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We investigate a 15‐day period in October 2011. Auroral observations by the Special Sensor Ultraviolet Spectrographic Imager instrument onboard the Defense Meteorological Satellite Program F16, F17, and F18 spacecraft indicate that the polar regions were covered by weak cusp‐aligned arc (CAA) emissions whenever the interplanetary magnetic field (IMF) clock angle was small, |θ| < 45°, which amounted to 30% of the time. Simultaneous observations of ions and electrons in the tail by the Cluster C4 and Geotail spacecraft showed that during these intervals dense (≈1 cm−3) plasma was observed, even as far from the equatorial plane of the tail as |ZGSE| ≈ 13 RE. The ions had a pitch angle distribution peaking parallel and antiparallel to the magnetic field and the electrons had pitch angles that peaked perpendicular to the field. We interpret the counter‐streaming ions and double loss‐cone electrons as evidence that the plasma was trapped on closed field lines, and acted as a source for the CAA emission across the polar regions. This suggests that the magnetosphere was almost entirely closed during these periods. We further argue that the closure occurred as a consequence of dual‐lobe reconnection. Our finding forces a significant re‐evaluation of the magnetic topology of the magnetosphere during periods of northwards IMF. Plain Language Summary: The magnetosphere is usually assumed to contain both open and closed magnetic flux. Closed magnetic field lines have both ends connected to the Earth; open field lines connect to the Earth at one end and into the interplanetary medium at the other. There tends to be little plasma on open field lines as the particles escape down the magnetotail, whereas plasma on closed field lines is trapped. Open flux near the poles naturally explains the oval configuration of Earth's auroras, with a lack of auroras at very high latitudes where there is no plasma to cause emissions. Somewhat unexpectedly, we show that auroral emission near the poles is common and that at these times there is significant plasma in the magnetotail, indicating that the magnetosphere contains only closed flux. We propose that this magnetic configuration is formed by a process known as dual‐lobe magnetic reconnection which occurs when the interplanetary magnetic field (IMF) within the solar wind points northwards. We must re‐evaluate the standard picture of magnetospheric structure during these periods of northwards IMF. Key Points: Cusp‐aligned arcs (CAAs) observed by the Defense Meteorological Satellite Program spacecraft occur frequently for northward interplanetary magnetic fieldCluster and Geotail observations show that the arcs are accompanied by trapped plasma at high latitudes in the magnetotailWe interpret CAAs as a signature of a magnetosphere almost entirely closed by dual‐lobe reconnection [ABSTRACT FROM AUTHOR]

Skin conditions are a common problem addressed by medical providers. Up to 25% of individuals in the United States will seek attention for these conditions each year. The same problem occurs in the athletic training room, where athletes with infectious skin conditions can be seen. Most conditions are simple and can be treated without concern for spread to susceptible athletes. However, others can be quite serious and spread rapidly through a team and opponents during competition. Knowledge of the different types of skin infections is necessary to help treat these athletes and prevent spread to others. With proper diagnosis and treatment, certified athletic trainers can keep the athlete off the field of play for a minimum period and prevent transmission. [ABSTRACT FROM AUTHOR]

The effects of a solar wind pressure pulse on the terrestrial magnetosphere have been observed in detail across multiple datasets. The communication of these effects into the magnetosphere is known as a positive geomagnetic sudden impulse (+SI), and are observed across latitudes and different phenomena to characterize the propagation of +SI effects through the magnetosphere. A superposition of Alfvén and compressional propagation modes are observed in magnetometer signatures, with the dominance of these signatures varying with latitude. For the first time, collocated lobe reconnection convection vortices and region 0 field aligned currents are observed preceding the +SI onset, and an enhancement of these signatures is observed as a result of +SI effects. Finally, cusp auroral emission is observed collocated with the convection and current signatures. For the first time, simultaneous observations across multiple phenomena are presented to confirm models of +SI propagation presented previously. Key Points: A positive sudden impulse (SI) is observed on 16 June 2012 at 20:19 UT in SYM‐HA superposition of Alfvén and compressional propagation modes are observed in magnetometer dataCollocated lobe reconnection vortices and region 0 field aligned currents are enhanced by the SI [ABSTRACT FROM AUTHOR]

We examine the average evolution of precipitation‐induced height‐integrated conductances, along with field‐aligned currents (FACs), in the nightside sector of the polar cap over the course of a substorm. Conductances are estimated from the average energy flux and mean energies derived from auroral emission data. Data are binned using a superposed epoch analysis on a normalized time grid based on the time between onset and recovery phase (δt) of each contributing substorm. We also examine conductances using a fixed time binning of width 0.25 hr. We split the data set by magnetic latitude of onset. We find that the highest conductances are observed for substorms with onsets that occur between 63 and 65° magnetic latitude, peaking at around 11 mho (Hall) and 4.8 mho (Pedersen). Substorms with onsets at higher magnetic latitudes show lower conductances and less variability. Changes in conductance over the course of a substorm appear primarily driven by changes (about 40% at onset) in the average energy flux, rather than the average energy of the precipitation. Average energies increase after onset slower than energy flux, later these energies decrease slowly for the lowest latitude onsets. No clear expansion of the main region 1 and region 2 FACs is observed. However, we do see an ordering of the current magnitudes with magnetic latitude of onset, particularly for region 1 downwards FAC in the morning sector. Peak current magnitudes occur slightly after or before the start of the recovery phase for the normalized and fixed‐time grids. Plain Language Summary: Particles precipitate from Earth's magnetosphere into the upper ionosphere causing auroral emissions. A comparison of these auroral emissions, taken at different wavelengths, can be used to estimate the mean energy of the particles, as well as the flux, or number of precipitating particles in an area per unit time. From this mean energy and flux, we can estimate changes in the conductance of the ionosphere. Here, we examine how the conductance varies during the course of a substorm; when increased auroral emissions are seen suddenly on the nightside of the Earth. For this work we use imaging data from low‐altitude spacecraft that give reasonable spatial coverage of the nightside ionosphere. We compare the changes in conductance over the course of an average substorm, to those seen in electrical currents that flow in the Earth's magnetosphere. The currents respond in a similar manner to the parameters derived from the auroral emissions. Key Points: We track the progression of mean energy and mean energy flux, derived conductances, and field‐aligned currents (FACs), during substormsLow‐latitude onsets exhibit the largest and longest‐lived changes to height‐integrated conductanceLatitude of substorm onset has less control on FAC magnitudes [ABSTRACT FROM AUTHOR]

Remote observations by the Mariner 10 and MErcury Surface, Space ENvironment, GEophysics, and Ranging (MESSENGER) spacecraft have shown the existence of hydrogen in the exosphere of Mercury. However, to date the hydrogen number densities could only be estimated indirectly from exospheric models, based on the remotely observed Lyman‐α radiances for atomic H, and the detection threshold of the Mariner 10 occultation experiment for molecular H2. Here, we show the first on‐site determined altitude‐density profile of atomic H, derived from in situ magnetic field observations by MESSENGER. The results reveal an extended H exosphere with densities that are ∼1–2 orders of magnitude larger than previously predicted. Using an exospheric model that reproduces the H altitude‐density profile allows us to constrain the so far unknown H2 density at the surface, which is ∼2–3 orders of magnitude smaller than previously assumed. These findings demonstrate the importance of (a) in situ measurements supporting remote observations of Mercury's exosphere that will be realized in the near future by the BepiColombo mission and (b) that dissociation processes play a crucial role in Mercury's exosphere. Plain Language Summary: Mercury has an exosphere that contains a variety of species. So far, only two spacecraft have probed the space environment around Mercury: Mariner 10 in 1974–1975 and MErcury Surface, Space ENvironment, GEophysics, and Ranging (MESSENGER) four decades later. Optical observations by Mariner 10 and MESSENGER showed that Mercury has a thin collisionless atmosphere (exosphere) which is abundant in hydrogen. To date the hydrogen number density at Mercury could only be estimated from exospheric models that use the optical observations as constrains, since no in situ measurements of hydrogen are available to determine its exact number density. For the first time, we derive an altitude‐density profile of Mercury's H exosphere from in situ magnetic field measurements by MESSENGER. From the observations of so‐called pick‐up ion cyclotron waves in the magnetic field data, it is possible to derive the local H number density, necessary to excite these waves. The results reveal an extended atomic H exosphere with densities decreasing from ∼100 to 10 cm−3 between 2,400 and 15,000 km above the surface. The unexpected large H densities can only be explained by dissociation processes of H2 molecules. Here, we introduce an exospheric model that includes such dissociation processes, which allows us for the first time to constrain the H2 number density. The results suggests that atomic H has additional sinks near the surface, most likely through chemical reactions with OH and O, and that the photochemistry of H2O in general play an important role for Mercury's exospheric composition. Key Points: First on‐site determined hydrogen number density in Mercury's exosphereDemonstration that dissociation processes play an important role in Mercury's exosphereBased on the results, we can constrain the so far unknown and overestimated molecular surface number density of hydrogen [ABSTRACT FROM AUTHOR]

Following the St. Patrick's Day (17 March) geomagnetic storm of 2013, the interplanetary magnetic field had near‐zero clock angle for almost two days. Throughout this period multiple cusp‐aligned auroral arcs formed in the polar regions; we present observations of, and provide a new explanation for, this poorly understood phenomenon. The arcs were observed by auroral imagers onboard satellites of the Defense Meteorological Satellite Program. Ionospheric flow measurements and observations of energetic particles from the same satellites show that the arcs were produced by inverted‐V precipitation associated with upward field‐aligned currents (FACs) at shears in the convection pattern. The large‐scale convection pattern revealed by the Super Dual Auroral Radar Network and the corresponding FAC pattern observed by the Active Magnetosphere and Planetary Electrodynamics Response Experiment suggest that dual‐lobe reconnection was ongoing to produce significant closure of the magnetosphere. However, we propose that once the magnetosphere became nearly closed complicated lobe reconnection geometries arose that produced interleaving of regions of open and closed magnetic flux and spatial and temporal structure in the convection pattern that evolved on timescales shorter than the orbital period of the DMSP spacecraft. This new model naturally explains many features of cusp‐aligned arcs, including why they focus in from the nightside toward the cusp region. Plain Language Summary: The geomagnetic storm that occurred on St. Patrick's Day in 2013 was followed by a period of almost two days during which the magnetic field embedded within the solar wind was pointing purely northwards, a rare occurrence. Auroral observations reveal that a series of auroral arcs formed near the geomagnetic poles which were aligned along the sunwards direction. Measurements of flows within the polar ionosphere reveal that these arcs were associated with shears in the convection pattern and electrical currents linking the magnetosphere and ionosphere. We propose a mechanism by which these flows could be produced, invoking complicated patterns of magnetic reconnection occurring at high latitudes on the dayside magnetopause. These observations shed light on the poorly understood solar wind‐magnetosphere‐ionosphere coupling processes that occur when the interplanetary magnetic field is directed northwards. Key Points: Cusp‐aligned arcs were observed for 2 days of near‐zero clock angle interplanetary magnetic field following the St. Patrick's Day storm of 2013Arcs were produced by upwards field‐aligned currents associated with vorticity within a highly structured ionospheric convection patternWe propose that the magnetosphere was nearly closed, but complicated lobe reconnection geometries interleaved open and closed flux [ABSTRACT FROM AUTHOR]

Upstream solar wind measurements from near the L1 Lagrangian point are commonly used to investigate solar wind‐magnetosphere coupling. The off‐Sun‐Earth line distance of such solar wind monitors can be large, up to 100 RE. We investigate how the correlation between measurements of the interplanetary magnetic field and associated ionospheric responses deteriorates as the off‐Sun‐Earth line distance increases. Specifically, we use the magnitude and polarity of the dayside region 0 field‐aligned currents (R0 FACs) as a measure of interplanetary magnetic field (IMF) BY‐associated magnetic tension effects on newly‐reconnected field lines, related to the Svalgaard‐Mansurov effect. The R0 FACs are derived from Advanced Magnetosphere and Planetary Electrodynamics Response Experiment measurements by a principal component analysis, for the years 2010–2016. We perform cross‐correlation analyses between time‐series of IMF BY, measured by the Wind spacecraft and propagated to the nose of the bow shock by the OMNI technique, and these R0 FAC measurements. Typically, in the summer hemisphere, cross‐correlation coefficients between 0.6 and 0.9 are found. However, there is a reduction of order 0.1–0.15 in correlation coefficient between periods when Wind is close to (within 45 RE) and distant from (beyond 70 RE) the Sun‐Earth line. We find a time‐lag of around 17 min between predictions of the arrival of IMF features at the bow shock and their effect in the ionosphere, irrespective of the location of Wind. Plain Language Summary: Space weather within the Earth's geospace environment is driven by the interaction of the solar wind with the magnetosphere. Measurements of the solar wind upstream of the Earth are crucial for understanding this interaction and for providing some advanced warning of hazardous conditions about to arrive. Such measurements are typically made by spacecraft located in orbit about the L1 Lagrangian point, sometimes far from the Sun‐Earth line, and it is uncertain how representative these measurements are of the solar wind that actually hits the Earth. In this study we investigate how predictions degrade as the off‐Sun‐Earth line distance increases. We use measurements of the east‐west component of the interplanetary magnetic field measured by the Wind spacecraft and observations of magnetic field‐aligned electrical currents within the magnetosphere, to assess how well these are correlated. We find that the correlation does indeed decrease somewhat as Wind wanders far from the Sun‐Earth line. This study will help provide confidence in using these upstream monitors, but also allow quantification of what discrepancies can be expected. It also allows the scale‐size of features in the solar wind to be estimated. Key Points: We compare OMNI measurements of interplanetary magnetic field BY by Wind with associated variations in the dayside ionosphereThe cross‐correlation between these measurements deteriorates as the off‐Sun‐Earth line distance of Wind increasesA delay of around 17 min is found between OMNI predictions of arrival at the bow shock and the ionospheric response [ABSTRACT FROM AUTHOR]

Horse collar aurora (HCA) are an auroral feature where the dawn and dusk sector auroral oval moves polewards and the polar cap becomes teardrop shaped. They form during prolonged periods of northward interplanetary magnetic field (IMF), when the IMF clock angle is small. Their formation has been linked to dual‐lobe reconnection (DLR) closing magnetic flux at the dayside magnetopause. The conditions necessary for DLR are currently not well‐understood therefore understanding HCA statistics will allow DLR to be studied in more detail. We have identified over 600 HCA events between 2010 and 2016 in UV images captured by the Special Sensor Ultraviolet Spectrographic Imager instrument on‐board the Defense Meteorological Satellite Program spacecraft F16, F17 and F18. As expected, there is a clear preference for HCA occurring during northward IMF. We find no clear seasonal dependence in their occurrence, with an average of 8 HCA events per month. The occurrence of HCA events does not appear to depend on the Bx component of the IMF. Considering the average radiance intensity across the dusk‐dawn meridian shows the HCA as a separate bulge inside the auroral oval and that the dawn side arc of the HCA is usually brighter than the dusk in the Lyman‐Birge‐Hopfield short band. We relate this to the expected field aligned current pattern of HCA formation. We further suggest that transpolar arcs observed in the dawn sector simultaneously in both northern and southern hemispheres are misidentified HCA. Plain Language Summary: Horse collar auroras (HCA) form when the auroras move to high latitudes at dawn and dusk. They have been proposed to be formed by a process called dual‐lobe reconnection, which takes place when the interplanetary magnetic field (IMF) embedded in the solar wind is directed almost exactly northwards. We study the occurrence of HCA in auroral observations from the Special Sensor Ultraviolet Spectrographic Imager instrument onboard satellites of the Defense Meteorological Satellite Program for the years 2010 to 2016. Studying the occurrence of HCA and the solar wind conditions under which they form allows us to gain new insights into the conditions necessary for dual‐lobe reconnection (DLR) to occur which are currently not well‐understood. We find that there are approximately 8 HCA events per month, with no seasonal dependence, and that the IMF must be within 30 degrees of northwards. When looked at by season no variation is seen in the IMF Bx component therefore suggesting that Bx is not an important factor in the occurrence of HCA. We also note a dawn‐dusk asymmetry in the brightness of the HCAs, which we attribute to the polarity of the field‐aligned electrical currents which produce the auroras. Key Points: Horse collar aurora, teardrop shaped polar cap, occur frequently approximately 8 times per month and are linked to dual lobe reconnectionInterplanetary magnetic field Bx does not appear to be an important factor in determining the occurrence of horse collar auroraThe dawn arc of the horse collar auroras is usually brighter than the dusk [ABSTRACT FROM AUTHOR]

For over a decade, the Super Dual Auroral Radar Network and the Active Magnetosphere and Planetary Electrodynamics Response Experiment have been measuring ionospheric convection and field‐aligned currents in the high‐latitude regions, respectively. Using both, high‐latitude maps of the magnetosphere‐ionosphere energy transfer rate (the Poynting flux) have been generated with a time resolution of 2 min between 2010 and 2017. These data driven Poynting flux (PF) patterns are used in this study to perform a superposed epoch analysis of the northern hemisphere ionospheric response to transitions of the interplanetary magnetic field Bz component, upwards of 60° geomagnetic latitude. We discuss the difference in the distribution of PF between the magnetosphere‐ionosphere Dungey cycle "switching on" and "switching off" to solar wind driving, revealing that they are not symmetric temporally or spatially. Plain Language Summary: The Earth's high‐latitude upper atmosphere (the ionosphere, upwards of 100 km in altitude) is consistently bombarded with solar energy that takes the form of electric currents aligned with Earth's magnetic field. The magnetosphere has two generalized states, "open" and "closed." Open is when the Earth and interplanetary magnetic fields (IMFs) connect to each other on the dayside, allowing energy into the atmosphere from the solar wind. Closed is when the fields do not connect (or do not connect simply) and thus not as much energy enters the atmosphere. The aforementioned open or closed states depend on the direction of the IMF, which varies constantly, as well as the history of the field as the magnetosphere takes time to reconfigure. In this study, we utilize nearly 8 years of data to generate average patterns of ionospheric energy input at various intervals before and after the IMF flips in direction. We discuss the spatial and temporal timescales upon which the energy varies in response to the relatively symmetric IMF transitions, finding that they do not result in symmetric changes in the ionospheric energy input patterns. Key Points: The Super Dual Auroral Radar Network and the Active Magnetosphere and Planetary Electrodynamics Response Experiment derived Poynting flux (PF) distributions are used to perform a superposed epoch analysis of interplanetary magnetic field (IMF) Bz transitionsChanges in the IMF Bz component have immediate responses, but continue to evolve for several tens of minutesThere is spatial and temporal asymmetry to how the PF morphology responds to opposite Bz transitions [ABSTRACT FROM AUTHOR]

A new technique has been developed to determine the high‐latitude electric potential from observed field‐aligned currents (FACs) and modeled ionospheric conductances. FACs are observed by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), while the conductances are modeled by Sami3 is Also a Model of the Ionosphere (SAMI3). This is a development of the Magnetosphere‐Ionosphere Coupling approach first demonstrated by Merkin and Lyon (2010), https://doi.org/10.1029/2010ja015461. An advantage of using SAMI3 is that the model can be used to predict total electron content (TEC), based on the AMPERE‐derived potential solutions. 23 May 2014 is chosen as a case study to assess the new technique for a moderately disturbed case (min Dst: −36 nT, max AE: 909 nT) with good GPS data coverage. The new AMPERE/SAMI3 solutions are compared against independent GPS‐based TEC observations from the Multi‐Instrument Data Analysis Software (MIDAS) by Mitchell and Spencer (2003), and against Defense Meteorological Satellite Program (DMSP) ion drift data. The comparison shows excellent agreement between the location of the tongue of ionization in the MIDAS GPS data and the AMPERE/SAMI3 potential pattern, and good overall agreement with DMSP drifts. SAMI3 predictions of high‐latitude TEC are much improved when using the AMPERE‐derived potential as compared to Weimer's (2005), https://doi.org/10.1029/2005ja011270 model. The two potential models have substantial differences, with Weimer producing an average 77 kV cross‐cap potential versus 60 kV for the AMPERE‐derived potential. The results indicate that the 66‐satellite Iridium constellation provides sufficient resolution of FACs to estimate large‐scale ionospheric convection as it impacts TEC. Plain Language Summary: Plasma in the ionosphere convects across high latitudes in response to electric fields generated by solar wind driving of Earth's magnetosphere. Magnetic data from the 66‐satellite Iridium satellite constellation is routinely analyzed to measure the electric currents flowing between the ionosphere and magnetosphere. In this paper, we combined these observed field‐aligned currents with a model of the ionospheric conductance to estimate the corresponding electric field in the ionosphere. We used these electric field estimates to specify the plasma convection in a first‐principles ionosphere model, and compared the results against independent data in a moderately disturbed period. The results of our case study indicate that the technique works well, although plasma convection is not the only factor that causes uncertainty into predictions of the high‐latitude ionosphere. The data used here can be made available in near‐real‐time, so it is possible that our technique could be adopted for operational space weather prediction. Key Points: Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) field‐aligned currents have been combined with the Sami3 is Also a Model of the Ionosphere (SAMI3) model to estimate the high‐latitude potentialIndependent validation of the new technique using satellite drifts and ground‐based total electron content indicates good agreement overallThe SAMI3 model performs better in this case when using AMPERE‐derived potentials than when using the empirical Weimer potential [ABSTRACT FROM AUTHOR]

We model an interval of remarkable interplanetary magnetic field (IMF), for which we have a comprehensive set of observational data. This interval is associated with the arrival of an interplanetary coronal mass ejection. The solar wind densities at the time are particularly high and the IMF is primarily northward over many hours. This results in strong auroral emissions within the polar cap in a cusp spot, which we associate with lobe reconnection at the high‐latitude magnetopause. We also observe areas of upwards field‐aligned current (FAC) within the summer Northern Hemisphere polar cap that exhibit large current magnitudes. The model can reproduce the spatial distribution of the FACs well, even under changing conditions in the incoming IMF. Discrepancies exist between the modeled and observed current magnitudes. Notably, the winter Southern Hemisphere exhibits much lower current magnitudes overall. We also model a sharp transition of the location of magnetopause reconnection at the beginning of the interval, before the IMF remained northward for many hours. The reconnection location changed rapidly from a subsolar location at the low‐latitude magnetopause under southward IMF conditions, to a high‐latitude lobe reconnection location when the field is northward. This occurs during a fast rotation of the IMF at the shock front of a magnetic cloud. Plain Language Summary: Under extreme incoming interplanetary magnetic field conditions following the impact of an Interplanetary Coronal Mass Ejection (CME) on the Earth's system, we observe a range of phenomena in the Northern Hemisphere ionosphere. This includes auroral emissions in the form of a cusp spot and associated precipitating particles, ionospheric flows, and strong field‐aligned currents (FACs) in the high‐latitude polar cap. These phenomena change in orientation and strength following variations in the incoming solar wind. We model the state of the magnetosphere during these observations. The modeled currents correspond well spatially with the observed currents, however, the current magnitudes are very different. The modeled FACs indicate that the site of magnetic reconnection can change rapidly from a lower‐latitude dayside position to a high‐latitude location in the magnetospheric lobes, which is reflected in field orientation within the magnetic cloud associated with the passing CME. Key Points: We model an interval of interplanetary BY‐dominated field and high solar wind densities during the impact of a Coronal Mass EjectionThe reconnection site moves rapidly from the subsolar to the high‐latitude magnetopause during a rotation of interplanetary magnetic fieldModeled and observed currents are spatially consistent in the polar cap although modeled to observed current magnitudes are often discrepant [ABSTRACT FROM AUTHOR]

The sub‐auroral polarization stream (SAPS) is a region of westward high velocity plasma convection equatorward of the auroral oval that plays an important role in mid‐latitude space weather dynamics. In this study, we present observations of SAPS flows extending across the North American sector observed during the recovery phase of a minor geomagnetic storm. A resurgence in substorm activity drove a new set of field‐aligned currents (FACs) into the ionosphere, initiating the SAPS. An upward FAC system is the most prominent feature spreading across most SAPS local times, except near dusk, where a downward current system is pronounced. The location of SAPS flows remained relatively constant, firmly inside the trough, independent of the variability in the location and intensity of the FACs. The SAPS flows were sustained even after the FACs weakened and retreated polewards with a decline in geomagnetic activity. The observations indicate that the mid‐latitude trough plays a crucial role in determining the location of the SAPS and that SAPS flows can be sustained even after the magnetospheric driver has weakened. Plain Language Summary: The sub‐auroral polarization stream (SAPS) is an important phenomenon that controls the dynamics of the sub‐auroral region. While SAPS has been studied for several decades, our understanding of the drivers of this phenomenon is still limited. In this study, we analyze an event to examine the validity of different SAPS mechanisms, and to determine the importance of the ionosphere‐thermosphere system in driving and sustaining SAPS flows. We find that the ionosphere can play an important role in determining SAPS location and that SAPS can be sustained even after the magnetospheric driver has weakened. Key Points: During the recovery phase of a small geomagnetic storm, substorm activity drove a new set of FACs into the ionosphere, initiating a sub‐auroral polarization stream (SAPS)The SAPS location remained relatively constant, firmly inside the trough, independent of the variability in the field‐aligned currents (FACs)The SAPS flows were sustained for almost an hour after the FACs weakened and retreated polewards [ABSTRACT FROM AUTHOR]

Large thermospheric neutral density enhancements in the cusp region have been examined for many years. The Challenging Minisatellite Payload (CHAMP) satellite for example has enabled many observations of the perturbation, showing that it is mesoscale in size and exists statistically over solar cycle timescales. Further studies examining the relationship with magnetospheric energy input have shown that fine‐scale Poynting fluxes are associated with the density perturbations on a case‐by‐case basis, whilst others have found that mesoscale downward fluxes also exist in the cusp region statistically. In this study, we use nearly 8 years of the overlapping Super Dual Auroral Radar Network and Active Magnetosphere and Planetary Electrodynamics Response Experiment datasets to generate global‐scale patterns of the high‐latitude and height‐integrated Poynting flux into the ionosphere, with a time resolution of 2 min. From these, average patterns are generated based on the interplanetary magnetic field orientation. We show the cusp is indeed an important feature in the Poynting flux maps, but the magnitude does not correlate well with statistical neutral mass density perturbations observed by the CHAMP satellite on similar spatial scales. Importantly, the lack of correlation between mesoscale height‐integrated Poynting fluxes and the cusp neutral mass density enhancement gives possible insight into other processes that may account for the discrepancy, such as energy deposition at finer scale sizes or at higher altitudes than captured. Key Points: Statistical patterns of the total downward Poynting flux into the atmosphere have been derived using Super Dual Auroral Radar Network and Active Magnetosphere and Planetary Electrodynamics Response Experiment dataStatistical patterns of neutral mass density perturbations as a percentage of the background density have been derived using Challenging Minisatellite Payload dataMesoscale downward Poynting flux in the cusp region do not correlate very well with neutral mass density enhancements at a similar scale [ABSTRACT FROM AUTHOR]

Electromagnetic ion cyclotron (EMIC) waves play important roles in particle loss processes in the magnetosphere. Determining the evolution of EMIC waves as they propagate and how this evolution affects wave-particle interactions requires accurate knowledge of the wave vector, k. We present a technique using the curl of the wave magnetic field to determine k observationally, enabled by the unique configuration and instrumentation of the Magnetospheric MultiScale (MMS) spacecraft. The wave curl analysis is demonstrated for synthetic arbitrary electromagnetic waves with varying properties typical of observed EMIC waves. The method is also applied to an EMIC wave interval observed by MMS on October 28, 2015. The derived wave properties and k from the wave curl analysis for the observed EMIC wave are compared with the Waves in Homogenous, Anisotropic, Multi-component Plasma (WHAMP) wave dispersion solution and with results from other single- and multi-spacecraft techniques. We find good agreement between k from the wave curl analysis, k determined from other observational techniques, and k determined from WHAMP. Additionally, the variation of k due to the time and frequency intervals used in the wave curl analysis is explored. This exploration demonstrates that the method is robust when applied to a wave containing at least 3-4 wave periods and over a rather wide frequency range encompassing the peak wave emission. These results provide confidence that we are able to directly determine the wave vector properties using this multi-spacecraft method implementation, enabling systematic studies of EMIC wave k properties with MMS. [ABSTRACT FROM AUTHOR]

We propose a mechanism for the formation of the horse‐collar auroral configuration during periods of strongly northward interplanetary magnetic field (IMF), invoking the action of dual‐lobe reconnection (DLR). Auroral observations are provided by the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) satellite and spacecraft of the Defense Meteorological Satellite Program (DMSP). We also use ionospheric flow measurements from DMSP and polar maps of field‐aligned currents (FACs) derived from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). Sunward convection is observed within the dark polar cap, with antisunward flows within the horse‐collar auroral region, together with the NBZ FAC distribution expected to be associated with DLR. We suggest that newly closed flux is transported antisunward and to dawn and dusk within the reverse lobe cell convection pattern associated with DLR, causing the polar cap to acquire a teardrop shape and weak auroras to form at high latitudes. Horse‐collar auroras are a common feature of the quiet magnetosphere, and this model provides a first understanding of their formation, resolving several outstanding questions regarding the nature of DLR and the magnetospheric structure and dynamics during northward IMF. The model can also provide insights into the trapping of solar wind plasma by the magnetosphere and the formation of a low‐latitude boundary layer and cold, dense plasma sheet. We speculate that prolonged DLR could lead to a fully closed magnetosphere, with the formation of horse‐collar auroras being an intermediate step. Plain Language Summary: During quiet geomagnetic conditions, the global distribution of auroras can acquire a "horse‐collar" configuration, in which regions of weak auroral emission appear at dawn and dusk poleward of the main auroral oval. We propose a new model to explain the formation of this configuration, which provides new insights into magnetospheric dynamics during periods of northward‐directed interplanetary magnetic field. To support our proposal, we use observations of the auroras, ionospheric convection, and estimations of the pattern of electrical currents flowing between the ionosphere and magnetosphere from a suite of spacecraft. Our proposed model resolves a 40‐year‐old question regarding the nature of the horse‐collar auroras and many other aspects of magnetospheric dynamics. Key Points: We propose that horse‐collar auroras, which occur during prolonged periods of northward IMF, are a signature of dual‐lobe reconnectionThe dayside polar cap is eroded and replaced by magnetic flux newly closed by dual‐lobe reconnection, filled with sun‐aligned arcsWe discuss the implications for NBZ phenomena, including solar wind capture, the low‐latitude boundary layer and cold, dense plasma sheet [ABSTRACT FROM AUTHOR]

The formation of polar cap density enhancements, such as tongues‐of‐ionization (TOIs), are often attributed to enhanced dayside reconnection and convection due to solar wind changes. However, ionospheric poleward moving density enhancements can also form in the absence of changes in the solar wind. This study examines how TOI and patch events that are not triggered by solar wind changes relate to magnetospheric processes, specifically substorms. Based on total electron content and Super Dual Auroral Radar Network (SuperDARN) observations, we find substorms that occur at the same time as TOIs are associated with sudden enhancements in dayside poleward flows during the substorm expansion phase. Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) observations also show enhanced field‐aligned currents (FACs) that extend into the dayside ionosphere during this period. We suggest that the global enhancement of FACs and convection during these substorms are the drivers of these TOIs by enhancing dayside convection and transporting high‐density lower‐latitude plasma into the polar cap. However, we also find that not all substorms are coincident with polar cap density enhancements. A superposed epoch study showed that the AL index for TOIs during substorms is not particularly stronger than substorms without TOIs, but epoch studies of AMPERE observations do show events with TOIs to have a higher total FAC on both the dayside and nightside. Our results show the importance of TOI formation during substorms when solar wind drivers are absent, and the importance of considering substorms in the global current system. This work also shows the need to incorporate substorms into models of high‐latitude global convection and currents. Key Points: Enhanced dayside flows without interplanetary magnetic field changes can occur during substormsDayside plasma flows during substorms can transport photoionized plasma into the polar capSubstorms impact dayside currents and patches despite being typically associated with the nightside [ABSTRACT FROM AUTHOR]

An understanding of ionospheric conductances is important for models of large‐scale dynamics in the Earth's magnetosphere. We parameterize height‐integrated Pedersen and Hall conductances in the ionosphere, derived from images of auroral emissions obtained by the Defense Meteorological Satellite Programme low‐altitude orbiting spacecraft, under different interplanetary and solar wind conditions. For the dayside, conductances are parameterized by interplanetary magnetic field clock angle and magnitude, and by season. These dayside conductances are compared to distributions of field‐aligned currents determined from measurements of the Active Magnetosphere and Planetary Electrodynamic Response Experiment. We use these currents to spatially determine a return flow region. We find that the return flow regions exhibit marginally larger conductances than those observed in the polar cap. Conductances in summer exceed those in winter for both the return flow and polar cap regions, on average by a factor of 1.2. On the nightside, we track changes in height‐integrated conductance across the Southern Hemisphere polar regions during an average substorm, following a substorm onset list derived from the SuperMAG database. Mean conductances peak approximately 0.75 hr after substorm onset, with maximum conductances seen in the 23 hr magnetic local time sector. Plain Language Summary: Low‐altitude spacecraft are able to build up images of aurora in the polar regions with low temporal resolution. Using a combination of these images taken in different wavebands, and after applying a model of atmospheric conditions, we can derive parameters such as ionospheric conductance, integrated along the line of sight from the spacecraft. Knowledge of conductances in the high‐latitude polar regions is important for our understanding of the large‐scale dynamics of the Earth's magnetosphere. We divide our results into two sections. The first section explores conductances in the dayside Northern Hemisphere under the effects of the incoming solar wind and interplanetary magnetic field. We compare these conductances to large‐scale patterns of ionospheric currents derived from a separate constellation of satellites. The currents can be used to define certain polar regions, and we compare conductances between these regions. The second section examines conductances under different phases of a substorm, when the magnetosphere is rearranged under the influence of particular driving conditions in the incoming solar wind. The behavior of the conductances is consistent with known patterns of substorm progression, and we show that peak conductances are seen half an hour after substorm onset. Key Points: The return flow region exhibits marginally larger conductances than those in the polar capPeak conductances occur at 23 hr magnetic local time within 1 hr after substorm onsetConductances remain elevated at 02 hr magnetic local time for a prolonged period after substorm onset [ABSTRACT FROM AUTHOR]