Michael James Wiltberger

MHD simulation of the magnetotail during the December 10, 1996, substorm

This paper presents results of a global MHD simulation of a substorm that occurred on December 10, 1996. We concentrate on the relationship between the simulation results and the magnetotail observations during the growth and expansion phases of the substorm. In general, we find excellent agreement between the single point observations made by various spacecraft in both the geosynchronous and mid-tail regions: the simulation accurately represented the energy loading (lobe field increase), small-scale activations (partial dipolarizations), and a global substorm onset (large dipolarizations and fast flows). The global view presented by the simulation shows complex series of discrete flow channels during the expansion phase prior to the onset of global reconnection. It is these flows channels that disrupt the thin current sheets present during the expansion phase of the substorm.

Pseudobreakup and substorm onset: Observations and MHD simulations compared

The global conditions during a moderate geomagnetic disturbance event on May 15, 1996, are examined by comparing data from several ground-based instruments and inner tail satellites with global MHD simulations of the same event. The ground-based data show two substorm intensifications about 40 min apart, the first one being small and localized (a pseudobreakup) and the second leading to a major rearrangement of both the ionospheric auroral distribution and the magnetotail configuration. The simulation shows that during the pseudobreakup, open field lines were reconnecting in the midtail, but the flows were mainly tailward and very few effects were observable in the inner magnetosphere. The result that pseudobreakups can be associated with activity producing topological changes in the tail is an important new aspect that has not been discussed in earlier studies. Both the observations and the simulation show two distinct regions of activity: a thin current sheet in the inner tail magnetically connected with the auroral bulge and a reconnection region in the midtail associated with the most intense electrojet currents.

An overview of the impact of the January 10–11 1997 magnetic cloud on the magnetosphere via global MHD simulation

The results of a 3D MHD simulation of the January 10-11, 1997 geomagnetic storm are presented. The simulation results agree well with ground-based and geosynchronous observations. The 28 hours modeled by the simulation include the magnetic cloud responsible for the storm, the shock preceding the cloud, and the dense plasma filament following it. The simulation shows that during the period of southward (MF ionospheric activity was strongly correlated to the solar wind density. The arrival of the plasma filament during northward IMF pushed the dayside magnetopause well within geosynchronous orbit, but generated little ionospheric activity. It appears that n,w as well as the orientation of Bsw plays a role in controlling the intensity of ionospheric and magnetospheric activity.

MHD simulations of the response of high‐latitude potential patterns and polar cap Boundaries to sudden southward turnings of the interplanetary magnetic field

3-D MHD simulations were used to investigate the behavior of the high-latitude convection and the polar cap variations during two events characterized by sudden southward IMF turnings. In agreement with recent observations the simulation results indicate that the convection pattern across the entire polar cap begins to change a few minutes after the arrival of the southward IMF. In contrast, the onset of the equatorward motion of the open closed field-line boundary depends on the local time, with equatorward motion of the midnight boundary delayed by about 20 minutes relative to the the onset of the boundary motion at noon. We interpret this delay as the time required to convect newly merged flux from the dayside to the nightside. We belive that these two different responses can reconcile apparent contradictions in studies of ionospheric reconfigurations in response to changes in the IMF.

The physics of substorms as revealed by the ISTP

Abstract Spacecraft and ground data combined with multiscale computer models developed by the ISTP program are providing a new and coherent understanding of the magnetospheric substorms and storms. Global MHD simulations that include ionospheric response are dynamically driven by upstream satellite data and allow for direct comparison with the field and flow quantities measured by magnetospheric satellites, ground data and images from the POLAR satellite. Through the combined analysis of the simulations and observations, the first unified picture of a substorm from the magnetospheric and ionospheric viewpoint is currently emerging. Here we use MHD simulations of two particularly well observed and analyzed events to explore the factors that trigger and organize the substorm elements into a coherent entity. The first event — March 9, 1995 — produces clear evidence that impulsive penetration of a large electric field in the vicinity of -8 to -10 RE, possibly associated with magnetosonic energy focusing, acts as a trigger for substorm initiation. It is the element that connects the ionospheric to magnetospheric substorm. Particularly impressive is the timing of the chain of events and indices observed on the ground and their proxies computed in the simulation. This simulation is complemented by a “theoreticians” simulation, a step function transition of the IMF from northward to southward, which clarifies the physics of energy penetration into the magnetosphere and demonstrates Poynting flux focusing in the near earth tail. The second event — January 10–11, 1997 — was driven by the impact of a magnetic cloud in the magnetosphere. It induced major disturbances in the magnetosphere and the groundand resulted in the loss of a geosynchronous ATT satellite. It is a simulation “tour de force” and used continuous upstream data over 36 hours as input. The results provide a graphical and fascinating view of the global magnetospheric and tail response to a magnetic cloud impinging upstream, illustrate the importance of dynamics and indicate that pressure impulses play a key role in providing the coherence required for substorms.

Three‐dimensional MHD simulations of the steady state magnetosphere with northward interplanetary magnetic field

In the recent past, three-dimensional global MHD codes have successfully demonstrated that they are invaluable tools for simulating global magnetospheric phenomena. In this paper, using the Lyon-Fedder-Mobarry code, we present results on the spatial structure of the steady state of Earths magnetosphere for northward interplanetary magnetic field (IMF). We have shown that the steady state tail length of the Earths magnetosphere for northward IMF is short and that the three dimensional structure of the tail lobe is of concave shape. This is in contradiction to the results presented by Fedder and Lyon [1995]. Explanation for this discrepancy is given in this paper. By varying the simple time-independent solar wind velocity and interplanetary magnetic field, we present the parametric (power law) dependence of the geometrical aspects of the tail lobe on solar wind condition. In general, with higher solar wind velocity the radius of the tail lobe cross section is smaller and the tail length is longer. With higher northward IMF the whole tail lobe is smaller. These numerical studies have guided us in building analytical models based on global conservation constraints and force balance. We conclude that the lobe size is mainly determined by the solar wind velocity and the reconnection process in the cusp region which is affected by the magnitude of the IMF. We also show that the ionospheric effects are not significant in determining the lobe size. The evolution history of the tail length for magnetotail to reach steady state is also presented in this paper. Emphasis is placed on the fact that the steady state is a dynamical steady state.

3D MHD simulations of the heliosphere-VLISM interaction

We present results of a global 3D MHD simulation of the interaction of the solar wind with the very local interstellar medium (VLISM). Three different cases are studied: purely hydrodynamic, MHD without charge exchange and MHD with charge exchange with the VLISM neutrals. Latitudinal differences in the solar wind are included as well as the solar cycle dependence of the Interplanetary Magnetic field (IMF). The latitudinal variation in the solar wind causes the polar termination shock at a considerably larger distance than in the ecliptic at the same angle with respect to the VLISM direction. As expected, the presence of magnetic pressure in the VLISM moves the termination shock inward, as does the presence of charge exchange with the neutrals.

Investigation of 3D Energetic Particle Transport Inside Quiet‐Time Magnetosphere using Particle Tracing in Global MHD Model

Due to the presence of a magnetic field minimum in the outer cusp region, energetic particles drifting toward dayside may experience large scale transport toward high latitude. Some particle maybe trapped at high latitude and then be scattered back. These particle orbits are termed as Shabansky orbits [Shabansky, 1971]. Particle trajectories inside the magnetosphere can be grouped into three classes: bouncing around the equator (trapped), going through Shabansky orbit or being elevated at dayside, and lost. Characterizing these three types of particle trajectory and their dependence on solar wind conditions can help understand the trapping and loss of energetic particles in the radiation belt. We developed 3D test-particle tracing codes to investigate particle transport in global MHD model magnetosphere. In the code, protons are traced with full-motion and electrons are traced with guiding-center approximation. In this paper, we lay out the framework of studying the trapping and lost regions systematically and effects of the enhancement of the solar wind velocity on these regions. We derived the so-called Shabansky Orbit Accessibility Map (SOAM) for both electrons and protons to visualize the three orbital characteristic regions as a function of the particles initial position and pitch angle inside quiet-time magnetosphere.

Global magnetospheric response to IMF driving: ISTP observations, empirical modeling, and MHD simulations

Abstract The energy flow through the magnetosphere-ionosphere system is examined during an isolated substorm event (May 15, 1996) using multi-spacecraft ISTP observations, empirical modeling techniques, and global MHD simulations. Detailed timings show that the growth phase signatures were evident in the ionosphere ∼5 min after the IMF southward turning — ∼7 min before the energy loading on open tail lobe field lines was initiated. After this initial period, the energy input and storage derived using existing empirical formulas are shown to be in balance. Based on both empirical model results and MHD simulations for this interval, it is argued that the magnetotail has two active regions that develop during the growth phase: The one in the inner magnetotail is necessary for the substorm pre-conditioning, but it is the one in the midtail region which initiates the global instability growth. The differences between pseudobreakups and full substorm onsets are discussed; it is suggested that although the pseudobreakup signatures in the inner tail appear localized, the midtail may already be undergoing large-scale reconfiguration.

Correction to “Magnetospheric cavity modes driven by solar wind dynamic pressure fluctuations”

[1] In the paper “Magnetospheric cavity modes driven by solar wind dynamic pressure fluctuations” by S. G. Claudepierre et al. (Geophysical Research Letters, 36, L13101, doi:10.1029/2009GL039045) the acknowledgments should appear as follows. The authors are grateful for thoughtful discussions with R. E. Denton and J. G. Lyon. This work was supported by the New Hampshire NASA Space grant NNG05GG76H and by the Center for Integrated Space Weather Modeling funded by the NSF STC program under cooperative agreement ATM‐0120950.