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A characteristic feature of the main phase of geomagnetic storms is the dawn‐dusk asymmetric depression of low‐ and mid‐latitude ground magnetic fields, with largest depression in the dusk sector. Recent work has shown, using data taken from hundreds of storms, that this dawn‐dusk asymmetry is strongly correlated with enhancements of the dawnside westward electrojet and this has been interpreted as a "dawnside current wedge" (DCW). Its ubiquity suggests it is an important aspect of stormtime magnetosphere‐ionosphere (MI) coupling. In this work we simulate a moderate geomagnetic storm to investigate the mechanisms that give rise to the formation of the DCW. Using synthetic SuperMAG indices we show that the model reproduces the observed phenomenology of the DCW, namely the correlation between asymmetry in the low‐latitude ground perturbation and the dawnside high‐latitude ground perturbation. We further show that these periods are characterized by the penetration of mesoscale bursty bulk flows (BBFs) into the dawnside inner magnetosphere. In the context of this event we find that the development of the asymmetric ring current, which inflates the dusk‐side magnetotail, leads to asymmetric reconnection and dawnward‐biased flow bursts. This results in an eastward expansion and multiscale enhancement of the dawnside electrojet. The electrojet enhancement extends across the dawn quadrant with localized enhancements associated with the wedgelet current systems of the penetrating BBFs. Finally, we connect this work with recent studies that have shown rapid, localized ground variability on the dawnside which can lead to hazardous geomagnetically induced currents. Plain Language Summary: During geomagnetic storms, electric currents in space can have a dramatic effect on the magnetic field on the ground, causing so‐called geomagnetic disturbances (GMDs). Storm‐time GMDs exhibit a lopsided asymmetry: dusk‐biased near the equator and dawn‐biased at high latitudes where aurora usually occur. This asymmetry has been interpreted as a giant wedge‐like current system, a dawnside current wedge (DCW). Using a high‐resolution supercomputer model, we successfully reproduced the DCW and showed that it occurred during a period of intense, localized flow bursts, akin to bubbles, on the nightside of near‐Earth space. The bubbles' buoyancy propels them from the nightside inwards toward dawn, driving intense currents into the Earth's atmosphere. Our simulations suggest that the causal agent of these dawnside bubbles is magnetic reconnection, typically symmetric but skewed dawnward due to asymmetry in the ring current, a crescent‐shaped population of energetic ions in space which intensifies during geomagnetic storms. Understanding the cause of stormtime GMD asymmetry is not only important to characterize how electric currents bind the magnetosphere and upper atmosphere, but also to mitigate space weather hazards, as intense GMDs can disrupt and damage power systems on Earth. Key Points: Global model reproduces correlation between ring current asymmetry and dawnside electrojet inferred from hundreds of geomagnetic stormsAnalysis of the model reveals a dawnside current wedge mediated by mesoscale flow bursts and driven by an asymmetric substorm‐like processModel reveals multiscale enhancement of dawnside electrojet with space weather implications due to rapid, localized ground variability [ABSTRACT FROM AUTHOR]

Recent advances in reconstructing Earth's magnetic field and associated currents by utilizing data mining of in situ magnetometer observations in the magnetosphere based on geomagnetic indices and solar wind parameters have proven remarkably accurate at reproducing observed ion diffusion regions. We investigate the effect of placing regions of localized resistivity in global simulations of the magnetosphere at specific locations inspired by the data mining results for the substorm occurring on 6 July 2017. When explicit resistivity is included, the simulation forms an x‐line at the same time and location as the Magnetospheric Multiscale Mission observation of an ion diffusion region at 15:35 UT on that day. Without this explicit resistivity, reconnection forms later in the substorm and far too close to Earth (≳−15RE), a common problem with global simulations of Earth's magnetosphere. A consequence of reconnection taking place farther down the tail due to localized resistivity is that the reconnection outflows transport magnetic flux Earthward and thus prevent the current sheet from thinning enough for reconnection to take place nearer Earth. As these flows rebound tailward from the inner magnetosphere, they can temporarily and locally (in the dawn‐dusk direction) stretch the magnetic field allowing for small scale x‐lines to form in the near Earth region. Due to the narrow cross‐tail extent of these x‐lines (≲5RE) and their short lifespan (≲5 min), they would be difficult to observe with in situ measurements. Future work will explore time‐dependent resistivity using 5 min cadence data mining reconstructions. Plain Language Summary: Recently, data mining of spacecraft magnetometers have created highly accurate reconstructions of Earth's magnetic field. These reconstructions capture the location and timing of so‐called x‐lines on the night side of Earth. Magnetic field lines reconnect at x‐lines and convert magnetic energy to plasma energy via heating and acceleration of flows. Nightside reconnection is a crucial part of geomagnetic storms and substorms that pose a danger to spacecraft, astronauts, and the power grid on Earth. By identifying x‐lines in reconstructions we can influence simulations of specific substorms to increase their accuracy and make them more physically realistic. We demonstrate this ability by matching simulation results with observations and find that reconnection further from Earth suppresses extended x‐line formation near Earth. Additionally, the flows from reconnection can bounce off the strong magnetic field close to Earth (a well‐known effect) and locally stretch the magnetic field creating small scale x‐lines near Earth. Key Points: Localized Resistivity in global magnetohydrodynamic simulations can "encourage" magnetic reconnection in specific locations to match data mining resultsActive x‐lines at XGSM ≲ −20RE can suppress the formation of extended x‐lines at XGSM ≳ −15REReconnection outflows rebound from the near‐Earth region causing transient and narrow (in YGSM) secondary reconnection [ABSTRACT FROM AUTHOR]

Earth's magnetotail plays a critical role in the dynamics of the magnetosphere, particularly during intervals of geomagnetic activity. To improve our understanding of the ion dynamics in this region, energetic neutral atom (ENA) imaging can provide global measurements to place in situ measurements in context and validate simulations. The NASA Two Wide‐angle Imaging Neutral‐atom Spectrometers mission provided near‐continuous observations using ENA imagers. ENA data can be used to calculate maps of equatorial ion temperatures that often show observations of regions of enhanced temperatures associated with phenomena in the magnetotail such as magnetic reconnection and narrow flow channels. We present an algorithm that can be used to search through a collection of these maps to identify intervals with such enhancements for further study. The algorithm results are validated against two sets of related phenomena: (a) a database of dipolarizing flux bundle (DFB) measurements from THEMIS and (b) a list of substorm onsets from SuperMAG. We demonstrate that the algorithm is very good at identifying intervals when there are DFB measurements or substorm onsets as long as there sufficient ENA data. We discuss some potential scientific studies that can result from use of the algorithm. We also show a preliminary application of the algorithm to simulation output to demonstrate the usefulness for other datasets, facilitate comparative studies, and introduce a new method for model validation. Key Points: An algorithm has been developed to identify regions of increased ion temperatures in maps calculated from Two Wide‐angle Imaging Neutral‐atom Spectrometers energetic neutral atom dataThe algorithm is validated against a list of dipolarizing flux bundle measurements and a list of substorm onsetsThe algorithm will be useful for future case and statistical studies of mesoscale phenomena as well as model validation [ABSTRACT FROM AUTHOR]

The present paper demonstrates the first observations by the Magnetospheric Multiscale (MMS) mission of the counter‐streaming energetic electrons and trapped energetic protons, localized in the magnetic field depressions between the mirror mode peaks, in the Earth's dusk sector high‐latitude magnetosphere. This region is characterized by high plasma beta, strong ion temperature anisotropy, and intermediate plasma density between magnetospheric and magnetosheath plasma. We show that these plasma conditions are unstable for the drift mirror instability. The counter‐streaming electron feature resembles those of the previously reported energetic electron microinjections, but without the energy‐time dispersion signature. This suggests that MMS is passing through one of the potential microinjection source regions. The energetic ion data in the present study is mainly used to estimate the scale size of the mirror mode structures. Plain Language Summary: Understanding the physical mechanisms that result in energetic electron acceleration and loss within the Van Allen radiation belts has been an active area of research for decades, and due to advances made possible by the Van Allen probe mission are now relatively well understood. However, the origin of the several 10–100s of keV seed population that can be accelerated to relativistic energies has remained more elusive. It is well known that magnetic reconnection and related secondary processes in the Earth's magnetotail during substorms can accelerate particles and inject them inward toward the radiation belts. In this paper we show the first observations of a possible source region of 10–100s of keV electrons and protons at the dayside of the Earth's high‐latitude magnetosphere. Four MMS spacecraft periodically encountered high fluxes of energetic electrons at wide energy range which were streaming both parallel and anti‐parallel to the magnetic field. Enhanced fluxes of counter‐streaming energetic electrons and trapped protons were observed between magnetic field peaks of the ULF waves identified as mirror mode peaks. The source region of these electrons and protons are likely the large diamagnetic cavities created by magnetic reconnection. Key Points: MMS detected dispersionless electron microinjections in ULF magnetic field wave depressions at the high‐latitude magnetosphereULF waves were consistent with mirror mode waves created by the drift mirror instability [ABSTRACT FROM AUTHOR]