  This paper investigates the incidence of coherent emission in solar radio
bursts, using a revised catalog of 3800 solar radio bursts observed by the
Nobeyama Radio Polarimeters from 1988 to 2023. We focus on the 1.0 and 2.0 GHz
data, where radio fluxes of order 10 billion Jansky have been observed.
Previous work has suggested that these bursts are due to electron cyclotron
maser (ECM) emission. In at least one well studied case, the bright emission at
1 GHz consists of narrowband spikes of millisecond duration. Coherent emission
at 1 GHz can be distinguished from traditional incoherent gyrosynchrotron flare
emission based on the radio spectrum: gyrosynchrotron emission at 1 GHz usually
has a spectrum rising with frequency, so bursts in which 1 GHz is stronger than
higher frequency measurements are unlikely to be incoherent gyrosynchrotron.
Based on this criterion it is found that, for bursts exceeding 100 sfu,
three-quarters of all bursts at 1 GHz and half of all 2 GHz bursts have a
dominant coherent emission component, assumed to be ECM. The majority of the
very bright bursts at 1 GHz are highly circularly polarized, consistent with a
coherent emission mechanism, but not always 100% polarized. The frequency range
from 1 to 2 GHz is heavily utilized for terrestrial applications, and these
results are relevant for understanding the extreme flux levels that may impact
such applications. Further, they provide a reference for comparison with the
study of ECM emission from other stars and potentially exoplanets.
