56 research outputs found
Center-to-Limb Variation of Radio Emissions from Thermal-Rich and Thermal-Poor Solar Flares
A statistical analysis of radio flare events was performed by using the event
list of Nobeyama Radioheliograph in 1996-2009. We examined center-to-limb
variations of 17GHz and 34GHz flux by dividing the flare events into different
groups with respect to the 'thermal plasma richness' (ratio of the peak flux of
soft X-ray to non-thermal radio emissions) and the duration of radio bursts. It
is found that peak flux of 17 and 34GHz tend to be higher toward the limb for
thermal-rich flares with short durations. We propose that the thermal-rich
flares, which are supposed to be associated with an efficient precipitation of
high energy particles into the chromosphere, have a pitch angle distribution of
non-thermal electrons with a higher population along the flare loop.Comment: 12 pages, 5 figure
Prominence Activation by Coronal Fast Mode Shock
An X5.4 class flare occurred in active region (AR) NOAA11429 on 2012 March 7.
The flare was associated with very fast coronal mass ejection (CME) with its
velocity of over 2500 km/s. In the images taken with STEREO-B/COR1, a dome-like
disturbance was seen to detach from expanding CME bubble and propagated
further. A Type-II radio burst was also observed at the same time. On the other
hand, in EUV images obtained by SDO/AIA, expanding dome-like structure and its
foot print propagating to the north were observed. The foot print propagated
with its average speed of about 670 km/s and hit a prominence located at the
north pole and activated it. While the activation, the prominence was strongly
brightened. On the basis of some observational evidence, we concluded that the
foot print in AIA images and the ones in COR1 images are the same, that is MHD
fast mode shock front. With the help of a linear theory, the fast mode mach
number of the coronal shock is estimated to be between 1.11 and 1.29 using the
initial velocity of the activated prominence. Also, the plasma compression
ratio of the shock is enhanced to be between 1.18 and 2.11 in the prominence
material, which we consider to be the reason of the strong brightening of the
activated prominence. The applicability of linear theory to the shock problem
is tested with nonlinear MHD simulation
Solar Energetic Particle Events with Short and Long Onset Times
Gradual solar energetic particle (SEP) events, usually attributed to shock
waves driven by coronal mass ejections (CMEs), show a wide variety of temporal
behaviors. For example, TO, the >10 MeV proton onset time with respect to the
launch of the CME, has a distribution of at least an order of magnitude, even
when the source region is not far from the so-called well-connected longitudes.
It is important to understand what controls TO, especially in the context of
space weather prediction. Here we study two SEP events from the western
hemisphere that are different in TO on the basis of >10 MeV proton data from
the Geostationary Operations Environmental Satellite, despite similar in the
CME speed and longitude of the source regions. We try to find the reasons for
different TO, or proton release times, in how the CME-driven shock develops and
the Alfv\'en Mach number of the shock wave reaches some threshold, by combining
the CME height-time profiles with radio dynamic spectra. We also discuss how
CME-CME interactions and active region properties may affect proton release
times.Comment: 14 pages, 8 figures, accepted for publication in Ap
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 between the electron spectral indices derived from
hard X-rays and those from microwaves
() of about 1.6. Furthermore, from
the start to the peak times of the HXR bursts, the time profiles of the HXR
spectral index evolve synchronously with those of the microwave
spectral index , keeping the constant gap. We also examined the
spatially resolved distribution of the microwave spectral index by using NoRH
data. The microwave spectral index 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.Comment: 24 pages, 6 figures, accepted for publication in Ap
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 () is the same value as that from
microwaves () 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 cm 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 () is much different, while the gap of the
number density of the accelerated electrons is somewhat reduced.Comment: 21 pages, 6 figures, ApJ accepte
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