GEOTAIL Observations of the Earth's Bow Shock: Whistler Critical Mach Number and the Diffusive Shock Acceleration of Electrons

Details GEOTAIL Observations of the Earth's Bow Shock: Whistler Critical Mach Number and the Diffusive Shock Acceleration of Electrons GEOTAIL Observations of the Earth's Bow Shock: Whistler Critical Mach Number and the Diffusive Shock Acceleration of Electrons

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmsh53b..07o&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #SH53B-07

Other

7815 Electrostatic Structures, 7845 Particle Acceleration, 7851 Shock Waves (4455), 7867 Wave/Particle Interactions (2483, 6984)

Scientific paper

It has theoretically been expected that whistler waves are emitted upstream of shocks whenever Mach number falls below a critical Mach number. By performing statistical analysis of the upstream waves, we have observationally confirmed the `whistler critical Mach number' at the Earth's bow shock using the GEOTAIL spacecraft. We have further found that the critical Mach number regulates electron acceleration. The spectral index Γ of electron energy spectrum f(E)∝ E-Γ varied greatly between 3.5 and 5.0 in the sub-critical regime while Γ were concentrated between 3.0 and 3.5 in the super-critical regime. The result suggests different acceleration mechanism for below and above the critical Mach number. In order to understand acceleration mechanisms, we performed detailed studies of two distinct events: One is sub-critical (obtained on 11 February 1995) and the other is super-critical (obtained on 1 July 1996) to the whistler critical Mach number. For the sub-critical shock, we found gradual spatial profile of non-thermal electron flux well associated with the upstream waves, and the event was interpreted by means of the Diffusive Shock Acceleration. For the super-critical shock, we found non-thermal electron flux enhancement at the shock ramp region associated with electrostatic waves. This result will be compared with conventional acceleration models.

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