Formation of Proton Beams in High-Speed Solar Wind

Details Formation of Proton Beams in High-Speed Solar Wind Formation of Proton Beams in High-Speed Solar Wind

Dec 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufmsh32a0734t&link_type=abstract

American Geophysical Union, Fall Meeting 2001, abstract #SH32A-0734

Physics

2164 Solar Wind Plasma

Scientific paper

A new mechanism is suggested to explain the formation of proton beams in high-speed solar wind streams. Proton beams were already observed in the early days of solar wind in-situ measurements ( Feldman et al., 1973, Marsch et al., 1982). The proton beam is found to move faster than the core part of the distribution by about the Alfven speed. The density ratio between beam and core is observed to increase radially from 0.3 AU to 1 AU. Several papers tried to explain this phenomenon. However, till today none of these two major properties has been explained properly. We suggest here that outward propagating RHP waves and LHP waves, for which the second branch of the dispersion relation is determined by including also the drifting alpha particles, may together play an important role in the formation of proton beams. The time-dependent two-dimensional diffusion equation, as determined from the quasi-linear theory of ion-cyclotron-wave resonance, is solved numerically to illustrate the evolution of the proton velocity distribution. Assuming the wave spectra to be given, a proton beam is shown to evolve by diffusion in the wave field from an initial bi-Maxwellian velocity distribution function. The drift velocity of the model beam is about the Alfven speed, and its density is found to increase with decreasing Alfven speed, in accord with the in-situ observations. Limitations of the present model and work that should be done in the future are also discussed. Feldman, W.C., et al., J. Geophys. Res., 78, 2017, 1973. Marsch, E., et.al., J. Geophys. Res., 87, 52, 1982.

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