Observations by the Magnetospheric Multiscale spacecraft (MMS) of an unusual layer, located between the dayside magnetosheath and the magnetosphere, alternating with encounters with the magnetosheath during an extended time period between December 31, 2015 and January 01, 2016, when the interplanetary magnetic field was strongly southward and the Earth's dipole tilt large and negative, are presented. It appears to have been magnetically connected to both magnetosphere and magnetosheath. The layer appears to be located mostly on closed field lines and was bounded by a rotational discontinuity (RD) at its magnetosheath edge and by the magnetosphere on its earthward side. A separatrix layer, with heated magnetosheath electrons streaming unidirectionally along the field lines, was present sunward of the RD. We infer that the layer was started by a dominant reconnection site well north of the spacecraft and that it may have gained additional width, from a large drop in solar wind density and ram pressure, which preceded the beginning of the event by more than an hour. Relative to the magnetosheath, in which the magnetic field was strongly southward, this unusual layer was characterized by a less southward, more dawnward magnetic field of lower magnitude. The plasma density and flow speed in the region were lower than in the magnetosheath, albeit with Alfvénic jetting occurring at the magnetosheath edge as well as at the magnetospheric edge of the layer. The closing of the magnetic field lines requires the existence of another reconnection site, located southward/tailward of MMS.

Observations from the Magnetospheric Multiscale (MMS) mission are used to quantify the maximum effect of magnetospheric H+ and He+ on dayside magnetopause reconnection. A data base of current‐sheet crossings from the first 2 years of the MMS mission is used to identify magnetopause crossings with the highest He+ concentrations. While all of these magnetopause crossings exhibit evidence of plasmaspheric plume material, only half of the crossings are directly associated with plasmaspheric plumes. The He+ density varies dramatically within the magnetosphere adjacent to the magnetopause, with density variations of an order of magnitude on timescales as short as 10 s, the time resolution of the composition instrument on MMS. Plasma wave observations are used to determine the total electron density, and composition measurements are used to determine the mass density in the magnetosheath and magnetosphere. These mass densities are then used with the magnetic field observations to determine the theoretical reduction in the reconnection rate at the magnetopause. The presence of high‐density plasmaspheric plume material at the magnetopause causes transient reductions in the reconnection rate of up to ∼40%.

The standard method for identifying magnetohydrodynamic rotational discontinuities in spacecraft data has been to examine how well the Walén relation is satisfied. In this paper, we apply two different versions of the Walén test to a database of nearly 1,000 dayside magnetopause crossings by the Magnetospheric Multi‐Scale spacecraft, with the objective of comparing their performance. The first approach is to evaluate the Walén relation as a jump condition, by determining the level of agreement between the change in plasma velocity across a discontinuity with the corresponding change in the Alfvén velocity. For this purpose, we use a recently developed quality index, Q, for which Q=±1 indicates perfect agreement. As was the case for a previously used quality index, ΔV∗, this new index employs data from two carefully chosen measurement times, located on opposite sides of the discontinuity. The second approach is to check the level of Alfvénicity of the flow for all measurements between those two points. Here, the quality index used is Wsl, the slope of the regression line in a scatter plot of plasma velocity components (after transformation into the deHoffmann‐Teller frame) versus the corresponding Alfvén velocity components, with Wsl=±1 indicating perfect agreement. For the two indices to give comparable numbers of rotational discontinuity candidates, a substantially higher threshold value is needed for |Q| than for |Wsl|. Even so, the events selected by the two methods are not identical. We also identify statistical relationships between Wsl and its associated correlation coefficient, Wcc, as well as between Wsl and Q and between Wsl and ΔV∗.

We have used a large number of magnetopause crossings by the Magnetospheric Multiscale (MMS) mission to investigate macroscopic properties of this current sheet, with emphasis on the flanks of the magnetopause. Macroscopic features such as thickness, location, and motion of the magnetopause were calculated as a function of local time sector. The results show that the flanks of the magnetopause are significantly thicker than the dayside magnetopause. Thicknesses vary from about 650 km near noon to over 1,000 km near the terminator. Current densities vary in a similar manner, with average current densities around noon almost twice as high as near the terminator. We also find a dawn‐dusk asymmetry in many of the macroscopic parameters; the dawn magnetopause is thicker than at dusk, while the dusk flank is more dynamic, with a higher average normal velocity.

This paper describes the generation and initial utilization of a database containing 80 vector and scalar quantities, for a total of 8,670 magnetopause and magnetosheath current sheet crossings by MMS1, using plasma and magnetic field data from the Fast Plasma Investigation, Fluxgate Magnetometer, and Hot Plasma Composition Analyzer instruments, augmented by solar wind and interplanetary magnetic field data from CDAWeb. Based on a determination of the current sheet width, measured and calculated vector and scalar quantities are stored for the two sides of the current sheet and for selected times within the current sheet. The only manual operations were the classification of the current sheets according to the type of boundary, the character of the magnetic field transition, and the quality of the current sheet fit. To characterize the database, histograms of selected key quantities are presented. We then give the statistics for the duration, motion, and thicknesses of the magnetopause current sheet, using single-spacecraft techniques for the determination of the normal velocities, obtaining median results of 12.9 s, 38.5 km/s, and 705.4 km, respectively. When scaled to the ion inertial length, the median thickness became 12.6; there were no thicknesses less than one. Next, we apply the Walen relation to find crossings that are rotational discontinuities and thus may indicate ongoing magnetic reconnection. For crossings where the velocities in the outflow region exceed the velocity on the magnetosheath side by at least 250 km/s, 47% meet our rotational discontinuity criteria. If we require the outflow to exceed 250 km/s along the L direction, then the percentage rises to 68%.

The jump conditions are analyzed in detail for two slow shocks bounding a reconnection plasma jet, observed on 3 August 2008 by the spacecraft THEMIS D (Time History of Events and Macroscale Interactions during Substorms) on the dayside, low‐latitude magnetopause. Both shocks are near the switch‐off limit. They have been previously examined by Sonnerup et al. (2016), on the basis of the simplest MHD version of the jump conditions. In the present paper, those conditions now include the pressure anisotropy, normal heat fluxes, and a finite normal magnetic field component, the effects of all of which are found to be small. We also present and discuss the, mostly field‐aligned, measured total heat fluxes, which are found to be substantial and directed away from the reconnection site. We show that the double‐adiabatic (Chew‐Goldberger‐Low) invariants are far from invariant. Their combination indicates a large entropy increase across the shock on the magnetospheric side with a much smaller increase across the shock on the magnetosheath side. The detailed cause of the entropy changes remains unclear but appears to involve irreversible transfer of energy between thermal motion parallel and perpendicular to the magnetic field. The new results confirm the previously found presence of heavy ions and the values of the effective ion mass on both sides of the event. They also confirm the need for an ion pressure correction in the shock on the magnetospheric side.

Rotational discontinuities (RDs) are governed by two relations: the Walén relation predicting that the plasma velocity observed in the deHoffmann–Teller frame equals the local Alfvén velocity and another relation that connects the variation in plasma mass density, ρ, to variations in the pressure anisotropy factor, α, defined as α: ≡(p∥ − p⊥) μ0/B2, so that ρ(1 − α) is constant. While the Walén relation has become a standard tool for classifying magnetopause crossings as RDs , the ρ(1 − α)= const. condition has never been directly verified at the same time, largely due to problems with determining ρ when no ion composition measurements were available. In fact, to overcome the lack of composition information, the validity of the relation has often been assumed and the Walén relation reformulated so that variations in ρ are replaced by variations in α. In this paper we exploit the availability of high-time-resolution composition measurements on the Cluster spacecraft to directly test the ρ (1− α)= const. condition for three magnetopause crossings, identified as RDs from the application of the Walén relation to measurements of plasma ions and magnetic field by the CIS (Cluster Ion Spectrometry) and FGM (flux-gate magnetometer) instruments, respectively. We find that the relation is not fulfilled in either case. In one event, with a fairly large content of oxygen ions, the Walén test improved when the contribution from these ions was taken into account. Through comparisons of the measured ion densities with simultaneously measured total electron densities by the Waves of HIgh frequency and Sounder for Probing of Electron density by Relaxation (WHISPER) instrument, we were able to exclude the possibility that ion populations hidden to the CIS instrument because of their very low energies could have changed ρ to match the ρ(1 − α)= const. condition. We also excluded the possibility that energetic ions above the CIS energy range could have sufficiently changed the true α. It thus appears that the ρ(1 − α)= const. condition, for reasons not presently understood, is not valid for the kind of RD-like structures we observe.

We examine the Alfvénicity of a set of 188 solar wind directional discontinuities (DDs) identified in the Cluster data from 2003 by Knetter (2005), with the objective of separating rotational discontinuities (RDs) from tangential ones (TDs). The DDs occurred over the full range of solar wind velocities and magnetic shear angles. By performing the Walén test in the de Hoffmann–Teller (HT) frame, we show that 77 of the 127 crossings for which a good HT frame was found had plasma flow speeds exceeding 80% of the Alfvén speed at an average angular deviation of 7.7°; 33 cases had speeds exceeding 90% of the Alfvén speed at an average angle of 6.4°. We show that the angular deviation between flow velocity (in the HT frame) and the Alfvén velocity can be obtained from a reduced form of the Walén correlation coefficient. The corresponding results from the Walén test expressed in terms of jumps in flow speed and corresponding jumps in Alfvén speed are similar: 66 of the same 127 cases had velocity jumps exceeding 80% with average angular deviation of 5.8°, and 22 exceeding 90% of the jump in Alfvén speed, with average angular deviation 6.2°. We conclude that a substantial fraction of the 127 events can be identified as RDs. We present further evidence for coupling across the DDs by showing that, for most of the 127 crossings, the HT frame velocities, evaluated separately on the two sides of the DD, are nearly the same – a result required for RDs but not for TDs. We also show that the degree of Alfvénicity is nearly the same for the DDs and fluctuations in which the DDs are embedded. Whatever process causes deviations from ideal Alfvénicity appears to operate equally for the DDs as for the surrounding fluctuations. Finally, our study has established a unique relation between the strahl electron pitch angle and the sign of the Walén slope, implying antisunward propagation in the plasma frame for all 127 cases.

We report results from a Cluster-based study of the properties of 28 arc-polarized magnetic structures (also called rotational discontinuities) in the solar wind. These Alfvénic events were selected from the database created and analyzed by Knetter (2005) by use of criteria chosen to eliminate ambiguous cases. His studies showed that standard, four-spacecraft timing analysis in most cases lacks sufficient accuracy to identify the small normal magnetic field components expected to accompany such structures, leaving unanswered the question of their existence. Our study aims to break this impasse. By careful application of minimum variance analysis of the magnetic field (MVAB) from each individual spacecraft, we show that, in most cases, a small but significantly non-zero magnetic field component was present in the direction perpendicular to the discontinuity. In the very few cases where this component was found to be large, examination revealed that MVAB had produced an unusual and unexplained orientation of the normal vector. On the whole, MVAB shows that many verifiable rotational discontinuities (Bn ≠ 0) exist in the solar wind and that their eigenvalue ratio (EVR = intermediate/minimum variance) can be extremely large (up to EVR = 400). Each of our events comprises four individual spacecraft crossings. The events include 17 ion-polarized cases and 11 electron-polarized ones. Fifteen of the ion events have widths ranging from 9 to 21 ion inertial lengths, with two outliers at 46 and 54. The electron-polarized events are generally thicker: nine cases fall in the range 20–71 ion inertial lengths, with two outliers at 9 and 13. In agreement with theoretical predictions from a one-dimensional, ideal, Hall-MHD description (Sonnerup et al., 2010), the ion-polarized events show a small depression in field magnitude, while the electron-polarized ones tend to show a small enhancement. This effect was also predicted by Wu and Lee (2000). Judging only from the sense of the plasma flow across our DDs, their propagation appears to be sunward as often as anti-sunward. However, we argue that this result can be misleading as a consequence of the possible presence of magnetic islands within the DDs. How the rotational discontinuities come into existence, how they evolve with time, and what roles they play in the solar wind remain open questions.

Recent multi-spacecraft studies of solar wind discontinuity crossings using the timing (boundary plane triangulation) method gave boundary parameter estimates that are significantly different from those of the well-established single-spacecraft minimum variance analysis (MVA) technique. A large survey of directional discontinuities in Cluster data turned out to be particularly inconsistent in the sense that multi-point timing analyses did not identify any rotational discontinuities (RDs) whereas the MVA results of the individual spacecraft suggested that RDs form the majority of events. To make multi-spacecraft studies of discontinuity crossings more conclusive, the present report addresses the accuracy of the timing approach to boundary parameter estimation. Our error analysis is based on the reciprocal vector formalism and takes into account uncertainties both in crossing times and in the spacecraft positions. A rigorous error estimation scheme is presented for the general case of correlated crossing time errors and arbitrary spacecraft configurations. Crossing time error covariances are determined through cross correlation analyses of the residuals. The principal influence of the spacecraft array geometry on the accuracy of the timing method is illustrated using error formulas for the simplified case of mutually uncorrelated and identical errors at different spacecraft. The full error analysis procedure is demonstrated for a solar wind discontinuity as observed by the Cluster FGM instrument.

We report in-situ measurements by three THEMIS spacecraft showing the evolution of reconnection in a solar wind current sheet as the current sheet transited from the solar wind across the bow shock and close to the magnetopause on July 11, 2008. The observations suggest that the solar wind reconnection exhaust within the current sheet was disrupted by its interaction with the bow shock, while the subsequent compression of the current sheet against the magnetopause significantly reduced both the current sheet thickness and the plasma beta and initiated reconnection at a new X-line located within the magnetosheath. Furthermore, electrons were heated at the center of the magnetosheath exhaust, in contrast to the previously reported absence of electron heating in solar wind exhausts, but consistent with electron heating occasionally observed in association with magnetopause reconnection. This suggests that the level of electron heating in reconnection exhausts depends strongly on the boundary conditions. Citation: Phan, T. D., T. E. Love, J. T. Gosling, G. Paschmann, J. P. Eastwood, M. Oieroset, V. Angelopoulos, J. P. McFadden, D. Larson, and U. Auster (2011), Triggering of magnetic reconnection in a magnetosheath current sheet due to compression against the magnetopause, Geophys. Res. Lett., 38, L17101, doi:10.1029/2011GL048586.

[1] We examine local structures of three directional discontinuities (DDs) observed by Cluster in the solar wind, using reconstruction based on the ideal 2-D MHD equations in a steady state. In this novel application of the technique, our goals are the following: (1) to explain why the minimum variance analysis of the magnetic field (MVAB) often fails to meaningfully predict the vector normal to a DD and (2) to use the reconstructed field maps as an aid in interpreting the differences in the magnetic field profiles recorded by the four Cluster spacecraft. From the maps, we learn that the failure of MVAB as a predictor of the normal direction is due to internal structure such as magnetic islands (flux ropes) within the DDs and also that we can partly understand the differences in the fields observed by the four spacecraft. We find fairly good agreement between the normal directions determined from the four-point timing approach and from MVAB, provided the constraint 〈Bn〉 = 0 is imposed on MVAB. Because of the island structures, the DDs cannot be readily identified as either tangential or rotational discontinuities, although the approximately Alfvenic flows on both sides favor the latter interpretation.

A theoretical model is proposed to account for some of the behavior of arc-polarized magnetic structures seen in the solar wind. To this end, an exact analytical solution is developed that describes infinite plane wave trains of arbitrary amplitude in a plasma governed by ideal Hall MHD. The main focus is on intermediate-mode wave trains, which display double-branched magnetic hodogram signatures similar to those seen in the solar wind. The theoretically derived hodograms have field rotation in the ion-polarized sense at a slightly depressed field magnitude on one branch and an electron-polarized rotation at a slightly enhanced field magnitude on the other branch. The two branches are joined at the two "turning points", at which the normal flow is exactly Alfvénic. The behavior is accounted for in terms of the opposite dispersive properties of ion and electron whistlers. The hodograms derived from the theory are shown to compare favorably with those of one event, observed by the Cluster spacecraft near the ecliptic plane, and one event at high heliographic latitude observed by the Ulysses spacecraft. However, these two observed structures comprise only a single full wave period, approximately from one turning point to the other and then back again. The theory can be used to predict propagation direction (away from, or towards, the sun) from magnetic data alone, provided the sign of the magnetic field component along the wave normal can be reliably determined. Under the same condition, it also predicts whether the ion-polarized branch should precede or follow the electron-polarized branch. Both behaviors are seen in the solar wind. The major shortcoming of the theory is that it fails to reproduce the observed saw-tooth like time series for the magnetic field, in which the field rotation is rapid in the ion sense and slow in the electron sense. Instead, the theory gives about the same rotation rates. Possible explanations for this discrepancy are discussed. Also discussed is the fact that the magnetic field measurements by Cluster, while giving high quality determinations of normal direction and normal field component for each of the four spacecraft, indicate a reversal of the normal field component and the predicted propagation sense during the event, as well as a wide spread in the four normal vector orientations.

The Electron Drift Instrument (EDI) contribution to the Cluster Active Archive (CAA) is described. Presented are descriptions of the EDI instrument, the various CAA/EDI data products, the CAA ingestion schedule and the current EDI status. An example of a science application is given for one of the main EDI data products available in the CAA.

We investigate an event with oscillations of electron counts at 500 eV, with a period of ∼3 s measured by the electron drift instrument (EDI) on Cluster. These oscillations are detected at the overshoot of a quasi‐perpendicular bow shock and in its downstream region. During this event, the Alfvén Mach number is supercritical, and the proton β is >1. The correlation of the electron counts between three pitch angle ranges (∼10°, ∼90°, and ∼170°) is in‐phase. However, the correlation between the electron counts and the magnitude of the magnetic fields is antiphase. As EDI has a higher time resolution than that of conventional particle instruments, the correlation at a period of ∼3 s is examined for the first time. The oscillations are inferred as mirror mode waves with oblique propagation angles to the magnetic field. The oscillation of electron counts in the parallel direction might indicate the oscillation of the parallel electric potential. We have estimated this electric potential by Liouville's mapping, taking into account the shape of the electron distribution function as well as using the linear hot plasma dispersion relation. We have found that this kind of parallel electric potential is related to heating of particles with a comparable magnitude as the cross‐shock potential. The elliptical polarization of these waves indicates a possible interaction with gyrating particles so that the waves would be related to the particle dynamics. Finally, in‐phase correlation between parallel and perpendicular counts is explained in terms of Liouville's mapping.

A unified minimum‐residue approach is presented to the use of classical conservation laws for determination of the orientation and motion of a plasma discontinuity, using data from a single spacecraft that traverses the discontinuity and makes measurements, not only on its two sides but also within it. The method is a generalization of the minimum Faraday residue (MFR) analysis technique described by Khrabrov and Sonnerup (1998a). It includes not only the standard MHD conservation laws for mass, momentum, total energy, and (where applicable) entropy, but also magnetic flux conservation from Faraday's law, absence of magnetic poles from ∇ · B = 0, and electric charge conservation from Ampère's law. A method, denoted by COM, for combining the results from more than one conservation law into a single optimal determination of the orientation and motion is presented, along with a general approach to the application of a certain class of constraints that can be placed on the vector normal to the discontinuity. The methodology, which is applicable to many types of discontinuity, including shocks, is illustrated by analysis of one magnetopause encounter by two of the four Cluster spacecraft (C1 and C3). The results from the various individual methods have considerable spread. However, in favorable circumstances and by exercising considerable care, the vector normal to the magnetopause from COM can be accurate to within a couple of degrees. For the C1 crossing, believed to be nearly a tangential discontinuity, albeit with signatures of incipient reconnection, the magnetopause speed (−56 km s−1) from COM appears accurate to within a few km s−1. The plasma flow across the magnetopause and the normal field component are both very nearly zero, and the results are consistent with those obtained from timing of the layer as it crosses the four Cluster spacecraft (assuming a constant thickness of the layer). The results for the velocity of the magnetopause and for the plasma flow across the layer are less consistent for the C3 crossing, believed to be a rotational discontinuity. For this crossing the presence of a component of the magnetic field along the normal direction could not be established with certitude. It is likely that the lower quality of the results for this crossing is caused by local multidimensional structure of the magnetopause.

This paper presents a detailed study of the Feb. 14, 2001 Cluster northern auroral pass at mid‐altitudes (4–5 RE), characterized by observations of a series of energy‐dispersed ion structures in a region of poleward convection. In contradiction with one current view, that ions populating these energy‐dispersed signatures originate sporadically in the magnetotail, Cluster directly observed energetic (0.2–15 keV), field‐aligned H+ ions of ionospheric origin. The ions were ejected at the top of a steady auroral acceleration region near 72.5° ILAT, then bounced on closed field‐lines, and were finally dispersed poleward in latitude by the E × B drift effect. Simple but realistic latitudinal drift computations demonstrate that the anticipated location of successive bouncing echoes coincides rather well with the Cluster observations. Best agreement is reached when the particles are further accelerated (presumably nonadiabatically) by 1–2 keV, as they periodically cross the tail neutral sheet.

Magnetic field lines are known to reorganize themselves in plasmas, converting magnetic to particle energy. Evidence harvested from the solar wind implies that the scale of the effect is larger than was thought. Magnetic reconnection is a process in which pairs of magnetic field lines merge to convert magnetic energy into particle energy. Kinks formed in the merged field lines produce a slingshot effect that accelerates high-speed plasma jets away from the merger site. The process supplies energy to solar flares and the space storms near Earth that interfere with electric power grids and telecommunications. Space physicists have long debated whether reconnection occurs over great distances, or randomly in localized patches. On 2 February 2002, the Cluster, ACE and Wind spacecraft, widely separated in interplanetary space, all detected similar plasma jets within the same passing current sheet. It was direct evidence of a 2.5-million-kilometre reconnection region, confirming that magnetic reconnection can occur on a very large scale over long periods. On the cover, kinked magnetic field lines accelerate a pair of particle jets.