On the generation of cusp HF backscatter irregularities

Abstract

[1] HF radar observations, 630.0 nm cusp auroral emission, and an ionospheric tomography scan are compared to investigate the mechanism by which radar cusp echoes are generated. A collocation is reported between the equatorward boundaries of broad spectral width HF radar backscatter and 630.0 nm cusp auroral precipitation. In addition, the radio tomographic image reveals spatial structures in ionospheric electron density that map closely to the precipitation. However, the large-scale horizontal gradient appears to be insufficiently steep to account alone for the rapid growth of irregularities by the gradient drift mechanism. It is proposed that the initial source of the decameter-scale features responsible for the backscatter may result from fine structure within the precipitation itself, or alternatively, from the cascade down of unstable intermediate-scale gradients created in the plasma convecting poleward from the source region. The latter process may explain the ongoing backscatter found poleward of the auroral band.

On a new process for cusp irregularity production

“…4g have been determined to be characteristic of the cusp region ionosphere (e.g. Moen et al, 2001Moen et al, , 2002. Figures 2, 3, and 4a-d show five patches initiated by the reconnection events discussed here in the time interval 07:00 to 07:40 UT.…”

“…Figure 7 illustrates that, beyond the 1-D Hankasalmi data presented here, such SuperDARN data can be a tool to map very rapid onsets of cusp irregularities across a large area of the cusp. Moen et al (2001Moen et al ( , 2002 showed a collocation of broad Super-DARN echoes with boundaries in the cusp aurora. Carlson et al (2007) interpreted those data as a confirmation of the onset of SuperDARN irregularities that were too rapid to be explained without the framework of the new process.…”

Factors determining spectral width of HF echoes from high latitudes

“…A feasible candidate for the generation mechanism might be particle precipitation, which exhibits a close but still poorly understood relationship with W (Dudeney et al, 1998;Lester et al, 2001;Moen et al, 2001Moen et al, , 2002Woodfield et al, 2002;Chisham et al, 2005a). Based on the observed co-location of the equatorward boundary for 630 nm emis- sion and the SWB, Moen et al (2001) suggested that particle precipitation can be structured on the decameter scale providing effective targets for HF radiowaves.…”

“…Moen et al (2001Moen et al ( , 2002 demonstrated that the dayside OCB coincides with the equatorward boundary of 630 nm optical emission caused by "soft" electron precipitation. Recently, Chisham et al (2005a) concluded that the magnitude of W shows anti-correlation with the total electron energy flux, but no explanation of these relationships has been provided.…”

“…Several studies have hinted at a close relationship between W and particle precipitation (Baker et al, 1986(Baker et al, , 1990Dudeney et al, 1998;Lester et al, 2001;Moen et al, 2001Moen et al, , 2002Woodfield et al, 2002;Chisham et al, 2005a). Moen et al (2001Moen et al ( , 2002 demonstrated that the dayside OCB coincides with the equatorward boundary of 630 nm optical emission caused by "soft" electron precipitation.…”