Mysteries of the 17 May 2012 Solar Event Responsible for GLE71: II. Features of the Flare and Its Atypical Microwave Emission

Abstract

As known, large near-Earth proton enhancements usually occur after major eruptive solar flares accompanied by strong microwave bursts. Typically, the spectral-maximum frequency of such a burst exceeds 10 GHz, and the flux exceeds 104 sfu. Ground-level cosmic-ray enhancements (GLEs) are the most energetic subset of large proton events, and it seems that microwave bursts in GLE-associated flares should follow this pattern. This is true in most cases, but in individual events that have produced GLEs, only moderate microwave bursts have been observed. In particular, in the SOL2012-05-17 event responsible for GLE71, the spectral-maximum frequency of the microwave burst did not exceed 10 GHz, and the flux did not reach 103 sfu. We found that the temporal profile of the microwave burst followed the smoothed magnetic-reconnection rate, lagging behind it by about 50 s and that the burst properties were determined by the following circumstances: i) the magnetic configuration was asymmetric, and ii) the sources of the gyrosynchrotron emission were the entire flare arcade and a compact region above the sunspot umbra. Observations directly demonstrated these features, which were previously inferred for the SOL2001-12-26 event responsible for GLE63. A long-known discrepancy was observed between the estimates of the electron spectrum obtained from hard X-rays and microwaves. However, the hardening of the spectrum of trapped electrons that has been invoked to explain this discrepancy was not found in this event. Indications of a relationship between flare processes and proton acceleration are discussed.