Interplanetary Magnetic Field Case Research Articles

Solar wind interaction with Mars: electric field morphology and source terms

ABSTRACTThe correlation between space environment conditions and the properties of escaping ions is a central topic of Mars research. Although empirical correlations have been visible in the data, a physics-based interpretation, rather than statistics-based pictures, has not been established yet. As a first effort, we investigate the electric field, the direct contributor to ion acceleration, in the Mars plasma environment from a hybrid plasma model (particle ions and fluid electrons). We use Amitis, a hybrid model combined with an observation-based ionospheric model, to simulate the Mars–solar wind interaction under nominal solar wind plasma conditions for perpendicular and Parker spiral directions of the interplanetary magnetic field (IMF). The simulations show following results: (1) the electric field morphology is structured by the IMF direction and the different plasma domains in the solar wind–Mars interaction; (2) asymmetry of the electric field between the hemispheres where the convective electric field points inward and outward, respectively, due to the mass loading and asymmetric draping of the magnetic field lines; (3) the motional electric field dominates in most regions, especially in the dayside magnetosheath; and (4) the Hall term is an order of magnitude weaker and significant in the magnetotail and plasma boundaries for a perpendicular IMF case. The Hall term is relatively stronger for the Parker spiral case. (5) The ambipolar electric field, in principle, agrees with Mars Atmosphere and Volatile Evolution measurements in the magnetosheath.

The Solar Wind Interaction with (1) Ceres: The Role of Interior Conductivity

As a potential “ocean world,” (1) Ceres’ interior may possess relatively high electrical conductivities on the order of 10−4–100 S m−1, suggesting that the solar wind interaction with Ceres may differ from other highly resistive objects such as the Moon. Here, we use a hybrid plasma model to quantify the solar wind interaction with Ceres over a range of scenarios for Ceres’ internal conductivity structure and the upstream solar wind and interplanetary magnetic field (IMF) conditions. Internal models for Ceres include one-, two-, and three-layer conductivity structures that variously include a crust, mantle, and/or subsurface ocean, while modeled solar wind conditions include a nominal case, a high IMF case, and an “extreme” space weather case. To first order, Ceres’ interaction with the solar wind is governed by the draping and enhancement of the IMF over its interior, whether from a moderate-conductivity mantle or a high-conductivity ocean. In turn, IMF draping induces compressional wings in the solar wind density and deceleration in the solar wind speed outside of Ceres. Together, all three effects are readily observable by a hypothetical orbital or landed mission with standard plasma and magnetic field instrumentation. Finally, we also consider the possible effects of unipolar induction within Ceres, which has been previously suggested as a mechanism for conducting bodies in the solar wind. Our model results show that the efficacy of unipolar induction is highly suppressed by the slow magnetic field-line diffusion through Ceres’ interior and, thus, is not a significant contributor to Ceres’ overall interaction with the solar wind.

Penetrating Electric Field Simulated by the MAGE and Comparison With ICON Observation

AbstractUsing the newly developed, Multiscale Atmosphere‐Geospace Environment (MAGE) model, we simulated the penetrating electric field in the equatorial region under different interplanetary magnetic field (IMF) BZ conditions during September 2020. Two intervals were selected for detailed analysis and the latter one was compared with the vertical ion drift data from the NASA Ionospheric Connection Explorer (ICON) satellite. The MAGE simulations show that in southward IMF (S‐IMF) cases, the dawn‐dusk electric potential drop at the equator is about 12% of the cross polar cap potential difference. Based on the MAGE simulation, the dawn‐dusk potential drop at the equator varies nearly instantaneously on the order of a few minutes with the changes in the IMF BZ or interplanetary electric field, which in turn alters the vertical ion drift. The daytime changes of the equatorial vertical ion drift in response to the penetrating electric field related to the IMF BZ are only half of that during the nighttime. ICON data, though not inconsistent with the simulation, were not able to verify the occurrence of penetrating electric field because of its unfavorable location at the time. The MAGE simulation shows a pre‐reversal enhancement (PRE) during southward IMF cases, but the PRE was absent in the ICON IVM observations. Further observations and modeling are needed to resolve this discrepancy.

Effects of the IMF Direction on Atmospheric Escape From a Mars‐like Planet Under Weak Intrinsic Magnetic Field Conditions

AbstractDirection of the upstream interplanetary magnetic field (IMF) significantly changes the magnetospheric configuration, influencing the atmospheric escape mechanism. This paper investigates effects of IMF on the ion escape mechanism from a Mars‐like planet that has a weak dipole magnetic field directing northward on the equatorial surface. The northward (parallel to the dipole at subsolar), southward (antiparallel), and Parker‐spiral IMFs under present solar wind conditions are compared based on multispecies magnetohydrodynamics simulations. In the northward IMF case, molecular ions escape from the high‐latitude lobe reconnection region, where ionospheric ions are transported upward along open field lines. Atomic oxygen ions originating either in the ionosphere or oxygen corona escape through a broader ring‐shaped region. In the southward IMF case, the escape flux of heavy ions increases significantly and has peaks around the equatorial dawn and dusk flanks. The draped IMF can penetrate into the subsolar ionosphere by erosion, and the IMF becomes mass‐loaded as it is transported through the dayside ionosphere. The mass‐loaded draped IMF is carried to the tail, contributing to ion escape. The escape channels in the northward and southward IMF cases are different from those in the Parker‐spiral IMF case. The escape rate is the lowest in the northward IMF case and comparable in the Parker‐spiral and southward IMF cases. In the northward IMF case, a weak intrinsic dipole forms a magnetosphere configuration similar to that of Earth, quenching the escape rate, while the Parker‐spiral and southward IMFs cause reconnection and erosion, promoting ion escape from the upper atmosphere.

Asymmetric Transport of the Earth’s Polar Outflows by the Interplanetary Magnetic Field

Abstract The polar outflows, as an important plasma source of the Earth’s magnetosphere, usually exhibit significant north–south asymmetries, which can strongly affect the plasma distributions in the magnetotail lobe and perhaps contribute to the substorm triggering. But the mechanism of the asymmetric transport of these outflows is still unclear. In this Letter, 3D global magnetohydrodynamic (MHD) simulations are performed to investigate the development of the polar outflows after their escapes from the inner boundary under influences of the interplanetary magnetic field (IMF) B x . It is found that the velocity of northern polar outflows is much stronger than the south. We suggest that the IMF B x causes the north–south asymmetries in the magnetospheric configuration, and subsequently, great differences of the force and mass distributions appear between the two hemispheres, which lead to the significant north–south asymmetries in the transport of the polar outflows. We also discuss the differences in the acceleration mechanisms of the polar outflows between the northward and southward IMF cases.

Double-cusp simulation during northward IMF using 3D PIC global code

The cusp has important effects on the transportation of particles and their energy from the solar wind to the magnetosphere, and ionosphere, and high-altitude atmosphere. The cusp can be considered to be a part of the magnetospheric boundary layer with weaker magnetic fields. It has been studied since 1971 by different satellite observations. Despite many years of investigation, some problems, such as the boundaries, shapes, and method of construction, remain to be solved. The double cusp was first reported by Wing using the observation of the DMSP satellite. He also compared the results of observations with the results of a 2D MHD simulation. In this study, by performing simulations and analyzing the results, we report the observation of a V-shaped double-cusp structure under the northward Interplanetary Magnetic Field (IMF). In our simulation, the double cusp was seen only for electrons, although a weak double cusp was observed for ions as well. We showed that this double cusp occurred because of electron precipitation from different sources of solar wind and magnetosphere with different magnetic field strengths. In previous studies of the double cusp, there were debates on its spatial structure or on its temporal behavior due to the cusp movement caused by the sharp solar wind effects on the magnetosphere shape. Here we report the spatial detection of the double cusp similar to the one observed by the DMSP satellite, but for the northward IMF case. Also, we investigate the asymmetry along the dawn-dusk side of the magnetosphere using our 3D PIC simulation code.

Separator reconnection at the magnetopause for predominantly northward and southward IMF: Techniques and results

AbstractIn this work, we demonstrate how to track magnetic separators in three‐dimensional simulated magnetic fields with or without magnetic nulls, apply these techniques to enhance our understanding of reconnection at the magnetopause. We present three methods for locating magnetic separators and apply them to 3‐D resistive MHD simulations of the Earth's magnetosphere using the Block‐Adaptive‐Tree Solar‐wind Roe‐type Upwind Scheme code. The techniques for finding separators and determining the reconnection rate are insensitive to interplanetary magnetic field (IMF) clock angle and can in principle be applied to any magnetospheric model. Moreover, the techniques have a number of advantages over prior separator finding techniques applied to the magnetosphere. The present work examines cases of high and low resistivity for two clock angles. We go beyond previous work examine the separator during Flux Transfer Events (FTEs). Our analysis of reconnection on the magnetopause yields a number of interesting conclusions: Reconnection occurs all along the separator even during predominately northward IMF cases. Multiple separators form in low‐resistivity conditions, and in the region of an FTE the separator splits into distinct branches. Moreover, the local contribution to the reconnection rate, as determined by the local parallel electric field, drops in the vicinity of the FTE with respect to the value when there are none.

MHD simulations using average solar wind conditions for substorms observed under northward IMF conditions

AbstractSubstorms are known to sometimes occur even under northward interplanetary magnetic field (IMF) conditions. In this paper, we perform three‐dimensional global magnetohydrodynamic simulations to examine dayside reconnection, tail, and ionospheric signatures for two cases of substorm observations under prolonged northward and dawnward IMF conditions: (1) a strongly northward/dawnward IMF case with BIMF = (0, −20, 20) nT; (2) a weakly northward/dawnward IMF case with BIMF = (0, −2, 2) nT. Throughout the simulations, we used the constant solar wind conditions to reflect the prolonged solar wind conditions around the substorm times. We found that, in both cases, the tail reconnection occurred after the usual high‐latitude reconnection on the dayside, providing a possible energy source for later triggered substorm observations under northward IMF conditions. The presence of an equal amount of IMF By allows the high‐latitude reconnected magnetic field lines to transport to the tail lobe, eventually leading to the tail reconnection. The simulation results also revealed the following major differences between the two cases: First, the reconnection onset (both on dayside and in the tail) occurs earlier in the strongly northward IMF case than in the weakly northward IMF case. Second, the polar cap size, which is finite in both cases despite the northward IMF conditions and thus supports the lobe energy buildup needed for the substorm occurrences, is larger in the strongly northward IMF case. Accordingly, the polar cap potential is far larger in the strongly northward IMF case (hundreds of kilovolt) than in the weakly northward IMF case (tens of kilovolt). Third, in the strongly northward IMF case, the strong earthward tail plasma flow appears to be caused by the enhanced convection (so enhanced duskward Ey) due to the tail reconnection. In contrast, in the weakly northward IMF case, the earthward tail plasma flow increases gradually in association with a modestly increased duskward electric field. In addition, the inner plasma pressure and the cross tail current near the reconnection site increase significantly in the strongly northward IMF case but less significantly in the weakly northward IMF case after the onset of the tail reconnection. In conclusion, the simulation results support observations of the substorms under northward IMF conditions in the presence of an equal amount of IMF By by demonstrating the energy input via dayside reconnection and the subsequent occurrence of the tail reconnection.

Pressure balance across the magnetopause: Global MHD results

The equilibrium magnetopause location on the dayside is determined by the balance of pressure forces. In this paper we use a physics-based global magnetohydrodynamic (MHD) to examine contributions of different terms to this balance. We calculate the total pressure, as well as thermal and magnetic pressures just inside and just outside the magnetopause. Our results show that (1) the total pressure just outside the magnetopause is enhanced with the increasing of BZ in both northward and southward interplanetary magnetic field (IMF). For southward IMF the thermal pressure is dominant in the total pressure while the magnetic pressure is dominant for northward IMF when BZ is not small (BZ>5nT). For southward IMF larger solar wind dynamic pressure further enhances thermal pressure near the magnetopause while for northward IMF the dynamic pressure is more effectively converted to the magnetic pressure; (2) the thermal pressure just outside the magnetopause is significantly enhanced with the increasing southward IMF as compared to the northward IMF case. However, this enhanced thermal pressure is not the only reason for the well-known earthward displacement of the magnetopause under southward IMF conditions. The dayside magnetic reconnection for southward IMF dramatically decreases magnetic pressure just inside the magnetopause. The combination of these two factors explains the Earthward motion of the magnetopause for southward IMF.

Hemispheric asymmetries of the Venus plasma environment

We study the Venus‐solar wind interaction and the hemispheric asymmetries of the Venus plasma environment in the global HYB‐Venus hybrid simulation. We concentrate especially on the role of the flow‐aligned interplanetary magnetic field (IMF) component (i.e., the Parker spiral angle or the IMF cone angle) and analyze the dawn‐dusk and Esw asymmetries between four magnetic quadrants around Venus. Using the simulation model, we study two upstream condition cases in detail: the perpendicular IMF to the solar wind flow case and the nominal Parker spiral case (dominant flow‐aligned IMF component). Several differences and similarities were found in these two simulation runs. Common features of the Venus plasma environment between the two cases include asymmetric magnetic barrier and tail lobes and asymmetric planetary ion escape in the direction of the solar wind convection electric field. Further, protons of planetary origin and of solar wind origin were found to follow similar velocity patterns in the Venus plasma wake in both cases. The differences when the IMF flow‐aligned component is dominating compared to the perpendicular IMF case, the so‐called (magnetic) dawn‐dusk asymmetries, include the parallel bow shock and the foreshock region, the asymmetric magnetic barrier, the asymmetric tail current system, and the asymmetric central tail current sheet. Further, the escaping planetary H+ and O+ ion fluxes are concentrated more on the hemisphere of the parallel bow shock. When interpreting in situ plasma and magnetic observations from Venus, the features of at least these two basic IMF configurations should be considered.

Dependence of O+ escape rate from the Venusian upper atmosphere on IMF directions

AbstractWe investigate the dependence of the O+ escape rate on the upstream interplanetary magnetic field (IMF) direction by using data from the Analyser of Space Plasma and Energetic Atoms (ASPERA‐4) instrument and the magnetometer (MAG) onboard Venus Express. We consider two cases, namely the perpendicular IMF case (167 events) and the parallel IMF case (82 events), where IMF is nearly perpendicular to the Venus‐Sun line and nearly parallel to it. By integrating O+ fluxes observed on the nightside, total O+ escape rates of (5.8 ± 2.9) × 1024 s−1 (perpendicular IMF case) and (4.9 ± 2.2) × 1024 s−1 (parallel IMF case) are obtained. As these values are not significantly different, the upstream IMF direction does not affect the total amount of O+ outflow from Venus. The different acceleration mechanisms must balance each other in order to keep the escape rate the same.

Momentum transfer of solar wind plasma in a kinetic scale magnetosphere

Solar wind interaction with a kinetic scale magnetosphere and the resulting momentum transfer process are investigated by 2.5-dimensional full kinetic particle-in-cell simulations. The spatial scale of the considered magnetosphere is less than or comparable to the ion inertial length and is relevant for magnetized asteroids or spacecraft with mini-magnetosphere plasma propulsion. Momentum transfer is evaluated by studying the Lorentz force between solar wind plasma and a hypothetical coil current density that creates the magnetosphere. In the zero interplanetary magnetic field (IMF) limit, solar wind interaction goes into a steady state with constant Lorentz force. The dominant Lorentz force acting on the coil current density is applied by the thin electron current layer at the wind-filled front of the magnetosphere. Dynamic pressure of the solar wind balances the magnetic pressure in this region via electrostatic deceleration of ions. The resulting Lorentz force is characterized as a function of the scale of magnetosphere normalized by the electron gyration radius, which determines the local structure of the current layer. For the finite northward IMF case, solar wind electrons flow into the magnetosphere through the reconnecting region. The inner electrons enhance the ion deceleration, and this results in temporal increment of the Lorentz force. It is concluded that the momentum transfer of solar wind plasma could take place actively with variety of kinetic plasma phenomena, even in a magnetosphere with a small scale of less than the ion inertial length.

O+outflow channels around Venus controlled by directions of the interplanetary magnetic field: Observations of high energy O+ions around the terminator

[1] Using the plasma and the magnetic field data measured by the ASPERA-4 (Analyzer of Space Plasma and Energetic Atoms) and the magnetometer (MAG) onboard Venus Express between June 2006 and December 2008, positions of high energy O+ fluxes (>100 eV) near the Venus terminator are examined for two different interplanetary magnetic field (IMF) configurations: IMF nearly perpendicular to the Venus-Sun line (perpendicular IMF case) and IMF nearly parallel to it (parallel IMF case). In most of the perpendicular IMF case, the high energy O+ fluxes are observed only near the magnetic poles. In contrast, they are observed regardless of a convection electric field around the terminator in most of the parallel IMF case. Energy of the flux depends on the convection electric field direction in the former case. In contrast, it has no dependence on the field direction in the latter case. We attribute these results to more complicated draping pattern of IMF around the ionosphere in the parallel IMF case than for that of the perpendicular IMF case. In the perpendicular IMF case, the IMF drapes around the ionosphere, forming a single plasma sheet. In contrast, in the parallel IMF case, the IMF drapes complicatedly, creating many antiparallel configuration of local magnetic field. Such multiple antiparallel configuration results in a local acceleration of O+ and more outflow channels. Thus, the ion acceleration region and its mechanism can be essentially different between the perpendicular IMF case and the parallel IMF case.

Impact of solar wind depression on the dayside magnetosphere under northward interplanetary magnetic field

Abstract. We present a follow up study of the sensitivity of the Earth's magnetosphere to solar wind activity using a particles-in-cell model (Baraka and Ben Jaffel, 2007), but here during northward Interplanetary Magnetic Field (IMF). The formation of the magnetospheric cavity and its elongation around the planet is obtained with the classical structure of a magnetosphere with parallel lobes. An impulsive disturbance is then applied to the system by changing the bulk velocity of the solar wind to simulate a decrease in the solar wind dynamic pressure followed by its recovery. In response to the imposed drop in the solar wind velocity, a gap (abrupt depression) in the incoming solar wind plasma appears moving toward the Earth. The gap's size is a ~15 RE and is comparable to the sizes previously obtained for both Bz<0 and Bz=0. During the initial phase of the disturbance along the x-axis, the dayside magnetopause (MP) expands slower than the previous cases of IMF orientations as a result of the abrupt depression. The size of the MP expands nonlinearly due to strengthening of its outer boundary by the northward IMF. Also, during the initial 100 Δt, the MP shrank down from 13.3 RE to ~9.2 RE before it started expanding, a phenomenon that was also observed for southern IMF conditions but not during the no IMF case. As soon as they felt the solar wind depression, cusps widened at high altitude while dragged in an upright position. For the field's topology, the reconnection between magnetospheric and magnetosheath fields is clearly observed in both the northward and southward cusps areas. Also, the tail region in the northward IMF condition is more confined, in contrast to the fishtail-shape obtained in the southward IMF case. An X-point is formed in the tail at ~110 RE compared to ~103 RE and ~80 RE for Bz=0 and Bz<0, respectively. Our findings are consistent with existing reports from many space observatories (Cluster, Geotail, Themis, etc.) for which predictions are proposed to test furthermore our simulation technique.

Rotational Asymmetry of Earth's Bow Shock

AbstractIn terms of global magnetohydrodynamic (MHD) simulations of the solar wind‐magnetosphere‐ionosphere system, this paper investigates the rotational asymmetry of the Earth's bow shock with respect to the Sun‐Earth line. We are limited to simple cases in which the solar wind is along the Sun‐Earth Line; and both the Earth's magnetic dipole moment and the interplanetary magnetic field (IMF) are perpendicular to the Sun‐Earth line. It is shown that even for the case of vanishing IMF strength the bow shock is not rotationally symmetric with respect to the Sun‐Earth line: the east‐west width of the cross section of the bow shock exceeds the north‐south width by about 9%~11% on the terminator plane (dawn‐dusk meridian plane) and its sunward side, and becomes smaller than the north‐south width by about 8% on the tailward side of the terminator plane. In the presence of the IMF, the configuration of the bow shock is affected by both the shape of the magnetopause and the anisotropy of fast magnetosonic wave speed. The magnetopause expands outward, being stretched along the IMF, and the extent of its expansion and stretch increases when the IMF rotates from north to south. In the magnetosheath, the fast magnetosonic wave speed is higher in the direction perpendicular to the magnetic field than that in the parallel direction. Therefore, the stretch direction of the magnetopause is perpendicular to the maximum direction of the fast magnetosonic wave speed, and their effects on the bow shock position are exactly opposite. The eventual shape of the bow shock depends on which effect dominates. On the tailward side of the terminator plane, the anisotropy of fast magnetosonic wave speed dominates, so the cross section of the bow shock is wider in the direction perpendicular to the IMF. On the terminator plane and its sunward side, the shape of the bow shock cross section depends on the orientation of the IMF: the bow shock cross section is still wider in the direction perpendicular to the IMF under generic northward or dawn‐dusk IMF cases, but it becomes narrower in the direction perpendicular to the IMF instead under generic southward IMF cases. In light of the intimate relationship between the shape of the bow shock and the orientation of the IMF, it is proposed to take the IMF as the datum direction so as to extract the parallel half width Rb// and the perpendicular half width Rb┴ as the scale parameters. In comparison with the commonly used east‐west half width yb and the north‐west half width zb, these parameters provide a more reasonable description of the geometry of the bow shock. Simulation data show that under the assumption of isotropic orientation of the IMF, the statistical averages of yb/zb and Rb‌‌/Rb┴ are both smaller than 1 on the terminator plane, which agrees with relevant observational conclusions.

Effects of the dipole tilt and northward and duskward IMF on dayside magnetic reconnection in a global MHD simulation

By using a three‐dimensional global magnetohydrodynamic simulation of the Earth's magnetosphere, we have studied whether antiparallel or subsolar reconnection better describes magnetospheric dynamics at the dayside magnetopause for the case of northward and duskward interplanetary magnetic field (IMF) and a positive dipole tilt. The simulation shows that the reconnection dominantly occurs at high latitudes where antiparallel field regions appear in the northern dusk sector and in the southern dawn sector. For the northward and duskward IMF case of 30° dipole tilt, the reconnection region in the dusk sector moves tailward at high magnetic latitudes in the Northern Hemisphere along the magnetopause further than that for the southward IMF case. Both the convective electric field and the resistive electric field are peaked in the region where the field lines are antiparallel. Moreover the perpendicular and parallel components of the current in the antiparallel field region are greater than those of the current in the subsolar and magnetic equator regions. The field‐aligned current, in the Southern (winter) Hemisphere on the dawn region is stronger than that in the Northern (summer) Hemisphere on the dusk region because the reconnection site shifts toward the tail in the winter hemisphere, which is just opposite to the case of southward IMF studied by Park et al. (2006). In the Northern (summer) Hemisphere, a three‐cell pattern appears in the ionosphere because the reconnection site shifts tailward and duskward from the cusp. In the Southern (winter) Hemisphere, the three‐cell pattern is distorted and has a weaker cross‐polar cap potential than that in the Northern Hemisphere due to the effects of the positive dipole tilt.

Cluster observations of the entry layer equatorward of the cusp under northward interplanetary magnetic field

Various boundary crossings in the vicinity of the high‐altitude cusp region were experienced by the Cluster spacecraft when the interplanetary magnetic field (IMF) was northward. In contrast to the southward IMF cases, in which a turbulent and diffusive entry layer is present equatorward of the cusp, a transition layer (without significant turbulence and diffusive properties) that shows clear differences in plasma parameters (sometimes step‐like profile) compared to the adjacent regions was observed. We suggest that this transition layer, which contains both magnetosheath and magnetospheric populations, is the entry layer during northward IMF conditions. This transition layer is possibly formed by dual‐lobe reconnection when the IMF is northward. The plasma property and the closed field line geometry of this layer indicate that it is possibly linked to the low‐latitude boundary layer. The width of this layer varies from 480 to 2200 km. The results support the notion that high‐latitude dual‐lobe reconnection is a potential mechanism of the transport of solar wind into the magnetosphere during northward IMF through the formation of a high‐altitude entry layer. The observations of different sublayers with evident density and temperature differences are consistent with the view that the reconnection process at the magnetopause is not steady.

Influence of the Ionospheric Conductance on the Size of the Rarth's Magnetopause and Bow Shock

AbstractThis paper studies the influence of the ionospheric conductance on the size of the Earth's magnetopause and bow shock under the following assumptions: (1) the ionosphere, treated as a spherical shell, has a uniform Pedersen conductance ΣP and a zero Hall conductance, and (2) the Earth's dipole moment is due southward and the interplanetary magnetic field (IMF) has only south component (Bz < 0). The size of the magnetopause and bow shock is characterized by the geocentric distances of their intersection points with the three axes of the GSE frame, i.e., the subsolar point, the dawn‐dusk flank point, and the north‐south top point. Given the solar wind conditions, and the values of Bz and ΣP, a quasi‐steady state of the system is obtained by 3‐D global MHD simulations. It is shown that the influence of ΣP on the size of the magnetopause and bow shock is significant in the range of about 1~5 S but negligible otherwise. As ΣP increases, the magnetopause and bow shock expand outward as a whole, and the former expands less than the latter so that the magnetosheath widens. The variation of the flank point position of the magnetopause with ΣP depends on the magnitude of Bz: the flank point shifts inward with increasing ΣP for weak southward IMF cases and outward otherwise. The above‐mentioned results indicate that the effect of the ionospheric conductance should be incorporated in constructing empirical models of the magnetopause and bow shock.

MHD flow visualization of magnetopause boundary region vortices observed during high‐speed streams

We present statistics for a total of 304 vortices found near the ecliptic plane on the magnetopause flanks, using simulated magnetohydrodynamics (MHD) data driven by real solar wind conditions. The study concentrates on 9 hours, from 29 March, 2000 UT to 30 March, 0500 UT, during the onset of a high‐speed stream that existed from 29 March to 5 April 2002. Magnetopause crossings seen by the Geotail spacecraft for the 9‐hour time interval were also analyzed and compared with the MHD simulation to validate our results. Vortices were classified by solar wind input provided by the Wind satellite located 60–80 RE upstream from Earth. Two hundred seventy‐three of the vortices were generated under northward Interplanetary Magnetic Field (IMF), and 31 were generated under southward IMF. The vortices generated under northward IMF were more prevalent on the dawnside than on the duskside and were substantially less ordered on the dawnside than on the duskside. Most of the vortices were large in scale, up to 10 RE, and with a rotation axis closely aligned with the ZSM direction. They rotated preferentially clockwise on the dawnside, and counterclockwise on the duskside. Those generated under southward IMF were less ordered, fewer in number, and also smaller in diameter. Significant vortex activity occurred on the nightside region of the magnetosphere for these southward cases in contrast to the northward IMF cases on which most of the activity was on the magnetopause flanks. The IMF is primarily northward for our time interval, and the development of these vortices with their rotation preference depending on their local time position suggest that a Kelvin‐Helmholtz (KH) instability is likely present.

Magnetospheric Cusps under Extreme Conditions: Cluster Observations and MHD Simulations Compared

MHD simulations are here applied to aid in the interpretation of three apparent cusp encounters by the Cluster 4 spacecraft in unusual places when the magnetosphere was under extreme solar wind and interplanetary magnetic field (IMF) conditions associated with the passage of magnetic clouds imbedded within fast ICMEs. At the time of each cusp encounter the IMF was very strong, generally northward in one case, generally equatorial in a second case, and generally southward in the third case. In the southward IMF case, the MHD models locate the origin of the cusp-like plasma by showing that the position of the spacecraft at the time of encounter was engulfed in a tongue of high-pressure plasma extending from the magnetopause into the magnetosphere. This tongue points to the northern-hemisphere cusp as the source of the feature. In the equatorial IMF case an elevated-pressure feature that apparently marked a cusp encounter in the computations coincided, however, with a passage in the solar wind of a dynamic pressure pulse, thus giving an alternative interpretation of the feature. However, Cluster data unambiguously identified the event as an encounter with magnetosheath-like plasma. Given that the Cluster observations classify the event as a true encounter with a cusp-like plasma feature (and not a compression event), the model simulations can be interpreted as identifying the origin of the feature to have been the northern-hemisphere cusp even though — and this is the interesting point — the observation point was in the southern hemisphere. In the northward IMF case, neither cusp (defined as a magnetic funnel linking the magnetopause to the Earth) was directly connected to the observation point. Instead, this encounter of magnetosheath-like plasma appears to be an instance of boundary-layer formation by means of the Song – Russell mechanism in which two-point magnetic reconnection entrains magnetosheath plasma on closed field lines when the IMF is northward.