Abstract
The total solar eclipse over the continental United States on 21 August 2017 offered a unique opportunity to study the dependence of the ionospheric density and morphology on incident solar radiation at different local times. The Super Dual Auroral Radar Network (SuperDARN) radars in Christmas Valley, Oregon, and Fort Hays, Kansas, are located slightly southward of the line of totality; they both made measurements of the eclipsed ionosphere. The received power of backscattered signal decreases during the eclipse, and the slant ranges from the westward looking radar beams initially increase and then decrease after totality. The time scales over which these changes occur at each site differ significantly from one another. For Christmas Valley the propagation changes are fairly symmetric in time, with the largest slant ranges and smallest power return occurring coincident with the closest approach of totality to the radar. The Fort Hays signature is less symmetric. In order to investigate the underlying processes governing the ionospheric eclipse response, we use a ray-tracing code to simulate SuperDARN data in conjunction with different eclipsed ionosphere models. In particular, we quantify the effect of the neutral wind velocity on the simulated data by testing the effect of adding/removing various neutral wind vector components. The results indicate that variations in meridional winds have a greater impact on the modeled ionospheric eclipse response than do variations in zonal winds. The geomagnetic field geometry and the line-of-sight angle from each site to the Sun appear to be important factors that influence the ionospheric eclipse response.
Highlights
The plasma density in the ionosphere is dependent on the intensity of incident sunlight, which creates plasma by ionizing neutral atmospheric particles
The low velocities and small spectral widths of these SuperDARN data confirm that the majority of observed scatter before, during, and after the eclipse was ground scatter
The CV radars observed decreases in angles of arrival of returned scatter during the eclipse, which is generally consistent with longer propagation paths through the ionospheric F region
Summary
The plasma density in the ionosphere is dependent on the intensity of incident sunlight, which creates plasma by ionizing neutral atmospheric particles. The onset of an eclipse is more gradual than sunrise/sunset effects at low latitudes and midlatitudes, and eclipse paths have an apparent eastward motion that contrasts sharply with the steady westward motion associated with the terminators All of these factors suggest that during an eclipse the ionosphere will not respond spatially or temporally as it does during sunrise/sunset conditions and will not experience decreases in electron density that are as significant as those that occur overnight. Experiments during eclipses have previously shown that their ionospheric effects can vary as functions of magnetic latitude, time of day, solar zenith angle, and geomagnetic conditions (Bowhill, 1970; Hulburt, 1939; Rishbeth, 1969; Stankov et al, 2017) Results from such studies indicate that there are significant large-scale spatial effects on the ionosphere that extend well outside the umbra region. We first describe the experiment and the radars and interpret the radar signatures using a ray-tracing code coupled with a first-principles flux tube-coupled model of ionospheric responses to variations in sunlight during the eclipse
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