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Solar eclipses present a rare glimpse into the impact of ionospheric electrodynamics on the magnetosphere independent of other well studied seasonal influences. Despite decades of study, we still do not have a complete description of the conditions for geomagnetic substorm onset. We present herein a mutual information based study of previously published substorm onsets and the past two decades of eclipses which indicates the likelihood of co‐occurrence is greater than random chance. A plausible interpretation for this relation suggests that the abrupt fluctuations in ionospheric conductivity during an eclipse may influence the magnetospheric preconditions of substorm initiation. While the mechanism remains unclear, this study presents strong evidence of a link between substorm onset and solar eclipses.

A chain of magnetometers has been placed in Antarctica for comparisons with magnetic field measurements taken in the Northern Hemisphere. The locations were chosen to be on magnetic field lines that connect to magnetometers on the western coast of Greenland, despite the difficulty of reaching and working at such remote locations. We report on some basic comparisons of the similarities and differences in the conjugate measurements. Our results presented here confirm that the conjugate sites do have very similar (symmetric) magnetic perturbations in a handful of cases, as expected. Sign reversals are required for two components in order to obtain this agreement, which is not commonly known. More frequently, a strongYcomponent of the Interplanetary Magnetic Field (IMF) breaks the symmetry, as well as the unequal conductivities in the opposite hemispheres, as shown in two examples. In one event the IMFYcomponent reversed signs twice within 2 hours, while the magnetometer chains were approaching local noon. This switch provided an opportunity to observe the effects at the conjugate locations and to measure time lags. It was found that the magnetic fields at the most poleward sites started to respond to the sudden IMF reversals 20 min after the IMF reaches the bow shock, a measure of the time it takes for the electromagnetic signal to travel to the magnetopause, and then along magnetic field lines to the polar ionospheres. An additional 9–14 min is required for the magnetic perturbations to complete their transition.

On 04 December 2021, a total solar eclipse occurred over west Antarctica. Nearly an hour beforehand, a geomagnetic substorm onset was observed in the northern hemisphere. Eclipses are suggested to influence magnetosphere‐ionosphere (MI) coupling dynamics by altering the conductivity structure of the ionosphere by reducing photoionization. This sudden and dramatic change in conductivity is not only likely to alter global MI coupling, but it may also introduce a variety of localized instabilities that appear in both hemispheres. Global navigation satellite system (GNSS) based observations of the total electron content (TEC) in the southern high latitude ionosphere during the December 2021 eclipse show signs of wave activity coincident with the eclipse peak totality. Ground magnetic observations in the same region show similar activity, and our analysis suggest that these observations are due to an “eclipse effect” rather than the prior substorm. We present the first multi‐point interhemispheric study of a total south polar eclipse with local TEC observational context in support of this conclusion.

Long‐lasting Pc5 ultralow frequency (ULF) waves spanning the dayside and extending fromL ∼ 5.5into the polar cap region were observed by conjugate ground magnetometers. Observations from MMS satellites in the magnetosphere and magnetometers on the ground confirmed that the ULF waves on closed field lines were due to fundamental toroidal standing Alfvén waves. Monochromatic waves at lower latitudes tended to maximize their power away from noon in both the morning and afternoon sectors, while more broadband waves at higher latitudes tended to have a wave power maximum near noon. The wave power distribution and MMS satellite observations during the magnetopause crossing indicate surface waves on a Kelvin‐Helmholtz (KH) unstable magnetopause coupled with standing Alfvén waves. The more turbulent ion foreshock during an extended period of radial interplanetary magnetic field (IMF) likely plays an important role in providing seed perturbations for the growth of the KH waves. These results indicate that the Pc5 waves observed on closed field lines and on the open field lines of the polar cap were from the same source.

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 () in each hemisphere separately. Next, we examine the ratio of Northern to Southern Hemisphereintensities and the time differences between the maximumimmediately 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 inand (2) the most intense response in. 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.