Ion thermalization and wave excitation downstream of Earth's bow shock: Cluster observations of He2+ acceleration and theory

Details Ion thermalization and wave excitation downstream of Earth's bow shock: Cluster observations of He2+ acceleration and theory Ion thermalization and wave excitation downstream of Earth's bow shock: Cluster observations of He2+ acceleration and theory

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsm54a..06l&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #SM54A-06

Physics

7827 Kinetic And Mhd Theory, 7829 Kinetic Waves And Instabilities, 7851 Shock Waves (4455), 7867 Wave/Particle Interactions (2483, 6984)

Scientific paper

It has been well documented that the plasma immediately downstream of Earth's quasi-perpendicular bow shock, which consists of reflected protons and directly transmitted ions with large temperature anisotropies, is unstable to the excitation of ion-cyclotron waves. These waves in turn scatter the protons and ions to marginal stability. Using Cluster data following the inbound shock crossing at 17:17:48 on 31 March 2001, we investigate the joint evolution of the proton and helium distribution functions. The He2+ ions are observed to be heated perpendicular to the magnetic field. Within a short distance downstream, the perpendicular heating is faster than parallel, so that the temperature anisotropy of helium first increases near the shock before decreasing further downstream, where perpendicular heating is slower. The observed spectra of magnetic fluctuations, which are dominated by left-hand circularly polarized waves, display one hump at frequency smaller than He2+ gyrofrequency just downstream of the shock, and two humps with a slot near helium gyrofrequency further downstream . We present a quasilinear theory which explains the observed evolution of the He2+ temperature anisotropy, and excited wave spectrum. The predicted temperature anisotropy and the general shape of the excited wave spectrum match the observations remarkably well. Nevertheless, certain features of the observations, such as the larger growth rate of the lower frequency hump relative to the higher frequency hump, require further work.

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