First measurements of the effective angular size of solar flares at 210 GHz

T Luethi

Andreas Luedi, Andreas Magun

A 210 GHz multibeam receiver for the observation of solar flares was installed in the KOSMA 3m telescope on Gornergrat. It consists of three radiometer channels, with a fourth beam synthesised from the other three. The four intersecting beams allow measurements of source locations with arcsecond resolution and, for the first time, also the determination of the effective source size at short millimeter wavelengths. The typical sensitivity is better than 1.5 sfu and thus allows also the detection of weak flares at millisecond time resolution.

First observational results were obtained for the GOES X17.2 flare on October 28, 2003. They are complemented with observations by patrol telescopes of the University of Bern between 19.6 and 89.4 GHz and at 230 and 345 GHz with KOSMA. The event reached a peak flux density of 11000 sfu at 210 GHz and exhibited a slowly varying, time-extended emission from an extended source (effective diameter approx. 60 arcsec), as well as a short-lived component from a compact source (effective diameter below 10 arcsec) originating from a different location. There appears to be a component with a rising spectrum in the submillimeter range.

Time profiles and instantaneous frequency spectra of the burst will be presented, along with a preliminary interpretation and a short description of the new 210 GHz instrument.