Statistics – Applications
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsm11c..06l&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #SM11C-06
Statistics
Applications
2407 Auroral Ionosphere (2704), 2471 Plasma Waves And Instabilities (2772), 2706 Cusp, 2772 Plasma Waves And Instabilities (2471)
Scientific paper
The Twin Rockets to Investigate Cusp Electrodynamics (TRICE) were launched Dec. 10, 2007, from Andoya into the active cusp over Svalbard. The high-frequency electric field waveform receiver on the higher flying payload detected Langmuir waves concentrated at 900-1145 km on the downleg. The waves occurred in bursts having durations between tens of ms and a few hundred ms. From analysis of selected sample intervals, it is estimated that the rocket encountered more than 1000 such bursts ranging in amplitude from about 1 mV/m, near the detection threshold, to nearly 1000 mV/m, though only a few bursts approached this upper bound. Waveform analysis showed that the waves were modulated at kHz to tens of kHz frequencies. The modulation was not monochromatic but consisted of many nonstationary frequency components giving rise to a complex envelope waveform. The modulated waves were sufficiently oversampled to clearly observe 180-degree phase shifts when the envelopes of the modulations passed through zero, as expected if a superposition of modulating waves is responsible. Dynamic spectra of the bursts showed that the bandwidth of each event was related to the modulation frequencies. The dynamic spectra also revealed a repeatable pattern whereby the highest Langmuir wave frequency and largest bandwidth occurred in the center of each burst, with frequency and bandwidth decreasing toward either side of each burst. For the conditions of the observations (fpe ≤ fce/2), Langmuir waves convert to whistler mode waves, and the observed pattern may be explained by trapping of the Langmuir/whistler waves in density enhancements. The observed wave electric fields were statistically analyzed in two ways: First, the distribution of the absolute values of all of the electric field measurements was determined, hereafter referred to as the "waveform distribution"; second, the envelope of the Langmuir waves was extracted and the distribution of this field was determined, hereafter referred to as the "envelope distribution". The waveform distribution showed a characteristic E+1 dependence at low electric fields. Bandpass-filtering the waves with progressively narrower passbands preserved the E+1 dependence but with a sharper cutoff at the high-electric field end, approaching the analytical expectation for a rectified sine wave. The envelope distribution showed a steeper power-law index at low electric fields. Averaging the waveforms by factors of 10-300 produced waveform distributions with a power index comparable to that of the envelope distribution, as expected since averaging by 10 corresponded to half the wave period so that the resulting statistics were dominated by the envelope. Averaging by factors greater than about 300, corresponding to averaging times exceeding about half of the modulation period, led to waveform distributions that appeared Gaussian although they were not well defined due to the small number of points. The relation between the waveform and envelope distributions appeared to be well explained by recent analytical predictions confirmed by numerical calculations [Cairns et al., Statistics of Waveform and Envelope Fields: Theory, Simulations and Initial Applications to TRICE Data, EOS Trans. Am. Geophys. Union, 2008, this issue]. The observed envelope distributions also appear consistent with theory [Kletzing et al., Determination of the Envelope Distribution for Langmuir Waves in the Topside Ionosphere, EOS Trans. Am. Geophys. Union, 2008, this issue].
Cairns Iver H.
Kletzing Craig A.
LaBelle James
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