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High latitude Ulysses observations of CIR accelerated ions and electrons
K.A. Sayle and
Received 11 August 1997 / Accepted 20 October 1997
From the comparison of the intensity maxima of 1 MeV/nuc. ions with those of the 40-65 keV electrons and higher energy ions ( 2-4 and 4-8 MeV/nuc.), during Ulysses' ascent to the solar south pole, 1992-1994, evidence has been found for the 1 MeV/nuc. ions observed at high latitude by the Ulysses spacecraft to have originated at mid-latitudes and greater radial distances. The key observations presented in this paper are; i) The 1 MeV/nuc. ion intensity maxima were observed to decay exponentially with latitude, above the streamer belt, whereas the electron intensity maxima and higher energy ( 4-8 MeV/nuc.) ion intensity maxima varied about a constant level, ii) When the ratios were taken of the ion intensity maxima to the electron intensity maxima for each of the CIR events, above the streamer belt, the ratios decayed exponentially with latitude, and iii) Upon the spacecraft's departure from the streamer belt, the electron maxima were observed to be delayed by 1-4 days, with respect to the 1 MeV/nuc. ion maxima. Within the streamer belt they had been typically simultaneous. Evidence was also found for a delay in the higher energy ( 2 MeV/nuc.) ions with respect to the lower energy 1 MeV/nuc. ions. From the exponential decay of the ion/electron ratios above the streamer belt, we concluded that the 1 MeV/nuc. ions originated from the CIR reverse shock at lower latitudes and greater radial distances, along with the electrons.
The observed delays in the electrons and higher energy ions were a consequence of the particles travelling from the distant reverse shock. Hence, it appears that the ions were accelerated at the reverse shock at lower latitudes, and not at the local poleward propagating reverse shocks as had been previously suggested.
Key words: acceleration of particles interplanetary medium
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© European Southern Observatory (ESO) 1998
Online publication: February 4, 1998