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 observations at 210 GHz taken by the Bernese Multibeam Radiometer for KOSMA ( BEMRAK ) are combined with hard X-ray and -ray observations from the SONG instrument on board CORONA-F and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI ) to investigate high-energy particle acceleration during the energetic solar flare of 2003 October 28. Two distinct components at submillimeter wavelengths are found. The first is a gradual, long-lasting (>30 minutes) component with large apparent source sizes ($60 00 ). Its spectrum below $200 GHz is consistent with synchrotron emission from flare-accelerated electrons producing hard X-ray and -ray bremsstrahlung assuming a magnetic field strength of !200 G in the radio source and a confinement time of the radioemitting electrons in the source of less than 30 s. The other component is impulsive and starts simultaneously with high-energy (>200 MeV nucleon À1 ) proton acceleration and the production of pions. The derived radio source size is compact ( 10 00 ), and the emission is cospatial with the location of precipitating flare-accelerated >30 MeV protons as seen in -ray imaging. The close correlation in time and space of radio emission with the production of pions suggests that synchrotron emission of positrons produced in charged-pion decay might be responsible for the observed compact radio source. However, order-of-magnitude approximations rather suggest that the derived numbers of positrons from charged-pion decay are probably too small to account for the observed radio emission. Synchrotron emission from energetic electrons therefore appears as the most likely emission mechanism for the compact radio source seen in the impulsive phase, although it does not account for its close correlation, in time and space, with pion production.

Abstract. We report on the study and successful application of an improved measurement method for solar flares at millimeter wavelengths. A 210 GHz multibeam receiver for the observation of solar bursts was installed in the KOSMA 3 m telescope on the 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 waves. The typical sensitivity of <1.5 sfu allows also the detection of weak flares at millisecond time resolution. In this paper we present the instrument and the numerical method for the determination of the source flux density, position and effective size, as well as simulations to asses the validity of the method. First observational results were obtained for the GOES X17.2 flare on October 28, 2003. The event reached a peak flux density of 11 000 sfu at 210 GHz and exhibited a slowly varying, time-extended emission from an extended source (effective diameter ≈ 60 arcsec), as well as a short-lived component from a compact source ( < ∼ 10 arcsec) originating from a different location.

Gamma-ray emission extending to energies greater than 2 GeV and lasting at least for two hours as well as 0.8-8.1 MeV nuclear line emission lasting 40 rain were observed with very sensitive telescopes aboard the GAMMA and CGRO satellites for the well-developed post-flare loop formation phase of the 3B/X12 flare on June 15, 1991. We undertook an analysis of optical, radio, cosmic-ray, and other data in order to identify the origin of the energetic particles producing these unusual gamma-ray emissions. The analysis yields evidence that the gamma-rays and other emissions, observed well after the impulsive phase of the flare, appear to be initiated by prolonged nonstationary particle acceleration directly during the late phase of the flare rather than by a long-term trapping of energetic electrons and protons accelerated at the onset of the flare. We argue that such an acceleration, including the acceleration of protons up to GeV energies, can be caused by a prolonged post-eruptive energy release following a coronal mass ejection (CME), when the magnetic field above the active region, strongly disturbed by the CME eruption, relaxes to its initial state through magnetic reconnection in the coronal vertical current sheet.

Selected high-resolution spectrograms of solar fast-drift bursts in the 6. 2-8.4 GHz range are presented. The bursts have similar characteristics as metric and decimetric type III bursts: rise and decay in a few thermal collision times, total bandwidth > 3 % of the center frequency, low polarization, drift rate of the order of the center frequency per second, and flare association. They appear in several groups per flare, each group consisting of some tens of single bursts. Fragmentation is also apparent in frequency; there are many narrowband bursts randomly scattered in the spectrum. The maximum frequency of the bursts is highly variable.The radiation is interpreted in terms of plasma emission of electron beams at plasma densities of more than 10 I1 cm -3. At this extremely high frequency, emission from the plasma level even at the harmonic is only possible in a very anisotropic plasma. The scale lengths perpendicular and parallel to the magnetic field can be estimated. A model of the source region and its environment is presented.