A. A. Samsonov

Anisotropic MHD model of the dayside magnetosheath downstream of the oblique bow shock

Large-scale flow in the dayside magnetosheath is calculated by using a three-dimensional anisotropic MHD model for the case when the angle between the interplanetary magnetic field and the solar wind velocity is 45°. The behavior of plasma and magnetic field parameters downstream of the quasi-perpendicular and quasi-parallel bow shocks is compared in the results from a single calculation. The model includes a limit on the proton temperature anisotropy based upon thresholds for onset of the ion cyclotron and mirror instabilities. Results are presented for three different values of the isotropization rate. The model shows the existence of the plasma depletion layer, corresponding to an increase of the magnetic field intensity and a decrease of the plasma density near the magnetopause, for all angles of the bow shock normal relative to the interplanetary magnetic field. There is a thin layer downstream of the quasi-parallel shock where T‖p > T⊥p. The magnetosheath regions are shown where the threshold conditions for ion cyclotron and mirror instabilities are satisfied.

Application of the bounded anisotropy model for the dayside magnetosheath

Experimental data show existence of the proton temperature anisotropy in the dayside magnetosheath (T⊥p > T‖p where ⊥ and ‖ denote directions perpendicular and parallel, respectively, to the background magnetic field). The adiabatic MHD model for an anisotropic plasma is known as CGL equations. However, an application of the CGL equations for magnetosheath modeling overestimates the temperature anisotropy T⊥p/T‖p. To improve this situation, the CGL model was modified into the bounded anisotropy model [Denton et al., 1994; Gary et al., 1994]. This new class of model takes into account proton pitch angle scattering resulting from a growth of plasma anisotropy instabilities. The present paper contains the calculations of three-dimensional MHD flow around a spherical body with the use of the bounded anisotropy model as well as a comparison between the obtained results and satellite observations of the dayside magnetosheath. The calculated proton temperature anisotropy obtained by the bounded anisotropy model corresponds reasonably to the observed data. The results of the isotropic and anisotropic MHD models are compared. The calculated magnetosheath width is showrn to be greater for the anisotropic double adiabatic model.

Propagation of a sudden impulse through the magnetosphere initiating magnetospheric Pc5 pulsations

[1] We compare multipoint observations of an interplanetary shock’s interaction with the Earth’s magnetosphere on 29 July 2002 with results from global MHD simulations. The sudden impulse associated with the shock’s arrival initiates global ultralow‐frequency waves with periods from 2 to 5 min. We interpret four cycles of Bz oscillations with T= ∼3 min at Geotail in the postdawn magnetosphere as radial magnetopause oscillations. GOES 8, in the same late morning sector, observed compressional and toroidal waves with the same frequency at the same time. GOES 10, in the early morning sector, observed toroidal waves with a slightly lower period. We suggest that these observations confirm the mode coupling theory. The interplanetary shock initiates compressional magnetospheric waves which, according to our estimates, oscillate between the ionosphere and magnetopause and gradually convert their energy into that of standing Alfven waves. At the same time, Polar in the outer predawn magnetosphere observed strong velocity oscillations and weak magnetic field oscillations with a ∼4 min period. Global MHD models successfully predict these oscillations and connect them to the Kelvin‐Helmholtz instability which results in large flow vortices with sizes of about ten Earth radii. However, the global models do not predict the multiple compressional oscillations with the observed periods and therefore cannot readily explain the GOES observations.

Asymmetric magnetospheric compressions and expansions in response to impact of inclined interplanetary shock

We use global MHD simulations to model the magnetospheric response to an inclined shock that first strikes the duskside magnetosphere. The simulations predict several phenomena related specifically to the inclined shocks. The magnetospheric compression on the duskside exceeds that on the dawnside, and the geocentric distance to the dusk magnetopause varies in a simple step-like form. The compression on the dawnside is preceded and followed by expansions. For a moderately strong shock, the expansion magnitude reaches several RE behind the terminator plane. The magnetopause and cross-tail currents in the magnetotail are significantly deformed during and after the shock passage. The position of the magnetotail magnetopause moves by more than 10 RE. This asymmetric magnetopause deformation is mainly related to a strong |Vy| downstream from the inclined shock. The magnetospheric expansion results in a decrease in the horizontal magnetic field at low-latitude stations, as confirmed by observations.

Sudden impulse observations in the dayside magnetosphere by THEMIS

We present a study of the magnetospheric response to interplanetary shocks. We show eight events with simultaneous observations of sudden impulses in the dayside magnetosphere and interplanetary shocks in the solar wind. The spacecraft measurements in the equatorial plane, even those very close to the Earth, can be interpreted in terms of the vortices predicted by previous studies employing global MHD models. In fact, these vortices are velocity oscillations with the properties of Alfven waves. The amplitude and frequency of the oscillations depend on radial distance from the Earth. The amplitude of the velocity perturbations decreases with increase of the density and magnetic field magnitude, but the velocity amplitude shows no dependence on magnitude of the solar wind dynamic pressure change attending the interplanetary shocks. The oscillations are observed both in the outer magnetosphere and the plasmasphere, but they become less sinusoidal near the plasmapause, i.e., in the region with a large-density gradient. The amplitude of the magnetic field enhancement in the sudden impulses also depends on the radial distance. The MHD simulations successfully predict the amplitudes of magnetic field increase and the first cycles of the velocity oscillations in these events.

MHD-modelling of the magnetosheath

Abstract A short discussion of some problems of magnetosheath physics is presented. In particular, anisotropic MHD models of the magnetosheath are discussed. A method to estimate the value of the characteristic relaxation time (τ) of the proton temperature anisotropy from experimental data is proposed. Another problem considered in the review concerns the conditions of formation of a magnetic barrier within the magnetosheath. The existing controversy in this question is explained in the authors’ opinion by different definitions of the term “magnetic barrier” used in papers by Pudovkin et al. J. Geophys. Res., 87 (1982) 8131; Ann. Geophys. 13 (1995) 828) and Phan et al. J. Geophys. Res. 99 (1994) 121). Experimental data on the magnetic barrier dependence on the IMF orientation are discussed.

A method to predict magnetopause expansion in radial IMF events by MHD simulations

This paper presents a method for taking into account changes of solar wind parameters in the foreshock using global MHD simulations. We simulate four events with very distant subsolar magnetopause crossings that occurred during quasi-radial interplanetary magnetic field (IMF) intervals lasting from one to several hours. Using previous statistical results, we suggest that the density and velocity in the foreshock cavity decrease to ∼60% and ∼94% of the ambient solar wind values when the IMF cone angle falls below 50°. This diminishes the solar wind dynamic pressure to 53% and causes a corresponding magnetospheric expansion. We change the upstream solar wind parameters in a global MHD model to take these foreshock effects into account. We demonstrate that the modified model predicts magnetopause distances during radial IMF intervals close to those observed by THEMIS. The strong total pressure decrease in the data seems to be a local, rather than a global, phenomenon. Although the simulations with decreased solar wind pressure generally reproduce the observed total pressure in the magnetosheath well, the total pressure in the magnetosphere often agrees better with results for nonmodified boundary conditions. The last result reveals a limitation of our method: we changed the boundary conditions along the whole inflow boundary, although a more correct approach would be to vary parameters only in the foreshock. A model with the suggested global modification of the boundary conditions better predicts the location of part of the magnetopause behind the foreshock but may fail in predicting the rest of the magnetopause.

Why does the total pressure on the subsolar magnetopause differ from the solar wind dynamic pressure

Based on analysis of MHD equations and the results of numerical simulation in the magneto-sheath it is demonstrated that the total pressure on the magnetopause differs from the solar wind dynamic pressure in the majority of cases. From the equation of motion it follows that the total pressure is reduced due to deflection from the Sun-Earth line. At the same time, it increases because of formation of a magnetic barrier. This result is consistent with experimentally observed expansion of the magnetosphere for the radial direction of the interplanetary magnetic field, when no magnetic barrier is formed. In this paper we compare the behavior of pressure along the Sun-Earth line for the northward and radial interplanetary field, using the results of numerical MHD simulation and observational data from THEMIS. In the isotropic MHD approximation, the difference between the total pressure on the subsolar magnetopause at northern and radial IMFs does not exceed 10–12 percent. However, in the anisotropic approximation this difference increases up to 15–20 percent. The results of anisotropic modeling well agree with observed averaged profiles of pressure components in the subsolar magnetosheath.

Specific Features of Magnetic Barrier Formation Near the Magnetopause

The numerical three-dimensional MHD model is used to study the formation of the magnetic barrier in the inner part of the magnetosheath near the magnetopause. The set of the quasistationary solutions for several characteristic directions of the interplanetary magnetic field (IMF) has been obtained: for northward and southward IMF, for the direction along the Parker helix (at an angle of 45° with respect to the Sun-Earth line), and for the predominantly radial direction (at an angle of 20° with respect to the Sun-Earth line). The mechanism used to take into account the effect of magnetic reconnection at the magnetopause on a flow in the magnetosheath is introduced in the case of southward IMF. The results of the calculations indicate that the magnetic field absolute value in the magnetic barrier reaches its maximal value when IMF is northward. The introduction of magnetic reconnection at southward IMF can result in an insignificant decrease in the field value. However, the model predicts that a decrease in the magnetic field is much more substantial when the IMF direction is close to radial.

Anisotropic MHD model for the magnetosheath of Saturn

Abstract The ion temperature anisotropy in the magnetosheath of Saturn is estimated by three-dimensional MHD modelling using theoretical thresholds of the mirror and proton cyclotron instabilities obtained from kinetic theory. The calculation has been performed for typical parameters of the supersonic solar wind, and an angle between the interplanetary magnetic field and the solar wind velocity equal to 45°. It is found that in the Saturns magnetosheath the plasma β is bigger and the ratio T⊥/T∥ is less than in the Earths magnetosheath. The maximum of T⊥/T∥ at the subsolar point near the Saturns magnetopause is about 1.5. The ion cyclotron instability is found to be the main source of the pitch-angle diffusion in the Saturns plasma depletion layer, whereas the threshold of the mirror instability is lower in the magnetosheath proper.