K. D. Siebert

The Magnetospheric Sash and the Cross‐Tail S

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 role of E ‖ in magnetopause reconnection: An explicit demonstration

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.

Simulations of the magnetosphere for zero interplanetary magnetic field: The ground state

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).

MHD properties of magnetosheath flow

Abstract 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 magnetosheaths 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.

Response of ionospheric convection to changes in the interplanetary magnetic field: Lessons from a MHD simulation

Characteristics of magnetospheric and high-latitude ionospheric convection pattern responses to abrupt changes in the interplanetary magnetic field (IMF) orientation have been investigated using an MHD model with a step function reversal of IMF polarity (positive to negative By) in otherwise steady solar wind conditions. By examining model outputs at 1 min intervals, we have tracked the evolution of the IMF polarity reversal through the magnetosphere, with particular attention to changes in the ionosphere and at the magnetopause. For discussion, times are referenced relative to the time of first contact (t = 0) of the IMF reversal with the subsolar nose of the magnetopause at ∼ 10.5 R E . The linear change in large-scale ionospheric convection pattern begins at t = 8 min, reproducing the difference pattern results of Ridley et al. [1997, 1998]. Field-aligned current difference patterns, similarly derived, show an initial two-cell pattern earlier, at t = 4 min. The current difference two-cell pattern grows slowly at first, then faster as the potential pattern begins to change. The first magnetic response to the impact of the abrupt IMF transition at the magnetopause nose is to reverse the tilt of the last-closed field lines and of the first-open field lines. This change in tilt occurs within the boundary layer before merging of IMF with closed magnetospheric field lines starts. In the case of steady state IMF By, IMF field lines undergo merging or changing partners with other IMF field lines, as they approach the nose and tilt in response to currents. When the By reversal approaches the magnetopause nose, IMF field lines from behind the reversal overtake and merge with those in front of the reversal, thus puncturing the reversal front and uncoupling the layer of solar wind plasma in the reversal zone from the magnetosphere. The uncoupled layer propagates tailward entirely within the magnetosheath. Merging of closed magnetospheric field lines with the new polarity IMF begins at t = 3 min and starts to affect local currents near the cusp 1 min later. While merging starts early and controls the addition of open flux to the polar cap, large-scale convection pattern changes are tied to the currents, which are controlled in the boundary layers. The resulting convection pattern is an amalgamation of these diverse responses. These results support the conclusion of Maynard et al. [2001b], that the small convection cell is driven from the opposite hemisphere in By-dominated situations.

Relation between cusp and mantle in MHD simulation

A global MHD simulation illustrates the relative contributions to magnetotail structure of two mantle models. One model is the data-based picture that Rosenbauer et al. [1975] originally put forth showing the cusp to be the source of the mantle. In this model the cusp is populated by magnetosheath particles that enter it as a consequence of magnetic reconnection on the day side magnetopause. The other model, which derives from MHD considerations, notes that since the mantle lies on open magnetic field lines, the solar wind should be able to enter it along the tail magnetopause downwind from the cusp. In the simulation results presented here, both modes of plasma entry into the mantle occur and their relative contributions are seen. The mantle, identified in the simulation as the volume of the tail occupied by a slow-mode expansion wave filled with magnetosheath plasma, lies mostly on streamlines that originate in the dayside cusp (cusp model), but the bulk of plasma inflow comes directly from the magnetosheath (magnetosheath model). The demarcation between streamlines originating in the cusp (cusp model) and streamlines originating in the solar wind (magnetosheath model) is sharp. We call this demarcation the fluopause. The fluopause is the downwind extension of the solar wind stagnation streamline. At 100 RE downtail the fluopause lies several RE inside the magnetopause identified by the peak in the electrical current that defines the magnetotail boundary. The spatial separation between the fluopause and the magnetopause defines the volume to which the magnetosheath model refers. Whereas the magnetopause clearly marks the start of the slow-mode expansion wave that defines the mantle, all MHD parameters and their slopes are continuous across the fluopause. The fluopause is only determinable by following streamlines. Streamlines in the mantle that originate in the solar wind (magnetosheath model) do not converge on the plasma sheet and, so, are not a likely source of particles for the plasma sheet. Streamlines in the mantle that originate in the cusp (cusp model) do converge on the plasma sheet and, so, are likely candidates for the particles that supply the plasma sheet. At their earthward ends, streamlines that originate in the cusp reach low altitudes and might, in nature, be populated with ionospheric plasma. An implication of the present result is that the inner portions of mantle encounters that have been identified in IMP 8 and Geotail data refer to plasma that has spent part of its life in the dayside cusp and possibly in the ionosphere. The latter possibility might account for Geotail observations that identify O+ as a common constituent of deep-mantle plasma.

Magnetospheric sash dependence on IMF direction

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.

Deflected magnetosheath flow at the high‐latitude magnetopause

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.

Theory of the Low Latitude Boundary Layer and Its Coupling to the Ionosphere: a Tutorial Review

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.

Ezekiel and the Northern Lights: Biblical aurora seems plausible

Auroral specialists have suggested that in the Bibles 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 Ezekiels 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 Ezekiels 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 Ezekiels 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.