Observations of a sharp boundary between field-aligned beams and gyrating ions in the foreshock

Details Observations of a sharp boundary between field-aligned beams and gyrating ions in the foreshock Observations of a sharp boundary between field-aligned beams and gyrating ions in the foreshock

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmsh21b0123m&link_type=abstract

American Geophysical Union, Fall Meeting 2003, abstract #SH21B-0123

Physics

2116 Energetic Particles, Planetary, 2134 Interplanetary Magnetic Fields, 2154 Planetary Bow Shocks, 2164 Solar Wind Plasma, 7867 Wave/Particle Interactions

Scientific paper

We report an observation by Cluster-CIS of an energetic (2--30~keV) upstream ion event presenting a clear double-peak spectrum. The lower-energy (E ˜~3.5~keV) peak is associated with an ion beam propagating along the magnetic field direction, while the higher-energy peak is associated with gyrating ions. Our analysis indicates that the gyrating ions had guiding centers on field lines downstream of the field-aligned component, but that both populations could be sampled simultaneously due to gyroradius effect. We find that downstream limit of the field-aligned beams is populated with protons having a speed 1.5 times the solar wind velocity, which is inconsistent with any known shock-related emission mechanisms. The spatial boundary separating the field-aligned beams from the gyrating ion guiding centers corresponds well with the ULF and intermediate ion foreshock boundary reported in previous studies. Like the field-aligned beams, the gyrating ions reported here have streaming speeds inconsistent with any known shock emission mechanisms. They probably result from instabilities excited by the propagation of field-aligned beams present just upstream. Our interpretation of these observations provides a refinement of the usual framework for foreshock morphology.

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