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Ann. Geophys., 21, 761-777, 2003
www.ann-geophys.net/21/761/2003/
© European Geosciences Union 2003
Multi-frequency observations of E-region HF radar aurora
S. E. Milan1, M. Lester1, and N. Sato2
1Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
2National Institute of Polar Research, Tokyo 173-8515, Japan
Abstract. Multi-frequency
observations of E-region coherent backscatter from decametre waves reveal that
auroral echoes tend to comprise two spectral components superimposed, one at
low Doppler shifts, below 250 ms-1, and the other Doppler shifted to
near the ion-acoustic speed or above, up to 800 ms-1. The low
Doppler shift component occurs at all look directions; Doppler shifts near the
ion acoustic speed occur when looking at low flow angles along the direction of
the electron drift in the electrojet, and Doppler shifts in excess of the ion
acoustic speed occur at intermediate flow angles. The latter population appears
most commonly at radar frequencies near 10–12 MHz, with its occurrence
decreasing dramatically at higher frequencies. The velocity of the high Doppler
shift echoes increases with increasing radar frequency, or irregularity wave
number k. The velocity of the low Doppler shift population appears to be
suppressed significantly below the line-of-sight component of the electron
drift. Initial estimates of the altitude from which scatter occurs suggest that
the high Doppler shift echoes originate from higher in the E-region than the
low Doppler shift echoes, certainly in the eastward electrojet. We discuss
these observations with reference to the theories of de/stabilization of
two-stream waves by electron density gradients and electrostatic ion cyclotron
waves excited by field-parallel electron drifts.
Key words. Ionosphere (ionospheric irregularities)
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