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Electromagnetic Ion Cyclotron (EMIC) waves mediate energy transfer from the solar wind to the magnetosphere, relativistic electron precipitation, or thermalization of the ring current population, to name a few. How these processes take place depends on the wave properties, such as the wavevector and polarization. However, inferring the wavevector from in‐situ measurements is problematic since one needs to disentangle spatial and time variations. Using 8 years of Magnetospheric Multiscale (MMS) mission observations in the dayside magnetosphere, we present an algorithm to detect proton‐band EMIC waves in the Earth's dayside magnetosphere, and find that they are present roughly 15% of the time. Their normalized frequency presents a dawn‐dusk asymmetry, with waves in the dawn flank magnetosphere having larger frequency than in the dusk, subsolar, and dawn near subsolar region. It is shown that the observations are unstable to the ion cyclotron instability. We obtain the wave polarization and wavevector by comparing Single Value Decomposition and Ampere methods. We observe that for most waves the perpendicular wavenumber (k⊥) is larger than the inverse of the proton gyroradius (ρi), that is, k⊥ρi> 1, while the parallel wavenumber is smaller than the inverse of the ion gyroradius, that is, k‖ρi< 1. Left‐hand polarized waves are associated with small wave normal angles (θBk< 30°), while linearly polarized waves are associated with large wave normal angles (θBk> 30°). This work constitutes, to our knowledge, the first attempt to statistically infer the full wavevector of proton‐band EMIC waves observed in the outer magnetosphere. We conduct a statistical analysis of proton‐band Electromagnetic Ion Cyclotron (EMIC) waves in dayside magnetosphere using 8 years of Magnetospheric Multiscale data and measure full wavevectorsThe normalized frequency of EMIC waves presents a dawn‐dusk asymmetryThe perpendicular wavevector is, for most of the waves, larger than the inverse of proton gyroradius and ion inertial lengths We conduct a statistical analysis of proton‐band Electromagnetic Ion Cyclotron (EMIC) waves in dayside magnetosphere using 8 years of Magnetospheric Multiscale data and measure full wavevectors The normalized frequency of EMIC waves presents a dawn‐dusk asymmetry The perpendicular wavevector is, for most of the waves, larger than the inverse of proton gyroradius and ion inertial lengths

The warm plasma cloak is a source of magnetospheric plasma that contain significant O+. When the O+density in the magnetosphere near the magnetopause is >0.2 cm‐3and the H+density is <1.5 cm‐3, then O+dominates the magnetospheric ion mass density by more than a factor of 2. A survey is conducted of such O+‐rich warm plasma cloak intervals and their effect on reconnection at the Earth's magnetopause. The survey uses data from the Magnetospheric Multiscale mission (MMS) and the results are compared and combined with a previous survey of the warm plasma cloak. Overall, the warm plasma cloak and the O+‐rich warm plasma cloak reduce the magnetopause reconnection rate by >20% due to mass‐loading only about 2% to 4% of the time. However, during geomagnetic storms, O+dominates the mass density of the warm plasma cloak and these mass densities are very high. Therefore, a separate study is conducted to determine the effect of the warm plasma cloak on magnetopause reconnection during geomagnetically disturbed times. This study shows that the warm plasma cloak reduces the reconnection rate significantly about 25% of the time during disturbed conditions. The magnetospheric warm plasma cloak is O+‐rich during geomagnetically active timesThe warm plasma cloak reduces the magnetic reconnection rate at the magnetopause ~2‐4% of the timeDuring geomagnetic storms, the O+‐rich warm plasma cloak reduces the reconnection rate by >20% sometime during 25% of the storms

At the Earth's magnetopause, a low‐energy ion population of ionospheric origin is commonly observed at the magnetospheric side. In this work we use a 2‐D fully kinetic simulation to identify several original signatures related to the dynamics of cold ions involved in magnetic reconnection at the asymmetric dayside magnetopause. We identify several original signatures of the cold ions dynamics driven by the development of magnetic reconnection at the asymmetric dayside magnetopause. We find that cold ions tend to rarefy in the diffusion region, while their density is enhanced as a result of compression along magnetospheric separatrices. We also observe the formation of crescent‐shaped cold ion distribution functions along the separatrices in the near‐exhaust region, and we present an analytical model to explain this signature. Finally, we give evidence of a localized parallel heating of cold ions. These signatures should be detected with the magnetospheric multiscale mission high‐resolution observations. We make PIC simulations of asymmetric magnetic reconnection including both hot and cold magnetospheric ion populationsWe study cold ions' specific signatures along magnetospheric separatricesWe develop a theoretical model for the crescent‐shaped distribution functions of cold ions observed along the separatrices separatrix

We explore the structure of the magnetopause using a crossing observed by the Magnetospheric Multiscale (MMS) spacecraft on 16 October 2015. Several methods (minimum variance analysis, BV method, and constant velocity analysis) are first applied to compute the normal to the magnetopause considered as a whole. The different results obtained are not identical, and we show that the whole boundary is not stationary and not planar, so that basic assumptions of these methods are not well satisfied. We then analyze more finely the internal structure for investigating the departures from planarity. Using the basic mathematical definition of what is a one‐dimensional physical problem, we introduce a new single spacecraft method, called LNA (local normal analysis) for determining the varying normal, and we compare the results so obtained with those coming from the multispacecraft minimum directional derivative (MDD) tool developed by Shi et al. (2005). This last method gives the dimensionality of the magnetic variations from multipoint measurements and also allows estimating the direction of the local normal when the variations are locally 1‐D. This study shows that the magnetopause does include approximate one‐dimensional substructures but also two‐ and three‐dimensional structures. It also shows that the dimensionality of the magnetic variations can differ from the variations of other fields so that, at some places, the magnetic field can have a 1‐D structure although all the plasma variations do not verify the properties of a global one‐dimensional problem. A generalization of the MDD tool is proposed. The internal structure of the magnetopause is investigated, using new analysis tools allowed by the high‐performance MMS instrumentsIn a case study, the observed boundary is shown to be nonplanar and nonstationaryQuasi 1‐D thin sub layers are identified separated by regions that are mainly 2‐D

At the dayside magnetopause, the magnetosphere often contains a cold ion population of ionospheric origin. This population is not always detectable by particle instruments due to its low energy, despite having an important contribution to the total ion density and therefore an impact on key plasma processes such as magnetic reconnection. The exact role and implications of this low‐temperature population are still not well known and has not been addressed with numerical simulation before. We present 2‐D fully kinetic simulations of asymmetric magnetic reconnection with and without a cold ion population on the magnetospheric side of the magnetopause, but sharing the same total density, temperature, and magnetic field profiles. The comparison of the simulations suggests that cold ions directly impact signatures recently suggested as a good marker of the X line region: the Larmor electric field. Our simulations reveal that this electric field, initially present all along the magnetospheric separatrix, is related to the bounce of magnetosheath ions at the magnetopause magnetic field reversal through Speiser‐like orbits. Once reconnection widens the current sheet away from the X line, the bouncing stops and the electric field signature remains solely confined near the X line. When cold ions are present, however, their very low temperature enables them to E× Bdrift in the electric field structure. If their density is large enough compared to other ions, their contribution to the momentum equation is capable of maintaining the signature away from the X line. This effect must be taken into account when analyzing in situ spacecraft measurements. PIC simulations of asymmetric magnetic reconnection including both hot and cold magnetospheric ion populationsCold ions sustain electromagnetic structures at scales lower than the hot ion Larmor radius in magnetic reconnectionExplanation of the mechanism leading to an extended Larmor electric field region in the presence of cold ions

The Magnetospheric Multiscale mission has demonstrated the frequent presence of reconnection exhausts at thin current sheets within Kelvin‐Helmholtz (KH) waves at the flank magnetopause. Motivated by these recent observations, we performed a statistical analysis of the boundary layers on the magnetosheath side of all KH current sheets on 8 September 2015. We show 86% consistency between the exhaust flows and particle leakage in the magnetosheath boundary layers but further highlight the very frequent presence of additional boundary layer signatures that do not come from the locally observed reconnection exhausts. These additional electron and ion boundary layers, of various durations and at various positions with respect to the leading and trailing boundaries of the KH waves, signal connections to reconnection sites at other locations. Based on the directionality and extent of these layers, we provide an interpretation whereby complex magnetic topologies can arise within KH waves from the combination of reconnection in the equatorial plane and at midlatitudes in the Southern and Northern Hemispheres, where additional reconnection sites are expected to be triggered by the three‐dimensional field lines interweaving induced by the KH waves at the flanks (owing to differential flow and magnetic field shear with latitude). The present event demonstrates that the three‐dimensional development of KH waves can induce plasma entry (through reconnection at both midlatitude and equatorial regions) already sunward of the terminator where the instability remains in its linear stage. Statistical study of magnetosheath boundary layers at Kelvin‐Helmholtz wave boundariesParticle leakage statistics consistent with local reconnection exhausts within KH wavesAdditional signatures suggest complex topologies with reconnection sites at higher latitudes

In‐situ spacecraft missions are powerful assets to study processes that occur in space plasmas. One of their main limitations, however, is extrapolating such local measurements to the global scales of the system. To overcome this problem at least partially, multi‐point measurements can be used. There are several multi‐spacecraft missions currently operating in the Earth's magnetosphere, and the simultaneous use of the data collected by them provides new insights into the large‐scale properties and evolution of magnetospheric plasma processes. In this work, we focus on studying the Earth's magnetopause (MP) using a conjunction between the Magnetospheric Multiscale and Cluster fleets, when both missions skimmed the MP for several hours at distant locations during radial interplanetary magnetic field (IMF) conditions. The observed MP positions as a function of the evolving solar wind conditions are compared to model predictions of the MP. We observe an inflation of the magnetosphere (∼0.7 RE), consistent with magnetosheath pressure decrease during radial IMF conditions, which is less pronounced on the flank (<0.2 RE). There is observational evidence of magnetic reconnection in the subsolar region for the whole encounter, and in the dusk flank for the last portion of the encounter, suggesting that reconnection was extending more than 15 RE. However, reconnection jets were not always observed, suggesting that reconnection was patchy, intermittent or both. Shear flows reduce the reconnection rate up to ∼30% in the dusk flank according to predictions, and the plasma βenhancement in the magnetosheath during radial IMF favors reconnection suppression by the diamagnetic drift. Simultaneous observations of the equatorial subsolar magnetopause and dusk flank during time‐extended radial interplanetary magnetic fieldThe magnetosphere enlarges ∼0.7 REin the subsolar region but <0.2 REin the flank due to the reduced pressure exerted by the solar windSimultaneous reconnection evidence in the subsolar and flank regions more than 15 REapart is observed during part of the encounter Simultaneous observations of the equatorial subsolar magnetopause and dusk flank during time‐extended radial interplanetary magnetic field The magnetosphere enlarges ∼0.7 REin the subsolar region but <0.2 REin the flank due to the reduced pressure exerted by the solar wind Simultaneous reconnection evidence in the subsolar and flank regions more than 15 REapart is observed during part of the encounter

We investigate both large‐ and small‐scale properties of a Kelvin‐Helmholtz (KH) event at the dusk flank magnetopause using Magnetospheric Multiscale observations on 8 September 2015. We first use two types of 3‐D simulations (global and local) to demonstrate that Magnetospheric Multiscale is close to the most KH unstable region, and so the occurrence of vortex‐induced reconnection may be expected. Because they produce low‐shear current sheets, KH vortices constitute a perfect laboratory to investigate magnetic reconnection with large guide field and low asymmetry. Recent works suggest that magnetic reconnection may be suppressed when a current sheet combines large guide field and pressure gradient (which induces a diamagnetic drift). We thus perform a statistical analysis of high‐resolution data for the 69 KH‐induced low‐shear magnetic reconnection events observed on that day. We find that the suppression mechanism is not at work for most of the observed reconnecting current sheets, as predicted, but we also find that almost all nonreconnecting current sheets should be reconnecting according to this model. This confirms the fact that the model provides a necessary but not sufficient condition for reconnection to occur. Finally, based on the same data set, we study the latitudinal distribution of these magnetic reconnection events combined with global magnetospheric modeling. We find that reconnection associated with KH vortices occurs over a significant range of latitudes at the flank magnetopause. It is not confined to the plane where the growth rate is maximum, in agreement with recent 3‐D simulations. We use two types of global simulations to locate the KH unstable region at the magnetopause and determine its dynamicsWe test the diamagnetic‐drift reconnection suppression condition over 69 very low shear current sheets and find an overall agreementWe report the broad latitudinal extent of magnetic reconnection locations triggered by KH vortices, consistent with bifluid 3‐D simulations