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Astron. Astrophys. 342, 575-582 (1999)

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Hard X-ray and gamma-ray observations of an electron dominated event associated with an occulted solar flare

Nicole Vilmer 1, Gérard Trottet 1, Claude Barat 2, Richard A. Schwartz 3, Shinzo Enome 4, Alexandr Kuznetsov 5, Rachid Sunyaev 5 and Oleg Terekhov 5

1 DASOP, CNRS-URA 2080, Observatoire de Paris, Section de Meudon, F-92195 Meudon, France
2 Centre d'Etude Spatiale des Rayonnements, B.P. 4346, F-31029 Toulouse, France
3 Hughes STX and LASP, NASA/Goddard Space Flight Center, Code 682.3, Greenbelt, MD 20771, USA
4 Nobeyama Radio Observatory, Minami-maki, Nagano 384-13, Japan
5 Space Science Institute, Profsoyouznaya 84/32, 117810 Moscow, Russia

Received 5 October 1998 / Accepted 12 November 1998


This paper reports hard X-ray (HXR) and gamma-ray (GR) observations of an impulsive electron-dominated event which occurred on 1991 June 30 at [FORMULA] 0256 UT. This event is associated with an optical flare produced in an active region located behind the east solar limb. When observed from the earth's direction, it appears as a moderate HXR/GR burst but still shows significant emission in the 10-100 MeV range. During most of the event, the photon spectrum exhibits a hardening above a break energy of 0.5 MeV and no significant GR line (GRL) emission is detected. When viewed from a largely different direction, this impulsive event corresponds to one of the giant flares observed with ULYSSES. The main results of the present analysis are:

(i) the HXR/GR emission observed from the earth's direction represents only a small fraction of the total X-ray emission; while the unocculted emission below 0.5 MeV is likely produced in the corona at heights greater than 104 km the higher energy emission probably originates from thick target interaction on the visible disk;

(ii) although associated with an occulted optical flare, this event exhibits at high energies spectral characteristics similar to what is observed for a disk electron dominated event;

(iii) even if no significant GRL emission is detected, the observations cannot exclude that a [FORMULA] 1MeV/nuc ion energy content comparable to the [FORMULA] 20 keV electron content is produced in the flare; the comparable energy contents between electrons and ions found for strong GRL flares and for one electron dominated event associated with a disk flare thus also holds for the unocculted part of this event.

Key words: Sun: activity – Sun: corona – Sun: flares – Sun: X-rays, gamma rays

Present address: NAOJ, VSOP Office, Osawa 2-21-1, Mitaka 181-8588, Japan

Send offprint requests to: N. Vilmer

© European Southern Observatory (ESO) 1999

Online publication: February 22, 1999