Observations of Electromagnetic Ion Cyclotron Waves in the Earth's Equatorial Outer Magnetosphere and the Proton Cyclotron Instability

Details Observations of Electromagnetic Ion Cyclotron Waves in the Earth's Equatorial Outer Magnetosphere and the Proton Cyclotron Instability Observations of Electromagnetic Ion Cyclotron Waves in the Earth's Equatorial Outer Magnetosphere and the Proton Cyclotron Instability

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmsm51a1266p&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #SM51A-1266

Physics

2772 Plasma Waves And Instabilities (2471)

Scientific paper

A study using 60 days of magnetic field data from the SCATHA (P78-2) satellite has been conducted to evaluate the occurrence of electromagnetic ion cyclotron (EMIC) waves in the Earth's magnetosphere. Based upon wave spectrograms and dynamic ellipticity data, EMIC waves were observed on 40 days. Thirty (30) events were chosen for analysis because they showed clear spectra over a 2 minute interval from which the peak frequency and the maximum frequency were clearly defined. These events occurred between early morning and late afternoon, i.e., the dayside sector, with L values from 5.6 to 8.02 between magnetic latitudes -8° and 13°. The peak frequencies ranged from 0.28 to 0.79 Hz, i.e., 0.176 to 0.6 normalized to the local proton cyclotron frequency. Half of the events were left-handed polarized, with 13 linearly polarized, and 2 right-handed polarized. Assuming a pure proton plasma, the proton temperature anisotropy, Ap= Tperp/Tpara -1, is determined from the expression, Ap= Xmax/(1-Xmax) [Kennel and Petschek,1966], where Xmax is the maximum observed frequency normalized to the local gyrofrequency calculated from the measured local magnetic field. The local plasma number density is obtained from the empirical models of Chappell [1974], Shelley et al. [2001], and Denton et al. [2002]. Using the formula established by Gomberoff and Neira [1983], the cold plasma convective growth rate for EMIC waves is then a function of Tpara and frequency. Assuming that the peak frequency in the observed emission spectrum corresponds to the calculated peak in the convective growth rate allows the determination of Tpara and then Tperp for each event. We also calculated the parallel resonant energy of the interacting proton Epara,R for each event. Analysis of the results leads to the following conclusions. (1) The plot of Ap versus the proton parallel beta, βpara,p, where βpara,p=2μonpkB Tpara/Bo2, shows a best fit function of the form Ap=0.7347/βpara,p0.3930 for βpara,p from 0.093 to 10.198, which is similar to that found by Gary et al. [1994]; this suggests that the proton cyclotron instability actually operated in the events. (2) The plot of Ap versus Tperp shows that Ap clearly decreases with increasing Tperp, suggesting that for EMIC waves to be destabilized, lower Tperp requires higher Ap; this is consistent with a finding that the free energy source of EMIC waves are energetic protons with an anisotropic distribution. (3) The Tperp versus X plot shows that Tperp decreases strongly with increasing X, approaching small values when X is nearer to X ~ 0.6; this is in agreement with the theoretical finding of Gendrin et al. [1971]. They used kinetic theory and assumed a bi-Maxwellian anisotropic distribution of hot protons with Tperp > Tpara and found that EMIC waves can be amplified when X is between 0.2 and 0.7. (4) The plot of Ap versus Epara,R shows that Ap also clearly decreases with increasing Epara,R suggesting that for EMIC waves to occur lower Epara,R requires higher Ap; this is again consistent with the finding that the free energy source of EMIC waves are energetic protons with an anisotropic distribution.

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