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Results of global MHD simulations are used to examine the susceptibility of magnetosheath plasma to anisotropy-driven instabilities, specifically the mirror mode, electromagnetic ion cyclotron (EMIC), and firehose instabilities. Advection of magnetosheath plasma is followed along MHD-specified streamlines assuming double-adiabatic equations of state, while the anisotropic pressure is constrained by the instability threshold criteria and the condition that the trace of the gyrotropic pressure tensor be three times the MHD specification for the (isotropic) pressure. Following this procedure, mirror mode and EMIC turbulence is indicated in the dayside magnetosheath and in a layer adjacent to the dayside magnetopause, generally in accord with theoretical and observational expectations. The new result is the prediction of strong firehose or Alfvenic turbulence in the magnetosheath adjacent to the magnetospheric sash. Alfven waves will propagate downwind from the sash. Also, as magnetic field strength increases downwind from the sash, mirror mode and EMIC waves are again indicated.

We discuss four aspects of magnetosheath flow that require MHD for their calculation and understanding. We illustrate these aspects with computations using a numerical MHD code that simulates the global magnetosphere and its magnetosheath. The four inherently MHD aspects of magnetosheath flow that we consider are the depletion layer, the magnetospheric sash, MHD flow deflections, and the magnetosheath's slow-mode expansion into the magnetotail. We introduce new details of these aspects or illustrate known details in a new way, including the dependence of the depletion layer on interplanetary magnetic filed clock angle; agreement between the locations of the antiparallel regions of Luhmann et al. (J. Geophys. Res. 89 (1984) 1739) and the magnetospheric sash, and deflections corresponding separately to a stagnation line and magnetic reconnection.

A global MHD simulation code, the Integrated Space Weather Prediction Model, is used to examine the steady state properties of the magnetosphere for zero interplanetary magnetic field. In this “ground state” of the system, reconnection at the magnetopause is absent. Topics reported here include (1) qualitative description of global magnetic field, plasma flow, and current systems (Chapman-Ferraro, geotail, Region 1 and Region 2 currents) (2) quantitative parametric studies of shock jump conditions, magnetopause and shock standoff distance, polar cap voltage, and total Region 1 current for different solar wind speeds and ionospheric Pedersen conductances; and (3) quantitative analysis of the low-latitude boundary layer (LLBL) and its coupling to the ionosphere. The central part of the geomagnetic tail is found to be very long, extending beyond the downstream end of the simulation box at X = −300 RE. Along each flank a “wing-like” region containing closed, albeit strongly stretched, field lines is present. Each such region contains a narrow convection cell, consisting of the tailward flowing LLBL and an adjoining narrow channel of sunward return flow. These cells are the result of viscous-like interaction along the magnetospheric flanks, with an effective kinematic viscosity, entirely of numerical origin, estimated to be ν = 1.8 × 108m2 s−1. Except in certain regions near the magnetopause, the magnetosheath flow is steady and laminar while the internal motion in the tail displays turbulent vortical motion in the plasma sheet. Plasma transport in the tail occurs as a result of this turbulence, and substantial turbulent plasma entry across the equatorial magnetopause is seen in the region −10 RE < X < 0 RE behind the torus of dipolar field lines. The polar cap potential ΔϕPC is 29.9 ± 1.4 kV for VSW = 400 km s−1 and ∑P = 6 mho, which is in reasonable agreement with results inferred from satellite observations. About half of ΔϕPC can be attributed to the LLBLs with the remainder coming from a dawn-to-dusk potential drop along the dayside magnetopause, caused by nonlinearly switched resistivity, added explicitly to the MHD equations, and/or by numerical diffusion. The magnetospheric voltage-current relation at VSW = 400 km s−1 has a constant negative slope with an open circuit voltage of ΔϕPC = 38.5kV. The total Region 1 current (into the northern dawn hemisphere) is 0. 66 MA (at VSW = 400 km s−1 and ∑P = 6 mho). It maximizes at about 2. 83 MA during short-circuit conditions (∑P = ∞; ΔϕPC = 0).

We use a global MHD simulation to compute the distribution of E‖ on the face of the magnetopause as represented by the last closed field line surface. In MHD codes, E‖ is a proxy for magnetic reconnection. Integrating E‖ along the topological separator line between open and closed magnetic field lines gives the global reconnection rate at the magnetopause. In the case studied here, where the interplanetary magnetic field (IMF) is precisely duskward, we find the global reconnection rate to be ∼49 kV, comparable to potentials inferred from measurements made in the polar cap. The exercise demonstrates an application of a general reconnection theorem that, in effect, equates reconnection with E‖. It prepares the way for MHD imaging of reconnection in terms of contours of E‖ on the magnetopause. The result also illustrates a property of parallel potentials in the global context that is not generally recognized. Nearly the full magnetopause reconnection voltage exists on some closed field lines between the northern and southern polar caps, so that they leave the dawn, southern hemisphere with a sizable positive polarity and enter the dusk, northern hemisphere with a sizable negative polarity. An unexpected finding is a substantial parallel potential (between 10 and 15 kV) between the magnetopause and the ionosphere in northern dawn and southern dusk sectors. (Interchange “dawn” and “dusk” for dawnward IMF.) This potential has the polarity that accelerates electrons into the ionosphere in the dusk sector and, so, might be the origin of the “hot spot” seen there in precipitating electrons.

The magnetospheric sash is a ribbon of weak field shaped like a horseshoe with its open ends adjacent to the north and south dayside, magnetopause cusps and its closed end forming the cross-tail current sheet. The clock angle of the sash in the dawn-dusk meridian plane (as seen from the sun) rotates from 0° to 90° as the clock angle of the interplanetary magnetic field (IMF) rotates from 0° to 180°. We use a global MHD simulation to obtain the sash clock angles for IMF clock angles of 45°, 90°, and 135°. Remarkably, the results are very close to the clock angle of the magnetic null points obtained by superposing a uniform field representing the IMF on a dipole field representing the earth. Contours of magnetic field strength on cross sections perpendicular to the solar wind flow direction show how the sash evolves tailward from the dayside cusps.

This paper presents results of a study that uses the output of a global MHD simulation to investigate the geometry and dynamics of solar wind deflection from tangential magnetic force acting at the magnetopause. Tangential magnetic force at the magnetopause is a consequence of merging between the geomagnetic field and the interplanetary magnetic field at the dayside magnetopause. This is the first study explicitly to show deflection of magnetosheath flow at the magnetopause and to compute deflection phenomenon quantitatively using self-consistent MHD. The presentation connects magnetopause deflection with polarcap magnetic perturbations called the Svalgaard-Mansurov effect.

As revealed in MHD simulation, the magnetospheric sash is a band of weak magnetic field that, for the usual case in which the IMF is approximately perpendicular to the geomagnetic dipole, runs tailward along the high-latitude magnetopause flanks from one dayside cusp to the other, closing via the cross-tail neutral sheet. On the magnetopause flanks, it contains the magnetic separator line, at which all three topological types of field lines meet. Seen in a cross-sectional plane through the near-Earth tail, the magnetospheric sash takes the form of the cross-tail S, a weak-field feature comprised of the tail neutral sheet with diagonally symmetric extensions along the magnetopause flanks connecting it to the separator line. The cross-tail S is evident in the MHD results and in cross-sectional maps based on IMP 8 data. The magnetopause expression of the sash is latent in prior works that described the geometry of antiparallel fields across the magnetopause and the consequent cancellation of the fields within the magnetopause layer. The sash picture bears a strong resemblance to antiparallel merging geometry.

Global MHD simulations of the magnetosphere reveal unsteady vortical plasma flows at the mesoscale, similar in appearance to fluid turbulence at moderate Reynolds numbers, in the magnetotail under a variety of solar-wind conditions and interplanetary magnetic field (IMF) orientations. Such results are not unexpected in view of satellite data for magnetotail plasma-flow properties and the chaotic nature of the model MHD equations. Simulations generated with the Integrated Space Weather Prediction Model that are driven by either steady-state or highly variable solar-wind and IMF conditions suggest mesoscale vortical flows to be pervasive and robust features of magnetotail dynamics. Intermittent surges in simulated plasma flows are suggestive of bursty bulk flows. Autocorrelation functions for fluctuating properties in the simulated plasma sheet compare favorably to corresponding statistical reductions of satellite data.

[1] On 12 March 2001, the Polar and Cluster spacecraft were at subsolar and cusp latitudes in the dayside magnetosheath, respectively, where they monitored the passage by Earth of a large-scale planar structure containing the high-density heliospheric plasma sheet (HPS) and the embedded current sheet. Over significant intervals, as the magnetic hole of the HPS passed Cluster and Polar, magnetic field strengths |B| were much smaller than expected for the shocked interplanetary magnetic field. For short periods, |B| even fell below values measured by ACE in the upstream solar wind. Within the magnetic hole the ratio of plasma thermal and magnetic pressures (plasma β) was consistently >100 and exceeded 1000. A temporary increase in lag times for identifiable features in B components to propagate from the location of ACE to those of Cluster and Polar was associated with the expansion (and subsequent compression) of the magnetic field and observed low |B|. Triangulation of the propagation velocity of these features across the four Cluster spacecraft configuration showed consistency with the measured component of ion velocity normal to the large-scale planar structure. B experienced large-amplitude wave activity, including fast magnetosonic waves. Within the low |B| region, guiding center behavior was disrupted and ions were subject to hydrodynamic rather than magnetohydrodynamic forcing. Under the reported conditions, a significant portion of the interplanetary coupling to the magnetosphere should proceed through interaction with the low-latitude boundary layer. Data acquired during a nearly simultaneous high-latitude pass of a Defense Meteorological Satellites Program satellite are consistent with this conjecture.

[1] The interaction of a tangential discontinuity (TD) and accompanying dynamic pressure increase with the Earth's bow shock launches a fast shock that travels ahead of the TD in the magnetosheath and carries a significant portion of the pressure change. In this event study, we use observations from the Cluster spacecraft and magnetohydrodynamic simulations to identify the fast shock and its properties and to track the TD in the magnetosheath. Velocities of the fast shock and the TD were determined by triangulation using the four distant Cluster spacecraft. The fast shock is a planar structure, traveling nearly perpendicular to B at the magnetosonic speed in the plasma rest frame. Changes in density and ∣B∣ are correlated, with about a 20% increase in each. A current was observed tangential to the plane of the fast shock, and the positive E •J there provided an electromagnetic energy source for the observed heating of the ions. The fast shock is generated by the pressure change and determines the timing of the initial response of the magnetopause to that change. The TD was moving nearly in the −XGSE direction and was being compressed as it moved inward. The passage of the TD ushered in large-scale compressive structure in the magnetosheath magnetic field, which satisfied the mirror mode instability criterion. Velocities of a fast rarefaction wave, reflected from the magnetopause, and an additional slow-mode structure, which was not a product of the initial interaction with the bow shock, were determined by triangulation.

[1] Shortly after 0600 UT on 7 April 2000 a tangential discontinuity (TD) in the solar wind passed the Advanced Composition Explorer satellite (ACE). It was characterized by a rotation of the interplanetary magnetic field (IMF) by ∼145° and more than a factor-of-2 decrease in the plasma density. About 50 min later, Polar encountered more complex manifestations of the discontinuity near noon in the magnetosheath outside the Northern Hemisphere cusp. On the basis of Polar observations, theoretical modeling, and MHD simulations we interpret the event as demonstrating that (1) a fast mode rarefaction wave was generated during the TD-bow shock interaction, (2) the fast wave carried a significant fraction of the density change to the magnetopause while the remainder stayed with the transmitted discontinuity, and (3) magnetic merging occurred between IMF field lines within the magnetosheath on opposite sides of the discontinuity's surface as it approached the magnetopause. Before the discontinuity passed the spacecraft, Polar detected ions accelerated antiparallel to B in the fast wave and perpendicular to B in a weak slow mode structure located adjacent to and just downstream of the fast wave. The antiparallel accelerated ions in the fast wave had no measurable ion-velocity dispersion signature, placing their source a few RE equatorward of Polar. Simulation results, a Walen test, detections of wave Poynting flux parallel to B, bidirectional electron heat flux, and ion velocity enhancements all indicate that the three ion bursts associated with the passage of the discontinuity were signatures of time-dependent, magnetic merging events within the magnetosheath.

[1] Magnetohydrodynamic (MHD) simulations are used to examine the response of the transpolar potential (Φ TP ) to changes in the solar wind density during periods of constant solar wind electric field. For increases (decreases) in the solar wind density Φ TP responds immediately by increasing (decreasing) from the steady state values. In both cases the response of Φ TP is transient, decaying to near initial steady state values even when the density change persists. The magnitude of the Φ TP response is proportional to both the rate of erosion of the dayside magnetopause and the ionospheric Pedersen conductance. In our MHD simulations Φ TP is driven entirely by the dayside merging rate and is insensitive to changes in the nightside reconnection rate. The observed relationship between the modeled dayside merging rate and Φ TP is well characterized by an L-R circuit equation derived from integrating Faraday's Law around the Region 1 current loop. The inductive time constant for variations in the transpolar potential was found to be 6.5 (13) minutes for simulations using ionospheric Pedersen conductances of 6 (12) mhos. This corresponds in both cases to a magnetosphere-ionosphere inductance of 65 Henries. Observations of the transpolar potential derived using the assimilative mapping of ionospheric electrodynamics (AMIE) model are presented and shown to be consistent with the simulation results.

[1] MHD simulations predict that under strongly southward interplanetary magnetic field (IMF) conditions the polar cap potential saturates and attendant Region 1 currents affect the dayside magnetosphere by weakening subcusp magnetic fields and enhancing them in the high-latitude lobe. Consequently, “shoulder-like” structures appear as sunward projections of the high-latitude magnetopause relative to the location of the subsolar magnetopause that retreats earthward. We present observations from the Polar satellite, acquired during a period of strong southward IMF and saturated transpolar potential, which illustrate the predicted lobe field enhancement and resulting “shoulder” structure. At about the same time the GOES satellite observed weakening magnetic fields near the subsolar magnetopause. Similar types of observations, obtained under weakly and strongly driven magnetospheric conditions, are shown to be consistent with the MHD predictions. We demonstrate that including the Region 1 current generated magnetic field in the force balance equation at the magnetopause provides a quantitative basis for predicting magnetopause erosion during strongly southward IMF intervals.

The MHD simulations described here show that the latitude of the high-altitude cusp decreases as the IMF swings from North to South, that there is a pronounced dawn-dusk asymmetry at high-altitude associated with a dawn-dusk component of the IMF, and that at the same time there is also a pronounced dawn-dusk asymmetry at low-altitude. The simulations generate a feature that represents what has been called the cleft. It appears as a tail (when the IMF has a B y component) attached to the cusp, extending either toward the dawn flank or the dusk flank depending on the dawn-dusk orientation of the IMF. This one-sided cleft connects the cusp to the magnetospheric sash. We compare cusp geometry predicted by MHD simulations against published observations based on Hawkeye and DMSP data. Regarding the high-altitude predictions, the comparisons are not definitive, mainly because the observations are incomplete or mutually inconsistent. Regarding the low-altitude prediction of a strong dawn-dusk asymmetry, the observations are unambiguous and are in good qualitative agreement with the prediction.

[1] On 10 January 1997 an isolated solar wind density pulse, with other interplanetary parameters remaining steady, led to an enhanced transpolar potential (ΦTP) that is not predicted by either the Weimer or Siscoe-Hill models. However, a magnetohydrodynamic simulation of the magnetosphere's response to a sudden increase in solar wind density does predict an ΦTP increase consistent with satellite observations during the 10 January 1997 event. The response is transient, decaying after several tens of minutes toward an equilibrium that is close to the original potential. The simulated ΦTP increase derives from erosion of the dayside magnetopause and transport of newly opened flux across the polar cap. The exponential decay of ΦTP results from inductance in the magnetosphere-ionosphere circuit.

[1] Near magnetopause current layers the Polar satellite often detects field-aligned fluxes of >0.5 keV electrons that carry heat flux away from merging sites along inner/outer separatrices between newly opened and closed/interplanetary magnetic flux. Close to separator lines, electron gyrotropy weakens to break trapping on closed field lines of the outer plasma sheet followed by attachment to newly opened flux at random pitch angles. Energetic electrons in the “tails” of distribution functions quickly move great distances along separatrices to act as surgical indicators of active merging. If IMF BY is large, field rotation in the magnetopause current layer can be ≪180°, obscuring relationships between merging sources and separatrices. Suprathermal electrons and accelerated ions moving along nearby inner and outer edges of moderate shear current layers are remote signatures of merging at widely separate locations. Detecting accelerated ions at a moderate shear magnetopause is insufficient evidence for component merging occurring near a spacecraft. During magnetopause-skimming passes Polar sampled electron tail events characteristic of inner and outer separatrices that typically lasted from a few tens of seconds to several minutes. The 557.7-nm emissions observed at cusp boundaries suggest that their sources operate on similar timescales and are active simultaneously at multiple locations. The magnetic conjugacy of inner separatrices with cusp boundaries as well as the similar timescales of electron tail and 557.7-nm emission events strongly indicates that they are causally connected. Combining the two types of measurements offers an effective technique for separating temporal and spatial variability in magnetopause dynamics.

We present observations from the Polar satellite that confirm the existence of two types of depletion layers predicted under southward interplanetary magnetic field (IMF) conditions in magnetohydrodynamic simulations. The first depletion type occurs along the stagnation line when IMF BX and/or dipole tilt are/is present. Magnetic merging occurred away from the equator (Maynard et al., 2003) and flux pile-ups developed while the field lines drape to the high-latitude merging sites. This high-shear type of depletion is consistent with the depletion layer model suggested by Zwan and Wolf(1976) for low-shear northward IMF conditions. Expected sites for depletion layers are associated with places where IMF tubes of force first impinge upon the magnetopause. The second depletion type develops poleward of the cusp. Under strongly driven conditions, magnetic fields from Region1 current closure over the lobes (Siscoe et al., 2002c) cause the high-latitude magnetopause to bulge outward, creating a shoulder above the cusp. These shoulders present the initial obstacle with which the IMF interacts. Flow is impeded, causing local flux pile-ups and low-shear depletion layers to form poleward of the cusps. Merging at the high-shear dayside magnetopause is consequently delayed. In both low- and high-shear cases, we show that the depletion layer structure is part of a slow mode wave standing in front of the magnetopause. As suggested by Southwood and Kivelson(1995), the depletions are rarefactions on the magnetopause side of slow-mode density compressions. While highly sheared magnetic fields are often used as proxies for ongoing local magnetic merging, depletion layers are prohibited at merging locations. Therefore, the existence of a depletion layer is evidence that the location of merging must be remote relative to the observation.

A tutorial overview is given of theoretical models that describe the plasma flow across closed field lines in the low latitude boundary layer and the perfect or imperfect coupling of the layer to the dayside auroral ionosphere by means of field-aligned currents. Forces that control the flow include j x B forces, viscous forces, pressure forces, and inertia forces. Characteristic scale sizes for the boundary layer width are derived and important dimensionless groups are identified. A comparison of model results is made with results from a global numerical simulation, using the ISM code for the case of vanishing interplanetary magnetic field. Application of the theory when most of the boundary layer is on open field lines is discussed.

[1] Simultaneous measurements from the Polar satellite and by ground-based optical sensors suggest that brief variations of the poleward auroral boundary on the nightside correlate with changes in the interplanetary magnetic field (IMF) about an hour after a structure has propagated in the solar wind past the Earth. Short-lived Sun-aligned arcs may emerge along the open–closed magnetic field line boundary (OCB) and then disappear after ∼10 min. The arcs are fueled by energetic particles whose spectral characteristics are similar to those of the of boundary plasma sheet (BPS). Polar measurements confirm that these auroral protrusions into the polar cap occur on nearly isolated closed magnetic flux. Optical emissions from these arcs appear strongest at their intersection with the poleward boundary of the auroral oval. Detailed magnetohydrodynamic (MHD) simulations of dayside interactions, when the dominant IMF component BY changes sign, indicate that near the polarity reversal merging can occur between interplanetary field line segments within the magnetosheath [Maynard et al., 2001b]. Newly formed loops of interplanetary flux sweep past the Earth without interacting with the magnetosphere. Here, we consider some consequences within the distant magnetotail as the loops of disconnected flux propagate to XGSM ≈ −200 RE. The MHD simulations indicate that about an hour after intra-IMF merging events fingers of closed field lines protrude from the OCB into the polar cap. Like the observed Sun-aligned arc, these simulated auroral features grow and decay on scales of ∼10 min and have ionospheric footprints that are nearly surrounded by open magnetic flux. The simulated auroral fingers are conjugate to high-pressure channels in the distant plasma sheet. We suggest that the short-lived Sun-aligned arcs are created via an interchange process similar to that proposed by Kan and Burke [1985] to explain one class of theta auroral forms. The continuity of magnetotail currents across a high-pressure channel requires the development of field-aligned currents carried by obliquely propagating Alfven waves. Plasma drifts associated with the dusk-to-dawn electric fields of the Alfven waves are away from the Earth in the magnetotail and poleward in the nightside ionosphere. The correlation of phenomena at the nightside OCB with variations in the IMF indicate that processes other than substorms can influence boundary dynamics. Effects of self-interactions of the IMF within the dayside magnetosheath may be felt along the OCB as much as 1 hour later.

[1] We present a comparison between a simple but general model of solar wind-magnetosphere-ionosphere coupling (the Hill model) and the output of a global magnetospheric MHD code, the Integrated Space Weather Prediction Model (ISM). The Hill model predicts transpolar potential and region 1 currents from environmental conditions specified at both boundaries of the magnetosphere: at the solar wind boundary, electric field strength, ram pressure, and interplanetary magnetic field direction; at the ionospheric boundary, conductance and dipole strength. As its defining feature, the Hill model predicts saturation of the transpolar potential for high electric field intensities in the solar wind, which accords with observations. The model predicts how saturation depends on boundary conditions. We compare the output from ISM runs against these predictions. The agreement is quite good for non-storm conditions (differences less than 10%) and still good for storm conditions (differences up to 20%). The comparison demonstrates that global MHD codes (like ISM) can also exhibit saturation of transpolar potential for high electric field intensities in the solar wind. We use both models to explore how the strength of solar wind-magnetosphere-ionosphere coupling depends on the strength of Earth's magnetic dipole, which varies on short geological timescales. As measured by power into the ionosphere, these models suggest that magnetic storms might be considerably more active for high dipole strengths.

[1] From the perspective of electric circuit theory a question regarding the existence of magnetopause reconnection arises because in circuit theory reconnection is a load and a load needs a dynamo to supply it. However, among current systems that have been identified that thread the presumed regions of magnetopause reconnection, none also treads a dynamo. Yet there is overwhelming observational evidence that magnetopause reconnection occurs. This paper attempts to reconcile the circuit theory viewpoint with observations of reconnection. The procedure is first to show that results of MHD simulation can be translated into circuit theory language. Then MHD simulation is applied to show that magnetopause reconnection is associated with current systems that close through the bow shock or the magnetosheath, which from the viewpoint of circuit theory are dynamos. Thus the reconciliation is simply that reconnection current systems are different from the standard set of current systems that have traditionally been associated with the magnetopause.

[1] This note reports properties of a little-studied but possibly common mode of magnetic reconnection—“flow-through reconnection” or “FTR”—characterized by plasma flowing at high speed right through a stationary reconnection site. FTR is readily identified in global MHD simulations by the following diagnostic properties, which affect slab-like portions (tongues) of the magnetopause, usually at high latitudes and along the flanks. 1. A magnetic null point lies near the downwind extreme of an FTR tongue. 2. Elsewhere within the tongue the magnetic field is also weak. 3. Electric current density is strong. 4. “J-dot-E” dissipation is strong. 5. Most remarkable, the electric field is antiparallel to and stronger than −V × B, the motional electric field. This last property implies that plasma and magnetic field flow in opposite directions—plasma downwind, magnetic field upwind. Here we discuss FTR for strictly northward interplanetary magnetic field, which manifests FTR properties in idealized geometry.

[1] MHD simulations give about the same dependence of transpolar potential on solar wind electric field (IEF) as the Hill model of transpolar potential, including saturation and dependence on ram pressure. In the Hill model, feedback of the region 1 current system is presumed to limit the rate of reconnection at the magnetopause thereby causing transpolar potential saturation. MHD simulations add as relevant information that in the saturation domain the region 1 current system usurps the role of the Chapman-Ferraro current system, which disappears. This means that the region 1 current system takes on the role of providing the current and generating most of the magnetic field in the J × B force at the magnetopause that balances solar wind ram pressure. Viewed from this perspective, transpolar potential saturation results not from the region 1 current system limiting the rate of reconnection at the magnetopause but instead from ram pressure (more accurately, total solar wind stresses) limiting the total amount of current that can flow in the region 1 current system. Transpolar potential saturation is then the limit on transpolar potential that corresponds to the ram-pressure limit on total region 1 current.

This paper discusses the global geometry of magnetospheric current systems as determined by a global MHD code simulating the magnetosphere for four elemental settings of the interplanetary magnetic field: 1. zero field strength (giving the Chapman-Ferraro and tail current systems); 2. straight north (giving the NBZ current system); 3, straight south (giving the region 1 current system); and 4. straight dawn to dusk (giving the cusp-mantle current system). The Chapman-Ferraro and tail current systems have the topologies usually ascribed to them, but the MHD simulation reveals that they overlap across a significant area of the magnetopause. The NBZ current system closes in the distant magnetotail. The region 1 current system closes across the high-latitude magnetopause. The cusp-mantle current system threads most of the magnetosphere. It spirals earthward around the south magnetotail lobe until it reaches the magnetosphere proper where it catapults over the South Pole to descend into the ionosphere along the polar border of the dayside polar cusp where it becomes the mantle part of the cusp-mantle system. Once in the ionosphere, it jogs a little north then lifts off again along the equatorward border of the cusp as the cusp part of the cusp-mantle system. From there, it follows magnetic field lines in a dipolar arch to the equatorial magnetopause beyond which it mirrors its southern career in the northern hemisphere. The cusp-mantle current system joins smoothly and continuously onto the region 1 current system. They appear to form a single system.

[1] Magnetohydrodynamic (MHD) simulations using the Integrated Space Weather Prediction Model (ISM) show merging in the magnetosheath between interplanetary magnetic field (IMF) field lines on opposite sides of a directional discontinuity. As the discontinuity passes the bow shock and traverses the magnetosheath, the magnetic field gradients on either side become steeper, and the geometry is distorted by the nonuniform flow speed in the magnetosheath. Associated current densities are intensified. In a location isolated from the magnetopause, we use the appearance of an X magnetic field configuration, associated dissipation electric fields, and increases in plasma velocity in the exhaust direction from the X as evidence for merging in the magnetosheath. We suggest that these signatures might be observable by Polar, as it now traverses the magnetosheath near the nose or by the Cluster spacecraft when the discontinuity is tilted away from the YZ plane. Merging signatures are seen in simulations with both 90° and 180° rotations of the field across the discontinuity. Magnetosheath merging creates a hole in the directional discontinuity, allowing the magnetosphere to penetrate through the structure as the discontinuity passes downstream. Magnetosheath merging has the potential to affect ionospheric convection pattern changes, open–closed boundaries, and magnetotail dynamics. A possible association with hot flow anomalies (HFA) is also indicated.

Auroral specialists have suggested that in the Bible's Old Testament book of Ezekiel, the opening vision of a “storm cloud out of the north” depicts imagery inspired by a low-latitude auroral display [Link, 1967; Eather, 1980; Silverman, 1998]. Naturally, other interpretations have been suggested, including a true epiphany, a sandstorm, a thunderstorm, a tornado, a solar halo, a hallucination, and a UFO. Biblical scholars place the site of the Ezekiel's vision about 100 km south of Babylon near Nippur, latitude ˜32°, longitude ˜45°, and the date is within a year or two of 593 B.C., or about 2600 years ago. An auroral interpretation of the vision is subject to possible refutation due to several geophysical considerations. Can auroras be seen at Ezekiel's latitude? More important, can they reach a coronal stage of development, which is what the vision requires? Was the tilt of the dipole axis favorable? Was the general level of solar activity favorable? And finally, What effect does a larger dipole moment in Ezekiel's time have on the question? All but the last question could have been answered on the basis of geophysical data a decade ago or earlier.