First detection of the impulsive and extended phases of a solar radio burst above 200 GHz

Abstract

We present a detailed analysis of radio observations obtained at 212 and 405 GHz during the 2000 March 22 Hα 2N flare that occured in AR8910 at ∼1834 UT. These data are compared with microwave, soft X-ray and hard X-ray measurements of this flare, While the flare emission is not clearly detected at 405 GHz, the time profile of the 212 GHz emission exhibits an impulsive burst, associated in time with the 1-18 GHz impulsive microwave burst and a long-lasting thermal burst which finishes at about the same time as the soft X-ray emission but reaches its maximum later. The 212 GHz impulsive emission and the lack of detection at 405 GHz are consistent with synchrotron radiation from a population of ultrarelativistic electrons in an average magnetic field of 400-600 G. This radiating population of electrons has a hard energy spectrum (power law index ≈ 2.7). The expected > 1 MeV gamma-ray continuum emission from the radio emitting electrons is comparable to that detected for mid-size electron-dominated events and the hard X-ray flux they would produce at 100 keV is consistently lower than the upper limit inferred from the observations. It is shown that the 212 GHz thermal source has to be different from that radiating the soft X-ray and microwave thermal emission. The present observations of a solar burst provide the first evidence of the extension of the gyrosynchrotron spectrum of an impulsive radio burst in the synchrotron domain above 200 GHz.