Darren L. Dezeeuw

ERUPTION OF A BUOYANTLY EMERGING MAGNETIC FLUX ROPE

We present a three-dimensional numerical magnetohydrodynamic simulation designed to model the emergence of a magnetic flux rope passing from below the photosphere into the corona. For the initial state, we prescribe a plane-parallel atmosphere that comprises a polytropic convection zone, photosphere, transition region, and corona. Embedded in this system is an isolated horizontal magnetic flux rope located 10 photospheric pressure scale heights below the photosphere. The flux rope is uniformly twisted, with the plasma temperature inside the rope reduced to compensate for the magnetic pressure. Density is reduced in the middle of the rope, so that this section buoyantly rises. The early evolution proceeds with the middle of the rope rising to the photosphere and expanding into the corona. Just as it seems the system might approach equilibrium, the upper part of the flux rope begins to separate from the lower, mass-laden part. The separation occurs through stretching of the field, which forms a current sheet, where reconnection severs the field lines to form a new system of closed flux. This flux then erupts into the corona. Essential to the eruption process are shearing motions driven by the Lorentz force, which naturally occur as the rope expands in the pressure-stratified atmosphere. The shearing motions transport axial flux and energy to the expanding portion of the magnetic field, driving the eruption.

Three‐dimensional multispecies MHD studies of the solar wind interaction with Mars in the presence of crustal fields

interaction of the solar wind with Mars. The three ions considered are H + ,O 2 , and O + , representing the solar wind and the two major ionospheric ion species, respectively. The calculations indicate that the presence of a hot oxygen corona does not, within the resolution and accuracy of the model, lead to any significant effect on the dayside bow shock and ionopause positions. Next the trans-terminator fluxes and escape fluxes down the tail were calculated neglecting the effects of the crustal magnetic field. The calculated flux values are consistent with the measured escape fluxes and the calculated limiting fluxes from the dayside ionosphere. Finally, a 60-order harmonic expansion model of the measured magnetic field was incorporated into the model. The crustal magnetic field did not cause major distortions in the bow shock but certainly had an important effect within the magnetosheath and on the apparent altitude of the ionopause. The model results also indicated the presence of ‘‘minimagnetocylinders,’’ consistent with the MGS observations. We also recalculated the trans-terminator and escape fluxes, for the nominal solar wind case, in the presence of the crustal magnetic field and found, as expected, that there is a decrease in the calculated escape flux; however, it is still reasonably close to the value estimated from the Phobos-2 observations. INDEX TERMS: 2780 Magnetospheric Physics: Solar wind interactions with unmagnetized bodies; 2459 Ionosphere: Planetary ionospheres (5435, 5729, 6026, 6027, 6028); 5440 Planetology: Solid Surface Planets: Magnetic fields and magnetism; 2728 Magnetospheric Physics: Magnetosheath; KEYWORDS: Mars, MHD, bow shock, escape flux, solar wind interaction, crustal magnetic field Citation: Ma, Y., A. F. Nagy, K. C. Hansen, D. L. DeZeeuw, T. I. Gombosi, and K. G. Powell, Three-dimensional multispecies MHD studies of the solar wind interaction with Mars in the presence of crustal fields, J. Geophys. Res., 107(A10), 1282, doi:10.1029/2002JA009293, 2002.

Heliosphere in the magnetized local interstellar medium : Results of a three-dimensional MHD simulation

The results of a three-dimensional adaptive magnetohydrodynamic (MHD) model of the interaction of a magnetized solar wind with a magnetized very local interstellar medium in the presence of neutral interstellar hydrogen are presented. The interplanetary magnetic field is approximated by the Parker spiral, and the direction of the interstellar magnetic field is taken to be arbitrary. It is demonstrated that magnetic field interaction has a very pronounced effect on the structure of the global heliosphere. In particular, it is shown that the interaction of the interstellar wind with the shocked solar wind significantly depends on the direction of the interstellar magnetic field. This effect is mainly manifested in the distances to the heliospheric boundaries and the shape of the heliosphere. Depending on the orientation of the interstellar magnetic field the upstream location of the termination shock is expected to be at 80 ± 10 AU. The termination shock is predicted to be weak in agreement with the available body of observations. It is found that under quiet solar conditions the spiral structure of the interplanetery magnetic field remains imprinted in the solar wind far beyond the termination shock. Numerical simulations indicate that magnetic fields have a stabilizing effect on the heliopause.

A THREE-DIMENSIONAL FLUX ROPE MODEL FOR CORONAL MASS EJECTIONS BASED ON A LOSS OF EQUILIBRIUM

A series of simulation runs are carried out to investigate the loss of equilibrium of the three-dimensional flux rope configuration of Titov & Demoulin as a suitable mechanism for the initiation of coronal mass ejections. By means of these simulations, we are able to determine the conditions for which stable equilibria no longer exist. Our results imply that it is possible to achieve a loss of equilibrium even though the ends of the flux rope are anchored to the solar surface. However, in order to have the flux rope escape, it is necessary to modify the configuration by eliminating the arcade field.

A numerical study of solar wind—magnetosphere interaction for northward interplanetary magnetic field

The solar wind-magnetosphere interaction for northward interplanetary magnetic field (IMF) is studied using a newly developed three-dimensional adaptive mesh refinement (AMR) global MHD simulation model. The simulations show that for north-ward IMF the magnetosphere is essentially closed. Reconnection between the IMF and magnetospheric field is limited to finite regions near the cusps. When the reconnection process forms newly closed magnetic field lines on the day side, the solar wind plasma trapped on these reconnected magnetic field lines becomes part of the low-latitude boundary layer (LLBL) plasma and it convects to the nightside along the magnetopause. The last closed magnetic field line marks the topological boundary of the magnetospheric domain. When the last closed magnetic field line disconnects at the cusps and reconnects to the IMF, its plasma content becomes part of the solar wind. Plasma convection in the outer magnetosphere does not directly contribute to the reconnection process. On the dayside the topological boundary between the solar wind and the magnetosphere is located at the inner edge of the magnetopause current layer. At the same time, multiple current layers are observed in the high-altitude cusp region. Our convergence study and diagnostic analysis indicate that the details of the diffusion and the viscous interaction do not play a significant role in controlling the large-scale configuration of the simulated magnetosphere. It is sufficient that these dissipation mechanisms exist in the simulations. In our series of simulations the length of the magnetotail is primarily determined by the balance between the boundary layer driving forces and the drag forces. With a parametric study, we find that the tail length is proportional to the magnetosheath plasma beta near the magnetopause at local noon. A higher solar wind density, weaker IMF, and larger solar wind Mach number results in a longer tail. On the nightside downstream of the last closed magnetic field line the plasma characteristics are similar to that in the magnetotail, posing an observational challenge for identification of the topological status of the corresponding field lines.

Magnetospheric configuration for Parker-spiral IMF conditions: Results of a 3D AMR MHD simulation

Abstract The global configuration and topology of the terrestrial magnetosphere for typical IMF conditions is simulated with a new 3D MHD model using adaptive mesh refinement (AMR). The paper briefly describes the main elements of the solution method and presents a steady-state solution for nominal Parker-spiral IMF conditions. The simulated magnetic field topology in the magnetotail is in excellent agreement with the recent empirical model derived from four years of IMP 8 observations (Kaymaz and Siscoe, 1998).

Io's plasma environment during the Galileo flyby: Global three‐dimensional MHD modeling with adaptive mesh refinement

The first results for applying a three-dimensional multiscale ideal MHD model for the mass-loaded flow of Jupiters corotating magnetospheric plasma past Io are presented. The model is able to consider simultaneously physically realistic conditions for ion mass loading, ion-neutral drag, and intrinsic magnetic field in a full global calculation without imposing artificial dissipation. Io is modeled with an extended neutral atmosphere which loads the corotating plasma torus flow with mass, momentum, and energy. The governing equations are solved using adaptive mesh refinement on an unstructured Cartesian grid using an upwind scheme for MHD. For the work described in this paper we explored a range of models without an intrinsic magnetic field for Io. We compare our results with particle and field measurements made during the December 7, 1995, flyby of Io, as published by the Galileo Orbiter experiment teams. For two extreme cases of lower boundary conditions at Io, our model can quantitatively explain the variation of density along the spacecraft trajectory and can reproduce the general appearance of the variations of magnetic field and ion pressure and temperature. The net fresh ion mass-loading rates are in the range of ∼300–650 kg s−1, and equivalent charge exchange mass-loading rates are in the range ∼540–1150 kg s−1 in the vicinity of Io.

The interaction between the magnetosphere of Saturn and Titan's ionosphere

A three-dimensional (3-D) multi-species magnetohydrodynamic model was used to study the interaction of Titans ionosphere and Saturns magnetosphere. The three generic species which were considered are light (e.g., H + , H 2 + , and H 3 + ), medium (e.g., N + and CH 5 + ), and heavy (e.g., N 2 + and HCNH + ) ion species. The effects of exospheric mass loading, major chemical reactions, and ion-neutral collisions were considered. The upstream parameters were selected to be the nominal values for the case when Titan is in the magnetosphere of Saturn. The simulation results are compared with Voyager measurements as well as related model calculations. The 3-D three-species model results reproduce reasonably well the global features such as magnetic barrier, magnetotail, and the distributions of the major ionospheric species. The outward escape flux of the major ionospheric species (i.e., the heavy ion species) from the tail is calculated to be approximately 6.5 × 10 24 s -1 .

Global MHD modeling of the impact of a solar wind pressure change

[1] A sudden increase in the solar wind dynamic pressure compresses the magnetosphere and launches compressional waves into the magnetosphere. The global response of the magnetosphere, including the ionosphere and the location of the field-aligned current (FAC) generation, to a step increase in the solar wind density has been studied using a global three-dimensional adaptive MHD model. As the density increase propagated along the flanks of the magnetopause, a two-phased response was seen in the ionosphere. The first response was an increase in FACs near the polar cap. For this response we found the location of FACs to lie just inside the magnetosphere. The second response was an increase in FACs at lower latitudes. The increase in FACs was in the same direction as region 1 currents. For the second response we found the location of FACs to fall well within the magnetosphere.

3D multi‐fluid MHD studies of the solar wind interaction with Mars

The interaction of the solar wind with planets with no or only very weak intrinsic magnetic fields, such as Mars and Venus, involves their ionospheres. Single fluid MHD models, which incorporate some of the important mass-loading processes, have been useful in reproducing numerous observed features at these planets, such as the location of the bow shock, but they do have obvious limitations. Our present 3D MHD model is a two-fluid one, which considers protons and the dominant heavy ions in the ionosphere, separately. We have used this model to study the interaction processes at Mars. The model results are in general agreement with the average observed bow shock shape and position and predict reasonable locations for the ionopause. The calculated oxygen escape flux down the tail was estimated to be 2.7 × 10 25 s -1 , which is consistent with Phobos-2 estimates.