Magneto-seismology using spacecraft observations

Abstract

The frequencies of toroidal Alfven waves can be used to infer the mass density along magnetospheric field lines. While ground-based measurements have a greater potential for global coverage, there are several advantages to using spacecraft data. These are the increased accuracy of the magnetic field line mapping in the vicinity of the magnetic equator, the greater ability to observe multiple harmonic frequencies, and the possibility of comparing the inferred mass density to in situ measurements of electron density. Here we review the methods which are used to determine the mass density from the frequencies, and review results obtained from studies using spacecraft data. These results cluster into three groups, the spatial and temporal distribution of mass density, the field line dependence of mass density, and the average ion mass (ratio of mass density to electron density) inferred at the spacecraft location.