Magnetopause Surface Research Articles

Exterior and interior polar cusps: Observations from Hawkeye

Hawkeye plasma, magnetic field, and plasma wave instruments directly sampled the throat of the northern polar cusp as the orientation of the interplanetary magnetic field (IMF) changed from southward to northward on July 3, 1974. Two distinct regions in the polar cusp were identified based on magnetic field, plasma flow and magnetic and electric noise: the interior and exterior cusps. The observations show highly variable flows in the exterior portion of the cusp and constantly strong dawn‐dusk flows in the interior portion during periods of strong IMF By component. Results of a minimum variance analysis of the magnetic field at each cusp interface crossing provides evidence that the magnetopause surface normal deviated highly from empirical models. During intervals of relatively steady solar wind dynamic pressure, the motion of the cusp relative to the slow moving spacecraft was modulated by the varying IMF clock angle as observed by IMP 8 in the upstream solar wind. The motion did not show a correlation with internal processes monitored by the AE index. We propose that observed plasma flow patterns and cusp motion are results of reconnection between the IMF and the magnetospheric magnetic field. Flow velocity observed in the interior cusp is consistent with stress balance for a reconnection process. This unique interval provides an opportunity for detailed studies of the plasma, magnetic field, and plasma wave properties in both the exterior and interior cusp.

The average shape of the Magnetopause: A comparison of three‐dimensional global MHD and empirical models

We present a predictive model of the magnetopause, constructed using three‐dimensional global MHD simulations of the magnetosphere over a range in solar wind dynamic pressure and the north‐south component of the interplanetary magnetic field (IMF BZ). The magnetopause is identified in the simulations using electric current intensity, thermal pressure, and magnetic field line mapping. The magnetopause is fitted by least squares to an ellipse in each plane, and a fully three‐dimensional magnetopause surface is constructed by assuming an elliptical cross section at intermediate clock angles. There is considerable flaring of the magnetopause dependent on BZ in the noon‐midnight meridian plane due to reconnection but virtually no flaring in the equatorial plane. Despite this asymmetry, when averaged over clock angle, the model shows a general agreement with recent (axisymmetric) empirical magnetopause models and predicts observed magnetopause crossings with about the same accuracy as the empirical models. Unlike these empirical models, our model can directly investigate asymmetries between the meridian and equatorial planes. The subsolar distance, directly measured from the simulations, varies linearly with BZ for both northward and southward interplanetary magnetic field, as do distances to the magnetopause on the flanks. Magnetospheric plasma pressure causes these distances to scale with dynamic pressure by a power law whose exponent is a bit less than −1/6 on the dayside, but little deviation from this ideal Chapman‐Ferraro value is seen in the magnetotail. These uniform changes with Bz lead to approximate conservation of (1) magnetopause shape in the dayside equatorial plane, (2) projected area in the dayside meridian plane, and (3) magnetopause cross‐sectional area in the dawn‐dusk plane. The dawn‐dusk magnetopause cross section is nearly circular on average, but cross sections in the tail are elongated in the Z direction on average.

The shape and field of the magnetopause as determined from pressure balance

This article presents a theoretical study of the dependence of the magnetopause shape and field on the dynamic pressure of the solar wind. The magnetopause surface (assumed to be axisymmetric) is determined by requiring the plasma pressure outside, estimated by the “Newtonian approximation,” to be balanced by the magnetic pressure inside. The magnetic field inside is given by the sum of (1) the Earth's internal field, (2) the field from various magnetospheric current systems (cross‐tail current, ring current, etc.) as given by an empirical model, and (3) the field due to magnetopause currents. The magnetopause field is a function of the magnetopause shape and is updated each time this shape is adjusted toward pressure balance. In previous studies of this kind, only dipole and magnetopause fields were incorporated. This is the first three‐dimensional solution of the problem to include the cross‐tail current and thus to obtain a realistic magnetotail configuration. The results of this study are twofold. First, the shape of the magnetopause surface is computed under a variety of conditions. The resulting shapes are compared with fits to observed magnetopause crossings, and the scaling of magnetopause position on solar wind dynamic pressure is determined. Second, the model that results can be viewed as an improved version of the empirical model that was used as input, in that the magnetic field in the vicinity of the magnetopause (where empirical models have less guidance from data) is made more realistic.

A statistical study of transient events in the outer dayside magnetosphere

The AMPTE CCE satellite frequently observed transient (1 ≤ τ ≤ 8 min) events marked by magnetic field strength increases and bipolar magnetic field signatures (peak‐to‐peak amplitudes ≥ 4 nT) while in the outer dayside magnetosphere. We report a survey of 59 prominant events observed from August to November 1984. The bipolar signatures and minimum variance analysis reveal that most events move poleward and antisunward, except in the immediate vicinity of local noon. Here the motion of the events appears to be better governed by the spiral/orthospiral interplanetary magnetic field (IMF) orientation than magnetic curvature forces associated with IMF By. The IMF orientation appears to have little or no influence on event occurrence or orientation. We interpret the events in terms of solar wind/foreshock pressure pulse induced ripples on the magnetopause surface. Our results can be reconciled with those obtained in previous studies which made use of ISEE 1/2, AMPTE IRM, and AMPTE UKS observations if pressure pulses produce large‐amplitude events and bursty merging (or reconnection) produces small‐amplitude events.

Jovian magnetopause breathing

Five closely spaced magnetic field directional discontinuities observed by Voyager 1 late on 2 March 1979 are analyzed using a variance technique. These discontinuities showed magnetic field signatures characteristic of magnetopause crossings occurring periodically at ≈ 36 min intervals but without dramatic changes in the low energy (<6 keV) plasma density. The locally obtained variance estimates of the discontinuity normal directions show no systematic variation consistent with magnetopause surface waves, are always within 19° of the average normal direction, and their average gives excellent agreement, to within 2°, with a well-known large-scale model of Jupiter's magnetopause. In addition, energetic ions (>5 MeV) of magnetospheric origin were present at elevated levels relative to typical magnetosheath rates throughout all crossings. These results suggest the discontinuities are magnetopause crossings resulting from a possibly semi-global breathing mode of the magnetopause boundary with a period of 66 min, an amplitude of 1.2 R J, and a mean position of 49.9 R J. The maximum boundary speed is 140 km s −1 with a magnetopause thickness of 21 ± 11 × 10 3 km.

A linear perturbation analysis of magnetopause motion in the Newton-Busemann limit

Abstract. The response of the magnetopause surface to time-varying solar wind dynamic pressure is examined. We argue that to a first approximation the magnetopause surface may be considered as analogous to an elastic membrane. Upon displacement from equilibrium resulting from a change in applied external pressure, it moves to a new equilibrium under the equation of motion of a forced, damped, simple harmonic oscillator. We derive this equation of motion by linearising for small perturbations the momentum equation for flow past a nonrigid ellipsoidal body in the Newton-Busemann limit. Though our approach is only an approximation to the real dynamics of the magnetopause boundary, it serves to demonstrate the importance of inertia in the system response. It allows us to estimate the natural eigenperiod of magnetopause oscillation as typically around 7 min, the precise value depending on solar wind conditions. However, the magnetopause eigenoscillation is furthermore found to be strongly damped, regardless of solar wind conditions. One consequence of these properties is that short-period fluctuations in the solar wind dynamic pressure elicit a suppressed magnetospheric response. We outline other theoretical expectations by which our model may be tested against observation, and discuss the implications of our findings for current interpretations of spacecraft observations made in the dynamic magnetopause environment.

A linear perturbation analysis of magnetopause motion in the Newton-Busemann limit

<strong class="journal-contentHeaderColor">Abstract.</strong> The response of the magnetopause surface to time-varying solar wind dynamic pressure is examined. We argue that to a first approximation the magnetopause surface may be considered as analogous to an elastic membrane. Upon displacement from equilibrium resulting from a change in applied external pressure, it moves to a new equilibrium under the equation of motion of a forced, damped, simple harmonic oscillator. We derive this equation of motion by linearising for small perturbations the momentum equation for flow past a nonrigid ellipsoidal body in the Newton-Busemann limit. Though our approach is only an approximation to the real dynamics of the magnetopause boundary, it serves to demonstrate the importance of inertia in the system response. It allows us to estimate the natural eigenperiod of magnetopause oscillation as typically around 7 min, the precise value depending on solar wind conditions. However, the magnetopause eigenoscillation is furthermore found to be strongly damped, regardless of solar wind conditions. One consequence of these properties is that short-period fluctuations in the solar wind dynamic pressure elicit a suppressed magnetospheric response. We outline other theoretical expectations by which our model may be tested against observation, and discuss the implications of our findings for current interpretations of spacecraft observations made in the dynamic magnetopause environment.

Multiple, discrete arcs on sunward convecting field lines in the 14‐15 MLT region

Ionospheric plasma flow measurements and simultaneous observations of thin (∼0.2° invariant latitude (ILAT)), multiple, longitudinally extended auroral arcs of transient nature within 74°‐76° ILAT and 1030‐1130 UT (∼14‐15 MLT) on January 12, 1989, are reported. The auroral structures appeared within the luminous belt of strong 630.0‐nm emissions located predominantly on sunward convecting field lines equatorward of the convection reversal boundary as identified by the European Incoherent Scatter UHF radar. The events occurred during a period of several hours quasi‐steady solar wind speed (∼ 700 km s−1) and a radially orientated interplanetary magnetic field (IMF) with a weak northward tilt (IMF Bz&gt;0). These typical dayside auroral features are related to previous studies of auroral activity related to the upward region 1 current in the postnoon sector. The discrete auroral events presented here may result from magnetosheath plasma injections into the low‐latitude boundary layer (LLBL) and an associated dynamo mechanism. An alternative explanation invokes kinetic Alfvén waves, triggered either by Kelvin‐Helmholtz instability at the inner (or outer) edge of the LLBL or by pressure pulse induced magnetopause surface waves.

Observational differences between flux transfer events and surface waves at the magnetopause

Whether the phenomenon known as a flux transfer event (FTE) is caused by a special process acting at the magnetopause such as transient reconnection or whether it is simply a double crossing of the magnetopause caused by a pressure pulse has become controversial. Theoretical investigations have shown that there are some similarities in the flow and field perturbation between a magnetopause surface wave and a converting cylindrical structure. This result by itself does not distinguish the two possibilities. We investigate the observed differences between FTEs and surface waves. In general, a realistic magnetopause surface wave has a much longer wavelength than the displacement, which is different from some models of surface waves. The scales of an FTE along the convection direction and normal to the magnetopause are of the same order. Spectral analysis of the soalr wind and upstream waves shows no evidence of pressure variations of the duration and intensity to generate the magnetopause surface wave of the required wavelength of few Earth radii and a displacement of an Earth radius. The difference between a surface wave and a series of FTEs can be easily recognized in the profiles of the magnetic field and plasma parameters. The FTEs look like a series of pulses, whereas multiple crossings of the magnetopause occur with a changing ratio of the duration of magnetosheath and magnetospheric entry. The timing difference between two spacecraft during magnetopause crossings is significantly greater than the timing difference of FTEs. For typical magnetopause crossings the perturbations in the normal component and magnitude of the magnetic field is not more than background magnetosheath fluctuations; but those for FTEs are often much greater. The reinterpretation of FTEs as surface waves arose without a quantitative assessment of the observed perturbations of the magnetopause and quantitative effects of proposed sources. When such a quantitative assessment is performed, it is clear that FTEs are not simply magnetopause crossings.

Remote sensing of two‐dimensional magnetopause structures

A technique is developed for analyzing remote measurements made by a single spacecraft of two dimensional disturbances in the ambient magnetosheath or magnetospheric magnetic field, caused, for example, by flux transfer events or pressure pulses. The methodology is based on a recent linear theory for isentropic field‐aligned MHD flow over gently sloping two‐dimensional obstacles (Sonnerup et al., 1992). Using only magnetic field measurements, the analysis technique can provide information about the orientation and actual cross‐sectional shape of the event, as well as information about the spacecraft trajectory relative to the bulge. If three‐dimensional plasma velocity measurements are also available, the technique provides the velocity and size of the event as well, and it allows one to determine whether the current sources causing the disturbance were in fact located on the side of the spacecraft trajectory facing the magnetopause. Analysis of two sample events, one recorded by the spacecraft AMPTE/IRM (Active Magnetospheric Particle Tracer Explorer/Ion Release Module) in the magnetosheath and the other by AMPTE/CCE (Charge Composition Explorer) in the magnetosphere, indicates that the bulges on the magnetopause surface causing the magnetic field and flow perturbations for these events did not have the semicircular cross section suggested in previous work; instead they had a more elongated shape, the dimension tangential to the magnetopause being substantially larger than that normal to it. The calculated invariant axes of the two events were found to differ substantially from the corresponding minimum variance directions of the measured magnetic field. The IRM event was found to move at a speed of 227 km/s away from the subsolar region. For the CCE event, plasma flow data were not available, but it was deduced indirectly that the event moved at a speed of about 90 km/s, presumably away from the subsolar region.

The field of the Jovian magnetosphere, including contributions of the magnetopause surface currents

An idealized large-scale Jovian magnetospheric field has been computed by calculating the contribution due to the currents on the surface of the magnetopause (calculated from theory). This contribution is added to those of a model current system in the magnetic equatorial plane and the intrinsic dipole field of the planet. Convenient spherical harmonic representations of the field in the region inside the magnetopause have been developed. The Voyager observation model is compared with the Pioneer observation model.

Transient events in the outer magnetosphere: Boundary waves or flux transfer events?

We present simultaneous solar wind and magnetospheric observations for a series of flux transfer events (FTEs) observed by the UKS and IRM spacecraft in the outer prenoon magnetosphere on October 28, 1984 (day 302). The CCE satellite, located near local noon, observed an enhancement in the magnetospheric magnetic field strength 4 min prior to each IRM/UKS event, suggesting the antisunward propagation of a compression in the magnetopause surface. We suggest that these compressions resulted from wavy magnetopause motion driven by variations in the solar wind dynamic pressure applied to the magnetosphere, rather than the motion of FTEs formed by magnetic merging. In support of this hypothesis we note that the events previously identified as FTEs occurred during a sequence of quasi‐periodic (5–6 min) oscillations in the magnetospheric plasma velocity and magnetic field. The oscillations exhibited all the characteristics expected for antisunward moving magnetopause boundary motion and those cycles identified as FTEs simply exhibited larger amplitudes. The magnetospheric compressions observed by the CCE and IRM/UKS corresponded to enhancements in the interplanetary magnetic field strength (IMF) observed by the ISEE 2 satellite, located just upstream of the subsolar bow shock. Some of the magnetospheric compressions occurred for a strongly northward IMF. There was no evidence for the type of accelerated flows commonly associated with magnetic merging at the magnetopause on this day, and the orientation of the magnetic field inside the UKS/IRM events was not consistent with that expected for FTEs.

Generation of Pc 1 waves by the ion temperature anisotropy associated with fast shocks caused by sudden impulses

Observations have reported on the high correlation of Pc 1 events with magnetospheric compressions. A number of mechanisms have been suggested for the generation of the Pc 1 waves. In this paper, we propose a new mechanism which leads to the generation of Pc 1 waves. The interaction of a dynamic pressure pulse (Δρυ²) with the Earth's bow shock leads to the formation of a weak fast mode shock propagating into the magnetosheath. The shock wave can pass right through a tangential discontinuity (magnetopause) and into the magnetosphere, without disturbing either of the structures. In a quasi‐perpendicular geometry, the shock wave exhibits anisotropic heating with T⊥ &gt; T∥. This anisotropy drives unstable ion cyclotron waves which we believe can contribute to the generation of the Pc 1 waves which are detected. The viability of the mechanism is demonstrated with simulations. This mechanism could explain peak in the occurrence of observed Pc 1 waves in the postnoon sector where a field‐aligned discontinuity in the solar wind would most often be parallel to the magnetopause surface due to the average Parker spiral magnetic field configuration.

Idealized Voyager Jovian magnetosphere shape and field

A magnetic field arising from the Jovian equatorial sheet current deduced from Voyager 1 and 2 observations has been added to a planetary dipole field to provide a model of magnetic field inside the magnetopause. This internal field was used to calculate the magnetopause surface in a cyclic process. During each cycle, the surface was calculated, and the resulting field due to currents on the magnetopause was calculated for inclusion in the total field used to calculate the next‐order surface. The resulting magnetopause is, as anticipated, flatter in shape than one resulting primarily from a dipole internal field source, but not dissimilar in overall height‐to‐width configuration to that of the magnetopause calculated for the larger inflated magnetopause observed by Pioneer 10. An array of magnetic field values for locations internal and external to the magnetopause due to currents on the surface has been computed by integrating over the entire magnetopause. A model for the total magnetospheric field of this semi‐inflated magnetosphere has been constructed by adding this latter contribution to the internal source fields to obtain a global model of a semi‐inflated Jovianlike magnetospheric field. The magnitude of the contribution of the surface currents to the total magnetic field in the region of the orbits of the Galilean satellites is calculated to be considerably larger for this Voyager model than for the Pioneer model.

Ion Reflection and transmission during reconnection at the Earth's subsolar magnetopause

Composition measurements in an accelerated flow event at the Earth's dayside magnetopause show evidence for reflection and transmission of magnetospheric and magnetosheath ion species. Furthermore, a single velocity transformation approximately tangent to the magnetopause surface orders the individual transmitted and reflected ion distributions on both sides of the magnetopause into field‐aligned flow at VA, the local Alfven speed. These observations provide strong evidence for a kinetic description of magnetic reconnection at the magnetopause.

A global model of gyroviscous field line merging at the magnetopause

A gyroviscous model of the magnetopause current layer is applied to the entire magnetopause with the inner boundary condition provided by a quantitative model of the magnetospheric field (BM) and the outer boundary condition given by the interplanetary magnetic field (IMF) draped against the magnetopause surface. It is found that the magnetopause surface is divided into at least four different regions by a tangential singularity line (Bn = 0) that separates regions with different signs of the normal component and rotation singularity lines (BM ∥ BIMF) that separate regions with different magnetic polarizations (rotations). The tangential singularity line (TSL) has topological properties similar to the classical reconnection line and the rotation singularity lines (RSL) are similar to lines of “antiparallel merging.” The field topology in the regular regions is determined by field line tracing using the model solutions. It is argued that the whole magnetopause surface, except the singular separation regions, possesses a stable rotational structure that can be described locally by a planar model. It is proposed that the singular regions on the magnetopause surface (TSL and RSL) play the role of gates for solar wind plasma entry into the magnetosphere and that rotational singularities are observed as flux transfer events. Observations of velocity jumps at the magnetopause that were previously classified as “high‐speed reconnection flows,” “tangential discontinuity” crossings as well as “diffusive entry” crossings can all be explained as special cases of the gyroviscous merging theory. The paper offers a unified view of such disparate concepts as steady state reconnection, antiparallel merging, viscous interactions, patchy reconnection, flux transfer events, and impulsive penetration.

Observational features of field line resonances excited by solar wind pressure variations on 4 September 1984

In the course of the magnetic storm of 4 September 1984, after an inverse sudden impulse (SI), geomagnetic pulsations in the Pc5-frequency range were observed at magnetometer stations in the local evening sector. They occurred at L-values of ∼6, and lasted for several hours, their period increasing from about 320 to 550 s. In this study, two events of enhanced activity are discussed in some detail. During the 16:00 U.T. event, a favourable position of the AMPTE/IRM spacecraft allows conjugate observations in the Northern and Southern Hemispheres and in the magnetosphere. This constellation permits a precise determination of the wave mode. During a later intensification around 18:00 U.T., the AMPTE/CCE spacecraft near local noon monitored poloidal waves, obviously driving the pulsations on the ground. Generally, the observations are consistent with the theory of field line resonance. They are interpreted as being excited by pressure variations in the solar wind. The hydromagnetic cavity mode is assumed to link the magnetopause surface motions to the field line resonances.

Quantitative models of the magnetospheric magnetic field: Methods and results

The paper reviews various approaches to the problem of evaluation and numerical representation of the magnetic field distributions produced within the magnetosphere by the main electric current systems including internal Earth's sources, the magnetopause surface current, the tail plasma sheet, the large-scale systems of Birkeland current, the currents due to radiation belt particles, and the partial ring current circuit. Some basic physical principles as well as mathematical background for development of magnetospheric magnetic field models are discussed. A special emphasis is placed on empirical modeling based on datasets created from large bodies of spacecraft measurements. A review of model results on the average magnetospheric configurations and their dependence on the geomagnetic disturbance level and the state of interplanetary medium is given. Possibilities and perspectives for elaborating the ‘instantaneous’ models capable of evaluating a current distribution of magnetic field and force line configuration based on a synoptic monitoring the intensity of the main magnetospheric electric current systems are also discussed. Some areas of practical use of magnetospheric models are reviewed in short. Magnetospheric plasma and energetic particle measurements are considered in the context of their use as an independent tool for testing and correcting the magnetic field models.

On mapping flux transfer events to the ionosphere

If a flux transfer event is modeled as a round patch of open field lines on an otherwise closed magnetopause, it maps to the ionosphere as an elongated shape pointing radially away from the cusp. We demonstrate this distortion by superposing a cylindrical flux rope field with the shielded Chapman‐Ferraro magnetospheric field. The axial component of the flux rope is normal to the planar magnetopause and decreases to zero at the edge of the rope. Field lines mapped from the edge nearest the cusp trace along the magnetopause surface to the cusp and then follow the singular cusp field line down to the ionosphere. However, field lines traced from the far edge must pass through the flux rope, where the axial component is strong. Consequently, they extend deeper into the magnetosphere, where the field is less cusp‐configured and more dipolar, and map down to the ionosphere away from the cusp, elongating the footprint of the round patch. Elongation increases with displacement of the patch from the subsolar region of the magnetopause. A series of patches mapped from distances successively further from the subsolar point forms a pattern of striations radiating from the cusp that resembles an observed pattern of auroral arcs in the midday oval, previously interpreted as signatures of magnetosheath plasma injection at the cusp by an unspecified mechanism. The mapping suggests that the mechanism could be patchy reconnection at distances which can be considerably removed from the cusp.

The magnetospheric response to 8‐minute period strong‐amplitude upstream pressure variations

This paper documents a series of brief, strong (Δp/p = 1), dynamic pressure oscillations that occurred in the region upstream of the Earth's bow shock during a period of radial interplanetary magnetic field (IMF). The analyzed set of oscillations, which may be either intrinsic solar wind or bow shock‐related phenomena, recur approximately every 8–10 min, and their magnetic field signatures occur nearly simultaneously over great distances transverse to the Earth‐Sun line. The pressure oscillations appear to drive tailward‐moving magnetopause surface wavelets. In turn, the surface wavelets can be identified as hydromagnetic waves with strong compressional components in the outer magnetosphere and as quasi‐periodic variations in electron precipitation and high‐latitude ground pulsations. We use observations by spacecraft in the outer dayside magnetosphere to predict geosynchronous and subsolar magnetic field strengths, the location of the subsolar magnetopause, the solar wind dynamic pressure, and variations in the energetic magnetospheric ion flux.