Magnetic local time, substorm, and particle precipitation-related variations in the behaviour of SuperDARN Doppler spectral widths

Abstract

Abstract. Super Dual Auroral Radar Network (DARN) radars often detect a distinct transition in line-of-sight Doppler velocity spread, or spectral width, from <50 m s −1 at lower latitude to >200 m s −1 at higher latitude. They also detect a similar boundary, namely the range at which ionospheric scatter with large spectral width suddenly commences (i.e. without preceding scatter with low spectral width). The location and behaviour of the spectral width boundary (SWB) (and scatter boundary) and the open-closed magnetic field line boundary (OCB) are thought to be closely related. The location of the nightside OCB can be inferred from the poleward edge of the auroral oval determined using energy spectra of precipitating particles measured on board Defence Meteorology Satellite Program (DMSP) satellites. Observations made with the Halley SuperDARN radar (75.5 • S, 26.6 • W, geographic; −62.0 • ) and the Tasman International Geospace Environment Radar (TIGER) (43.4 • S, 147.2 • E; −54.5 • ) are used to compare the location of the SWB with the DMSP-inferred OCB during 08:00 to 22:00 UT on 1 April 2000. This study interval was chosen because it includes several moderate substorms, whilst the Halley radar provided almost continuous high-time resolution measurements of the dayside SWB location and shape, and TIGER provided the same in the nightside ionosphere. The behaviour of the day-and nightside SWB can be understood in terms of the expanding/contracting polar cap model of high-latitude convection change, and the behaviour of the nightside SWB can also be organised according to substorm phase. Previous comparisons with DMSP OCBs have proven that the radar SWB is often a reasonable proxy for the OCB from dusk to just past midnight . However, the present case study actually suggests that the nightside SWB is often a better proxy for the poleward edge of Pedersen conductance enhanced by hot particle precipitation in the auroral zone. Simple modeling implies that the large spectral widths must be caused by ∼10-km scale velocity fluctuations.

Factors determining spectral width of HF echoes from high latitudes

“…Indeed, the scope of previous research (e.g. Parkinson et al, 2004;Chisham et al, 2004Chisham et al, , 2005a was limited to a boundary between relatively large and small spectral width regions. This has influenced both data processing algorithms and the physical interpretation of the results.…”

“…Hanuise et al, 1993;Baker et al, 1995;André et al, 2000;Moen et al, 2001;Parkinson et al, 2004;Chisham et al, 2005b), they generally failed to satisfactory explain the whole set of experimental results, including the existence of large spectral width values, W ≥200 m/s. Recently, Ponomarenko and Waters (2006) revised the software package FITACF used for processing the SuperDARN data and found that the software produces W values that are generally overestimated by 20-40%.…”

Dynamical critical scaling of electric field fluctuations in the greater cusp and magnetotail implied by HF radar observations of F-region Doppler velocity

“…3.1, Population B echoes are more likely to be found on open field lines in the polar cap, but also on closed field lines just equatorward of the open-closed boundary, whereas Population A echoes are more likely to be found on closed field lines in the auroral and sub-auroral ionosphere. Population A echoes are also more likely to be found in regions of high Pedersen conductance which often corresponds to the auroral ionosphere (Parkinson et al, 2004). The PDFs of v LOS for Population B (Fig.…”

“…The daytime sector was biased toward pre-noon because this was better aligned with the observed distribution of echoes with large spectral width, and to preserve symmetry with the nighttime sector used for TIGER. The latter was biased toward pre-midnight where the spectral width boundary is a better proxy for the open-closed field line boundary (Parkinson et al, 2004;Chisham et al, 2005). Note that we also aim to separate the analysis according to open and closed flux.…”

“…Magnetospheric electric fields map to ionospheric electric fields, except at short scales in regions of strong Pedersen conductance and field-aligned potential drops (Weimer et al, 1985). Hence the characteristics of electric field fluctuations may differ depending on whether they occur within regions of low Pedersen conductance which tend to occur in the weakly illuminated polar cap ionosphere (open field lines), or within regions of high Pedersen conductance which tend to occur in nightside auroral regions (closed field lines) (see Parkinson et al, 2004).…”