Statistical Study of Magnetic Fluctuation Features Associated with Near-tail Dipolarizations Observed by the THEMIS Spacecraft

Details Statistical Study of Magnetic Fluctuation Features Associated with Near-tail Dipolarizations Observed by the THEMIS Spacecraft Statistical Study of Magnetic Fluctuation Features Associated with Near-tail Dipolarizations Observed by the THEMIS Spacecraft

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsm41a1852p&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #SM41A-1852

Physics

Plasma Physics

[2752] Magnetospheric Physics / Mhd Waves And Instabilities, [2768] Magnetospheric Physics / Plasmasphere, [2790] Magnetospheric Physics / Substorms, [7836] Space Plasma Physics / Mhd Waves And Instabilities

Scientific paper

In this work we examined a number of magnetic dipolarizations that occurred at X > ~ -11 RE in the near-Earth plasma sheet. These dipolarization events were observed by the three near-tail THEMIS spacecraft. By employing the continuous wavelet transform technique, we found several interesting features. First, for the low frequency regime that we conveniently defined here, 0.005Hz to 0.03Hz (typically well below local proton gyro-frequency), we found that a significant percentage of dipolarizations indicate that the fluctuations are predominantly localized at one or more discrete frequencies. Many of them grow with growth time less than 2 min across onset time. Statistically, f ~ 0.012-0.014Hz is the dominant frequency, which is similar to frequency that has been referred to as “ballooning mode frequency” (e.g., Cheng and Lui, 1998; Horton et al., 1999). So, many of the dipolarizations can be viewed as manifestation of low frequency instability phenomenon. Further, we analyzed the polarization features (instability mode structure) of the dipolarization fluctuations in the low frequency domain, and found that the compressional component of magnetic fluctuations is significant for most of the studied events. It implies that the instability inevitably involves “compression” effect to a large extent. But it was also found that the relative significance of the perpendicular component of the fluctuations (so, the field line bending effect) increases with plasma beta which is thought to be proportional to the degree of field line stretching. When a perpendicular component is non-negligible, many events indicate that the magnetic oscillation on the perpendicular plane is mostly linearly polarized, and it is mostly in pshi direction, which is perpendicular to the azimuthal direction. This mode structure seems to be consistent with the conventional ballooning limit. Lastly, we have also studied the fluctuation features in the high frequency regime which we defined here is from 0.03 Hz to 0.1 Hz. Most importantly we found the tendency that the magnetic fluctuation intensity in the high frequency regime is localized near the neutral sheet, decreasing away from the neutral sheet. We have also estimated the relative significance between the low and high frequency fluctuations, the details of which will be presented in this paper.

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