Magnetosheath Magnetic Field Research Articles

The low-latitude flank magnetosheath, magnetopause and boundary layer: WIND observations

This paper presents highlights of a survey of the WIND spacecraft crossings of the low-latitude dawn and dusk magnetosheath, magnetopause (MP), and the low-latitude boundary layer (LLBL). In the magnetosheath the plasma flow direction is found to be remarkably steady, irrespective of the local magnetic field direction. In the LLBL, the flow direction in a substantial portion of the region is nearly aligned with that in the magnetosheath, while the flow tangential to the MP slows down gradually across the LLBL. This flow characteristic seems to be independent of the external magnetosheath magnetic field orientation, and may suggest that the LLBL is locally coupled to the adjacent magnetosheath, irrespective of the magnetic shear across the local MP.

Numerical modelling of solar wind flow about nonaxisymmetric magnetospheres: Planets Jupiter and Saturn

We discuss results on the solar wind flow past the non-axisymmetric magnetospheres of planets Jupiter and Saturn obtained by integrating numerically the dissipationless MHD equations under simplifying assumptions. We model these equatorially broadened magnetospheres as paraboloids with two different radii of curvature at the subsolar point. The thickness of the magnetosheath and the width and structure of the plasma depletion layer are found to be strong functions of the orientation of the interplanetary magnetic field (IMF). The effect of the IMF on the magnetosheath is strongest (weakest) when the IMF is directed perpendicular (parallel) to the rotational equator. For any intermediate IMF orientation, a smooth rotation of the magnetosheath magnetic field towards the direction of the planet's rotational axis is superimposed on the field strength enhancement (and the density reduction) as the respective magnetopauses are approached. These effects are more pronounced at Jupiter than at Saturn.

INTERBALL magnetotail boundary case studies

We present two examples of INTERBALL-1 data near both the high and low-latitude tail magnetopause (MP) under disturbed conditions. For the high-latitude case, MAGION-4 data determine the scales of the MP current sheets which are in the order of 100–500 km for the main ones, 50–200 km for Flux Transfer Events (FTEs) and a few km for the fine structures and ULF turbulence. The MP speed was 15–30 km/s. The energetic protons in the magnetosheath (MSH) provide evidence of reconnection upstream of the spacecraft (S/C). The tailward flows grow for the northward MSH magnetic field when the reconnection site is believed to be shifted tailward of the cusp. The inner boundary layer (BL) after the disturbance consists of tailward and earthward flowing plasma of MSH origin and cold mantle plasma flowing tailward The earthward flow is evidence of reconnection tailward of the S/C, which is regarded as a specific feature of the disturbed conditions. Local production of a plasma-sheet-like plasma at high latitudes is argued based on the inner BL plasma characteristics. The following features are observed in both cases: (a) FTEs for both northward and southward MSH fields; (b) waves in the current sheet vicinities over ten mV/m and 15 nT peak-to-peak; (c) electron fluxes with scales down to a few km with extra heating especially parallel to the magnetic field; (d) outer turbulent boundary layers with a deflected magnetic field; (e) ions with time-energy dispersion-like features and deflected ion fluxes. In the downstream dawn region at the transition between the low-latitude boundary layer and the plasma sheet (LLBL/PS), multiple MP encounters are observed. In the LLBL parallel electron intensifications correlate with ULF magnetic fluctuations.

Effects on the Jovian magnetosheath arising from solar wind flow around nonaxisymmetric bodies

We investigate the MHD structure of the Jovian magnetosheath along the Sun‐Jupiter line and, particularly, the region where the interplanetary magnetic field (IMF) exerts a large influence on the magnetosheath flow (the “magnetic barrier”). We do this by integrating numerically the dissipationless MHD equations in their “magnetic string” formulation. The lack of axisymmetry of the magnetospheric obstacle introduces corresponding asymmetries in the Jovian magnetosheath. The dominant effect on the flow is produced by the IMF component orthogonal to Jupiter's rotational equator. The thicknesses of the magnetosheath and magnetic barrier depend sensitively on the orientation of the IMF, decreasing monotonically as the inclination of the IMF to the rotational equator decreases. The magnetic barrier practically disappears when the IMF vector lies in the equator. For an arbitrary orientation of the IMF the magnetosheath magnetic field along the stagnation streamline is not only compressed as the magnetopause is approached but also rotates smoothly toward the direction of the Jovian rotation axis. This effect is absent in the case of flow around axisymmetric obstacles, such as the terrestrial magnetosphere.

Large scale structures in the magnetosheath: Exogenous or endogenous in origin?

Observations of the solar wind and the interplanetary magnetic field from ISEE‐3 are used as input to the gasdynamic convected field model, as implemented in a new space weather forecast model. Then the model output, for three case studies, is compared with the magnetosheath quantities observed at ISEE‐2 in order to identify the sources of the observed variations of the magnetosheath. It is found that some variations in the magnetosheath plasma and magnetic field are well correlated with corresponding variations in the solar wind and hence have their sources in the solar wind. However, some variations in the magnetosheath magnetic field correlate well with those in the solar wind but not variations in plasma density. Finally, we find that other variations in both plasma and magnetic field in the magnetosheath do not have appreciable correlations with variations in the solar wind. Most of these latter variations occur in the inner magnetosheath, indicating that they are endogenous in origin. Our results show that the forecast model can provide an accurate estimate of the timeshift from the solar wind monitor to the magnetosheath, of the instantaneous locations of the bow shock and magnetopause, and of the properties of the plasma and magnetic field in the outer and middle magnetosheath.

Generation of the magnetic field at the magnetopauses of the rapidly rotating planets

This paper is a study of a possible mechanism of magnetic field amplification in the thin transition layer just outside the magnetopause of a quickly spinning planet. Here we present analysis of a new magnetospheric magnetic field source arising due to the rapid rotation. When the ionosphere and magnetopause conductivities of the rapidly roating planet are sufficiently high (but less than the field‐aligned conductivity) the circulation exists at the magnetopause in the open field line bundles. The toroidal magnetic field is generated there from the poloidal ambient magnetosheath magnetic field by differential rotation. The requirement of the α effect existence is satisfied due to the deviation from axial symmetry about the planet's rotation axis of the initial magnetosheath velocity and magnetic fields. The increasing poloidal magnetic field can be regenerated from the toroidal one by the α effect. This mechanism may act as a hydromagnetic dynamo. The generated magnetic field penetrates into the magnetosphere and adds to the magnetospheric magnetic field.

Dependence of the magnetopause Kelvin‐Helmholtz instability on the orientation of the magnetosheath magnetic field

It is shown by means of a 2‐D MHD simulation that the observed rotation of the unperturbed magnetic field within the magnetopause (MP), which is assumed to be a tangential discontinuity (TD), leads to a significant dependence of the magnetopause Kelvin‐Helmholtz (K‐H) instability on the orientation of the sheath field. That is, northward sheath Bz is more favorable to the instability than southward Bz and the MP undulates most strongly when the sheath field points due north. When the sheath field does not point due north, a slow rarefaction region is formed in the velocity boundary layer (BL) inside the MP, wherein there are minima of the plasma pressure and temperature accompanied by a maximum of the magnetic pressure.

Oxygen outflow in the Martian magnetotail

Characteristic features of the oxygen ion (O+) outflow in the nightside of Mars have been studied. The data set used includes all high telemetry O+ energy spectra observations made by the ASPERA particle instrument onboard the Phobos 2 spacecraft. Oxygen outflow within the optical shadow of Mars was detected on all orbits, but no clear structure was found when the ions were organised according to the cross‐flow component of the interplanetary magnetic field (IMF). Oxygen ions in the tail outside the shadow were, in contrast, detected only on a few orbits and were related to fluctuating magnetosheath magnetic and electric fields with non‐Parker spiral IMF orientation. The analysis indicates that O+ ions within the optical shadow from a “steady”, ever persisting outflow feature. The high energy O+ ions in the tail outside the optical shadow are in contrast possibly a manifestation of the dynamic nature of the Martian tail, although effects associated to an asymmetric tail can not be fully excluded.

Magnetopause magnetic barrier parameters in dependence on the solar wind magnetic field orientation

Abstract. Some theories predict the magnetosheath magnetic field strength will decrease and the density increase just outside the dayside magnetopause as the interplanetary magnetic field turns southward. Two studies have recently reported results which confirm these expectations. In contrast, we briefly review our own theoretical predictions which indicate that precisely the opposite effect is expected. We survey new and previously reported magnetosheath observations and demonstrate that they are consistent with the predictions of our model. The conflicting results indicate a need for further theoretical and observational work.

Interplanetary magnetic field orientation for transient events in the outer magnetosphere

It is generally believed that flux transfer events (FTEs) in the outer dayside magnetosphere, usually identified by transient (∼1 min) bipolar magnetic field perturbations in the direction normal to the nominal magnetopause, occur when the magnetosheath magnetic field has a southward component. We compare the results of three methods for determining the magnetosheath magnetic field orientation at the times of previously identified UKS/IRM events: (1) the average magnetosheath magnetic field orientation in the 30‐min period adjacent to the nearest magnetopause crossing, (2) the magnetosheath magnetic field orientation observed just outside the magnetopause, and (3) the lagged interplanetary magnetic field (IMF) orientation at the time of the transient events. Whereas the results of method 2 indicate that the events tend to occur for a southward magnetosheath magnetic field, the results of methods 1 and 3 show no such tendency. The fact that the three methods yield significantly different results emphasizes the need for caution in future studies.

Prognoz 10 energetic particle data: Leakage from the magnetosphere versus bow shock acceleration

We present a case study and a statistical survey of Prognoz 10 energetic particle and magnetic field observations from the solar wind to the magnetosphere. The results provide evidence for both Fermi acceleration and leakage from the magnetosphere. Energetic ion (E > 15 keV) flux levels upstream of the bow shock and in the outer magnetosphere generally exceed those in the magnetosheath. Energetic electrons rarely accompany ion fluxes in the outer magnetosheath and upstream of the bow shock, but commonly accompany those in the inner magnetosheath. Upstream flux levels increase as the interplanetary magnetic field (IMF) rotates toward the bow shock normal, consistent with Fermi acceleration. Although the north/south magnetosheath magnetic field orientation has no effect upon energetic particle flux levels in the outer magnetosheath, flux levels in the inner magnetosheath increase during periods of southward IMF. We interpret the results as indicating that Fermi acceleration supplies most of the ions observed in the outer dayside magnetosheath but that leakage from the magnetosphere supplies most of the ions observed in the inner magnetosheath.

Magnethosheath magnetic field variability

A case study using simultaneous IRM and CCE observations demonstrates that transient magnetospheric events correspond to pressure pulses in the magnetosheath, inward bow shock motions and magnetopause compression. Statistical surveys indicate that the magnetosheath magnetic field orientation rarely remains constant during periods of magnetopause and bow shock motion (both characterized by periods of 1–10 min). There is no tendency for bow shock motion to occur for southward IMF orientations.

The magnetosheath region adjacent to the dayside magnetopause: AMPTE/IRM observations

We have studied 38 low‐latitude, dayside (0800‐1600 LT) magnetopause crossings by the AMPTE/IRM satellite to investigate the variations of key plasma parameters and the magnetic field in the magnetosheath region adjacent to the dayside magnetopause. We find that the structures of the key plasma parameters and the magnetic field and the dynamics of plasma flows in this region depend strongly on the magnetic shear across the magnetopause, that is, on the angle between the magnetosheath magnetic field and the geomagnetic field. When the magnetic shear is low (<30°), a magnetosheath transition layer, also called the “plasma depletion layer,” of 10‐min average width exists where the magnetosheath magnetic field piles up against the magnetopause. In this region the plasma density and plasma β as well as the proton and electron temperatures are lower than in the magnetosheath proper. The condition for the onset of the mirror instability is generally not met in the magnetosheath transition layer, where the plasma β often falls below 1, while it is marginally satisfied in the magnetosheath proper, where usually β>1. When the magnetic shear across the magnetopause is high (>60°), the near‐magnetopause magnetosheath is more disturbed. The magnetic field in this case does not pile up in the immediate vicinity of the magnetopause, and no systematic variations in the plasma parameters are observed in this region until the encounter of the magnetopause current layer; that is, there is no magnetosheath transition layer. Also in contrast to the low‐shear case, the mirror instability threshold is marginally satisfied throughout the magnetosheath. The plasma flow pattern in the magnetosheath region adjacent to the dayside magnetopause is also found to depend strongly on the magnetic shear across the magnetopause: the magnetosheath flow component tangential to the magnetopause is enhanced and rotates to become more perpendicular to the local magnetic field as the low‐shear magnetopause is approached. This flow behavior may be consistent with the formation of a stagnation line instead of a stagnation point at the subsolar magnetopause. Enhancement and rotation of the magnetosheath flow on approach to the magnetopause are rarely observed when the magnetic shear across the magnetopause is high. In essence, our observations provide evidence for high (low) rate of transfer of magnetic flux and mass across the magnetopause when the magnetic shear is high (low). The relationships between the electron and proton temperature anisotropies and β in the near‐magnetopause magnetosheath region are also examined. It is found that Te⊥/Te∥ remains close to 1 for the entire range of βe, whereas Tp⊥/Tp∥ is generally anticorrelated with βp∥. However, no universal relationship seems to exist between Tp⊥/Tp∥ and βp∥.

Energetic electrons and ions in the magnetosheath at low and medium latitudes: Prognoz 10 data

We present a survey of Prognoz 10 energetic ion (>15 keV) and electron (>30 keV) observations at low and middle latitudes in the dayside magnetosheath. At low latitudes, peak fluxes are observed inside the magnetopause, whereas at middle latitudes the peak fluxes are generally observed in the magnetosheath at some distance from the magnetopause. Both electron and ion fluxes tend to be greater outside the dawnside magnetopause than outside the duskside magnetopause. The flux of energetic particles in the outer magnetosheath is almost invariably less than that within the inner magnetosheath. Although the particle flux exhibits a clear dependence upon geomagnetic activity, no such dependence upon the local magnetosheath magnetic field orientation was noted. We interpret the observations as indicating that leakage of magnetospheric particles is the dominant source of energetic particles in the magnetosheath, but do not rule out Fermi acceleration at the bow shock as a possible subsidiary contributor to the population of ions with energies of about 15 keV.

Distant magnetotails of the outer magnetic planets

We assume that the distant planetary magnetotails of Jupiter, Saturn, Uranus, and Neptune are partially open, hot, long plasma cavities generally in pressure equilibrium with the solar wind. Most of the magnetosheath magnetic field lines drape around the magnetotails. We invoke conservation of momentum density, magnetic field, plasma density and energy density fluxes at the tail boundaries to determine the shape of the magnetotails and the variations of plasma and magnetic field characteristics with distance down the magnetotail. We use the Voyager observations to initialize all of our calculations in the near-planet portions of each magnetotail. We present information on estimates of magnetotail cross-sections, magnetic field strengths, and plasma densities as a function of downstream distance. Comparison of our results for Jupiter with the Voyager 2 observations suggests that the model accurately predicts properties of the Jovian magnetotail at least as far as Saturn's orbit.

IMF draping around the Geotail: IMP 8 observations

This paper gives maps of magnetosheath magnetic field lines in a roughly 12RE thick annulus circling the tail approximately 30RE downwind from Earth. There are maps for northward and southward IMF and for quasi‐equatorial IMF (the usual case). The maps are based on IMP 8 data taken during the 127 orbits (four years) when ISEE 3 measured the upstream IMF. In these four years, IMP 8's orbit swept the mapped annulus eight times. The maps project the field onto a plane perpendicular to the sun‐earth line, after correcting for solar wind aberration and tail flaring. Each shows draping around the tail: The field enters from one side, bifurcates at the tail boundary, “flows” around in both directions, rejoins at the opposite side, and exits. A new finding is that, contrary to expectations, the draping pattern is rotated relative to the plane formed by the IMF and the aberrated x‐axis. For example, it is rotated relative to the equatorial plane when the IMF lies in the equatorial plane. The degree of rotation varies from essentially zero for strongly northward and southward IMF to a peak ∼17° for moderately southward IMF. The sense of rotation is clockwise for toward IMF sectors and counterclockwise for away IMF sectors—the same as the dayside merging line. We also find that the tail is fatter and the magnetosheath field less ordered for strongly southward IMF than for strongly northward IMF. The new findings are understandable in terms of dayside magnetic merging.

Multipoint observations of planar interplanetary magnetic field structures

We present interplanetary magnetic field (IMF) data made on 1 November 1984 by three spatially well-separated spacecraft in the solar wind: the IMP 8, AMPTE-UKS and -CCE spacecraft. The IMF measured by each of the spacecraft is found to consist of a multiplicity of structures within which the magnetic field varies in parallel planes. The orientations of these planes at the three spacecraft locations are similar. The planes are inclined at a large angle to the ecliptic, and they lie almost perpendicular to the nominal Parker spiral direction in the ecliptic. Intercomparisons of the measurements at the various spacecraft show that the IMF features at one spacecraft are clearly reproduced at another with, however, time delays required for the propagation of signals. Using these time delays and the mutual separations of the spacecraft, we infer that the structures are convecting with the ambient flow. Simultaneous observations made downstream of the bow shock in the magnetosheath reveal that the magnetosheath magnetic field, too, is planar. However, the plane of maximum variation of the magnetosheath magnetic field is sheared with respect to the corresponding plane in the solar wind, and the field now lies approximately tangential to the magnetopause. Finally, our observations are contrasted to another IMF configuration showing gross departures from the nominal Parker spiral orientation: magnetic clouds.

Venus ionospheric “clouds”: relationship to the magnetosheath field geometry

Venus plasma “clouds,” apparently detached regions of ionospheric plasma above the main ionosphere of Venus observed by the Langmuir probe on the Pioneer Venus orbiter, are thought to be evidence for bulk removal of ionospheric ions by the solar wind interaction. In addition to the primary feature of an electron density enhancement above the ionopause, the characteristic signature of a cloud may include a change in the sign of the Bx (sunward/antisunward) component of the surrounding magnetosheath magnetic field. The plasma wave instrument also sometimes observed enhanced electric field noise in the clouds. In an effort to learn more about clouds and their cause(s), we analyzed the magnetic field control of cloud positions relative to the planet by rotating their observed locations into a coordinate system in which all of the transverse upstream (interplanetary) magnetic fields were aligned. The results indicate that clouds are scattered around the periphery of the planet in the terminator plane. There is no evidence of a concentration of clouds where the magnetosheath magnetic field is most strongly “draped,” as suggested by some earlier studies of selected occurrences. On the other hand, statistics show that the change in the orientation of the transverse upstream magnetic field between the inbound and outbound bow shock crossings, for the orbits where clouds are seen, is approximately 30° greater than the average upstream change at Venus over the same time intervals. The Spreiter and Stahara gasdynamic model of the magnetosheath provides further support for the idea that a rotation in the upstream transverse magnetic field accompanies cloud formation. Modeling of the magnetic field time series in the magnetosheath for a “cloud” orbit is possible only when a sequence of upstream fields is used to reconstruct the observed time series. Thus, clouds are associated not simply with normal magnetosheath field draping but with interplanetary field orientation changes.

Plasma structure near the low‐latitude boundary layer: A rebuttal

A recent reanalysis of a well‐documented interval of plasma and magnetic field data led its authors to offer a new model for the structure of the outer magnetosphere and the magnetosheath on the northern dawnside. On November 6, 1977, ISEE 1 and 2 had observed a series of quasi‐periodic pulses of magnetosheath‐like plasma on northward oriented magnetic field lines which were originally interpreted as repeated encounters of a pulsed low‐latitude boundary layer inside a smooth magnetopause followed by a single outward crossing of the magnetopause. D. G. Sibeck and coworkers reinterpreted the ISEE observations as being due to quasi‐periodic magnetopause motion causing the satellites to repeatedly exit the magnetosphere and to observe draped northward magnetosheath magnetic field lines in the plasma depletion layer. Their model is based on qualitative arguments concerning the amount of field line draping in the magnetosheath as well as the behavior of energetic electrons near the magnetopause. It is shown in this paper that both arguments are not in accordance with the available evidence.

A comparison of ULF fluctuations in the solar wind, magnetosheath, and dayside magnetosphere: 2. Field and plasma conditions in the magnetosheath

Data from three spacecraft (AMPTE IRM, AMPTE CCE, and ISEE 1 or 2) are used to study the correlation among the field and plasma conditions in the subsolar magnetosheath region, ULF wave activity in the magnetosphere, and the cone angle of the IMF. A disturbance parameter, R, which is the magnitude of the normalized resultant of unit vectors (calculated from measurements of the magnetic field or the plasma bulk velocity in a time interval), is used to describe the disturbance of the magnetosheath region. A “quiet” state has R values close to unity. We have studied five time intervals and found that when the R values of the magnetosheath magnetic field were below 0.8, indicative of a disturbed magnetosheath near local noon, transverse harmonic oscillations of magnetic field lines in the Pc 3, 4 range were observed in the magnetosphere and small cone angles were observed in the solar wind. We have also investigated the variation of other magnetosheath parameters (such as the magnetic pressure, the thermal pressure, the dynamic pressure, and the perturbation energy, etc.) under various magnetosheath conditions by comparing them with the disturbance parameter. It is found that the thermal beta (the ratio of the thermal pressure to the magnetic pressure) and the perturbation energy increase greatly as R decreases (i.e., as the magnetosheath region becomes more disturbed). The total pressure, which is the sum of the magnetic, thermal, and dynamic pressure of the subsolar magnetosheath region, decreases as the region becomes more disturbed. The dynamic pressure and the dynamic beta (the ratio of the dynamic pressure to the magnetic pressure) measured in the magnetosheath are poorly correlated with R, indicating that the changes in magnitude of the plasma bulk velocity in the subsolar magnetosheath have little effect on the occurrence of Pc 3‐4 waves in the outer magnetosphere. All magnetosheath parameters we examined became more variable during disturbed periods than during quiet ones. The implication of the above results to the transport of wave energy from the solar wind to the magnetosphere is discussed.