D region HF radar echoes associated with energetic particle precipitation and pulsating aurora

S E Milan,M Lester,F Honary,K Hosokawa,N Sato,H Yamagishi

doi:10.5194/angeo-26-1897-2008

Abstract

Abstract. Milan et al. (2001) identified a class of narrow, slow-moving HF radar backscatter echoes which originate between altitudes of 80 and 100 km, the ionospheric D- and lower E-regions. These echoes appeared to be associated with the occurrence of pulsating aurora, which are known to be created by energetic electrons capable of penetrating to D region altitudes. In this study we show that these echoes are observed in tandem with enhancements in cosmic noise absorption (auroral absorption), additional evidence that energetic (>30 keV) particle precipitation is responsible for generating the irregularities from which a radar can scatter. In addition, we show that the D region backscatter echoes occur predominantly in the post-midnight sector during substorm recovery phase, in common with auroral absorption events and pulsating aurora.

Highlights

The SuperDARN radars (Greenwald et al, 1995; Chisham et al, 2007) are designed to detect HF radiowave coherent backscatter from ionospheric plasma irregularities at F region altitudes which act as tracers of the polar ionospheric convection pattern
Most of these occur in tandem with absorption of the cosmic noise background observed by the riometer; coincident events are marked by horizontal bars for clarity. On occasions these are associated with overall decreases in the number of E region echoes observed, as might be expected if the radar signals are being attenuated in the enhanced D region. These results graphically demonstrate the association between enhancements of the D region electron density, giving rise to auroral absorption of the cosmic noise background, and the observation of backscatter echoes from D region altitudes
Such echoes occur predominantly in the postmidnight sector, in association with enhancements of the D region electron density. Such enhancements lead to the attenuation of HF radio signals, often referred to as auroral absorption, which we measure as cosmic noise absorption (CNA)

Summary

Introduction

The SuperDARN radars (Greenwald et al, 1995; Chisham et al, 2007) are designed to detect HF radiowave coherent backscatter from ionospheric plasma irregularities at F region altitudes which act as tracers of the polar ionospheric convection pattern. First identified by Milan et al (2001), based upon 30 min of observations, spatiallyextensive regions of D region echoes appeared to be associated with pulsating aurora In addition to their altitude of origin, the echoes observed were unusual in that their spectra were exceptionally narrow, having spectral widths less than 20 m s−1, which is in contrast to the bulk of E region spectra which have widths in excess of 100 m s−1. Such spectral widths are comparable to those of meteor echoes, in which the decorrelation of the radar auto correlation function is governed by the rate of ambipolar diffusion of the plasma trail rather than ionospheric instability growth and decay times (Hall et al, 1997; Milan et al, 2001). We further show that during an additional 18 h of riometer observations, from 3 separate nights, other occurrences of cosmic noise absorption are associated with detection of this low-altitude echo population

Observations

Findings

Discussion

Conclusions