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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 often associated with omega band activity, and the pre-noon sector associated with the Kelvin-Helmholtz instability (KHI) 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 maximize 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. publishedVersion

Mercury's flux transfer event (FTE) showers are dayside magnetopause crossings accompanied by large numbers (≥ 10) of magnetic flux ropes (FRs). These shower events are common, occurring during 52% (1953/3748) of the analyzed crossings. Shower events are observed with magnetic shear angles (θ) from 0° to 180° across the magnetopause and magnetosheath plasma β from 0.1 to 10, but are most prevalent for high θ and low plasma β. Individual FR duration correlates positively, while spacing correlates negatively, with θ and plasma β. FR flux content and core magnetic field intensity correlate negatively with plasma β, but they do not correlate with θ. During shower intervals, FRs carry 60% to 85% of the magnetic flux required to supply Mercury's Dungey cycle. The FTE showers and the large amount of magnetic flux carried by the FTE‐type FRs appear quite different from observations at Earth and other planetary magnetospheres visited thus far.

Under periods of strong solar wind driving, the magnetopause can become compressed, playing a significant role in draining electrons from the outer radiation belt. Also termed “magnetopause shadowing,” this loss process has traditionally been attributed to a combination of magnetospheric compression and outward radial diffusion of electrons. However, the drift paths of relativistic electrons and the location of the magnetopause are usually calculated from statistical models and, as such, may not represent the time‐varying nature of this highly dynamic process. In this study, we construct a database ∼20,000 spacecraft crossings of the dayside magnetopause to quantify the accuracy of the commonly used Shue et al. (1998, https://doi.org/10.1029/98JA01103) model. We find that, for the majority of events (74%), the magnetopause model can be used to estimate magnetopause location to within ±1 RE. However, if the magnetopause is compressed below 8 RE, the observed magnetopause is greater than 1 RE inside of the model location on average. The observed magnetopause is also significantly displaced from the model location during storm sudden commencements, when measurements are on average 6% closer to the radiation belts, with a maximum of 42%. We find that the magnetopause is rarely close enough to the outer radiation belt to cause direct magnetopause shadowing, and hence rapid outward radial transport of electrons is also required. We conclude that statistical magnetopause parameterizations may not be appropriate during dynamic compressions. We suggest that statistical models should only be used during quiescent solar wind conditions and supplemented by magnetopause observations wherever possible.

The equatorial region of the Earth's ionosphere is one of the most complex ionospheric regions due to its interactions, instabilities, and several unresolved questions regarding its dynamics, electrodynamics, and physical processes. The equatorial ionosphere overall spans three continents with the longest region being that over the African continent. Satellite observations have demonstrated that very large differences exist in the formation of ionospheric irregularities over the African sector compared with other longitudinal sectors. This may be a consequence of the symmetric shape of the magnetic equator over the continent and the lack of variability in latitude. In this paper, we propose a science campaign to equip the African sector of the magnetic equator with ground-based instruments, specifically magnetometers and radars. The network of radars proposed is similar in style and technique to the high-latitude SuperDARN radar network, while the magnetometers will form an array along the equatorial belt. These two proposed space physics instruments will be used to study this region of the equatorial ionosphere over a long interval of time, at least one solar cycle. The deployment of an array of magnetometers (AfrequaMA) and a radar network (AfrequaRN) in the African sector of the magnetic equator is jointly called the Africa Equatorial Magnetometer Array and Radar Network (AfrequaMARN), which will provide simultaneous observations of both electric and magnetic variations over the African sector. We also examine the possible science questions such a magnetometer array and radar network would be able to address, both individually and in conjunction with other space-based and ground-based instrumentation. The proposed projects will clearly improve our understanding of the dynamics of the equatorial ionosphere and our understanding of its role in balancing the large-scale ionospheric current system, and will contribute to our ability to adequately model ionospheric and plasmaspheric densities. It will also enhance our understanding of global ionospheric processes, which will improve the space weather capabilities of the African and international space science communities.

[1] McComas and Bagenal [2007] (hereinafter referred to as MB) have presented a discussion of the reconnectionmediated interaction of the Jovian magnetosphere with the interplanetary medium, which they suggest to be significantly different to that at Earth. In the latter case, it is well established that ‘open’ flux is produced at the magnetopause when the interplanetary magnetic field (IMF) is directed opposite to the equatorial planetary field, is transported to the tail by the solar wind, and returns as closed flux via plasma sheet reconnection preferentially during substorms, thus forming the Dungey cycle of flux transport [e.g., Dungey, 1961]. MB propose that the consequences of open flux production at Jupiter are different, however, due to a suggested difficulty of closed flux tube return from the tail against a substantial down-tail flow of iogenic plasma. They suggest instead that open flux is effectively removed by two-lobe reconnection when the IMF has the opposite polarity, such that the open flux in the system remains small. Two-lobe reconnection has been discussed theoretically for many years [e.g., Dungey, 1963; Cowley, 1981], though convincing evidence for its occurrence at Earth has only recently been found [e.g., Imber et al., 2006, 2007]. Here, however, we question both aspects of MB’s discussion. [2] With regard to the return of tail flux by plasma sheet reconnection, MB characterise the process as requiring closed field contraction over distances of 1500–2000 RJ at speeds of 40 km s , thus requiring 30–40 days. They suggest this to be unlikely given the surrounding fast down-tail flow of iogenic plasma. However, we regard this scenario as being unduly pessimistic, first because the estimate of the distance to the tail reconnection site is unrealistically large, and second because the closed flux tube contraction speed is unrealistically small, both contributing to unrealistically large estimates of the transport time. MB’s estimate of the distance to the tail reconnection site is essentially the length of the entire tail of open field lines, obtained by multiplying the solar wind speed by the residence time of open flux tubes in the lobe. This time is estimated to be 3–4 days on the basis that open field lines flow toward the plasma sheet at 10% of the solar wind speed ( 40 km s ) for 10% reconnection efficiency with the IMF, leading to a tail length of 1500–2000 RJ as indicated above. In fact, this significantly underestimates the length of the Jovian tail, since the lobe flow speed is slowed relative to MB’s estimate by the ratio of the lobe and IMF field strengths, i.e. factors of two to three, while an overall magnetopause reconnection efficiency of 10% seems optimistic. A more realistic residence time is 10– 20 days [Nichols et al., 2006], leading to tail lengths of 5000–10000 RJ in agreement with Lepping et al. [1983]. [3] The main point to emphasise, however, is not the inaccuracy of MB’s tail length estimate, but that such estimates provide no information about the location of the tail reconnection sites, other than an upper limit. For Earth, for example, similar estimates produce tail lengths of 1000 RE [e.g., Milan, 2004], while substorm-related reconnection is typically initiated at down-tail distances of 20–30 RE [e.g., Nagai and Machida, 1998]. While flux return from the distant tail may be unlikely as MB suggest, a reasonable conclusion is that open flux will then accumulate until reconnection occurs substorm-like sufficiently close to the planet that the closed flux is indeed able to return. The return flow speeds are then expected to be comparable to the lobe Alfven speed [e.g., Badman and Cowley, 2007], at least an order of magnitude faster than the return speeds employed in MB’s estimate. [4] Significant evidence indeed exists for sporadic reconnection in the Jovian nightside plasma sheet at distances of 100 RJ, resulting in ion jets directed both toward and away from the planet [e.g., Woch et al., 2002]. These dynamics are generally assumed to relate to pinch-off of distended closed field lines and the down-tail release of iogenic plasma occurring as part of the Vasyliunas cycle [Vasyliunas, 1983]. However, supposing that after plasmoid release the reconnection continues into the tail lobe, then closed flux is generated that will clearly flow back to the planet unencumbered by surrounding down-tail flow, whether the combined reconnection is envisaged as largescale [Cowley et al., 2003], or occurs more sporadically and multiply on smaller scales [Kivelson and Southwood, 2005]. While Dungeyand Vasyliunas-cycle tail reconnection need not be coherently related in this way, the argument is sufficient to show that open flux return from the Jovian tail by plasma sheet reconnection is not as problematic as MB suggest. [5] We now turn to MB’s second argument, that open flux can instead be effectively removed from the tail by two-lobe reconnection poleward of the cusp, such that the amount of open flux in the system remains small. This requires the open flux removal rate by two-lobe reconnection for southward-directed IMF, averaged over typical GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L10101, doi:10.1029/2007GL032645, 2008

This paper was published as Astronomy and Geophysics, 2008, 49 (4), pp. 4.16-4.18. It is available from http://www3.interscience.wiley.com/journal/120751339/abstract. Doi: 10.1111/j.1468-4004.2008.49416.x