Physics
Scientific paper
Nov 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990jgr....9519099b&link_type=abstract
Journal of Geophysical Research, Volume 95, Issue A11, p. 19099-19114
Physics
12
Magnetospheric Physics: Magnetopause, Cusp, And Boundary Layers
Scientific paper
Magnetic holes are occasionally observed in the vicinity of the magnetopause. These holes are similar to crater flux transfer events in size and magnetic signature, but have a lower magnetic field and different wave spectra. An exemplary magnetic hole has been observed by the AMPTE IRMspacecraft on September 4, 1984, in the early evening sector of the magnetosphere. We here investigate the plasma wave spectra inside and in the immediate vicinity of the hole and their relation to the plasma properties of the hole. Inside the hole where magnetic field levels are lowest, wave emissions between a few times the ion cyclotron frequency and the plasma frequency drop to levels close to the instrumental noise level. This behavior is similar to that observed in the artifical comet and plasma cloud releases in the solar wind, magnetosheath, and magnetospheric tail produced by the same satellite. Electromagnetic waves with frequency up to several times the ion cyclotron frequency are found inside the hole with relatively strong amplitudes (δB/B~1). These could represent either whistler waves or ion cyclotron waves. Estimations of the local electron plasma frequency suggest that this hole has a plasma density as high as 200 cm-3, whereas the ambient plasma maintains a density of 10-30 cm-3. In contrast to the low activity in the parts of the whole where the magnetic field is lowest, the boundaries contain very intense broadband noise from less than 1 Hz to several kHz. These could be generated by the lower hybrid drift instability. The energy density of the boundary waves, assumming they are lower hydrid waves, if far greater than the energy density of the low-frequency waves inside the hole; hence, it is plausible that the interior waves represent the leakage of waves from the boundary region into the hole. Some ideas about the origin of the magnetic holes are discussed.
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