The onset of solar energetic particle events: prompt release of deka-MeV protons and associated coronal activity

Abstract

Between 1996 and 2002, the COSTEP particle instrument aboard the SoHO mission observed 18 solar energetic particle events where the release times of protons up to 54 MeV could be determined to within an uncertainty of about ten minutes, and where metre wave imaging observations with the Nancay Radioheliograph were available. In six of them the proton release starts, within the uncertainty of the method, simultaneously with the release of electron beams observed through their type III emission at decametric-to-kilometric wavelengths, during flares in Ha or EUV. These well-connected events are studied to identify the coronal processes that occur when the particle release starts at the Sun. Given that big events are not reliably measured by COSTEP due to its large geometric factor, they are rather weak with peak intensities (9 x 10 -4 - 5 x 10 -2 ) [(cm 2 s sr MeV) -1 ] in the (8-25) MeV range. All are accompanied by fast (400-1300 km s -1 ) and narrow coronal mass ejections (CMEs). While half of the events is accompanied by significant microwave bursts, the other half shows little or no evidence of electron acceleration in the low corona and rather weak soft X-ray bursts. The association with weak flares may point to different acceleration sites of the interacting and the escaping particles. From the radio observations of the type III bursts and the associated metre wave emission, the escaping electrons, and by inference also the first escaping protons, are accelerated roughly between 0.1 R ○. and 0.5 R ○. above the photosphere, a height range shown to be well behind the front of the CMEs at the time of acceleration. The data show no evidence that the shock waves, which are presumably driven by at least the fastest of these CMEs, participate in the early acceleration of the escaping protons or affect the escape of the protons accelerated at lower height.