On Improvement in Normal-mode Magnetoseismology with Network Observations by Ground-based Magnetometers

Abstract:

Estimating magnetospheric density by the measurements of field line resonance (FLR) frequencies, a subject also known as normal-mode magnetoseismology, is one of the most important modern uses of ground-based magnetometer observations. Nowadays, ground detection of FLR frequencies is typically made by using the gradient technique, which compares the wave phase and sometimes amplitude at two stations located on the same meridian but separated by one or a few degrees in latitude. The calculation of the equatorial plasma mass density from the FLR frequency, if only the fundamental mode is observed, requires an assumption on the field-aligned distribution of density.

This paper proposes two improvements over the present practice of normal-mode magnetoseismology. First, if there are multiple pairs of stations on the same meridian, such as those enabled by the Mid-continent MAgnetoseismic Chain (McMAC), it is possible to use the observations to infer not only the dependence of plasma density on L-value but also the field-aligned distribution of density. Second, the gradient technique can still be performed when the two stations have a small separation in longitude. Several event studies using the observations by McMAC and THEMIS ground magnetometers found that the azimuthal wavenumbers associated with the FLR in the plasmasphere were no greater than 2, suggesting a limited effect of the azimuthal phase drift due to the longitudinal separation between stations. These improvements enable a better use of network observations by ground-based magnetometers in normal-mode magnetoseismology, such as monitoring the distribution of plasma density and plasmaspheric dynamics during space weather events.

Image: A two-dimensional snapshot of equatorial plasma density inferred from 10 minutes of the FLR observations by McMAC and other ground magnetometers in North America.