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Routine ultraviolet imaging of the Sun's upper atmosphere shows the spectacular manifestation of solar activity; yet we remain blind to its main driver, the magnetic field. Here we report unprecedented spectropolarimetric observations of an active region plage and its surrounding enhanced network, showing circular polarization in ultraviolet (Mg II $h$ & $k$ and Mn I) and visible (Fe I) lines. We infer the longitudinal magnetic field from the photosphere to the very upper chromosphere. At the top of the plage chromosphere the field strengths reach more than 300 gauss, strongly correlated with the Mg II $k$ line core intensity and the electron pressure. This unique mapping shows how the magnetic field couples the different atmospheric layers and reveals the magnetic origin of the heating in the plage chromosphere., 50 pages, 11 figures, 1 table, published in Science Advances

We are planning a new solar satellite mission, "PhoENiX", for understanding of particle acceleration during magnetic reconnection. The main observation targets of this mission are solar flares. The scientific objectives of this mission are (1) to identify particle acceleration sites, (2) to investigate temporal evolution of particle acceleration, and (3) to characterize properties of accelerated particles, during solar flares. In order to achieve these science objectives, the PhoENiX satellite is planned to be equipped with three instruments of (1) Photon-counting type focusing-imaging spectrometer in soft X-rays (up to ~10 keV), (2) Photoncounting type focusing-imaging spectrometer in hard X-rays (up to ~30 keV), and (3) Spectropolarimeter in soft gamma-rays (spectroscopy is available in the energy range of from > 20 keV to 60 keV to < 600 keV). We plan to realize PhoENiX satellite mission around next solar maximum (around 2025).

We present ongoing efforts on the development of precision Wolter mirrors for the Soft X-ray Imaging Spectrometer (SXIS) aboard PhoENiX mission proposed to JAXA for studying mechanism(s) of particle acceleration and its relationship with magnetic reconnection in solar flares. The Wolter mirrors for PhoENiX/SXIS are made by direct polishing of glass-ceramic substrates. So far, we succeeded in fabricating a small size of high precision Wolter surfaces (e.g., PSF core size of ~0.2 arcsec HPD at 8 keV) as well as good indication of extending the mirror area along the cylindrical direction. Recent status of the mirror development will be reported.

We succeeded in observing two large spicules simultaneously with the Atacama Large Millimeter/submillimeter Array (ALMA), the Interface Region Imaging Spectrograph (IRIS), and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory. One is a spicule seen in the IRIS Mg II slit-jaw images and AIA 304\AA\ images (MgII/304A spicule). The other one is a spicule seen in the 100GHz images obtained with ALMA (100GHz spicule). Although the 100GHz spicule overlapped with the MgII/304A spicule in the early phase, it did not show any corresponding structures in the IRIS Mg II and AIA 304A images after the early phase. It suggests that the spicules are individual events and do not have a physical relationship. To obtain the physical parameters of the 100GHz spicule, we estimate the optical depths as a function of temperature and density using two different methods. One is using the observed brightness temperature by assuming a filling factor, and the other is using an emission model for the optical depth. As a result of comparing them, the kinetic temperature of the plasma and the number density of ionized hydrogens in the 100GHz spicule are ~6800 K and 2.2 x 10^10 cm^-3. The estimated values can explain the absorbing structure in the 193A image, which appear as a counterpart of the 100GHz spicule. These results suggest that the 100GHz spicule presented in this paper is classified to a macrospicule without a hot sheath in former terminology., Comment: 8 pages, 4 figures, Accepted to ApJL

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

High angular-resolution imagery (~1” or better) together with good off-axis scattering performance (

The X-ray Telescope (XRT) aboard Hinode was designed to observe the solar corona in its entire temperature range, spanning from below 1 million Kelvin (MK) to beyond 20 MK. In particular, the capability of observing low-temperature (below 2 MK) plasmas that were not accessible with the Yohkoh Soft X-ray Telescope (SXT), coupled with an order of magnitude higher cadence than SXT, has enabled XRT to make various discoveries on coronal activities. Furthermore, XRT has the unique strength of observing high-temperature plasmas in the corona that are not necessarily well observed with other coronal imagers. The scientific considerations made while designing the XRT and the key points of the instrumental features are reviewed, followed by some highlights of the scientific results obtained with XRT.

The imaging spectroscopic observations for solar soft X-rays are expected to provide us novel and valuable information about the plasma activity in the solar corona, e.g., particle acceleration, heating, shock, etc. However, this type of observations has not been performed yet with enough energy, spatial, and temporal resolutions. In this situation, we plan to realize the imaging spectroscopic observations for solar soft X-rays with a high speed soft X-ray camera and grazing incidence mirrors. Our developing camera consists of a back-illuminated CMOS sensor. This censor has a sensitivity to soft X-rays (0.5 keV - 10 keV), and can perform continuous exposures of 1,000 frame per second for the imaging area of 1k x 100 pixels. We will mount this camera on the FOXSI-3 sounding rocket that is planned to be launched in the summer of 2018. By the combination of our camera and the X-ray mirror on the FOXSI, we can achieve an energy resolution of 0.2 keV, a spatial resolution of ~5 arcsec (1 arcsec sampling), and the temporal resolution of ~10 seconds in an energy range of 0.5 keV - 10 keV. In this presentation, we will explain the science goal, the instrumental design, and the developments of the solar soft X-ray imaging spectrometer.

Taro Sakao, Satoshi Matsuyama, Takumi Goto, Jumpei Yamada, Shuhei Yasuda, Kazuto Yamauchi, Yoshiki Kohmura, Ayumi Kime, Akira Miyake, Tadakazu Maezawa, Hirokazu Hashizume, Yoshinori Suematsu, Noriyuki Narukage, and Shin-nosuke Ishikawa "Development of precision Wolter mirrors for solar x-ray observations", Proc. SPIE 10386, Advances in X-Ray/EUV Optics and Components XII, 103860E (23 August 2017); https://doi.org/10.1117/12.2273507.

High resolution imagery of solar X-ray corona provides a crucial key to understand dynamics and heating processes of plasmas there. However, imagery of the Sun with sub-arcsecond resolution in X-ray wavelengths has never been conducted due to severe technical difficulty in fabricating precision Wolter mirrors with a wide field of view exceeding several 100”. For future X-ray observations of the Sun, we are attempting to realize precision Wolter mirrors with sub-arcsecond resolution by adopting state-of-the-art surface polish and measurement methods to segmented mirrors which consist of a portion of an entire circle. Following evaluation of X-ray focusing performance of the first engineering Wolter mirror using BL29XUL coherent X-ray beam line at SPring-8 synchrotron facility, the second engineering mirror was fabricated with improvements in precision polish from the first mirror incorporated. X-ray evaluation of the second mirror at SPring-8 was conducted in February 2015, yielding FWHM size of ~0.2” for the PSF core at 8 keV while its HPD (half power diameter) size still remained at ~3” due to a large amount of small-angle scattering right outside the PSF core. Further improvements in the precision polish for the second mirror, in particular in the spatial scale from 0.3 mm to 5 mm, is currently under way with another X-ray evaluation at SPring-8 planned in spring 2016. Progress in our development activities for precision Wolter mirrors will be reported including at-wavelength evaluation results.

資料番号: SA1004443000

The X-ray Telescope (XRT) aboard the Hinode satellite is a grazing incidence X-ray imager equipped with a 2048×2048 CCD. The XRT has 1 arcsec pixels with a wide field of view of 34×34 arcmin. It is sensitive to plasmas with a wide temperature range from < 1 to 30 MK, allowing us to obtain TRACE-like low-temperature images as well as Yohkoh/SXT-like high-temperature images. The spacecraft Mission Data Processor (MDP) controls the XRT through sequence tables with versatile autonomous functions such as exposure control, region-of-interest tracking, flare detection, and flare location identification. Data are compressed either with DPCM or JPEG, depending on the purpose. This results in higher cadence and/or wider field of view for a given telemetry bandwidth. With a focus adjust mechanism, a higher resolution of Gaussian focus may be available on-axis. This paper follows the first instrument paper for the XRT (Golub et al., Solar Phys. 243, 63, 2007) and discusses the design and measured performance of the X-ray CCD camera for the XRT and its control system with the MDP.

The Hinode (Solar-B) was launched by M-V rocket on 22 September 2006 UT. The telemetry data of the Hinode X-ray Telescope (XRT) showed that the X-ray count rate detected with the XRT had decreased rapidly since the operational heaters on the XRT telescope tube were turned on. This is attributed to the fact that molecular contaminants accumulated onto the CCD with the temperature of −60oC resulting in the degradation of the XRT sensitivity. We baked the CCD at the temperature of 35oC in order to remove the contaminants from the CCD surface. However many contaminant spots appeared on the surface. We found that major contaminant source existed in the telescope tube, and identified the contaminants as diethylhexyl phthalate (DEHP) or DEHP-like organics. The mechanisms to yield the contaminant spots were discussed.

著者人数：15名, Accepted: 2007-09-11, 資料番号: SA1000268000

Accepted: 2007-08-17, 資料番号: SA1000267000

著者人数：25名, Accepted: 2007-07-30, 資料番号: SA1000203000

We present the H$\alpha$ observations from ARIES, Nainital of a compact and impulsive solar flare occurred on March 10, 2001 and associated with a CME. We have also analysed HXT, SXT/Yohkoh observations as well as radio observations from Nobeyama Radio Observatory to derive the energetics and dynamics of this impulsive flare. We co-align the H$\alpha$, SXR, HXR, MW and magnetogram images within the instrumental spatial resolution limit. We detect a single HXR source in this flare, which is found spatially associated with one of the H$\alpha$ bright kernel. The unusual feature of HXR and H$\alpha$ sources, observed for the first time, is the rotation during the impulsive phase in clockwise direction. We propose that the rotation may be due to asymmetric progress of the magnetic reconnection site or may be due to the change of peak point of the electric field. In MW emission we found two sources, one is main source which is at the main flare site and another is remote source located in South-West direction. It appears that the remote source is formed by the impact of accelerated energetic electrons from the main flare site. From the spatial co-relation of multi-wavelength images of th, Comment: Accepted in Solar Physics, 20 pages, 12 Figures

Coronal hard X-ray (HXR) sources were discovered by the Yohkoh HXT telescope in about two dozen limb flares: Impulsive and gradual ones. On the basis of HXT data, we investigated the spatial evolution of coronal sources. Slow ascending motions of sources are seen in several flares. In five events, it was possible to estimate the velocity of the upward motion with values between 10 and 30 km/s. We present these observational results and conclude that coronal source motions should be studied statistically using the RHESSI high-resolution HXR imaging data. We discuss the possibility that coronal HXR emission is generated as bremsstrahlung of the fast electrons accelerated in collapsing magnetic traps due to joint action of the Fermi-type first-order mechanism and betatron acceleration.

Using the Yohkoh Hard X-Ray Telescope (HXT) data, we have examined motions of the hard X-ray (HXR) sources during 72 solar flares occurred from 1991 September to 2001 December. In these flares, we have found 198 intense sources that are presumably the chromospheric footpoints (FPs) of flare loops. The average velocity V and the velocity dispersion σ were determined by a linear regression for these sources. For 80% of them, the ratio of V to 3 σ is larger than 1, strongly suggesting that the regular motions of the HXR sources dominate their chaotic motions. For 43 of 72 flares, coalignment of the HXT images with the photospheric magnetograms allows us to consider the HXR sources located on the both sides of the photospheric neutral line (NL) as the FP sources, and to distinguish between three main types of the FP motions. The type I is the motions of the HXR sources preferentially away from and nearly perpendicular to the NL. Less than 5% of the flares show this pattern of motion. In the type II, the sources move mainly along the NL in anti-parallel directions. Such motions have been found in 26% of flares. The type III involves a similar pattern of motions as the type II but all the HXR sources move in the same direction along the NL. Flares of this type constitute 30% of the flares. About 19% of flares can be described as a combination of these basic types. The remaining 20% of flares seem to be more complicated or less regular in the motion scale under consideration. An interpretation of results is suggested.

We present the results of a detailed analysis of multi-wavelength observations of a very impulsive solar flare 1B/M6.7, which occurred on 10 March, 2001 in NOAA AR 9368 (N27 W42). The observations show that the flare is very impulsive with very hard spectrum in HXR that reveal non-thermal emission was most dominant. On the other hand this flare also produced type II radio burst and coronal mass ejections (CME), which are not general characteristics for impulsive flares. In H$\alpha$ we observed the bright mass ejecta (BME) followed by drak mass ejecta (DME). Based on the consistence of the onset times and direction of BME and CME, we conclude that these two phenomena are closely associated. It is inferred that the energy build-up took place due to photospheric reconnection between emerging positive parasitic polarity and predominant negative polarity, which resulted as a consequence of flux cancellation. The shear increased to $>80^o$ due to further emergence of positive parasitic polarity causing strongly enhanced cancellation of flux. It appears that such enhanced magnetic flux cancellation in a strongly sheared region triggered the impulsive flare., Comment: 14 pages, 8 Figures, Accepted for the publication in Solar Physics

The tip-tilt mirror (TTM) system was developed for the extreme ultraviolet (XUV) Cassegrain telescope aboard the Institute of Space and Astronautical Science sounding rocket. The spatial resolution of the telescope is about 5 arcsec, whereas the rocket pointing is only controlled to be within ±0.5 deg of the target (sun) without additional stability control. To stabilize the XUV image within about 5 arcsec on the focal plane, the TTM system controls the tilt of the secondary mirror with two-axis fixed-coil magnetic actuators. The TTM system has a wide tilt angle and can drive the large secondary mirror at high frequency. The two position-sensitive detectors, one placed in the telescope and the other in the TTM mechanical structure, are used for closed-loop control of the TTM. The closed-loop control system, which has command and telemetry, is executed by the flight software on the digital signal processor. The TTM has a launch-lock mechanism to protect against launch vibrations up to about 16G. The sounding rocket was launched from the Kagoshima Space Center on 31 January 1998. The TTM worked perfectly during the flight and achieved better than the expected 5-arcsec stability on the focal plane during CCD charge-coupled device exposures.

A suite of images from the XUV Doppler Telescope (XDT), the Yohkoh Soft X-ray Telescope (SXT), and the Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO) allow us to see the whole (T > 1 MK) temperature evolution of coronal loops. The detailed morphological comparison of an active region shows that hot loops seen in SXT (T > 3 MK) and cool loops seen in the the EIT 195 A band (T ~ 1.5 MK) are located in almost alternating manner. The anticoincidence of the hot and the cool loops is conserved for a duration much longer than the estimated cooling timescale. However, both hot and cool loops have counterparts in the intermediate-temperature images. The cross-correlation coefficients are higher for neighboring temperature pairs and lower for pairs with larger temperature differences. These results suggest that loops are not isothermal but rather have a differential emission measure distribution of modest but finite width that peaks at different temperatures for different loops.

On the basis of Yohkoh Hard X-Ray Telescope data and the magnetograms taken by the SOHO Michelson Doppler Imager and the Solar Magnetic Field Telescope at Huairou Solar Observing Station, we suggest an interpretation of the well-observed "Bastille Day 2000" flare. The large-scale structure and dynamics of the flare, as seen in hard X-rays, can be explained in terms of the three-dimensional reconnection at a separator in the corona. More specifically, we suggest that before occurrence of two-ribbon flares with significant decrease of a distance between the hard X-ray (HXR) footpoints, like the Bastille Day flare, the bases of magnetic field separatrices are moved by the large-scale photospheric flows of two types. First, the shear flows, which are parallel to the photospheric neutral line, increase the length of field lines in the corona and produce an excess of magnetic energy. Second, the converging flows, i.e., the flows directed to the neutral line, create the preflare current layers in the corona and provide an excess of energy sufficient to produce a large flare. During the flare, both excesses of magnetic energy are released completely or partially. In the Bastille Day flare, the model describes two kinds of apparent motions of the HXR kernels. One is an increase of a distance between the flare ribbons in which the HXR kernels appear. The effect results from fast reconnection in a coronal current layer. The second effect is a decrease of the distance between the kernels moving to each other as a result of relaxation of magnetic tensions generated by the photospheric shear flows.

The Yohkoh data on the site and mechanism of magnetic energy transformation into kinetic and thermal energies of a superhot plasma at the Sun require new models of reconnection under conditions of highlyanomalous resistivity, which are similar to that one observed in toroidal devices performed to study turbulent heating of a collisionless plasma, and its anomalous heat-conductive cooling. The models make intelligible the observed properties of the three-dimensional reconnection in solar flares.

We present overview and development activities of a soft X-ray photon-counting spectroscopic imager for the solar corona that we conceive as a possible scientific payload for future space solar missions including Japanese Solar-C. The soft X-ray imager will employ a Wolter I grazing-incidence sector mirror with which images of the corona (1 MK to beyond 10 MK) will be taken with the highest-ever angular resolution (0.5"/pixel for a focal length of 4 m) as a solar Xray telescope. In addition to high-resolution imagery, we attempt to implement photon-counting capability for the imager by employing a backside-illuminated CMOS image sensor as the focal-plane device. Imaging-spectroscopy of the X-ray corona will be performed for the first time in the energy range from ~0.5 keV up to 10 keV. The imaging-spectroscopic observations with the soft X-ray imager will provide a noble probe for investigating mechanism(s) of magnetic reconnection and generation of supra-thermal (non-thermal) electrons associated with flares. Ongoing development activities in Japan towards the photon-counting imager is described with emphasis on that for sub-arcsecond-resolution grazing-incidence mirrors.

The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a sounding-rocket instrument currently under development at the National Astronomical Observatory of Japan (NAOJ) as a part of an international collaboration. CLASP’s optics are composed of a Cassegrain telescope and a spectro-polarimeter which are designed to achieve an unprecedentedly accurate polarization measurement of the Ly-α line at 121.6nm emitted from the solar upper-chromosphere and transition region. CLASP’s first flight is scheduled for August 2015. Reaching such accuracy requires a careful alignment of the optical elements to optimize the image quality at 121.6 nm. However Ly-α is absorbed by air and therefore the optics alignment has to be done under vacuum condition which makes any experiment difficult. To bypass this issue, we proposed to align the telescope and the spectrograph separately in visible light. Hence we present our alignment procedure for both telescope and spectro-polarimeter. We will explain details about the telescope preliminary alignment before mirrors coating, which was done in April 2014, present the telescope combined optical performance and compare them to CLASP tolerance. Then we will present details about an experiment designed to confirm our alignment procedure for the CLASP spectro-polarimeter. We will discuss the resulting image quality achieved during this experiment and the lessons learned.

Particle acceleration is one of the most significant features that are ubiquitous among space and cosmic plasmas. It is most prominent during flares in the case of the Sun, with which huge amount of electromagnetic radiation and high-energy particles are expelled into the interplanetary space through acceleration of plasma particles in the corona. Though it has been well understood that energies of flares are supplied by the mechanism called magnetic reconnection based on the observations in X-rays and EUV with space telescopes, where and how in the flaring magnetic field plasmas are accelerated has remained unknown due to the low plasma density in the flaring corona. We here report the first observational identification of the energetic non-thermal electrons around the point of the ongoing magnetic reconnection (X-point); with the location of the X-point identified by soft X-ray imagery and the localized presence of non-thermal electrons identified from imaging-spectroscopic data at two microwave frequencies. Considering the existence of the reconnection outflows that carries both plasma particles and magnetic fields out from the X-point, our identified non-thermal microwave emissions around the X-point indicate that the electrons are accelerated around the reconnection X-point. Additionally, the plasma around the X-point was also thermally heated up to 10 MK. The estimated reconnection rate of this event is ~0.017., 24 pages, 8 figures, accepted for publication in The Astrophysical Journal

On the basis of the Yohkoh data on the site and mechanism of magnetic energy transformation into thermal and kinetic energies of superhot plasmas and accelerated particles, a model is developed that explains observed properties of collisionless 3D reconnection in active regions. The model makes intelligible the observed relation between the S-like morphology and eruptive activity.

It is observed in impulsive flares with the Hard X-ray Telescope (HXT) onboard Yohkoh that a hard X-ray source is located above the apex of a soft X-ray flaring loop in addition to double footpoint sources (Masuda et al. 1994, 1995). This observation suggests that the flare energy-release, probably magnetic reconnection, takes place not in the soft X-ray loop, but above the loop. It is important to derive the hard X-ray spectrum of this source accurately in order to understand how electrons are energized there. Using an improved image-synthesis technique with new modulation patterns (Sato 1997), we obtained hard X-ray images of the 13 January 1992 flare in the four HXT energy bands with a common photon-accumulation time. The spectrum of the above-the-looptop source in the energy range of 14–93 keV seems to be well fit by emission from an isothermal plasma whose temperature is about 100 MK, rather than by a non-thermal, single power-law spectrum. Although this is consistent with the observational result that the density of the ambient plasma is not high enough to stop high energy electrons, we need to study other events, especially intense events from which more accurate spectra can be derived, in order to confirm this result.

We present hard X-ray imaging observations of impulsive solar flares with Yohkoh HXT, focusing on the evolution of hard X-ray double footpoint sources seen in the energy range above 30 keV. In 7 out of the 14 events studied, the separation between the double sources increases (at the 3σ confidence level) as the flare progresses, with the double sources moving nearly anti-parallel to each other in most cases. Implications of these results to the global magnetic field structure responsible for solar flare energy release / particle acceleration are discussed.

New results concerning magnetic reconnection in the solar corona are reviewed. We apply the collisionless 3D reconnection theory — more exactly, the model of a high-temperature turbulent-current non-neutral current sheet — to the coronal conditions derived from the observations with the HXT and SXT onboard Yohkoh . New interpretations are discussed of the Yohkoh data on the site and mechanism of magnetic energy transformation into kinetic and thermal energies of superhot plasmas and accelerated high-energy particles. Open issues are the focus of our attention.

We analyze the footpoint separation d and flux asymmetry A of magnetically conjugate double footpoint sources in hard X-ray images from the Yohkoh Hard X-Ray Telescope (HXT). The data set of 54 solar flares includes all events simultaneously observed with the Compton Gamma Ray Observatory (CGRO) in high time resolution mode. From the CGRO data we deconvolved the direct-precipitation and trap-precipitation components previously (in Paper II). Using the combined measurements from CGRO and HXT, we develop an asymmetric trap model that allows us to quantify the relative fractions of four different electron components, i.e., the ratios of direct-precipitating (qP1, qP2) and trap-precipitating electrons (qT1, qT2) at both magnetically conjugate footpoints. We find mean ratios of qP1=0.14 ± 0.06, qP2=0.26 ± 0.10, and qT=qT1+qT2=0.60 ± 0.13. We assume an isotropic pitch-angle distribution at the acceleration site and double-sided trap precipitation (qT2/qT1=qP2/qP1) to determine the conjugate loss-cone angles (α1=42° ± 11° and α2=52° ± 10°) and magnetic mirror ratiosat both footpoints (R1=1.6,...,4.0 and R2=1.3,...,2.5). From the relative displacement of footpoint sources we also measure altitude differences of hard X-ray emission at different energies, which are found to decrease systematically with higher energies, with a statistical height difference of hLo-hM1=980 ± 250 km and hM1-hM2=310 ± 300 km between the three lower HXT energy channels (Lo, M1, M2).

We present an overview and instrumental details of the solar XUV Doppler Telescope (XDT) launched in January 1998 with the S520CN-22 sounding rocket of the Institute of Space and Astronautical Science. The XDT observes nearly single-temperature solar corona at 1.8 MK with angular resolution of ≈ 5′′ pixel size, together with the ability to detect the coronal velocity field with a full-Sun field of view. By use of normal incidence optics whose primary and secondary mirrors are coated with multilayer materials in two sectors, the XDT takes images of the Sun in a set of shorter and longer wavelength bands around the Fe xiv 211.3 A emission line. Summation of a pair of images in the two bands provides an image of the 1.8 MK-corona while the difference between the two provides velocity images of the Fe xiv-emitting plasma. A brief description on the observation sequence together with the flight result is also given.

著者人数：12名, 資料番号: SA1000194000

Two subclasses of impulsive solar flares, observed with the Hard X-Ray Telescope (HXT) onboard Yohkoh, have been discovered by Sakao et al. The two subclasses can be characterized as more impulsive (MI) and less impulsive (LI) flares, the former having a shorter total duration of the impulsive phase in the hard X-ray emission than the latter. We assume that in both subclasses, the collisionless three-dimensional reconnection process occurs at the separator with a longitudinal magnetic field. The high-temperature turbulent-current sheet (HTTCS), located along the separator, generates accelerated particles and fast outflows of "superhot" (T ≥ 30 MK) plasma. Powerful anomalous heat-conductive fluxes along the reconnected field lines maintain a high temperature in the superhot plasma. The difference between the LI and MI flares presumably appears because the footpoint separation (the distance between two brightest hard X-ray sources) increases in time in the LI flares, but decreases in the MI flares. According to our model, in the LI flares the three-dimensional reconnection process accompanies an increase in the longitudinal magnetic field at the separator. In contrast, in the MI flares the reconnection proceeds with a decrease of the longitudinal field; hence, the reconnection rate is higher in the MI flares. Since reconnection in the MI flares proceeds with a decrease of the longitudinal field, the reconnected field lines become shorter in this process. As the reconnected lines become shorter, accelerated electron beams arrive at the upper chromosphere faster. So, in the MI flares chromospheric evaporation begins earlier than in the LI flares. The evaporation process driven by accelerated electron beams generates upflows of "warm" (T ≤ 10 MK) plasma that interacts with downflows of superhot plasma and can switch off the accumulation of superhot plasma in the MI flares during the impulsive phase. In the LI flares, however, an observable amount of superhot plasma is accumulated even during the impulsive phase. Moreover, since the cooling timescales increase with the length of the reconnected field lines, our argument for the association of superhot plasma with longer lines may remain valid to a reasonable extent even if the chromospheric evaporated plasma mixes with the reconnected outflow and superhot temperatures are reached in this mixture. Further analysis of the Yohkoh data obtained simultaneously with the Hard and Soft X-Ray Telescopes and the bent crystal spectrometer (BCS) is necessary to distinguish the superhot components of chromospheric and coronal origins in different classes of flares as well as at different phases of their development.

We present science and development activities of the soft X-ray photon-counting spectroscopic imager for the solar corona that we conceive as a possible scientific payload for the Japanese Solar-C mission. The imager employs a grazing-incidence sector mirror of Wolter-I type with which images of the corona are to be taken in a wide temperature range (1 MK to beyond 10 MK) with the highest-ever angular resolution (0.5"/pixel for a focal length of 4 m) as an Xray telescope for the Sun. Moreover, by employing a back-thinned CMOS image sensor as the focal-plane device, we attempmt to implement photon-counting capability with which imaging-spectroscopy of the X-ray corona will be performed for the first time, in the energy range from ~0.5 keV up to 10 keV. The imaging-spectroscopic observations will provide totally-new information on mechanism(s) for magnetic reconnection, generation of supra-thermal electrons in the reconnecting magnetic structure during flares, and for the generation of hot coronal plasmas (heated beyond a few MK) which may be responsible for the formation of the hot cores of solar active regions.

Imaging observations of solar flare hard X-ray sources with the Hard X-ray Telescope (HXT) aboard the Yohkoh satellite have revealed that hard X-ray emissions (greater than 30 ke V) originate most frequently from double sources. The double sources are located on both sides of the magnetic neutral line, suggesting that the bulk of hard X-rays is emitted from footpoints of flaring magnetic loops. We also found that hard X-rays from the double sources are emitted simultaneously within a fraction of second and that the weaker source tends to be located in the stronger magnetic field region, showing a softer spectrum. Physcial implications on the observed characteristics of the hard X-ray double sources are discussed.

Solar-C is the third generation solar observatory led by JAXA. The accepted ‘Plan-B’ payload calls for a radiation-hard solar-staring photon-counting x-ray spectrometer. CMOS APS technology offers advantages over CCDs for such an application such as increased radiation hardness and high frame rate (instrument target of 1000 fps). Looking towards the solution of a bespoke CMOS APS, this paper reports the x-ray spectroscopy performance, concentrating on charge collection efficiency and split event analysis, of two baseline e2v CMOS APSs not designed for x-ray performance, the EV76C454 and the Ocean Colour Imager (OCI) test array. The EV76C454 is an industrial 5T APS designed for machine vision, available back and front illuminated. The OCI test arrays have varying pixel design across the chips, but are 4T, back illuminated and have thin low-resistivity and thick high-resistivity variants. The OCI test arrays’ pixel variants allow understanding of how pixel design can affect x-ray performance.

We report science and development activities of the X-ray/EUV telescope for the Japanese Solar-C mission whose projected launch around 2019. The telescope consists of a package of (a) a normal-incidence (NI) EUV telescope and (b) a grazing-incidence (GI) soft X-ray telescope. The NI telescope chiefly provides images of low corona (whose temperature 1 MK or even lower) with ultra-high angular resolution (0.2-0.3"/pixel) in 3 wavelength bands (304, 171, and 94 angstroms). On the other hand, the GI telescope provides images of the corona with a wide temperature coverage (1 MK to beyond 10 MK) with the highest-ever angular resolution (~0.5"/pixel) as a soft X-ray coronal imager. The set of NI and GI telescopes should provide crucial information for establishing magnetic and gas-dynamic connection between the corona and the lower atmosphere of the Sun which is essential for understanding heating of, and plasma activities in, the corona. Moreover, we attempt to implement photon-counting capability for the GI telescope with which imaging-spectroscopy of the X-ray corona will be performed for the first time, in the energy range from ~0.5 keV up to 10 keV. The imaging-spectroscopic observations will provide totally-new information on mechanism(s) for the generation of hot coronal plasmas (heated beyond a few MK), those for magnetic reconnection, and even generation of supra-thermal electrons associated with flares. An overview of instrument outline and science for the X-ray photoncounting telescope are presented, together with ongoing development activities in Japan towards soft X-ray photoncounting observations, focusing on high-speed X-ray CMOS detector and sub-arcsecond-resolution GI mirror.

One of the biggest challenges in heliophysics is to decipher the magnetic structure of the solar chromosphere. The importance of measuring the chromospheric magnetic field is due to both the key role the chromosphere plays in energizing and structuring the outer solar atmosphere and the inability of extrapolation of photospheric fields to adequately describe this key boundary region. Over the last few years, significant progress has been made in the spectral line formation of UV lines as well as the MHD modeling of the solar atmosphere. It is found that the Hanle effect in the Lyman-alpha line (121.567 nm) is a most promising diagnostic tool for weaker magnetic fields in the chromosphere and transition region. Based on this groundbreaking research, we propose the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) to NASA as a sounding rocket experiment, for making the first measurement of the linear polarization produced by scattering processes and the Hanle effect in the Lyman-alpha line (121.567 nm), and making the first exploration of the magnetic field in the upper chromosphere and transition region of the Sun. The CLASP instrument consists of a Cassegrain telescope, a rotating 1/2-wave plate, a dual-beam spectrograph assembly with a grating working as a beam splitter, and an identical pair of reflective polarization analyzers each equipped with a CCD camera. We propose to launch CLASP in December 2014.

Particle acceleration is intrinsic to the primary energy release in the impulsive phase of solar flares, and we cannot understand flares without understanding acceleration. New observations in soft and hard X-rays, γ-rays and coherent radio emissions are presented, suggesting flare fragmentation in time and space. X-ray and radio measurements exhibit at least five different time scales in flares. In addition, some new observations of delayed acceleration signatures are also presented. The theory of acceleration by parallel electric fields is used to model the spectral shape and evolution of hard X-rays. The possibility of the appearance of double layers is further investigated.

Yohkoh observations of an impulsive solar flare which occurred on 16 December, 1991 are presented. This flare was a GOES M2.7 class event with a simple morphology indicative of a single flaring loop. X-ray images were taken with the Hard X-ray Telescope (HXT) and soft X-ray spectra were obtained with the Bragg Crystal Spectrometer (BCS) on board the satellite. The spectrometer observations were made at high sensivity from the earliest stages of the flare, are continued throughout the rise and decay phases, and indicate extremely strong blueshifts, which account for the majority of emission in Caxix during the initial phase of the flare. The data are compared with observations from other space and ground-based instruments. A balance calculation is performed which indicates that the energy contained in non-thermal electrons is sufficient to explain the high temperature plasma which fills the loop. The cooling of this plasma by thermal conduction is independently verified in a manner which indicates that the loop filling factor is close to 100%. The production of ‘superhot’ plasma in impulsive events is shown to differ in detail from the morphology and mechanisms appropriate for more gradual events.

We report instrument outline as well as science of the photon-counting soft X-ray telescope that we have been studying as a possible scientific payload for the Japanese Solar-C mission whose projected launch around 2019. Soft X-rays (~1- 10 keV) from the solar corona include rich information on (1) possible mechanism(s) for heating the bright core of active regions seen in soft X-rays (namely, the hottest portion in the non-flaring corona), (2) dynamics and magnetohydrodynamic structures associated with magnetic reconnection processes ongoing in flares, and even (3) generation of supra-thermal distributions of coronal plasmas associated with flares. Nevertheless, imaging-spectroscopic investigation of the soft X-ray corona has so far remained unexplored due to difficulty in the instrumentation for achieving this aim. With the advent of recent remarkable progress in CMOS-APS detector technology, the photon-counting X-ray telescope will be capable of, in addition to conventional photon-integration type exposures, performing imaging-spectroscopic investigation on active regions and flares, thus providing, for example, detailed temperature information (beyond the sofar- utilized filter-ratio temperature) at each spatial point of the observing target. The photon-counting X-ray telescope will emply a Wolter type I optics with a piece of a segmented mirror whose focal length 4 meters, combined with a focal-plane CMOS-APS detector (0.4-0.5"/pixel) whose frame read-out rate required to be as high as 1000 fps.

Two mission concepts (plan A: out-of-ecliptic mission and plan B: high resolution spectroscopic mission) have been studied for the next Japanese-led solar mission Solar-C, which will follow the scientific success of the Hinode mission. The both mission concepts are concluded as equally important and attractive for the promotion of space solar physics. In the meantime we also had to make efforts for prioritizing the two options, in order to proceed to next stage of requesting the launch of Solar-C mission at the earliest opportunity. This paper briefly describes the two mission concepts and the current status on our efforts for prioritizing the two options. More details are also described for the plan B option as the first-priority Solar-C mission. The latest report from the Solar-C mission concept studies was documented as "Interim Report on the Solar-C Mission Concept."

The solar chromosphere is an important boundary, through which all of the plasma, magnetic fields and energy in the corona and solar wind are supplied. Since the Zeeman splitting is typically smaller than the Doppler line broadening in the chromosphere and transition region, it is not effective to explore weak magnetic fields. However, this is not the case for the Hanle effect, when we have an instrument with high polarization sensitivity (~ 0.1%). "Chromospheric Lyman- Alpha SpectroPolarimeter (CLASP)" is the sounding rocket experiment to detect linear polarization produced by the Hanle effect in Lyman-alpha line (121.567 nm) and to make the first direct measurement of magnetic fields in the upper chromosphere and lower transition region. To achieve the high sensitivity of ~ 0.1% within a rocket flight (5 minutes) in Lyman-alpha line, which is easily absorbed by materials, we design the optical system mainly with reflections. The CLASP consists of a classical Cassegrain telescope, a polarimeter and a spectrometer. The polarimeter consists of a rotating 1/2-wave plate and two reflecting polarization analyzers. One of the analyzer also works as a polarization beam splitter to give us two orthogonal linear polarizations simultaneously. The CLASP is planned to be launched in 2014 summer.