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Collisionless shock waves, found in supernova remnants, interstellar, stellar, and planetary environments, and laboratories, are one of nature’s most powerful particle accelerators. This study combines in situ satellite measurements with recent theoretical developments to establish a reinforced shock acceleration model for relativistic electrons. Our model incorporates transient structures, wave-particle interactions, and variable stellar wind conditions, operating collectively in a multiscale set of processes. We show that the electron injection threshold is on the order of suprathermal range, obtainable through multiple different phenomena abundant in various plasma environments. Our analysis demonstrates that a typical shock can consistently accelerate electrons into very high (relativistic) energy ranges, refining our comprehension of shock acceleration while providing insight on the origin of electron cosmic rays. The mechanisms resulting in particle acceleration to relativistic energies in space plasmas are an open question. Here, the authors show a reinforced shock acceleration model which enables electrons to efficiently achieve relativistic energies and reveal a low electron injection threshold.

Numerous studies have explored modeling the shape of Earth's magnetopause (MP) and bow shock (BS) as a function of incident solar wind conditions, utilizing either magnetohydrodynamic model data or observations of boundary crossings from orbiting spacecraft. Here we present a novel method for modeling the variance of the location of the BS and MP location as a function of solar wind variability. We utilize 27 years of OMNI solar wind data to drive a Monte Carlo simulation in which 1,000 solar wind conditions are randomly drawn and the MP and BS locations are simulated. Leveraging ray tracing, we have aggregated the Lin et al. (2010, https://doi.org/10.1029/2009ja014235) and Nguyen et al. (2022c, https://doi.org/10.1029/2021ja030112) models of the MP and Jeřáb et al. (2005, https://doi.org/10.1016/j.pss.2004.09.032) and Chapman and Cairns (2003, https://doi.org/10.1029/2002ja009569) models of the BS to produce 3D models for the 75%, 90%, 95%, and 99% quantiles of both surfaces, along with average and median locations.

The Van Allen radiation belts are two rings of charged particles encircling the Earth. Therefore the transient appearance in 2012 of a third ring between the inner and outer belts was a surprise. A study of the ultrarelativistic electrons in this middle ring reveals new physics for particles above 2 MeV. Radiation in space was the first discovery of the space age. Earth’s radiation belts consist of energetic particles that are trapped by the geomagnetic field and encircle the planet 1 . The electron radiation belts usually form a two-zone structure with a stable inner zone and a highly variable outer zone, which forms and disappears owing to wave–particle interactions on the timescale of a day, and is strongly influenced by the very-low-frequency plasma waves. Recent observations revealed a third radiation zone at ultrarelativistic energies 2 , with the additional medium narrow belt (long-lived ring) persisting for approximately 4 weeks. This new ring resulted from a combination of electron losses to the interplanetary medium and scattering by electromagnetic ion cyclotron waves to the Earth’s atmosphere. Here we show that ultrarelativistic electrons can stay trapped in the outer zone and remain unaffected by the very-low-frequency plasma waves for a very long time owing to a lack of scattering into the atmosphere. The absence of scattering is explained as a result of ultrarelativistic particles being too energetic to resonantly interact with waves at low latitudes. This study shows that a different set of physical processes determines the evolution of ultrarelativistic electrons.

Earth's magnetotail, a night‐side region characterized by stretched magnetic field lines and strong plasma currents, is the primary site for the release of magnetic field energy and its transformation into plasma heating and kinetic energy plus charged particle acceleration during magnetic reconnection. In this study, we demonstrate that the efficiency of this acceleration can be sufficiently high to produce populations of relativistic and ultra‐relativistic electrons, with energies up to several MeV, which exceeds all previous theoretical and simulation estimates. Using data from the low‐altitude ELFIN and CIRBE CubeSats, we show multiple events of relativistic electron bursts within the magnetotail, far poleward of the outer radiation belt. These bursts are characterized by power‐law energy spectra and can be detected during even moderate substorms. Plain Language Summary Charged particle acceleration during magnetic reconnection is a universal process occurring in various space plasma environments. Traditionally, theoretical and simulation models of this acceleration are verified using data from the reconnection region in the near‐Earth magnetosphere, where in situ spacecraft measurements are most accessible. In this study, we demonstrate that the efficiency of this acceleration can significantly exceed previous estimates, leading to the formation of electron populations with energies up to several MeV, even in regions where thermal electron energies are below 1 keV. These observations of highly energetic electron bursts are made available by new low‐altitude CubeSat missions monitoring magnetotail electron fluxes. Key Points We report observations of relativistic and ultra‐relativistic electrons in near‐Earth magnetotail We show energy spectra of relativistic electron bursts in the magnetotail We discuss potential mechanisms responsible for the formation of relativistic and ultra‐relativistic electrons

In situ exploration of Uranus has been limited to a single flyby encounter by the Voyager 2 spacecraft in January 1986. Nonetheless, new investigation has led to significant questions about the origin of energetic ions observed in the region between its moons Miranda and Ariel. Radial and pitch angle diffusion modeling suggests that typical magnetospheric sources cannot explain the observed characteristics of these energetic ions. We suggest that these are likely being introduced by a source from one of these moons and give rise to waves that could result in the observed particle distribution characteristics. This may reveal that internal plasma sources in the system may be important for Uranus' magnetospheric dynamics and may contribute to its unexpectedly strong radiation belts. Plain Language Summary Uranus is an oddity in the solar system for a variety of reasons, but mostly as a result of its perpendicular rotation relative to the rest of the planets in the solar system. During its approximately 3‐day flyby of Uranus in 1986, Voyager 2 captured the only in situ observations of the planet and its system. New analysis of these three‐decade‐old observations have revealed a mysterious source of energetic ions in the planet's magnetosphere. These ions were originally explained by dynamics of the system, but new understanding suggests that this is probably unlikely. New simple modeling of the expected behavior of such energetic particles show that sustaining such a population requires a very strong source and specific energization mechanism. Both would potentially be consistent with the ions originating from either Miranda or Ariel. This potentially hints that the Uranian magnetosphere may harbor an ocean world like those known or believed to exist at the other Giant Planets. Key Points Analysis of Voyager 2 observations revealed a localized source of energetic ions in the region between the moons Miranda and Ariel Diffusive transport modeling suggests that typical magnetospheric sources cannot explain the observed characteristics of the energetic ions Additional in situ ion composition and plasma wave observations are necessary to confirm whether these ions are coming from an active moon

Magnetic reconnection is a fundamental mechanism for the transport of mass and energy in planetary magnetospheres and astrospheres. While the process of reconnection is itself ubiquitous across a multitude of systems, the techniques used for its analysis can vary across scientific disciplines. Here we frame the latest understanding of reconnection theory by missions such as NASA’s Magnetospheric Multiscale (MMS) mission for use throughout the solar system and beyond. We discuss how reconnection can couple magnetized obstacles to both sub- and super-magnetosonic upstream flows. In addition, we address the need to model sheath plasmas and field-line draping around an obstacle to accurately parameterize the possibility for reconnection to occur. We conclude with a discussion of how reconnection energy conversion rates scale throughout the solar system. The results presented are not only applicable to within our solar system but also to astrospheres and exoplanets, such as the first recently detected exoplanet magnetosphere of HAT-11-1b.

Geomagnetic storms driven by the solar wind can cause a dramatic drop in the flux of high-energy electrons in the Earth’s outer Van Allen belt. Analysis of data obtained during such an event by three different sets of spacecraft suggests that these electrons are directed into space rather than lost to the atmosphere. The Van Allen radiation belts were first discovered in 1958 by the Explorer series of spacecraft 1 . The dynamic outer belt consists primarily of relativistic electrons trapped by the Earth’s magnetic field. Magnetospheric processes driven by the solar wind 2 cause the electron flux in this belt to fluctuate substantially over timescales ranging from minutes to years 3 . The most dramatic of these events are known as flux ’dropouts’ and often occur during geomagnetic storms. During such an event the electron flux can drop by several orders of magnitude in just a few hours 4 , 5 and remain low even after a storm has abated. Various solar wind phenomena, including coronal mass ejections and co-rotating interaction regions 6 , can drive storm activity, but several outstanding questions remain concerning dropouts and the precise channels to which outer belt electrons are lost during these events. By analysing data collected at multiple altitudes by the THEMIS, GOES, and NOAA–POES spacecraft, we show that the sudden electron depletion observed during a recent storm’s main phase is primarily a result of outward transport rather than loss to the atmosphere.

Particles are accelerated to very high, non-thermal energies during explosive energy-release phenomena in space, solar, and astrophysical plasma environments. While it has been established that magnetic reconnection plays an important role in the dynamics of Earth’s magnetosphere, it remains unclear how magnetic reconnection can further explain particle acceleration to non-thermal energies. Here we review recent progress in our understanding of particle acceleration by magnetic reconnection in Earth’s magnetosphere. With improved resolutions, recent spacecraft missions have enabled detailed studies of particle acceleration at various structures such as the diffusion region, separatrix, jets, magnetic islands (flux ropes), and dipolarization front. With the guiding-center approximation of particle motion, many studies have discussed the relative importance of the parallel electric field as well as the Fermi and betatron effects. However, in order to fully understand the particle acceleration mechanism and further compare with particle acceleration in solar and astrophysical plasma environments, there is a need for further investigation of, for example, energy partition and the precise role of turbulence.

Electrons in Earth's magnetic field often exhibit a striped pattern of intensity as a function of electron energy and altitude. A model that factors in some unexpectedly important effects can now explain this feature. See Letter p.338 'Zebra stripes' in Earth's radiation belt Earth's radiation belts are populated by electrons and ions that are held in place by a magnetic field. Structured features in these belts have previously been attributed to enhanced solar wind activity. Although planetary rotation is considered to have an important role in driving belt dynamics around Jupiter and Saturn, it has been thought inconsequential for Earth's radiation belts, where the forces involved are much smaller. A new analysis of data from the Van Allen Probes mission shows that the distributions of energetic electrons across the entire spatial extent of Earth's inner radiation belt are organized in regular, highly structured, and unexpected 'zebra stripes' even when the solar wind activity is low. Modelling reveals that the patterns are produced by planetary rotation, which induces global diurnal variations of magnetic and electric fields that resonantly interact with electrons whose drift period is close to 24 hours.