HR: 0830h
AN: SM31B-1111    [PDF]
TI: Electromagnetic Ion Cyclotron Waves in the Subsolar Region Under Normal Dynamic Pressure: Wind Observations and Theory
AU: Graciela, G
EM: ggnavi@arnet.com.ar
AF: Institute of Plasma Physics, University of Buenos Aires, Buenos Aires, 1114 Argentina
AU: * Farrugia, C J
EM: charlie.farrugia@unh.edu
AF: Space Science Center, University of New Hamsphere, Durham, NH 03824
AU: Gratton, F T
EM: fgratton@arnet.com.ar
AF: Institute of Plasma Physics, University of Buenos Aires, Buenos Aires, 1114 Argentina
AU: Matsui, H
EM: hiroshi.matsui@unh.edu
AF: Space Science Center, University of New Hamsphere, Durham, NH 03824
AU: Torbert, R B
EM: roy.torbert@unh.edu
AF: Space Science Center, University of New Hamsphere, Durham, NH 03824
AU: Lepping, R P
EM: RPL@leprpl1.gsfc.nasa.gov
AF: NASA Goddard Space Flight Center, Greenbelt, Greenbelt, MD 20771 United States
AU: Oieroset, M
EM: oieroset@ssl.berkeley.edu
AF: Space Science Lab, University of California, Berkeley, CA 94720 United States
AU: Lin, R P
EM: boblin@ssl.berkeley.edu
AF: Space Science Lab, University of California, Berkeley, CA 94720 United States
AB: We analyze observations on electromagnetic ion cyclotron waves (EICWs) in the plasma depletion layer (PDL) made by Wind on 3 inbound passes of the magnetosheath near the stagnation streamline, and model the observations using recent theory ({\it Gnavi et al}., 2000). While one pass was under substantially high solar wind dynamic pressure $P_{dyn}$ ($\sim$ 6.4 nPa), two passes were under typical $P_{dyn}$ at 1 AU ($\sim$ 2.2 nPa) which allows us to extend the study of EICWs into this lower pressure regime. Two passes were under steady, and one under time varying, conditions. We subdivide the PDL into inner, middle and outer regions and compute in each the power spectral density (PSD) of the magnetic fluctuations transverse to the background field, using high resolution ($\sim$ 11 samples/s) data from the Magnetic Field Investigation (MFI). The theory solves the kinetic dispersion relation in a plasma composed of electrons, protons, and $\alpha$ particles, each species modeled by biMaxwellian distribution functions. Parameters for calculating growth rates are taken from the observations, where available, and from average values quoted in the literature otherwise. Qualitative, and on occasion even quantitative, agreement between theory and data is very good. The observations show (i) at the inner PDL position the spectral PSD weakens as $P_{dyn}$ decreases, and (ii) the frequency range of emission shifts downward with diminishing $P_{dyn}$. Comparing observations with the classification of spectral types in the PDL under compressed conditions proposed by {\it Anderson et al}. [1994], we find a preponderance of so-called low and continuous spectra and only marginally bifurcated spectra with activity peaks below and above the alpha-resonance frequency. In general, near the magnetopause our temperature anisotropies $A_p$ are at the lower end of values characterizing bifurcated spectra in AMPTE/CCE. Using bipolytropic laws for the anisotropic magnetosheath, we argue that the effect of a reduction of $P_{dyn}$ is to lower $A_p$, thereby weakening the driver of EICWs leading to marginally bifurcated spectra and weaker EICW activity in the PDL under typical conditions. This work is partially supported by NASA grants NAG5-13116, NAG5-11803, and and the Argentinean UBACYT grant X059.
DE: 2728 Magnetosheath
DE: 2772 Plasma waves and instabilities
DE: 2784 Solar wind/magnetosphere interactions
DE: 2799 General or miscellaneous
SC: SPA - Magnetospheric Physics [SM]
MN: 2003 Fall Meeting