S. P. Slinker

Global numerical simulation of the growth phase and the expansion onset for a substorm observed by Viking

We report the first global magnetohydrodynamic (MHD) simulation of an actual magnetospheric substorm, which was recorded by the Viking spacecraft on October 19, 1986. The simulation is driven by IMP 8 solar wind parameters measured upstream of the Earths bow shock. The substorm, which had expansion onset at 1132 UT, was caused by a brief period of southward interplanetary magnetic field (IMF) and two weak solar wind shocks. The simulation model includes a self-consistent auroral ionospheric conductance depending directly on the MHD magnetospheric plasma parameters and magnetic field. Synthetic auroral emissions, derived from simulation results, are compared to the Viking images, which show considerable dayside activity preceding the substorm. We also compare model-derived synthetic AU and AL indices to geomagnetic measurements. The simulation results are seen to be in reasonable agreement with the observations throughout the growth phase and expansion onset. Moreover, the results allow us to form conclusions concerning which essential processes were responsible for the substorm occurence. These results are a highly encouraging first step leading toward development of a space weather forecasting methodology based on the directly measured solar input.

Topological structure of the magnetotail as a function of interplanetary magnetic field direction

Magnetic reconnection between the interplanetary magnetic field (IMF) and the geomagnetic field is thought to play a major role in the transfer of solar wind momentum and energy to the magnetosphere. As the angle between the IMF and the geomagnetic field is changed at the bow of the magnetosphere, the topological record of the location of the reconnection region should be recorded in the magnetosheath and on the magnetopause along the flanks of the tail, because the super fast flow freezes strong magnetic gradients formed in the bow reconnection regions into the plasma downstream. In this report, we present results from a three-dimensional, magnetohydrodynamic (MHD), global numerical simulation code for the location of the separatrix between unconnected IMF magnetosheath field lines and reconnected field lines which penetrate the magnetopause and connect to the polar ionosphere. The angle between the IMF direction and the line where the separatrix crosses the magnetopause is shown to be a sensitive function of the IMF clock angle. We also explain how this behavior can be used to derive an approximate relation for the dependence of the cross-polar voltage on the IMF clock angle. We conclude with a note of caution concerning the importance of physical boundary conditions in magnetoplasma simulations.

Impulsive enhancements of oxygen ions during substorms.

It has been observed that H+ is the dominant ion species in the plasma sheet and the ring current during quiet times. However, the O+/H+ density ratio increases with increasing geomagnetic storm and substorm activity. Energetic neutral atom (ENA) images from Imager for Magnetopause-to-Aurora Global Exploration/High Energy Neutral Atom (IMAGE/HENA) reveal the rapid increase of O+ ring current at substorm expansion. Finding the cause of this substorm-associated O+ enhancement is the main focus of this paper. Two possible sources are suggested: direct injection from the ionosphere and energization of the preexisting oxygen ions in the magnetosphere. We perform numerical simulations to examine these two mechanisms. Millions of O+ are released from the auroral region during a simulated substorm by the Lyon-Fedder-Mobarry MHD model. The subsequent trajectories of these outflowing ions are calculated by solving the full equation of particle motion. A few minutes into the substorm expansion phase, an enhancement in O+ pressure is found on the nightside at ∼12 RE. After careful analysis, we conclude that this pressure peak is coming from energization of the preexisting O+ in the plasma sheet. The direct injection mechanism will introduce a significant time lag between strong ionospheric outflow and magnetospheric enhancement, so that it cannot explain the observed O+ bursts. Using the temperature and density established by the test-particle calculations as boundary conditions to a ring current model, we calculate the O+ fluxes and the corresponding ENA emissions during the model substorm. We are able to reproduce observable features of oxygen ENA enhancements as seen by IMAGE/HENA.

Response of the ionosphere to a density pulse in the solar wind: Simulation of traveling convection vortices

We show the response of the Earths magnetosphere and ionosphere to a density pulse in the solar wind using a global 3D MHD simulation model. Flow vortices are generated in the ionosphere and they exhibit many properties similar to those observed during impulsive traveling convection vortices. Two oppositely rotating flow vortices are formed at about 70° magnetic latitude near noon. They separate and move down the morning and evening flanks, greatly weakening as they pass the terminator. Meanwhile a second pair of flow vortices appears at noon at a slightly higher latitude and with opposite flow directions than the first pair. The second pair follows the first and also fades as it reaches the nightside. The results are interpreted as a hydromagnetic wave propagating in the inhomogeneous magnetosphere plasma.

Low Frequency Oscillations in A Plasma with Spatially Variable Field-Aligned Flow

The effects of a transverse gradient in the plasma flow velocity parallel to the ambient magnetic field are analyzed. A transverse velocity gradient in the parallel ion flow, even in small magnitude, can increase the parallel phase speed of the ion-acoustic waves sufficiently to reduce ion Landau damping. This results in a significantly lower threshold current for the current driven ion acoustic instability. Ion flow gradients can also give rise to a new class of ion cyclotron waves via inverse cyclotron damping. A broadband wave spectrum with multiple cyclotron harmonics is possible. A combination of the multiple cyclotron harmonic waves can result in spiky electric field structures with their peaks separated by an ion cyclotron period. A spatial gradient in the parallel electron flow is also considered but it is found to play a minimal role in the low frequency regime. Relevance of these to natural plasma environments is discussed.

Global MHD simulation of the magnetosphere for November 24, 1996

Using Wind-measured solar wind data, we have simulated the Earths magnetosphere and ionosphere for the period 1930–2330 UT on November 24, 1996. The simulation model is a global, three-dimensional, MHD formulation. This event is the focus of a Geospace Environment Modeling (GEM) substorm challenge. The event features a strongly northward interplanetary magnetic field (IMF) period, followed by a sudden rotation to steadily southward. About 80 min after the Wind-observed north-south transition a substorm was observed by the Polar-visible imaging system (VIS). There was good data coverage throughout this period, and the modeling community was challenged to simulate the event. During the northward period the simulation produces a small open polar cap at high magnetic latitudes, especially on the dawnside, where a polar arc was observed by the Polar-VIS and was reproduced in the simulation results. The simulation also shows the ionospheric response to the southward transition in the IMF propagate from the day side to the night side in only a few minutes, consistent with Super Dual Auroral Radar Network (SuperDARN) data. Later, the simulation also produces a substorm, but it occurs nearly a half hour earlier than was observed. Numerical experiments were performed by altering the solar wind IMF to investigate the sensitivity of substorm onset timing to this parameter. The model substorms occur spontaneously and do not show a strong correlation between an IMF-induced convection reduction and the onset. The initial event in the expansion onset of the simulation substorm is the beginning of fast magnetic reconnection at ∼ 20 RE in the stressed magnetotail.

Plasma sheet and (nonstorm) ring current formation from solar and polar wind sources

We consider the formation of the plasma sheet and geosynchronous region (nonstorm) ring current in the framework of collisionless test particle motions in three-dimensional magnetospheric fields obtained from self-consistent MHD simulations. Simulation results are compared with observations of the near-Earth plasma sheet from the Polar spacecraft during 2001 and 2002. Many particles were initiated in two regions representative of the solar wind source upstream of the bow shock and the polar wind source outside the plasmasphere, both of which are dominated by protons (H+). Proton trajectories are run until they precipitate into the atmosphere, escape from the simulation space, or become stably trapped. These calculations produce a database of proton characteristics in each 1 RE3 volume element of the magnetosphere and yield velocity distributions as well as bulk plasma properties. We report results reflecting steady growth phase conditions after 45 min of southward interplanetary field, BZ = −5 nT (BY = 0), and for conditions resulting after 2 hours of northward BZ = +5 nT. The results for simulated velocity distributions are consistent with the Polar soundings of the current sheet from lobe to lobe and with AMPTE/CCE observations of (nonstorm) ring current region protons. The simulations help us identify the differentiation between solar and polar wind H+ ions in observations. The weak NBZ ring current-like pressure is primarily polar wind protons, while the moderately active SBZ ring current-like pressure is primarily solar wind protons. The solar and polar wind contributions to the SBZ ring current are comparable in density, but the solar protons have a higher average energy. For SBZ, solar wind protons enter the nonstorm ring current region primarily via the dawn flank and to a lesser degree via the midnight plasma sheet. For NBZ, solar wind protons enter the ring current-like region via the cusp and flanks. Polar wind protons enter the nonstorm ring current through the midnight plasma sheet in both cases. Solar and ionospheric plasmas thus take different transport paths to the geosynchronous (nonstorm) ring current region and may thus be expected to respond differently to substorm dynamics of the magnetotail.

A comparison of global numerical simulation results to data for the January 27–28, 1992, Geospace Environment Modeling challenge event

A magnetohydrodynamic (MHD) numerical simulation model is used to study the magnetospheric state during the Geospace Environment Modeling (GEM) challenge period on January 27-28, 1992. Direct comparisons are made between the simulation results and the GEM Program data as summarized in the synoptic maps. The features we concentrate on are the cross-polar convection measurements and the ionospheric projection of magnetospheric boundaries. The results show good agreement at times and locations and poor agreement at other times. All in all, the results are rather mixed. We discuss the results and what they might mean for the accuracy of both the simulations and the synoptic maps. Moreover, we relate the results to other previous research on the cross-polar potential and draw guidance for future work.

Flux rope model of the 2003 October 28-30 coronal mass ejection and interplanetary coronal mass ejection

A numerical model of an erupting solar flux rope is shown to reproduce both quantitative near-Sun properties of the 2003 October 28 coronal mass ejection and the timing, strength, and orientation of the fields measured in situ at 1 AU. Using a simple erupting flux rope model, we determine the best-fit parameters for this event. Our analysis shows that the orientation of the magnetic axis of the flux rope in this case rotates smoothly through approximately 50 � as thefluxropeapex expandsfromthe solar surface to1AU. Using aglobal magnetospheric simulation code,we further show that the resulting model solar wind properties at 1 AU produce a magnetospheric response comparable to that computed using the actual solar wind data.

Entry of the POLAR spacecraft into the polar cusp under northward IMF conditions

On May 29, 1996 from 0200 to 0800 UT the solar wind dynamic pressure was high ranging from 6 to 8 nPa and the interplanetary field was almost due northward, ranging from 10 to 15 nT in BZ GSM. Even at apogee the POLAR spacecraft should not have entered the magnetosheath according to recent scaling laws. However, the magnetic fieid was greatly depressed below the value expected indicating the presence of significant plasma energy density throughout the high latitude magnetosphere surrounding the cusp. The presence of this plasma is confirmed by the plasma instrumentation on board the spacecraft. While the entry into this nearly stagnant plasma was gradual, the exit on to polar cap field lines was abrupt. We interpret these observations in terms of the post-cusp reconnection of the strongly northward interplanetary magnetic field.