HR: 16:15h
AN: SM44A-02    [Abstracts]
TI: Analysis of Upper Hybrid Wave Growth Rates From Measured Electron Distributions; An Encounter With the Source of Auroral Roar
AU: * Bounds, S R
EM: scott-bounds@uiowa.edu
AF: University of Iowa, Dept. Physics and Astronomy 210 Van Allen Hall, Iowa City, IA 52242 United States
AU: Kletzing, C A
SM44A-02 AF: University of Iowa, Dept. Physics and Astronomy 210 Van Allen Hall, Iowa City, IA 52242 United States
AU: Labelle, J W
SM44A-02 AF: Dartmouth College, Dept. Physics and Astronomy, Hanover, NH 03755 United States
AU: Samara, M
SM44A-02 AF: Southwest Research Institute, 6220 Culebra Road, San Antonia, TX 78238 United States
AU: Yoon, P H
SM44A-02 AF: University of Maryland, Inst. Physical Science & Tech., College Park, MD 20742 United States
AB: In January of 2003, the High Bandwidth Auroral Rocket (HIBAR) passed through two regions of strong upper hybrid wave emission associated with the approximate matching of the upper hybrid frequency to twice the electron cyclotron frequency (f_uh = 2 f_ce) (Samara 2004) These types of emission are believed to be the source of the HF auroral roar often observed by ground based receivers. The current model theorizes that the free space 0-mode waves observed on the ground are produced through mode conversion of strong emission of Z-mode, or upper hybrid waves. The relativistic electron cyclotron maser exhibits significant growth rates for the Z-mode when the local upper hybrid frequency is just below (~1%) twice the electron cyclotron frequency and with the appropriately unstable electron distribution (Yoon 1996, Yoon 1998, Yoon 2000). Though auroral roar is frequently observed from the ground, the source region has rarely been identified in-situ and even more rarely with sufficient bandwidth to analyze the underlying physical processes. Analysis of the electron distributions from HIBAR show good agreement with the theoretical distributions used by Yoon:98. HIBAR encountered three separate regions where f_uh ≈ 2 f_ce, two of these regions include strong upper hybrid emission, while the third is void of upper hybrid wave activity. The measured particle distributions demonstrate that, in the two regions with wave emission, the relativistic electron cyclotron maser instability produces Z mode wave growth rates at least an order of magnitude greater than the electron collision frequency. In the third region without wave emission, the growth rates are much smaller in both amplitude and the extent of occurance.
Samara, M., J. LaBelle, C. A. Kletzing, and S. R. Bounds, Rocket observations of structured upper hybrid wave at f_uh=2f_ce, Geophys. Res. Lett., 31, L22804, doi:10.1029/2004GL021043.
Yoon, P. H., A. T. Weatherwax, and T. J. Rosenberg, Lower ionospheric cyclotron maser theory: A possible source of 2f_ce and 3f_ce auroral radio emissions, J. Geophys. Res., 101, 27,015--27,025, 1996.
Yoon, P. H., A. T. Weatherwax, and T. J. Rosenberg, On the generation of auroral radio emission at harmonics of the lower ionospheric electron cyclotron frequency: X, O and Z mode maser calculations, J. Geophys. Res., 103, 4071--4078, 1998.
Yoon, P. H., A. T. Weatherwax, and J. LaBelle, Discrete electrostatic eigenmodes associated with ionospheric density structure: generation of auroral roar fine frequency structure, J. Geophys. Res., 105, 27,589--27,596, 2000.
DE: 2407 Auroral ionosphere (2704)
DE: 2471 Plasma waves and instabilities (2772)
DE: 2704 Auroral phenomena (2407)
DE: 7867 Wave/particle interactions (2483, 6984)
SC: SPA-Magnetospheric Physics [SM]
MN: Fall Meeting 2005