Charles C. Goodrich

Increase in relativistic electron flux in the inner magnetosphere: ULF wave mode structure

Abstract Pc 5 ULF waves are seen concurrently with the rise in radiation belt fluxes associated with CME magnetic cloud events. A 3D global MHD simulation of the 10–11 January, 1997 event has been analyzed for mode structure and shown to contain field line resonance components, both toroidal and poloidal, with peak power on the nightside during southward IMF conditions. A mechanism for inward radial transport and first-invariant conserving acceleration of relativistic electrons is assessed in the context of ULF mode structure analysis, and compared with groundbased and satellite observations.

Microinstabilities associated with a high Mach number, perpendicular bow shock

The instabilities associated with a high Mach number perpendicular shock are reexamined in light of recent enhanced understanding of the Earths bow shock. The insights provided by both the ISEE observations and hybrid simulations are reviewed and subsequently incorporated into the instability analyses. The discussion of the instabilities is divided according to their location in the shock layer. In the regions in front of and at the shock transition the cross-field instabilities are subdivided into low frequency modes (e.g. ion-ion streaming, kinetic cross-field streaming, drift lower hybrid instabilities) and high frequency modes (electron cyclotron drift, ion sound and electron whistler instabilities). Further downstream various ion ring-like and anisotropy driven instabilities are considered. In each case the instability analysis is reviewed and recent developments are emphasized. Implications of these results concerning the wave signatures and plasma heating and acceleration are also discussed.

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.

Effects of the solar wind electric field and ionospheric conductance on the cross polar cap potential: Results of global MHD modeling

[1] The behavior of the cross polar cap potential, PC, under strong solar wind conditions is studied using global MHD simulations. Simulations using two typical values of the ionospheric Pedersen conductance in agreement with others show that the cross polar cap potential is reduced compared to the corresponding potential in the solar wind due to the stagnation of the magnetosheath flow and the existence of parallel potentials. However, it is the ionospheric conductance that affects the value of PC the most: the transpolar potential saturates only for high enough ionospheric conductance. A mechanism in which the ionospheric conductance changes the properties of the magnetosheath flow is proposed. This mechanism assumes mapping of the electrostatic potential in the ideal MHD system and yields a self-consistent response of the reconnection and transpolar potentials to changes in the ionospheric conductance. INDEX TERMS: 2736 Magnetospheric Physics: Magnetosphere/ionosphere interactions; 2753 Magnetospheric Physics: Numerical modeling; 2776 Magnetospheric Physics: Polar cap phenomena; 2784 Magnetospheric Physics: Solar wind/magnetosphere interactions. Citation: Merkine, V. G., K. Papadopoulos, G. Milikh, A. S. Sharma, X. Shao, J. Lyon, and C. Goodrich, Effects of the solar wind electric field and ionospheric conductance on the cross polar cap potential: Results of global MHD modeling, Geophys. Res. Lett., 30(23), 2180, doi:10.1029/2003GL017903, 2003.

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.

Simulation of the heliosphere: Model

The problem of the interaction of the solar wind with the very local interstellar medium (VLISM) is complicated by the role played by collisions between the plasma components of the heliosphere and VLISM and the neutral component of the VLISM. We outline the inherent approximations in fluid descriptions of the problem and give formulas for the near-exact charge exchange and elastic collision transfer integrals for particles, momentum and energy for two drifting Maxwellians with different drift speeds and temperatures. To lowest order, all Boltzmann collision operators can be evaluated analytically in terms of exponentials and error functions. Analytic approximations that have relative errors of less than 2.62% compared with the exact expressions can be implemented in large simulation codes. Our formulation avoids approximations used previously by others and leads to a simplification of the formulation and increased accuracy for numerical simulations of the heliosphere/VLISM interaction.

Solar wind electric field driving of magnetospheric activity: Is it velocity or magnetic field?

[1 ] The Lyon-Fedder-Mobarry global magnetohydrodynamic simulation code is used to examine a period of steady magnetospheric convection driven by a moderately southward IMF and a steady and quite low solar wind speed. Two other runs were performed to test the effects of increasing solar wind driving: one with 50% increase in the IMF magnitude and one with 50% increase in the solar wind speed. Larger IMF magnitude leads to nightside reconnection closer to the Earth but the steady state of the magnetotail is not changed. On the other hand, higher solar wind speed enhances Earthward mass and Poynting flux transport as well as their variability. The increase in the solar wind E Y is the same for both runs, but the E Y in the magnetosheath is larger in the run in which the speed is enhanced, which is associated with the higher magnetotail activity in that simulation.

Midtail plasma flows and the relationship to near-Earth substorm activity : a case study

Recent simulations of magnetotail reconnection have pointed to a link between plasma flows, dipolarization, and the substorm current wedge. In particular, Hesse and Birn (1991) have proposed that earthward jetting of plasma from the reconnection region transports flux into the near-Earth region. At the inner edge of the plasma sheet this flux piles up, producing a dipolarization of the magnetic field. The vorticity produced by the east-west deflection of the flow at the inner edge of the plasma sheet gives rise to field-aligned currents that have region 1 polarity. Thus in this scenario the earthward flow from the reconnection region produces the dipolarization and the current wedge in a self-consistent fashion. In this study we examine observations made on April 8,1985 by the Active Magnetospheric Particle Tracer Explorers/Ion Release Module (IRM), the geosynchronous satellites 1979-053,1983-019, and 1984-037, and Syowa station, as well as AE. This event is unique because IRM was located near the neutral sheet in the midnight sector for an extended period of time. Ground data show that there was ongoing activity in the IRM local time sector for several hours, beginning at 1800 UT and reaching a crescendo at 2300 UT. This activity was also accompanied by energetic particle variations, including injections, at geosynchronous orbit in the nighttime sector. Significantly, there were no fast flows at the neutral sheet until the great intensification of activity at 2300 UT. At that time, IRM recorded fast earthward flow simultaneous with a dipolarization of the magnetic field. We conclude that while the aforementioned scenario for the creation of the current wedge encounters serious problems explaining the earlier activity, the observations at 2300 UT are consistent with the scenario of Hesse and Birn (1991). On that basis it is argued that the physics of substorms is not exclusively rooted in the development of a global tearing mode. Processes at the inner edge of the cross-tail current that cause a disruption of the current and a consequent dipolarization and current wedge may be unrelated to the formation of a macroscale reconnection region. Thus the global evolution of a substorm is probably a complicated superposition of such processes operating on a very localized scale and a global macroscale process that allows for such things as releasing the energy stored in lobe flux and the creation of plasmoids.