Features of MHD oscillations in the geomagnetic tail (Invited)

Abstract:

The features of the structures and spectra of MHD oscillations in the geotail are studied. Large-scale fast magnetosonic (FMS) waves can form the spectrum of the lowest-frequency magnetospheric resonator in the near-Earth part of the current sheet. A new concept is proposed for the emergence of ULF geomagnetic oscillations with a discrete spectrum of frequencies (0.8, 1.3, 1.9, 2.6 ... mHz) registered in the magnetosphere's midnight-morning sector. The wave confinement is a result of the velocity values of fast magnetosonic waves in the near-Earth plasma sheet differing greatly from those in the magnetotail lobes, leading to turning points forming in the tailward direction. The fundamental harmonics of this resonator's eigen-frequencies are shown to be capable of being clustered into groups with average frequencies matching, with good accuracy, the frequencies of the observed oscillations. A possible explanation for the stability of the observed oscillation frequencies is that such a resonator might only form when the magnetosphere is in a certain unperturbed state.

Coupled modes can be formed by azimuthally small-scale Alfven and slow magnetosonic (SMS) waves at the geomagnetic field lines crossing the plasma layer. It is shown that the linear transformation of these waves occurs in the current sheet on geomagnetic field lines stretched into the magnetotail. In most of the field lines their structure is determined by the large-scale Alfven wave structure. Near the ionosphere and in the current sheet, a small-scale SMS wave field starts to dominate. Such modes are neutrally stable on the field lines that do not cross the current sheet, but switch to the ballooning instability regime on field lines crossing the current sheet. In the direction across magnetic shells the coupled modes are waves running away from the magnetic shell on which they were generated. At the field lines crossing the current sheet the structure of the field components of coupled modes has four singularities at the inflection points of a field line, that look like as resonance peaks.