Particles are accelerated to very high, non-thermal energies in solar and space plasma environments. While energy spectra of accelerated electrons often exhibit a power law, it remains unclear how electrons are accelerated to high energies and what processes determine the power-law index δ . Here, we review previous observations of the power-law index δ in a variety of different plasma environments with a particular focus on sub-relativistic electrons. It appears that in regions more closely related to magnetic reconnection (such as the 'above-the-looptop' solar hard X-ray source and the plasma sheet in Earth's magnetotail),

We investigate the influence of the interplanetary magnetic field (IMF) clock angle ϕIMF on high‐latitude inertial Alfvén wave (IAW) activity in the magnetosphere‐ionosphere transition region using Fast Auroral SnapshoT (FAST) satellite observations. We find evidence that negative IMF Bz coincides with nightside IAW power generation and enhanced rates of IAW‐associated electron energy deposition, while positive IMF Bz coincides with enhanced dayside wave and electron energy deposition. Large ( ≳0.3em0.3em0.3em5 nT) negative IMF By coincides with enhanced postnoon IAW power, while large positive IMF By coincides with enhanced but relatively weaker prenoon IAW power. For each ϕIMF orientation we compare IAW Poynting flux and IAW‐associated electron energy flux distributions with previously published distributions of Alfvénic Poynting flux over ∼2–22 mHz, as well as corresponding wave‐driven electron energy deposition derived from Lyon‐Fedder‐Mobarry global MHD simulations. We also compare IAW Poynting flux distributions with distributions of broad and diffuse electron number flux, categorized using an adaptation of the Newell et al. (2009) precipitation scheme for FAST. Under negative IMF Bz in the vicinity of the cusp (9.5–14.5 magnetic local time), regions of intense dayside IAW power correspond to enhanced diffuse electron number flux but relatively weaker broadband electron precipitation. Differences between cusp region IAW activity and broadband precipitation illustrate the need for additional information, such as fields or pitch angle measurements, to identify the physical mechanisms associated with electron precipitation in the vicinity of the cusp.

We explore the global Alfvénic response of the transition region between the topside ionosphere and magnetosphere to geomagnetic storms. From superposed epoch and storm phase‐dependent analyses, it is found that subsequent to storm commencement the occurrence rate of Alfvénic field variations on electron inertial scales through this region increases by as much as a factor of 5 relative to prestorm levels. This increase is accompanied by order‐of‐magnitude enhancements in coincident energy deposition rates into the ionosphere and ion outflow rates into the magnetosphere, particularly near noon, premidnight, and on the dawn flank. During main phase on the dayside these waves shift to lower invariant latitudes (ILATs), expanding over a large range of ILATs and magnetic local times, where they are associated with significant enhancements in upward ion flux. Nightside storm‐enhanced occurrence probability of Alfvén waves and upward ion flux is lower than on the dayside, but the average precipitating electron energy flux is larger. There is also a localized region of intense ion outflow premidnight at low latitudes during storm main phase. Wave occurrence rates subside to prestorm levels about 20 h after storm commencement.

The Swarm satellites fly at altitudes that at polar latitudes are generally assumed to only contain currents that are aligned with the local magnetic field. Therefore, disturbances along the main field direction are mainly signatures of auroral electrojet currents, with a relatively smooth structure due to the distance from the currents. Here we show that superimposed on this smooth signal is an irregular pattern of small perturbations, which are anticorrelated with the plasma density measured by the Langmuir probe. We show that the perturbations can be remarkably well reproduced by assuming they represent a j × B force, which balances the plasma pressure gradient implied by the density variations. The associated diamagnetic current, previously reported to be most important near the equator, appears to be a ubiquitous phenomenon also at polar latitudes. A spectral analysis indicates that this effect dominates magnetic field intensity variations at small‐scale sizes of a few tens of kilometers.

A magnetospheric substorm is a process where magnetic flux and energy stored in the magnetotail lobes are unloaded by reconnection in the near-Earth tail, causing a global reconfiguration of the magnetosphere (Hones, 1979; review by Baker et al., 1996). The shape of the magnetotail changes from a stretched configuration to a more dipolar configuration during the unloading, and a field-aligned current system, known as the substorm current wedge, develops near midnight (McPherron et al., 1973; review by Kepko et al., 2015). The current wedge closes in the ionosphere, leading to an enhancement of the westward electrojet. This enhancement causes a pronounced negative bay in the northward component of magnetometers in the auroral zone, a signature that is directly linked to the auroral substorm, as first described by Akasofu (1964). The auroral substorm starts with an onset, which is a sudden, localized brightening of the aurora, typically located at the equatorial boundary of the discrete aurora. The intensified region then expands, both longitudinally and poleward; this period of the substorm is referred to as the expansion phase. The expansion phase is followed by a recovery phase, in which the magnetospheric system slowly reverts toward its preonset