Perpendicular Ion Heating: Observations at Earth and theory at the Sun

Transverse Ion Heating: Observations on Earth and Theory at the Sun

Bob Lysak (UMinn), GEM co-convener; Ben Chandran (UNH), SHINE co-convener

This session, somewhat misnamed since solar observations and auroral ion heating theories were also discussed, took place on Friday morning, June 27, in the Matterhorn room. Six presentations were made in this session, which was rather well attended considering its late placement in the week’s schedule. The session began with reviews of auroral zone heating observations and theory by Eric Lund (UNH) and Chris Chaston (UCB). Eric discussed heating at the ion cyclotron resonance by a variety of wave modes in the auroral zone, while Chris focused on stochastic ion heating by kinetic Alfvén waves of short perpendicular wavelength (comparable to the ion gyroradius) and large amplitude. This distinction between resonant and stochastic heating was a common theme that ran throughout the session.

John Kohl (CfA) followed with a presentation on observations from the SOHO UVCS instrument, which measures resonant line widths of the O+5 ion, as well as H, which is considered a proxy for protons. These observations indicate remarkably rapid acceleration of these ions to solar wind speeds, reaching 400 km/s by 2-3 RS. His observations, using a maximum probability technique, also indicated that the oxygen ions were predominantly heated perpendicular to the field, with temperature anisotropies ranging up to about 10. This talk was followed by two theoretical talks, the first from Tulasi Parashar (UDelaware), who presented hybrid simulations of perpendicular ion heating in turbulent plasmas, concluding that resonance was not necessary and that heating was stochastic. The second theoretical talk was from Liu Chen (UCI), who discussed a detailed theory of sub-harmonic resonant heating. This talk seemed to be at odds with the results of Chaston, which was possibly due to the distinction between Chaston dealing with short perpendicular wavelength kinetic Alfvén waves while Chen was discussing the pure shear Alfvén wave. Finally, Lan Jian (UCLA) discussed very recent STEREO observations at 1 AU of ion cyclotron waves presumably generated closer in to the Sun that propagated both Sunward and anti-Sunward in the solar wind frame, with the anti-Sunward propagating waves showing a right-handed polarization due to being convected outward in the solar wind flow.

Overall, the discussion illustrated the similarities and differences between the two plasma environments. It was suggested that the auroral environment was possibly similar to the colder plasma environments in coronal holes, although it was realized that one major difference is the strong, static background magnetic field in the Earth’s magnetosphere in contrast with the more variable magnetic fields in the corona. In general, however, the universality of the plasma physics phenomena was nicely demonstrated.