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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 21, issue 3
Ann. Geophys., 21, 761–777, 2003
https://doi.org/10.5194/angeo-21-761-2003
© Author(s) 2003. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 21, 761–777, 2003
https://doi.org/10.5194/angeo-21-761-2003
© Author(s) 2003. This work is distributed under
the Creative Commons Attribution 3.0 License.

  31 Mar 2003

31 Mar 2003

Multi-frequency observations of E-region HF radar aurora

S. E. Milan1, M. Lester1, and N. Sato2 S. E. Milan et al.
  • 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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