Physics
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..sh22e03t&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #SH22E-03
Physics
2159 Plasma Waves And Turbulence, 2164 Solar Wind Plasma
Scientific paper
Direct evidence from Helios observation is, for the first time, provided for the occurrence of pitch-angle diffusion of solar wind protons, induced by resonance with ion-cyclotron waves propagating away from the Sun. The iso-density contours of the sunward halo part of the proton velocity distribution function (VDF) are shown to be well outlined by a sequence of segments of circles centered at the wave phase speed, which is only slightly smaller than the local Alfven speed. These circular segments correspond to the plateaus as predicted by quasi-linear theory. A plateau is defined by a vanishing pitch-angle gradient of the VDF in the resonant regime, whereby the proton conserves during pitch-angle diffusion its total kinetic energy in the frame moving with the wave phase speed. The influence of the wave dispersion relation is studied by comparing the measured iso-density contours and the theoretical plateaus, as defined by the dispersion relation of either a cold or a warm plasma. When defined by the phase speed based on the cold-plasma dispersion, the plateau is shown to match the observed contours better than when defined by either the Alfven speed or the dispersive phase speed of the warm-plasma. The contours of the anti-sunward halo part of the proton VDF are also found to match the diffusion plateau as formed by the resonance with inward-propagating cyclotron waves. The implication of these results for the heating and kinetic transport of solar wind ions are discussed.
Marsch Eckart
Tu Chang-Cun
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