Earth's Magnetopause Research Articles

Cold Ionospheric Ions in the Magnetic Reconnection Outflow Region

AbstractMagnetosheath plasma usually determines properties of asymmetric magnetic reconnection at the subsolar region of Earth's magnetopause. However, cold plasma that originated from the ionosphere can also reach the magnetopause and modify the kinetic physics of asymmetric reconnection. We present a magnetopause crossing with high‐density (10–60 cm−3) cold ions and ongoing reconnection from the observation of the Magnetospheric Multiscale (MMS) spacecraft. The magnetopause crossing is estimated to be 300 ion inertial lengths south of the X line. Two distinct ion populations are observed on the magnetosheath edge of the ion jet. One population with high parallel velocities (200–300 km/s) is identified to be cold ion beams, and the other population is the magnetosheath ions. In the deHoffman‐Teller frame, the field‐aligned magnetosheath ions are Alfvénic and move toward the jet region, while the field‐aligned cold ion beams move toward the magnetosheath boundary layer, with much lower speeds. These cold ion beams are suggested to be from the cold ions entering the jet close to the X line. This is the first observation of the cold ionospheric ions in the reconnection outflow region, including the reconnection jet and the magnetosheath boundary layer.

Exact Vlasov‐Maxwell equilibria for asymmetric current sheets

AbstractThe NASA Magnetospheric Multiscale mission has made in situ diffusion region and kinetic‐scale resolution measurements of asymmetric magnetic reconnection for the first time, in the Earth's magnetopause. The principal theoretical tool currently used to model collisionless asymmetric reconnection is particle‐in‐cell simulations. Many particle‐in‐cell simulations of asymmetric collisionless reconnection start from an asymmetric Harris‐type magnetic field but with distribution functions that are not exact equilibrium solutions of the Vlasov equation. We present new and exact equilibrium solutions of the Vlasov‐Maxwell system that are self‐consistent with one‐dimensional asymmetric current sheets, with an asymmetric Harris‐type magnetic field profile, plus a constant nonzero guide field. The distribution functions can be represented as a combination of four shifted Maxwellian distribution functions. This equilibrium describes a magnetic field configuration with more freedom than the previously known exact solution and has different bulk flow properties.

Energy budget and mechanisms of cold ion heating in asymmetric magnetic reconnection

AbstractCold ions (few tens of eV) of ionospheric origin are commonly observed on the magnetospheric side of the Earth's dayside magnetopause. As a result, they can participate in magnetic reconnection, changing locally the reconnection rate and being accelerated and heated. We present four events where cold ion heating was observed by the Magnetospheric Multiscale mission, associated with the magnetospheric Hall E field region of magnetic reconnection. For two of the events the cold ion density was small compared to the magnetosheath density, and the cold ions were heated roughly to the same temperature as magnetosheath ions inside the exhaust. On the other hand, for the other two events the cold ion density was comparable to the magnetosheath density and the cold ion heating observed was significantly smaller. Magnetic reconnection converts magnetic energy into particle energy, and ion heating is known to dominate the energy partition. We find that at least 10–25% of the energy spent by reconnection into ion heating went into magnetospheric cold ion heating. The total energy budget for cold ions may be even higher when properly accounting for the heavier species, namely helium and oxygen. Large E field fluctuations are observed in this cold ion heating region, i.e., gradients and waves, that are likely the source of particle energization.

Nonlinear evolution of 3D whistler waves in space plasmas

The present model is proposed to study the nonlinear effects related to quasi-transverse 3D whistler waves in three different regions of space plasmas which are Earth's radiation belt, magnetopause, and solar wind plasma at 1 A.U. We have analysed the effect of considering the third dimension in the dynamics of whistler waves on Localization of whistler waves and associated magnetic field power spectra. Three dimensionally propagating oblique whistler waves get localized due to background density perturbation. This background density modulation is supposed to be originating due to the propagation of low frequency kinetic Alfven waves in the background. The ponderomotive nonlinearity originating due to high amplitude whistler waves has been taken into account to develop the model equations. Next, these coupled model equations have been solved numerically using the pseudo-spectral method. Simulation results are investigated to study the process of field localization and magnetic field power spectrum. The resulting magnetic field power spectrum is discussed in detail in view of their observation in all the three above mentioned regions of space plasmas.

Locating dayside magnetopause reconnection with exhaust ion distributions

AbstractMagnetic reconnection at Earth's dayside magnetopause is essential to magnetospheric dynamics. Determining where reconnection takes place is important to understanding the processes involved, and many questions about reconnection location remain unanswered. We present a method for locating the magnetic reconnection X line at Earth's dayside magnetopause under southward interplanetary magnetic field conditions using only ion velocity distribution measurements. Particle‐in‐cell simulations based on Cluster magnetopause crossings produce ion velocity distributions that we propagate through a model magnetosphere, allowing us to calculate the field‐aligned distance between an exhaust observation and its associated reconnection line. We demonstrate this procedure for two events and compare our results with those of the Maximum Magnetic Shear Model; we find good agreement with its results and show that when our method is applicable, it produces more precise locations than the Maximum Shear Model.

Electron diffusion region during magnetopause reconnection with an intermediate guide field: Magnetospheric multiscale observations

AbstractAn electron diffusion region (EDR) in magnetic reconnection with a guide magnetic field approximately 0.2 times the reconnecting component is encountered by the four Magnetospheric Multiscale spacecraft at the Earth's magnetopause. The distinct substructures in the EDR on both sides of the reconnecting current sheet are visualized with electron distribution functions that are 2 orders of magnitude higher cadence than ever achieved to enable the following new findings: (1) Motion of the demagnetized electrons plays an important role to sustain the reconnection current and contributes to the dissipation due to the nonideal electric field, (2) the finite guide field dominates over the Hall magnetic field in an electron‐scale region in the exhaust and modifies the electron flow dynamics in the EDR, (3) the reconnection current is in part carried by inflowing field‐aligned electrons in the magnetosphere part of the EDR, and (4) the reconnection electric field measured by multiple spacecraft is uniform over at least eight electron skin depths and corresponds to a reconnection rate of approximately 0.1. The observations establish the first look at the structure of the EDR under a weak but not negligible guide field.

Drift waves, intense parallel electric fields, and turbulence associated with asymmetric magnetic reconnection at the magnetopause

AbstractObservations of magnetic reconnection at Earth's magnetopause often display asymmetric structures that are accompanied by strong magnetic field (B) fluctuations and large‐amplitude parallel electric fields (E||). The B turbulence is most intense at frequencies above the ion cyclotron frequency and below the lower hybrid frequency. The B fluctuations are consistent with a thin, oscillating current sheet that is corrugated along the electron flow direction (along the X line), which is a type of electromagnetic drift wave. Near the X line, electron flow is primarily due to a Hall electric field, which diverts ion flow in asymmetric reconnection and accompanies the instability. Importantly, the drift waves appear to drive strong parallel currents which, in turn, generate large‐amplitude (~100 mV/m) E|| in the form of nonlinear waves and structures. These observations suggest that turbulence may be common in asymmetric reconnection, penetrate into the electron diffusion region, and possibly influence the magnetic reconnection process.

Comparative study of three reconnection X line models at the Earth's dayside magnetopause using in situ observations

AbstractThis work examines the large‐scale aspects of magnetic field reconnection at the Earth's dayside magnetopause. We use two sets of reconnection events, which are identified mostly by the in situ detection of accelerated and Alfvénic plasma flows. We intercompare three analytical models that predict the reconnection X line location and orientation, namely, the Trattner et al. (2007) and Swisdak and Drake (2007) models and also a modified version of the component merging model. In the first set of reconnection observations, we show three fortuitous, quasi‐simultaneous dayside magnetopause crossing events where two widely separated spacecraft detect reconnection signatures, and the X line location and orientation can be inferred from the observations. We compare X line model predictions to those inferred from observations. These three reconnection events indicate the presence of an extended (>7 Earth radii in length), component‐type reconnection X line on Earth's dayside magnetopause connecting and structuring the reconnection signatures at locations far apart. In the second set of reconnection events, we analyze the X line models' performance in predicting the observed reconnection outflow direction, i.e., its north‐south and/or east‐west senses, in a total of 75 single, rather than multiple and quasi‐simultaneous, magnetopause crossing events, where reconnection‐associated plasma flows were clearly present. We found that the Swisdak and Drake's (2007) X line model performs slightly better, albeit not statistically significant, when predicting both accelerated plasma flow north‐south and east‐west components in 73% and 53% of the cases, respectively, as compared to the Trattner et al. (2007) model (70% north‐south and 42% east‐west) and the modified component merging model (66% north‐south and 50% east‐west).

Occurrence frequency and location of magnetic islands at the dayside magnetopause

AbstractSimulations of magnetic reconnection often predict the formation of magnetic islands forming and propagating out through the reconnection exhaust region. However, to date, only a few observations of magnetic islands at the Earth's magnetopause have been identified and analyzed. Crossings near reconnection sites at the dayside magnetopause by Cluster from 2001 to 2009 are examined in a systematic effort to determine the frequency and location of magnetic islands. Using the maximum magnetic shear model as a guide for the distance to the reconnection site, 47 magnetopause crossings within 3 RE of the predicted reconnection site are examined. The occurrence of island structures in the reconnection exhaust is investigated in the context of how the type of reconnection, antiparallel or component, affects the frequency of formation of the islands. For Cluster magnetopause crossings confirmed to be near reconnection sites, island signatures, primarily bidirectional streaming electrons in the magnetosheath boundary layer, are seen for approximately 85% of the crossings and are common features for both antiparallel and component reconnection.

Enhanced electron mixing and heating in 3‐D asymmetric reconnection at the Earth's magnetopause

AbstractElectron heating and mixing during asymmetric reconnection are studied with a 3‐D kinetic simulation that matches plasma parameters from Magnetospheric Multiscale (MMS) spacecraft observations of a magnetopause diffusion region. The mixing and heating are strongly enhanced across the magnetospheric separatrix compared to a 2‐D simulation. The transport of particles across the separatrix in 3‐D is attributed to lower hybrid drift turbulence excited at the steep density gradient near the magnetopause. In the 3‐D simulation (and not the 2‐D simulation), the electron temperature parallel to the magnetic field within the mixing layer is significantly higher than its upstream value in agreement with the MMS observations.

Magnetospheric Multiscale mission observations of the outer electron diffusion region

AbstractThis paper presents Magnetospheric Multiscale mission (MMS) observations of the exhaust region in the vicinity of the central reconnection site in Earth's magnetopause current sheet. High‐time‐resolution measurements of field and particle distributions enable us to explore the fine structure of the diffusion region near the X line. Ions are decoupled from the magnetic field throughout the entire current sheet crossing. Electron jets flow downstream from the X line at speeds greater than the E × B drift velocity. At/around the magnetospheric separatrix, large‐amplitude electric fields containing field‐aligned components accelerate electrons along the magnetic field toward the X line. Near the neutral sheet, crescent‐shaped electron distributions appear coincident with (1) an out‐of‐plane electric field whose polarity is opposite to that of the reconnection electric field and (2) the energy transfer from bulk kinetic to field energy. The observations indicate that MMS passed through the edge of an elongated electron diffusion region (EDR) or the outer EDR in the exhaust region.

Pressure balance across the magnetopause during the solar wind event on 5 June 1998

A three-dimensional adaptive magnetohydrodynamic (MHD) model, SWMF, is used to simulate the interaction between the solar wind and magnetosphere for a particular event on 5 June 1998, and the simulated results of this event is used to investigate the balances of the dynamic, thermal and magnetic pressure along the Sun-Earth line for the different conditions of interplanetary magnetic field (IMF). The conclusions are as follows: (1) outside the magnetopause, the total and thermal pressures are clearly correlated with upstream solar wind dynamic pressure and increase with the solar wind dynamic pressure. In contrast, the magnetic pressure decreases with the increasing intensity of the southward IMF due to the magnetic reconnection and is enhanced with the increasing intensity of the northward IMF due to the magnetic accumulation. It is similar to the variation inside the magnetopause; (2) the solar wind pressure coefficient is larger for the northward IMF than that for the southward IMF, but it has no obvious dependence on the upstream solar wind dynamic pressure; (3) the magnetic field compression ratio just inside the magnetopause is larger and more stable in northward IMF than in southward IMF; and (4) along the Sun-Earth line, the thermal pressure is dominant on the magnetopause in southward IMF, while the magnetic pressure is dominant on the magnetopause in northward IMF. The magnetic reconnection easily occurs for southward IMF, and results in magnetic pressure decreasing just inside the magnetopause. This factor plays a crucial role in the earthward displacement of the Earth's magnetopause for southward IMF. And the increasing of thermal pressure just outside the magnetopause also has contribution to this displacement, especially for lower IMF.

On the origin of the crescent‐shaped distributions observed by MMS at the magnetopause

AbstractMMS observations recently confirmed that crescent‐shaped electron velocity distributions in the plane perpendicular to the magnetic field occur in the electron diffusion region near reconnection sites at Earth's magnetopause. In this paper, we reexamine the origin of the crescent‐shaped distributions in the light of our new finding that ions and electrons are drifting in opposite directions when displayed in magnetopause boundary‐normal coordinates. Therefore, E × B drifts cannot cause the crescent shapes. We performed a high‐resolution multiscale simulation capturing subelectron skin‐depth scales. The results suggest that the crescent‐shaped distributions are caused by meandering orbits without necessarily requiring any additional processes found at the magnetopause such as the highly asymmetric magnetopause ambipolar electric field. We use an adiabatic Hamiltonian model of particle motion to confirm that conservation of canonical momentum in the presence of magnetic field gradients causes the formation of crescent shapes without invoking asymmetries or the presence of an E × B drift. An important consequence of this finding is that we expect crescent‐shaped distributions also to be observed in the magnetotail, a prediction that MMS will soon be able to test.

On the occurrence of magnetic reconnection equatorward of the cusps at the Earth's magnetopause during northward IMF conditions

AbstractMagnetic reconnection changes the topology of magnetic field lines. This process is most readily observable with in situ instrumentation at the Earth's magnetopause as it creates open magnetic field lines to allow energy and momentum flux to flow from the solar wind to the magnetosphere. Most models use the direction of the interplanetary magnetic field (IMF) to determine the location of these magnetopause entry points, known as reconnection lines. Dayside locations of magnetic reconnection equatorward of the cusps are generally found during sustained intervals of southward IMF, while high‐latitude region regions poleward of the cusps are observed for northward IMF conditions. In this study we discuss Double Star magnetopause crossings and a conjunction with a Polar cusp crossing during northward IMF conditions with a dominant IMF BY component. During all seven dayside magnetopause crossings, Double Star detected switching ion beams, a known signature for the presence of reconnection lines. In addition, Polar observed a cusp ion‐energy dispersion profile typical for a dayside equatorial reconnection line. Using the cutoff velocities for the precipitating and mirrored ion beams in the cusp, the distance to the reconnection site is calculated, and this distance is traced back to the magnetopause, to the vicinity of the Double Star satellite locations. Our analysis shows that, for this case, the predicted line of maximum magnetic shear also coincides with that dayside reconnection location.

Nowcasting and forecasting of the magnetopause and bow shock—A status update

AbstractThere has long been interest in knowing the shape and location of the Earth's magnetopause and of the standing fast‐mode bow shock upstream of the Earth's magnetosphere. This quest for knowledge spans both the research and operations arenas. Pertinent to the latter, nowcasting and near‐term forecasting are important for determining the extent to which the magnetosphere is compressed or expanded due to the influence of the solar wind bulk plasma and fields and the coupling to other magnetosphere‐ionosphere processes with possible effects on assets. This article provides an update to a previous article on the same topic published 15 years earlier, with focus on studies that have been conducted, the current status of nowcasting and forecasting of geophysical boundaries, and future endeavors.

A new methodology for the development of high‐latitude ionospheric climatologies and empirical models

AbstractMany empirical models and climatologies of high‐latitude ionospheric processes, such as convection, have been developed over the last 40 years. One common feature in the development of these models is that measurements from different times are combined and averaged on fixed coordinate grids. This methodology ignores the reality that high‐latitude ionospheric features are organized relative to the location of the ionospheric footprint of the boundary between open and closed geomagnetic field lines (OCB). This boundary is in continual motion, and the polar cap that it encloses is continually expanding and contracting in response to changes in the rates of magnetic reconnection at the Earth's magnetopause and in the magnetotail. As a consequence, models that are developed by combining and averaging data in fixed coordinate grids heavily smooth the variations that occur near the boundary location. Here we propose that the development of future models should consider the location of the OCB in order to more accurately model the variations in this region. We present a methodology which involves identifying the OCB from spacecraft auroral images and then organizing measurements in a grid where the bins are placed relative to the OCB location. We demonstrate the plausibility of this methodology using ionospheric vorticity measurements made by the Super Dual Auroral Radar Network radars and OCB measurements from the IMAGE spacecraft FUV auroral imagers. This demonstration shows that this new methodology results in sharpening and clarifying features of climatological maps near the OCB location. We discuss the potential impact of this methodology on space weather applications.

Three‐scale structure of diffusion region in the presence of cold ions

AbstractKinetic simulations and spacecraft observations typically display the two‐scale structure of collisionless diffusion region (DR), with electron and ion demagnetization scales governing the spatial extent of the DR. Recent in situ observations of the nightside magnetosphere, as well as investigation of magnetic reconnection events at the Earth's magnetopause, discovered the presence of a population of cold (tens of eV) ions of ionospheric origin. We present two‐dimensional particle‐in‐cell simulations of collisionless magnetic reconnection in multicomponent plasma with ions consisting of hot and cold populations. We show that a new cold ion diffusion region scale is introduced in between that of hot ions and electrons. Demagnetization scale of cold ion population is several times (∼4–8) larger than the initial cold ion gyroradius. Cold ions are accelerated and thermalized during magnetic reconnection and form ion beams moving with velocities close to the Alfvén velocity.

Optimized merging of search coil and fluxgate data for MMS

Abstract. The Magnetospheric Multiscale mission (MMS) targets the characterization of fine-scale current structures in the Earth's tail and magnetopause. The high speed of these structures, when traversing one of the MMS spacecraft, creates magnetic field signatures that cross the sensitive frequency bands of both search coil and fluxgate magnetometers. Higher data quality for analysis of these events can be achieved by combining data from both instrument types and using the frequency bands with best sensitivity and signal-to-noise ratio from both sensors. This can be achieved by a model-based frequency compensation approach which requires the precise knowledge of instrument gain and phase properties. We discuss relevant aspects of the instrument design and the ground calibration activities, describe the model development and explain the application on in-flight data. Finally, we show the precision of this method by comparison of in-flight data. It confirms unity gain and a time difference of less than 100 µs between the different magnetometer instruments.

Observations of turbulence in a Kelvin‐Helmholtz event on 8 September 2015 by the Magnetospheric Multiscale mission

AbstractSpatial and high‐time‐resolution properties of the velocities, magnetic field, and 3‐D electric field within plasma turbulence are examined observationally using data from the Magnetospheric Multiscale mission. Observations from a Kelvin‐Helmholtz instability (KHI) on the Earth's magnetopause are examined, which both provides a series of repeatable intervals to analyze, giving better statistics, and provides a first look at the properties of turbulence in the KHI. For the first time direct observations of both the high‐frequency ion and electron velocity spectra are examined, showing differing ion and electron behavior at kinetic scales. Temporal spectra exhibit power law behavior with changes in slope near the ion gyrofrequency and lower hybrid frequency. The work provides the first observational evidence for turbulent intermittency and anisotropy consistent with quasi two‐dimensional turbulence in association with the KHI. The behavior of kinetic‐scale intermittency is found to have differences from previous studies of solar wind turbulence, leading to novel insights on the turbulent dynamics in the KHI.

The permeability of the magnetopause to a multispecies substorm injection of energetic particles

AbstractLeakage of ions from the magnetosphere into the magnetosheath remains an important topic in understanding the plasma physics of Earth's magnetopause and the interaction of the solar wind with the magnetosphere. Here using sophisticated instrumentation from two spacecraft (Radiation Belt Storm Probes Ion Composition Experiment on the Van Allen Probes and Energetic Ion Spectrometer on the Magnetospheric Multiscale) spaced uniquely near and outside the dayside magnetopause, we are able to determine the escape mechanisms for large gyroradii oxygen ions and much smaller gyroradii hydrogen and helium ions. The oxygen ions are entrained on the magnetosphere boundary, while the hydrogen and helium ions appear to escape along reconnected field lines. These results have important implications for not only Earth's magnetosphere but also other solar system magnetospheres.