We investigate the causes of large $dB/dt$ events observed by SuperMAG, by comparing with the time-series of different types of geomagnetic activity, or “convection state”, for the duration of 2010. Spikes are found to occur predominantly in the pre-midnight and dawn sectors. We find that pre-midnight spikes are associated with substorm onsets. Dawn sector spikes are not directly associated with substorms, but with auroral activity occurring within the westward electrojet region. Azimuthally-spaced auroral features drift sunwards, producing Ps6 (10-20 min period) magnetic perturbations on the ground. The magnitude of $dB/dt$ is determined by the flow speed in the convection return flow region, which in turn is related to the strength of solar wind-magnetospheric coupling. Pre-midnight and dawn sector spikes can occur at the same time, as strong coupling favours both substorms and westward electrojet activity; however, the mechanisms that create them seem somewhat independent. The dawn auroral features share some characteristics with omega bands, but can also appear as north-south aligned auroral streamers. We suggest that these two phenomena share a single underlying cause.

Magnetic reconnection occurring between the interplanetary magnetic field and the dayside magnetopause causes a circulation of magnetic flux and plasma within the magnetosphere, known as the Dungey cycle. This circulation is transmitted to the ionosphere via field-aligned currents (FACs). The magnetic flux transport within the Dungey cycle is quantified by the cross-polar cap potential (CPCP or transpolar voltage). Previous studies have suggested that under strong driving conditions the CPCP can saturate near a value of 250 kV. In this study we investigate whether an analogous saturation occurs in the magnitudes of the FACs, using observations from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). The solar wind speed, density and pressure, the Bz component of the interplanetary magnetic field, and combinations of these, were compared to the concurrent integrated current magnitude, across each hemisphere. We find that FAC magnitudes are controlled most strongly by solar wind speed and the orientation and strength of the interplanetary magnetic field. FAC magnitude increases monotonically with solar wind driving but there is a distinct knee in the variation around IMF Bz = -10 nT, above which the increase slows.

High-Intensity Long-Duration Continuous AE Activity (HILDCAA) intervals are driven by High Speed solar wind Streams (HSSs) during which the rapidly-varying interplanetary magnetic field (IMF) produces high but intermittent dayside reconnection rates. This results in several days of large, quasi-periodic enhancements in the auroral electrojet (AE) index. There has been debate over whether the enhancements in AE are produced by substorms or whether HILDCAAs represent a distinct class of magnetospheric dynamics. We investigate sixteen HILDCAA events using the expanding/contracting polar cap model as a framework to understand the magnetospheric dynamics occurring during HSSs. Each HILDCAA onset shows variations in open magnetic flux, dayside and nightside reconnection rates, the cross-polar cap potential, and AL that are characteristic of substorms. The enhancements in AE are produced by activity in the pre-midnight sector, which is the typical substorm onset region. The periodicities present in the intermittent IMF determine the exact nature of the activity, producing a range of behaviours from a sequence of isolated substorms, through substorms which merge into one-another, to almost continuous geomagnetic activity. The magnitude of magnetic fluctuations, $dB/dt$, in the pre-midnight sector during HSSs is sufficient to produce a significant risk of Geomagnetically Induced Currents, which can be detrimental to power-grids and pipelines.

We investigate sharp changes in magnetic field that can produce Geomagnetically Induced Currents (GICs) which damage pipelines and power grids. We use one-minute cadence SuperMAG observations to find the occurrence distribution of magnetic field “spikes”. Recent studies have determined recurrence statistics for extreme events and charted the local time distribution of spikes; however, their relation to solar activity and conditions in the solar wind is poorly understood. We study spike occurrence during solar cycles 23 and 24, roughly 1995 to 2020. We find three local time hotspots in occurrence: the pre-midnight region associated with substorm onsets, the dawn sector associated with omega band activity, and the pre-noon sector associated with the Kelvin-Helmholtz instability occurring at the magnetopause. Magnetic field perturbations are mainly North-South for substorms and KHI, and East-West for omega bands. Substorm spikes occur at all phases of the solar cycle, but maximise in the declining phase. Omega-band and KHI spikes are confined to solar maximum and the declining phase. Substorm spikes occur during moderate solar wind driving, omega band spikes during strong driving, and KHI spikes during quiet conditions but with high solar wind speed. We show that the shapes of these distributions do not depend on the magnitude of the spikes, so it appears that our results can be extrapolated to extreme events.

Upstream solar wind measurements from near the L1 Lagrangian point are commonly used to investigate solar wind-magnetosphere coupling. The off-Sun-Earth line distance of such solar wind monitors can be large, up to 100 RE. We investigate how the correlation between measurements of the interplanetary magnetic field and associated ionospheric responses deteriorates as the off-Sun-Earth line distance increases. Specifically, we use the magnitude and polarity of the dayside region 0 field-aligned currents (R0 FACs) as a measure of IMF BY-associated magnetic tension effects on newly-reconnected field lines, related to the Svalgaard-Mansurov effect. The R0 FACs are derived from Advanced Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) measurements by a principal component analysis, for the years 2010 to 2016. We perform cross-correlation analyses between time-series of IMF BY, measured by the Wind spacecraft and propagated to the nose of the bow shock by the OMNI technique, and these R0 FAC measurements. Typically, in the summer hemisphere, cross-correlation coefficients between 0.6 and 0.9 are found. However, there is a reduction of order 0.1 to 0.15 in correlation coefficient between periods when Wind is close to (within 45 RE) and distant from (beyond 70 RE) the Sun-Earth line. We find a time-lag of around 17 minutes between predictions of the arrival of IMF features at the bow shock and their effect in the ionosphere, irrespective of the location of Wind.