Computer Science – Sound
Scientific paper
Apr 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991angeo...9..273h&link_type=abstract
Annales Geophysicae (ISSN 0939-4176), vol. 9, April 1991, p. 273-285. Research supported by DRET and USAF.
Computer Science
Sound
34
Doppler Radar, E Region, Ionospheric Sounding, Radar Measurement, Statistical Analysis, Auroral Electrojets, Energy Spectra, Field Aligned Currents, High Frequencies
Scientific paper
A large number of Doppler spectra obtained at six HF frequencies in the E region with the SHERPA HF radar are analyzed statistically. They are characterized by their total power, their mean velocity and their spectral width. The observations cover geomagnetic conditions varying from quiet to disturbed. Due to the location of the auroral oval relative to the radar field of view, echoes are preferentially detected in the morning westward electrojet, with radial velocities away from the radar. This results in an asymmetrical velocity distribution. Both usual Type 1 and Type 2 Doppler spectra, associated with two-stream and gradient-drift waves, respectively, are at the origin of the two peaks in the distribution. Other spectral types do not appear in the statistics. Spectral widths remain narrow for both types. This is explained by a longer lifetime of wave trains compared to higher frequency waves detected by VHF radars. A decrease of the Doppler velocity with radar frequency is related to gradient effects on threshold, as predicted by the linear dispersion equation for the two-stream instability.
Baker Kile B.
Cerisier Jean-Claude
Greenwald Ray A.
Hanuise Ch.
Ruohoniemi Michael J.
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