Magnetosheath Parameters Research Articles

Monte Carlo Simulations of Electron Acceleration at Bow Waves Driven by Fast Jets in the Earth’s Magnetosheath

The shocked solar wind flows around the Earth’s magnetosphere in the magnetosheath downstream of the Earth’s bow shock. Within this region, faster flows of plasma, called magnetosheath jets, are frequently observed. These jets have been shown to sometimes exhibit supermagnetosonic speeds relative to the magnetosheath flow and to develop bow waves or shocks of their own. Such jet-driven bow waves have been observed to accelerate ions and electrons. We model electron acceleration by magnetosheath jet-driven bow waves using test-particle Monte Carlo simulations. Our simulations suggest that the energy increase of electrons with energies of a few hundred eV to 10 keV can be explained by a collapsing magnetic trap forming between the bow wave and the magnetopause with shock drift acceleration at the moving bow wave. Our simulations allow us to estimate the efficiency of acceleration as a function of different jet and magnetosheath parameters. Electron acceleration by jet-driven bow waves can increase the total acceleration in the parent shock environment, most likely also at shocks other than the Earth’s bow shock.

Solar Minimum Exospheric Neutral Density Near the Subsolar Magnetopause Estimated From the XMM Soft X‐Ray Observations on 12 November 2008

AbstractThe Earth's magnetosheath and cusps emit soft X‐rays due to the charge exchange between highly charged solar wind ions and exospheric hydrogen atoms. The Lunar Environment Heliospheric X‐ray Imager and Solar wind Magnetosphere Ionosphere Link Explorer missions are scheduled to image the Earth's dayside magnetosphere system in soft X‐rays to investigate global‐scale magnetopause reconnection modes under varying solar wind conditions. The exospheric neutral hydrogen density distribution, especially the value of this density at the subsolar magnetopause is of particular interest for understanding X‐ray emissions near this boundary. This paper estimates the exospheric density during solar minimum using the X‐ray Multimirror Mission (XMM) astrophysics observatory. We selected an event on 12 November 2008 from the XMM data archive, which detects soft X‐rays of magnetosheath origin while solar wind and interplanetary magnetic field conditions are relatively constant. During the event the location of the magnetopause was measured in situ by the THEMIS mission, thus the location of the solar wind ions responsible for the magnetosheath emission is well constrained by observation. We estimated the exospheric density using the Open Geospace Global Circulation Model (OpenGGCM) and a spherically symmetric exosphere model. The ratio of the magnetosheath plasma flux between the OpenGGCM model and the THEMIS, was nearly 1, which means the magnetohydrodynamic model reasonably reproduces the magnetosheath plasma conditions. The OpenGGCM magnetosheath parameters were used to deconvolve soft X‐rays of exospheric origin from the XMM signal. The lower‐limit of the exospheric density of this solar minimum event is 36.8 ± 11.7 cm−3 at 10 RE subsolar location.

Statistics on the Magnetosheath Properties Related to Magnetopause Magnetic Reconnection

Abstract Magnetosheath properties, particularly those related to magnetopause magnetic reconnection (MR), are investigated in this study. (1) Asymmetries are found to exist in the distributions of plasma and magnetic field parameters in the magnetosheath. These asymmetries are related to the interplanetary magnetic field (IMF) orientation, and they are produced either on the bow shock or inside the magnetosheath. Thus, one must be very cautious in directly using the upstream solar wind and IMF properties as the magnetopause MR initiation conditions, since the magnetosheath parameters are not the same everywhere. (2) A unique method is introduced to estimate how much IMF magnetic flux passes through the magnetosphere via MR on either the low-latitude or the high-latitude magnetopause. This flux mainly varies with three independent parameters: the IMF clock angle θ CL, the magnetosheath plasma β, and the solar wind sound Mach number M S . Surprisingly, the magnetic fluxes passing through the magnetosphere are comparable under the southward and northward IMF conditions. (3) The dipole tilt angle, the property from the inside of the magnetosphere, also controls the magnetosheath parameters. As the Earth’s dipole tilt angle varies, the plasma pressure ridge shifts its location but remains near the magnetic equator. The stagnation point of the magnetosheath flow on the magnetopause, however, remains at the subsolar point no matter how large the dipole tilt angle is. These behaviors may be determinative of the locations of MR and the generation of flux transfer events on the magnetopause.

Survey of Magnetosheath Plasma Properties at Saturn and Inference of Upstream Flow Conditions

AbstractA new Cassini magnetosheath data set is introduced that is based on a comprehensive survey of intervals in which the observed magnetosheath flow was encompassed within the plasma analyzer field of view and for which the computed numerical moments are therefore expected to be accurate. The data extend from 2004 day 299 to 2012 day 151 and comprise 19,155 416 s measurements. In addition to the plasma ion moments (density, temperature, and flow velocity), merged values of the plasma electron density and temperature, the energetic particle pressure, and the magnetic field vector are included in the data set. Statistical properties of various magnetosheath parameters, including dependence on local time, are presented. The magnetosheath field and flow are found to be only weakly aligned, primarily because of a relatively large z component of the magnetic field, attributable to the field being pulled out of the equatorial orientation by flows at higher latitudes. A new procedure for using magnetosheath properties to estimate the upstream solar wind speed is proposed and used to determine that the amount of electron heating at Saturn's high Mach‐number bow shock is ~4% of the dissipated flow energy. The data set is available as supporting information to this paper.

Investigation of a rare event where the polar ionospheric reverse convection potential does not saturate during a period of extreme northward IMF solar wind driving

AbstractA variety of statistical studies have shown that the ionospheric polar potential produced by solar wind‐magnetosphere‐ionosphere coupling is linear for weak to moderate solar wind driving but becomes nonlinear during periods of very strong driving. It has been shown that this applies to the two‐cell convection potential that develops during southward interplanetary magnetic field (IMF) and also to the reverse convection cells that develop during northward IMF. This has been described as polar potential saturation, and it appears to begin when the driving solar wind electric field becomes greater than 3 mV/m. Utilizing measurements from the Resolute Incoherent Scatter Radar (RISR‐N), we examine ionospheric data near local noon within the reverse convection cells that developed during a period of very strong northward interplanetary magnetic field (IMF) on 12 September 2014. During this period we measure the electric field within the throat of the reverse convection cells to be near 150 mV/m at a time when the IMF is nearly 28 nT northward. This is far in excess of the 30–40 mV/m expected for polar potential saturation of the reverse convection cells. In fact, the development of the electric field responds linearly to the IMF Bz component throughout this period of extreme driving. The conditions in the solar wind show the solar wind velocity near 600 km/s, number density near 20 ions/cm3, and the Alfvén velocity about 75 km/s giving an Alfvén Mach number of 8. A search of several years of solar wind data shows that these values occur together 0.035% of the time. These conditions imply a high plasma β in the magnetosheath. We believe that condition of high β along with high mass density and a strong merging electric field in the magnetosheath are the significant parameters that produce the linear driving of the ionospheric electric field during this unusual period of extreme solar wind conditions. A discussion of current theories to account for cross‐polar cap potential saturation is given with the conclusion that theories that utilize magnetosheath parameters as they affect the reconnection rate appear to be the most relevant to the cross‐polar cap potential saturation solution.

Correlation level between solar wind and magnetosheath plasma and magnetic field parameters

We conduct a statistical study of the correlation coefficients between solar wind and magnetosheath parameters using plasma and magnetic field data from THEMIS satellites. Correlation coefficients for high temporal resolution data are less than 0.5 in 70–80% of cases and remain the same for 30–40% of cases for 100-s smoothed data. The solar wind and magnetosheath parameters correlate better for larger solar wind density and interplanetary magnetic field magnitude values and for larger relative standard deviations of their parameters. Correlation level is higher in cases of quasi-perpendicular bow shock versus quasiparallel one. We consider correlation changes while magnetosheath satellite approaches the magnetopause and do not find correlation dependence on the satellite position inside the magnetosheath.

Dependence of the dayside magnetopause reconnection rate on local conditions

AbstractWe estimate the reconnection rates for eight dayside magnetopause reconnection events observed by the Cluster spacecraft and compare them with the predictions of the Cassak‐Shay Formula (Rcs) Cassak and Shay (2007). The measured reconnection rate is determined by calculating the product of the inflow velocity and magnetic field in the magnetosheath inflow region. The predicted reconnection rate is calculated using the plasma parameters on both sides of the current layer, including the contributions of magnetosheath H+, magnetospheric hot H+ and O+, and magnetospheric cold ions. The measured reconnection rates show clear correlations with Rcs with an aspect ratio of 0.07. The O+ and cold ions can contribute up to ~30% of the mass density, which may reduce the reconnection rate for individual events. However, the variation of the reconnection rate is dominated by the variation of the magnetosheath parameters. In addition, we calculated the predicted reconnection rate using only magnetosheath parameters (Rsh). The correlation of the measured rate with Rsh was better than the correlation with Rcs, with an aspect ratio of 0.09. This might indicate deviations from the Cassak‐Shay theory caused by the asymmetric reconnection structure and kinetic effects of different inflow populations. A better aspect ratio is expected to be between the ones determined using Rcs and Rsh. The aspect ratio does not show a clear dependence on the O+ concentration, likely because the O+ contribution is too small in these events. The aspect ratio also does not show a clear correlation with density asymmetry or guide field.

Determining the thickness of the low-latitude boundary layer in the Earth’s magnetosphere

We describe a method for determining the thickness of the low-latitude boundary layer (LLBL) of the Earth’s magnetosphere at the dayside near the equatorial plane based on the data gathered by a single satellite that traverses the layer and measures the plasma velocity. The method may be applied when the position of the magnetopause and the magnetosheath parameters fluctuate. The necessity of taking the presence of outer and inner LLBL regions into account is analyzed. The developed method is tested using the analysis results of two almost simultaneous close traverses of the magnetopause completed by the THEMIS mission satellites that provided relatively precise data on the LLBL thickness. It is shown that the developed method makes it possible to determine the LLBL thickness with an accuracy of ∼10%.

Magnetosheath cavities: case studies using Cluster observations

Abstract. This paper presents examples of magnetosheath cavities in Cluster spacecraft observations. The cavities are accompanied by high energy particles in the magnetosheath and characterized by depressed magnetic fields and densities. Flow speeds decrease and temperatures increase within the cavities. All magnetosheath parameters show increased variability within the cavities when the energetic particle flux is high. We predict outward motion of the magnetopause boundary in response to the decreases in the magnetosheath ram pressure caused by the high energy particles within the magnetosheath cavities. For our events, the magnetopause distance is predicted to be 30% larger during the times of high energy particle flux in the magnetosheath than that predicted using concurrent upstream solar wind pressure observations. Our events show no preference to occur for a particular IMF direction or solar wind plasma condition.

Correlation length of magnetosheath fluctuations: Cluster statistics

Abstract. Magnetosheath parameters are usually described by gasdynamic or magnetohydrodynamic (MHD) models but these models cannot account for one of the most important sources of magnetosheath fluctuations – the foreshock. Earlier statistical processing of a large amount of magnetosheath observations has shown that the magnetosheath magnetic field and plasma flow fluctuations downstream of the quasiparallel shock are much larger than those at the opposite flank. These studies were based on the observations of a single spacecraft and thus they could not provide full information on propagation of the fluctuations through the magnetosheath. We present the results of a statistical survey of the magnetosheath magnetic field fluctuations using two years of Cluster observations. We discuss the dependence of the cross-correlation coefficients between different spacecraft pairs on the orientation of the separation vector with respect to the average magnetic field and plasma flow vectors and other parameters. We have found that the correlation length does not exceed ~1 RE in the analyzed frequency range (0.001–0.125 Hz) and does not depend significantly on the magnetic field or plasma flow direction. A close connection of cross-correlation coefficients computed in the magnetosheath with the cross-correlation coefficients between a solar wind monitor and a magnetosheath spacecraft suggests that solar wind structures persist on the background of magnetosheath fluctuations.

Occurrence of reconnection jets at the dayside magnetopause: Double Star observations

We present a statistical study on reconnection occurrence at the dayside magnetopause performed using the Double Star TC1 plasma and magnetic field data. We examined the magnetopause crossings that occurred during the first year of the mission in the 0600–1800 LT interval and we identified plasma flows, at the magnetopause or in the boundary layer, with a different velocity with respect to the adjacent magnetosheath. We used the Walén relation to test which of these flows could be generated by magnetic reconnection. For some event we observed opposite‐directed reconnection jets, which could be associated with the passage of the X‐line near the satellite. We analyzed the occurrence of the reconnection jets and reconnection jet reversals in relation to the magnetosheath parameters, in particular the local Alfvèn Mach number, the plasma β, and the magnetic shear angle. We also studied the positions and velocities of the reconnection jets and jet reversals in relation to the magnetosheath magnetic field clock angle. We found that the observations indicate the presence of a reconnection line hinged near the subsolar point and tilted according to the observed magnetosheath clock angle, consistently with the component merging model.

Statistics of low-frequency variations in solar wind, foreshock and magnetosheath: INTERBALL-1 and CLUSTER data

The properties of small-scale ion flux and magnetic field fluctuations in the regions of undisturbed solar wind, foreshock and magnetosheath have been investigated. The systematic measurements of these parameters with very high time resolution, which were obtained onboard INTERBALL-1 spacecraft during 1996–2000, make it possible to examine their variations in a broad range of timescales (1–1800 s). The results from similar analysis of CLUSTER data are then compared with those from INTERBALL-1 data. The magnetic field variations from these two different satellites data are consistent not only in quality but also in quantity. Statistical results show that the level of the relative variations of parameters in magnetosheath and foreshock is about 3 times larger than those in undisturbed solar wind. Very intensive fluctuations in the magnetosheath are observed even under the “quiet” conditions of upstream solar wind. Properties of the small-scale fluctuations in the magnetosheath are strongly controlled by the angle Θ Bn between the interplanetary magnetic field vector and the bow shock normal, i.e. the level of fluctuations grows while the angle Θ Bn decreases. Wave properties of the plasma and magnetic field fluctuations are also very different in undisturbed solar wind, foreshock, quasi-parallel and quasi-perpendicular magnetosheath.

Interchange instability of a curved current layerconvecting in the magnetosheath from the bow shock towards themagnetopause

Abstract. This paper deals with nonsteady perturbations of the magnetosheath parameters which are related to variations of the interplanetary magnetic field from north to south under a constant solar wind dynamic pressure. The magnetic field changes its direction within a thin layer which is convected with the plasma from the bow shock to the ionopause. In the course of time, this current layer is amplified during its motion towards the magnetopause. The intensity of the current is increasing, the layer thickness is decreasing, and the gradients of parameters are becoming much sharper while the layer is approaching the magnetopause. The curvature radius of this layer is decreasing while it is draping around the magnetopause. This curved layer structure with reversed magnetic field in the magnetosheath is found to be unstable with respect to the interchange instability. The growth rate of the instability is obtained for different positions of the layer. Key words. Magnetospheric physics (magnetosheath)

Search for plasma and magnetic field cavities in magnetosheath

In the solar wind, foreshock cavities create depressed magnetic field and density regions associated with high energetic particles due bow shock related processes. In this study, we searched for the presence of foreshock-type transient events in the magnetosheath using Interball-1 spacecraft data. In our search of four years of magnetosheath magnetic field, density, temperature, velocity and proton flux data, we have not found signatures of these cavities. The relation between magnetosheath parameters and energetic particles is not clearly revelaed in Interball-1 data. While these events are very clear and commonly found in the solar wind, why they are not observed in the magnetosheath emposes an interesting and challenging question on the evolution of these solar wind structures and their relation to the magnetosheath dynamics.

High and low frequency large amplitude variations of plasma and magnetic field in the magnetosheath: Radial profile and some features

Variations in magnetosheath parameters, such as the ion flux and the magnitude of the magnetic field, over a wide range of frequencies are much larger than the variations in the undisturbed solar wind. We present the results of a statistical investigation of magnetosheath variations using a large database of high time resolution (ls) INTERBALL-1 measurements and 1 min resolution IMP 8 data. Radial profiles of the ion flux and the standard deviations of the ion flux on 1-minute intervals were obtained for dusk and dawn flanks of the magnetosheath. These profiles are similar on the dawn and dusk flanks of the magnetosheath and for high and low frequency variations. The amplitude of the high frequency variations depends on the angle between the bow shock normal and interplanetary magnetic field direction; variations are much larger behind quasi-parallel bow shocks.

Magnetosheath parameters near the subsolar line predicted by an MHD flow model with anisotropic pressure

We extend our “magnetic string” MHD model describing the flow of the shocked solar wind around the magnetosphere to include an anisotropic plasma pressure with p ⊥ > p ‖. Thermodynamic properties are determined by the law of conservation of energy of a magnetic string associated with a magnetic field line. The MHD equations are closed by a relation between p ⊥ and p ∥ corresponding to the threshold of the electromagnetic proton cyclotron wave instability. Assuming no flow across the magnetopause, we compare profiles of the steady-state magnetic field and plasma parameters along the subsolar line for upstream sonic and Alfvén Mach numbers = 10 with the isotropic case ( p ⊥ = p ‖). In the anisotropic model, besides the density, both temperatures, plasma pressures and betas decrease toward the magnetopause. The temperature and plasma pressure parallel to the magnetic field decrease more strongly than those perpendicular to the field. Profiles for temperature and pressure in the case of isotropy lie between those of corresponding parallel and perpendicular values, but closer to the latter. The gradient of B near the magnetopause is larger than for isotropic pressure.

On the effects of solar wind dynamic pressure on the anisotropic terrestrial magnetosheath

We apply our recent three‐dimensional anisotropic MHD model of magnetosheath flow [Erkaev et al., 1999] to study quantitatively effects of solar wind dynamic pressure (Pd∞ ) and Alfvén Mach number (Ma∞) on the anisotropic magnetosheath and the plasma depletion layer (PDL) in the subsolar region. Given the wide range over which these two parameters vary, their influence on the magnetosheath structure may be significant. Our analysis is applicable to quasi‐steady changes in the interplanetary medium. Following our earlier work, and in general agreement with the data, we define the sunward edge of the PDL by β‖ = 1, where β‖ is the proton beta parallel to the magnetic field. We first discuss changes in Pd∞ occurring under constant Ma∞. In this case, a rescaling of the parameters yields the effects on the magnetosheath. We then study quantitatively a changing dynamic pressure through a varying Alfvén Mach number. We obtain profiles of key magnetosheath parameters and the width of the PDL for Alfvén Mach numbers representative of the solar wind at Earth orbit. Gradients in parameter profiles become steeper and shift toward the magnetopause as Ma∞ increases. We find that PDL width varies as even in the anisotropic magnetosheath. Using our model to study the magnetosheath location where the electromagnetic ion cyclotron wave (EICW) instability dominates over the mirror instability, we find that this location occurs well inside the PDL. In addition, we estimated the fraction of the PDL width occupied by the EICWs as a function of solar wind Alfvén Mach number. We conclude that the EICW regime is contained in, but is not co‐extensive with, the PDL. Examining critically this issue by changing the PDL identification criterion to others based on a density decrease relative to the bow shock value and a systematic drop toward the magnetopause, we find that this result still holds, but the region where EICWs are destabilized occupies a different fraction of the PDL thus defined. Some model results are compared with documented data from an inbound crossing of the magnetosheath made on December 24, 1994. Good agreement with model predictions are obtained.

The effect of the magnetopause shapes of Jupiter and Saturn on magnetosheath parameters

The solar wind flow past nonaxisymmetric magnetospheres exhibits features which are absent in the case of axisymmetric magnetospheres such as that of Earth. We discuss results obtained by a numerical integration of the dissipationless MHD equations, under simplifying assumptions, and apply them to the two outer planets Jupiter and Saturn, both of whose magnetospheres depart substantially from axisymmetry. We model these magnetospheres as paraboloids with two different radii of curvature at the subsolar point, L 0 and L 1, where L 0 and L 1 refer to a magnetopause cut comtaining the rotational axis, and to the rotational equator, respectively ( L 0 < L 1). The degree of flattening is expressed by a parameter q:= L 0 L 1 , and, following earlier modelling and data analysis work, we set q = 0.35 for Jupiter and q = 0.64 for Saturn. We find here that the structure and behaviour of physical parameters in the magnetosheath of Saturn is intermediate to that at Earth and at Jupiter. For both planets, 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 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. For Saturn, we study the dependence of magnetosheath parameters for various IMF orientations at two Alfvén Mach numbers. Throughout, we compare our results to similar calculations of solar wind flow past Earth.

Relationships between plasma depletion and subsolar reconnection

Observations in the subsolar magnetosheath show that the plasma depletion layer (PDL) is less pronounced for southward than for northward IMF. Since subsolar plasma depletion indicates pile‐up of magnetic flux, the degree of plasma depletion should depend on the relative rates of flux transport via subsolar reconnection and flux advection by the solar wind flow. To identify the factors affecting plasma depletion, we consider the ratio D = Er/Esw where Er is the reconnection electric field and Esw is the imposed solar wind electric field. For a quasi‐perpendicular subsolar bow shock D can be expressed in terms of magnetosheath parameters. Since PDL formation is suppressed when the subsolar bow shock is quasi‐parallel, we restrict attention to quasi‐perpendicular conditions. We show that D increases with increasing reconnection efficiency, magnetic shear at the magnetopause, and the magnetosheath magnetic field but decreases with increasing total perpendicular pressure (particle plus magnetic field) in the magnetosheath. By combining observations of the subsolar quasi‐perpendicular magnetosheath from AMPTE/IRM and AMPTE/CCE we verify that the degree of plasma depletion is inversely correlated with D. Furthermore, we show that the greater prevalence of plasma depletion in the CCE data implies that the reconnection efficiency is lower for the CCE events and specifically that the reconnection efficiency depends roughly inversely on magnetosheath β, for β &gt; 1. Finally, the results show that all of the changes brought about by plasma depletion act to increase Er so that some subsolar reconnection is likely to occur even for low magnetic shear. Thus the percentage of the magnetosheath magnetic field that participates in reconnection near the subsolar region does not act as a rectifier but remains positive for all shear angles, decreasing monotonically as the magnetic shear at the subsolar magnetopause changes from high to low shear. This implies that the equatorward polar cap convection cells observed during northward IMF and conventionally thought to be driven by a viscous interaction may be due at least in part if not wholly to subsolar low shear reconnection.

Changes in the distant tail configuration during geomagnetic storms

Changes in the structure of the distant tail associated with geomagnetic storms are studied by using plasma and magnetic field data obtained from Geotail. Thirteen storm intervals between October 1993 and October 1994 are examined when the satellite was located in the distant tail between X = −83 RE and X = −210 RE. Geotail observed the magnetosheath during all storms including those when the satellite was located near the nominal tail axis. Assuming the flow direction in the magnetosheath to be parallel to the magnetotail axis, we estimated the dimension and the flux of the tail for the magnetopause crossing events. Systematic changes of the distant tail structure are found in association with the development of the storms. Before the main phase onset of the storms, when the Dst shows positive excursion, the enhanced solar wind pressure reduces the average radius of the distant tail to ∼23 RE as compared with the quiet time value, ∼31 RE. During the storm main phase the dimension of the tail is comparable to the quiet time value in spite of the high solar wind pressure. This is attributed to the enhanced magnetic flux in the tail in association with the southward interplanetary magnetic field (IMF) Bz. An average energy of ∼5 × 1015 J is stored also in the distant tail during the storm main phase, which is a comparable value to that stored in the midtail during an intense substorm growth phase reported in the previous studies. During storm time, when the Bz component of the magnetosheath field was larger compared with the By component, the average tail cross section has a north‐south elongated elliptical shape. The average tail for quiet time, however, was elongated toward the dawn‐dusk direction. By component was larger than Bz component in our quiet time data set. These observations imply that the anisotropy in the magnetosheath magnetic field pressure for different IMF orientation changes the shape of the tail, which was also shown in global MHD models. Changes in the average magnetosheath parameters (density, speed, direction) observed during the storms are found to be consistent with solar wind fluctuations expected from corotating interaction regions.