Computer Science – Sound
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmsm11c1184l&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #SM11C-1184
Computer Science
Sound
2471 Plasma Waves And Instabilities, 2487 Wave Propagation (6934), 2704 Auroral Phenomena (2407)
Scientific paper
The SIERRA sounding rocket was launched into an active substorm aurora north of Poker Flat, Alaska, on Jan 14, 2002, achieving an apogee of 735 km. Instrumentation included continuous waveform measurements of 100 kHz--5 MHz electric fields detected with a single 6-m double probe antenna which rotated in a plane containing the ambient magnetic field at the rocket spin rate of 0.33 Hz. For about 200 seconds on the downleg of the flight, corresponding to a range of about 200 km along the ground and vertically, this instrument detected whistler waves in the LF-MF range (up to about 1 MHz) which exhibit complex structure resembling AKR fine structure observed with satellite-borne instruments at higher altitudes. The most intense structured whistler mode waves occur at 250--500 kHz, the same frequency range as AKR. One type of structured whistler mode signal which is common in this data set consists of packets of descending tones separated by tens of kHz in frequency and 10--50 ms in time, with typical downward frequency slopes of 1 MHz/s. Ignoring wave dispersion and assuming excitation at the electron gyrofrequency would imply a height of 2800 km and upward velocity of 5000 km/s for the sources of these structures, though consideration of whistler wave dispersion could significantly affect the latter estimate. The occurrence and intensity of this type of structured whistler mode signal appear not to depend on the antenna orientation with respect to the background magnetic field. Another type of structured whistler mode which is common in this data set consists of signals whose frequency increases rapidly until just below the gyro- or plasma-frequency at which point the rate of change of frequency decreases. These structures may be a form of hissler observed at higher frequencies near the top of the whistler range rather than near the bottom of the whistler frequency range as in the original hisslers reported by Siren [1975]. In addition to these structured whistler mode signals, structured Langmuir/Upper Hybrid waves occurred at frequencies above 1 MHz during most of the flight. Structured LF-MF whistler mode signals have been detected on one other recent rocket flight [LaBelle et al., 1999] restricted to a few short time intervals, rather than dominating the LF-MF spectrum for over 200 seconds as observed during the SIERRA experiment. References: LaBelle, J., K.L. McAdams, and M.L. Trimpi, Structured low and medium frequency whistler mode emissions in the auroral ionosphere, J. Geophys. Res., 104, 28101, 1999. Siren, J., Fast Hisslers in substorms, J. Geophys. Res., 80, 93, 1975.
Kintner Paul M.
Klatt Eric
LaBelle James
Lynch Kristina Anne
MacDonald Elizabeth A.
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