Occurrence Distributions and Morphology of Large Geomagnetic Disturbances Observed in Arctic Canada
Mark Engebretson1, Lily Yang1, Erik Steinmetz1, Mark Moldwin2, Martin Connors3, James Weygand4, and Chris Russell4
1 Augsburg University, Minneapolis, MN
2 University of Michigan, Ann Arbor, MI
3 Athabasca University, Athabasca, AB, Canada
4 UCLA Department of Earth Planetary and Space Sciences, Los Angeles, CA
A Wednesday morning poster, SM31A-01, contains more detail.
Introduction
We present some recent findings from our continuing efforts to use ground magnetometer data to study the geomagnetic disturbances (GMDs), also known as magnetic perturbation events (MPEs), that can generate geomagnetically induced currents (GICs).
1a. We show that GMD occurrences strongly favor the declining phase of the sunspot cycle. This is evident in GMDs observed in Arctic Canada between 2013-2022, and is consistent with two earlier published studies using data from Scandinavia and Australia.
1b. However, we show that GMD occurrences align statistically with substorm occurrences (based on the Newell and Gjerloev (2011) substorm list on both a yearly and a monthly basis.
2. We investigate the differing occurrence patterns for isolated GMDs (one per day) and multiple events (up to 12 successive events within tens of minutes spacing).
3. We provide additional information on the differences between GMDs that occur predominantly premidnight and those occurring postmidnight.
1. GMDs and the Sunspot Cycle
We analyzed data from five stations from two magnetometer arrays in eastern Arctic Canada, MACCS and AUTUMNX, from near the start of the previous sunspot cycle through the early part of the current cycle (to June 2022).
We also show GMD data from Scandinavia (Kellinsalmi et al., 2022) and GIC data from Australia (Marshall et al., 2011), covering earlier segments of the multiyear sunspot cycle, that are consistent with the findings we present here.
1. GMDs are more likely to occur during the declining phase of the sunspot cycle.�
Scandinavia 1995-2018
Australia 1985-2010
However, neither GMD maxima nor substorm maxima coincide with sunspot maxima. GMD occurrences show a much better temporal association with substorms than with sunspots.
3. Local Time of GMDs: Two Populations
In previous work we showed that at the higher latitude stations most >6 nT/s GMDs occurred between 18 and 01 MLT, but that a separate population that occurred after 02 MLT was observed most often at the lowest latitude station (Engebretson et al., 2021).
Weygand et al. (2021) showed that the source of GMDs in the premidnight sector was somewhere between the inner to mid plasma sheet, and the source for the GMDs in the postmidnight sector was somewhere between the inner magnetosphere and the inner plasma sheet.
Premidnight GMDs were most often characterized by a ~5-10 min duration spike with broad frequency content, while postmidnight GMDs were in many cases trains of up to 2 hours of quasi-sinusoidal Ps6 or Pi3 (~5-10 min period) oscillations with little higher frequency content. Only some of the oscillations in each train had very large derivative amplitudes. An example is shown below.
Summary
1a. Both large and modest amplitude GMD occurrences as well as GICs peaked during the declining phase of recent solar cycles.
1b. Their occurrence patterns agreed better with substorm occurrences, on both yearly and monthly time scales. However, the yearly trend in GMDs did not fully follow trends in substorm occurrences, either. What can these temporal patterns teach us?
2. The most common number of large GMDs on a given day was 1; days with 3 or more GMDs occurred more rarely. Trains of closely spaced GMDs were more likely to occur at lower latitudes.
3. Postmidnight GMDs were rare above 70° MLAT, but constituted ~40% of the events at the lowest latitude station (64.7° MLAT). Premidnight GMDs were spiky and broadband in frequency, while postmidnight events were often quasi-sinusoidal and much more narrowband. These morphological differences suggest that different, more localized magnetospheric processes act to generate large GMDs before and after midnight, respectively.