Search

This study aimed to verify the accuracy of auto-contouring and auto-dose optimization for hippocampal-avoidance whole-brain radiation therapy (HA-WBRT). Head computed tomography (CT) images of 15 patients were selected. The regions of interest, containing the brain, hippocampus, eyes, and lacrimal glands, were contoured manually and automatically on CT images. They were compared and evaluated for concordance rates using the Simpson coefficient. To verify the performance of dose optimization, auto-dose planning was compared with manual planning for 15 cases. All optimization plans were performed using the volumetric modulated arc therapy technique. The automatically contoured brain showed a very high concordance rate with the manually contoured brain; the Simpson coefficient was 0.990 ± 0.01. Contrastingly, the concordance rate of the hippocampal contour was low at 0.642 ± 0.15 (right) and 0.500 ± 0.16 (left); however, the rate improved to 0.871 ± 0.09 (right) and 0.852 ± 0.11 (left) with an additional 3-mm margin. For 2% of each planning target volume with the prescribed dose (D2%) and Dmean, there was no significant difference between the automatic and manual plans (35.50 Gy vs 35.23 Gy; p = 0.233 and 33.09 Gy vs 32.84 Gy; p = 0.073, respectively). The D98% was significantly better for the manual plan than for the automatic plan (25.49 Gy vs 26.11 Gy; p0.01). Dmax and D100% for the hippocampus did not show any significant difference between the automatic and manual plans (15.65, 16.09 Gy (right, left) vs 15.51, 15.80 Gy; p = 0.804, 0.233 and 8.08, 8.03 Gy vs 8.13, 8.01 Gy; p = 0.495, 1 respectively). The accuracy of auto-contouring for HA-WBRT can be guaranteed by providing an appropriate margin, and the precision of the auto-dose optimization was comparable to that of the manual plan.

The purpose of this study was to evaluate the change in capillary blood volume in the muscles of the lower limbs, before and after exercise, using intravoxel incoherent motion (IVIM).Diffusion-weighted images were obtained (with 16 b values) from the thigh and leg muscles of 11 subjects. The imaging was obtained before and immediately exercise; additionally, imaging was performed at 3 h, 6 h, and 24 h after exercise, and an IVIM index was calculated. The exercise involved walking up and down a flight of stairs (140 steps) ten times. The IVIM of each time course-before and after the exercise-was compared. In addition, we examined the correlation of IVIM measurements with the degree of the muscle ache that occurred at 24 h following the exercise.The IVIM index significantly increased after exercise compared with that before exercise (P 0.01). IVIM decreased at 3 h following exercise, but increased again at 24 h. A correlation was found between the IVIM index at 24 h after exercise and the degree of the muscle ache (r = 0.80) CONCLUSIONS: The capillary blood volume significantly increased after exercise when compared to before exercise. The capillary blood volume decreased after exercise at 3 h and 6 h following exercise, but it increased again at 24 h. There was a correlation between the degree of muscle ache and the amount of capillary blood volume measured from the femoral muscle at 24 h after exercise.

We often find spectral signatures of chromospheric cold plasma ejections accompanied by flares in a wide range of spatial scales in the solar and stellar atmospheres. However, the relationship between physical quantities (such as mass, kinetic energy, and velocity) of cold ejecta and flare energy has not been investigated in a unified manner for the entire range of flare energies to date. This study analyzed the spectra of cold plasma ejections associated with small-scale flares and solar flares (energy $10^{25}-10^{29}\,\mathrm{erg}$) to supply smaller energy samples. We performed H$\alpha$ imaging spectroscopy observation by the Solar Dynamics Doppler Imager on the Solar Magnetic Activity Research Telescope (SMART/SDDI). We determined the physical quantities of the ejecta by cloud model fitting to the H$\alpha$ spectrum. We determined flare energy by differential emission measure analysis using Atmospheric Imaging Assembly onboard Solar Dynamics Observatory (SDO/AIA) for small-scale flares and by estimating the bolometric energy for large-scale flares. As a result, we found that the ejection mass $M$ and the total flare energy $E_{\mathrm{tot}}$ follow a relation of $M\propto E_{\mathrm{tot}}^{2/3}$. We show that the scaling law derived from a simple physical model explains the solar and stellar observations with a coronal magnetic field strength as a free parameter. We also found that the kinetic energy and velocity of the ejecta correlate with the flare energy. These results suggest a common mechanism driven by magnetic fields to cause cold plasma ejections with flares on the Sun and stars., Comment: 23 pages, 10 figures; accepted for publication in ApJ

Sun-as-a-star analyses, in which observational data is spatially integrated, are useful for interpreting stellar data. For future applications to stellar observations, we performed Sun-as-a-star analyses of H$\alpha$ spectra for various active events on the Sun, not only for flares and filament eruptions/surges on the solar disk, but also for eruptions of off limb prominences using H$\alpha$ spectral images taken by the Solar Magnetic Activity Research Telescope / Solar Dynamics Doppler Imager (SMART/SDDI) at Hida Observatory, Kyoto University. All the analyzed events show emission relative to the pre-event state and the changes in their H$\alpha$ equivalent widths are all on the orders of 10$^{-4}$ \r{A}. Sun-as-a-star H$\alpha$ spectra exhibit different features depending on the causes of the emission: (i) Flares show emission at the H$\alpha$ line center, together with red asymmetry and line broadening, as reported in a previous study. (ii) Filament eruptions with and without flares show emission near the H$\alpha$ line center, accompanied by blue-/red-shifted absorption. Notably, disappearance of dark filaments leads to the apparent enhancement of the H$\alpha$ line center emission. (iii) Eruptions of off limb prominences show blue-/red-shifted emission. These spectral features enable us to identify the active phenomena on Sun-like stars. We have also found that even the filament eruptions showing red-shifted absorptions in Sun-as-a-star H$\alpha$ spectra lead to coronal mass ejections (CMEs). This result suggests that even if the falling components of stellar filament eruptions are detected as red-shifted absorptions in H$\alpha$ spectra, such stellar filament eruptions may also develop into CMEs., Comment: 23 pages, 19 figures. Accepted for publication in The Astrophysical Journal

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.

It is widely recognised that filament disappearances or eruptions are frequently associated with Coronal Mass Ejections (CMEs). Since CMEs are a major source of disturbances of the space environment surrounding the Earth, it is important to investigate these associations in detail for the better prediction of CME occurrence. However, the proportion of filament disappearances associated with CMEs is under debate. The estimates range from $\sim$10% to $\sim$90% and could be affected by the manners to select the events. In this study, we aim to reveal what parameters control the association between filament eruptions and CMEs. We analysed the relationships between CME associations and the physical parameters of filaments including their length, maximum ascending velocity, and direction of eruptions using 28 events of filament eruptions observed in H$\alpha$. We found that the product of the maximum radial velocity and the filament length is well correlated with the CME occurrence. If the product is larger than 8.0$\times$10$^{6}$ km$^{2}$ s$^{-1}$, the filament will become a CME with a probability of 93%, and if the product is smaller than this value, it will not become a CME with a probability of 100%. We suggest a kinetic-energy threshold above which filament eruptions are associated with CMEs. Our findings also suggest the importance of measuring the velocity vector of filament eruption in three-dimensional space for the better prediction of CME occurrence., Comment: accepted for publication in Earth, Planets and Space

We present a detailed three-dimensional (3D) view of a prominence eruption, coronal loop expansion, and coronal mass ejections (CMEs) associated with an M4.4 flare that occurred on 2011 March 8 in the active region NOAA 11165. Full-disk H$\alpha$ images of the flare and filament ejection were successfully obtained by the Flare Monitoring Telescope (FMT) following its relocation to Ica University, Peru. Multiwavelength observation around the H$\alpha$ line enabled us to derive the 3D velocity field of the H$\alpha$ prominence eruption. Features in extreme ultraviolet were also obtained by the Atmospheric Imager Assembly onboard the {\it Solar Dynamic Observatory} and the Extreme Ultraviolet Imager on board the {\it Solar Terrestrial Relations Observatory - Ahead} satellite. We found that, following collision of the erupted filament with the coronal magnetic field, some coronal loops began to expand, leading to the growth of a clear CME. We also discuss the succeeding activities of CME driven by multiple interactions between the expanding loops and the surrounding coronal magnetic field., Comment: 17 pages, 9 figures; accepted for publication in PASJ

Background Abdominal/pelvic lymph node (LN) oligometastasis, a pattern of treatment failure, is observed occasionally, and radiotherapy may work as salvage therapy. The optimal prescription dose, however, is yet to be determined. This study assessed the efficacy of high-dose radiotherapy. Methods The medical records of 113 patients at 4 institutes were retrospectively analysed who had 1 to 5 abdominal/pelvic LN oligometastases and were treated with definitive radiotherapy between 2008 and 2018. The exclusion criteria included non-epithelial tumours, uncontrolled primary lesions, palliative intent, and re-irradiation. The prescription dose was evaluated by using the equivalent dose in 2 Gy fractions (EQD2). Patients receiving EQD2 ≥ 60 Gy were placed into the high-dose group, and the remaining others the low-dose group. Kaplan-Meier analyses were performed to evaluate overall survival (OS), local control (LC), and progression-free survival (PFS). Univariate log-rank and multivariate Cox proportional hazards model analyses were performed to explore predictive factors. Adverse events were compared between the high-dose and low-dose groups. Results The primary tumour sites included the colorectum (n = 28), uterine cervix (n = 27), endometrium (n = 15), and ovaries (n = 10). The rate of 2-year OS was 63.1%, that of LC 59.7%, and that of PFS 19.4%. On multivariate analyses, OS were significantly associated with solitary oligometastasis (hazard ratio [HR]: 0.48, p = 0.02), LC with high-dose radiotherapy (HR: 0.93, p p = 0.01). Whereas high-dose radiotherapy did not significantly improve 2-year OS in the entire cohort (74.8% in the high-dose vs. 52.7% in the low-dose; p = 0.08), it did in the subgroup of solitary oligometastasis (88.8% in the high-dose vs. 56.3% in the low-dose; p = 0.009). As for Late grade ≥ 3 adverse event, ileus was observed in 7 patients (6%) and gastrointestinal bleeding in 4 (4%). No significant association between the irradiation dose and adverse event incidence was found. Conclusions As salvage therapy, high-dose radiotherapy was recommendable for oligometastasis in the abdominal/pelvic LNs. For solitary oligometastasis, LC and OS were significantly better in the high-dose group.

This paper describes a new SMART/SDDI Filament Disappearance Catalogue, in which we listed almost all the filament disappearance events that the Solar Dynamics Doppler Imager (SDDI) has observed since its installation on the Solar Magnetic Activity Research Telescope (SMART) in May 2016. Our aim is to build a database that can help predict the occurrence and severity of coronal mass ejections (CMEs). The catalogue contains miscellaneous information associated with filament disappearance such as flare, CME, active region, three-dimensional trajectory of erupting filaments, detection in Interplanetary Scintillation (IPS), occurrence of interplanetary CME (ICME) and Dst index. We also provide statistical information on the catalogue data. The catalogue is available from the following website: this https URL.

To improve the forecasting capability of impactful solar energetic particle (SEP) events, the relation between coronal mass ejections (CMEs) and SEP events needs to be better understood. Here we present a statistical study of SEP occurrences and timescales with respect to the CME source locations and speeds, considering all 257 fast ($v_{CME}$ $\ge$ 900 km/s) and wide (angular width $\ge$ 60$^{\circ}$) CMEs that occurred between December 2006 and October 2017. We associate them with SEP events at energies above 10 MeV. Examination of the source region of each CME reveals that CMEs more often accompany a SEP event if they originate from the longitude of E20-W100 relative to the observer. However, a SEP event could still be absent if the CME is $, Comment: 23 pages, 10 figures, accepted for publication in ApJ

Recently, many superflares on solar-type stars were discovered as white-light flares (WLFs). A correlation between the energies (E) and durations (t) of superflares is derived as $t\propto E^{0.39}$, and this can be theoretically explained by magnetic reconnection ($t\propto E^{1/3}$). In this study, we carried out a statistical research on 50 solar WLFs with SDO/HMI to examine the t-E relation. As a result, the t-E relation on solar WLFs ($t\propto E^{0.38}$) is quite similar stellar superflares, but the durations of stellar superflares are much shorter than those extrapolated from solar WLFs. We present the following two interpretations; (1) in solar flares, the cooling timescale of WL emission may be longer than the reconnection one, and the decay time can be determined by the cooling timescale; (2) the distribution can be understood by applying a scaling law $t\propto E^{1/3}B^{-5/3}$ derived from the magnetic reconnection theory., 4 pages, 1 figures, paper of the Proceedings of the IAUS340, accepted

Recently, many imaging methods are developed in magnetic resonance imaging and in particular, the release of the 3D acquisition method is remarkable. The image distortion often becomes the problem by the gradient echo method and the echo planar imaging (EPI) -based method, but the image distortion of the 3D acquisition is not established. A purpose of this study is to examine the utility of the novel evaluation method of the image distortion for the 3D acquisition image. The spin echo image was used as a criteria image, and EPI was used as a target image for 3D acquisition imaging. Image J was used for the image processing. The value that divided the volume of the different shape of criteria image and the target image by the volume of the criteria image was defined as a distortion rate. Hence, we refer this method to the volume method. It is reported that the distortion rate of the EPI is proportional to a rectangle rate of rectangular field of view (RFOV). The distortion rate of the volume method for 50-100% of rectangle ratio was compared with the theoretical value. In addition, the dependence by the threshold of the binarization was considered. Furthermore, the comparison with the NEMA (National Electrical Manufacturers Association) method was carried out. As a result, the distortion rate decreased according to a rectangular rate of RFOV by the volume method, and the numerical value was equal with a theoretical value. In addition, the distortion rate did not have the effect by the thresholding of binarizing. The volume method had less error of measurement than the NEMA method.

Capillaries are the most basic and important blood vessel of the circulatory systems. The evaluation of the blood flow may contribute to many studies in future. We evaluated the capillary blood flow change of lower limb muscle over time before and after the exercise used by magnetic resonance imaging-intravoxel incoherent motion (MRI-IVIM) obtained perfusion information. Furthermore, we examined an association between the muscle pain after the exercise and the diffusion weighted image (DWI) indexes. DWI was imaged using multi-b values for a thigh and calf muscles. MRI was performed just after an exercise test, 3, 6, and 24 hours later, and the IVIM index and diffusion index were calculated. Furthermore, we interviewed the degree of the muscle ache 24 hours later. As a result, pseudo diffusion coefficient (D*) and f value as IVIM index increased after-exercise as compared with pre-exercise and decreased in 3 hours later. A similar tendency was found in the apparent diffusion coefficient and the diffusion coefficient as diffusion index. Furthermore, all indexes increased in after exercise from before exercise and decreased with time passed and increased again 24 hours later. In conclusion, IVIM could obtain capillary blood flow information, and it was suggested to contribute for sports medicine in future.

Apparent diffusion coefficient (ADC) values calculated from diffusion-weighted magnetic resonance imaging (DW-MRI) can be used for differentiation of tumors. Clinically, ADC values are used for monitoring treatment response after chemotherapy or radiation. However, it is reported that the threshold of the ADC value differs among institutions. In addition, there are reports regarding the change factor of the ADC value. Slice thickness may induce error in the ADC value by the influence of the partial volume effect in thicker objects, and by the influence of signal-to-noise ratio (SNR) in thinner objects. Therefore, in this study, the effect of slice thickness was examined. The signal body of spherical high-diffusion coefficients of 6, 7.9, and 9.3 mm in diameter was fixed in the low-circumference material of the diffusion coefficient. These phantoms were imaged using DW imaging (DWI) of 1, 1.5, 2, 2.5, 3, 4, 5, 6, 7, 8, 9, 10, 15, and 20 mm slice thickness using the multi-b values. In addition, different SNR were imaged by changing field-of-view and the number of additions. ADC was calculated by DWI of the different b values. As a result, slice thickness showed a peak at 50-65% of the diameter of the signal body. Furthermore, ADC values fluctuated in the slice thickness in front of the peak with a change in SNR. In conclusion, the ADC value was most accurate at a setting of 50-65% of slice thickness for the object diameter.

We performed coordinated observations of AR 12205, which produced a C-class flare on 2014 November 11, with the Interface Region Imaging Spectrograph (IRIS) and the Domeless Solar Telescope (DST) at Hida Observatory. Using spectral data in the Si IV 1403 \AA, C II 1335 \AA, and Mg II h and k lines from IRIS and the Ca II K, Ca II 8542 \AA, and H$\alpha$ lines from DST, we investigated a moving flare kernel during the flare. In the Mg II h line, the leading edge of the flare kernel showed the intensity enhancement in the blue wing, and the smaller intensity of the blue-side peak (h2v) than that of the red-side one (h2r). The blueshift lasted for 9-48 s with a typical speed of 10.1 $\pm$ 2.6 km s$^{-1}$ and it was followed by the high intensity and the large redshift with a speed of up to 51 km s$^{-1}$ detected in the Mg II h line. The large redshift was a common property for all six lines but the blueshift prior to it was found only in the Mg II lines. A cloud modeling of the Mg II h line suggests that the blue wing enhancement with such peak difference can be caused by a chromospheric-temperature (cool) upflow. We discuss a scenario in which an upflow of cool plasma is lifted up by expanding hot plasma owing to the deep penetration of non-thermal electrons into the chromosphere. Furthermore, we found that the blueshift persisted without any subsequent redshift in the leading edge of the flare kernel during its decaying phase. The cause of such long-lasting blueshift is also discussed., Comment: 19 pages, 13 figures, accepted for publication in PASJ, replaced for the correction of the name of one author

The total solar fluxes at 1, 2, 3.75, and 9.4 GHz were observed continuously from 1957 to 1994 at Toyokawa, and from 1994 until now at Nobeyama, Japan with the current Nobeyama Radio Polarimeters. We examined the multi-frequency and long-term datasets, and found that not only the microwave solar flux but also its monthly standard deviation well indicates the long-term variation of solar activity. Furthermore, we found that the microwave spectra at the solar minima of Cycle 20~24 agree with each other. These results show that the average atmospheric structure above the upper chromosphere in the quiet Sun has not varied for half a century, and suggest that the energy input for atmospheric heating from the sub-photosphere to the corona has not changed in the quiet Sun despite significantly differing strengths of magnetic activity in the last five solar cycles., 8 pages, 2 figures, accepted for publication in ApJ

We report a solar jet phenomenon associated with successive flares on November 10th 2014. These explosive events were involved with the satellite spots' emergence around a delta-type sunspot in the decaying active region NOAA 12205. The data of this jet was provided by Solar Dynamic Observatory (SDO), X-Ray Telescope (XRT) aboard Hinode, Interface Region Imaging Spectrograph (IRIS) and Domeless Solar Telescope (DST) at Hida Observatory, Kyoto University. These plentiful data enabled us to present this series of papers to discuss the entire processes of the observed phenomena including the energy storage, event trigger, and energy release. In this paper, we focus on the energy build-up and trigger phases, by analyzing the photospheric horizontal flow field around the active region with an optical flow method. The analysis reveals the following three. (i) The observed explosive phenomena involved three satellite spots, the magnetic fluxes of which successively reconnected with their pre-existing ambient fields. (ii) All of these satellite spots emerged in the moat region of a pivotal delta-type sunspot, especially near its convergent boundary with the neighboring supergranules or moat regions of adjacent sunspots. (iii) Around the jet ejection site, the positive polarities of satellite spot and adjacent emerging flux encountered the global magnetic field with negative polarity in the moat region of the pivotal delta-type sunspot, and thus the polarity inversion line was formed along the convergent boundary of the photospheric horizontal flow channels., Comment: 18 pages, 17 figures, accepted for publication in PASJ, replaced for the correction of reference

On 2011 September 6, we observed an X2.1-class flare in continuum and H$\alpha$ with a frame rate of about 30~Hz. After processing images of the event by using a speckle-masking image reconstruction, we identified white-light (WL) flare ribbons on opposite sides of the magnetic neutral line. We derive the lightcurve decay times of the WL flare kernels at each resolution element by assuming that the kernels consist of one or two components that decay exponentially, starting from the peak time. As a result, 42% of the pixels have two decay-time components with average decay times of 15.6 and 587 s, whereas the average decay time is 254 s for WL kernels with only one decay-time component. The peak intensities of the shorter decay-time component exhibit good spatial correlation with the WL intensity, whereas the peak intensities of the long decay-time components tend to be larger in the early phase of the flare at the inner part of the flare ribbons, close to the magnetic neutral line. The average intensity of the longer decay-time components is 1.78 times higher than that of the shorter decay-time components. If the shorter decay time is determined by either the chromospheric cooling time or the nonthermal ionization timescale and the longer decay time is attributed to the coronal cooling time, this result suggests that WL sources from both regions appear in 42% of the WL kernels and that WL emission of the coronal origin is sometimes stronger than that of chromospheric origin., Comment: 8 pages, 7 figures, and 1 table. Accepted by ApJ

Shibata & Yokoyama (1999, 2002) proposed a method of estimating the coronal magnetic field strengths ($B$) and magnetic loop lengths ($L$) of solar and stellar flares, on the basis of magnetohydrodynamic simulations of the magnetic reconnection model. Using the scaling law provided by Shibata & Yokoyama (1999, 2002), $B$ and $L$ are obtained as functions of the emission measure ($EM=n^2L^3$) and temperature ($T$) at the flare peak. Here, $n$ is the coronal electron density of the flares. This scaling law enables the estimation of $B$ and $L$ for unresolved stellar flares from the observable physical quantities $EM$ and $T$, which is helpful for studying stellar surface activities. To apply this scaling law to stellar flares, we discuss its validity for spatially resolved solar flares. $EM$ and $T$ were calculated from GOES soft X-ray flux data, and $B$ and $L$ are theoretically estimated using the scaling law. For the same flare events, $B$ and $L$ were also observationally estimated with images taken by Solar Dynamics Observatory (SDO)/ Helioseismic and Magnetic Imager (HMI) Magnetogram and Atmospheric Imaging Assembly (AIA) 94�� pass band. As expected, a positive correlation was found between the theoretically and observationally estimated values. We interpret this result as indirect evidence that flares are caused by magnetic reconnection. Moreover, this analysis makes us confident in the validity of applying this scaling law to stellar flares as well as solar flares., 17 pages, 6 figures, accepted for publication in PASJ

We report a strong association between the particle acceleration and plasma motions found in the 2010 August 18 solar flare. The plasma motions are tracked in the extreme-ultraviolet (EUV) images taken by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory and the Extreme UltraViolet Imager (EUVI) on the Solar Terrestrial Relation Observatory spacecraft Ahead, and the signature of particle acceleration was investigated by using Nobeyama Radioheliograph data. In our previous paper, we reported that in EUV images many plasma blobs appeared in the current sheet above the flare arcade. They were ejected bidirectionally along the current sheet, and the blobs that were ejected sunward collided with the flare arcade. Some of them collided or merged with each other before they were ejected from the current sheet. We discovered impulsive radio bursts associated with such plasma motions (ejection, coalescence, and collision with the post flare loops). The radio bursts are considered to be the gyrosynchrotron radiation by nonthermal high energy electrons. In addition, the stereoscopic observation by AIA and EUVI suggests that plasma blobs had a three-dimensionally elongated structure. We consider that the plasma blobs were three-dimensional plasmoids (i.e. flux ropes) moving in a current sheet. We believe that our observation provides clear evidence of particle acceleration associated with the plasmoid motions. We discuss possible acceleration mechanisms on the basis of our results., Comment: 29 pages, 10 figures. Published in ApJ

Recently, many superflares on solar-type stars have been discovered as white-light flares (WLFs). The statistical study found a correlation between their energies ($E$) and durations ($\tau$): $\tau \propto E^{0.39}$ (Maehara et al. 2017 $EP\& S$, 67, 59), similar to those of solar hard/soft X-ray flares: $\tau \propto E^{0.2-0.33}$. This indicates a universal mechanism of energy release on solar and stellar flares, i.e., magnetic reconnection. We here carried out a statistical research on 50 solar WLFs observed with \textit{SDO}/HMI and examined the correlation between the energies and durations. As a result, the $E$--$\tau$ relation on solar WLFs ($\tau \propto E^{0.38}$) is quite similar to that on stellar superflares ($\tau \propto E^{0.39}$). However, the durations of stellar superflares are one order of magnitude shorter than those expected from solar WLFs. We present the following two interpretations for the discrepancy. (1) In solar flares, the cooling timescale of WLFs may be longer than the reconnection one, and the decay time of solar WLFs can be elongated by the cooling effect. (2) The distribution can be understood by applying a scaling law ($\tau \propto E^{1/3}B^{-5/3}$) derived from the magnetic reconnection theory. In this case, the observed superflares are expected to have 2-4 times stronger magnetic field strength than solar flares., Comment: 43 pages, 15 figures, accepted for publication in ApJ

We present a detailed examination on the coronal nonthermal emissions during the preflare phase of the X4.8 flare that occurred on 2002 July 23. The microwave (17 GHz and 34 GHz) data obtained with Nobeyama Radioheliograph, at Nobeyama Solar Radio Observatory and the hard X-ray (HXR) data taken with {\it Reuven Ramaty High Energy Solar Spectroscopic Imager} obviously showed nonthermal sources that are located above the flare loops during the preflare phase. We performed imaging spectroscopic analyses on the nonthermal emission sources both in microwaves and in HXRs, and confirmed that electrons are accelerated from several tens of keV to more than 1 MeV even in this phase. If we assume the thin-target model for the HXR emission source, the derived electron spectral indices ($\sim 4.7$) is the same value as that from microwaves ($\sim 4.7$) within the observational uncertainties, which implies that the distribution of the accelerated electrons follows a single power-law. The number density of the microwave-emitting electrons is, however, larger than that of the HXR-emitting electrons, unless we assume low ambient plasma density of about $1.0 \times 10^9$ cm$^{-3}$ for the HXR-emitting region. If we adopt the thick-target model for the HXR emission source, on the other hand, the electron spectral index ($\sim 6.7$) is much different, while the gap of the number density of the accelerated electrons is somewhat reduced., 21 pages, 6 figures, ApJ accepted

We report a detailed examination of the fine structure inside flare ribbons and the temporal evolution of this fine structure during the X2.5 solar flare that occurred on 2004 November 10. We examine elementary bursts of the C IV (1550{\AA}) emission lines seen as local transient brightenings inside the flare ribbons in the ultraviolet (1600{\AA}) images taken with Transition Region and Coronal Explorer, and we call them C IV kernels. This flare was also observed in Ha with the Sartorius 18 cm Refractor telescope at Kwasan observatory, Kyoto University, and in hard X-rays (HXR) with Reuven Ramaty High Energy Solar Spectroscopic Imager. Many C IV kernels, whose sizes were comparable to or less than 2", were found to brighten successively during the evolution of the flare ribbon. The majority of them were well correlated with the Ha kernels in both space and time, while some of them were associated with the HXR emission. These kernels were thought to be caused by the precipitation of nonthermal particles at the footpoints of the reconnecting flare loops. The time profiles of the C IV kernels showed intermittent bursts, whose peak intensity, duration, and time interval were well described by power-law distribution functions. This result is interpreted as evidence for "self-organized criticality" in avalanching behavior in a single flare event, or for fractal current sheets in the impulsive reconnection region., Comment: (11 pages, 3 figures)

We have studied the characteristics of the non-Gaussian line profile of the Fe XIV 274.20 A line in and around a flare arcade. We found that broad non-Gaussian line profiles associated with redshifts are observed in the flare arcade. There were two typical types of broad line profiles. One was a distorted line profile caused by multiple flows, and the other was a symmetric line profile without any additional component. We successfully distinguished those two types using higher order statistical moments or M—the additional component contribution—defined in this Letter. The distorted/symmetric broad line profiles were preferentially observed in new/old flare loops, respectively.

Coronal structure of active regions appearing in coronal holes is studied by using the data obtained with the Soft X-Ray Telescope (SXT) aboard {\it Yohkoh} from 1991 November to 1993 March. The following characteristics are found; Many of active regions appearing in coronal holes show a structure that looks like a ``sea-anemone''. Such active regions are called {\it anemone ARs}. About one-forth of all active regions that were observed with SXT from their births showed the anemone structure. For almost all the anemone ARs, the order of magnetic polarities is consistent with the Hale-Nicholson's polarity law. These anemone ARs also showed more or less east-west asymmetry in X-ray intensity distribution, such that the following (eastern) part of the ARs is brighter than its preceding (western) part. This, as well as the anemone shape itself, is consistent with the magnetic polarity distribution around the anemone ARs. These observations also suggest that an active region appearing in coronal holes has simpler (less sheared) and more preceding-spot-dominant magnetic structure than those appearing in other regions., Comment: 11 pages, 3 tables, 4 figures

We present an examination of multiwavelength observations of an M8.2 long-duration flare which occurred on 2000 November 25. During the flare, we can see a hard X-ray (HXR) source on one Hα flare ribbon in the HXR images obtained with the Hard X-ray Telescope aboard Yohkoh, and a compact microwave emission source on the other flare ribbon in the data taken with the Nobeyama Radioheliograph, while we can also see an extended microwave emission source that connects both of these emission sources. The compact microwave and HXR sources clearly showed gradual hardening tendencies in their spectra. In addition, we found energy-dependent delays of the peak times in the HXR bursts and concluded that almost all of the accelerated electrons are trapped in magnetic loops to generate the extended microwave source and are dripping into the chromosphere at the compact microwave and the HXR emission sites. We then performed imaging spectroscopic analyses to the microwave emission sources. The temporal evolutions of the flux and the spectral index of the compact microwave footpoint source are quite similar to those of the HXR source, which is mainly emitted at the other footpoint, while those at the loop-top extended source do not show this similarity. Moreover, there is a constant gap between the electron spectral index derived from the microwave footpoint source and that from the HXR source. We also discuss the constant gap, based on the trapped and dripping model.

We studied the M3.7 class flare which occurred on 2005 July 27, in the active region NOAA 10792. This flare is an over-the-limb flare, and the footpoints are entirely occulted by the solar disk. The microwave and the hard X-ray images obtained with the Nobeyama Radioheliograph and the RHESSI satellite, respectively, clearly showed emission sources above the post-flare loop system. We examined the emission sources in detail spatially, temporally, and spectroscopically. As a result, one of the hard X-ray emission sources and the microwave emission source are nonthermal.

Aims. We aim to examine the precursor phases and early evolution of a prominence eruption associated with a M4-class flare and a partial halo coronal mass ejection (CME) observed on 2005 July 27. Our main goal is to investigate the precursor eruption signatures observed in EUV, X-ray and microwave emission and their relation to the prominence destabilisation. Methods. We perform a multi-wavelength study of the prominence morphology and motion using high-cadence and spatial resolution EUV 171 A images from the TRACE satellite. The high-temperature flare radiative emission in soft and hard X-rays are analysed through imaging and spectral modeling with RHESSI. Complementary microwave images (17 GHz and 34 GHz) from NoRH are also investigated. Results. The activation of the filament proceeds from one anchored footpoint. We observe "pre-eruption" brightenings in X-ray and EUV images, close to the erupting footpoint of the prominence, being temporally correlated to the point when the prominence first enters a slow-rise phase, and then an accelerated fast-rise phase. The brightness temperature (T b ) of the prominence at 34 GHz is increasing during the eruption. We also find very good correlation between the prominence height-time profile and the spatially integrated soft X-ray (SXR) emission. Conclusions. We discuss the observed precursor brightenings with respect to possible mechanisms that might be responsible for the prominence destabilisation and acceleration. Our observations suggest that reconnection events localised beneath the erupting footpoint may eventually destabilise the entire prominence, causing the eruption.

We report a detailed examination about the relationship between the evolution of the Hα flare ribbons and the released magnetic energy during the April 10 2001 flare. In the Hα images, several bright kernels are observed in the flare ribbons. We identified the conjugated foot-points, by analyzing the lightcurves at each Hα kernels, and showed their connectivities during the flare. Then, based on the magnetic reconnection model, we calculated quantitatively the released energy by using the photospheric magnetic field strengths and separation speeds of the Hα flare ribbons. Finally, we examined the downward motions which are observed at the Hα kernels. We found that the stronger the red-asymmetry tends to be associated with the brighter the Hα kernel.

We newly developed a method of limb-event detection for the Nobeyama Radiograph, and show the results over one solar-cycle, 1992 July–2004 December. We detected 785 prominence activities and 31 flares on the limb by this method. We investigated the relationship between the distributions of the prominence activities and the solar cycle. As a result, we found the following facts: 1) The variation in the number of prominence activities is similar to that of sunspots during one solar cycle. 2) There are differences between the peak times of prominence activities and sunspots. 3) The frequency distribution as a function of the magnitude of the prominence activities (the size of activated prominences) at each phase shows a power-law distribution. The power-law index of the distribution does not change, except around the solar minimum. 4) The number of prominence activities has a dependence on the latitude. On the other hand, the average magnitude is independent of the latitude. 5) During the rise phase of the solar cycle, the location of the high-latitude prominence activities migrates to the pole region. 6) After a solar polarity reversal, the location of the prominence activities in the northern hemisphere migrates to the equator. On the other hand, the prominence activities in the southern hemisphere occurred in the high-latitude region until the decay phase of Cycle 23.

From 1926 to 1969, a long term solar full disk observation was done by Kyoto University. Daily, Ca II K (393.4 nm) spectroheliographic images and white light images were recorded on photographic plates. In this report, we will give the current status of our project to digitize all these images and to construct a database of these images for public use through the IUGONET system. In addition, we will discuss our perspective on the scientific analysis of the database by taking the solar CaII K brightness as a proxy measure of the solar UV irradiance on the terrestrial upper atmosphere.

We present an examination of multiwavelength observations of three X-class homologous flares that occurred on 2000 November 24. By investigating the behavior of the two-ribbon flares in Hα and the ultraviolet (1600 A), we found that the temporal variation of the distance between the two ribbons shows a good correlation with the soft X-ray light curve. From this finding we can derive the relation dISXR(t)/dt ∝ Vrib(t), where ISXR(t) is the soft X-ray intensity and Vrib(t) is the separation velocity of the two ribbons. This relation is similar to the well-known empirical law, the Neupert effect. We also measured the rise time, velocity of the plasmoid/filament ejection, and separation velocity of the two ribbons for each of these homologous flares. Since the magnetic reconnection model predicts that each of these physical parameters has a dependence on the coronal magnetic field strength Bc, we derived the relative Bc between the three flares from each of the parameters. We compared the relative Bc values, which are derived from those parameters, and found that they are roughly equal. Our results successfully support the magnetic reconnection model. Moreover, the relative hard X-ray maximum intensity in each flare is consistent with the relative Bc derived above, if we assume that the hard X-ray intensity is proportional to the energy release rate as implied by the Neupert effect.

We have examined the relation between the evolution of the Hα flare ribbons and the released magnetic energy in a solar flare that occurred on 2001 April 10. Based on the magnetic reconnection model, the released energy was quantitatively calculated by using the photospheric magnetic field strengths and separation speeds of the fronts of the Hα flare ribbons. We compared the variation of the released energy with the temporal and spatial fluctuations in the nonthermal radiation observed in hard X-rays and microwaves. These nonthermal radiation sources indicate when and where large energy releases occur. We also estimated the magnetic energy released during the flare. The estimated energy release rates in the Hα kernels associated with the hard X-ray sources are locally large enough to explain the difference between the spatial distributions of the Hα kernels and the hard X-ray sources. We also reconstructed the peaks in the nonthermal emission by using the estimated energy release rates.

We present a detailed examination of downflow motions above flare loops observed in the 2002 July 23 flare. The extreme-ultraviolet images obtained with the Transition Region and Coronal Explorer show dark downflow motions (sunward motions) above the postflare loops, not only in the decay phase but also in the impulsive and main phases. We also found that the times when the downflow motions start to be seen correspond to the times when bursts of nonthermal emissions in hard X-rays and microwaves are emitted. This result implies that the downflow motions occurred when strong magnetic energy was released and that they are, or are correlated with, reconnection outflows.

We report a detailed examination of the fine structure inside flare ribbons and the temporal evolution of such structure during an X2.3 solar flare, which occurred on 2001 April 10. We examined fine structures, such as systems of conjugate footpoints, inside flare ribbons by using the Hα images obtained with the Sartorius telescope at Kwasan Observatory, Kyoto University. We identified the conjugate footpoints of each Hα kernel in both flare ribbons by a new method that uses cross-correlation functions of the light curves. We also compared the sites of the Hα kernels with the spatial configurations of flare loops seen in the extreme-ultraviolet images obtained with the Transition Region and Coronal Explorer. We found that the highly correlated pairs of Hα kernels were connected by flare loops seen in the 171 A images. Investigating such fine structures inside the flare ribbons, we can follow the history of energy release and perhaps acquire key information about particle acceleration.

We present the relation of the spatial distribution of H-alpha kernels with the distribution of hard X-ray (HXR) sources seen during the 2001 April 10 solar flare. This flare was observed in H-alpha with the (Sartorius) telescope at Kwasan Observatory, Kyoto University, and in hard X-rays (HXRs) with the Hard X-ray Telescope (HXT) onboard Yohkoh. We compared the spatial distribution of the HXR sources with that of the H-alpha kernels. While many H-alpha kernels are found to brighten successively during the evolution of the flare ribbons, only a few radiation sources are seen in the HXR images. We measured the photospheric magnetic field strengths at each radiation source in the H-alpha images, and found that the H-alpha kernels accompanied by HXR radiation have magnetic strengths about 3 times larger than those without HXR radiation. We also estimated the energy release rates based on the magnetic reconnection model. The release rates at the H-alpha kernels with accompanying HXR sources are 16-27 times larger than those without HXR sources. These values are sufficiently larger than the dynamic range of HXT, which is about 10, so that the difference between the spatial distributions of the H$\alpha$ kernels and the HXR sources can be explained., Comment: 10 pages, 3 figures