K. Jelínek

Magnetopause expansions for quasi‐radial interplanetary magnetic field: THEMIS and Geotail observations

We report THEMIS and Geotail observations of prolonged magnetopause (MP) expansions during long-lasting intervals of quasi-radial interplanetary magnetic field (IMF) and nearly constant solar wind dynamic pressure. The expansions were global: the magnetopause was located more than 3 RE and ~7 RE outside its nominal dayside and magnetotail locations, respectively. The expanded states persisted several hours, just as long as the quasi-radial IMF conditions, indicating steady-state situations. For an observed solar wind pressure of ~1.1-1.3 nPa, the new equilibrium subsolar MP position lay at ~14.5 RE, far beyond its expected location. The equilibrium position was affected by geomagnetic activity. The magnetopause expansions result from significant decreases in the total pressure of the high-beta magnetosheath, which we term the low-pressure magnetosheath (LPM) mode. A prominent LPM mode was observed for upstream conditions characterized by IMF cone angles less than 20 ~ 25 grad, high Mach numbers and proton plasma beta<1.3. The minimum value for the total pressure observed by THEMIS in the magnetosheath adjacent to the magnetopause was 0.16 nPa and the fraction of the solar wind pressure applied to the magnetopause was therefore 0.2, extremely small. The equilibrium location of the magnetopause was modulated by a nearly continuous wavy motion over a wide range of time and space scales.The pressure balance at the magnetopause is formed by magnetic field and plasma in the magnetosheath, on one side, and inside the magnetosphere, on the other side. In the approach of dipole earths magnetic field configuration and gas-dynamics solar wind flowing around the magnetosphere, the pressure balance predicts that the magnetopause distance R depends on solar wind dynamic pressure Pd as a power low R ~ Pd^alpha, where the exponent alpha=-1/6. In the real magnetosphere the magnetic filed is contributed by additional sources: Chapman-Ferraro current system, field-aligned currents, tail current, and storm-time ring current. Net contribution of those sources depends on particular magnetospheric region and varies with solar wind conditions and geomagnetic activity. As a result, the parameters of pressure balance, including power index alpha, depend on both the local position at the magnetopause and geomagnetic activity. In addition, the pressure balance can be affected by a non-linear transfer of the solar wind energy to the magnetosheath, especially for quasi-radial regime of the subsolar bow shock formation proper for the interplanetary magnetic field vector aligned with the solar wind plasma flow.

Low-frequency variations of the ion flux in the magnetosheath

Abstract The paper presents a statistical study of INTERBALL-1 ion flux fluctuations in the magnetosheath. We concentrated on low-frequency variations and their changes from the magnetopause up to the bow shock region. The study is based on relative standard deviations of one minute data computed over 30-min intervals and thus it reveals properties of the fluctuations with periods ranging from units to tens of minutes. The results provide no evidence for any amplification of the solar wind variations in the magnetosheath. The level of magnetosheath fluctuations increases from the bow shock toward the magnetopause. The direction of the interplanetary magnetic field orders fluctuation levels in both the dawn and dusk flanks of the magnetosheath. A significantly higher level of variations is observed in the dawn magnetosheath. The difference is caused by the predominantly Parker spiral orientation of the upstream magnetic field.

Analysis of temperature versus density plots and their relation to the LLBL formation under southward and northward IMF orientations

The paper reports a clear dependence of a basic structure of two sublayers of the low-latitude boundary layer (LLBL) on northward and southward interplanetary magnetic field (IMF) orientations. Regardless of different processes responsible for a formation and evolution of the entire LLBL, the outer part of the LLBL is significantly influenced by the sign of the IMF BZ component. Under northward IMF conditions this layer is present, whereas it is missing during a southward pointing IMF. This behavior can be understood in terms of a motion of reconnection spots due to the changes of the orientation of the magnetosheath magnetic field in the vicinity of the magnetopause. Our case and statistical studies demonstrate that the changes of the LLBL structure can be observed in the subsolar region as well as on flanks near the dawn-dusk meridian. Moreover, the study emphasizes a role of magnetosheath magnetic field variations on the boundary layer formation.

The bow shock velocity from two-point measurements in frame of the interball project

The bow shock is a highly dynamic boundary controlled by steady and transient variations in solar wind parameters. It has been found that both shape and position of this boundary are determined mainly by the dynamic pressure and by the upstream Mach number of the incoming solar wind. Intervals of multiple bow shock crossings, often lasting over intervals from minutes to hours, are currently interpreted in terms of bow shock motions with respect to the observing spacecraft. In the present paper, we examine the bow shock velocity based on several series as well as several single bow shock crossings observed by two closely separated spacecraft (MAGION-4/INTERBALL-1). Our estimations of the bow shock velocity typically ranges from several tens of kilometers per second to approx. 120 km/s. These results correspond to those previously published for quasiperpendicular shocks but our set contains both quasiparallel and quasiperpendicular shocks. An analysis shows that about 80% of crossings can be explained by radial expansion/compression of the bow shock surface. The timing of the rest of events requires another mechanisms for explanation.

Solar wind modification upstream of the bow shock

A spacecraft configuration with two monitors near L1 and a fleet of the spacecraft orbiting in front of the bow shock brings a great opportunity to test the propagation techniques for the solar wind and the assumption on a negligible solar wind parameter evolution. We use multi-point observations of the THEMIS-ARTEMIS mission and compare them with data from the Wind solar wind monitor in order to estimate different factors influencing solar wind speed evolution. We have found a significant deceleration (up to 6%) of the solar wind close to the bow shock and the effect extends up to 30 RE from the Earth. It is controlled by the level of magnetic field fluctuations and by the flux of reflected and accelerated particles. We can conclude that the reflected particles not only excite waves of large amplitudes but also modify mean values of the solar wind speed measured in an unperturbed solar wind.

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.