D. R. Weimer

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.

Geospace Environment Modeling 2008–2009 Challenge: Ground magnetic field perturbations

helps the users of the modeling products to better understand the capabilities of the models and to choose the approach that best suits their specific needs. Further, metrics!based analyses are important for addressing the differences between various modeling approaches and for measuring and guiding the progress in the field. In this paper, the metrics!based results of the ground magnetic field perturbation part of the Geospace Environment Modeling 2008‐2009 Challenge are reported. Predictions made by 14 different models, including an ensemble model, are compared to geomagnetic observatory recordings from 12 different northern hemispheric locations. Five different metrics are used to quantify the model performances for four storm events. It is shown that the ranking of the models is strongly dependent on the type of metric used to evaluate the model performance. None of the models rank near or at the top systematically for all used metrics. Consequently, one cannot pick the absolute“winner”: the choice for the best model depends on the characteristics of the signal one is interested in. Model performances vary also from event to event. This is particularly clear for root!mean!square difference and utility metric!based analyses. Further, analyses indicate that for some of the models, increasing the global magnetohydrodynamic model spatial resolution and the inclusion of the ring current dynamics improve the models’capability to generate more realistic ground magnetic field fluctuations.

Modeling studies of the impact of high-speed streams and co- rotating interaction regions on the thermosphere-ionosphere

Received 28 November 2011; revised 14 June 2012; accepted 14 June 2012; published 1 August 2012. [1] Changes in the thermosphere-ionosphere system caused by high-speed streams in the solar wind, and the co-rotating interaction regions they engender, are studied using a combination of model simulations and data analysis. The magnetospheric responses to these structures and consequent ionospheric drivers are simulated using the numerical Coupled Magnetosphere-Ionosphere-Thermosphere model and the empirical Weimer 2005 model, finding that the interplanetary magnetic field (IMF) is more important than solar wind speed and density per se in controlling magnetosphere-ionosphere coupling. The NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model is then employed to calculate neutral density, nitric oxide cooling, and electron density, for comparison to space-based measurements from the STAR instrument on the CHAMP satellite, the SABER instrument on the TIMED satellite, and GPS occultations from the COSMIC mission, respectively. The recurrent, periodic changes observed under solar minimum conditions during 2008, and particularly during the Whole Heliospheric Interval (March–April of 2008), are simulated by the model and compared to these measurements. Numerical experiments were conducted to elucidate the mechanisms of solar wind and IMF forcing, setting the solar wind speed and density to nominal values, smoothing the IMF, and also setting it to zero. The results confirm the importance of IMF variations, particularly its north-south component (Bz), but also show that when the average Bz values are negative (southward), the interaction with increased solar wind speed amplifies the magnetosphere-ionosphere-thermosphere response. Conversely, during events when Bz is on average positive (northward), even large increases in solar wind speed have small effects on the system.

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).

Polar observations of convection with northward interplanetary magnetic field at dayside high latitudes

This study of dayside electrodynamics provides a first view of quasi-dc electric fields measured by Polar at middle altitudes. Three high-latitude passes chosen for analysis, occurred while Polar was in the midday sector and the interplanetary magnetic field (IMF) maintained a steady northward orientation at clock angles near 40° in the YGSM – ZGSM plane. At different times during the passes, Polar encountered electron and ion fluxes from the central plasma sheet (CPS), the boundary plasma sheet, the polar rain, and various magnetospheric boundary regions. Although electric fields measured within the boundary layers were highly variable, we derived average components of plasma convection. The results compare favorably with convection characteristics of four-cell patterns observed at ionospheric altitudes with northward IMF. We show that Polar encountered lobe cells of both positive and negative polarity in the early postnoon sector. The negative potential extends well into the prenoon sector. Inverse, energy-dispersed fluxes of He++ were detected in regions of sunward convection. Sunward convection extended into the region of polar rain north of the merging lines projection. The magnetic merging signatures indicate that the process is both time varying and patchy. Particle and field measurements are consistent with the afternoon, auroral convection cell closing in the low-latitude boundary layer, which extend many tens of RE tailward of the dusk terminator. Taken in a larger context, the Polar measurements provide new insight concerning the evolution of high-latitude convection from distorted two-cell patterns at large IMF clock angles to four cells at low clock angles.

Energy coupling during the August 2011 magnetic storm

Abstract : We present results from an analysis of high-latitude ionosphere-thermosphere (IT) coupling to the solar wind during a moderate magnetic storm which occurred on 5 6 August 2011. During the storm, a multipoint set of observations of the ionosphere and thermosphere was available. We make use of ionospheric measurements of electromagnetic and particle energy made by the Defense Meteorological Satellite Program and neutral densities measured by the Gravity Recovery and Climate Experiment satellite to infer (1) the energy budget and (2) timing of the energy transfer process during the storm. We conclude that the primary location for energy input to the IT system may be the extremely high latitude region. We suggest that the total energy available to the IT system is not completely captured either by observation or empirical models.

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.

How wide in magnetic local time is the cusp? An event study

A unique pass of the DMSP F11 satellite, longitudinally cutting through the cusp and mantle, combined with simultaneous optical measurements of the dayside cusp from Svalbard has been used to determine the width in local time of the cusp. We have shown from this event study that the cusp was at least 3.7 hours wide in magnetic local time. These measurements provide a lower limit for the cusp width. The observed cusp optical emissions are relatively constant, considering the processes which lead to the 630.0 nm emissions, and require precipitating electron flux to be added each minute during the DMSP pass throughout the local time extent observed by the imaging photometer and probably over the whole extent of the cusp defined by DMSP data. We conclude that the electron fluxes which produce the cusp aurora are from a process which must have been operable sometime during each minute but could have had both temporal and spatial variations. The measured width along with models of cusp precipitation provide the rationale to conclude that the region of flux tube opening in the dayside merging process involves the whole frontside magnetopause and can extend beyond the dawn-dusk terminator. The merging process for this event was found to be continuous, although spatially and temporally variable.

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.