Mass Density Models Based on the Frequency of Observed Standing Alfven Waves: AMPTE/CCE Results for L = 6-10

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2720 Energetic Particles, Trapped, 2740 Magnetospheric Configuration And Dynamics, 2752 Mhd Waves And Instabilities

Scientific paper

The frequency of magnetospheric standing Alfven waves depends on the magnetic field and plasma along the path of the waves. This fact has been used to estimate the plasma mass density from the frequency of magnetic pulsations observed either on ground or in space. However, construction of global mass density models with this approach is yet be achieved due to uncertainty in interpreting ground observations and sparse spatial coverage of reported satellite observations except at geostationary orbit. Meanwhile, there are mass density models derived from satellite observation of plasma along with theoretical considerations. In this study we compare the frequency of toroidal standing Alfven waves observed by AMPTE/CCE in the middle magnetosphere (L = 6-10) with the theoretical frequency obtained using an empirical mass density model [Gallagher et al., 2000] and the T89c magnetic field model. We determine the toroidal frequency using particle and magnetic field time series. From 4 years of data we obtained approximately 4000 20-min intervals containing a clear signature of the standing waves. The spatial map of the frequency constructed from these events indicates a clear local time and radial distance dependence. Regardless of local time, the L dependence of the rate for frequency change is in excellent agreement between the observation and model. As for the frequency we find a good agreement for 12-24 MLT but a large discrepancy near midnight: the model mass density tends to be too small. These results imply that the existing plasma mass model is quite good for the dayside, but that it may require adjustment on the nightside. On the other hand, our theoretical calculations of frequency are least reliable in this region. Gallagher, D. L., P. D. Craven, and R. H. Comfort, Global core plasma model, J. Geophys. Res., 105, 18,819, 2000.

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