Estimation of Plasma Mass Density Using Toroidal Oscillations Observed by CRRES

Plasma mass density is a quantity that is difficult to determine using particle instruments. However, with proper models of the ambient magnetic field and density variation along the field lines, one can relate the frequency of observed field line eigenoscillations to the density [Denton et al., JGR, page 29,925, 2001]. In the present study we estimate the density using the toroidal oscillations detected by the magnetic (B) and electric (E) field experiments on the CRRES spacecraft. A period including the geomagnetic storm of October 9,1990, is chosen for analysis because the spacecraft was located on the dawn sector where toroidal waves are routinely excited. Dynamic spectra of the B and E fields are generated for each CRRES orbit for identification of the presence and harmonic mode of the toroidal oscillations. The fundamental mode is found to the easiest to identify when the spacecraft was at magnetic latitude higher than 15 degrees. Consequently, we follow the frequency of this mode for the selected storm period. At L = 7, the frequency was 6 mHz before the storm and it decreased to 3 mHz a few days after the main phase of the storm. This frequency change corresponds to an increase of mass density by a factor of four. The electron number density at the same L shell, determined from the CRRES plasma wave spectra, did not show a similar change. This result suggests that heavy ions increased during the storm. We use the Denton et al. [2001] technique to estimate the mass density, and then combine the mass density and electron density to obtain the effective ion mass/charge ratio.