Abstract
Substorm-associated radar auroral surges (SARAS) are a short lived (15â90 minutes) and spatially localised (~5° of latitude) perturbation of the plasma convection pattern observed within the auroral E-region. The understanding of such phenomena has important ramifications for the investigation of the larger scale plasma convection and ultimately the coupling of the solar wind, magnetosphere and ionosphere system. A statistical investigation is undertaken of SARAS, observed by the Sweden And Britain Radar Experiment (SABRE), in order to provide a more extensive examination of the local time occurrence and propagation characteristics of the events. The statistical analysis has determined a local time occurrence of observations between 1420 MLT and 2200 MLT with a maximum occurrence centred around 1700 MLT. The propagation velocity of the SARAS feature through the SABRE field of view was found to be predominately L-shell aligned with a velocity centred around 1750 m sâ1 and within the range 500 m sâ1 and 3500 m sâ1. This comprehensive examination of the SARAS provides the opportunity to discuss, qualitatively, a possible generation mechanism for SARAS based on a proposed model for the production of a similar phenomenon referred to as sub-auroral ion drifts (SAIDs). The results of the comparison suggests that SARAS may result from a similar geophysical mechanism to that which produces SAID events, but probably occurs at a different time in the evolution of the event.Key words. Substorms · Auroral surges · Plasma con-vection · Sub-auroral ion drifts
Highlights
The understanding of short lived or spatially localised variations in the auroral ionospheric convectionowCorrespondence to: M
Results from the statistical study, along with information obtained by Freeman and coworkers, are further employed in an investigation of a generation mechanism for substorm-associated radar auroral surge (SARAS) based on a model proposed by Anderson et al (1993) for sub-auroral ion drifts (SAIDs; e.g. Anderson et al, 1991; Spiro et al, 1979)
A time series of one of the measured parameters from two positions, separated in either latitude or longitude, for a SARAS event was cross-correlated with the corresponding data taken from a central point over a time interval represented by t to Æ 10 minutes, where to is de®ned as the start epoch of the SARAS determined by the selection algorithm
Summary
Lester has important rami®cations concerning the investigation of the larger scale plasma convection system, and the coupling of the solar wind, magnetosphere and ionosphere system One such transient auroral feature known as a substorm-associated radar auroral surge (SARAS) was ®rst identi®ed by Freeman et al (1992) in data from the bistatic Sweden And Britain Radar Experiment (SABRE; Nielsen et al, 1983a). Shand et al (1996) investigated the statistical behavior of backscatter intensity as a function of line of sight (l.o.s) velocity for the entire SABRE data set. Results from the statistical study, along with information obtained by Freeman and coworkers, are further employed in an investigation of a generation mechanism for SARAS based on a model proposed by Anderson et al (1993) for sub-auroral ion drifts All ideal functions employed by the algorithm were constructed from sinusoids with base values appended to either end
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