Low-frequency electromagnetic waves near and below the proton cyclotron frequency at the AMPTE Ba release: Relevance to comets and Mars

Details Low-frequency electromagnetic waves near and below the proton cyclotron frequency at the AMPTE Ba release: Relevance to comets and Mars Low-frequency electromagnetic waves near and below the proton cyclotron frequency at the AMPTE Ba release: Relevance to comets and Mars

Apr 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999jgr...104.6763s&link_type=abstract

Journal of Geophysical Research, Volume 104, Issue A4, p. 6763-6772

Physics

Plasma Physics

3

Space Plasma Physics: Active Perturbation Experiments, Interplanetary Physics: Plasma Waves And Turbulence, Space Plasma Physics: Kinetic And Mhd Theory

Scientific paper

``Nongyrotropic massloading'' is a characteristic signature of the AMPTE barium release in the solar wind because of the large pickup radius compared with the size of the interaction region. In the fluid picture this is equivalent to a beam-plasma configuration where the solar wind protons have a large relative velocity to the heavies, which can be considered as unmagnetized. Dispersion analysis on the basis of bi-ion Hall-MHD equations gives that two types of low-frequency electromagnetic waves are excited. The ``upper'' instability occurs in the branch of whistler/magnetoacoustic waves with the remarkable feature that the frequency in the beam (spacecraft) frame of the most unstable wave is close to the proton cyclotron frequency. Its growth rate strongly depends on the electron plasma beta (βe) which preferentially determines the whistler damping at oblique propagation. In consequence, the increase of the growth rate with the propagation angle is limited and an ``instability gap'' results at intermediate angles (40°<=θ<=70°). For βe~1, only small ranges near parallel and transverse propagation remain. For a high beam velocity the ``lower'' instability is determined by the cutoff frequency of the bi-ion system. Both lines were clearly seen in the spectra of magnetic field data of the AMPTE Ba release during the transition phase when the Ba ion density decreases below the solar wind density. These results may be relevant for other situations in space: comets with low gas production rates (q<=1028s-1), nonmagnetized planets (Venus, Mars) and active moons (Phobos, Deimos) including their gas tori.

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