Time dependence of ponderomotive plasma energization in the magnetosphere

Details Time dependence of ponderomotive plasma energization in the magnetosphere Time dependence of ponderomotive plasma energization in the magnetosphere

Nov 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994jgr....9921281a&link_type=abstract

Journal of Geophysical Research (ISSN 0148-0227), vol. 99, no. A11, p. 21,281-21,290

Physics

9

Earth Magnetosphere, Magnetohydrodynamic Waves, Plasma Acceleration, Plasma Oscillations, Ponderomotive Forces, Space Plasmas, Standing Waves, Time Dependence, Geosynchronous Orbits, Oxygen Ions, Positive Ions

Scientific paper

The time-dependent second-order force generated by standing Alfven waves in a dipolar magnetospheric geometry is used to derive the time-dependent specific parallel energy (parallel energy per proton mass) of magnetospheric plasma particles in the wave fields. Time-dependent energy signatures are evaluated at the dipole equatorial plane (EP), in order to provide a means of identifying ponderomotive energization of plasma in spacecraft data sets. Starting magnetic latitudes for particles arriving at the EP are also derived. In general, specific energies increase rapidly at the EP soon after the driving wave is switched on, reaching a plateau as particles arrive from increasingly higher latitudes. Oscillations in specific energy occur at twice the driving wave frequency. At geosynchronous orbit these oscillations are unlikely to be detectable, but at high L values (where L is the McIlwain parameter) the oscillation amplitudes can be significant. For the fundamental mode with reasonable amplitude, ions from the ionosphere can reach the EP in a few wave cycles at high L values. Higher harmonic standing waves include negative acceleration regions (or potential barriers) on a magnetic field line which create 'stop bands' of magnetic latitudes from which particles cannot reach the EP. (In some cases ionospheric ions are 'shielded' from reaching the EP). The stop bands in turn create characteristic gaps in the time series of specific energy at the EP. At high L values, O(+) ions from the ionosphere can just overcome the third harmonic potential barrier and can be accelerated to the EP with energies approximately 2 keV for reasonable wave amplitudes of the order of 5 mV/m.

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