We discussWindobservations of a long and slow magnetic cloud (MC) propagating through large-amplitude Alfven waves (LAAWs). The MC axis has a strong component along GSEX, as also confirmed by a Grad-Shafranov reconstruction. It is overtaking the solar wind at a speed roughly equal to the upstream Alfven speed, leading to a weak shock wave 17 hr ahead. We give evidence to show that the nominal sheath region is populated by LAAWs: (i) a well-defined de Hoffmann-Teller frame in which there is excellent correlation between the field and flow vectors, (ii) constant field and total pressure, and (iii) an Alfven ratio (i.e., ratio of kinetic-to-magnetic energy of the fluctuations) near unity at frequencies much lower than the ion cyclotron frequency in the spacecraft frame. In the region where the LAAWs approach the MC's front boundary there are field and flow discontinuities. At the first, magnetic reconnection is taking place, as deduced from a stress balance test (Walen test). This severs connection of some field lines to the Sun and the solar wind strahl disappears. There follows a 2-hr interval where the magnetic field strength is diminished while pressure balance is maintained. Here the bidirectionality of the suprathermal electron flows is intermittently disrupted. This interval ends with a slow expansion fan downstream of which there is a dropout of halo electrons just inside the front boundary of the MC. This study illustrates an untypical case of a slow MC interacting with LAAWs in the slow solar wind.

In this paper we report on a sequence of large‐amplitude Alfvénic fluctuations terminating in a field and flow discontinuity and their effects on electromagnetic fields and plasmas in the near‐magnetopause magnetosheath. An arc‐polarized structure in the magnetic field was observed by the Time History of Events and Macroscale Interactions during Substorms‐C in the solar wind, indicative of nonlinear Alfvén waves. It ends with a combined tangential discontinuity/vortex sheet, which is strongly inclined to the ecliptic plane and at which there is a sharp rise in the density and a drop in temperature. Several effects resulting from this structure were observed by the Magnetospheric Multiscale spacecraft in the magnetosheath close to the subsolar point (11:30 magnetic local time) and somewhat south of the geomagnetic equator (−33° magnetic latitude): (i) kinetic Alfvén waves; (ii) a peaking of the electric and magnetic field strengths where E · J becomes strong and negative (−1 nW/m 3 ) just prior to an abrupt dropout of the fields; (iii) evolution in the pitch angle distribution of energetic (a few tens of kilo‐electron‐volts) ions (H + , He n + , and O n + ) and electrons inside a high‐density region, which we attribute to gyrosounding of the tangential discontinuity/vortex sheet structure passing by the spacecraft; (iv) field‐aligned acceleration of ions and electrons that could be associated with localized magnetosheath reconnection inside the high‐density region; and (v) variable and strong flow changes, which we argue to be unrelated to reconnection at partial magnetopause crossings and likely result from deflections of magnetosheath flow by a locally deformed, oscillating magnetopause.

We present an example of a boundary layer tailward of the dawn terminator which is entirely populated by rolled‐up flow vortices. Observations were made by Wind on 24 October 2001 as the spacecraft moved across the region at X ∼−13 RE. Interplanetary conditions were steady with a near‐radial interplanetary magnetic field (IMF). Approximately 15 vortices were observed over the 1.5 h duration of Wind's crossing, each lasting ∼5 min. The rolling up is inferred from the presence of a hot tenuous plasma being accelerated to speeds higher than in the adjoining magnetosheath, a circumstance which has been shown to be a reliable signature of this in single‐spacecraft observations. A blob of cold dense plasma was entrained in each vortex, at whose leading edge abrupt polarity changes of field and velocity components at current sheets were regularly observed. In the frame of the average boundary layer velocity, the dense blobs were moving predominantly sunward and their scale size along X was ∼7.4 RE. Inquiring into the generation mechanism of the vortices, we analyze the stability of the boundary layer to sheared flows using compressible magnetohydrodynamic Kelvin‐Helmholtz theory with continuous profiles for the physical quantities. We input parameters from (i) the exact theory of magnetosheath flow under aligned solar wind field and flow vectors near the terminator and (ii) the Wind data. It is shown that the configuration is indeed Kelvin‐Helmholtz (KH) unstable. This is the first reported example of KH‐unstable waves at the magnetopause under a radial IMF.

Se estudia la condición para la cual haces de electrones acelerados a energías relativisticas e inyectados en una cámara de deriva entre electrodos cilíndricos equipotenciales desarrollan una inestabilidad, que limita la corriente máxima que puede fluir radialmente. El umbral de la inestabilidad (de Pierce) correspondiente a electrodos planos es modificado fuertemente por la curvatura. La corriente límite pasa de variar como L-2 a ser proporcional a L-1 , donde L es la distancia interelectródica. Se encuentra que la corriente límite (para radio de electrodo de entrada y velocidad de inyección prefijadas) es mayor para haces divergentes que para el caso plano, y que a su vez la corriente crítica de este caso es mayor que la correspondiente a haces convergentes. Se observa que los efectos relativistas, por su parte, ayudan a obtener corrientes críticas más altas.

Investigamos la influencia estabilizadora de un flujo de plasma paralelo a una lámina de corriente formada por un movimiento tipo punto de estancamiento, sobre la inestabilidad de "tearing". Damos el análisis de estabilidad para una extensión de esta configuración, la cual es solución exacta de la MHD. El espectro se calcula usando un método matricial con diferencias finitas. Se da la tasa de crecimiento y en ficción de los números de Mach-Alfvénico MA, Lundquist S y Prandtl magnético Pm. Mostramos (para Pm=1) la estabilización del modo "tearing" por Kelvin-Helmholtz cuando MA<1. El efecto de la viscosidad tiende a reducir las tasas de crecimiento. Se confirma, en el caso v0=0, la dependencia de y con el número de Hartmann Ha=S/Pm1/2.

Se aplica nuestra teoría de la inestabilidad de Kelvin-Helmholtz compresible en presencia de cizalla magnética y diferencias de densidad y temperatura de ambos lados de la discontinuidad de velocidad, para estudiar la estabilidad de configuraciones típicas de la magnetopausa terrestre. Se muestra que la inclusión de los efectos de la compresibilidad lleva a desestabilizar el plasma para bajas velocidades relativas en situaciones que son estables en el límite incompresible, lo que muestra que la teoría incompresible da predicciones incorrectas acerca de la estabilidad.

The Kelvin-Helmoltz instability (KH) with formation of vortices appears in a wide variety of terrestrial, interplanetary, and astrophysical contexts. We study a series of iterated rolled-up coherent plasma structures (15) that flow in the equatorial Earth's boundary layer (BL), observed on October 24, 2001. The data were recorded during a 1.5 hour-long Wind crossing of the BL at the dawn magnetospheric flank, tailward of the terminator (X approximate to-13 R-E). The interplanetary magnetic field (IMF) was radially directed, almost antiparallel to the magnetosheath (MS) flow. This configuration is expected to be adverse to the KH instability because of the collinearity of field and flow, and the high compressibility of the MS. We analyze the BL stability with compressible MHD theory using continuous profiles for the physical quantities. Upstream, at near Earth sites, we input parameters derived from an exact MHD solution for collinear flows. Further downtail at Wind position we input measured parameters. The BL is found KH unstable in spite of unfavorable features of the external flow. On the experimental side, the passage of vortices is inferred from the presence of low density hot plasma being accelerated to speeds higher than that of the contiguous MS. It is further supported by the peculiar correlation of relative motions (in the bulk velocity frame): cold-dense plasma drifts sunward, while hot-tenuous plasma moves tailward. This event differs from many other studies that reported BL vortices under strongly northward IMF orientations. This is a case of KH vortices observed under an almost radial IMF, with implicit significance for the more common Parker's spiral fields, and the problem of plasma entry in the magnetosphere.

The work is about the evolution of large vortices in the boundary layer of the terrestrial magnetopause studied by computational physics, in support of interpretive analysis of spacecraft data from an event observed on December 7, 2000. The distinctive features of the configuration examined are i) the very small magnetic shear across the boundary, and ii) the dynamics concerns mainly the vorticity. The magnetic field is only fluted during the process, but it determines the direction of the vortex axis. Considering new elements from computer simulations and observations, an extension of the research of a recent publication in this journal is reported. A magnetohydrodynamic code for large eddy simulations is used to examine the influence of supersonic, and superAlfvénic speeds, on the dynamics of the boundary layer. Correlations of data are studied to identify signatures of the Kelvin-Helmholtz instability, and the presence of whirling plasma. The aim is to improve the understanding of factors that govern the tailward growth of the low latitude boundary layer, and the potentiality of plasma entry into the magnetosphere, during periods of northward interplanetary magnetic field.

While there are many approximations describing the flow of the solar wind past the magnetosphere in the magnetosheath, the case of perfectly aligned (parallel or anti‐parallel) interplanetary magnetic field (IMF) and solar wind flow vectors can be treated exactly in a magnetohydrodynamic (MHD) approach. In this work we examine a case of nearly‐opposed (to within 15°) interplanetary field and flow vectors, which occurred on October 24–25, 2001 during passage of the last interplanetary coronal mass ejection in an ejecta merger. Interplanetary data are from the ACE spacecraft. Simultaneously Wind was crossing the near‐Earth (X ∼ −13 Re) geomagnetic tail and subsequently made an approximately 5‐hour‐long magnetosheath crossing close to the ecliptic plane (Z = −0.7 Re). Geomagnetic activity was returning steadily to quiet, “ground” conditions. We first compare the predictions of the Spreiter and Rizzi theory with the Wind magnetosheath observations and find fair agreement, in particular as regards the proportionality of the magnetic field strength and the product of the plasma density and bulk speed. We then carry out a small‐perturbation analysis of the Spreiter and Rizzi solution to account for the small IMF components perpendicular to the flow vector. The resulting expression is compared to the time series of the observations and satisfactory agreement is obtained. We also present and discuss observations in the dawnside boundary layer of pulsed, high‐speed (v ∼ 600 km/s) flows exceeding the solar wind flow speeds. We examine various generating mechanisms and suggest that the most likely cause is a wave of frequency 3.2 mHz excited at the inner edge of the boundary layer by the Kelvin‐Helmholtz instability.

The Kelvin-Helmholtz (KH) instability can operate in various situations in the solar wind, but at the boundaries of planetary obstacles, for example the Earth's magnetopause, it is most amenable to investigation. Reliable estimates of wave characteristics are essential for comparison with theoretical and numerical models and for understanding the non-linear development of KH waves and their role in the plasma entry into the magnetosphere. After discussing their typical conditions of appearance in KH unstable domains at the magnetopause, both theoretically and observationally, we outline recent results of multi-spacecraft analysis with Cluster giving accurate, albeit spatially limited, determination of surface wave characteristics. Those characteristics (wavelength and propagation direction), close to the terminator on the nightside, are likely to be prescribed by the 3-D geometry and the bending of field lines developed by the KH waves, rather than by the magnitude and the direction of the magnetosheath or background flow. An unprecedented number of satellites provides now the opportunity to extend the analysis of source regions of KH waves and their domains of development.

Observations of surface waves on the magnetopause indicate a wide range of phase velocities and wavelengths. Their multispacecraft analysis allows a more precise determination of wave characteristics than ever before and reveal shortcomings of approximations to the phase speed that take a predetermined fraction of the magnetosheath speed or the average flow velocity in the boundary layer. We show that time lags between two or more spacecraft can give a qualitative upper estimate, and we confirm the unreliability of flow approximations often used by analyzing a few cases. Using two‐point distant magnetic field observations and spectral analysis of the tailward magnetic field component, we propose an alternative method to estimate the wavelength and phase speed at a single spacecraft from a statistical fit to the data at the other site.

Compressibility has a strong influence on the stability of velocity shear layers when the difference of velocity ΔV across the flow becomes supersonic. The flanks of the Earth's magnetopause are normally supersonic Ms > 1, and super-Alfvénic MA > 1, depending on the distance from the dayside terminator (Ms and MA are the sonic and Alfvén Mach numbers of the magnetosheath plasma, respectively). The stability of MHD supersonic flows depends, also on several other features, such as the finite thickness Δ of the boundary layer, the relative orientation of velocity and magnetic fields, the density jump across the boundary and the magnetic shear angle. We analyze the MHD stability of some representative flank sites modeled after data from spacecraft crossings of the magnetopause under different interplanetary conditions, complementing these cases with extrapolations of likely conditions upstream, and downstream of the crossing site. Under northward interplanetary magnetic field conditions, there are solar wind regimes such that the near, but already supersonic, flank of the magnetopause may be locally stable. Stability is possible, e.g., when Ms becomes larger than ∼1.2–1.4 while MA remains smaller than 1.2, and there is magnetic shear between the geomagnetic and the interplanetary magnetic field. Solar winds favouring local stability of the boundary layer are cold, not-too-dense plasmas, with strong magnetic fields, so that MA is smaller, while Ms is larger, than normal values of the magnetosheath flow. A gap between dayside and tail amplifying regions of Kelvin-Helmholtz disturbances over the magnetopause may exist when the above conditions are realized.

A study of the magnetohydrodinamic (MHD) development of coherent structures in compressible, inhomogeneous, mixing layers due to the velocity shear instability is reported. The non-linear evolution of the original vorticity sheet is computed with 3-D large eddy simulations (LES) of temporal mixing layers tailored to represent distinctive conditions at the terrestrial magnetopause. We find that the boundary layer is characterized by the growth of large-scale vortices and becomes a site of mass mixing and enhanced plasma diffusion. In MHD the Lorentz force and its associated baroclinic term, together with the ordinary baroclinic term, and stratified entropy across the mixing layer, conspire to hinder vorticity flux conservation. In our LES the non-conservation of vorticity becomes manifest after ∼ one rollover time when in addition to vortices with positive rotation (the same as the original vorticity sheet) other coherent structures with strong negative vorticity also arise, a noteworthy effect examined here. It is found that the vorticity is concentrated in cores of both signs with absolute values ∼ 4–5 × ωi, (maximum vorticity of the initial shear layer). Concomitant with 3-D vortex stretching, the kinetic helicity also rises at vorticity cores. Furthermore, high temperature occurs in the cores, ∼ 3xTi (magnetospheric temperature) correlated with local density depletion, ∼ 0.4xni (magnetospheric density), while gas and magnetic pressure remain close to surrounding values. The study is intended as a contribution to the understanding of solar wind interaction with the magnetosphere during periods of northward interplanetary magnetic field.

We demonstrate, on experimental grounds, that the justifications for the comment by Hasegawa [2009], hereinafter H09, on work done by Foullon et al. [2008], hereinafter F08, are not well founded.

We describe (i) perturbations of the magnetopause/magnetosphere elicited by an interplanetary discontinuity and (ii) the production of Kelvin–Helmholtz waves on the magnetopause. These are two large topics, so for reasons of space we combine both features in a single data example, supporting the observations by theory. Correspondingly, the observations, made by ACE, consist of an interval in which a current sheet is followed by a period of strongly northward IMF. In view of recent attention directed at the effect of variations of the azimuthal component of the solar wind velocity on the magnetosphere, we chose a current sheet (CS) across which the east–west components of both field and flow vectors change polarity. A two-stage response is evident in the records of Cluster, outbound at the dusk terminator at 27° MLAT: (i) Four cycles of large-amplitude, ∼3min oscillations during which the spacecraft sample alternately the cold, dense magnetosheath and the hot and tenuous magnetosphere plasmas. We argue that these motions are likely due to tangential stresses applied to the magnetopause. (ii) Soon thereafter the oscillatory character changes dramatically, and ∼80s small-amplitude undulations appear which we argue to be magnetopause surface waves. Applying linear MHD theory we show these waves are due to a locally Kelvin–Helmholtz unstable boundary. As input parameters, we take values during the preceding large oscillations at the same magnetopause locale. An aspect of the non-linear phase of this instability is illustrated by a numerical simulation: the reduced duration of the evolution into large vortices by a strong initial perturbation.

We discuss the motion and structure of the magnetopause/boundary layer observed by Cluster in response to a joint tangential discontinuity/vortex sheet (TD/VS) observed by the Advanced Composition Explorer spacecraft on 7 December 2000. The observations are then supplemented by theory. Sharp polarity reversals in the east‐west components of the field and flow By and Vy occurred at the discontinuity. These rotations were followed by a period of strongly northward interplanetary magnetic field (IMF). These two factors elicited a two‐stage response at the magnetopause, as observed by Cluster situated in the boundary layer at the duskside terminator. First, the magnetopause suffered a large deformation from its equilibrium position, with large‐amplitude oscillations of ∼3‐min period being set up. These are argued to be mainly the result of tangential stresses associated with ΔVy the contribution of dynamic pressure changes being small in comparison. This strengthens recent evidence of the importance to magnetospheric dynamics of changes in azimuthal solar wind flow. The TD/VS impact caused a global response seen by ground magnetometers in a magnetic local time range spanning at least 12 h. The response monitored on ground magnetometers is similar to that brought about by magnetopause motions driven by dynamic pressure changes. Second, Cluster recorded higher‐frequency waves (∼79 s). Two clear phases could be distinguished from the spectral power density, which decreased by a factor of ∼3 in the second phase. Applying compressible linearized MHD theory, we show that these waves are generated by the Kelvin‐Helmholtz (KH) instability. Varying the local magnetic shear at the Cluster locale, as suggested by the temporal profile of the IMF clock angle, we find that locally stability was reinstated, so that the reduced power in the second phase is argued to be due residual KH activity arriving from locations farther to the dayside.

Our purpose is to characterize the evolution of the magnetopause Kelvin-Helmholtz (KH) wave activity with changes in thickness of the adjacent boundary layer, geomagnetic latitude and interplanetary magnetic field (IMF) orientation. As the IMF turns northward, wave activity may be generated at the dayside before propagating down the tail, where the boundary layer is expected to support longer wavelengths. We use two-point observations on the dusk magnetopause at low latitudes, from Geotail on the dayside and Cluster tailward of the dusk terminator. We quantify the wavelength, power, wavefront steepness and propagation direction at Cluster. An estimate of the thickness of the low-latitude boundary layer (LLBL) is obtained by correlating normal distances to the magnetopause, derived from two empirical solar-wind-driven models, with a systematic relationship (the "transition parameter'') found between the electron number density and temperature; the correlation factor is used to infer the temporal evolution of the thickness of the locally sampled layer. We find that wavelengths are controlled by the IMF clock angle, as expected when generated by the KH mechanism at the dayside, although amplitudes, wavefront steepness and propagation directions are more closely correlated with the layer thickness. A survey of parameter space provides evidence of the contribution of the KH mechanism to the widening of the electron LLBL.

During solar cycle 23 quasi-dropouts of the solar wind (density <1cm-3) were observed. These tenuous winds allow us to probe properties of the magnetosphere and its coupling to the solar wind which would otherwise be obscured by the effect of high density. We focus on five areas which provided new insights into the response of geospace to solar wind variations: (i) the magnetospheric magnetic configuration; (ii) the polar rain; (iii) dayside flux erosion; (iv) magnetosheath waves; and (v) ring current constants. We find: (i) the geostationary field had dipolar strength and was inclined by ⩽5∘ to the dipolar direction; (ii) the solar wind strahl, and consequently the polar rain, were intensified; (iii) the depression of the geostationary field (ΔBGS) due to dayside flux erosion could be measured and was related to IMF Bz by ΔBGS=-2.8+2.3Bz (nT); (iv) right-hand electromagnetic ion cyclotron waves were excited alone in the magnetosheath and were generated directly from the temperature anisotropy of the solar wind; (v) ring and magnetopause currents decreased to asymptotic values of 5 and 3 nT, respectively, which are substantially smaller than quiet-time values obtained from statistics.