Drivers of rapid geomagnetic variations at high latitudes

Abstract

Abstract. We have examined the most intense external (magnetospheric and ionospheric) and internal (induced) |dH/dt| (amplitude of the 10 s time derivative of the horizontal geomagnetic field) events observed by the high-latitude International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers between 1994 and 2018. While the most intense external |dH/dt| events at adjacent stations typically occurred simultaneously, the most intense internal (and total) |dH/dt| events were more scattered in time, most likely due to the complexity of induction in the conducting ground. The most intense external |dH/dt| events occurred during geomagnetic storms, among which the Halloween storm in October 2003 featured prominently, and drove intense geomagnetically induced currents (GICs). Events in the prenoon local time sector were associated with sudden commencements (SCs) and pulsations, and the most intense |dH/dt| values were driven by abrupt changes in the eastward electrojet due to solar wind dynamic pressure increase or decrease. Events in the premidnight and dawn local time sectors were associated with substorm activity, and the most intense |dH/dt| values were driven by abrupt changes in the westward electrojet, such as weakening and poleward retreat (premidnight) or undulation (dawn). Despite being associated with various event types and occurring at different local time sectors, there were common features among the drivers of most intense external |dH/dt| values: preexisting intense ionospheric currents (SC events were an exception) that were abruptly modified by sudden changes in the magnetospheric magnetic field configuration. Our results contribute towards the ultimate goal of reliable forecasts of dH/dt and GICs.

Extreme Geomagnetic Disturbances (GMDs) Observed in Eastern Arctic Canada: Occurrence Characteristics and Solar Cycle Dependence

“…Ngwira et al (2018) showed observations of a geomagnetic storm on March 17, 2015 during which strong dB/dt events appeared from 7 to 12 min after a substorm onset, and Engebretson et al (2019b) showed that on November 11, 2015 there was a ~10 min delay between substorm onset and the appearance of GMDs. This delay was also noted by Juusola et al (2023) in a study of five GMD events that were responsible for the most intense derivative magnitudes in external sources (due to ionospheric and magnetospheric electric currents) observed by the IMAGE array in Scandinavia between 1994 and 2018. They found that there were no substorm onsets or sudden intensifications of the WEJ among them.…”

“…All available daily data files obtained from January 2011 through December 2022 (covering approximately one solar sunspot cycle) from each of the three MACCS stations (RBY, PGG, and CDR) and two AUTUMNX stations (SALU, KJPK) were analyzed to identify GMDs with ≥ 6 nT/s amplitude. GMD amplitude thresholds of 1 nT/s have been used in many studies (e.g., Viljanen et al, 2001) and more recently by Juusola et al (2023). A higher threshold level for GIC hazards of 5 nT/s was identified by Molinski et al (2000), Boteler (2001) and Woodroffe et al (2016), so the ≥ 6 nT/s events identified in this study would pose significant threats to electrical infrastructure if any were present near these sites.…”

“…Al noted that the difficulty in predicting high-latitude regional disturbances reflects the tendency of this model to miss strong auroral zone latitude activity associated with substorms or other localized magnetotail processes that drive currents that couple to the ionosphere. Juusola et al2019Juusola et al (2023 noted that Pulkkinen et al (2003) and more recently Dimmock et al (2019) have suggested that GICs are primarily driven by smallscale spatiotemporal structures superimposed on the large-scale westward electrojet (WEJ). Weygand et al (2021) confirmed this using a large statistical study finding that most nighttime GMDs occurred under a westward electrojet, including many within the Harang current system.…”

Analysis of the Geoelectric Field in Sweden Over Solar Cycles 23 and 24: Spatial and Temporal Variability During Strong GIC Events

“…The shock of the first ICME arrived on 29 October at about 05:59 UT with a sudden increase of the IMF. The consequent SSC at about 06:11 UT caused record‐large d B / dt at several IMAGE sites (Juusola et al., 2023, Table 1). The following magnetic sheath and ejecta produced a clear impact on the magnetosphere, causing a two‐step decrease in the Dst index (Figure 6b).…”

“…The potential effects of GICs on electrical power transmission systems in Sweden are of great concern due to its geographic location (55-70°N) near high-latitude regions where the largest geomagnetic variations are recorded. In a recent study of Juusola et al (2023), the authors confirm that the most intense rapid geomagnetic variations at high latitudes in Fennoscandia are typically attributed to sudden storm commencements (SSCs), pulsations, and substorms during geomagnetic storms preceded by an SSC and driven by intense southward interplanetary magnetic field (IMF) and often fast solar wind. Moreover, the authors report the M-I process for the most intense magnetic disturbance for each driver: (a) in the SSC event, an abrupt compression of the magnetopause caused an intensification of the eastward electrojet (EEJ); (b) in the pulsation event, wave activity during a magnetopause expansion caused a disruption of the EEJ; (c) in the premidnight substorm event, wave activity during expansion of the transition region in the magnetosphere caused a sudden weakening and poleward retreat of the westward electrojet (WEJ).…”