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Abstract 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. 

Abstract Foreshock transient (FT) events are frequently observed phenomena that are generated by discontinuities in the solar wind. These transient events are known to trigger global‐scale magnetic field perturbations (e.g., ULF waves). We report a series of FT events observed by the Magnetospheric Multiscale mission in the upstream bow shock region under quiet solar wind conditions. During the event, ground magnetometers observed significant Pc1 wave activity as well as magnetic impulse events in both hemispheres. Ground Pc1 wave observations show ∼8 min time delay (with some time differences) from each FT event which is observed at the bow shock. We also find that the ground Pc1 waves are observed earlier in the northern hemisphere compared to the southern hemisphere. The observation time difference between the hemispheres implies that the source region of the wave is the off‐equatorial region. 

Abstract 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. 

Abstract 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. 

Abstract Nearly all studies of impulsive geomagnetic disturbances (GMDs, also known as magnetic perturbation events MPEs) that can produce dangerous geomagnetically induced currents (GICs) have used data from the northern hemisphere. In this study, we investigated GMD occurrences during the first 6 months of 2016 at four magnetically conjugate high latitude station pairs using data from the Greenland West Coast magnetometer chain and from Antarctic stations in the conjugate AAL‐PIP magnetometer chain. Events for statistical analysis and four case studies were selected from Greenland/AAL‐PIP data by detecting the presence of >6 nT/s derivatives of any component of the magnetic field at any of the station pairs. For case studies, these chains were supplemented by data from the BAS‐LPM chain in Antarctica as well as Pangnirtung and South Pole in order to extend longitudinal coverage to the west. Amplitude comparisons between hemispheres showed (a) a seasonal dependence (larger in the winter hemisphere), and (b) a dependence on the sign of theBycomponent of the interplanetary magnetic field (IMF): GMDs were larger in the north (south) when IMFBywas >0 (<0). A majority of events occurred nearly simultaneously (to within ±3 min) independent of the sign ofByas long as |By| ≤ 2 |Bz|. As has been found in earlier studies, IMFBzwas <0 prior to most events. When IMF data from Geotail, Themis B, and/or Themis C in the near‐Earth solar wind were used to supplement the time‐shifted OMNI IMF data, the consistency of these IMF orientations was improved. 

Abstract We report the first lidar observations of regular occurrence of mid‐latitude thermosphere‐ionosphere Na (TINa) layers over Boulder (40.13°N, 105.24°W), Colorado. Detection of tenuous Na layers (∼0.1–1 cm⁻³from 150 to 130 km) was enabled by high‐sensitivity Na Doppler lidar. TINa layers occur regularly in various months and years, descending from ∼125 km after dusk and from ∼150 km before dawn. The downward‐progression phase speeds are ∼3 m/s above 120 km and ∼1 m/s below 115 km, consistent with semidiurnal tidal phase speeds. One or more layers sometimes occur across local midnight. Elevated volume mixing ratios above the turning point (∼105–110 km) of Na density slope suggest in situ production of the dawn/dusk layers via neutralization of converged Na⁺layers. Vertical drift velocity of TINa⁺calculated with the Ionospheric Connection Explorer Hough Mode Extension tidal winds shows convergent ion flow phases aligned well with TINa, supporting this formation hypothesis. 

Abstract 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. 

Abstract Throat auroras frequently observed near local noon have been confirmed to correspond to magnetopause indentations, but the generation mechanisms for these indentations and the detailed properties of throat aurora are both not fully understood. Using all‐sky camera and magnetometer observations, we reported some new observational features of throat aurora as follows. (1) Throat auroras can occur under stable solar wind conditions and cause clear geomagnetic responses. (2) These geomagnetic responses can be simultaneously observed at conjugate geomagnetic meridian chains in the Northern and Southern Hemispheres. (3) The initial geomagnetic responses of throat aurora show concurrent onsets that were observed at all stations along the meridians. (4) Immediately after the concurrent onsets, poleward moving signatures and micropositive bays were observed in theXcomponents at higher‐ and lower‐latitude stations, respectively. We argue that these observations provide evidence for throat aurora being generated by low‐latitude magnetopause reconnection. We suggest that the concurrent onsets reflect the instantaneous responses of the reconnection signal arriving at the ionosphere, the followed poleward moving signatures reflect the antisunward dragging of the footprint of newly opened field lines, and the micropositive bays may result from a pair of field‐aligned currents generated during the reconnection. This study may shed new light on the geomagnetic transients observed at cusp latitude near magnetic local noon. 

Abstract Geomagnetic pulsations in Pc5‐6 band (~3–20 min) are persistent feature of ULF activity at dayside high latitudes. Magnetopause surface eigenmodes may be suggested as potential mechanism of these pulsations. One might expect the ground response of these modes to be near ionospheric projection of the open‐closed field line boundary (OCB). Using data from instruments located at Svalbard we study transient geomagnetic response to impulsive “intrusion” of magnetosheath plasma into the dayside magnetosphere. These intrusions are triggered by modest changes of interplanetary magnetic field to southward, and observed as sudden shifts of equatorward red aurora boundary to lower latitudes and green line emission intensification. Each auroral disturbance is accompanied by burst of ~1.7–2.0‐mHz geomagnetic pulsations. Near‐cusp latitudinal structure of ULF pulsations is compared with instant location of equatorward boundary of the red aurora, assumed to be a proxy of the OCB. Optical OCB latitude has been identified using data from the meridian scanning photometer. The latitudinal maximum of the transient geomagnetic response tends to be located near disturbed OCB proxy, within the error ~1°–2° of the photometer and magnetometer methods. Recorded transient pulsations may be associated with the ground image of the magnetopause surface mode harmonic. Theoretical consideration indicates that after an initial excitation, surface large‐scale mode converts into localized Alfvén oscillations and thus can exist for limited time only. Therefore, MHD surface modes in realistic inhomogeneous plasma cannot be considered in isolation, but as a combined system of modes with discrete and continuous spectra with irreversible transformation between them. 

Abstract 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 Hemisphere intensities and the time differences between the maximum 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 and (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.