TIME-DEPENDENCE OF PONDEROMOTIVE PLASMA ENERGIZATION IN THE MAGNETOSPHERE

The time-dependent second-order force generated by standing Alfven wav es in a dipolar magnetospheric geometry is used to derive the time-dep endent specific parallel energy (parallel energy per proton mass) of m agnetospheric plasma particles in the wave fields. Time-dependent ener gy signatures are evaluated at the dipole equatorial plane (EP), in or der to provide a means of identifying ponderomotive energization of pl asma in spacecraft data sets. Starting magnetic latitudes for particle s arriving at the EP are also derived. In general, specific energies i ncrease rapidly at the EP soon after the driving wave is switched on, reaching a plateau as particles arrive from increasingly higher latitu des. Oscillations in specific energy occur at twice the driving wave f requency. At geosynchronous orbit these oscillations are unlikely to b e detectable, but at high L values (where L is the McIlwain parameter) the oscillation amplitudes can be significant. For the fundamental mo de with reasonable amplitude, ions from the ionosphere can reach the E P in a few wave cycles at high L values. Higher harmonic standing wave s include negative acceleration regions (or potential barriers) on a m agnetic field line which create ''stop bands'' of magnetic latitudes f rom which particles cannot reach the EP. (In some cases ionospheric io ns are ''shielded'' from reaching the EP.) The stop bands in turn crea te characteristic gaps in the time series of specific energy at the EP . At high L values, Of ions from the ionosphere can just overcome the third harmonic potential barrier and can be accelerated to the EP with energies similar to 2 keV for reasonable wave amplitudes of the order of 5 mV m(-1).