  This paper presents a solely thermal flare, which we detected in the
microwave range from the thermal gyro- and free-free emission it produced. An
advantage of analyzing thermal gyro emission is its unique ability to precisely
yield the magnetic field in the radiating volume. When combined with
observationally-deduced plasma density and temperature, these magnetic field
measurements offer a straightforward way of tracking evolution of the magnetic
and thermal energies in the flare. For the event described here, the magnetic
energy density in the radio-emitting volume declines over the flare rise phase,
then stays roughly constant during the extended peak phase, but recovers to the
original level over the decay phase. At the stage where the magnetic energy
density decreases, the thermal energy density increases; however, this increase
is insufficient, by roughly an order of magnitude, to compensate for the
magnetic energy decrease. When the magnetic energy release is over, the source
parameters come back to nearly their original values. We discuss possible
scenarios to explain this behavior.
