First observation of a solar X-class flare in the submillimeter range with KOSMA

Abstract

Abstract. We present the first solar flare observations with the KOSMA submillimeter telescope at 230 and 345 GHz. The GOES X2.0 flare on April 12, 2001 was also observed at millimeter and centimeter wavelengths, as well as in soft and hard X-rays. It exhibits both an impulsive phase of nonthermal gyrosynchrotron radiation and an extended phase of strong thermal free-free emission in the millimeter and submillimeter range. As in previous observations, a mismatch between the electron energy spectral indices, inferred from the millimeter and hard X-ray data, exists and is interpreted as a flattening of the energy spectrum above a break energy of several hundred keV. The observed thermal emission closely follows the shape of the mm/submm flux density time profile predicted from soft X-ray observations. As the observed absolute flux densities exceed the predicted ones by a factor of ∼1.5-3.4, both the mm/submm emission and the soft X-rays must be thermal bremsstrahlung with a common energy source, but from locations with different plasma parameters. KOSMA observations allowed an estimate of source locations and sizes for the nonthermal and thermal sources. All of them coincide within 0.2 arcmin and with those seen in soft and hard X-rays. Surprisingly, the thermal submillimeter source diameters at 230 and 345 GHz (42 and 70 arcsec respectively) increase with frequency.

Radio Submillimeter and γ‐Ray Observations of the 2003 October 28 Solar Flare

“…Hence, it is not known whether the radio spectrum above 210 GHz continues to decrease, as was observed for a few flares ( Trottet et al 2002;Lüthi et al 2004b;Guiménez de Castro et al 2005), or increases with frequency, as was reported for very large flares like the present one ( Kaufmann et al 2004( Kaufmann et al , 2007Silva et al 2007). In the latter case, the emission at 210 GHz could contain contributions from both the decreasing component and the increasing component.…”

“…Both SST at 212 GHz and BEMRAK at 210 GHz provide multibeam measurements that allow us to estimate the location (center of mass) and the averaged size of the millimeter-waveYemitting region. For small X-class flares, these observations show that the synchrotron component extends up to 200 GHz (Trottet et al 2002) and possibly to higher frequencies ( Lüthi et al 2004b). However, for large X-class flares such as the 2003 November 4 flare ( Kaufmann et al 2004), the 2003 November 2 flare (Silva et al 2007), and the 2006 December 6 flare ( Kaufmann et al 2007), the radio spectrum above 200 GHz is not the continuation of the synchrotron spectrum measured at lower frequencies, but surprisingly increases with increasing frequency.…”

“…Total flux densities have been measured at 19.6, 35, and 50 GHz by the Bumishus patrol telescopes, at 89.4 GHz by a nulling interferometer, and at 230 and 345 GHz by KOSMA ( Lüthi et al 2004b). In addition, observations at 210 GHz were made by BEMRAK (Lüthi et al 2004a), a multibeam instrument that allows us to estimate the location and the size of the 210 GHz emitting region.…”

“…No absolute pointing was available due to a constant but unknown instrumental offset ( 60 00 ), and no comparison with imaging observations in other domains such as EUV and HXR could be done. In order to get absolute pointing for this work we used a full-disk map of the Sun at 210 GHz taken at 11: 40 UT (half-power beam width is 88 00 ; see Lüthi et al 2004b) that still shows flare emission. Together with the location of the solar limb, the absolute position of the 210 GHz flare relative to the Sun's center could be determined.…”

“…These observations are routinely performed by the Solar Submillimeter Telescope (SST) at 212 and 405 GHz (Kaufmann et al 2001). Furthermore, observations at 230 and 345 GHz have been obtained for two flares by the KOSMA ( Köln Observatory for Submillimeter and Millimeter Astronomy) telescope, and one of these flares (2003 October 28) has additionally been observed at 210 GHz by BEMRAK (Bernese Multibeam Radiometer for KOSMA; Lüthi et al 2004aLüthi et al , 2004b. Both SST at 212 GHz and BEMRAK at 210 GHz provide multibeam measurements that allow us to estimate the location (center of mass) and the averaged size of the millimeter-waveYemitting region.…”