Magnetospheric toroidal Alfvén wave harmonics and the field line distribution of mass density

Abstract

Measurements of electric and magnetic field taken by instruments on the Combined Release and Radiation Effects Satellite (CRRES) spacecraft on 29 July 1991 reveal seven or eight toroidal Alfvén wave frequency bands. By using observations of the waves near the magnetic equator and the symmetry of the theoretical modes as a function of magnetic latitude, we can identify which theoretical Alfvén wave harmonic corresponds to a particular frequency band. This, along with the L shell frequency dependence, gives convincing evidence that we are in fact observing the toroidal Alfvén mode. Because the harmonics of the toroidal Alfvén wave have a different response to mass density at different points along a magnetic field line, the frequencies of these harmonics can be used to infer the distribution of mass density along the field line. While there is a significant uncertainty in the results due to the uncertainty in the observed frequencies, it is nevertheless true that both the solution based on the peak (mean) frequencies and the majority of solutions using a Monte Carlo simulation of the effects of uncertainty in frequency show the same result, that there is a local maximum in ρ within about 30° of the magnetic equator. A similar result is found for the wave event observed on 28 August 1990, a local maximum in ρ within about 15° of the magnetic equator. These results imply that heavy ions are preferentially concentrated at the magnetic equator.