R. A. Wolf

Space Weather Modeling Framework: A new tool for the space science community

[1] The Space Weather Modeling Framework (SWMF) provides a high-performance flexible framework for physics-based space weather simulations, as well as for various space physics applications. The SWMF integrates numerical models of the Solar Corona, Eruptive Event Generator, Inner Heliosphere, Solar Energetic Particles, Global Magnetosphere, Inner Magnetosphere, Radiation Belt, Ionosphere Electrodynamics, and Upper Atmosphere into a high-performance coupled model. The components can be represented with alternative physics models, and any physically meaningful subset of the components can be used. The components are coupled to the control module via standardized interfaces, and an efficient parallel coupling toolkit is used for the pairwise coupling of the components. The execution and parallel layout of the components is controlled by the SWMF. Both sequential and concurrent execution models are supported. The SWMF enables simulations that were not possible with the individual physics models. Using reasonably high spatial and temporal resolutions in all of the coupled components, the SWMF runs significantly faster than real time on massively parallel supercomputers. This paper presents the design and implementation of the SWMF and some demonstrative tests. Future papers will describe validation (comparison of model results with measurements) and applications to challenging space weather events. The SWMF is publicly available to the scientific community for doing geophysical research. We also intend to expand the SWMF in collaboration with other model developers.

Interpretation of high‐speed flows in the plasma sheet

Pursuing an idea suggested by Pontius and Wolf, the authors propose that the {open_quotes}bursty bulk flows{close_quotes} are {open_quotes}bubbles{close_quotes} in the Earth`s plasma sheet. Specifically, they are flux tubes that have lower values of pV{sup 5/3} than their neighbors, where p is the thermal pressure of the particles and V is the volume of a tube containing one unit of magnetic flux. Whether they are created by reconnection or some other mechanism, the bubbles are propelled earthward by a magnetic buoyancy force, which is related to the interchange instability. Most of the major observed characteristics of the bursty bulk flows can be interpreted naturally in terms of the bubble picture. They propose a new {open_quotes}stratified fluid{close_quotes} picture of the plasma sheet, based on the idea that bubbles constitute the crucial transport mechanism. Results from simple mathematical models of plasma sheet transport support the idea that bubbles can resolve the pressure balance inconsistency, particularly in cases where plasma sheet ions are lost by gradient/curvature drift out the sides of the tail or bubbles are generated by reconnection in the middle plasma sheet. 23 refs., 8 figs., 1 tab.

Comprehensive computational model of Earth's ring current

A comprehensive ring current model (CRCM) has been developed that couples the Rice Convection Model (RCM) and the kinetic model of Fok and coworkers. The coupled model is able to simulate, for the first time using a self-consistently calculated electric field, the evolution of an inner magnetosphere plasma distribution that conserves the first two adiabatic invariants. The traditional RCM calculates the ionospheric electric fields and currents consistent with a magnetospheric ion distribution that is assumed to be isotropic in pitch angle. The Fok model calculates the plasma distribution by solving the Boltzmann equation with specified electric and magnetic fields. To combine the RCM and the Fok model, the RCM Birkeland current algorithm has been generalized to arbitrary pitch angle distributions. Given a specification of height-integrated ionospheric conductance, the RCM component of the CRCM computes the ionospheric electric field and currents. The Fok model then advances the ring current plasma distribution using the electric field computed by the RCM and at the same time calculates losses along particle drift paths. We present the logic of CRCM and the first validation results following the H+ distribution during the previously studied magnetic storm of May 2, 1986. The H+ fluxes calculated by the coupled model agree very well with observations by AMPTE/CCE. In particular, the coupled model is able to reproduce the high H+ flux seen on the dayside at L ∼ 2.3 that the previous simulation, which employed a Stern-Volland convection model with shielding factor 2, failed to produce. Though the Stern-Volland and CRCM electric fields differ in several respects, the most notable difference is that the CRCM predicts strong electric fields near Earth in the storm main phase, particularly in the dusk-midnight quadrant. Thus the CRCM injects particles more deeply and more quickly.

Ionosphere-Magnetosphere Coupling

The large-scale electrical coupling between the ionosphere and magnetosphere is reviewed, particularly with respect to behavior on time scales of hours or more. The following circuit elements are included: (1) the magnetopause boundary layer, which serves as the generator for the magnetospheric-convection circuit; (2) magnetic field lines, usually good conductors but sometimes subject to anomalous resistivity; (3) the ionosphere, which can conduct current across magnetic field lines; (4) the magnetospheric particle distributions, including tail current and partial-ring currents. Magnetic merging and a viscous interaction are considered as possible generating mechanisms, but merging seems the most likely alternative. Several mechanisms have been proposed for causing large potential drops along magnetic field lines in the upper ionosphere, and many isolated measurements of parallel electric fields have been reported, but the global pattern and significance of these electric fields are unknown. Ionospheric conductivities are now thoroughly measured, but are highly variable. Simple self-consistent theoretical models of the magnetospheric-convection system imply that the magnetospheric particles should shield the inner magnetosphere and low-latitude ionosphere from most of the time-average convection electric field.

The physics of the Harang discontinuity

Absent a source of energetic ions at the flanks of the tail, the westward gradient/curvature drift of E×B-convecting plasma results in the depletion of energetic ions from the dawnside of the plasma sheet. This dawnside depletion effect means that, on average, the duskside of the plasma sheet will have higher ion temperatures, pressures, and flux tube contents, and hence, stronger westward cross-tail drift current than the dawnside. The resulting cross-tail divergence of drift current must find closure by means of Birkeland currents connecting to the ionosphere. Tailward and poleward of the inner edge region, the divergence of cross-tail current requires upward current from the ionosphere. In the ionosphere, current closure requires electric fields that are directed toward the center of the upward current, i.e., directed equatorward on the poleward side, poleward on the equatorward side. This is precisely the nature of the Harang discontinuity. The region poleward of the Harang discontinuity maps well out into the plasma sheet and provides an eastward component of E×B drift to oppose the westward gradient/curvature drift of the ions. This helps keep the flow of plasma sheet ions directed toward the inner plasma sheet, rather than toward the dusk flank of the tail, a point originally made by Atkinson. The region equatorward of the Harang discontinuity maps close to the inner edge of the plasma sheet and results in westward E×B drift, increasing the westward flow of plasma azimuthally around the duskside of the inner magnetosphere and toward the dayside magnetopause. Although this scenario can be understood qualitatively, runs were carried out using the Rice convection model (RCM) to examine the ionospheric-magnetospheric coupling implications of this dawnside depletion effect. These runs confirm the above scenario, generally. They show that dawnside ion depletion results in a band of upward Birkeland current in the central auroral zone on the nightside, similar to what has been consistently observed by Iijima and Potemra and others. These currents modify the nightside, auroral electric field distribution to produce a strong reversal in the meridional electric field, similar to the classically observed Harang discontinuity. Inclusion of dawnside ion depletion also results in a major improvement in the agreement between the RCM and observations with regard to the latitudinal distribution of Birkeland currents. Finally, the dawnside depletion effect results in a reduction of plasma sheet pressures in the near-Earth, midnight sector of the plasma sheet. However, this reduction is significantly less than that suggested by Kivelson and Spence.

Global ENA IMAGE Simulations

The energetic neutral atom (ENA) images obtained by the ISEE and POLAR satellites pointed the way toward global imaging of the magnetospheric plasmas. The Imager for Magnetopause to Aurora Global Exploration (IMAGE) is the first mission to dedicate multiple neutral atom imagers: HENA, MENA and LENA, to monitor the ion distributions in high-, medium- and low-energy ranges, respectively. Since the start of science operation, HENA, MENA and LENA have been continuously sending down images of the ring current, ionospheric outflow, and magnetosheath enhancements from high pressure solar wind. To unfold multiple-dimensional (equal or greater than 3) plasma distributions from 2-dimensional images is not a trivial task. Comparison with simulated ENA images from a modeled ion distribution provides an important basis for interpretation of features in the observed images. Another approach is to develop image inversion methods to extract ion information from ENA images. Simulation studies have successfully reproduced and explained energetic ion drift dynamics, the transition from open to closed drift paths, and the magnetosheath response to extreme solar wind conditions. On the other hand, HENA has observed storm-time ion enhancement on the nightside toward dawn that differs from simple concepts but can be explained using more sophisticated models. LENA images from perigee passes reveal unexpected characteristics that now can be interpreted as evidence for a transient superthermal exospheric component that is gravitationally-influenced if not bound. In this paper, we will report ENA simulations performed during several IMAGE observed events. These simulations provide insight and explanations to the ENA features that were not readily understandable previously.

Is the Earth's magnetotail balloon unstable?

The authors consider the ideal MHD stability of two-dimensional equilibrium models of the geomagnetic tail, where the magnetic field is in the xz plane and nothing depends on the coordinate y. Ballooning-type perturbations are considered for which [vert bar]k[sub y][vert bar]L[much gt]1, where L is the scale length for variation in the xz plane. They employ the energy principle to test the stability of each flux tube of the plasma sheet for a hard ionospheric boundary condition (v = 0) and present a physical argument suggesting that it is the most appropriate simple boundary condition for the Earths plasma sheet. Numerical results are presented for compressible ballooning modes that are symmetric about the center of the current sheet. For such a hard boundary condition, they found no reasonable magnetotail configuration that was unstable to compressible symmetric ballooning but stable against interchange. Therefore compressible symmetric ideal-MHD ballooning seems incapable of substantially altering the large-scale configuration of the magnetotail.

Numerical simulation of torus‐driven plasma transport in the Jovian magnetosphere

The Rice convection model has been modified for application to the transport of Io-generated plasma through the Jovian magnetosphere. The new code, called the RCM-J, has been used for several ideal-MHD numerical simulations to study how interchange instability causes an initially assumed torus configuration to break up. In simulations that start from a realistic torus configuration but include no energetic particles, the torus disintegrates too quickly (∼50 hours). By adding an impounding distribution of energetic particles to suppress the interchange instability, reasonable lifetimes were obtained. For cases in which impoundment is insufficient to produce ideal-MHD stability, the torus breaks up predominantly into long fingers, unless the initial condition strongly favors some other geometrical form. If the initial torus has more mass on one side of the planet than the other, fingers form predominantly on the heavy side (which we associate with the active sector). Coriolis force bends the fingers to lag corotation. The simulation results are consistent with the idea that the fingers are formed with a longitudinal thickness that is roughly equal to the latitudinal distance over which the invariant density declines at the outer edges of the initial torus. Our calculations give an average longitudinal distance between plasma fingers of about 15°, which corresponds to 20 to 30 minutes of rotation of the torus. We point to some Voyager and Ulysses data that are consistent with this scale of torus longitudinal irregularity.

Calculations of Magnetospheric Electric Fields

This paper will describe efforts to compute theoretically the electric field and plasma distributions in the magnetosphere. In the last few years, such efforts have been made by Block (1966), Iwasaki and Nishida (1967), Karlson (1971), Swift (1971), Vasyliunas (1970, 1972), Wolf (1970), and Jaggi and Wolf (1973). The physical basis of all this work is pretty much the same; the differences lie in the details of the approximations.

Understanding storm‐time ring current development through data‐model comparisons of a moderate storm

[1] With three components, global magnetosphere (GM), inner magnetosphere (IM), and ionospheric electrodynamics (IE), in the Space Weather Modeling Framework (SWMF), the moderate storm on 19 May 2002 is globally simulated over a 24-hour period that includes the sudden storm commencement (SSC), initial phase, and main phase of the storm. Simulation results are validated by comparison with in situ observations from Geotail, GOES 8, GOES 10, Polar, LANL MPA, and the Sym-H and Dst indices. It is shown that the SWMF is reaching a sophistication level for allowing quantitative comparison with the observations. Major storm characteristics at the SSC, in the initial phase, and in the main phase are successfully reproduced. The simulated plasma parameters exhibit obvious dawn-dusk asymmetries or symmetries in the ring current region: higher density near the dawn and higher temperature in the afternoon and premidnight sectors; the pressure is highest on the nightside and exhibits a near dawn-dusk symmetry. In addition, it is found in this global modeling that the upstream solar wind/ IMF conditions control the storm activity and an important plasma source of the ring current is in the solar wind. However, the ionospheric outflow can also affect the ring current development, especially in the main phase. Activity in the high-latitude ionosphere is also produced reasonably well. However, the modeled cross polar cap potential drop (CPCP) in the Southern Hemisphere is almost always significantly larger than that in the Northern Hemisphere during the May storm.