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Browsing by Author "Shepherd, Simon G."

Now showing 1 - 4 of 4
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    Effects of Subauroral Polarization Streams on the Upper Thermospheric Winds During Non-Storm Time
    Zou, Ying; Lyons, Larry R.; Shi, Xueling; Liu, Jiang; Wu, Qian; Conde, Mark; Shepherd, Simon G.; Mende, Stephen; Zhang, Yongliang; Coster, Antea (American Geophysical Union, 2022-05)
    Intense sunward (westward) plasma flows, named Subauroral Polarization Stream (SAPS), have been known to occur equatorward of the electron auroras for decades, yet their effect on the upper thermosphere has not been well understood. On the one hand, the large velocity of SAPS results in large momentum exchange upon each ion-neutral collision. On the other hand, the low plasma density associated with SAPS implies a low ion-neutral collision frequency. We investigate the SAPS effect during non-storm time by utilizing a Scanning Doppler Imager (SDI) for monitoring the upper thermosphere, SuperDARN radars for SAPS, all-sky imagers and DMSP Spectrographic Imager for the auroral oval, and GPS receivers for the total electron content. Our observations suggest that SAPS at times drives substantial (>50 m/s) westward winds at subauroral latitudes in the dusk-midnight sector, but not always. The occurrence of the westward winds varies with AE index, plasma content in the trough, and local time. The latitudinally averaged wind speed varies from 60 to 160 m/s, and is statistically 21% of the plasma. These westward winds also shift to lower latitude with increasing AE and increasing MLT. We do not observe SAPS driving poleward wind surges, neutral temperature enhancements, or acoustic-gravity waves, likely due to the somewhat weak forcing of SAPS during the non-storm time.
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    GPS phase scintillation at high latitudes during geomagnetic storms of 7-17 March 2012-Part 1: The North American sector
    Prikryl, P.; Ghoddousi-Fard, R.; Thomas, E. G.; Ruohoniemi, J. Michael; Shepherd, Simon G.; Jayachandran, P. T.; Danskin, D. W.; Spanswick, E.; Zhang, Y.; Jiao, Y.; Morton, Y. T. (European Geosciences Union, 2015)
    The interval of geomagnetic storms of 7-17 March 2012 was selected at the Climate and Weather of the Sun-Earth System (CAWSES) II Workshop for group study of space weather effects during the ascending phase of solar cycle 24 (Tsurutani et al., 2014). The high-latitude ionospheric response to a series of storms is studied using arrays of GPS receivers, HF radars, ionosondes, riometers, magnetometers, and auroral imagers focusing on GPS phase scintillation. Four geomagnetic storms showed varied responses to solar wind conditions characterized by the interplanetary magnetic field (IMF) and solar wind dynamic pressure. As a function of magnetic latitude and magnetic local time, regions of enhanced scintillation are identified in the context of coupling processes between the solar wind and the magnetosphere-ionosphere system. Large southward IMF and high solar wind dynamic pressure resulted in the strongest scintillation in the nightside auroral oval. Scintillation occurrence was correlated with ground magnetic field perturbations and riometer absorption enhancements, and collocated with mapped auroral emission. During periods of southward IMF, scintillation was also collocated with ionospheric convection in the expanded dawn and dusk cells, with the antisunward convection in the polar cap and with a tongue of ionization fractured into patches. In contrast, large northward IMF combined with a strong solar wind dynamic pressure pulse was followed by scintillation caused by transpolar arcs in the polar cap.
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    Observations and Modeling Studies of Solar Eclipse Effects on Oblique High Frequency Radio Propagation
    Moses, M. L.; Kordella, L. J.; Earle, Gregory D.; Drob, Douglas P.; Huba, J. D.; Ruohoniemi, John M.; Shepherd, Simon G.; Sivakumar, V (2021-03)
    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.
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    Review of the accomplishments of mid-latitude Super Dual Auroral Radar Network (SuperDARN) HF radars
    Nishitani, Nozomu; Ruohoniemi, J. Michael; Lester, Mark; Baker, Joseph B. H.; Koustov, Alexandre V.; Shepherd, Simon G.; Chisham, Gareth; Hori, Tomoaki; Thomas, Evan Grier; Makarevich, Roman A.; Marchaudon, Aurélie; Ponomarenko, Pavlo V.; Wild, James A.; Milan, Stephen E.; Bristow, William A.; Devlin, John; Miller, Ethan; Greenwald, Raymond A.; Ogawa, Tadahiko; Kikuchi, Takashi (2019-03-18)
    The Super Dual Auroral Radar Network (SuperDARN) is a network of high-frequency (HF) radars located in the high- and mid-latitude regions of both hemispheres that is operated under international cooperation. The network was originally designed for monitoring the dynamics of the ionosphere and upper atmosphere in the high-latitude regions. However, over the last approximately 15 years, SuperDARN has expanded into the mid-latitude regions. With radar coverage that now extends continuously from auroral to sub-auroral and mid-latitudes, a wide variety of new scientific findings have been obtained. In this paper, the background of mid-latitude SuperDARN is presented at first. Then, the accomplishments made with mid-latitude SuperDARN radars are reviewed in five specified scientific and technical areas: convection, ionospheric irregularities, HF propagation analysis, ion-neutral interactions, and magnetohydrodynamic (MHD) waves. Finally, the present status of mid-latitude SuperDARN is updated and directions for future research are discussed.