The terrestrial magnetopause forms the boundary between the solar wind plasma with its embedded interplanetary magnetic field on one side, and the terrestrial magnetosphere, dominated by Earth's dipole field, on the other side. It is therefore a key region for the transfer of mass, momentum, and energy from the solar wind to the magnetosphere. The Cluster mission, comprising a constellation of four spacecraft flying in formation was launched more than 20 years ago to study boundaries in space. During its lifetime, Cluster has provided a wealth of new knowledge about the magnetopause. In this presentation, we give an overview of Cluster-based studies of this boundary, and highlight a selection of interesting results.

Electron inflow and outflow velocities during magnetic reconnection at and near the dayside magnetopause are measured using satellites from NASA's Magnetospheric Multiscale (MMS) mission. A case study is examined in detail, and three other events with similar behavior are shown, with one of them being a recently published electron‐only reconnection event in the magnetosheath. The measured inflow speeds of 200–400 km/s imply dimensionless reconnection rates of 0.05–0.25 when normalized to the relevant electron Alfvén speed, which are within the range of expectations. The outflow speeds are about 1.5–3 times the inflow speeds, which is consistent with theoretical predictions of the aspect ratio of the inner electron diffusion region. A reconnection rate of 0.04 ± 25% was obtained for the case study event using the reconnection electric field as compared to the 0.12 ± 20% rate determined from the inflow velocity.

When the supersonic solar wind encounters the Earth's magnetosphere a shock, called bow shock, is formed and the plasma is decelerated and thermalized in the magnetosheath downstream from the shock. Sometimes, however, due to discontinuities in the solar wind, bow shock ripples or ionized dust clouds carried by the solar wind, high speed jets (HSJs) are observed in the magnetosheath. These HSJs have typically a V-x component larger than 200 km s(-1) and their dynamic pressure can be a few times the solar wind dynamic pressure. They are typically observed downstream from the quasi-parallel bow shock and have a typical size around one Earth radius (R-E) in X-GSE. We use a conjunction of Cluster and MMS, crossing simultaneously the magnetopause, to study the characteristics of these HSJs and their impact on the magnetopause. Over 1 h 15 min interval in the magnetosheath, Cluster observed 21 HSJs. During the same period, MMS observed 12 HSJs and entered the magnetosphere several times. A jet was observed simultaneously by both MMS and Cluster and it is very likely that they were two distinct HSJs. This shows that HSJs are not localized into small regions but could span a region larger than 10 R-E, especially when the quasi-parallel shock is covering the entire dayside magnetosphere under radial IMF. During this period, two and six magnetopause crossings were observed, respectively, on Cluster and MMS with a significant angle between the observation and the expected normal deduced from models. The angles observed range between from 11 degrees up to 114 degrees. One inbound magnetopause crossing observed by Cluster (magnetopause moving out at 142 km s(-1)) was observed simultaneous to an outbound magnetopause crossing observed by MMS (magnetopause moving in at -83 km s(-1)), showing that the magnetopause can have multiple local indentation places, most likely independent from each other. Under the continuous impacts of HSJs, the magnetopause is deformed significantly and can even move in opposite directions at different places. It can therefore not be considered as a smooth surface anymore but more as surface full of local indents. Four dust impacts were observed on MMS, although not at the time when HSJs are observed, showing that dust clouds would have been present during the observations. No dust cloud in the form of Interplanetary Field Enhancements was however observed in the solar wind which may exclude large clouds of dust as a cause of HSJs. Radial IMF and Alfven Mach number above 10 would fulfill the criteria for the creation of bow shock ripples and the subsequent crossing of HSJs in the magnetosheath.

A new quality index is proposed for quantitatively assessing the level of Alfvenicity of the flow in the magnetopause and other quasi one-dimensional plasma/field structures. Perfect Alfvenicity is expressed by the so-called Walen relation, which describes the agreement of the observed jump in velocity across a suitable portion of the magnetopause and the corresponding jump in Alfven velocity. In the past, approximate agreement with this relation has been used as an important indicator of ongoing magnetic reconnection. The new index incorporates the effects of deviations in direction as well as in magnitude of the two vectors. Its properties and utility are illustrated by application to a large set of dayside magnetopause crossings by the Magnetospheric Multiscale spacecraft MMS 1. Time series for two of these events are also presented and discussed.

We discuss observations of reconnection, obtained by Time History of Events and Macroscale Interactions during Substorms (THEMIS) D during an outward bound traversal of the low‐latitude dayside magnetopause. The reconnection signatures include high magnetic shear, a southward directed Alfvénic jet, bounded by slow‐mode shocks near the switch‐off limit (as in the symmetric Petschek geometry), a small, sunward directed normal magnetic field and plasma inflow into the jet from both sides. We conclude that cold, unmeasured ionospheric ions helped establish the symmetry. The effective ion mass, estimated from the switch‐off condition, was 2.39 amu on the magnetospheric side, where the number density was inferred from the spacecraft potential, and 1.09 amu on the magnetosheath side. After a modest pressure correction in the magnetospheric shock, the MHD jump conditions for density, pressure, temperature, and entropy were well satisfied. The shock jumps were much larger on the magnetosphere side than on the magnetosheath side; we show this to be a plasma β effect. The main dissipation mechanism appears to be irreversible transfer between thermal motion parallel and perpendicular to the field, such that both shocks bring about approximate downstream temperature isotropy. Hall currents and electric fields were present, albeit in a strongly asymmetric configuration. The magnetospheric shock had longer duration than the magnetosheath one, possibly as a result of a nonconstant magnetopause speed. We infer an average earthward magnetopause speed (14 km/s), corresponding nominal shock thicknesses (12 and 6 λ i ), dimensionless reconnection rates (0.061–0.085), and reconnection wedge angles (5° between shocks; 13° between separatrices).

We surveyed 87 magnetopause reconnection exhausts detected by the THEMIS spacecraft to investigate how the amount and anisotropy of ion bulk heating depend on the inflow boundary conditions. We find that the heating, Δ T i , is correlated with the asymmetric Alfvén speed, V AL ,asym , based on the reconnecting magnetic field and the plasma number density measured in both inflow regions. Best fit to the data produces the empirical relation Δ T i = 0.13 m i V AL ,asym 2 , where m i is the proton mass, indicating that the increase in the ion internal (thermal) energy, 3Δ T i /2, is 20% of the available magnetic energy per proton‐electron pair. The observed parallel heating generally exceeds perpendicular heating (by a factor of ~2), and there are some indications that the heating is reduced in the presence of a strong guide field. Finally, the ratio of ion to electron bulk heating is ~8 on average.

Described is a technique used by the Freja TESP top hat-style electron spectrometer to measure rapidly the structure of field-aligned electron beams. The technique utilizes the instrument's entrance aperture deflector to sweep the look direction across the direction of B, providing a measurement of the pitch-angle structure in 8 ms. The technique is described in detail and measurements from its application are shown. The advantages and disadvantages of such a measurement scheme are discussed in light of the returned measurements.

AMPTE-IRM Observations of Particles and Fields at the Dayside Low-latitude Magnetopause T. M. Bauer, T. M. Bauer Max-Planck-Institut für Extraterrestrische Physik, Garching, GermanySearch for more papers by this authorG. Paschmann, G. Paschmann Max-Planck-Institut für Extraterrestrische Physik, Garching, GermanySearch for more papers by this authorN. Sckopke, N. Sckopke Max-Planck-Institut für Extraterrestrische Physik, Garching, GermanySearch for more papers by this authorW. Baumjohann, W. Baumjohann Max-Planck-Institut für Extraterrestrische Physik, Garching, GermanySearch for more papers by this authorR. A. Treumann, R. A. Treumann Max-Planck-Institut für Extraterrestrische Physik, Garching, GermanySearch for more papers by this authorT.-D. Phan, T.-D. Phan Space Sciences Laboratory, University of California, BerkeleySearch for more papers by this author T. M. Bauer, T. M. Bauer Max-Planck-Institut für Extraterrestrische Physik, Garching, GermanySearch for more papers by this authorG. Paschmann, G. Paschmann Max-Planck-Institut für Extraterrestrische Physik, Garching, GermanySearch for more papers by this authorN. Sckopke, N. Sckopke Max-Planck-Institut für Extraterrestrische Physik, Garching, GermanySearch for more papers by this authorW. Baumjohann, W. Baumjohann Max-Planck-Institut für Extraterrestrische Physik, Garching, GermanySearch for more papers by this authorR. A. Treumann, R. A. Treumann Max-Planck-Institut für Extraterrestrische Physik, Garching, GermanySearch for more papers by this authorT.-D. Phan, T.-D. Phan Space Sciences Laboratory, University of California, BerkeleySearch for more papers by this author Book Editor(s):James L. Horwitz, James L. HorwitzSearch for more papers by this authorDennis L. Gallagher, Dennis L. GallagherSearch for more papers by this authorWilliam K. Peterson, William K. PetersonSearch for more papers by this author First published: 01 January 1998 https://doi.org/10.1029/GM104p0051Citations: 3Book Series:Geophysical Monograph Series AboutPDFPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShareShare a linkShare onEmailFacebookTwitterLinkedInRedditWechat Summary This chapter contains sections titled: Introduction Predictions for a Rotational Discontinuity Case Study Comparison with Other Magnetopause Passes Estimates of Diffusion Coefficients Conclusions References T. M. Bauer, Particles and Fields at the Dayside Low-latitude Magnetopause, Dissertation, Ludwig-Maximilians-Universität Munchen, Germany, 1997. Google Scholar T. M. Bauer, G. Paschmann, R. A. Treumann, W. Baumjohann, N. Sckopke, Ion signatures of reconnection at the magnetopause, Adv. Space Res., 19, 12, 1947– 1950 1997. 10.1016/S0273-1177(97)00107-5 Web of Science®Google Scholar S. W. H. 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We have performed a statistical study of THEMIS spacecraft crossings of the asymmetric dayside magnetopause to test the prediction that the diamagnetic drift of the X-line due to a plasma pressure gradient across the magnetopause can suppress magnetic reconnection. The study includes crossings both when reconnection exhausts were present and when they were absent in the current sheet. When we restrict the survey to the subsolar region (10 < MLT < 14), we find that for low Delta beta (the difference of plasma beta on the two sides of the current sheet) the majority of reconnection events occurred over a large range of magnetic shears, whereas when Delta beta was high reconnection events occurred only for high shears. Furthermore, nonreconnection events occurred primarily in the Delta beta-shear regime in which reconnection is predicted to be suppressed, in good agreement with theory. The Delta beta-shear condition should have general consequences for the occurrence of reconnection in space and laboratory plasmas. Citation: Phan, T. D., G. Paschmann, J. T. Gosling, M. Oieroset, M. Fujimoto, J. F. Drake, and V. Angelopoulos (2013), The dependence of magnetic reconnection on plasma beta and magnetic shear: Evidence from magnetopause observations, Geophys. Res. Lett., 40, 11-16, doi: 10.1029/2012GL054528.

[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 hBni = 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 Alfvénic flows on both sides favor the latter interpretation.

We address the conditions for the onset of magnetic reconnection based on a survey of 197 reconnection events in solar wind current sheets observed by the Wind spacecraft. We report the first observational evidence for the dependence of the occurrence of reconnection on a combination of the magnetic field shear angle, θ, across the current sheet and the difference in the plasma β values on the two sides of the current sheet, Δβ. For low Δβ, reconnection occurred for both low and high magnetic shears, whereas only large magnetic shear events were observed for large Δβ: Events with shears as low as 11° were observed for Δβ < 0.1, but for Δβ > 1.5 only events with θ > 100° were detected. Our observations are in quantitative agreement with a theoretical prediction that reconnection is suppressed in high β plasmas at low magnetic shears due to super-Alfvénic drift of the X-line caused by plasma pressure gradients across the current sheet. The magnetic shear-Δβ dependence could account for the high occurrence rate of reconnection observed in current sheets embedded within interplanetary coronal mass ejections, compared to those in the ambient solar wind. It would also suggest that reconnection could occur at a substantially higher rate in solar wind current sheets closer to the Sun than at 1 AU and thus may play an important role in the generation and heating of the solar wind.

For a four-point mission like Cluster, the differences in position and time when the satellites detect the magnetopause or any other discontinuity, can be used to infer the discontinuity local orientation, thickness and motion. This timing technique, commonly assuming a planar geometry, offers an independent check for various single-spacecraft techniques. In the present paper we propose an extension of the timing method, capable of determining in a self-consistent way the macroscopic parameters of a two-dimensional, non-planar discontinuity. Such a configuration can be produced by a local bulge or indentation in the magnetopause, or by a large amplitude wave traveling on this surface, and is recognized in Cluster data when the single spacecraft techniques provide different individual normals contained roughly in the same plane. The model we adopted for the magnetopause assumes a layer of constant thickness of either cylindrical or parabolic shape, which has one or two degrees of freedom for the motion in the plane of the individual normals. The method was further improved by incorporating in a self-consistent way the requirement of minimum magnetic field variance along the magnetopause normal. An additional assumption, required in a previously proposed non-planar technique, i.e. that the non-planarity has negligible effects on the minimum variance analysis, is thus avoided. We applied the method to a magnetopause transition for which the various planar techniques provided inconsistent results. By contrast, the solutions obtained from the different implementations of the new 2-D method were consistent and stable, indicating a convex shape for the magnetopause. These solutions perform better than the planar solutions from the normal magnetic field variance perspective. The magnetopause dynamics and the presence of a non-zero normal magnetic field component in the analyzed event are discussed.