HR: 0830h
AN: SM11C-1189    [PDF]
TI: Diurnal Variations of Quasi-Periodic and Periodic VLF emissions in the Outer Magnetosphere
AU: * Engebretson, M J
EM: engebret@augsburg.edu
AF: Department of Physics, Augsburg College, Minneapolis, MN 55454 United States
AU: Posch, J L
AF: Department of Physics, Augsburg College, Minneapolis, MN 55454 United States
AU: Shelburne, G A
AF: Department of Physics, Augsburg College, Minneapolis, MN 55454 United States
AU: Halford, A J
AF: Department of Physics, Augsburg College, Minneapolis, MN 55454 United States
AU: Halford, A J
AF: LASP, University of Colorado, Boulder, CO 80309 United States
AU: Smith, A J
AF: British Antarctic Survey, Madingley Road, Cambridge, CB3 0ET United Kingdom
AU: Spasojevich, M
AF: Star Laboratory, Stanford University, Stanford, CA 94305 United States
AU: Inan, U S
AF: Star Laboratory, Stanford University, Stanford, CA 94305 United States
AU: Arnoldy, R L
AF: Space Science Center, University of New Hampshire, Durham, NH 03824 United States
AB: ELF-VLF receiver and search coil magnetometer data from four Antarctic stations during 1998 have been analyzed to study quasi-periodic emissions (QPs) and periodic emissions (PEs), which occur as ULF-range modulations of ELF-VLF signals between 0.5 kHz and $\sim$4 kHz. QPs are modulated at frequencies of 15-50 mHz, and PEs are modulated at frequencies of 100-500 mHz. The stations used covered a range of magnetic latitudes from -62$\deg$ (Halley) to -74$\deg$ (South Pole Station); two automated geophysical observatories (AGOs) were located at intermediate latitudes. Consistent with earlier studies, most QPs were observed with magnetic pulsations of identical period in the Pc 3-4 range (type I QPs). Of those QPs not observed with simultaneous magnetic pulsations (type II QPs), nearly all were accompanied by PEs. It is also notable that no PE events simultaneous with Pc 3-4 magnetic pulsations but without QPs were observed during the entire year at any station. Diurnal patterns of all categories of QP events show occurrence maxima in the noon or post-noon sectors. The total absence of type I QP occurrences without PEs on the nightside is consistent with the expected absence of Pc 3-4 pulsations there, because of their origin at the upstream bow shock. PEs, on the other hand, could be seen at all local times. Diurnal profiles showed no latitudinal or seasonal differences for most categories, except for a clear reversal of the profile of PE occurrence with latitude. In every multistation event studied, we have found QP and/or PE modulations to be simultaneous at all stations to within the 1-s sampling time of the data. This suggests that all such events originate a single, localized region, most probably near the plasmapause. In contrast, the magnetic pulsations showed little or no detailed correlation between stations. Both statistical and single event studies support the hypothesis that highly localized field-aligned currents stimulated by echoing whistlers (PEs) may be responsible for producing type II QPs on the same flux tubes, typically in the subauroral or auroral regions. In addition, we have noted that the small number of type II QP events we have observed without accompanying PEs is tightly clustered near local noon. The existence of a set of equatorially-localized, near-noon, compressional Pc 3-4 waves that evidently do not appear in ground records suggests that this category as well might be generated by ULF wave modulation of equatorial ELF/VLF wave growth.
DE: 2731 Magnetosphere--outer
DE: 2736 Magnetosphere/ionosphere interactions
DE: 2752 MHD waves and instabilities
DE: 2772 Plasma waves and instabilities
SC: SPA - Magnetospheric Physics [SM]
MN: 2003 Fall Meeting