Observations of ion cyclotron waves in the solar wind near 0.3 AU

Abstract

Using 2 Hz magnetic field data from the MESSENGER mission, we have investigated more than 300 strong narrowband ion cyclotron wave (ICW) events at a heliocentric distance of about 0.3 AU during 31 May to 9 June 2008. These nearly circularly polarized transverse waves are observed extensively and discretely in the solar wind, with a median duration of 21 s. They are preferentially observed when the magnetic field is more radial than the ambient solar wind. The waves appear both left‐handed and right‐handed in the spacecraft frame. Their wave frequencies in the spacecraft frame are generally larger than the local proton cyclotron frequency (fpc), with a median of 1.44 fpc. The wave power spectra do not cutoff at the local fpc. On the basis of their wave characteristics, we conclude that they are intrinsically left‐handed in the solar wind frame and they are generated closer to the Sun and carried out to the spacecraft by the super Alfvénic solar wind. After removing the Doppler shift, the wave frequencies in the solar wind frame are all below the local fpc, with a median of 0.35 fpc. The ICWs propagate nearly parallel to the magnetic field, and the median wave amplitude is about 0.73 nT, 3% of the background magnetic field. We compare these observations with earlier Helios observations at 0.3 AU in 1976 and contemporary 1 AU observations.