Field-aligned and Gyrophase-bunched Ion Beams in the Earth's Foreshock: "Pitch-angle" Wave Trapping

Details Field-aligned and Gyrophase-bunched Ion Beams in the Earth's Foreshock: "Pitch-angle" Wave Trapping Field-aligned and Gyrophase-bunched Ion Beams in the Earth's Foreshock: "Pitch-angle" Wave Trapping

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmsh51b1214m&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #SH51B-1214

Physics

2134 Interplanetary Magnetic Fields, 2154 Planetary Bow Shocks, 7836 Mhd Waves And Instabilities (2149, 2752, 6050), 7839 Nonlinear Phenomena (4400, 6944), 7867 Wave/Particle Interactions (2483, 6984)

Scientific paper

One major characteristics of the Earth's foreshock is the occurrence of large-amplitude quasi-monochromatic low-frequency waves. They reveal the existence of coherent wave-particle interaction which is an efficient process to dissipate the energy of the particles reflected at the collisionless bow shock. The backstreaming ion distributions associated with the highly coherent waves are revealed as gyrating ions with well-defined pitch-angle and gyrophase organization around the local magnetic field. Previous experimental results from a few case studies have shown the possibility of producing the gyrophase-bunched ion distributions from local nonlinear wave-particle interaction. This process involves initially field-aligned beam ions observed at the edge of the gyrating ions/coherent wave region. From a much larger data set mainly including recent multi-spacecraft observations by Cluster, the physical properties (principally the parallel and perpendicular velocities) of both the field-aligned and the gyrophase-bunched ion beams are used to qualify a theoretical model of nonlinear "pitch-angle" trapping by the waves. First, the field-aligned ion beams observed just both at the edge of the gyrating ions region and the onset of the waves are quantitatively shown to be in cyclotron resonance with the waves and thus can generate them via the ion/ion right-hand mode instability. Since multi-spacecraft analysis with Cluster allows the unambiguous determination of both the plasma frame frequency and the wave vector, this is the first unambiguous quantitative test of the cyclotron resonance in the foreshock. Second, the observed pitch-angles of the gyrophase-bunched distributions are in good agreement with the theoretical values which is dependent on the wave transverse magnetic field amplitude, for both a large range of pitch-angles and wave amplitude. Third, the conservation of energy in the "wave frame" from the initial field-aligned beam to the gyrating ion distribution is also verified experimentally. The reduction of the parallel velocity in the plasma rest frame is also accompanied by a heating of the distributions. Such results show the relevance of kinetic processes in the foreshock which also influence the dayside magnetosphere (e.g. magnetopause motion linked to pressure variations).

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