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We report on prolonged enhancements of electron fluxes at energies at or above 500 keV, observed in the magnetotail by the lunar‐orbiting Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) during the recovery phase of a magnetic storm with minimum Dst${D}_{st}$≈${\\approx} $−200 nT during periodic auroral electrojet (AE$AE$ ) activations. The enhanced energetic electron fluxes were omnidirectional and observed near the magnetic equator. No solar energetic particle background was detected. Although ARTEMIS detected earthward magnetic flux transport impulses exceeding 2 mV/m, along with associated broadband electrostatic fluctuations, no correlation was evident between these phenomena and the relativistic electron flux enhancements. Spectra obtained during the relativistic electron flux enhancements are fit by the Kappa function, κ$\\kappa $= 3.75, similar to that of the quiet‐time plasma sheet electron population at lunar distance. Multiple reconnection events at large distances are, most likely, responsible for the electron heating.

Motivated by recent observations of intense electric fields and elevated energetic particle fluxes within flow bursts beyond geosynchronous altitude (Runov et al., 2009, 2011), we apply modeling of particle guiding centers in prescribed but realistic electric fields to improve our understanding of energetic particle acceleration and transport toward the inner magnetosphere through model‐data comparisons. Representing the vortical nature of an earthward traveling flow burst, a localized, westward‐directed transient electric field flanked on either side by eastward fields related to tailward flow is superimposed on a nominal steady state electric field. We simulate particle spectra observed at multiple THEMIS spacecraft located throughout the magnetotail and fit the modeled spectra to observations, thus constraining properties of the electric field model. We find that a simple potential electric field model is capable of explaining the presence and spectral properties of both geosynchronous altitude and “trans‐geosynchronous” injections at higher L‐shells (L > 6.6 RE) in a manner self‐consistent with the injections' inward penetration. In particular, despite the neglect of the magnetic field changes imparted by dipolarization and the inductive electric field associated with them, such a model can adequately describe the physics of both dispersed injections and depletions (“dips”) in energy flux in terms of convective fields associated with earthward flow channels and their return flow. The transient (impulsive), localized, and vortical nature of the earthward‐propagating electric field pulse is what makes this model particularly effective. Key Points We adapted a numerical model of particle GC motion in prescribed electric fields We simulate (trans‐)geosynchronous injection features: eflux enhancements & dips We explain e‐ acceleration & transport by impulsive, localized electric E‐fields

Measurements from NASA’s Van Allen Probes have transformed our understanding of the dynamics of Earth’s geomagnetically-trapped, charged particle radiation. The Van Allen Probes were equipped with the Magnetic Electron Ion Spectrometers (MagEIS) that measured energetic and relativistic electrons, along with energetic ions, in the radiation belts. Accurate and routine measurement of these particles was of fundamental importance towards achieving the scientific goals of the mission. We provide a comprehensive review of the MagEIS suite’s on-orbit performance, operation, and data products, along with a summary of scientific results. The purpose of this review is to serve as a complement to the MagEIS instrument paper, which was largely completed before flight and thus focused on pre-flight design and performance characteristics. As is the case with all space-borne instrumentation, the anticipated sensor performance was found to be different once on orbit. Our intention is to provide sufficient detail on the MagEIS instruments so that future generations of researchers can understand the subtleties of the sensors, profit from these unique measurements, and continue to unlock the mysteries of the near-Earth space radiation environment.

We examined how much large-scale and localized upward and downward currents contribute to the substorm current wedge (SCW), and how they evolve over time, using the THEMIS all-sky imagers (ASIs) and ground magnetometers. One type of events is dominated by a single large-scale wedge, with upward currents over the surge and broad downward currents poleward-eastward of the surge. The other type of events is a composite of large-scale wedge and wedgelets associated with streamers, with each wedgelet having comparable intensity to the large-scale wedge currents. Among 17 auroral substorms with wide ASI coverage, the composite current type is more frequent than the single large-scale wedge type. The dawn–dusk size of each wedgelet is ~ 600 km in the ionosphere (~ 3.2 RE in the magnetotail, comparable to the flow channel size). We suggest that substorms have more than one type of SCW, and the composite current type is more frequent.

During substorms, Earth's magnetotail undergoes rapid dipolarization, driving Earthward plasma flows that decelerate and dissipate energy upon encountering the dipole magnetic field in the nightside transition region. This region mediates the interaction between the magnetotail, inner magnetosphere, and the ionospheric auroral zone, though significant mapping uncertainties obscure the precise link and particle acceleration processes. Using data from THEMIS, TREx, and ELFIN, we analyze a storm‐time substorm on 4 September 2022, establishing a relationship, that is, not a causation, between magnetospheric and ionospheric dynamics. Following a dipolarization, the auroral bulge overlapped with the footprints of the electron isotropy boundary (IB) and the outer radiation belt. Notably, the precipitating electron energies reached at least 2 MeV in the bulge, exceeding previous reports. By comparing the latitudes of the electron IB with respect to the auroral bulge, we deduce that the sources of both auroral and relativistic precipitation were confined in the dipolarized region.

Cumulative magnetic flux transport earthward/tailward of the reconnection site in the plasma sheet or equatorward toward the neutral sheet (Φ) has been shown to be one of the most useful quantities for remotely sensing reconnection onset in the magnetotail. We examine the behavior of Φ during substorms near the onset meridian using superposed epoch analysis of Time History of Events and Macroscale Interactions during Substorms (THEMIS) probe observations at different downtail distances. Observational data come from the THEMIS Substorm Database, assembled under the auspices of the Geospace Environment Modeling (GEM) program (http://www.igpp.ucla.edu/themis/events/). We find that Φ starts to increase a few minutes prior to ground midlatitude Pi2 onset. Although our study cannot monitor regions beyond 30 RE, the apogee of the most distant probe (P1), enhanced transport tends to begin at 20–30 RE and moves progressively inward just prior to ground Pi2 onset. Our results are consistent with recent THEMIS case studies showing that reconnection initiates the substorm expansion phase process.

On 16 February 2008, the THEMIS spacecraft (probes) bracketed the near‐Earth signatures of a substorm onset as identified in the THEMIS ground‐based observatories measuring an AETH index up to 180 nT. The main onset was associated with the formation and tailward release of a plasmoid (a proto‐plasmoid) at XGSM = −18.3 RE and a dipolarization in the inner part of the plasma sheet at XGSM = −11.0 RE. The time history and geometry of the event in the tail are consistent with magnetic reconnection as the cause of the substorm expansion onset process. Two activations of the plasma sheet, evidenced by tailward streaming of energetic ions and southward or bipolar signatures of the magnetic field, preceded the main substorm. The first activation was associated with an intensification of a high‐latitude arc, while the second was associated with the onset of ULF pulsations at midlatitude and low‐latitude stations. We conclude that near‐Earth plasma sheet activity that may also be due to reconnection and may be related to nonsubstorm arc intensifications can precede substorm onset by several minutes. In particular, high‐latitude arcs do not appear to result in substorm initiation even though they may occur in close temporal and spatial proximity to the substorm arc.

Dbf4p is an essential regulatory subunit of the Cdc7p kinase required for the initiation of DNA replication. Cdc7p and Dbf4p orthologs have also been shown to function in the response to DNA damage. A previous Dbf4p multiple sequence alignment identified a conserved ∼40-residue N-terminal region with similarity to the BRCA1 C-terminal (BRCT) motif called “motif N.” BRCT motifs encode ∼100-amino-acid domains involved in the DNA damage response. We have identified an expanded and conserved ∼100-residue N-terminal region of Dbf4p that includes motif N but is capable of encoding a single BRCT-like domain. Dbf4p orthologs diverge from the BRCT motif at the C terminus but may encode a similar secondary structure in this region. We have therefore called this the BRCT and DBF4 similarity (BRDF) motif. The principal role of this Dbf4p motif was in the response to replication fork (RF) arrest; however, it was not required for cell cycle progression, activation of Cdc7p kinase activity, or interaction with the origin recognition complex (ORC) postulated to recruit Cdc7p–Dbf4p to origins. Rad53p likely directly phosphorylated Dbf4p in response to RF arrest and Dbf4p was required for Rad53p abundance. Rad53p and Dbf4p therefore cooperated to coordinate a robust cellular response to RF arrest.