Browsing by Author "Oliveira, Denny M."

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    Impact Angle Control of Local Intense dB/dt Variations During Shock-Induced Substorms
    Oliveira, Denny M.; Weygand, James M.; Zesta, Eftyhia; Ngwira, Chigomezyo M.; Hartinger, Michael D.; Xu, Zhonghua; Giles, Barbara L.; Gershman, Daniel J.; Silveira, Marcos V. D.; Souza, Vítor M. (American Geophysical Union, 2021-12-01)
    The impact of interplanetary shocks on the magnetosphere can trigger magnetic substorms that intensify auroral electrojet currents. These currents enhance ground magnetic field perturbations (dB/dt), which in turn generate geomagnetically induced currents (GICs) that can be detrimental to power transmission infrastructure. We perform a comparative study of dB/dt variations in response to two similarly strong shocks, but with one being nearly frontal and the other highly inclined. Multi-instrument analyses by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) and Los Alamos National Laboratory spacecraft show that nightside substorm-time energetic particle injections are more intense and occur faster in the case of the nearly head-on impact. The same trend is observed in dB/dt variations recorded by THEMIS ground magnetometers. THEMIS all-sky imager data show a fast and clear poleward auroral expansion in the first case, which does not clearly occur in the second case. Strong field-aligned currents computed with the spherical elementary current system (SECS) technique occur in both cases, but the current variations resulting from the inclined shock impact are weaker and slower compared to the nearly frontal case. SECS analyses also reveal that geographic areas with dB/dt surpassing the thresholds 1.5 and 5 nT/s, usually linked to high-risk GICs, are larger and occur earlier due to the symmetric compression caused by the nearly head-on impact. These results, with profound space weather implications, suggest that shock impact angles affect the geospace driving conditions and the location and intensity of the subsequent dB/dt variations during substorm activity.
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    Interhemispheric Asymmetries in the Ground Magnetic Response to Interplanetary Shocks: The Role of Shock Impact Angle
    Xu, Z.; Hartinger, Michael D.; Oliveira, Denny M.; Coyle, Shane; Clauer, C. Robert; Weimer, Daniel R.; Edwards, T. R. (2020-03)
    Interplanetary (IP) shocks drive magnetosphere-ionosphere (MI) current systems that in turn are associated with ground magnetic perturbations. Recent work has shown that IP shock impact angle plays a significant role in controlling the subsequent geomagnetic activity and magnetic perturbations; for example, highly inclined shocks drive asymmetric MI responses due to interhemispherical asymmetric magnetospheric compressions, while almost head-on shocks drive more symmetric MI responses. However, there are few observations confirming that inclined shocks drive such asymmetries in the high-latitude ground magnetic response. We use data from a chain of Antarctic magnetometers, combined with magnetically conjugate stations on the west coast of Greenland, to test these model predictions (Oliveira & Raeder, 2015, https://doi.org/10.1002/2015JA021147; Oliveira, 2017, https://doi.org/10.1007/s13538-016-0472-x). We calculate the time derivative of the magnetic field (partial derivative B/partial derivative t) in each hemisphere separately. Next, we examine the ratio of Northern to Southern Hemisphere partial derivative B/partial derivative t intensities and the time differences between the maximum. partial derivative B/partial derivative t immediately following the impact of IP shocks. We order these results according to shock impact angles obtained from a recently published database with over 500 events and discuss how shock impact angles affect north-south hemisphere asymmetries in the ground magnetic response. We find that the hemisphere the shock strikes first usually has (1) the first response in partial derivative B/partial derivative t and (2) the most intense response in partial derivative B/partial derivative t. Additionally, we show that highly inclined shocks can generate high-latitude ground magnetic responses that differ significantly from predictions based on models that assume symmetric driving conditions.