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Astron. Astrophys. 316, 481-486 (1996)

The regular structure of shock-accelerated ~40-100 keV electrons in the high latitude heliosphere

E.C. Roelof1, G.M. Simnett, and S.J. Tappin2

1 APL/JHU, Laurel, MD, 20723, USA
2 School of Physics & Space Research, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

Received 28 March 1996 / Accepted 15 May 1996

Abstract. The passage of Ulysses through the high latitude heliosphere has given us a new insight into the interplanetary dynamical processes which are occurring, at any rate near solar minimum, in the heliosphere above the streamer belt. We report here observations of img1.gif 40-100 keV electrons and img1.gif 0.5 MeV protons detected by HI-SCALE from 1992-96. Apart from a few increases associated with solar energetic particle events and coronal mass ejections, the dominant events were recurrent and associated with a long-lived corotating interaction region (CIR). Furthermore, we show that for the majority of the high latitude events the acceleration region is at radial distances many AU beyond the spacecraft. Beginning with the hypothesis that the acceleration is taking place at the reverse shock of a regular, but expanding CIR, we show that the appearance of the electron increases at Ulysses is ordered by the right ascension and radial distance of Ulysses in the frame of reference corotating with the Sun. The timing of the maxima of the electron recurrences is predicted sufficiently accurately with this model, so that the accelerated electrons can be used as a clock during the high latitude phase of the mission. Although the recurrent events are not as strong in the northern hemisphere as in the south, those seen up to the middle of March, 1996 are consistent with the model when known changes in the coronal structure are taken into account.

Key words: interplanetary medium - acceleration of particles - shock waves

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