Dayside Magnetosheath Research Articles

Variations and Effects of the Venusian Bow Shock from VEX Mission

AbstractThe upper atmosphere of Venus is not shielded by planetary magnetic field from direct interaction with the solar wind. The interaction of shocked solar wind and the ionosphere results in ionopause. Magnetic barrier, the inner region of dayside magnetosheath with the dominated magnetic pressure deflects the solar wind instead of the ionopause at solar maximum. Therefore, the structure and interaction of venusian ionosphere is very complex. Although the Venus Express (VEX) arrived at Venus in April 2006 provides more knowledge on the Venusian ionosphere and plasma environment, compared to Pioneer Venus Orbiter (PVO) with about 14 years of observations, some important details are still unknown (e.g., long Venusian bow shock variations and effects). In this paper, the bow shock positions of Venus are determined and analyzed from magnetometer (MAG) and ASPERA-4 of the Venus Express mission from May 28, 2006 to August 17, 2010. Results show that the altitude of BS was mainly affected by SZA (solar zenith angle) and Venus bow shocks inbound and outbound are asymmetry.

Observations of oxygen ions in the dayside magnetosheath associated with southward IMF

We present a case study of high energy oxygen ions (O+) observed in the dayside terrestrial magnetosheath, in the southern hemisphere. It is shown that the presence of O+ is strongly correlated to the IMF direction: O+ is observed only for Bz < 0. Three satellites observe O+ immediately at both sides of the magnetopause and about 2 RE outside the magnetopause. These conditions indicate escape along open magnetic field lines. We show that if outflowing O+ is heated and accelerated sufficiently in the cusp, it takes 15–20 minutes for it to reach the magnetopause, allowing the ions to escape along newly opened field lines on the dayside. Earlier studies show evidence of strong heating and high velocities of O+ in the cusp and mantle at high altitudes, strengthening our interpretation. The observed magnetosheath O+ fluxes are of the same order as measured in the ionospheric upflow, which indicates that this loss mechanism is significant when it takes place.

Prototyping a global soft X‐ray imaging instrument for heliophysics, planetary science, and astrophysics science

AbstractWe describe current progress in the development of a prototype wide field‐of‐view soft X‐ray imager that employs Lobster‐eye optics and targets heliophysics, planetary, and astrophysics science. The prototype will provide proof‐of‐concept for a future flight instrument capable of imaging the entire dayside magnetosheath from outside the magnetosphere. Such an instrument was proposed for the ESA AXIOM mission (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Scientific visualization to study Flux Transfer Events at the Community Coordinated Modeling Center

In this paper we present recent additions to the visualization toolset offered by the Community Coordinated Modeling Center (CCMC). Two suites of visualization tools are available that can address different needs during the analysis of model simulations of the magnetosphere that are provided by the CCMC. The online, server-side visualization allows the user to quickly browse through simulation runs and now can create maps of magnetic field line topology in the magnetosphere. The second tool, SWX, can be used on the client computer after data have been downloaded. With this second tool the user can interact directly with the three-dimensional objects that are being rendered. We present results from a simulation of a Flux Transfer Event that was performed at the CCMC using a magnetohydrodynamic model of the Earth’s magnetosphere with a high resolution grid focused on the dayside magnetosheath and dayside magnetopause. The simulation shows that the FTE that results from localized magnetic reconnection is a complicated three-dimensional structure that requires modern visualization techniques. Visualization techniques that are presented here allow the researcher to fully appreciate the complexity contained in magnetospheric simulation results.

Interactions of the heliospheric current and plasma sheets with the bow shock: Cluster and Polar observations in the magnetosheath

[1] On 12 March 2001, the Polar and Cluster spacecraft were at subsolar and cusp latitudes in the dayside magnetosheath, respectively, where they monitored the passage by Earth of a large-scale planar structure containing the high-density heliospheric plasma sheet (HPS) and the embedded current sheet. Over significant intervals, as the magnetic hole of the HPS passed Cluster and Polar, magnetic field strengths |B| were much smaller than expected for the shocked interplanetary magnetic field. For short periods, |B| even fell below values measured by ACE in the upstream solar wind. Within the magnetic hole the ratio of plasma thermal and magnetic pressures (plasma β) was consistently >100 and exceeded 1000. A temporary increase in lag times for identifiable features in B components to propagate from the location of ACE to those of Cluster and Polar was associated with the expansion (and subsequent compression) of the magnetic field and observed low |B|. Triangulation of the propagation velocity of these features across the four Cluster spacecraft configuration showed consistency with the measured component of ion velocity normal to the large-scale planar structure. B experienced large-amplitude wave activity, including fast magnetosonic waves. Within the low |B| region, guiding center behavior was disrupted and ions were subject to hydrodynamic rather than magnetohydrodynamic forcing. Under the reported conditions, a significant portion of the interplanetary coupling to the magnetosphere should proceed through interaction with the low-latitude boundary layer. Data acquired during a nearly simultaneous high-latitude pass of a Defense Meteorological Satellites Program satellite are consistent with this conjecture.

Unusual nonlinear waves in the Venusian magnetosheath

[1] Within the Venusian magnetosheath, Venus Express has observed the existence of high-amplitude, nonlinear waves which have been interpreted as large-scale nonlinear rotational structures within the magnetic field. A number of mechanisms have been proposed that can be used to explain their origin. Venus Express data have been searched for other examples of the similar structures. It appears that such structures are quite rare, being observed during only 8 of the dayside magnetosheath crossings among 676 that took place between April 2006 and February 2008. All together, only 50 such waves have been identified during these 8 crossings. The properties of these structures were studied. After considering a number of possible generation mechanisms, it is concluded that the most probable generation mechanism is the Kelvin-Helmholtz Instability; however, other mechanisms such as shock-related processes cannot be completely discounted.

Thin magnetosheath as a consequence of the magnetopause deformation: THEMIS observations

This paper presents a simultaneous observation of the bow shock and magnetopause by THEMIS probes that allows determination of the actual magnetosheath thickness at the subsolar point. Moreover, Geotail located at the dusk dayside magnetosheath registered a brief excursion to the magnetosphere in this time. The spacecraft configuration reveals a significant deformation of the magnetopause surface that locally decreases its curvature radius. The highly curved magnetopause results in the decrease of the magnetosheath thickness to about half of its standard value in a particular observation point. The observed phenomenon is attributed to a rotation of the interplanetary magnetic field (IMF). Although it is generally expected that the bow shock and magnetopause move in accord, being driven mainly by the solar wind dynamic pressure, we suggest that the local and transient thinning of the magnetosheath can result from different responses of its boundaries to a sudden change of the pressure and/or IMF orientation.

High kinetic energy density jets in the Earth’s magnetosheath: A case study

We study several high kinetic energy density jets observed during a traversal of the dayside magnetosheath by the Cluster spacecraft on March 17, 2001, at various distances from the magnetopause, generally characterised by anomalously high values of the local magnetosonic Mach number. We concentrate on two jets observed just outside the magnetopause, the first almost parallel to the GSM x axis and the second directed northward–tailward along the nominal magnetopause surface. We present evidence that none of them can be ascribed to magnetic reconnection at the magnetopause and show that the magnetopause is severely deformed by the jets, so that its local normal forms an angle of 97° with the quiet time magnetopause normal. On these grounds, we suggest that the indentation of the magnetopause is caused by an anti-sunward jet ramming into the magnetopause slightly equatorward of the northern cusp and that the northward–tailward jet is the result of its reflection at the deformed magnetopause. Finally, we briefly discuss our results by comparing them with past studies of events which in some way recall the one analysed herein.

Low-frequency magnetic field fluctuations in Venus' solar wind interaction region: Venus Express observations

Abstract. We investigate wave properties of low-frequency magnetic field fluctuations in Venus' solar wind interaction region based on the measurements made on board the Venus Express spacecraft. The orbit geometry is very suitable to investigate the fluctuations in Venus' low-altitude magnetosheath and mid-magnetotail and provides an opportunity for a comparative study of low-frequency waves at Venus and Mars. The spatial distributions of the wave properties, in particular in the dayside and nightside magnetosheath as well as in the tail and mantle region, are similar to observations at Mars. As both planets do not have a global magnetic field, the interaction process of the solar wind with both planets is similar and leads to similar instabilities and wave structures. We focus on the spatial distribution of the wave intensity of the fluctuating magnetic field and detect an enhancement of the intensity in the dayside magnetosheath and a strong decrease towards the terminator. For a detailed investigation of the intensity distribution we adopt an analytical streamline model to describe the plasma flow around Venus. This allows displaying the evolution of the intensity along different streamlines. It is assumed that the waves are generated in the vicinity of the bow shock and are convected downstream with the turbulent magnetosheath flow. However, neither the different Mach numbers upstream and downstream of the bow shock, nor the variation of the cross sectional area and the flow velocity along the streamlines play probably an important role in order to explain the observed concentration of wave intensity in the dayside magnetosheath and the decay towards the nightside magnetosheath. But, the concept of freely evolving or decaying turbulence is in good qualitative agreement with the observations, as we observe a power law decay of the intensity along the streamlines. The observations support the assumption of wave convection through the magnetosheath, but reveal at the same time that wave sources may not only exist at the bow shock, but also in the magnetosheath.

Slow-mode shock candidate in the Jovian magnetosheath

We discuss some interesting plasma observations in the Jovian magnetosheath by the onboard plasma instruments of the Cassini spacecraft during the 2000–2001 Jupiter flyby. We propose that the observations are consistent with a slow-mode shock transition. In the terrestrial magnetosheath, a number of observations have been made that are consistent with slow-mode waves or shocks. In addition, a number of observations have established that, at least occasionally, slow-mode structures form at the plasma sheet-lobe boundary in the terrestrial magnetotail, related to X lines associated with reconnection. There has been only one previously reported observation of a slow-mode shock-like transition in the Jovian plasma environment. This observation was made in the dayside magnetosheath. The observation we report here was made well downstream of the magnetosphere in Jupiter’s magnetosheath, at local time ∼19:10. For our analysis we have used the data from the Cassini Plasma Spectrometer (CAPS) the Magnetospheric Imaging Instrument (MIMI) and the Magnetometer (MAG). The bow shock crossings observed by Cassini ranged downstream to −600 R J from the planet

Substorm evolution as revealed by THEMIS satellites and a global MHD simulation

The major substorm that occurred on 1 March 2008 had excellent spacecraft coverage by the THEMIS spacecraft in the magnetotail, GOES 11, and GOES 12 at geosynchronous orbit and Geotail in the dayside magnetosheath. A global magnetohydrodynamic simulation of this substorm, driven by Wind solar wind observations, accurately reproduced the magnetospheric observations. The simulation revealed the complexity of magnetotail dynamics during the substorm, in particular, in the near‐Earth plasma sheet. Reconnection began prior to the substorm on closed field lines and a flux rope formed there. Around substorm onset, the simulation exhibited flow vortices near the locations of THEMIS P3 and P4, in agreement with observations at P3 and P4. These vortices were associated with a duskside neutral line that formed early in the substorm. Six minutes later, another neutral line formed on the dawnside of the tail. These neutral lines then merged to form a single large reconnection region that extended across the tail and greatly expanded the flux rope. The least active part of the tail was the region around midnight. Strong flows were seen in the observations and in the simulation during the two intensifications of this substorm; in particular, tailward flows were seen at THEMIS P1 and P2. Reconnection on closed field lines, vortices in the near‐Earth region, a channel of strong tailward flow, and enhanced precipitation into the ionosphere all contributed to substorm development.

Escape of high-energy oxygen ions through magnetopause reconnection under northward IMF

Abstract. During a storm recovery phase on 15 May 2005, the Geotail spacecraft repeatedly observed high-energy (>180 keV) oxygen ions in the dayside magnetosheath near the equatorial plane. We focused on the time period from 11:20 UT to 13:00 UT, when Geotail observed the oxygen ions and the interplanetary magnetic field (IMF) was constantly northward. The magnetic reconnection occurrence northward and duskward of Geotail is indicated by the Walén analysis and convective flows in the magnetopause boundary layer. Anisotropic pitch angle distributions of ions suggest that high-energy oxygen ions escaped from the northward of Geotail along the reconnected magnetic field lines. From the low-energy particle precipitation in the polar cap observed by DMSP, which is consistent with magnetic reconnection occurring between the magnetosheath field lines and the magnetospheric closed field lines, we conclude that these oxygen ions are of ring current origin. Our results thus suggest a new escape route of oxygen ions during northward IMF. In the present event, this escape mechanism is more dominant than the leakage via the finite Larmor radius effect across the dayside equatorial magnetopause.

Surveys on magnetospheric plasmas based on the Double Star Project (DSP) exploration

The equatorial and polar satellites of the Double Star Project (DSP) were launched successfully on December 29, 2003 and July 25, 2004, respectively, and both of them are operating smoothly. The DSP provides a good opportunity for investigating the structure of the magnetosphere. Based on the DSP data collected during 2004, we have surveyed the distribution of the magnetic fields and plasmas in the magnetosphere. It is found that: (1) Near the Earth’s equatorial plane within geocentric distances of less than 7 RE, the Earth’s magnetic field is dipolar. In the vicinity of the magnetopause, the magnetic field is enhanced by a factor of about 1.5, and on the nightside, the magnetic field can vary significantly from the Earth’s dipole field, likely caused by the presence of the near-Earth tail current sheet. (2) In the day-side magnetosheath, the electron and ion densities are usually both in the range of 10–30 cm−3; the ion and electron temperatures are usually about 200 and 50 eV, respectively. The flow pattern is usually smooth, with a low velocity in the subsolar region and with significantly higher velocities in the dawn and dusk flanks. (3) In the region between the magnetopause and plasmasphere the density is low, approximately 0.5–5 cm−3, and the temperature is high, about 1–10 keV for ions and 0.1–5 keV for electrons. The ion temperature has an apparent anisotropy, with the ratio of the perpendicular and parallel temperatures being about 1.0–1.3 for the night-and dusk-side magnetosphere and about 1.3–2.0 for the day-and dawn-side magnetosphere. There is an evident sunward convection of about 50 km/s in the magnetosphere. On the dawn side, the flow becomes somewhat turbulent, and in the vicinity of the night-noon meridian plane, the convection is rather slow. (4) The high-energy electrons with energies higher than 2 MeV are mainly located in the regions with 3 < L < 4.5; the size of the high-energy electrons area varies with time, it may expand and shrink occasionally according to different solar wind conditions and magnetic activities.

Mirror‐mode‐like structures in Venus' induced magnetosphere

The solar wind interaction with Venus creates an induced magnetosphere around the planet. It is shown that within the space bound by Venus' bow shock and ionopause, there is a rich occurrence of mirror‐mode‐like structures in the magnetic field data. The dayside magnetosheath and nightside magnetosheath/wake regions are investigated separately. It is shown that the probability to observe mirror mode structures is much higher at the dayside, where it is also strongly dependent on the angle between the solar wind magnetic field and the bow shock normal. In Venus' wake the chance to observe these structures is low, most likely because of the fully developed turbulence in this region, which will decrease temperature anisotropies. The results stand in contrast to the very low occurrence rate claimed from data taken by the Pioneer Venus Orbiter mission.

High energy jets in the Earth’s magnetosheath: Implications for plasma dynamics and anomalous transport

High energy density jets in the magnetosheath near the Earth magnetopause were observed by Interball-1 [1]. In this paper, we continue the investigation of this important physical phenomenon. New data provided by Cluster show that the magnetosheath kinetic energy density during more than one hour exhibits an average level and a series of peaks far exceeding the kinetic energy density in the undisturbed solar wind. This is a surprising finding because the kinetic energy of the upstream solar wind in equilibrium should be significantly diminished downstream in the magnetosheath due to plasma braking and thermalization at the bow shock. We suggest resolving the energy conservation problem by the fact that the nonequilibrium jets appear to be locally superimposed on the background equilibrium magnetosheath, and, thus, the energy balance should be settled globally on the spatial scales of the entire dayside magnetosheath. We show that both the Cluster and Interball jets are accompanied by plasma superdiffusion and suggest that they are important for the energy dissipation and plasma transport. The character of the jet-related turbulence strongly differs from that of known standard cascade models. We infer that these jets may represent the phenomenon of the general physical occurrence observed in other natural systems, such as heliosphere, astrophysical, and fusion plasmas [2–10].

Coordinated Cluster and Double Star observations of the dayside magnetosheath and magnetopause at different latitudes near noon

We present results of a favorable conjunction where the equatorial spacecraft (TC‐1) of the Double Star mission exits the dayside magnetopause near the equator, while Cluster is inbound, near the southern cusp. This configuration makes it possible to compare observations of the magnetopause, around the same magnetic local time but at different latitudes. In this paper, we report on the general properties of the magnetosheath plasma at the two latitudes: unlike predictions from gasdynamic modeling, the density is found lower near the nose of the magnetopause than further downstream. Then, we present three interesting events. First, an FTE is observed at TC‐1 and not at Cluster; we discuss the implications this has on the evolution of FTEs and on the size of the reconnection site. Then, a structure observed at both spacecraft is interpreted as a bulge progressing along the magnetopause. It is not clear whether this bulge is actually the remnant of an FTE or a running pulse that makes Cluster sense the reconnection layer. In any case, a rotational discontinuity is observed within it. At last, a northward turning of the magnetosheath magnetic field is observed at TC‐1 and a reverse FTE is subsequently seen at Cluster, suggesting that magnetic reconnection is very fast to set up following a change in the IMF orientation.

Specific features of magnetic barrier formation near the magnetopause

It is well known that a detached shock wave is formed when the solar wind supersonic stream flows around the Earth’s magnetosphere. A layer of several Earth’s radii in thickness, which is called the magnetosheath, exists between a detached shock wave and the magnetopause. In the absence of magnetic reconnection at the magnetopause, the latter can be considered relatively impermeable for the solar wind stream in the dayside region. To describe a stream in the magnetosheath, it is necessary to solve the problem of supersonic stream that flows around a blunt body. A similar problem can be solved analytically only in the case of an incompressible fluid [Alexeev, 1984]. However, a shock cannot exist in this case; therefore, numerical simulation has been the main approach for the last several decades. In the 1960s, Spreiter et al. [1966] developed the hydrodynamic model used to study flow around a paraboloid of revolution, conditionally corresponding to the Earth’s magnetosphere. This model made it possible to calculate density, velocity, and temperature in the magnetosheath and to find the magnetic field vector values using the obtained velocity distribution with the help of the rot ( V × B ) = 0 relationship. This model rather exactly predicted the velocity distribution within the magnetosheath and satisfactorily described the position of a detached shock. This model was used to predict that a layer with an increased magnetic field strength should be formed near a flown-around surface. Pudovkin et al. [1982] and Pudovkin and Lebedeva [1987] made a similar conclusion based on the numerical quasi-two-dimensional MHD modeling. Pudovkin [1987] defined “magnetic barrier” as a region in the inner dayside magnetosheath where the Ampere force exceeds the force related to the thermal pressure gradient. At the same time, Erkaev [1988] characterized the magnetic barrier using the condition P B > P (i.e., magnetic pressure is higher than thermal pressure). Two different definitions of the magnetic barrier can result (and really results) in an ambiguous definition of the magnetic barrier region (see our numerical calculations). Two-dimensional [Pudovkin and Lebedeva, 1987; Zwan and Wolf, 1976], three-dimensional [Erkaev,

Plasma wave source location using CLUSTER as a spherical wave telescope

The use of sensor arrays in space opens up the possibility to investigate the source location of plasma waves. In order to do so, we generalize the wave telescope technique to use spherical waves instead of plane waves. The new tool determines the center of the wavefronts locally measured by the sensor array. This virtual source can be related with the position of the source generating the detected waves provided the wave propagation mode and medium properties are known. Moreover, the motion of the source region can be derived, as well as its basic geometrical characteristics. In this work we give the theoretical background for the new tool and examples of location analysis based on synthetic data. An example based on CLUSTER magnetic field data in the dayside magnetosheath reveals a virtual source region elongated in the magnetic field direction, moving with the plasma flow in the vicinity of the spacecraft configuration.

Rotation of the magnetic field in Earth's magnetosheath by bulk magnetosheath plasma flow

Abstract. Orientations of the observed magnetic field in Earth's dayside magnetosheath are compared with the predicted field line-draping pattern from the Kobel and Flückiger static magnetic field model. A rotation of the overall magnetosheath draping pattern with respect to the model prediction is observed. For an earthward Parker spiral, the sense of the rotation is typically clockwise for northward IMF and anticlockwise for southward IMF. The rotation is consistent with an interpretation which considers the twisting of the magnetic field lines by the bulk plasma flow in the magnetosheath. Histogram distributions describing the differences between the observed and model magnetic field clock angles in the magnetosheath confirm the existence and sense of the rotation. A statistically significant mean value of the IMF rotation in the range 5°-30° is observed in all regions of the magnetosheath, for all IMF directions, although the associated standard deviation implies large uncertainty in the determination of an accurate value for the rotation. We discuss the role of field-flow coupling effects and dayside merging on field line draping in the magnetosheath in view of the evidence presented here and that which has previously been reported by Kaymaz et al. (1992).

Statistical phase propagation and dispersion analysis of low frequency waves in the magnetosheath

Abstract. We present the results of a statistical analysis of low-frequency fluctuations in the high latitude regions of the dayside magnetosheath using CLUSTER as a wave telescope. Magnetic field observations are used to determine wave propagation directions and wave numbers for selected frequencies. Using observations of the plasma flow velocity we correct for the Doppler shift, in order to calculate frequencies and phase velocities in the plasma rest frame. This provides us with the possibility to perform a statistical dispersion analysis and to investigate various wave properties, such as the phase velocity and the propagation angle between k and B. The analysis of dispersion distributions and Friedrichs diagrams results in the identification of different wave populations. We find a multiplicity of standing structures (mirror modes) convected with the plasma flow and a large number of Alfvénic waves. The results confirm previous magnetosheath wave studies, such as ISSE or AMPTE spacecraft observations, but we also find a small number of mirror mode-like waves that have propagation speeds up to the local Alfvén velocity, quasi-perpendicular to the magnetic field.