up to ~20 R☉ and then decelerate to the velocity of the ambient slow solar wind. The analysis clearly shows that this particular CME is a typical case of a very slow or gradual CME for which it is difficult to define an exact onset time. The CME could be tracked for about 30 hours until it crossed a distance of 30 R☉ and disappeared from the field of view of the LASCO-C3 coronagraph. The height-time profiles of various features of this CME suggest that the leading edge of the CME and the top of the prominence or the core follow similar pattern, implying a common driver for both the CME and the eruptive prominence. The observations provide strong evidence that the CME and the prominence eruption resulted from a common cause which is the global restructuring of the magnetic field in the corona in an extensive volume of space near and including the CME. The restructuring in turn was a consequence of newly emerging flux regions near and within the neighboring active regions close to the base of the CME.

We analyze the three-dimensional geometry of solar flares that show so-called interacting flare loops in soft X-ray, hard X-ray, and radio emission, as previously identified by Hanaoka and Nishio. The two flare loops that appear brightest after the flare are assumed to represent the outcome of a quadrupolar magnetic reconnection process, during which the connectivity of magnetic polarities is exchanged between the four loop footpoints. We parameterize the three-dimensional geometry of the four involved magnetic field lines with circular segments, additionally constrained by the geometric condition that the two pre-reconnection field lines have to intersect each other at the onset of the reconnection process, leading to a 10 parameter model. We fit this 10 parameter model to Yohkoh Soft and Hard X-Ray Telescopes (SXT and HXT) data of 10 solar flares and determine in this way the loop sizes and relative orientation of interacting field lines before and after reconnection. We apply a flare model by Melrose to calculate the magnetic flux transfer and energy released when two current-carrying field lines reconnect to form a new current-carrying system in a quadrupolar geometry. The findings and conclusions are the following. (1) The pre-reconnection field lines always show a strong asymmetry in size, consistent with the scenario of newly emerging small-scale loops that reconnect with preexisting large-scale loops. (2) The relative angle between reconnecting field lines is nearly collinear in half of the cases, and nearly perpendicular in the other half, contrary to the antiparallel configuration that is considered to be most efficient for magnetic reconnection. (3) The angle between interacting field lines is reduced by ≈10°-50° after quadrupolar reconnection. (4) The small-scale flare loop experiences a shrinkage by a factor of 1.31 ± 0.44, which is consistent with the scaling law found from previous electron time-of-flight measurements, suggesting that electron acceleration occurs near the cusp of quadrupolar configurations. (5) The large-scale loop is found to dominate the total induction between current-carrying loops, providing a simple estimate of the maximum magnetic energy available for flare energy release because of current transfer, which scales as ΔEI ≈ 1029.63(r2/109 cm)(I2/1011A)2 (with r2 the curvature radius and I2 the current of the large-scale loop) and is found to correlate with observed flare energies deduced from soft X-ray and hard X-ray fluxes. Most of the energy is transferred to small-scale loops that have one-half of the large-scale current (I1 = I2/2). (6) The quadrupolar reconnection geometry provides also a solution of Canfield's dilemma of the offset between the maximum of vertical currents and the HXR flare loop footpoints. (7) The quadrupolar geometry provides not only a framework for interacting double-loop flares, but it can also be considered as a generalized version of (cusp-shaped) single-loop flares.

We have found evidence that heating of erupting prominences occurs at the beginning of two events on 1992 November 5 and 1994 February 20, which were observed with the Nobeyama Radioheliograph at 17 GHz. Both of the events are eruptions of prominences on the disk, which are seen as eruptions of dark filaments. The radio images of the events show very similar changes. Shortly after the filaments begin to be accelerated rapidly and detach from the disk, the filaments apparently disappear. However, when the moving filaments pass above bright plage regions, the plage regions become dark. These facts indicate that the brightness temperature of the dark filaments increases to the temperature of the quiet disk at the beginning of the abrupt eruptions, while the filaments keep their large optical thickness. After the rapid increase, the brightness temperature does not change significantly during the motion of the filaments. In contrast to Hα, the visibility of prominences in 17 GHz images is not affected by Doppler shifts. Therefore, changes in the brightness temperature reflect changes in the physical parameters of prominences. Heating of the surface of the cool core of the prominences up to the transition-region temperature is the most plausible explanation of the observed increase in the brightness temperature at 17 GHz.

We have studied the time evolution of Bragg crystal spectrometer spectra of He-like Ca XIX and Fe XXV ions observed by the Yohkoh satellite for a solar flare on 1992 September 6 at 05:05 UT (M2.4 class). Electron temperatures are derived through a fit of synthetic spectra to the observed Ca XIX and Fe XXV spectra using new evaluated atomic data. Ion density ratios derived from spectra without assuming ionization equilibrium show a time-dependent, nonequilibrium ionization. The shift from equilibrium values indicates an ionizing plasma. For the first time the emission measures are derived from the spectra without assuming ionization equilibrium and solar abundances. Apparent ion temperatures are derived from the line widths of Ca XIX spectra. In the rising phase a blueshifted component is separated from the main component of the Ca XIX spectra. The time variation of these parameters are compared with hard X-rays, soft X-rays, and radio measurements. In the preheating phase, a moderate increase of the amount of thermal plasma is observed with turbulence indicated by the line width. The time evolution of the blueshifted component coincides with that of a burst in hard X-rays and microwaves which are produced by high-energy, nonthermal electrons.

The main aim of this paper is to estimate, from multispectral observations, the plasma parameters in a microwave burst source which was also the site of spike emission. This information is essential for the determination of the spike emission process. By analyzing one-dimensional source distributions observed with the SSRT at 5.7 GHz and correlating them with Yohkoh X-ray and Nobeyama 17 GHz images, we have concluded that the microwave emitting region was larger than the soft X-ray loop-top source, and that the origin of the burst could be explained by gyrosynchrotron emission of non-thermal electrons in a magnetic field of approximately 100 G. It has been shown that the source of 5.7 GHz spikes observed during the burst was located close to an SXR-emitting loop with high density and temperature and a relatively low magnetic field. Thus, plasma emission is the most favourable radiation mechanism for the generation of the sub-arc-second microwave pulses.

We analyzed several flares, which are presumed to be caused by interactions between an emerging loop and an overlying loop. We call such a basic combination of loops a ‘double-loop configuration’, and we reveal its topology on the basis of the microwave and soft X-ray observations of the flares and the magnetograms. In many cases, the magnetic field of the flare loops shows a ‘bipolar + remote unipolar’ structure, rather than a quadrapole structure. The footpoints of two loops are distributed in three magnetic patches, and two of the footpoints of the loops, one from the emerging loop and the other from the overlying loop, are included in a single magnetic polarity patch. Therefore, the two loops form a ‘three-legged’ structure, and the two loops are not anti-parallel as assumed in the traditional reconnection models. Typically, the emergence of a parasitic polarity near the major preceding-polarity region or the following one in an active region creates this configuration, but, in one of the analyzed flares, two active regions are involved in the configuration. Not only the flares, but various other active phenomena – microflares, thermal plasma flows like jets, and surges – occur in the same magnetic configuration. Hence, the interaction between two loops, which forms the three-legged structure, is an important source of the various types of activity.

We carried out white-light observations of the total solar eclipse on 13 November 2012 at two sites, where the totality occurred 35 minutes apart. We caught an ongoing coronal mass ejection (CME) and a pre-CME loop structure just before the eruption in the height range between 1-2 R_sun. The source region of CMEs was revealed to be in this height range, where the material and the magnetic field of CMEs were located before the eruption. This height range includes the gap between the extreme ultraviolet observations of the low corona and the spaceborne white-light observations of the high corona, but the eclipse observation shows that this height range is essentially important to study the CME initiation. The eclipse observation is basically just a snapshot of CMEs, but it indicates that future continuous observations of CMEs within this height range are promising., Comment: submitted to "Solar Physics"

Active region NOAA 7360 was observed in 1992 December with various instruments including the Yohkoh satellite. In this region, a small loop emerged near one of the footpoints of a pre-existing large coronal loop. These loops show evidence that interactions between coronal loops cause flares, microflares, and plasma flow. All of the four flares observed in this region show that brightenings in the small loop occurred first, and then the large loop flared up. The brightenings in the large loop can not occur by themselves, but must be triggered by the brightenings in the small loop. There must be interactions between the loops to cause these flares. As well as the flares, many microflares occurred in the small loop. More than half of them are accompanied by plasma ejection phenomena from the small loop into the large loop. The large loop is filled with ejected plasma with velocities of about 1000 km s−1. These ejection phenomena are considered as X-ray jets. The associated occurrences of the microflares and the jets suggest that they are also caused by interactions between the loops. The recurrent occurrences of the homologous flares and microflares mean that the magnetic field structure in this region inevitably causes the activity due to loop-loop interactions; the flares and jets occur under a common magnetic field structure.

Radio images and spectra of an eruptive prominence were obtained from simultaneous multifrequency observations at 36 GHz, 89 GHz, and 110 GHz on May 28, 1991 with the 45-m radio telescope at Nobeyama Radio Observatory (NRO), the National Astronomical Observatory, Japan (NAOJ). The radio spectra indicated that the optical depth is rather thick at 36 GHz whereas it is thin at 89 and 110 GHz. The Hα data, taken at Norikura Solar Observatory, NAOJ, suggest that the eruption of an active region filament was triggered by an Hα flare. The shape and position of the radio prominence generally coincided with those of Hα images. The radio emission is explained with an isothermal cool thread model. A lower limit for the electron temperature of the cool threads is estimated to be 6100 K. The range of the surface filling factors of the cool threads is 0.3–1.0 after the Hα flare, and 0.2–0.5 in the descending phase of the eruptive prominence. The column emission measure and the electron number density are estimated to be of the order of 1028 cm−5 and 1010 cm−3, respectively. The physical parameters of a quiescent prominence are also estimated from the observations. The filling factors of the eruptive prominence are smaller than those of the quiescent prominence, whereas the emission measures and the electron densities are similar. These facts imply that each cool thread of the prominence did not expand after the eruption, while the total volume of the prominence increased.

We developed a polarimeter for the ground-based solar observation using a high-speed rotating waveplate (typically 12.5–25 revolutions s −1 ) and a high-speed camera (typically 200–400 frames s −1 ) with commercially available devices. Fast polarization modulation is required for the ground-based solar polarimetry to avoid producing seeing-induced false polarization signals. Modulation with a high-speed rotating waveplate realizes not only fast full-Stokes modulation but also wide coverage of the wavelength range, and therefore, a polarimeter with a highspeed rotating waveplate is most suitable for the simultaneous polarimetry observations at multi-wavelengths with a spectrograph. A comprehensive description of the instrument and some results of the solar spectropolarimetry with this polarimeter are given in this paper.

Solar adaptive optics (AO) systems are developed at the 60cm domeless solar telescope in the Hida Observatory, Japan. An AO system currently used has a deformable mirror with high-speed 97 electromagnetic actuators and a Shack- Hartmann wavefront sensor with a 10x10-microlens array and 4000fps-CMOS camera. Its control frequency is about 1100-1400 Hz, and hence the -3dB cutoff frequency of the system is theoretically above 100 Hz. In parallel to developing the system, a new full-scaled AO system is designed to be applicable to various observations, such as highdispersion spectroscopy and simultaneous wide-range spectroscopy. The new system will work as classical AO at first. The details of the current system, observational results using it, and the design of the new AO system are described. © (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

The early phases of three flares, observed by the Nobeyama Radio Heliograph, are studied. Nonthermal and thermal radio sources are identified by comparison with soft X-ray images taken by the Soft X-ray Telescope onboard the Yohkoh satellite. Two of the flares are mainly of nonthermal origin and their location coincides with one of the footpoints of soft X-ray loops. Another flare has both thermal and nonthermal components which start to brighten simultaneously. This suggests that particle acceleration and plasma compression develop simultaneously.

We developed a practical method to derive response functions that convert the amount of incident light to the counts of analog-to-digital conversion (A/D) of cameras for scientific imaging. In this method, we need a mechanism to accurately control the amount of incident light into cameras just within a limited dynamic range and at a limited number of steps. A variable brightness light source, which supplies the incident light into cameras, is also necessary, but we do not need to know its accurate brightness. Thus, this method enables us to derive the nonlinear response functions accurately with such a simple setup.

We carried out a multi-wavelength study of a CME and a medium-size 1B/C1.3 flare occurring on 12 May 1997. We present the investigation of magnetic-field variations in the NOAA Active Region 8038 which was observed on the Sun during 7--16 May 1997. Analyses of H{\alpha} filtergrams and MDI/SOHO magnetograms revealed continual but discrete surge activity, and emergence and cancellation of flux in this active region. The movie of these magnetograms revealed two important results that the major opposite polarities of pre-existing region as well as in the emerging flux region (EFR) were approaching towards each other and moving magnetic features (MMF) were ejecting out from the major north polarity at a quasi-periodicity of about ten hrs during 10--13 May 1997. These activities were probably caused by the magnetic reconnection in the lower atmosphere driven by photospheric convergence motions, which were evident in magnetograms. The magnetic field variations such as flux, gradient, and sunspot rotation revealed that free energy was slowly being stored in the corona. The slow low-layer magnetic reconnection may be responsible for this storage and the formation of a sigmoidal core field or a flux rope leading to the eventual eruption. The occurrence of EUV brightenings in the sigmoidal core field prior to the rise of a flux rope suggests that the eruption was triggered by the inner tether-cutting reconnection, but not the external breakout reconnection. An impulsive acceleration revealed from fast separation of the H{\alpha} ribbons of the first 150 seconds suggests the CME accelerated in the inner corona, which is consistent with the temporal profile of the reconnection electric field. In conclusion, we propose a qualitative model in view of framework of a solar eruption involving, mass ejections, filament eruption, CME, and subsequent flare., Comment: 8 figures, accepted for publication in Solar Physics

We develop a solar adaptive optics system at the Hida Observatory in Japan. We report the details of the system and observation results. Solar images observed with the system demonstrate better contrast and finer structures.

The new Nobeyama Radioheliograph was completed in March 1992 after two years of construction. It is a T-shaped array operating as a multiple spacing grating-type radio interferometer at 17 GHz and is dedicated to full disk solar observations. Routine observations began in late June, 1992, after three months of system integration, fine tuning, and test observations. During the course of test observations it was shown that major items of the system performance exceeded the designed values, and that the image quality or the dynamic range of the images is better than the designed value. In the three months of routine observations two X-class flares, several M-class flares and a number of small flares were observed. In this report we present a summary of initial observational results and preliminary comparisons with YOHKOH HXT and SXT observations.

A solar adaptive optics system for a high-dispersion spectrograph is developed at the 60 cm domeless solar telescope of the Hida Observatory in Japan. Details of its optical setup are described for implementing a scanning slit spectroscopy with wavefront correction. A wavefront sensor used in the system is specified and a technique of reducing computational cost in wavefront sensing is also described. In solar observations, the improvement of contrast in images obtained with the adaptive optics system was demonstrated when a sunspot was used as a target of wavefront sensing.