Prolonged millimeter-wave radio emission from a solar flare near the limb

¹ Tuorla Observatory, Väisälä Institute for Space Physics and Astronomy, 21500 Piikkiö, Finland
² Observatoire de Paris, DASOP, 92195 Meudon, France
³ Astrophysical Institute Potsdam, 14482 Potsdam, Germany
⁴ Astronomical Institute Ondřejov, 25165 Ondrejov, Czech Republic
⁵ Institute of Applied Physics, 3012 Bern, Switzerland
⁶ IZMIRAN, Russian Academy of Sciences, Troitsk, Moscow Region, 142190 Russia

Corresponding author: S. Pohjolainen, spo@astro.utu.fi

Received: 3 November 2000
Accepted: 26 September 2002

Abstract

We present a multi-wavelength analysis of a gradual radio flare on June 27, 1993 which showed emission at millimeter waves long after the soft X-ray flux had peaked. The radio flare located at S12 E75 was associated with a GOES class M3.6 flare that lasted for more than one hour and hard X-ray emission during the rising phase of the soft X-ray/radio emission. The maximum radio flux density at 35 GHz was 60 sfu, but the calculated thermal bremsstrahlung flux from the GOES soft X-rays was less than half of that. The possible explanations for this prolonged millimeter wave emission could be accelerated high-energy electrons gyrating along the field-lines (nonthermal gyrosynchrotron emission) or thermal bremsstrahlung from evaporating chromospheric warm and dense plasma (cool enough to go undetected by GOES), or a mixture of these. Our model calculations show that even an inhomogeneous source containing both kinds of particles would not be able to produce such a spectral shape. A second source with extremely high electron densities (>10¹⁶ m^-3), large source dimensions (>10¹⁵ m²), and very low temperatures (<10⁶ K) must be assumed to explain the observed radio spectra.