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Fundamental processes in plasmas act to convert energies into different forms, for example, electromagnetic, kinetic and thermal. Direct derivation from the Vlasov‐Maxwell equation yields sets of equations that describe the temporal evolution of magnetic, kinetic and internal energies in either the monofluid or multifluid frameworks. In this work, we focus on the main terms affecting the changes in kinetic energy. These are pressure‐gradient‐related terms and electromagnetic terms. The former account for plasma acceleration/deceleration from a pressure gradient, while the latter from an electric field. Although limited spatial and temporal deviations are expected, a statistical balance between these terms is fundamental to ensure the overall conservation of energy and momentum. We use in‐situ observations from the Magnetospheric MultiScale (MMS) mission to study the relationship between these terms. We perform a statistical analysis of those parameters in the context of magnetic reconnection by focusing on small‐scale Electron Diffusion Regions and large‐scale Flux Transfer Events. The analysis reveals a correlation between the two terms in the monofluid force balance, and in the ion force and energy balance. However, the expected relationship cannot be verified from electron measurements. Generally, the pressure‐gradient‐related terms are smaller than their electromagnetic counterparts. We perform an error analysis to quantify the expected underestimation of gradient values as a function of the spacecraft separation compared to the gradient scale. Our findings highlight that MMS is capable of capturing energy and force balance for the ion fluid, but that care should be taken for energy conversion terms based on electron pressure gradients. We analyze energy and force balance in the monofluid and multifluid frameworks using MMS dataWe show that MMS is able to capture energy and force balance in the monofluid and ion fluid frameworksThe ratio of spacecraft separation to local gradient scale limits the proper estimation of electron pressure gradient terms We analyze energy and force balance in the monofluid and multifluid frameworks using MMS data We show that MMS is able to capture energy and force balance in the monofluid and ion fluid frameworks The ratio of spacecraft separation to local gradient scale limits the proper estimation of electron pressure gradient terms

While double ion populations, with a cold population originating from the solar wind and a hotter one from the magnetosphere, are frequently observed in the low‐latitude boundary layers at the Earth's magnetopause, similar double electron populations are observed less often. A preliminary study of magnetopause crossings characteristics was used to determine the typical locations and energy spectra of ion and electron populations near the magnetopause. Then, we set up an automated detection algorithm for identifying regions with concomitant double populations in both ion and electron energy spectra. The statistical study was carried out on 7 years of Time History of Events and Macroscale Interactions during Substorms particle data, to determine the interplanetary magnetic field conditions in the upstream solar wind for these double populations. The results suggest that such concomitant ion and electron double population boundary layers form preferentially in the subsolar region and under northward interplanetary magnetic field but with a significant BYcomponent. We interpret this finding as the result of reconnection of the same magnetosheath field line in both hemispheres with at least one end reconnecting in its hemisphere at lower latitude with a closed magnetospheric field line that already contains a hot electron source. Frequent observation of concomitant double electron and ion population in Earth's boundary layersStatistical analyses show that double populations in both ions and electrons occur mainly at dayside during northward IMF with a large ByResult of reconnection between a magnetosheath field line and a magnetospheric field lines already containing hot electrons

Four‐spacecraft missions are probing the Earth's magnetospheric environment with high potential for revealing spatial and temporal scales of a variety of in situ phenomena. The techniques allowed by these four spacecraft include the calculation of vorticity and the magnetic curvature analysis (MCA), both of which have been used in the study of various plasma structures. Motivated by curved magnetic field and vortical structures induced by Kelvin‐ Helmholtz (KH) waves, we investigate the robustness of the MCA and vorticity techniques when increasing (regular) tetrahedron sizes, to interpret real data. Here for the first time, we test both techniques on a 2.5‐D MHD simulation of KH waves at the magnetopause. We investigate, in particular, the curvature and flow vorticity across KH vortices and produce time series for static spacecraft in the boundary layers. The combined results of magnetic curvature and vorticity further help us to understand the development of KH waves. In particular, first, in the trailing edge, the magnetic curvature across the magnetopause points in opposite directions, in the wave propagation direction on the magnetosheath side and against it on the magnetospheric side. Second, the existence of a “turnover layer” in the magnetospheric side, defined by negative vorticity for the duskside magnetopause, which persists in the saturation phase, is reminiscent of roll‐up history. We found significant variations in the MCA measures depending on the size of the tetrahedron. This study lends support for cross‐scale observations to better understand the nature of curvature and its role in plasma phenomena. Varying tetrahedron sizes are used to study magnetic curvature and vorticity analyses in a MHD simulation of KH waves at the magnetopauseIn the trailing edge of the KH wave, the magnetic curvature across the magnetopause points in opposite directionsThe development of negative vorticity on the dusk magnetospheric side is contrary to expected and persists in the saturation phase

Simulations and observations of collisionless shocks have shown that deviations of the nominal local shock normal orientation, that is, surface waves or ripples, are expected to propagate in the ramp and overshoot of quasi‐perpendicular shocks. Here we identify signatures of a surface ripple propagating during a crossing of Earth's marginally quasi‐parallel (θBn∼45∘) or quasi‐parallel bow shock on 27 November 2015 06:01:44 UTC by the Magnetospheric Multiscale (MMS) mission and determine the ripple's properties using multispacecraft methods. Using two‐dimensional hybrid simulations, we confirm that surface ripples are a feature of marginally quasi‐parallel and quasi‐parallel shocks under the observed solar wind conditions. In addition, since these marginally quasi‐parallel and quasi‐parallel shocks are expected to undergo a cyclic reformation of the shock front, we discuss the impact of multiple sources of nonstationarity on shock structure. Importantly, ripples are shown to be transient phenomena, developing faster than an ion gyroperiod and only during the period of the reformation cycle when a newly developed shock ramp is unaffected by turbulence in the foot. We conclude that the change in properties of the ripple observed by MMS is consistent with the reformation of the shock front over a time scale of an ion gyroperiod. A surface ripple has been observed by MMS at Earth's quasi‐parallel bow shockHybrid simulations show that these ripples are transients modulated by shock reformationChanges in the observed ripple are consistent with reformation processes

Cold 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 Efield 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 Efield fluctuations are observed in this cold ion heating region, i.e., gradients and waves, that are likely the source of particle energization. The magnetic field of Earth creates a natural shield that isolates and protects us from the particles and fields coming from our star, the Sun. This natural shield is called the magnetosphere and is filled by plasma. The particles coming from the Sun form another plasma called the solar wind and are usually deviated around the magnetosphere. However, under certain circumstances these two plasmas can reconnect (magnetic reconnection), and part of the energy and mass of the two plasmas is interchanged. Magnetic reconnection is the driver of storms and substorms inside the magnetosphere. In this work, we investigate what occurs to particles of very low energy (cold ions) of ionospheric origin when they reach the reconnecting boundary of the magnetosphere. It is found that they are energized and take an important part of the energy spent in reconnecting the plasmas. The plasma boundary develops spatial structures and emits waves that are able to heat the cold ions. Once heated, these cold ions irreversibly will escape the Earth's magnetosphere to never come back to Earth. Observations of cold ion heating associated with magnetic reconnectionLarge electric field fluctuations (gradients and waves) in association with the cold ion heatingAt least 10 to 25% of the energy that goes into heating is used to heat cold ions

We have studied two events in which relatively superdense plasma appeared near geosynchronous orbit after the interplanetary magnetic field (IMF) changed from a long period of northward condition to southward condition. We used the magnetosphere MHD model Open Geospace General Circulation Model to simulate these two events and compared the simulation results with observations. The observations and the simulation results show that an extended period of northward IMF caused a cold dense plasma sheet (CDPS) to form in the near tail, and a subsequent southward IMF caused a highly stretched near‐tail dipole field, which enhanced the earthward convection and eventually set off reconnection. Some of the cold dense plasma was thus consequently transported to geosynchronous orbit and formed a superdense plasma sheet (SDPS). CDPS is the main source of SDPS in its early stage after the IMF southward turning. An event with both CDPS and SDPS observations is reproduced in an OpenGGCM simulationCDPS is the main source of SDPS in its early stageCDPS and a subsequent IMF southward turning causes the formation of SDPS

We present the design study of an electrostatic analyzer that permits combined high temporal, energy, and angular resolution measurements of solar wind ions. The requirements for high temporal, energy, and angular resolutions, combined with the need for sufficient counting statistics, lead to an electrostatic analyzer with large radius (140 mm) and large geometric factor. The resulting high count rates require the use of channel electron multipliers (CEMs), instead of microchannel plates, to avoid saturation. The large radius further permits the placement of 32 CEM detectors at the analyzer focal plane, thereby providing very high angular resolution in azimuth (1.5°). Electrostatic simulations were performed to define the analyzer geometric factor, energy resolution, analyzer constant (K), elevation response, etc. Simulations were also performed to define the geometry of the deflectors and collimator that are used to provide the proper energy resolution, field of view, and angular resolution (1.5°) in elevation as well (the total field of view of the design is ±24° × ±24°). We show how this design permits unprecedented measurements of the fine structure of the solar wind proton beam and other important features such as temperature anisotropy. This design is used for the Cold Solar Wind instrument of the medium‐class Turbulent Heating ObserveR mission, currently in phase A at the European Space Agency. These unprecedented measurement capabilities are in accordance with and even beyond the requirements of the mission. A new solar wind ion instrument design is presentedSimulations are used to define a design with unprecedented temporal and angular resolutionsA plasma turbulence simulation is used to show the measurement capabilities achieved

Lower hybrid waves (LHWs) are frequently observed associated with magnetic reconnection. A general picture of the statistical properties of LHWs in magnetopause reconnection, however, has not yet been established. Here we report such a statistical study of LHWs using magnetospheric multiscale mission data in the magnetopause reconnection exhaust region. LHWs are identified in reconnection outflow regions over a broad range of magnetic local time. Most of the LHWs are near strong density gradients located on the low‐density magnetospheric side of the reconnection layer. The wave amplitude is typically several mV/m but can occasionally reach a few tens of mV/m. Compared with intervals that do not exhibit LHWs, intervals with LHWs show a ∼50% increase in parallel electron temperature, providing statistical evidence that LHWs contribute to parallel electron heating in reconnection exhausts and the dayside low‐latitude boundary layer. In this study, we provide a statistical study of the wave strength, the location, and the potential effect of lower hybrid waves (LHWs) on particles in reconnection exhaust regions. We find that LHWs are mainly located near strong density gradients on the low‐density magnetospheric side of the reconnection layer. The wave strength and distribution do not depend on the magnetic shear condition. Te,‖and Te,‖/Te,⊥show significant increases associated with LHWs, suggesting that LHWs contribute to electron heating in reconnection exhaust regions and the low‐latitude boundary layer. Lower hybrid waves (LHWs) in reconnection outflow regions are found from 06 MLT to 18 MLT near the dayside magnetopauseMost of the LHWs are near strong density gradients located on the magnetospheric side of the reconnection layerSignificant increases in Te,‖and Te,‖/Te,⊥are observed during intervals of LHWs Lower hybrid waves (LHWs) in reconnection outflow regions are found from 06 MLT to 18 MLT near the dayside magnetopause Most of the LHWs are near strong density gradients located on the magnetospheric side of the reconnection layer Significant increases in Te,‖and Te,‖/Te,⊥are observed during intervals of LHWs

We first introduce previous works on spacecraft electrostatic charging and its effects on particle measurements and the calculation of moments from three‐dimensional distribution functions. We illustrate the fact that the lack of use, or misuse, of Liouville's theorem may lead to misinterpretations and inappropriate corrections to those effects. We emphasize in particular that its appropriate use naturally accounts for what is often called the “sheath focusing effect” in moment calculation. In the case of a “perfect” particle detector we show that there exists a trivial and essentially exact formulation for the calculation of moments of particle distribution functions that accounts for the spacecraft potential and that is particularly useful for onboard moment calculations. The main limitations, but that are not specific to this formulation, are inaccuracies in angle information (from blurring of acceptance angles at low energies or spacecraft‐skimming trajectory effects) and those arising from the detector properties (resolution, cutoffs, photoelectrons, etc.). We discuss this correction in the context of previous works and remind that it primarily affects low‐energy measurements and for populations whose temperature is comparable to the potential. Based on spacecraft observations, we show that this correction is most needed for regimes such as that of solar wind electrons but also for cold ions as often observed, for instance, in the magnetosphere near the Earth's magnetopause. Introduction of past work on spacecraft potential correctionDescription of proper method to correct particle moments from spacecraft chargingIllustration of the impact of this correction from spacecraft data

Solar wind slower than 300 km/s, hereafter termed very slow solar wind (VSSW), is seldom observed at 1 AU. It was, however, commonly measured inside 0.7 AU by the two Helios spacecraft, particularly during solar maximum. Magnetohydrodynamic (MHD) modeling reveals that the disappearance of VSSW at 1 AU is the result of its interaction with faster solar wind. The acceleration and compression of the VSSW contributes to the observed highly variable structure of the slow solar wind at 1 AU. The VSSW usually contains the heliospheric plasma sheet and current sheet. It has higher density and lower temperature than the regular slow solar wind, extending the known scaling laws below 300 km/s. Its helium abundance increases with solar activity even more significantly than the slow solar wind. Contrary to faster solar winds, the helium ions in the VSSW are slower than the dominant protons. Combining a Potential Field Source Surface (PFSS) model with ballistic back tracing, we study the source region of the VSSW. We show that the proton density flux for the VSSW is much higher than for the faster winds, particularly at solar maximum. The solar wind speed extends down to 200 km/s with similar scaling laws as beyond 300 km/sHelium abundance, proton mass flux above the photosphere, and photospheric magnetic field have similar solar cycle variabilityThe results point out toward a likely influence of collisions in the acceleration of the slow solar wind, regulating the helium abundance

Whistler waves are often observed in magnetopause reconnection associated with electron beams. We analyze seven MMS crossings surrounding the electron diffusion region (EDR) to study the role of electron beams in whistler excitation. Waves have two major types: (a) Narrow‐band waves with high ellipticities and (b) broad‐band waves that are more electrostatic with significant variations in ellipticities and wave normal angles. While both types of waves are associated with electron beams, the key difference is the anisotropy of the background population, with perpendicular and parallel anisotropies, respectively. The linear instability analysis suggests that the first type of wave is mainly due to the background anisotropy, with the beam contributing additional cyclotron resonance to enhance the wave growth. The second type of broadband waves are excited via Landau resonance, and as seen in one event, the beam anisotropy induces an additional cyclotron mode. The results are supported by particle‐in‐cell simulations. We infer that the first type occurs downstream of the central EDR, where background electrons experience Betatron acceleration to form the perpendicular anisotropy; the second type occurs in the central EDR of guide field reconnection. A parametric study is conducted with linear instability analysis. A beam anisotropy alone of above ∼3 likely excites the cyclotron mode waves. Large beam drifts cause Doppler shifts and may lead to left‐hand polarizations in the ion frame. Future studies are needed to determine whether the observation covers a broader parameter regime and to understand the competition between whistler and other instabilities. In EDRs observed by MMS, electron distributions of background plus beams excite whistler by beam drift and anisotropy of both populationsDifferent types of distributions and waves are inferred to depend on the distance from the X‐lineA parametric study with the linear instability analysis is used to discuss the competition between different whistler modes In EDRs observed by MMS, electron distributions of background plus beams excite whistler by beam drift and anisotropy of both populations Different types of distributions and waves are inferred to depend on the distance from the X‐line A parametric study with the linear instability analysis is used to discuss the competition between different whistler modes

At the beginning of 2007 the twin STEREO spacecraft provided a unique opportunity to study the global solar wind control of the terrestrial magnetotail under typical solar activity minimum conditions. The STEREO‐B (STB) spacecraft flew in the vicinity of the far terrestrial magnetotail, while the STEREO‐A (STA) spacecraft was located in front of the Earth performing measurements in the undisturbed solar wind. In February, the STB spacecraft was located in the magnetosheath most of the time but experienced several incursions into the distant magnetotail. Comparison of STA and STB observations determines unambiguously whether solar wind events such as energetic particle enhancements observed by STB are of pure solar origin or due to the influence of the terrestrial magnetosphere. During this time period in 2007, there were solar minimum conditions with alternating fast and slow solar wind streams that formed corotating interaction regions, which were the dominating source of magnetospheric disturbances encountering the Earth almost every week. Under these conditions, STB experienced multiple bow shock and magnetopause crossings due to the induced highly dynamic behavior of the terrestrial magnetotail and detected bursts of tailward directed energetic ions in the range of 110–2200 keV accompanied by suprathermal electrons of ~700–1500 eV, which were not seen in the undisturbed solar wind by STA. The corotating interaction regions triggered these energetic particle enhancements, and we demonstrate their magnetosphere‐related origin. Even after leaving the magnetosheath in March 2007, STB continued to observe antisunward directed energetic ion bursts until May up to a distance of ~ 800 REbehind Earth, the largest distance to which solar wind and magnetospheric interaction has been observed. These results show that Earth is a very significant source of energetic particles in its local interplanetary environment. Simultaneous observation of solar wind and terrestrial far tail by STEREOTerrestrial tail dynamics triggered by corotating interaction region passagesAccelerated ions observed up to ~ 800 REdistance from the Earth