HR: 0800h
AN: SM51A-1266    [Abstracts]
TI: Observations of Electromagnetic Ion Cyclotron Waves in the Earth­_s Equatorial Outer Magnetosphere and the Proton Cyclotron Instability
AU: * Perez, J D
EM: perez@physics.auburn.edu
AF: Physics Department, Auburn University, 206 Allison Lab, Auburn, AL 36849 United States
AU: Nguyen, S T
EM: nguyest@auburn.edu
AF: Physics Department, Auburn University, 206 Allison Lab, Auburn, AL 36849 United States
AU: Fennell, J F
EM: Joseph.F.Fennell@aero.org
AF: Space Science Laboratory, Aerospace Corporation, El Segundo, CA 90245 United States
AB: 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, A_p= T_perp/T_para -1, is determined from the expression, A_p= X_max/(1-X_max) [Kennel and Petschek,1966], where X_max 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 T_para 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 T_para and then T_perp for each event. We also calculated the parallel resonant energy of the interacting proton E_para,R for each event. Analysis of the results leads to the following conclusions. (1) The plot of A_p versus the proton parallel beta, β_para,p, where β_para,p=2μ_on_pk_B T_para/B_o², shows a best fit function of the form A_p=0.7347/β_para,p^0.3930 for β_para,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 A_p versus T_perp shows that A_p clearly decreases with increasing T_perp, suggesting that for EMIC waves to be destabilized, lower T_perp requires higher A_p; this is consistent with a finding that the free energy source of EMIC waves are energetic protons with an anisotropic distribution. (3) The T_perp versus X plot shows that T_perp 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 T_perp > T_para and found that EMIC waves can be amplified when X is between 0.2 and 0.7. (4) The plot of A_p versus E_para,R shows that A_p also clearly decreases with increasing E_para,R suggesting that for EMIC waves to occur lower E_para,R requires higher A_p; this is again consistent with the finding that the free energy source of EMIC waves are energetic protons with an anisotropic distribution.
DE: 2772 Plasma waves and instabilities (2471)
SC: SPA-Magnetospheric Physics [SM]
MN: Fall Meeting 2005