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Recent observations show very near‐Earth reconnection (∼8–13RE) could efficiently power the ring current during the main phase of geomagnetic storms, but whether the recovery phase might be contributed remains unclear. During the recovery phase of the May 2024 major geomagnetic storm, intense auroral brightening and geomagnetic disturbances were observed at midnight, indicative of particle injections. Current wedges observed by mid‐latitude ground magnetometers around midnight suggest dipolarizing flux bundles (DFBs). The latitude of the auroral brightening was clearly lower than usual, suggesting near‐Earth reconnection (NERX) was closer to Earth than during substorms (∼20–30RE). GOES‐18 at midnight detected magnetic field and plasma signatures consistent with DFBs, following an extremely thin current sheet likely compressed by strong upstream dynamic pressure. These results indicate NERX could have been close enough for resultant DFBs to penetrate geosynchronous orbit and contribute to the ring current during the recovery phase. This scenario deserves further examination in future. Plain Language Summary: When a coronal mass ejection hits Earth's magnetic field, significant disturbances in the near‐Earth space environment occur, namely geomagnetic storms, causing many hazards to our power systems and space missions. It is thus important to understand the underlying processes, especially how energetic particles are transported from the nightside to energize these disturbances. Nightside magnetic reconnection, which converts magnetic energy to particle energy, was not widely considered as an efficient contributor because it typically occurs too far from Earth. However, by identifying observational characteristics using ground and spacecraft measurements, we find that such magnetic reconnection could be sufficiently close to Earth to transport energy and particles to geosynchronous orbit during the recovery phase of a geomagnetic storm. Our results improve our understanding of energy transport during the recovery phase, which will help understand and mitigate space weather hazards in the future. Key Points: During the recovery phase of the May 2024 major storm, near‐Earth reconnection was likely close enough to contribute to the ring currentDipolarizing flux bundles penetrated to ∼6.6RE at midnight with current wedges, auroral brightening, and ionospheric currents identifiedThe driver was likely a large dynamic pressure that strongly compressed the magnetosphere, causing an extremely thin current sheet at ∼6.6RE [ABSTRACT FROM AUTHOR]

Abstract Recent observations show very near‐Earth reconnection (∼8–13RE) could efficiently power the ring current during the main phase of geomagnetic storms, but whether the recovery phase might be contributed remains unclear. During the recovery phase of the May 2024 major geomagnetic storm, intense auroral brightening and geomagnetic disturbances were observed at midnight, indicative of particle injections. Current wedges observed by mid‐latitude ground magnetometers around midnight suggest dipolarizing flux bundles (DFBs). The latitude of the auroral brightening was clearly lower than usual, suggesting near‐Earth reconnection (NERX) was closer to Earth than during substorms (∼20–30RE). GOES‐18 at midnight detected magnetic field and plasma signatures consistent with DFBs, following an extremely thin current sheet likely compressed by strong upstream dynamic pressure. These results indicate NERX could have been close enough for resultant DFBs to penetrate geosynchronous orbit and contribute to the ring current during the recovery phase. This scenario deserves further examination in future.

This study presents a re‐evaluation of the Kullen and Janhunen (2004, https://doi.org/10.5194/angeo-22-951-2004) global northward interplanetary magnetic field (IMF) simulation, using the Grand Unified Magnetosphere–Ionosphere Coupling Simulation version 4 (GUMICS‐4), a global MHD model. We investigate the dynamic coupling between northward IMF conditions and the Earth's magnetotail and compare the results to observation‐based mechanisms for the formation of transpolar arcs. The results of this study reveal that under northward IMF conditions (and northward IMF initialization), a large closed field line region forms in the magnetotail, with similarities to transpolar arc structures observed from spacecraft data. This interpretation is supported by the simultaneous increase of closed flux measured in the magnetotail. However, the reconnection configuration differs in several respects from previously theorized magnetotail structures that have been inferred from both observations and simulations results and associated with transpolar arcs. We observe that dawn–dusk lobe regions form as a result of high‐latitude reconnection during the initialization stages, which later come into contact as the change in the IMF By component causes the magnetotail to twist. We conclude that in the GUMICS simulation, transpolar arc‐like structures are formed as a result of reconnection in the magnetotail, rather than high‐latitude reconnection or due to the mapping of the plasma sheet through a twisted magnetotail as interpreted from previous analysis of GUMICS simulations. Plain Language Summary: When the magnetic field associated with the solar wind is directed "northward," the Earth's aurora can adopt a formation where a "bar" of emission crosses the otherwise dim region that lies poleward of the usual auroral emissions. These phenomena are called "theta auroras" (due to their resemblance to the Greek letter), or "transpolar arcs," and have been observed from spacecraft observing the auroral regions. As spacecraft cannot directly observe the global configuration of magnetic field lines within planetary magnetospheres, simulations can be useful to gain insight into the possible structures that occur during different solar wind conditions. We use a global simulation to model the interaction between the solar wind and the Earth's magnetosphere (the region of space around our planet) and see that a large‐scale field line structure can form during certain northward‐directed magnetic field conditions. When we trace these field lines to the ionospheric boundary, we find that they resemble the global structures that have been observed for auroral arcs that stretch across the polar cap (also named theta aurora or transpolar arcs). We also note some key differences in their formation process from observational results. Key Points: Signatures consistent with magnetotail reconnection are observed in a simulation driven by northward interplanetary magnetic field conditionsThe simulation results in the production of a closed magnetotail structure and polar cap arcsThe above structure occurs within a magnetotail that is highly twisted such that open lobe regions cross the equatorial plane [ABSTRACT FROM AUTHOR]

We investigate the accuracy with which the reconnection electric field E M can be determined from in situ plasma data. We study the magnetotail electron diffusion region observed by National Aeronautics and Space Administration's Magnetospheric Multiscale (MMS) on 11 July 2017 at 22:34 UT and focus on the very large errors in E M that result from errors in an L M N boundary normal coordinate system. We determine several L M N coordinates for this MMS event using several different methods. We use these M axes to estimate E M. We find some consensus that the reconnection rate was roughly E M = 3.2 ± 0.6 mV/m, which corresponds to a normalized reconnection rate of 0.18 ± 0.035. Minimum variance analysis of the electron velocity (MVA-v e), MVA of E, minimization of Faraday residue, and an adjusted version of the maximum directional derivative of the magnetic field (MDD-B) technique all produce reasonably similar coordinate axes. We use virtual MMS data from a particle-in-cell simulation of this event to estimate the errors in the coordinate axes and reconnection rate associated with MVA-v e and MDD-B. The L and M directions are most reliably determined by MVA-v e when the spacecraft observes a clear electron jet reversal. When the magnetic field data have errors as small as 0.5% of the background field strength, the M direction obtained by MDD-B technique may be off by as much as 35°. The normal direction is most accurately obtained by MDD-B. Overall, we find that these techniques were able to identify E M from the virtual data within error bars ≥20%.

We investigate the accuracy with which the reconnection electric field E M can be determined from in situ plasma data. We study the magnetotail electron diffusion region observed by National Aeronautics and Space Administration's Magnetospheric Multiscale (MMS) on 11 July 2017 at 22:34 UT and focus on the very large errors in E M that result from errors in an L M N boundary normal coordinate system. We determine several L M N coordinates for this MMS event using several different methods. We use these M axes to estimate E M. We find some consensus that the reconnection rate was roughly E M = 3.2 ± 0.6 mV/m, which corresponds to a normalized reconnection rate of 0.18 ± 0.035. Minimum variance analysis of the electron velocity (MVA-v e), MVA of E, minimization of Faraday residue, and an adjusted version of the maximum directional derivative of the magnetic field (MDD-B) technique all produce reasonably similar coordinate axes. We use virtual MMS data from a particle-in-cell simulation of this event to estimate the errors in the coordinate axes and reconnection rate associated with MVA-v e and MDD-B. The L and M directions are most reliably determined by MVA-v e when the spacecraft observes a clear electron jet reversal. When the magnetic field data have errors as small as 0.5% of the background field strength, the M direction obtained by MDD-B technique may be off by as much as 35°. The normal direction is most accurately obtained by MDD-B. Overall, we find that these techniques were able to identify E M from the virtual data within error bars ≥20%.

One of the most significant observations associated with a sharp enhancement in solar wind dynamic pressure, PSW, is the poleward expansion of the auroral oval and the closing of the polar cap. The polar cap shrinking over a wide range of magnetic local times (MLTs), in connection with an observed increase in ionospheric convection and the transpolar potential, led to the conclusion that the nightside reconnection rate is significantly enhanced after a pressure front impact. However, this enhanced tail reconnection has never been directly measured. We demonstrate the effect of a solar wind dynamic pressure front on the polar cap closure, and for the first time, measure the enhanced reconnection rate in the magnetotail, for a case occurring during southward background Interplanetary Magnetic Field (IMF) conditions. We use Polar Ultra‐Violet Imager (UVI) measurements to detect the location of the open‐closed field line boundary, and combine them with Assimilative Mapping of Ionospheric Electrodynamics (AMIE) potentials to calculate the ionospheric electric field along the polar cap boundary, and thus evaluate the variation of the dayside/nightside reconnection rates. We find a strong response of the polar cap boundary at all available MLTs, exhibiting a significant reduction of the open flux content. We also observe an immediate response of the dayside reconnection rate, plus a phased response, delayed by ∼15–20 min, of the nightside reconnection rate. Finally, we provide comparison of the observations with the results of the Open Geospace General Circulation Model (OpenGGCM), elucidating significant agreements and disagreements. Plain Language Summary: This study provides valuable information on how the Earth's magnetosphere (the magnetized protective bubble around the Earth) is eroded by powerful explosions at the Sun. The response of the polar cap size and the reconnection rates in the magnetosphere to a solar wind high density front are investigated for an event with Interplanetary Magnetic Field orientation anti‐parallel to the Earth's magnetic field. We make a data‐based assessment, using Polar spacecraft ultraviolet images and assimilative model‐generated potentials, plus a model‐based comparison using a global magnetospheric model. An immediate response is observed at the dayside ionosphere for both the polar cap boundary and the reconnection rate mapped to the ionosphere. We also observe about 15–20 min delayed effect on various sectors of the nightside ionosphere. The comparison with the model reveals considerable discrepancies on the dayside ionosphere and significant agreements at the nightside ionosphere. Key Points: Effect of a solar wind dynamic pressure front on the open‐closed polar cap boundaryDetermination of the magnetic reconnection rate for a southward interplanetary magnetic field caseResponse of the dayside/nightside reconnection rate to a solar wind dynamic pressure front [ABSTRACT FROM AUTHOR]

We investigate the accuracy with which the reconnection electric field EM can be determined from in situ plasma data. We study the magnetotail electron diffusion region observed by National Aeronautics and Space Administration's Magnetospheric Multiscale (MMS) on 11 July 2017 at 22:34 UT and focus on the very large errors in EM that result from errors in an LMN boundary normal coordinate system. We determine several LMN coordinates for this MMS event using several different methods. We use these M axes to estimate EM. We find some consensus that the reconnection rate was roughly EM = 3.2 ± 0.6 mV/m, which corresponds to a normalized reconnection rate of 0.18 ± 0.035. Minimum variance analysis of the electron velocity (MVA‐ve), MVA of E, minimization of Faraday residue, and an adjusted version of the maximum directional derivative of the magnetic field (MDD‐B) technique all produce reasonably similar coordinate axes. We use virtual MMS data from a particle‐in‐cell simulation of this event to estimate the errors in the coordinate axes and reconnection rate associated with MVA‐ve and MDD‐B. The L and M directions are most reliably determined by MVA‐ve when the spacecraft observes a clear electron jet reversal. When the magnetic field data have errors as small as 0.5% of the background field strength, the M direction obtained by MDD‐B technique may be off by as much as 35°. The normal direction is most accurately obtained by MDD‐B. Overall, we find that these techniques were able to identify EM from the virtual data within error bars ≥20%. Key Points: The reconnection rate EM is estimated for one event using several techniques to find an M directionThe error bars in EM and the LMN coordinate directions are estimated from virtual dataThe reconnection rate is likely EM = 3.2 mV/m ± 0.6 mV/m, which corresponds to a normalized rate of 0.18 ± 0.035 [ABSTRACT FROM AUTHOR]

An improved algorithm was developed to estimate the polar rain electron path length from the magnetotail X-line to the polar ionosphere using the information of polar rain electron energy-latitude dispersion. Recent particle tracing simulations using APLOPM model (Applied Physics Laboratory – Open-field line particle Precipitation Model) ( Wing et al., 2001 ; Wing and Zhang, 2015 ) indicate that an existing or traditional method underestimates the path length by at least 33%. A new method for estimating electron path length that introduces a new parameter (energy parameter) is proposed. The improved algorithm has been validated using the APLOPM simulation data. By applying the new algorithm to two real events measured by DMSP satellites, we found the polar rain electron path lengths of 67 and 114 R E , (X-lines estimated at X =−54 and −91 R E ), respectively (assuming the distance from the X-line to the Earth is 80% of the electron path length). The associated IMF B z were 1 and –11 nT, respectively for the two events. This is consistent with the expected stretched magnetotail configuration under a strongly southward IMF. The results are also consistent with statistical results of the X-line locations from Geotail measurements. [ABSTRACT FROM AUTHOR]

Consecutive bursts of low-latitude Pi2 pulsations are examined with magnetic field data obtained by the Sino Magnetic Array at Low Latitudes (SMALL) in 1999. To ascertain the relationship with the substorm onset, 33 events consisting of two consecutive Pi2 bursts are selected from the observations at the Wuhan and Beijing stations, with reference to H-component magnetic bays in high-latitude magnetograms. The dominant frequencies of these consecutive Pi2 bursts are mostly in the range 10–20 mHz. The frequencies of the two Pi2 bursts are less correlated with each other at the lower latitude station, Wuhan, than at Beijing. The dominant frequency increases as the Kp index increases. Observations with ACE and Wind satellite show that the first Pi2 burst occurs significantly after the southward turning of the interplanetary magnetic field (IMF) and the second Pi2 often occurs shortly after a northward turning of the IMF. Thus, the first Pi2 burst is not a signature of the beginning of the flux transfer to the tail but perhaps signals the initial onset of reconnection in the tail. The interplanetary data can be used to estimate the amount of reconnected magnetic flux transported to the tail. The delay time of two consecutive bursts of low-latitude Pi2 is correlated with this estimated flux pileup. If the second burst of Pi2 signals the onset of reconnection of the open field lines in the tail lobes, then this observation implies that the rate of reconnection of the closed field lines in the plasma sheet is more rapid the greater is the rate of reconnection at the nose. Thus, we would expect plasmoids to be created more quickly for strong southward IMF than for weakly southward IMF. [Copyright &y& Elsevier]

A review of the impact of the cold-ion and cold-electron populations in the Earth's magnetosphere is presented. The cold populations are defined by total energy less than approximately 100 eV, i.e. in the energy range which is strongly affected by spacecraft charging and that often dominates the total plasma density. We also include the warm plasma cloak in the review, since it overlaps partially with the cold energy range and is a population that is still not well understood. The known impacts of cold ions and cold electrons that are discussed are: the source of hot magnetospheric plasma, solar-wind/magnetosphere coupling, magnetotail reconnection and substorms, Kelvin–Helmholtz instabilities on the magnetopause, chorus, hiss, electromagnetic-ion-cyclotron and ultra-low-frequency wave–particle interactions, aurora structuring and spacecraft charging. Other possible impacts are associated with refilling on open-drift trajectories, the remnant layer and plasmapause disruption. A discussion of the difficulty of cold-plasma measurements and the need for new measurement techniques that measure the full cold-ion and cold-electron distribution functions is also presented. There remain a lot of unknowns about the cold-ion and cold-electron populations, associated with their origin, properties, drivers and impacts. These populations will need to be fully understood before the magnetosphere–ionosphere system can be fully understood. • The cold-particle populations play a critical role in the Earth's magnetosphere. • This paper reviews their known and probable impacts in magnetospheric physics. • The need for new cold-plasma measurement techniques is also discussed. • The lack of understanding of the cold populations hinders magnetospheric research. [ABSTRACT FROM AUTHOR]