Ion cyclotron waves in the solar wind: generation mechanism and source region

Abstract:

Ion cyclotron waves have been observed in the solar wind at several heliocentric distances. The STEREO magnetic field observations are examined to understand the wave properties and the possible formation mechanisms. Statistical studies show that the waves are probably generated in the solar corona and carried outward by the solar wind. Among these ion cyclotron wave observations, there is a special group of events in which waves last for over half an hour (i.e. so-called storm events) and have co-existing right-handed and left-handed waves. We study such events and find that these waves have the properties expected for left-handed in the solar wind frame but are Doppler-shifted in the spacecraft frame, with Sunward-propagating waves shifted to higher frequency, and anti-Sunward-propagating waves shifted to lower frequency or even reversed in polarity. Assuming both left-handed and right-handed waves are generated by pickup ion at the same source location, we estimate the pickup ion's initial velocity at the source region is typically one third of the solar wind velocity and three times the Alfven velocity. We also use the frequencies of these waves to estimate the field strength and heliocentric distances of the source region.