Abstract
AbstractThe midlatitude Super Dual Auroral Radar Network (SuperDARN) radars regularly observe nighttime low‒velocity Sub‒Auroral Ionospheric Scatter (SAIS) from decameter‒scale ionospheric density irregularities during quiet geomagnetic conditions. To establish the origin of the density irregularities responsible for low‒velocity SAIS, it is necessary to distinguish between the effects of high frequency (HF) propagation and irregularity occurrence itself on the observed backscatter distribution. We compare range, azimuth, and elevation data from the Blackstone SuperDARN radar with modeling results from ray tracing coupled with the International Reference Ionosphere assuming a uniform irregularity distribution. The observed and modeled distributions are shown to be very similar. The spatial distribution of backscattering is consistent with the requirement that HF rays propagate nearly perpendicular to the geomagnetic field lines (aspect angle ≤1°). For the first time, the irregularities responsible for low‒velocity SAIS are determined to extend between 200 and 300 km altitude, validating previous assumptions that low‒velocity SAIS is an F‒region phenomenon. We find that the limited spatial extent of this category of ionospheric backscatter within SuperDARN radars' fields‒of‒view is a consequence of HF propagation effects and the finite vertical extent of the scattering irregularities. We conclude that the density irregularities responsible for low‒velocity SAIS are widely distributed horizontally within the midlatitude ionosphere but are confined to the bottom‒side F‒region.
Highlights
[2] The geomagnetic midlatitude ionosphere is typically defined as a buffer zone between the equatorial and auroral regions, with boundaries that vary with geomagnetic activity
[3] The ionosphere is populated by plasma density irregularities. These irregularities result from plasma instabilities driven by combinations of plasma drifts, density and temperature gradients, electric fields, and winds [e.g., Fejer and Kelley, 1980]. These plasma density fluctuations cover a wide range of scale sizes, spatial distributions, and time scales
The authors relied on colocated observations by the Millstone Hill Incoherent Scatter Radar (ISR) and the Wallops SuperDARN radar, which showed opposed temperature and density gradients, a geometry that yields a positive growth rate for the Temperature Gradient Instability (TGI)
Summary
[2] The geomagnetic midlatitude ionosphere is typically defined as a buffer zone between the equatorial and auroral regions, with boundaries that vary with geomagnetic activity. These plasma density fluctuations cover a wide range of scale sizes, spatial distributions, and time scales At midlatitudes, both plasma and neutral processes are believed to be involved in generating and sustaining ionospheric irregularities. Greenwald et al [2006] reported recurring decameter-scale irregularities with low drift velocities (< 100 m/s) in the quiet time midlatitude nightside ionosphere observed with the first midlatitude SuperDARN radar located at Wallops Flight Facility, Virginia. The purpose of this paper is to resolve the HF propagation effects in the observation of backscatter in order to determine the limiting factors in the horizontal and vertical extent of the irregularities To achieve this objective, we rely on data from the Blackstone SuperDARN radar and apply ray-tracing analysis of HF propagation based on empirical models of the ionosphere and geomagnetic field.
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