Substorm‐Time Ground dB/dt Variations Controlled by Interplanetary Shock Impact Angles: A Statistical Study.

In this study, we investigate the effects caused by interplanetary (IP) shock impact angles on the subsequent ground dB/dt variations during substorms. IP shock impact angles have been revealed as a major factor controlling the subsequent geomagnetic activity, meaning that shocks with small inclinations with the Sun‐Earth line are more likely to trigger higher geomagnetic activity resulting from nearly symmetric magnetospheric compressions. Such field variations are linked to the generation of geomagnetically induced currents (GICs), which couple to artificial conductors on the ground leading to deleterious consequences. We use a sub‐set of a shock data base with 237 events observed in the solar wind at L1 upstream of the Earth, and large arrays of ground magnetometers at stations located in North America and Greenland. The spherical elementary current system methodology is applied to the geomagnetic field data, and field‐aligned‐like currents in the ionosphere are derived. Then, such currents are inverted back to the ground and dB/dt variations are computed. Geographic maps are built with these field variations as a function of shock impact angles. The main findings of this investigation are: (a) typical dB/dt variations (5–10 nT/s) are caused by shocks with moderate inclinations; (b) the more frontal the shock impact, the more intense and the more spatially defined the ionospheric current amplitudes; and (c) nearly frontal shocks trigger more intense dB/dt variations with larger equatorward latitudinal expansions. Therefore, the findings of this work provide new insights for GIC forecasting focusing on nearly frontal shock impacts on the magnetosphere. Plain Language Summary: Space weather effects caused by solar perturbations can increase the intensity of electric currents flowing in the Earth's upper atmosphere. These currents in turn induce geoelectric currents on the ground that couple with ground infrastructure, such as power transmission lines, oil/gas pipelines, and railways, which further cause deleterious consequences. Recent studies have shown that the angle with which solar perturbations impact the Earth closely controls intensifications of electric currents in the upper atmosphere and the subsequent ground magnetic field perturbations causing geoelectric currents. When these space weather drivers impact Earth more frontally, the resulting atmospheric current enhancements and the subsequent ground magnetic field perturbations are generally more intense. In this study, we use a large amount of magnetic field data recorded in North America and Greenland. We find that the most intense geomagnetic field disturbances are caused by the impacts of nearly frontal solar perturbations on the Earth and cover large geographic areas reaching southern Canada and northern United States. The results of this work can be used by space weather forecasters when predicting events with significant magnetic field variations leading to intense geoelectric currents, particularly when tracking space weather drivers that are forecasted to impact Earth nearly frontally. Key Points: Typical substorm‐time ground dB/dt variations (5–10 nT/s) are caused by interplanetary shocks with moderate inclinations (140°–160°)The more frontal the shock, the more intense and the broader the ionospheric current amplitudesThe more frontal the shock, the more intense, larger the area coverage, and the lower the equatorward latitudinal extent of dB/dt variations [ABSTRACT FROM AUTHOR]