The Earth's radiation belts consist of electrons that range in energy from hundreds of keV to multiple MeV (Baker et al., 2017;Thorne, 2010), and represent the first major discovery of the space age (Van Allen & Frank, 1959;Van Allen et al., 1958). Different particle species and energy ranges, most notably the electrons in the outer radiation belt, behave quite differently with respect to one another since different physical processes dominate their dynamics. Outer radiation belt electron fluxes are highly variable during geomagnetic storms due to a competition between various loss and acceleration processes (Bortnik and Thorne
The Earth's energetic particle environment consists of electrons that range in energy from a few keV to multiple MeV (e.g., Baker et al., 2017;Thorne, 2010), and were discovered by Geiger counters flown on Explorer 1, launched in January 1958, which represents the first major discovery of the space age (Van Allen & Frank, 1959;Van Allen et al., 1958). The dynamics of the different electron energies that have been studied over several decades, but more recently in great detail facilitated by high quality data from the Van Allen Probes mission (Li & Hudson, 2019;Mauk et al., 2013). The exact response of the radiation belts to solar wind driving is difficult to predict due to various competing loss and acceleration processes, often giving vastly different responses to seemingly similar driving conditions (Reeves et al., 2003) but occasionally even being accelerated up to roughly 10 MeV. The generally accepted process of radiation belt flux enhancement is believed to proceed as follows: an increase in the strength of the convection electric field causes an enhanced drift of plasma-sheet electrons into the inner magnetosphere, where they gradient-drift eastwards, and become unstable to plasma wave excitation. A particular class of waves, called whistler-mode chorus waves is excited on the dawn side of the Earth by ∼10-100 keV "source" electrons, and these waves then transfer a portion of their energy to higher energy "seed" electrons (>100 keV), that are further accelerated to relativistic energies (∼MeV) facilitated by Ultra-Low Frequency waves (
The plasmasphere is a region in the near-Earth space, extending up to ∼7 Earth radii (R E ), filled with cold (0.1-10 eV) and dense (up to 10 4 cm −3 ) plasma. It is populated mostly by the outflow of ions and electrons from the ionosphere and typically has an irregular toroidal shape around the Earth (Carpenter & Anderson, 1992) with an exponentially decreasing density profile as the radial distance increases. Previous studies have suggested that the shape of the plasmasphere is correlated with geomagnetic activity (Carpenter & Seely, 1976;Goldstein, 2007), and the global distribution of cold plasma can become complex during geomagnetically disturbed periods
The Earth's radiation belts consist of energetic charged particles trapped by the geomagnetic field into two regions, a relatively stable inner zone, and a more dynamic outer zone. These particles range in energy from tens of keV to multiple MeV (e.g.,
Auroral beads are spatially wavy forms routinely seen before the onset of auroral substorms and are closely related to the onset‐related instabilities. To date, the acceleration mechanism of electrons that create auroral beads is not fully determined. Here, we present a fortuitous event when the Van Allen Probe A (RBSP‐A) was in magnetic conjunction with auroral beads. RBSP‐A observed Alfvén waves, locally generated kinetic Alfvén waves (KAWs) and Alfvénic accelerated electrons at several 100 eV. The Alfvén waves and KAWs carried sufficient Poynting flux to power visible aurora and may control the beads' motion. These observations and previous simulations support that the Alfvénic acceleration is the acceleration mechanism of the auroral beads. Specifically, KAWs are generated around the equator and accelerate local cold electrons to several 100 eV. The waves are suggested to propagate to both hemispheres and accelerate electrons to several keV, which directly account for the auroral beads.
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