3 research outputs found
3D Structure of Microwave Sources from Solar Rotation Stereoscopy vs Magnetic Extrapolations
We use rotation stereoscopy to estimate the height of a steady-state solar
feature relative to the photosphere, based on its apparent motion in the image
plane recorded over several days of observation. The stereoscopy algorithm is
adapted to work with either one- or two-dimensional data (i.e. from images or
from observations that record the projected position of the source along an
arbitrary axis). The accuracy of the algorithm is tested on simulated data, and
then the algorithm is used to estimate the coronal radio source heights
associated with the active region NOAA 10956, based on multifrequency imaging
data over 7 days from the Siberian Solar Radio Telescope near 5.7 GHz, the
Nobeyama Radio Heliograph at 17 GHz, as well as one-dimensional scans at
multiple frequencies spanning the 5.98--15.95 GHz frequency range from the
RATAN-600 instrument. The gyroresonance emission mechanism, which is sensitive
to the coronal magnetic field strength, is applied to convert the estimated
radio source heights at various frequencies, h(f), to information about
magnetic field vs. height B(h), and the results are compared to a magnetic
field extrapolation derived from photospheric magnetic field observations
obtained by Hinode and MDI. We found that the gyroresonant emission comes from
the heights exceeding location of the third gyrolayer irrespectively on the
magnetic extrapolation method; implications of this finding for the coronal
magnetography and coronal plasma physics are discussed.Comment: 26 pages, 13 figures, ApJ accepte
Thermal to Nonthermal Energy Partition at the Early Rise Phase of Solar Flares
In some flares the thermal component appears much earlier than the nonthermal
component in X-ray range. Using sensitive microwave observations we revisit
this finding made by Battaglia et al. (2009) based on RHESSI data analysis. We
have found that nonthermal microwave emission produced by accelerated electrons
with energy of at least several hundred keV, appears as early as the thermal
soft X-ray emission indicative that the electron acceleration takes place at
the very early flare phase. The non-detection of the hard X-rays at that early
stage of the flares is, thus, an artifact of a limited RHESSI sensitivity. In
all considered events, the microwave emission intensity increases at the early
flare phase. We found that either thermal or nonthermal gyrosynchrotron
emission can dominate the low-frequency part of the microwave spectrum below
the spectral peak occurring at 3-10 GHz. In contrast, the high-frequency
optically thin part of the spectrum is always formed by the nonthermal,
accelerated electron component, whose power-law energy spectrum can extend up
to a few MeV at this early flare stage. This means that even though the total
number of accelerated electrons is small at this stage, their nonthermal
spectrum is fully developed. This implies that an acceleration process of
available seed particles is fully operational. While, creation of this seed
population (the process commonly called `injection' of the particles from the
thermal pool into acceleration) has a rather low efficiency at this stage,
although, the plasma heating efficiency is high. This imbalance between the
heating and acceleration (in favor of the heating) is difficult to reconcile
within most of available flare energization models. Being reminiscent of the
tradeoff between the Joule heating and runaway electron acceleration, it puts
additional constraints on the electron injection into the acceleration process.Comment: 11 pages, 12 figures, accepted for Ap