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Astron. Astrophys. 360, 729-741 (2000)

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1. Introduction

There are a variety of acceleration processes that may produce suprathermal, energetic and relativistic particles in the solar corona and the solar wind. They include acceleration by direct electric fields, waves, and shock waves. Miller et al. (1997) discuss the performance of each of these candidate accelerators in the context of solar flares. Shock waves driven by fast coronal mass ejections (CMEs) are considered as an independent accelerator in the corona and interplanetary space (Reames 1999and references therein).

Observational constraints are provided on the one hand by remote sensing observations of particles interacting in the solar atmosphere, whereupon they emit at [FORMULA]-ray, hard X-ray (HXR) and radio wavelengths. With the exception of nuclear [FORMULA]-ray lines and neutrons (the latter are detectable in the most energetic events), these diagnostics probe electrons. Direct measurements of electrons and ions have been made aboard satellites at heliocentric distances greater than 0.3 AU. Ideally one would like to combine the different diagnostics to infer where, when and how the particles were accelerated. However, transport and probably re-acceleration modify the particle populations on their way to the detector, so that the populations measured in situ may be of different origin than the interacting particles. A scenario of this kind is the proposed acceleration of large solar energetic particle (SEP) events by the bow shocks of CMEs at heliocentric distances above 5-15[FORMULA] (Kahler 1994), where it is presumed that particles accelerated in the magnetically stressed corona in association with flares play a negligible role.

Few observational tests are at hand which might allow to probe the connection of SEPs with coronal acceleration sources. Similarities in the temporal evolution of hard X-rays, radio waves and particle fluxes measured close to the Sun (Bieber et al. 1980; Kallenrode & Wibberenz 1991; Akimov et al. 1996; Klein et al. 1999) suggest that such relationships do exist, but the quantitative correlation e.g. between the fluxes of protons interacting in the corona and those detected at 1 AU during given events is loose (Ramaty et al. 1993; Cliver et al. 1989). The particle signatures at 1 AU clearly depend on a range of processes on active region scales and on global scales, including acceleration in flare related small-scale structures as well as in widespread regions associated with a chromospheric or coronal "Moreton" wave (Kocharov et al. 1994; Torsti et al. 1998, 1999; Krucker et al. 1999) or a CME (e.g., Maia et al. 1999; Klein et al. 1999).

The acceleration and propagation of particles in the middle and high corona are difficult to probe by remote sensing techniques, since collisional radiation processes are inefficient due to the low target density. However, suprathermal electrons generate collective radio emission at frequencies from a few GHz to about 1 MHz, which is sensitive to the accelerated electrons as well as to the ambient medium from the low to the high corona. In the present paper we analyze the particle signatures related to a flare and CME on 1996 July 9. This SEP event was observed under particularly quiet conditions of interplanetary transport (Torsti et al. 1997; Kocharov et al. 1997), so that uncertainties introduced by the transport are minimal. We employ measurements of energetic protons and mildly relativistic electrons, respectively by the Energetic and Relativistic Nuclei and Electron (ERNE) instrument and the Comprehensive Suprathermal and Energetic Particle Analyser (COSTEP) aboard the SoHO spacecraft. Coronal electron acceleration and propagation is probed by radio observations using spectrography from ground (radio spectrographs at Bern, Hiraiso and Tremsdorf) and space (the WAVES experiment aboard Wind) and imaging observations (Nançay Radioheliograph). Sect. 2 of this paper presents the particle data and the derived injection functions. Sect. 3 describes the radio observations and identifies potential sites of coronal acceleration. These are compared with the inferred particle injections in Sect. 4.

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© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000