HR: 1340h
AN: SM33B-1341    [Abstracts]
TI: Occurrence Pattern of Whistler Mode (WM) Echoes Observed by RPI/IMAGE as a Function of Geomagnetic Activity
AU: * Reddy, A
EM: ftar1@uaf.edu
AF: University of Alaska Fairbanks, P.O.B. 755915, Electrical and Computer Engineering Department, University of Alaska Fairbanks, Fairbanks, AK 99775, United States
AU: Sonwalkar, V S
EM: ffvss@uaf.edu
AF: University of Alaska Fairbanks, P.O.B. 755915, Electrical and Computer Engineering Department, University of Alaska Fairbanks, Fairbanks, AK 99775, United States
AU: Carpenter, D L
EM: dlc@nova.stanford.edu
AF: Stanford University, Electrical Engineering Department, STAR Laboratory, Stanford University, Stanford, CA 94305, United States
AU: Reinisch, B W
EM: bodo_reinisch@uml.edu
AF: University of Massachusetts Lowell, Department of Environmental, Earth & Atmospheric Sciences, University of Massachusetts Lowell, Lowell, MA 01845, United States
AB: Discrete and diffuse magnetospherically reflected (MR) and specularly reflected (SR) whistler-mode (WM) echoes have been observed by the Radio Plasma Imager (RPI) on the IMAGE satellite ( Sonwalkar et al., J. Geophys. Res., 109, A11212, 2004; Sonwalkar et al., AGU Fall Meeting 2006, abstract SM11B-0323, 11-15 December, San Francisco, California, USA). Discrete WM echoes occur under relatively smooth plasma density variation along the field line passing through IMAGE (B₀) and diffuse WM occur when small scale (~10-100 m) field aligned irregularities (FAI) are present near B₀. The study presented here is based on WM echo observations during 2004-2005 when 3.2 ms short pulses in 6-63 kHz frequency range with 0.3 kHz linear stepping (Program 38) were transmitted at altitudes less than ~6000 km. We examined the occurrence of discrete and diffuse SR- and MR-WM echoes observed in Aug-Dec 2005 during geomagnetically quiet periods, Kp < 3-4 and Dst is varying between -10 nT and 40 nT, and during geomagnetically disturbed periods. The disturbed periods included three geomagnetic storms (Kp>4 and Dst below -100 nT at the onset of the storm) and their recovery periods. In all ~400 and ~600 transmissions of program 38 were made during the geomagnetically quiet and disturbed periods, respectively. The occurrence rate per transmission for a WM echo (irrespective of the type of echo) was around ~30% and ~25% for quiet and disturbed periods, respectively. We found that the occurrence rate per transmission of diffuse MR-WM echoes was not significantly affected but the occurrence rate of discrete MR-WM had doubled from quiet to disturbed periods. We found significantly fewer (one-fourth) discrete SR-WM echoes and slightly larger number of diffuse SR-WM echoes during the disturbed periods compared to that during the quiet periods. These observations suggest that geomagnetic storms lead to significant enhancements in density structures (FAIs) at altitudes lower than 1000 km, thereby affecting primarily the propagation of SR-WM echoes that are reflected at ~90 km but not that of MR-WM echoes because they are reflected at altitudes >1000 km. It is, however, interesting that the probability of discrete MR-WM has doubled from quiet to disturbed periods.
DE: 2403 Active experiments
DE: 2439 Ionospheric irregularities
DE: 2481 Topside ionosphere
DE: 2487 Wave propagation (0689, 3285, 4275, 4455, 6934)
DE: 2788 Magnetic storms and substorms (7954)
SC: SPA-Magnetospheric Physics [SM]
MN: 2007 Fall Meeting