Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Implications of Flat Optically Thick Microwave Spectra in Solar Flares
  for Source Size and Morphology   The study aims to examine the spectral dynamics of the low-frequency,
optically thick gyrosynchrotron microwave emission in solar flares to determine
the characteristics of the emitting source. We present the high-resolution
spectra of a set of microwave bursts observed by the Expanded Owens Valley
Solar Array (EOVSA) during its commissioning phase in the $2.5-18$ GHz
frequency range with $1$ second time resolution. Out of the 12 events analyzed
in this study, nine bursts exhibit a direct decrease with time in the optically
thick spectral index $\alpha_l$, an indicator of source morphology.
Particularly, five bursts display "flat" spectrum ($\alpha_l\leq1.0$) compared
to that expected for a homogeneous/uniform source ($\alpha_l\approx2.9$). These
flat spectra at the low-frequencies (<$10$ GHz) can be defined as the emission
from a spatially inhomogeneous source with a large area and/or with multiple
emission components. In a subset of six events with partial cross-correlation
data, both the events with flat spectra show a source size of $\sim120$ arcsec
at $2.6-3$ GHz. Modeling based on inhomogeneity supports the conclusion that
multiple discrete sources can only reproduce a flat spectrum. We report that
these flat spectra appear predominantly in the decay phase and typically grow
flatter over the duration in most of the bursts, which indicates the increasing
inhomogeneity and complexity of the emitting volume as the flare progresses.
This large volume of flare emission filled with the trapped energetic particles
is often invisible in other wavelengths, like hard X-rays, presumably due to
the collisionless conditions in these regions of low ambient density and
magnetic field strength.

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The categories of this paper are: solv-int
19
The categories of this paper are: eess
17
The categories of this paper are: q-fin
33
The categories of this paper are: plasm-ph
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The categories of this paper are: quant-ph
11
The categories of this paper are: hep-lat
4
The categories of this paper are: cond-mat
32
The categories of this paper are: atom-ph
25
The categories of this paper are: patt-sol
16
The categories of this paper are: stat
8
The categories of this paper are: hep-ex
34
The categories of this paper are: chem-ph
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The categories of this paper are: bayes-an
18
The categories of this paper are: econ
10
The categories of this paper are: q-bio
28
The categories of this paper are: alg-geom