Detection of a cyclotron line in the radio spectrum of a solar active region and its interpretation

Observations of the active region AR 7962 obtained at 2-32 cm on the RATAN- BOO radio telescope on May 10-12, 1996, are presented. The high-resolution measurements detected a narrow feature near 8.5 cm against the background o f the smooth spectrum of the local source associated with sunspots. This na rrow-band emission is identified with a bright, pointlike, high-frequency s ource at 1.7 cm recorded on maps made using the Nobeyama radio telescope. T he characteristics of the observed line (lifetime 3 days, brightness temper ature of the order of several million Kelvin, relative width of about 10%) suggest that it can be explained as thermal cyclotron radiation at the thir d harmonic of the electron gyrofrequency from a compact source containing a dense, hot plasma; the corresponding higher frequency emission could be du e to thermal Bremsstrahlung. Analysis of the RATAN-600 and Nobeyama data ca n be used to probe the magnetic field, kinetic temperature, and electron de nsity in the radiation source in the corona. (C) 2000 MAIK "Nauka/Interperi odica".