Interplanetary Magnetic Field Orientation Research Articles

Mirror mode structures in the asymmetric Hermean magnetosheath: Hybrid simulations

Results of two global three‐dimensional hybrid simulations of the solar wind interaction with the Hermean magnetosphere are presented for southward and northward interplanetary magnetic field (IMF) orientations. Important dawn‐dusk asymmetries of the Hermean bow shock and magnetosheath are observed depending on the IMF orientation. For the southward IMF, the dawnside has a thicker magnetosheath with higher β values and slower bulk velocities compared to the duskside, whereas for the northward IMF, the duskside has a thicker and higher β magnetosheath with slower bulk velocities. Mirror mode activity consequently appears at the dawnside for the southward IMF and at the duskside for the northward IMF. A mechanism for the bow shock and magnetosheath asymmetries is proposed and discussed in the context of the Hermean and terrestrial magnetosheaths.

Statistical maps of small‐scale electric field variability in the high‐latitude ionosphere

Statistical maps of small‐scale electric field variability in the high‐latitude ionosphere are derived for the Northern and Southern Hemispheres using 48 months of data from the Super Dual Auroral Radar Network (SuperDARN). Maps of variability magnitude (from scales of 45–450 km and 2–20 min) are derived for a range of interplanetary magnetic field (IMF) orientations and dipole tilt angles (the angle between the best fit dipole axis and the plane perpendicular to the Sun‐Earth line). It is found that the observed spatial distribution of average variability is significantly modified as the IMF and dipole tilt conditions change. Under negative (winter‐like) and neutral (equinox‐like) dipole tilt angles, variability is concentrated in the auroral and dayside cusp regions, and the spatial distributions of variability appear to be correlated to those of large‐ and small‐scale field‐aligned currents (FACs). Additionally, variability on the nightside is found to be more enhanced in the downward FAC region than it is in the upward FAC region. Under positive (summer‐like) dipole tilt angles, the average variability magnitudes across the high‐latitude regions are smaller than those observed under negative dipole tilt angles, and the spatial distributions are more uniform. These dipole tilt effects suggest that scale‐size‐ and conductivity‐dependent field‐aligned potential drops and conductivity‐dependent changes in the processes that generate variability are possible factors that impact the observed small‐scale electric field variability. In general, Southern Hemisphere maps appear very similar to Northern Hemisphere maps, although some minor differences are observed that may result from interhemispheric asymmetries in the geomagnetic field.

Influence of the solar wind magnetic field on the Earth and Mercury magnetospheres in the paraboloidal model

We study the dependence of Mercury's magnetospheric magnetic field structure on the interplanetary magnetic field (IMF). Special attention is paid to the case of radial IMF. Mercury is the smallest planet in the solar system and it does not have a substantial atmosphere or ionosphere. Mercury is the closest planet to the Sun, and it possesses a week intrinsic magnetic field. Due to these circumstances, IMF plays a major role in the hermean magnetospheric dynamics. Using a paraboloidal model of Mercury's magnetosphere, we study the magnetospheric magnetic field topology for different orientations of IMF including examples representative of the first MESSENGER's flyby. Variations in IMF lead to variations in the Mercury's magnetospheric magnetic field structure, which in turn, lead to changes in the distribution of open and closed magnetic field lines. Comparison with the much better investigated Earth's magnetosphere is fulfilled for clarifying the physical processes (mainly reconnection) existent in the hermean magnetosphere. We also consider the cases when MESENGER, being the Mercury's orbiter, observed flux transfer events (FTEs) in the hermean magnetosphere. When the radial IMF component (BIMFx) is significant, which is character to the Mercury, the quasi-neutral line is placed in one of the cusps (depending on the sign of BIMFx). The FTE generation at Mercury can be connected with this line, similarly to the case of southward IMF at Earth, when FTEs arise at the dayside magnetopause at the quasi-neutral line. We show examples of observations supporting this result for Mercury.

Control of lunar external magnetic enhancements by IMF polarity: A case study

We study an interaction between the solar wind and crustal magnetic fields on the lunar surface using SELENE (Kaguya) data. It has been known that magnetic enhancements are at times detected near the limb external to the lunar wake, which is thus called lunar external magnetic enhancement (LEME), as a result of direct interaction between the solar wind and lunar crustal fields. Although previous observational studies showed that LEMEs in the high solar zenith angle region favor stronger interplanetary magnetic field (IMF) and higher solar wind density, the relation between the IMF and the crustal field orientation has not been taken into account. We show evidence that the relation between the IMF and crustal field orientation is also one of the key factors that control the extent of LEME, focusing on one-day observations at 100km altitude that include data above strong crustal fields around South Pole-Aitken (SPA) basin. Strong LEMEs are detected at 100km altitude around SPA basin under the stronger and northward IMF condition, while they weaken under southward IMF. All LEME's peaks are located in the region where unperturbed crustal fields at 300km altitude are directed northward while they are less related to unperturbed crustal fields at 100km or lower, which suggests that lunar crustal fields are compressed by the solar wind dynamic pressure, and its large scale component parallel to the IMF is essential to the formation of the LEME.

The first in situ observation of Kelvin‐Helmholtz waves at high‐latitude magnetopause during strongly dawnward interplanetary magnetic field conditions

We report the first in situ observation of high‐latitude magnetopause (near the northern duskward cusp) Kelvin‐Helmholtz waves (KHW) by Cluster on January 12, 2003, under strongly dawnward interplanetary magnetic field (IMF) conditions. The fluctuations unstable to Kelvin‐Helmholtz instability (KHI) are found to propagate mostly tailward, i.e., along the direction almost 90° to both the magnetosheath and geomagnetic fields, which lowers the threshold of the KHI. The magnetic configuration across the boundary layer near the northern duskward cusp region during dawnward IMF is similar to that in the low‐latitude boundary layer under northward IMF, in that (1) both magnetosheath and magnetospheric fields across the local boundary layer constitute the lowest magnetic shear and (2) the tailward propagation of the KHW is perpendicular to both fields. Approximately 3‐hour‐long periods of the KHW during dawnward IMF are followed by the rapid expansion of the dayside magnetosphere associated with the passage of an IMF discontinuity that characterizes an abrupt change in IMF cone angle,ϕ = acos , from ∼90° to ∼10°. Cluster, which was on its outbound trajectory, continued observing the boundary waves at the northern evening‐side magnetopause during sunward IMF conditions following the passage of the IMF discontinuity. By comparing the signatures of boundary fluctuations before and after the IMF discontinuity, we report that the frequencies of the most unstable KH modes increased after the discontinuity passed. This result demonstrates that differences in IMF orientations (especially inϕ) are associated with the properties of KHW at the high‐latitude magnetopause due to variations in thickness of the boundary layer, and/or width of the KH‐unstable band on the surface of the dayside magnetopause.

Conditions at the expanded Jovian magnetopause and implications for the solar wind interaction

Using idealized models of the magnetosheath and magnetosphere magnetic fields, plasma densities, and plasma flow, we test for the steady state viability of processes mediating the interaction between the solar wind and the Jovian magnetosphere. The magnetopause is modeled as an asymmetric paraboloid with variable asymmetry. The subsolar standoff of the magnetopause has been shown to exhibit a bimodal probability distribution (Joy et al., 2002). Only the expanded magnetopause is considered, with a standoff of ∼90 RJ. We test where on the magnetopause surface large‐scale reconnection may be affected by either a shear flow or diamagnetic drift due to a pressure gradient across the magnetopause boundary. We also test for the onset of the Kelvin‐Helmholtz instability. We find that reconnection is inhibited on the dawn flank due to the large shear flows in this region, regardless of magnetopause shape or interplanetary magnetic field orientation. The presence of a high energy plasma population in the magnetosphere may inhibit reconnection over much of the magnetopause area, except when the fields are antiparallel. Additionally, most of the dawn flank of the magnetopause is Kelvin‐Helmholtz unstable, regardless of magnetopause asymmetry; and the dusk flank tailward of the planet is Kelvin‐Helmholtz unstable when the magnetopause is highly oblate.

Velocity shear instability and plasma billows at the Earth's magnetic boundary

The Kelvin-Helmoltz instability (KH) with formation of vortices appears in a wide variety of terrestrial, interplanetary, and astrophysical contexts. We study a series of iterated rolled-up coherent plasma structures (15) that flow in the equatorial Earth's boundary layer (BL), observed on October 24, 2001. The data were recorded during a 1.5 hour-long Wind crossing of the BL at the dawn magnetospheric flank, tailward of the terminator (X≈−13 RE). The interplanetary magnetic field (IMF) was radially directed, almost antiparallel to the magnetosheath (MS) flow. This configuration is expected to be adverse to the KH instability because of the collinearity of field and flow, and the high compressibility of the MS. We analyze the BL stability with compressible MHD theory using continuous profiles for the physical quantities. Upstream, at near Earth sites, we input parameters derived from an exact MHD solution for collinear flows. Further downtail at Wind position we input measured parameters. The BL is found KH unstable in spite of unfavorable features of the external flow. On the experimental side, the passage of vortices is inferred from the presence of low density - hot plasma being accelerated to speeds higher than that of the contiguous MS. It is further supported by the peculiar correlation of relative motions (in the bulk velocity frame): cold-dense plasma drifts sunward, while hot-tenuous plasma moves tailward. This event differs from many other studies that reported BL vortices under strongly northward IMF orientations. This is a case of KH vortices observed under an almost radial IMF, with implicit significance for the more common Parker's spiral fields, and the problem of plasma entry in the magnetosphere.

Why does the subsolar magnetopause move sunward for radial interplanetary magnetic field?

This paper analyzes the distribution of different pressure components (dynamic, thermal, magnetic) in the magnetosheath along the Sun‐Earth line for northward and radial interplanetary magnetic field (IMF) orientations with motivation to explain an unusual location and shape of the magnetopause often observed during the intervals of the radial IMF. The analysis employs isotropic and anisotropic MHD models, and their results are compared with the statistical processing of THEMIS observations. We have found that (1) the temperature anisotropy in the magnetosheath controls the pressure distribution near the magnetopause, and (2) the total pressure exerted on the subsolar magnetopause depends on the IMF orientation being significantly lower than the solar wind dynamic pressure for the radial IMF. The results of both MHD simulations and statistical investigations are quantitatively consistent with already published observations of the average magnetopause location as a function of the IMF orientation.

Magnetopause energy transfer dependence on the interplanetary magnetic field and the Earth's magnetic dipole axis orientation

Abstract. We examine the spatial variation of magnetospheric energy transfer using a global magnetohydrodynamic (MHD) simulation (GUMICS-4) and a large data set of flux transfer events (FTEs) observed by the Cluster spacecraft. Our main purpose is to investigate whether it is possible to validate previous results on the spatial energy transfer variation from the GUMICS-4 simulation using the statistical occurrence of FTEs, which are manifestations of magnetospheric energy transfer. Previous simulation results have suggested that the energy transfer pattern at the magnetopause rotates according to the interplanetary magnetic field (IMF) orientation, and here we investigate whether a similar rotation is seen in the locations at which FTE signatures are observed. We find that there is qualitative agreement between the simulation and observed statistics, as the peaks in both distributions rotate as a function of the IMF clock angle. However, it is necessary to take into account the modulation of the statistical distribution that is caused by a bias towards in situ FTE signatures being observed in the winter hemisphere (an effect that has previously been predicted and observed in this data set). Taking this seasonal effect into account, the FTE locations support the previous simulation results and confirm the earlier prediction that the energy transfers in the plane of the IMF. In addition, we investigate the effect of the dipole orientation (both the dipole tilt angle and its orientation in the plane perpendicular to the solar wind flow) on the energy transfer spatial distribution. We find that the energy transfer occurs mainly in the summer hemisphere, and that the dayside reconnection region is located asymmetrically about the subsolar position. Finally, we find that the energy transfer is 10% larger at equinox conditions than at solstice, contributing to the discussion concerning the semiannual variation of magnetospheric dynamics (known as "the Russell-McPherron effect").

3-D mesoscale MHD simulations of magnetospheric cusp-like configurations: cusp diamagnetic cavities and boundary structure

Abstract. We present results from mesoscale simulations of the magnetospheric cusp region for both strongly northward and strongly southward interplanetary magnetic field (IMF). Simulation results indicate an extended region of depressed magnetic field and strongly enhanced plasma β which exhibits a strong dependence on IMF orientation. These structures correspond to the Cusp Diamagnetic Cavities (CDC's). The typical features of these CDC's are generally well reproduced by the simulation. The inner boundaries between the CDC and the magnetosphere are gradual transitions which form a clear funnel shape, regardless of IMF orientation. The outer CDC/magnetosheath boundary exhibits a clear indentation in both the x-z and y-z planes for southward IMF, while it is only indented in the x-z plane for northward, with a convex geometry in the y-z plane. The outer boundary represents an Alfvénic transition, mostly consistent with a slow-shock, indicating that reconnection plays an important role in structuring the high-altitude cusp region.

Comparison of the observed dependence of large-scale Birkeland currents on solar wind parameters with that obtained from global simulations

Abstract. Spatial distributions of the large-scale Birkeland currents derived from magnetic field data acquired by the constellation of Iridium Communications satellites have been compared with global-magnetosphere magneto-hydrodynamic (MHD) simulations. The Iridium data, spanning the interval from February 1999 to December 2007, were first sorted into 45°-wide bins of the interplanetary magnetic field (IMF) clock angle, and the dependencies of the Birkeland currents on solar wind electric field magnitude, Eyz, ram pressure, psw, and Alfvén Mach number, MA, were then examined within each bin. The simulations have been conducted at the publicly-accessible Community Coordinated Modeling Center using the University of Michigan Space Weather modeling Framework, which features a global magnetosphere model coupled to the Rice Convection Model. In excess of 120 simulations with steady-state conditions were executed to yield the dependencies of the Birkeland currents on the solar wind and IMF parameters of the coupled model. Averaged over all IMF orientations, the simulation reproduces the Iridium statistical Birkeland current distributions with a two-dimensional correlation coefficient of about 0.8, and the total current agrees with the climatology averages to within 10%. The total current for individual events regularly exceeds those computed from statistical distributions by factors of ≥2, resulting in larger disparities between observations and simulations. The simulation results also qualitatively reflect the observed increases in total current with increasing Eyz and psw, but the model underestimates the rate of increase by up to 50%. The equatorward expansion and shift of the large-scale currents toward noon observed for increasing Eyz are also evident in the simulation current patterns. Consistent with the observations, the simulation does not show a significant dependence of the total current on MA.

Antiparallel and component reconnection at the dayside magnetopause

[1] Two types of reconnection occur at the Earth's magnetopause: component and antiparallel reconnection. Recently, an empirical model was developed that purportedly determines under what solar wind conditions one or the other type of reconnection is dominant. This empirical model is tested using observations at the magnetopause from the Cluster spacecraft. For a range of interplanetary magnetic field orientations, there are regions on the magnetopause where these two types of reconnection can be distinguished from one another by the direction of the flow of reconnection jets observed in magnetopause boundary layers. Cluster spacecraft observations at the magnetopause under these specific interplanetary magnetic field (IMF) orientations confirm model predictions of the type of reconnection. In addition, these observations show evidence of possible multiple reconnection (both component and antiparallel) occurring at the magnetopause.

Foreshock cavitons for different interplanetary magnetic field geometries: Simulations and observations

[1] Global hybrid (kinetic ions, fluid electrons) simulations have shown the existence of foreshock cavitons characterized by large depressions in plasma density and magnetic field magnitude, bounded by enhancements in these two parameters. Foreshock cavitons share some characteristics with reported foreshock cavities, but in contrast to the cavities that often appear as isolated structures, cavitons are always found surrounded by a sea of ultra low frequency (ULF) waves. They always occur in regions deep in the foreshock, downstream from the ion and ULF wave boundaries. We perform global hybrid simulations to show that cavitons can form for a variety of interplanetary magnetic field (IMF) geometries and different solar wind Mach numbers. We use Cluster data to show that cavitons are a common foreshock feature, and that they can form for different IMF orientations, consistent with our simulation results. We find that cavitons show density and field decrements as large as 60% of the ambient values. They are immersed in regions where waves show transverse and compressive components and the presence of diffuse ion distributions. We use linear kinetic theory to estimate wave growth for the two types of waves responsible for caviton formation, i.e., the weakly compressive waves and oblique propagating linearly polarized waves. Some of the cavitons observed by Cluster show trains of high-frequency waves inside them and this is consistent with the predictions of local higher-resolution hybrid simulations.

Flux transfer events: Motion and signatures

We present results from a 2.5 dimensional hybrid code simulation for the evolution of flux transfer events (FTEs) during intervals of due southward interplanetary magnetic field (IMF) orientation. The structures invariably form between pairs of reconnection lines, often remaining nearly stationary on the subsolar magnetopause before their motion begins. Although a few structures move sunward, ultimately coalescing with others, most eventually accelerate antisunward, reaching velocities many times greater than the sound speed in the magnetosheath but only about one Alfvén speed greater than the ambient magnetosheath flow. At these velocities, slow mode wakes (but not shocks) marked by density enhancements and magnetic field strength decreases extend outward from the structures into the magnetosheath. Upon encountering the cusps, the structures decelerate, undergo reconnection, and are destroyed.

Statistics of plasma sheet convection

[1] Determining the characteristics of plasma sheet convection and their response to changes in various solar wind parameters is important for understanding the energy and mass transport, as well as disturbance propagation, through geospace. We use 15 years of data obtained by Geotail, Cluster, and THEMIS to study statistically the characteristics of plasma sheet flows and the effect of the interplanetary magnetic field (IMF) on the convection. We find that plasma sheet convection is dominated by slow speed ( 500 km/s are directed almost purely sunward. Both IMF By and IMF Bz are observed to penetrate the plasma sheet. During southward IMF conditions, a channel of increased Bz is created in the nightside around the aberrated midnight axis. We suggest that the channel is caused by dipolarization and magnetic flux pileup related to fast flows. The nightside region of highest mean flow speed is located more duskward during dawnward IMF conditions than during duskward IMF conditions. For plasma sheet flows with speeds higher than 100 km/s, we find that the orientation of IMF (clock angle) controls the speeds, while the magnitude of the solar wind electric field plays a minor role. The increasing speed indicates that energy transfer per unit length of the nightside X line increases as IMF turns southward.

Concerning the motion and orientation of flux transfer events produced by component and antiparallel reconnection

We employ the Cooling et al. (2001) model to predict the location, orientation, motion, and signatures of flux transfer events (FTEs) generated at the solstices and equinoxes along extended subsolar component and high ]latitude antiparallel reconnection curves for typical solar wind plasma conditions and various interplanetary magnetic field (IMF) strengths and directions. In general, events generated by the two mechanisms maintain the strikingly different orientations they begin with as they move toward the terminator in opposite pairs of magnetopause quadrants. The curves along which events generated by component reconnection form bow toward the winter cusp. Events generated by antiparallel reconnection form on the equatorial magnetopause during intervals of strongly southward IMF orientation during the equinoxes, form in the winter hemisphere and only reach the dayside equatorial magnetopause during the solstices when the IMF strength is very large and the IMF points strongly southward, never reach the equatorial dayside magnetopause when the IMF has a substantial dawnward or duskward component, and never reach the equatorial flank magnetopause during intervals of northward and dawnward or duskward IMF orientation. Magnetosheath magnetic fields typically have strong components transverse to events generated by component reconnection but only weak components transverse to the axes of events generated by antiparallel reconnection. As a result, much stronger bipolar magnetic field signatures normal to the nominal magnetopause should accompany events generated by component reconnection. The results presented in this paper suggest that events generated by component reconnection predominate on the dayside equatorial and flank magnetopause for most solar wind conditions.

Displacement of conjugate points during a substorm in a global magnetohydrodynamic simulation

[1] We reproduce a magnetospheric reconfiguration under southward interplanetary magnetic field (IMF) conditions using numerical magnetohydrodynamic simulations. To investigate the relative displacements of the geomagnetic conjugate points, we trace both footprints of the geomagnetic field lines during the magnetospheric reconfiguration under positive and negative IMF By conditions. Several substormlike features, namely, the formation of a near-Earth neutral line, a fast earthward flow, and tailward releases of the plasmoid, occur about 1 h after a southward turning of the IMF. The surveyed field line traced from the near-Earth magnetotail was strongly distorted duskward in the north and south after the substorm onset for positive and negative IMF By, respectively. The maximum of the relative longitudinal displacement of both footprints is 4.5 and 5.5 h in magnetic local time for positive and negative IMF By, respectively. While observational studies have indicated that the IMF orientation is the main factor controlling the relative displacement of the conjugate points, the present simulation-based study with a constant IMF orientation shows for the first time that the combined effects of plasma pressure, magnetic field intensity, and the field-aligned current density distribution along the field line are likely to be major factors controlling the relative displacement of conjugate points.

The transmission of upstream waves to the magnetosphere: An analysis at widely separated ground stations

[1] On 18 February 2003 a long-duration, almost monochromatic, wave event was detected by Cluster at radial distances from the bow shock smaller than ≈10 Re. The interplanetary magnetic field orientation determined a wide and almost symmetric foreshock region around the bow shock nose, providing highly favorable conditions for a direct wave penetration into the magnetosphere. The general correspondence between the characteristics of the wave trains observed in the foreshock region and at two widely separated ground stations (namely, at low latitudes in the Northern Hemisphere and in Antarctica) confirms that the ground activity is ubiquitously determined and controlled by the upstream activity. At low latitudes, the direct propagation of the external waves through the subsolar point is sharply confined to the dayside hemisphere. In Antarctica, the wave activity is detected even on field lines stretched into the tail; here the polarization pattern suggests a significant contribution of sunward propagating waves in the postmidnight hours, consistent with a wave penetration through the magnetotail lobes. At both stations the wave energy is typically ≈5–10% of the external energy; in addition, in Antarctica the contribution of the cusp turbulence largely overcomes that one related to the penetration of the upstream waves.

Spatial dependence of magnetopause energy transfer: Cluster measurements verifying global simulations

Abstract. We investigate the spatial variation of magnetopause energy conversion and transfer using Cluster spacecraft observations of two magnetopause crossing events as well as using a global magnetohydrodynamic (MHD) simulation GUMICS-4. These two events, (16 January 2001, and 26 January 2001) are similar in all other aspects except for the sign of the interplanetary magnetic field (IMF) y-component that has earlier been found to control the spatial dependence of energy transfer. In simulations of the two events using observed solar wind parameters as input, we find that the GUMICS-4 energy transfer agrees with the Cluster observations spatially and is about 30 % lower in magnitude. According to the simulation, most of the the energy transfer takes place in the plane of the IMF (as previous modelling results have suggested), and the locations of the load and generator regions on the magnetopause are controlled by the IMF orientation. Assuming that the model results are as well in accordance with the in situ observations also on other parts of the magnetopause, we are able to pin down the total energy transfer during the two Cluster magnetopause crossings. Here, we estimate that the instantaneous total power transferring through the magnetopause during the two events is at least 1500–2000 GW, agreeing with ε scaled using the mean magnetopause area in the simulation. Hence the combination of the simulation results and the Cluster observations indicate that the ε parameter is probably underestimated by a factor of 2–3.

Electron transport and precipitation at Mercury during the MESSENGER flybys: Implications for electron-stimulated desorption

To examine electron transport, energization, and precipitation in Mercury's magnetosphere, a hybrid simulation study has been carried out that follows electron trajectories within the global magnetospheric electric and magnetic field configuration of Mercury. We report analysis for two solar-wind parameter conditions corresponding to the first two MESSENGER Mercury flybys on January 14, 2008, and October 6, 2008, which occurred for similar solar wind speed and density but contrasting interplanetary magnetic field (IMF) directions. During the first flyby the IMF had a northward component, while during the second flyby the IMF was southward. Electron trajectories are traced in the fields of global hybrid simulations for the two flybys. Some solar wind electrons follow complex trajectories at or near where dayside reconnection occurs and enter the magnetosphere at these locations. The entry locations depend on the IMF orientation (north or south). As the electrons move through the entry regions they can be energized as they execute non-adiabatic (demagnetized) motion. Some electrons become magnetically trapped and drift around the planet with energies on the order of 1–10 keV. The highest energy of electrons anywhere in the magnetosphere is about 25 keV, consistent with the absence of high-energy (>35 keV) electrons observed during either MESSENGER flyby. Once within the magnetosphere, a fraction of the electrons precipitates at the planetary surface with fluxes on the order of 10 9 cm −2 s −1 and with energies of hundreds of eV. This finding has important implications for the viability of electron-stimulated desorption (ESD) as a mechanism for contributing to the formation of the exosphere and heavy ion cloud around Mercury. From laboratory estimates of ESD ion yields, a calculated ion production rate due to ESD at Mercury is found to be on par with ion sputtering yields.