Hot-onset flare precursors viewed at cm-mm wavelengths

The GOES soft X-ray data have recently been interpreted to show a “hot onset precursor event” (HOPE for short) phenomenon prior to the impulsive phase of a flare (2021MNRAS.501.1273H). During the HOPE development, which occurs in most flares and lasts for tens of seconds to tens of minutes, the GOES isothermal fits show a smooth increase of emission measure in the range 0.001-0.1 EM_49, which corresponds to a negligible microwave (free-free) flux level for whole-Sun observations such as the RSTN and NoRP data. An X-ray emission measure of 10⁴⁹ cm^-3 corresponds to about 10 SFU in the optically-thin region of the Rayleigh-Jeans emission spectrum. The HOPE normally appears as a gradual increase, rather than a discrete event or set of events. We have found that the most powerful events may have cm-mm HOPE counterparts ranging up to 100 SFU, with microwave spectra peaking in the few GHz range but extending to 15.4~GHz in SOL2002-07-23. We tentatively interpret them as thermal gyroresonance emission resulting from the hot-onset process operating in strong magnetic fields in and near sunspots. In this presentation we report on a survey of Nobeyama Radio Polarimeter observations of these radio HOPE signatures and interpret them as characterizing the preflare magnetic structure. We also descrbe their possible utility in anticipating the magnitude of a solar flare by a few minutes.