Joint Measurements of Flare Flux Densities at 210 – 212 GHz by Two Different Radio Telescopes

Abstract

Multiple-beam observations of solar flares at submillimeter wavelengths need detection with at least four beams to derive the flux density \(\mbox{$F$} \) of the emitting source, its size, and centroid position. When this condition is not fulfilled, the assumptions on the location and/or size of the emitting source have to be made in order to compute \(\mbox{$F$}\). Otherwise, only a flux density range \(\mbox{$\Delta F$}\) can be estimated. We report on simultaneous flare observations at 212 and 210 GHz obtained by the Solar Submillimeter Telescope (SST) and the Bernese Multibeam Radiometer for Kosma (BEMRAK), respectively, during two solar events on 28 October 2003. For both events, BEMRAK utilized four beam information to calculate the source flux density F 210, its size and position. On the other hand, the SST observed the events with only one beam, at low solar elevation angles and during high atmospheric attenuation. Therefore, because of these poor observing conditions at 212 GHz, only a flux density range ΔF 212 could be estimated. The results show that ΔF 212 is within a factor of 2.5 of the flux density F 210. This factor can be significantly reduced (e.g. 1.4 for one of the studied events) by an appropriate choice of the 212 GHz source position using flare observations at other wavelengths. By adopting the position and size of the 210 GHz source measured by BEMRAK, the flux density at 212 GHz, F 212b, is comparable to F 210 within the uncertainties, as expected. Therefore our findings indicate that even during poor observing conditions, the SST can provide an acceptable estimate of the flux density at 212 GHz. This is a remarkable fact since the SST and BEMRAK use quite different procedures for calibration and flux density determination. We also show that the necessary assumptions made on the size of the emitting source at 212 GHz in order to estimate its flux density are not critical, and therefore do not affect the conclusions of previous studies at this frequency.