Free access article

A&A 409, 1107-1125 (2003)
DOI: 10.1051/0004-6361:20031187

A catalogue of white-light flares observed by Yohkoh

S. A. Matthews¹, L. van Driel-Gesztelyi^{1, 2, 3, 4}, H. S. Hudson⁵ and N. V. Nitta<sup>6

1</sup>  Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
²  Centre for Plasma Astrophysics, K.U. Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium
³  Observatoire de Paris, section Meudon, LESIA (CNRS), 92195 Meudon Principal Cedex, France
⁴  Konkoly Observatory, Budapest, 1526, Hungary
⁵  Space Science Laboratory, UC Berkeley, CA 92740, USA
⁶  Lockheed Martin Solar and Astrophysics Laboratory, O/L9-41, B/252, 3251 Hanover Street, Palo Alto, CA 94304, USA

(Received 16 June 2003 / Accepted 30 July 2003 )

Abstract
The aspect camera of the Soft X-ray Telescope (SXT) on Yohkoh provided the first systematic survey of white-light flares from an observatory in space. The observations were made in the Fraunhofer g-band at a pixel size of 2.46 arcsec and a typical sample interval on the order of ten seconds. A total of 28 flares with clear white-light signatures were detected, corresponding to GOES events down to the C7.8 level in one case. Above the X-class threshold, all 5 events observed by SXT were observed in white light, and the maximum average contrast observed was 30% relative to the pre-flare continuum brightness of the flare location. We have made comprehensive comparisons of Yohkoh soft X-ray and hard X-ray data for this list of flares. In addition we compare the properties of the WLF sample to a sample of 31 flares that showed no white-light emission. These comparisons show that while white-light continuum emission has a strong association with hard X-ray emission it is also strongly related to coronal overpressure, as determined from the soft X-ray spectrum, indicating a component with a thermal, rather than non-thermal origin.

Key words: Sun: flares -- Sun: X-rays, gamma rays

Offprint request: S. A. Matthews, sam@mssl.ucl.ac.uk

© ESO 2003

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