  The spectrum of gyrosynchrotron emission from solar flares generally peaks in
the microwave range. Its optically-thin, high-frequency component, above the
spectral peak, is often used for diagnostics of the nonthermal electrons and
the magnetic field in the radio source. Under favorable conditions, its
low-frequency counterpart brings additional, complementary information about
these parameters as well as thermal plasma diagnostics, either through
gyrosynchrotron self-absorption, free-free absorption by the thermal plasma, or
the suppression of emission through the so-called Razin effect. However, their
effects on the low-frequency spectrum are often masked by spatial
nonuniformity. To disentangle the various contributions to low-frequency
gyrosynchrotron emission, a combination of spectral and imaging data is needed.
To this end, we have investigated Owens Valley Solar Array (OVSA)
multi-frequency images for 26 solar bursts observed jointly with Reuven Ramaty
High Energy Solar Spectroscopic Imager (RHESSI) during the first half of 2002.
For each, we examined dynamic spectra, time- and frequency-synthesis maps,
RHESSI images with overlaid OVSA contours, and a few representative
single-frequency snapshot OVSA images. We focus on the frequency dependence of
microwave source sizes derived from the OVSA images and their effect on the
low-frequency microwave spectral slope. We succeed in categorizing 18 analyzed
events into several groups. Four events demonstrate clear evidence of being
dominated by gyrosynchrotron self-absorption, with an inferred brightness
temperature of $\geq10^8$~K. The low-frequency spectra in the remaining events
are affected to varying degree by Razin suppression. We find that many radio
sources are rather large at low frequencies, which can have important
implications for solar energetic particle production and escape.
