Physics
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmsm51a1279r&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #SM51A-1279
Physics
2731 Magnetosphere: Outer, 2772 Plasma Waves And Instabilities (2471), 7829 Kinetic Waves And Instabilities
Scientific paper
There are a number of mechanisms by which ULF oscillations can be excited in the magnetosphere, both externally (i.e. solar wind) or internally (e.g. via resonance with energetic particle sources). One mechanism that is not traditionally considered/observed is the mirror instability/mode. The mirror mode is a fundamental plasma instability and described as a compressible slow mode typically excited in high-beta plasmas where there is significant pressure anisotropy and grows if the free energy of the pressure anisotropy is sufficiently large and must be dissipated. Observational evidence of these mirror modes are rare, but mainly concentrated in the dayside magnetosheath, where the necessary temperature anisotropies can develop, and increase as the magnetopause is approached. However, it is possible to generate the mirror instability inside the magnetosphere, though as yet there are few examples. We present an interval on the 19th March 1998 when the Equator-S spacecraft is traversing the dawn-side magnetosphere above the ecliptic plane at radial distances of up to L ~ 11, and encounters quasi-monochromatic magnetic field fluctuations, and also similar variations in ion number density, temperatures and velocities. The magnetic field strength and number densities are anti-correlated, and the plasma and magnetic field pressures are in anti-phase also, which are all consistent with the mirror instability hypothesis. We calculate the mirror instability criterion (Hasegawa, 1969) and find that the condition for mirror-mode waves to grow is strongly met throughout the interval. We believe that this is a rare observation of a fundamental plasma instability in a natural environment at high temporal resolution. Observations such as these provide an invaluable natural tool in which to test current plasma theory.
Jonathan Rae I.
Kistler Lynn M.
Mann Ian R.
Watt C. E.
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