OBSERVATIONAL TEST OF LOCAL PROTON CYCLOTRON INSTABILITY IN THE EARTHS MAGNETOSPHERE

We present a study of the proton cyclotron instability in the Earth's outer magnetosphere, L > 7, using Active Magnetosphere Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) magnetic field, ion , and plasma wave data. The analysis addresses the energy of protons t hat generate the waves, the ability of linear theory to predict both i nstability and stability, comparison of the predicted wave properties with the observed wave polarization and frequency, and the temperature anisotropy/parallel beta relation. The data were obtained during 24 i ntervals of electromagnetic ion cyclotron (EMIC) wave activity (active ) and 24 intervals from orbits without EMIC waves (quiet). This is the same set of events used by Anderson and Fuselier [1994]. The active e vents are drawn from noon and dawn local times for which the wave prop erties are significantly different. For instability analysis, magnetos pheric hot proton distributions required the use of multiple populatio ns to analytically represent the data. Cyclotron waves are expected to limit the proton temperature anisotropy, A(p) = T-perpendicular to p/ T-parallel to p - 1, according to A(p) < alpha beta(parallel to p)(c) with a similar to 1 and c similar to 0.5, where T-perpendicular to p, T-parallel to p, and beta(parallel to p) are the perpendicular and par allel proton temperatures and the proton parallel beta, respectively. During cyclotron wave events, A(p) should be close to alpha beta(paral lel to p)(c) whereas in the absence of waves A(p) should be below alph a beta(parallel to p)(c). The active dawn cases yielded instability in 9 of 12 cases using the measured plasma data with an average growth r ate gamma/Omega(p) = 0.025 and followed the relation A(p) = 0.85 beta( parallel to p)(-0.52). The active noon events gave instability in 10 o f 12 cases, but only when an additional similar to 2 cm(-3) cold plasm a was assumed. The noon wave events fell well below the dawn events in A(p)-beta(parallel to p) space, slightly above the A(p) = 0.2 beta(pa rallel to p)(-0.5) curve. The lower A(p) limit for the noon cases is a ttributed to the presence of unmeasured cold plasma. The quiet events were all stable even for additional assumed cold ion densities of up t o 10 cm(-3), the upper limit implied by the plasma wave data. The quie t events gave A(p) < 0.2 beta(parallel to p)(-0.5). At noon, the unsta ble component has T-perpendicular to p similar to 20 keV and A(p) simi lar to 0.8. At dawn the unstable component has T-perpendicular to p si milar to 4 keV and A(p) similar to 2.3. Observed wave frequencies agre e with the frequencies of positive growth, and the difference in frequ ency between noon and dawn is attributable to the combined effects of the different hot proton T-perpendicular to p and A(p) and the inferre d higher cold plasma density at noon. The dawn events had significant growth for highly oblique waves, suggesting that the linear polarizati on of the dawn waves may be due to domination of the wave spectrum by waves generated with oblique wave vectors.