Interhemispheric Comparisons of Large Nighttime Magnetic Perturbation Events Relevant to GICs

Abstract

Nearly all studies of impulsive magnetic perturbation events (MPEs) with large magnetic field variability (dB/dt) that can produce dangerous geomagnetically induced currents (GICs) have used data from the Northern Hemisphere. Here we present details of four large‐amplitude MPE events (|ΔBx| > 900 nT and |dB/dt| > 10 nT/s in at least one component) observed between 2015 and 2018 in conjugate high‐latitude regions (65–80° corrected geomagnetic latitude), using magnetometer data from (1) Pangnirtung and Iqaluit in eastern Arctic Canada and the magnetically conjugate South Pole Station in Antarctica and (2) the Greenland West Coast Chain and two magnetically conjugate chains in Antarctica, AAL‐PIP and BAS LPM. From one to three different isolated MPEs localized in corrected geomagnetic latitude were observed during three premidnight events; many were simultaneous within 3 min in both hemispheres. Their conjugate latitudinal amplitude profiles, however, matched qualitatively at best. During an extended postmidnight interval, which we associate with an interval of omega bands, multiple highly localized MPEs occurred independently in time at each station in both hemispheres. These nighttime MPEs occurred under a wide range of geomagnetic conditions, but common to each was a negative interplanetary magnetic field Bz that exhibited at least a modest increase at or near the time of the event. A comparison of perturbation amplitudes to modeled ionospheric conductances in conjugate hemispheres clearly favored a current generator model over a voltage generator model for three of the four events; neither model provided a good fit for the premidnight event that occurred near vernal equinox.

Superposed Epoch Analysis of Nighttime Magnetic Perturbation Events Observed in Arctic Canada

“…The significantly larger range in SML values for postmidnight events than for premidnight events in all three time delay categories (also shown in Figure 11) is more difficult to understand. As Engebretson et al (2020) suggested in regard to the MPEs associated with an interval of omega bands, it is possible that two separate and highly localized magnetotail-magnetosphere-ionosphere coupling mechanisms may be responsible for generating the large, rapid geomagnetic perturbations that generate premidnight and postmidnight GICs, respectively.…”

Nighttime Magnetic Perturbation Events Observed in Arctic Canada: 3. Occurrence and Amplitude as Functions of Magnetic Latitude, Local Time, and Magnetic Disturbance Indices

“…The local time range of the "postmidnight" MPE population matches that of omega bands (Syrjäsuo & Donovan, 2004), which were identified along with other auroral phenomena by Akasofu and Kimball (1964) and Akasofu (1974). Engebretson et al (2020) reviewed many of the properties of omega bands, including the ionospheric currents associated with them, as part of a recent study of MPEs observed in conjugate hemispheres that included an interval of MPEs associated with omega bands, so these will not be repeated here. This study, however, pointed out that "postmidnight" MPEs exhibited even less temporal and spatial correlation between MPEs than "premidnight" MPEs, and suggested the possibility that the mesoscale mechanisms that produce MPEs in these two MLT ranges might be at least somewhat different.…”

“…A more recent paper (Engebretson et al., 2020) reported case studies of four intervals containing MPEs using data from magnetometers in Western Greenland and eastern Arctic Canada as well as magnetically conjugate sites in Antarctica. This paper found that conjugate premidnight MPEs were often but not always simultaneous to within 3 min over ∼100–700 km in latitude, while postmidnight MPEs associated with an omega band were very localized and independent in time.…”

Nighttime magnetic perturbation events observed in Arctic Canada: 3. Occurrence and amplitude as functions of magnetic latitude, local time, and magnetic disturbances

“…A more recent paper (Engebretson et al, 2020) reported case studies of four intervals containing MPEs using data from magnetometers in Western Greenland and eastern Arctic Canada as well as magnetically conjugate sites in Antarctica. This paper found that conjugate premidnight MPEs were often but not always simultaneous to within 3 min over ∼100-700 km in latitude, while postmidnight MPEs associated with an omega band were very localized and independent in time.…”