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The Earth's radiation belts are usually composed of an inner and an outer zone. Previous studies have reported a third belt of ultra‐relativistic electrons, but the mechanism requires to be further clarified. Here we report a very interesting feature of 3.4−6.3 $3.4-6.3$ MeV electrons during two storms in October 2018. During the October 7 storm the electrons in the region L≈3.5−5.0 $L\approx 3.5-5.0$ were quickly scattered by EMIC waves, leading to a storage ring left between L≈3.0−3.5 $L\approx 3.0-3.5$. Over the following days, due to the chorus‐driven acceleration, electron fluxes significantly increased in the region L>4 $L > 4$, consequently allowing for a three‐belt structure. During the October 13 storm the electrons in the region L>5 $L > 5$ were swept away by EMIC waves, leaving a narrow outer radiation belt confined within the region L≈4−5 $L\approx 4-5$ for several days. Numerical calculations demonstrate that the EMIC‐driven loss and chorus‐induced acceleration can account for the formation and evolution of the third‐belt morphology. Plain Language Summary: The Earth's electron radiation belts are usually composed of two high‐flux zones called the inner and outer belts, which are separated by a low‐flux slot region. However, previous studies have found that ultra‐relativistic (kinetic energy Ek>3 ${E}_{\mathrm{k}} > 3$ MeV) electrons sometimes exhibit a three‐belt morphology. Although some theories have been proposed, the mechanism underlying this rare phenomenon still requires further elucidation. Here we report a very interesting feature of 3.4–6.3 MeV electrons during two geomagnetic storms in October 2018. Prior to the first storm on October 7, the outer belt was present between the magnetic L $L$‐shell 3.0–5.0. During the storm main phase, electrons in the region L≈3.5−5.0 $L\approx 3.5-5.0$ were quickly lost by electromagnetic ion cyclotron (EMIC) waves, leaving a high‐flux zone between L≈3.0−3.5 $L\approx 3.0-3.5$. Over the following days, electrons in the region L>4 $L > 4$ were accelerated by chorus waves, consequently allowing for a three‐belt structure. During the second storm on October 13, electrons in the region L>5 $L > 5$ were swept away by EMIC waves, leaving a narrow outer belt confined within the region L≈4−5 $L\approx 4-5$ for several days. Numerical calculations demonstrate that the EMIC‐driven loss and chorus‐induced acceleration can account for the formation and evolution of the third‐belt morphology. Key Points: Ultra‐relativistic electrons exhibited three‐belt morphologies during storms in October 2018, corresponding to enhanced EMIC/chorus wavesCalculations show that electron loss/acceleration timescales induced by EMIC/chorus are about 0.01/10 days, comparable to observationsThe combined role of electromagnetic ion cyclotron (EMIC) and chorus waves can explain the formation and evolution of the three‐belt morphology [ABSTRACT FROM AUTHOR]

The physical mechanisms usually applied to explain the relativistic electron enhancement have been delved into to elucidate non-adiabatic electron acceleration resulting in the ultra-relativistic electron population observed in the outer radiation belt. We considered multisatellite observations of the solar wind parameters, magnetospheric waves, and particle flux to report an unusual local acceleration of ultra-relativistic electrons under a prolonged high-speed solar wind stream (HSS). A corotating interaction region reaches the Earth's bowshock on August 3, 2016, causing a minor geomagnetic storm. Following this, the magnetosphere was driven for 72 h by a long-term HSS propagating at 600 km/s. During this period, the magnetosphere sustained both ultra-low frequency (ULF) and very-low frequency (VLF) waves in the outer radiation belt region. Besides the waves, the relativistic and ultra-relativistic electron fluxes were enhanced with different time lags regarding the magnetic storm main phase. The efficiency of wave-particle interaction in enhancing ultrarelativistic electrons is evaluated by the diffusion coefficient rates, considering both ULF and VLF waves together with phase space density analyses. Results show that local acceleration by whistler mode chorus waves can occur in a time scale of 2–4 h, whereas ULF waves take around 10's of hours and magnetosonic waves take a time scale of days. This result is confirmed by the phase space density analysis. Accordingly, it shows that peaks of local acceleration of 1 MeV electrons are consistent with the observation of the highest chorus wave amplitude at the same L-shell and MLT. Thus, we argue that whistler mode chorus waves interacting with relativistic electrons are the main physical mechanisms leading to ultra-relativistic electron enhancement, while ULF and fast magnetosonic waves are found as secondary physical processes. Lastly, our analysis contributes to understanding how whistler and ULF waves can contribute to ultra-relativistic electrons showing up in the inner magnetosphere under the HSS driver. [ABSTRACT FROM AUTHOR]

Abstract Processes in the radiation belts under extreme geomagnetic conditions involve the interplay between acceleration and loss processes, both of which can be caused by wave‐particle interactions. Whistler mode waves play a critical role in these interactions, and up to now their properties during extreme events remained poorly sampled and understood. We employ extensive databases of spacecraft observations to specify their distribution. We show that under extreme geomagnetic conditions, lower‐band whistler mode chorus waves have a net effect of accelerating ultra‐relativistic electrons, which results in an increase of fluxes at multi‐MeV energies by several orders of magnitude. During future magnetic superstorms, the radiation levels in the outer zone could therefore experience a substantial increase beyond what has been previously observed during the space age.

Van Allen Probes measurements revealed the presence of the most unusual structures in the ultra‐relativistic radiation belts. Detailed modeling, analysis of pitch angle distributions, analysis of the difference between relativistic and ultra‐realistic electron evolution, along with theoretical studies of the scattering and wave growth, all indicate that electromagnetic ion cyclotron (EMIC) waves can produce a very efficient loss of the ultra‐relativistic electrons in the heart of the radiation belts. Moreover, a detailed analysis of the profiles of phase space densities provides direct evidence for localized loss by EMIC waves. The evolution of multi‐MeV fluxes shows dramatic and very sudden enhancements of electrons for selected storms. Analysis of phase space density profiles reveals that growing peaks at different values of the first invariant are formed at approximately the same radial distance from the Earth and show the sequential formation of the peaks from lower to higher energies, indicating that local energy diffusion is the dominant source of the acceleration from MeV to multi‐MeV energies. Further simultaneous analysis of the background density and ultra‐relativistic electron fluxes shows that the acceleration to multi‐MeV energies only occurs when plasma density is significantly depleted outside of the plasmasphere, which is consistent with the modeling of acceleration due to chorus waves. Plain Language Summary: The most energetic electrons in the Earth Van Allen radiation belts have not been accurately measured in the past. Observations for a recent NASA's Van Allen Probes missions reviled new unique structures, such as narrow rings, and posed further scientific questions. This review shows that, unlike relativistic electrons, ultra‐relativistic electrons can be very effectively locally scattered by plasma waves produced by ions, so‐called electromagnetic ion cyclotron waves. Observations also show that acceleration from MeV to multi‐MeV occurs locally by taking energy from another type of plasma wave. These waves are called whistler‐mode waves and can accelerate particles to such high energy when total plasma density is low. The difference between the relativistic and ultra‐relativistic particles justifies the classification of these particles into a different population from the bulk population of the outer radiation belt. Key Points: Electromagnetic ion cyclotron waves effectively scatter ultra‐relativistic electrons in the radiation beltsThe local acceleration produces acceleration from MeV to multi‐MeV in the regions of low densityThe difference between MeV and multi‐MeV electrons justifies the classification of these particles into a new population [ABSTRACT FROM AUTHOR]

Aims. To better understand the radiation environment in low Earth orbit (LEO), the analysis of in-situ observations of a variety of particles, at different atmospheric heights, and in a wide range of energies, is needed. Methods. We present an analysis of energetic particles, indirectly detected by the large yield radiometer (LYRA) instrument on board ESA's project for on-board autonomy 2 (PROBA2) satellite as background signal. Combining energetic particle telescope (EPT) observations with LYRA data for an overlapping period of time, we identified these particles as electrons with an energy range of 2 to 8 MeV. Results. The observed events are strongly correlated to geo-magnetic activity and appear even during modest disturbances. They are also well confined geographically within the L = 4–6 McIlwain zone, which makes it possible to identify their source. Conclusions. Although highly energetic particles are commonly perturbing data acquisition of space instruments, we show in this work that ultra-relativistic electrons with energies in the range of 2–8 MeV are detected only at high latitudes, while not present in the South Atlantic Anomaly region.

We present data showing hyperfine transitions in an atomic deuterium beam induced by the (476 MHz) radio-frequency field of a 704 MeV electron beam in a storage ring. A polarized deuterium beam, produced in an atomic beam source, was crossed with a stored electron beam and analyzed with a Breit--Rabi polarimeter. Electron-beam induced transitions were singled out by injecting different combinations of hyperfine states. Transition probabilities as high as 70% were measured at large currents (~ 100 mA). All possible deuterium transitions for a radio-frequency of 476 MHz were observed. In addition, a 1--6 transition resulting from the first harmonic (952 MHz) was observed. The effects of these transitions are of general importance for the polarized internal target technique applied in nuclear and particle physics experiments. The data are reasonably described by numerical estimates. The observed mechanism can be exploited to create nuclear polarized atoms when injecting electron polarized atoms with no net nuclear polarization into a storage cell. However, when nuclear polarized atoms are injected, care should be taken to avoid this mechanism, since it would result in depolarization of the atoms. The studies enabled us to choose the magnetic guide field during our spin-dependent electron--deuteron scattering experiments, such that electron-beam induced depolarizing effects were avoided. [ABSTRACT FROM AUTHOR]