The almost monoenergetic ion event on 19 October 2009: SEPT/STEREO observations

¹ Institut für Experimentelle und Angewandte Physik, Universität Kiel, 24118 Kiel, Germany
e-mail: klassen@physik.uni-kiel.de
² Centre d’Étude Spatiale des Rayonnements, Université de Toulouse (UPS), France

Received: 30 March 2010
Accepted: 31 January 2011

Abstract

The Solar Electron and Proton Telescope (SEPT) on board the twin spacecraft STEREO A and B measures electrons and ions in the energy range from 60 to above 400 keV with an energy resolution of 10%. On 19 October 2009, when STEREO-B was already 1.03 AU away from the Earth, a strong and prolonged almost monoenergetic ion event has been observed with the SEPT instrument. The event lasted 27 min and its energy spectrum contained a strong narrow peak at 235 keV with a relative full width at half maximum of 0.35. The event occurred during a period of slow solar wind in front of a weak ion increase associated with a distant corotating interaction region (CIR). Previously similar events containing spectral peaks were detected in the vicinity of the Earth’s magnetosphere using observations on Interball-1 and on both STEREO A & B spacecraft.

We present evidence that the narrow spectral peak is caused by a quasi-monoenergetic ion beam and suggest that the particles were accelerated at a distant CIR or CIR shock. We discuss the possible mechanisms that could be responsible for accelerating these ions: the shock drift acceleration, the surfatron mechanism and the acceleration in a large-scale electrostatic field.