The Teardrop Magnetosphere

1996 ◽  
Author(s):  
Charles F. Kennel
Keyword(s):  
Solar Wind ◽  
Dynamic Pressure ◽  
Confining Pressure ◽  
Weak Field ◽  
Bow Shock ◽  
Dayside Magnetopause ◽  
Blunt Bodies ◽  
Model Section ◽  
Plasma Depletion ◽  
Average Position

In this chapter, we try to infer from magnetohydrodynamic reasoning and observation how the magnetosphere might look and behave if the magnetopause were inactive. Since there probably never has been an occasion when both viscosity and reconnection were absent, all we can do is array observations of phenomena that do not depend on either mechanism for their existence. As a result, we end up focusing on how the magnetosphere arrives at a balance of pressure with the solar wind. How it responds to changes in its confining pressure will be the topic of the next chapter. All discussions of the magnetosphere start with the magnetopause, and, indeed, the first models of the magnetosphere were calculations of the shape of the magnetopause. Without reconnection and without viscosity, the magnetopause would be given by the Chapman-Ferraro model on the dayside and close due to the reexpansion of the finite-temperature solar wind on the nightside (Section 2.2). This magnetosphere has a teardrop shape. After the dependence upon the interplanetary field via the reconnection process is taken into account, the average position and shape of the dayside magnetopause is in general accord with the Chapman-Ferraro model (Section 2.3). Because the magnetopause is always in motion, the early estimates of its thickness were uncertain until the first twospacecraft observations were made (Section 2.4). The magnetopause current layer proved to be several ion Larmor radii thick, significantly thicker than the electron inertial length. Once the average position of the magnetopause is specified, the position of the bow shock can be calculated using methods first employed for hypersonic flow around blunt bodies, which are easily extended to a weak-field MHD regime. The measured average positions of the bow shock and magnetopause agree once variations in solar wind dynamic pressure are taken into account (Section 2.5). While weak-field MHD does a good job with the bow shock, it fails in the subsolar magnetosheath, where a plasma depletion layer forms just upstream of the magnetopause (Section 2.6). Full MHD theory suggests that as many as three shocks could be standing in the flow enclosing the magnetosphere, a fast bow shock, an intermediate shock, and a slow shock.

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

2011 ◽  
Vol 29 (1) ◽  
pp. 31-46 ◽  
Author(s):  
S. Baraka ◽  
L. Ben-Jaffel
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Interplanetary Magnetic Field ◽  
Cell Model ◽  
Dynamic Pressure ◽  
Outer Boundary ◽  
Solar Wind Plasma ◽  
Tail Region ◽  
Dayside Magnetopause

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

scholarly journals A new method for solving the MHD equations in the magnetosheath

2013 ◽  
Vol 31 (3) ◽  
pp. 419-437 ◽  
Author(s):  
C. Nabert ◽  
K.-H. Glassmeier ◽  
F. Plaschke
Keyword(s):  
Solar Wind ◽  
Order Approximation ◽  
Bow Shock ◽  
Mhd Equations ◽  
Pile Up ◽  
Solar Wind Flow ◽  
Flow Deceleration ◽  
Solar Wind Parameters ◽  
Plasma Depletion ◽  
Geomagnetic Dipole

Abstract. We present a new analytical method to derive steady-state magnetohydrodynamic (MHD) solutions of the magnetosheath in different levels of approximation. With this method, we calculate the magnetosheath's density, velocity, and magnetic field distribution as well as its geometry. Thereby, the solution depends on the geomagnetic dipole moment and solar wind conditions only. To simplify the representation, we restrict our model to northward IMF with the solar wind flow along the stagnation streamline. The sheath's geometry, with its boundaries, bow shock and magnetopause, is determined self-consistently. Our model is stationary and time relaxation has not to be considered as in global MHD simulations. Our method uses series expansion to transfer the MHD equations into a new set of ordinary differential equations. The number of equations is related to the level of approximation considered including different physical processes. These equations can be solved numerically; however, an analytical approach for the lowest-order approximation is also presented. This yields explicit expressions, not only for the flow and field variations but also for the magnetosheath thickness, depending on the solar wind parameters. Results are compared to THEMIS data and offer a detailed explanation of, e.g., the pile-up process and the corresponding plasma depletion layer, the bow shock and magnetopause geometry, the magnetosheath thickness, and the flow deceleration.

scholarly journals Evidence for transient, local ion foreshocks caused by dayside magnetopause reconnection

2016 ◽  
Vol 34 (11) ◽  
pp. 943-959 ◽  
Author(s):  
Yann Pfau-Kempf ◽  
Heli Hietala ◽  
Steve E. Milan ◽  
Liisa Juusola ◽  
Sanni Hoilijoki ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Interplanetary Magnetic Field ◽  
Ion Beams ◽  
Bow Shock ◽  
Flux Transfer Events ◽  
Dayside Magnetopause ◽  
Flux Transfer ◽  
Push Out ◽  
Magnetopause Reconnection

Abstract. We present a scenario resulting in time-dependent behaviour of the bow shock and transient, local ion reflection under unchanging solar wind conditions. Dayside magnetopause reconnection produces flux transfer events driving fast-mode wave fronts in the magnetosheath. These fronts push out the bow shock surface due to their increased downstream pressure. The resulting bow shock deformations lead to a configuration favourable to localized ion reflection and thus the formation of transient, travelling foreshock-like field-aligned ion beams. This is identified in two-dimensional global magnetospheric hybrid-Vlasov simulations of the Earth's magnetosphere performed using the Vlasiator model (http://vlasiator.fmi.fi). We also present observational data showing the occurrence of dayside reconnection and flux transfer events at the same time as Geotail observations of transient foreshock-like field-aligned ion beams. The spacecraft is located well upstream of the foreshock edge and the bow shock, during a steady southward interplanetary magnetic field and in the absence of any solar wind or interplanetary magnetic field perturbations. This indicates the formation of such localized ion foreshocks.

scholarly journals Magnetopause energy and mass transfer: results from a global MHD simulation

2006 ◽  
Vol 24 (12) ◽  
pp. 3467-3480 ◽  
Author(s):  
M. Palmroth ◽  
T. V. Laitinen ◽  
T. I. Pulkkinen
Keyword(s):  
Magnetic Field ◽  
Mass Transfer ◽  
Solar Wind ◽  
Energy Transfer ◽  
Dynamic Pressure ◽  
Solar Wind Dynamic Pressure ◽  
Closed Field ◽  
Dayside Magnetopause ◽  
Solar Wind Parameters ◽  
The Imf

Abstract. We use the global MHD model GUMICS-4 to investigate the energy and mass transfer through the magnetopause and towards the closed magnetic field as a response to the interplanetary magnetic field (IMF) clock angle θ=arctan (BY/BZ), IMF magnitude, and solar wind dynamic pressure. We find that the mass and energy transfer at the magnetopause are different both in spatial characteristics and in response to changes in the solar wind parameters. The energy transfer follows best the sin2 (θ/2) dependence, although there is more energy transfer after large energy input, and the reconnection line follows the IMF rotation with a delay. There is no clear clock angle dependence in the net mass transfer through the magnetopause, but the mass transfer through the dayside magnetopause and towards the closed field occurs preferably for northward IMF. The energy transfer occurs through areas at the magnetopause that are perpendicular to the subsolar reconnection line. In contrast, the mass transfer occurs consistently along the reconnection line, both through the magnetopause and towards the closed field. Both the energy and mass transfer are enhanced in response to increased solar wind dynamic pressure, while increasing the IMF magnitude does not affect the transfer quantities as much.

scholarly journals Saturation of the magnetosphere during superstorms: new results from magnetogram inversion technique

2017 ◽  
Vol 3 (3) ◽  
pp. 15-19
Author(s):  
Владимир Мишин ◽  
Vladimir Mishin ◽  
Юрий Караваев ◽  
Yuriy Karavaev
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Magnetic Flux ◽  
Interplanetary Magnetic Field ◽  
Geomagnetic Field ◽  
Dynamic Pressure ◽  
Magnetic Clouds ◽  
Polar Cap ◽  
Thermal Pressure ◽  
Dayside Magnetopause

From data of three three superstorms we study new features of the saturation process of the polar cap magnetic flux deceleration of its area at strengthening the solar wind (SW). It is shown that the saturation of the polar cap is observed at growth of the SW dynamic pressure and vertical IMF component for both signs. Saturation is realized not only during the passage of interplanetary magnetic clouds, but also at significant enhancement of SW density, when the SW thermal pressure is comparable with the pressure of the interplanetary magnetic field. We assume that at such condiitions the saturation is caused not only by a decrease in the efficiency of reconnection at the dayside magnetopause, but mainly by a finite magnetosphere compressibility –stopping the magnetopause compression due to the rapid Eathward growth of the geomagnetic field, ie, interior magnetospheric structure of the geomagnetic field

scholarly journals The responses of the earth’s magnetopause and bow shock to the IMF Bz and the solar wind dynamic pressure: a parametric study using the AMR-CESE-MHD model

2018 ◽  
Vol 8 ◽  
pp. A41 ◽  
Author(s):  
Juan Wang ◽  
Zhifang Guo ◽  
Yasong S. Ge ◽  
Aimin Du ◽  
Can Huang ◽  
...  
Keyword(s):  
Solar Wind ◽  
Mach Number ◽  
Dynamic Pressure ◽  
Bow Shock ◽  
Solar Wind Dynamic Pressure ◽  
The Earth ◽  
Shock Location ◽  
Mhd Model ◽  
Upstream Solar Wind ◽  
The Imf

We have used the AMR-CESE-MHD model to investigate the influences of the IMF Bz and the upstream solar wind dynamic pressure (Dp) on Earth’s magnetopause and bow shock. Our results present that the earthward displacement of the magnetopause increases with the intensity of the IMF Bz. The increase of the northward IMF Bz also brings the magnetopause closer to the Earth even though with a small distance. Our simulation results show that the subsolar bow shock during the southward IMF is much closer to the Earth than during the northward IMF. As the intensity of IMF Bz increases (also the total field strength), the subsolar bow shock moves sunward as the solar wind magnetosonic Mach number decreases. The sunward movement of the subsolar bow shock during southward IMF are much smaller than that during northward IMF, which indicates that the decrease of solar wind magnetosonic Mach number hardly changes the subsolar bow shock location during southward IMF. Our simulations also show that the effects of upstream solar wind dynamic pressure (Dp) changes on both the subsolar magnetopause and bow shock locations are much more significant than those due to the IMF changes, which is consistent with previous studies. However, in our simulations the earthward displacement of the subsolar magnetopause during high solar wind Dp is greater than that predicted by the empirical models.

Particle acceleration by transient structures around Earth’s bow shock

2020 ◽  
Author(s):  
Terry Zixu Liu ◽  
Vassilis Angelopoulos ◽  
Heli Hietala ◽  
San Lu ◽  
Drew Turner
Keyword(s):  
Solar Wind ◽  
Particle Acceleration ◽  
Dynamic Pressure ◽  
Bow Shock ◽  
Fermi Acceleration ◽  
Bow Wave ◽  
Nonlinear Transient ◽  
High Dynamic ◽  
Shock Environment ◽  
Transient Structures

&lt;p&gt;Upstream of Earth&amp;#8217;s bow shock, the foreshock is filled with particles that have been reflected at the bow shock and are streaming away from it. Interaction of these particles with solar wind particles and discontinuities within this region can cause foreshock transients to form. Downstream of Earth&amp;#8217;s bow shock, localized magnetosheath jets with high dynamic pressure are frequently observed. When such a fast magnetosheath jet compresses the ambient magnetosheath plasma, an earthward compressional bow wave/shock can form. Here we present that foreshock transients and magnetosheath jets can accelerate particles through shock drift acceleration, Fermi acceleration, and the betatron acceleration. Foreshock transients and magnetosheath jets therefore can increase the particle acceleration efficiency of the parent shock by providing additional acceleration. The shock environment relevant for particle acceleration is not just the shock itself, but also the nonlinear transient structures both upstream and downstream of it.&lt;/p&gt;

Magnetosheath high speed jets observed simultaneously by Cluster and MMS

2020 ◽  
Author(s):  
C.-Philippe Escoubet ◽  
Keyword(s):  
Solar Wind ◽  
Mach Number ◽  
High Speed ◽  
Smooth Surface ◽  
Dynamic Pressure ◽  
Dust Cloud ◽  
Bow Shock ◽  
Solar Wind Dynamic Pressure ◽  
Typical Size ◽  
Dust Clouds

&lt;p&gt;The supersonic solar wind is decelerated and thermalized when it encounters the Earth's magnetosphere and cross the bow shock. Sometimes, however, due to discontinuities in the solar wind, bow shock ripples or ionised dust clouds carried by the solar wind, high speed jets (HSJs) are observed in the magnetosheath. These HSJs have typically a Vx component larger than 200 km s-1 and their dynamic pressure can be a few times the solar wind dynamic pressure. They are typically observed downstream from the quasi-parallel bow shock and have a typical size around one Earth radius (RE) in XGSE. We use a conjunction of Cluster and MMS, crossing simultaneously the magnetopause, to study the characteristics of these HSJs and their impact on the magnetopause. Over one hour-fifteen minutes interval in the magnetosheath, Cluster observed 21 HSJs. During the same period, MMS observed 12 HSJs and entered the magnetosphere several times. A jet was observed simultaneously by both MMS and Cluster and it is very likely that they were two distinct HSJs. This shows that HSJs are not localised into small regions but could span a region larger than 10 RE, especially when the quasi-parallel shock is covering the entire dayside magnetosphere under radial IMF. During this period, two and six magnetopause crossings were observed respectively on Cluster and MMS with a significant angle between the observation and the expected normal deduced from models. The angles observed range between from 11&amp;#176; up to 114&amp;#176;. One inbound magnetopause crossing observed by Cluster (magnetopause moving out at 142 km s-1) was observed simultaneous to an outbound magnetopause crossing observed by MMS (magnetopause moving in at -83 km s-1), showing that the magnetopause can have multiple local indentation places, most likely independent from each other. Under the continuous impacts of HSJs, the magnetopause is deformed significantly and can even move in opposite directions at different places. It can therefore not be considered as a smooth surface anymore but more as surface full of local indents. Four dust impacts were observed on MMS, although not at the time when HSJs are observed, showing that dust clouds would have been present during the observations. No dust cloud in the form of Interplanetary Field Enhancements was however observed in the solar wind which may exclude large clouds of dust as a cause of HSJs. Radial IMF and Alfv&amp;#233;n Mach number above 10 would fulfill the criteria for the creation of bow shock ripples and the subsequent crossing of HSJs in the magnetosheath.&lt;/p&gt;

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

2004 ◽  
Vol 22 (3) ◽  
pp. 993-999 ◽  
Author(s):  
I. L. Arshukova ◽  
N. V. Erkaev ◽  
H. K. Biernat
Keyword(s):  
Magnetic Field ◽  
Growth Rate ◽  
Solar Wind ◽  
Layer Structure ◽  
Dynamic Pressure ◽  
Bow Shock ◽  
Curvature Radius ◽  
Current Layer ◽  
Interchange Instability ◽  
The Magnetic Field

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)

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