Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000apj...536..455r&link_type=abstract
The Astrophysical Journal, Volume 536, Issue 1, pp. 455-464.
Astronomy and Astrophysics
Astronomy
9
Ism: Cosmic Rays, Plasmas, Scattering, Sun: Solar Wind, Turbulence, Waves
Scientific paper
Isotropic, nonresonant fast magnetosonic waves (FMWs) very efficiently scatter low-rigidity cosmic rays through the zero pitch-angle cosine μ. The derivation of this effect is generalized here to an anisotropic, conic turbulence of FMWs with an arbitrary spectral shape. Compared to the one of the FMWs, the contributions to the nonresonant scattering from both a slab electromagnetic turbulence and oblique Alfvén waves are shown to be negligible. As a consequence, the nonresonant scattering rate by FMWs alone is combined with the scattering rates due to gyroresonance with the Alfvén waves and transit-time damping with the FMWs to fit the parallel mean free path obtained from measurements of solar cosmic rays as a function of the particles' rigidities. The global fit of a large set of data points, resulting from many solar events, displays three distinct regions (plus a transition region between the two first ones), where the transit-time damping at |μ|~1, the nonresonant scattering by FMWs at μ~0, and the gyroresonance with Alfvén waves at μ~0 successively determine the parallel mean free path for rigidities ranging from 10-2 to 105 MV. Very accurate fits are further obtained for several independent solar events, during which particles were detected in the whole range of rigidities-from 1 to ~5×102 MV-where the nonresonant scattering determines the mean free path. Finally, the use of a conic turbulence of FMWs with an opening angle deduced from the damping rate of the waves relates the form of the FMW turbulence to the parameters of the plasma during each of the solar events. It argues in favor of a very strong anisotropy of the FMW spectrum above about 10-2 times the ion gyrofrequency.
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