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

Abstract

Rapid changes of magnetic fields associated with nighttime magnetic perturbation events (MPEs) with amplitudes |ΔB| of hundreds of nT and 5–10 min duration can induce geomagnetically induced currents (GICs) that can harm technological systems. Here we present superposed epoch analyses of large nighttime MPEs (|dB/dt| ≥ 6 nT/s) observed during 2015 and 2017 at five stations in Arctic Canada ranging from 64.7° to 75.2° in corrected geomagnetic latitude (MLAT) as functions of the interplanetary magnetic field (IMF), solar wind dynamic pressure, density, and velocity, and the SML, SMU, and SYM/H geomagnetic activity indices. Analyses were produced for premidnight and postmidnight events and for three ranges of time after the most recent substorm onset: (a) 0–30 min, (b) 30–60 min, and (c) >60 min. Of the solar wind and IMF parameters studied, only the IMF Bz component showed any consistent temporal variations prior to MPEs: a 1–2 h wide 1–3 nT negative minimum at all stations beginning ∼30–80 min before premidnight MPEs, and minima that were less consistent but often deeper before postmidnight MPEs. Median, 25th, and 75th percentile SuperMAG auroral indices SML (SMU) showed drops (rises) before pre‐ and post‐midnight type A MPEs, but most of the MPEs in categories B and C did not coincide with large‐scale peaks in ionospheric electrojets. Median SYM/H indices were flat near −30 nT for premidnight events and showed no consistent temporal association with any MPE events. More disturbed values of IMF Bz, Psw, Nsw, SML, SMU, and SYM/H appeared postmidnight than premidnight.

Geomagnetic Disturbances That Cause GICs: Investigating Their Interhemispheric Conjugacy and Control by IMF Orientation

“…They suggested, consistent with the results of a resistive MHD study by Hesse and Birn (1990), that the magnetotail was more stable during conditions of large IMF | By |, requiring the magnetotail lobes and the polar cap to contain more flux to initiate a substorm compared to the situation when | By | was small. Our observations that GMDs were strongly suppressed under IMF conditions dominated by the By component (Engebretson, Ahmed, et al., 2021) and occurred only when preceded by intervals of IMF Bz < 0 and conditions when |IMF By | < 2 |IMF Bz | (this study) suggests that their generation is in some way linked to magnetotail reconnection.…”

“…This pattern provides an explanation for the distribution of IMF orientations in the ecliptic plane shown in the Engebretson, Ahmed et al. (2021) superposed epoch study of GMDs observed in Arctic Canada, because the northern hemisphere values would be stronger for By > 0, so more likely to exceed the 6 nT/s amplitude threshold. The distributions of IMF Bx and By , shown separately in Figure 9 of that paper, included both positive and negative values, but the median in Bx was <0 and that in By was >0, consistent with a Parker‐spiral oriented IMF vector directed toward Earth.…”

“…In addition, Workayehu et al (2021), using nearly 6 years of magnetic field measurements from the Swarm A and C satellites, reported that auroral currents were stronger in the northern hemisphere than the southern hemisphere for IMF By > 0 in most local seasons under both signs of IMF Bz. This pattern provides an explanation for the distribution of IMF orientations in the ecliptic plane shown in the Engebretson, Ahmed et al (2021) superposed epoch study of GMDs observed in Arctic Canada, because the northern hemisphere values would be stronger for By > 0, so more likely to exceed the 6 nT/s amplitude threshold. The distributions of IMF Bx and By, shown separately in Figure 9 of that paper, included both positive and negative values, but the median in Bx was <0 and that in By was >0, consistent with a Parker-spiral oriented IMF vector directed toward Earth.…”

“…In a more recent superposed epoch study, Engebretson, Ahmed et al. (2021) reported that the medians of nearly all the nearly 700 ≥ 6 nT/s GMDs observed at five stations in Arctic Canada during 2015 and 2017, both premidnight and postmidnight, were preceded by intervals of negative IMF Bz . This pattern held for the 25th and 75th percentile traces in most cases as well, but not every Bz trace was negative prior to GMD occurrence or showed a similar time dependence.…”

Review of Environmental Monitoring by Means of Radio Waves in the Polar Regions: From Atmosphere to Geospace

“…However, dangerously large MPEs have been shown to occur mostly at higher latitudes, where the auroral zone is more commonly situated, and the poorly understood physical mechanisms that produce MPEs and GICs appear to be the same under expanded or contracted auroral oval conditions. A number of large statistical and event studies of MPEs (and in some cases the associated GICs) using the IMAGE array in Scandinavia (ranging, for example, from Viljanen, 1997and Viljanen and Tanskanen, 2011to Belakhovsky et al 2019and Dimmock et al 2020) have now been extended to Arctic Canada (Engebretson et al 2021 andWeygand et al 2021). The latter study applied spherical elementary current systems (SECS) analysis and included all-sky imager data in statistical and case studies of MPEs in order to clarify their association with localised and/or extended ionospheric and field-aligned currents.…”