Earth’s magnetotail is filled with solar wind and ionospheric electrons, whose initial energies are significantly lower than the typical energies (temperatures) of plasmasheet electrons. One of the most common mechanisms responsible for heating of solar wind and ionospheric electrons in Earth’s magnetotail is adiabatic heating caused by earthward convection of these electrons from the deep tail (i.e., from the region of a weak magnetic field) towards the region of stronger magnetic fields closer to Earth. This heating is moderated by electron losses into the ionosphere due to local wave scattering. In this study, we compare electron spectra from simultaneous observations of The Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft at different radial distances with spectra obtained from a simple model that includes adiabatic heating and losses. Our comparison shows that the model heating significantly overestimates the increase in energetic (>1 keV) electron fluxes, indicating that losses are essential for accurate modelling of the observed spectra. The required electron losses are similar to or even greater than the losses in the strong diffusion limit (when the loss cone is full). The latter can be interpreted as loss cone widening by field-aligned electron acceleration.

Energetic electron precipitation to the Earth’s atmosphere is a key process controlling radiation belt dynamics and magnetosphere-ionosphere coupling. One of the main drivers of precipitation is electron resonant scattering by whistler-mode waves. Low-altitude observations of such precipitation often reveal quasi-periodicity in the ultra-low-frequency (ULF) range associated with whistler-mode waves, causally linked to ULF-modulated equatorial electron flux and its anisotropy. Conjunctions between ground-based instruments and equatorial spacecraft show that low-altitude precipitation concurrent with equatorial whistler-mode waves also exhibits a spatial periodicity as a function of latitude over a large spatial region. Whether this spatial periodicity might also be due to magnetospheric ULF waves spatially modulating electron fluxes and whistler-mode chorus has not been previously addressed due to a lack of conjunctions between equatorial spacecraft, LEO spacecraft, and ground-based instruments. To examine this question, we combine ground-based and equatorial observations magnetically conjugate to observations of precipitation at the low-altitude, polar-orbiting CubeSats ELFIN-A and -B. As they sequentially cross the outer radiation belt with a temporal separation of minutes to tens of minutes, they can easily reveal the spatial quasi-periodicity of electron precipitation. Our combined datasets confirm that ULF waves may modulate whistler-mode wave generation within a large MLT and $L$-shell domain in the equatorial magnetosphere, and thus lead to significant aggregate energetic electron precipitation exhibiting both temporal and spatial periodicity. Our results suggest that the coupling between ULF and whistler-mode waves is important for outer radiation belt dynamics.

Hot flow anomalies (HFAs) and foreshock bubbles (FBs) are frequently observed in Earth’s foreshock, which can significantly disturb the bow shock and therefore the magnetosphere-ionosphere system and can accelerate particles. Previous statistical studies have identified the solar wind conditions (high solar wind speed and high Mach number, etc.) that favor their generation. However, backstreaming foreshock ions are expected to most directly control how HFAs and FBs form, whereas the solar wind may partake in the formation process indirectly by determining foreshock ion properties. Using Magnetospheric Multiscale mission and Time History of Events and Macroscale Interactions during Substorms mission, we perform a statistical study of foreshock ion properties around 275 HFAs and FBs. We show that foreshock ions with a high foreshock-to-solar wind density ratio (>~3%), high kinetic energy (>~600eV), large ratio of kinetic energy to thermal energy (>~0.1), and large perpendicular temperature anisotropy (>~1.4) favor HFA and FB formation. We also examine how these properties are related to solar wind conditions: higher solar wind speed and larger (angle between the interplanetary magnetic field and the bow shock normal) favor higher kinetic energy of foreshock ions; foreshock ions are less diffuse at larger ; small , high Mach number, and closeness to the bow shock favor a high foreshock-to-solar wind density ratio. Our results provide further understanding of HFA and FB formation.

We report observational evidence for a class of coherent magnetic dipolarization structures that are long lived and radially extensive. The reported dipolarization structures, a subset of general dipolarizations, typically remain coherent over 20-30 min in real time, 3-6 hours in MLT, and 10-20 Re in radial distance. Arrays of more than three spacecraft in non-collinear geometry are used to determine the propagation vector, including both the normal speed and direction, of such dipolarizations in the equatorial plane. The determined azimuthal propagation is ~3 deg/min, which corresponds to ~50 km/s at 6.6 Re. This speed is consistent with those obtained from two azimuthally separated spacecraft in previous works. Further analysis suggests that these azimuthally propagating dipolarizations (APDs) are often finger-like in shape, ranging from 5 to 20 Re in length and several Re in width. The reported APD may accompany the earthward flow channel and dipolarizing flux bundle (DFB).

This White Paper outlines the importance of addressing the fundamental science theme “How are charged particles energized in space plasmas” through a future ESA mission. The White Paper presents five compelling science questions related to particle energization by shocks, reconnection, waves and turbulence, jets and their combinations. Answering these questions requires resolving scale coupling, nonlinearity, and nonstationarity, which cannot be done with existing multi-point observations. In situ measurements from a multi-point, multi-scale L-class Plasma Observatory consisting of at least seven spacecraft covering fluid, ion, and electron scales are needed. The Plasma Observatory will enable a paradigm shift in our comprehension of particle energization and space plasma physics in general, with a very important impact on solar and astrophysical plasmas. It will be the next logical step following Cluster, THEMIS, and MMS for the very large and active European space plasmas community. Being one of the cornerstone missions of the future ESA Voyage 2050 science programme, it would further strengthen the European scientific and technical leadership in this important field.

The substorm current wedge (SCW) is believed to be driven by pressure gradients and vortices associated with fast flows. Therefore, it is expected that relevant observations are organized by the SCW’s central meridian, which cannot be determined using in-situ observations. This study takes advantage of the SCW inversion technique, which provides essential information about an SCW (e.g., location and strengths of field-aligned currents (FACs) and investigates the generation mechanisms of the SCW. First, we have found good temporal and spatial correlations between earthward flows and substorm onsets identified using the midlatitude positive bay (MPB) index. Over half of the flows are observed within 10 minutes of substorm onsets. Most flows (85%) were located inside the SCW between its upward and downward FACs. Second, superposed epoch analysis (SPEA) shows that the onset-associated flow velocity has a flow-scale (3-min) peak, while the equatorial thermal pressure has a substorm-scale (>30 min) enhancement and a trend similar to the westward electrojet and FACs in the SCW. Third, the pressure gradient calculated using in-situ observations is well organized in the SCW frame and points toward the SCW’s central meridian. These facts suggest that the SCW is likely sustained by substorm-scale pressure gradient rather than flow-scale flow vortices. The nonalignment between the pressure gradient and flux tube volume could generate an SCW with a quadrupole FAC pattern, similar to that seen in global MHD and RCM-E simulations. Their magnetic effects on the ground and geosynchronous orbit resemble a classic one-loop SCW.