arXiv subjects

Satoshi Masuda

Publications and source records attributed to Satoshi Masuda.

26 records · Page 2Linked to original sources

arXiv2013

Systematic Microwave Source Motions along Flare-arcade Observed by Nobeyama Radioheliograph and AIA/SDO

We found systematic microwave source motions along a flare-arcade using Nobeyama Radioheliograph (NoRH) 17 GHz images. The motions were associated with a X-class disk flare which occurred on 15th February 2011. For this study, we also used EUV images from Atmospheric Imaging Assembly (AIA) and magnetograms from Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamics Observatory, and multi-channel microwave data from Nobeyama Radiopolarimeters (NoRP) and Korean Solar Radio Burst Locator (KSRBL). We traced centroids of the microwave source observed by NoRH 17 GHz during the flare and found two episodes of the motion with several facts: 1) The microwave source moved systematically along the flare-arcade, which was observed by the AIA 94 A in a direction parallel to the neutral line. 2) The period of each episode was 5 min and 14 min, respectively. 3) Estimated parallel speed was 34 km/s for the first episode and 22 km/s for the second episode. The spectral slope of microwave flux above 10 GHz obtained by NoRP and KSRBL was negative for both episodes, and for the last phase of the second episodes, it was flat with the flux of 150 sfu. The negative spectrum and the flat with high flux indicate that the gyrosynchrotron emission from accelerated electrons was dominant during the source motions. The sequential images from the AIA 304 A and 94 A channels revealed that there were successive plasma eruptions and each eruption was initiated just before the start time of the microwave sources motion. Based on the results, we suggest that the microwave source motion manifests the displacement of the particle acceleration site caused by plasma eruptions.

astro-ph.SR↗

arXiv2013

Emission Height and Temperature Distribution of White-Light Emission Observed by Hinode/SOT from the 2012 January 27 X-class Solar Flare

White-light emissions were observed from an X1.7 class solar flare on 27 January 2012, using three continuum bands (red, green, and blue) of the Solar Optical Telescope (SOT) onboard the Hinode satellite. This event occurred near the solar limb, and so differences in locations of the various emissions are consistent with differences in heights above the photosphere of the various emission sources. Under this interpretation, our observations are consistent with the white-light emissions occurring at the lowest levels of where the Ca II H emission occurs. Moreover, the centers of the source regions of the red, green, and blue wavelengths of the white-light emissions are significantly displaced from each other, suggesting that those respective emissions are emanating from progressively lower heights in the solar atmosphere. The temperature distribution was also calculated from the white-light data, and we found the lower-layer emission to have a higher temperature. This indicates that high-energy particles penetrated down to near the photosphere, and deposited heat into the ambient lower layers of the atmosphere.

astro-ph.SR↗

arXiv2012

Temporal and Spatial Analyses of Spectral Indices of Nonthermal Emissions Derived from Hard X-Rays and Microwaves

We studied electron spectral indices of nonthermal emissions seen in hard X-rays (HXRs) and in microwaves. We analyzed 12 flares observed by the Hard X-ray Telescope aboard {\it Yohkoh}, Nobeyama Radio Polarimeters (NoRP), and the Nobeyama Radioheliograph (NoRH), and compared the spectral indices derived from total fluxes of hard X-rays and microwaves. Except for four events, which have very soft HXR spectra suffering from the thermal component, these flares show a gap $Δδ$ between the electron spectral indices derived from hard X-rays $δ_{X}$ and those from microwaves $δ_μ$ ($Δδ= δ_{X} - δ_μ$) of about 1.6. Furthermore, from the start to the peak times of the HXR bursts, the time profiles of the HXR spectral index $δ_{X}$ evolve synchronously with those of the microwave spectral index $δ_μ$, keeping the constant gap. We also examined the spatially resolved distribution of the microwave spectral index by using NoRH data. The microwave spectral index $δ_μ$ tends to be larger, which means a softer spectrum, at HXR footpoint sources with stronger magnetic field than that at the loop tops. These results suggest that the electron spectra are bent at around several hundreds of keV, and become harder at the higher energy range that contributes the microwave gyrosynchrotron emission.

astro-ph.SR↗

arXiv2011

Coronal Electron Distribution in Solar Flares: Drift-Kinetic Model

Using a model of particle acceleration and transport in solar flares, we investigate the height distribution of coronal electrons by focusing on the energy-dependent pitch-angle scattering. When pitch-angle scattering is not included, the peak heights of loop-top electrons are constant, regardless of their energy, owing to the continuous acceleration and compression of the electrons via shrinkage of magnetic loops. On the other hand, under pitch-angle scattering, the electron heights are energy dependent; intermediate energy electrons are at a higher altitude, whereas lower and higher energy electrons are at lower altitudes. This implies that the intermediate energy electrons are inhibited to follow the shrinking field lines to lower altitudes because pitch-angle scattering causes efficient precipitation of these electrons into the footpoint and their subsequent loss from the loop. This result is qualitatively consistent with the position of the above-the-loop-top hard X-ray (HXR) source that is located above coronal HXR loops emitted by lower energy electrons and microwaves emitted by higher energy electrons. Quantitative agreement with observations might be achieved by considering primary acceleration before the onset of loop shrinkage and additional pitch-angle scattering via wave-particle interactions.

astro-ph.SR↗

arXiv2010

G-band and Hard X-ray Emissions of the 2006 December 14 flare observed by Hinode/SOT and RHESSI

We report on G-band emission observed by the Solar Optical Telescope onboard the Hinode satellite in association with the X1.5-class flare on 2006 December 14. The G-band enhancements originate from the footpoints of flaring coronal magnetic loops, coinciding with non-thermal hard X-ray bremsstrahlung sources observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager. At the available 2 minute cadence, the G-band and hard X-ray intensities are furthermore well correlated in time. Assuming that the G-band enhancements are continuum emission from a blackbody, we derived the total radiative losses of the white-light flare (white-light power). If the G-band enhancements additionally have a contribution from lines, the derived values are overestimates. We compare the white-light power with the power in hard X-ray producing electrons using the thick target assumption. Independent of the cutoff energy of the accelerated electron spectrum, the white-light power and the power of accelerated electrons are roughly proportional. Using the observed upper limit of ~30 keV for the cutoff energy, the hard X-ray producing electrons provide at least a factor of 2 more power than needed to produce the white-light emission. For electrons above 40 keV, the powers roughly match for all four of the time intervals available during the impulsive phase. Hence, the flare-accelerated electrons contain enough energy to produce the white-light flare emissions. The observed correlation in time, space, and power strongly suggests that electron acceleration and white-light production in solar flares are closely related. However, the results also call attention to the inconsistency in apparent source heights of the hard X-ray (chromosphere) and white-light (upper photosphere) sources.

astro-ph.SR↗

arXiv2009

Imaging Spectroscopy on Preflare Coronal Nonthermal Sources Associated with the 2002 July 23 Flare

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.

astro-ph.SR↗

arXiv2005

A Statistical Study of Gamma-Ray Emitting Solar Flares Observed with Yohkoh

Gamma-ray emitting solar flares observed with Yohkoh were analyzed from a statistical viewpoint. The four-band hard X-ray (15--95 keV) photometric data, taken with the Hard X-ray Telescope onboard Yohkoh, were utilized in combination with the spectro-photometric gamma-ray (0.2--30 MeV) data obtained with the Gamma-Ray Spectrometer. The GOES class was also incorporated. Out of 2788 X-ray flares observed from 1991 October to 2001 December, 178 events with strong hard X-ray emission were selected. Among them, 40 flares were further found to show significant gamma-ray emission. A fractal dimension analysis and multi-band color--color plots of the 40 flares suggest that their soft X-ray to MeV gamma-ray spectral energy distributions involve at least four independent parameters. These are: (1) the overall flare size; (2) the relative intensities of the thermal vs. non-thermal signals; (3) the gamma-ray to hard X-ray intensity ratio; and (4) the hard X-ray spectral slope. These results are examined for possible selection effects. Also, the meanings of the third parameter are briefly considered.

astro-ph↗

arXiv2002

Difference between Spatial Distributions of the H-alpha Kernels and Hard X-Ray Sources in a Solar Flare

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$α$ kernels and the HXR sources can be explained.

astro-ph↗