J. Šafránková

Comprehensive study of the magnetospheric response to a hot flow anomaly

We present a comprehensive observational study of the magnetospheric response to an interplanetary magnetic field (IMF) tangential discontinuity, which first struck the postnoon bow shock and magnetopause and then swept past the prenoon bow shock and magnetopause on July 24, 1996. Although unaccompanied by any significant plasma variation, the discontinuity interacted with the bow shock to form a hot flow anomaly (HFA), which was observed by Interball-1 just upstream from the prenoon bow shock. Pressures within and Earthward of the HFA were depressed by an order of magnitude, which allowed the magnetopause to briefly (∼7 min) move outward some 5 RE beyond its nominal position and engulf Interball-1. A timing study employing nearby Interball-1 and Magion-4 observations demonstrates that this motion corresponded to an antisunward and northward moving wave on the magnetopause. The same wave then engulfed Geotail, which was nominally located downstream in the outer dawn magnetosheath. Despite its large amplitude, the wave produced only minor effects in GOES-8 geosynchronous observations near local dawn. Polar Ultraviolet Imager (UVI) observed a sudden brightening of the afternoon aurora, followed by an even more intense transient brightening of the morning aurora. Consistent with this asymmetry, the discontinuity produced only weak near-simultaneous perturbations in high-latitude postnoon ground magnetometers but a transient convection vortex in the prenoon Greenland ground magnetograms. The results of this study indicate that the solar wind interaction with the bow shock is far more dynamic than previously imagined and far more significant to the solar wind-magnetosphere interaction.

Transient flux enhancements in the magnetosheath

This study presents INTERBALL-1, and MAGION-4 observations of transient ion flux variations in the magnetosheath which cannot be related to similar changes in the solar wind observed by WIND and GEOTAIL. The duration of the transient events varies from tens of seconds to a few minutes and their amplitude can exceed background levels by a factor of 2 or more. We use the closely-spaced INTERBALL-1 and MAGION-4 satellites to estimate that the spatial dimensions of the events are on the order of ∼ 1RE. The interaction of foreshock discontinuities with the bow shock is identified as a possible source of these events.

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.

Magnetopause motion driven by interplanetary magnetic field variations

We use previously reported observations of hot flow anomalies (HFAs) and foreshock cavities to predict the characteristics of corresponding features in the dayside magnetosheath, at the magnetopause, and in the outer dayside magnetosphere. We compare these predictions with Interball 1, Magion 4, and GOES 8/GOES 9 observations of magneto-pause motion on the dusk flank of the magnetosphere from 1800 UT on January 17 to 0200 UT on January 18, 1996. As the model predicts, strong (factor of 2 or more) density enhancements bound regions of depressed magnetosheath densities and/or outward magnetopause displacements. During the most prominent event, the geosynchronous spacecraft observe an interval of depressed magnetospheric magnetic field strength bounded by two enhancements. Simultaneous Wind observations indicate that the intervals of depressed magnetosheath densities and outward magnetopause displacements correspond to periods in which the east/west (By) component of the interplanetary magnetic field (IMF) decreases to values near zero rather than to variations in the solar wind dynamic pressure, the north/south component of the IMF, or the IMF cone angle.

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].

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.

Multispacecraft measurements of plasma and magnetic field variations in the magnetosheath: Comparison with Spreiter models and motion of the structures

Abstract Large (from tens of percent up to several times) ion flux (or density) and magnetic field magnitude variations are typical magnetosheath features. Case and statistical comparisons of simultaneous solar wind observations, magnetosheath observations, and the gasdynamic model for the magnetosheath flow of Spreiter et al. (Planet. Space Sci. 14 (1966) 223) show that two types of magnetosheath plasma and magnetic field variations exist: • in some cases, they are a repetition or amplification of solar wind or IMF disturbances which pass through the bow shock; • but in the most cases, these variations are endogenous; i.e., they originate inside the magnetosheath. Persistence times and/or correlation lengths for the magnetosheath plasma variations were investigated via detailed comparison of simultaneous magnetosheath measurements from several spacecraft. For small separation distances (about 0.5RE) we used the satellite pair INTERBALL-1/MAGION-4; for larger distances (up to 10–30RE on the same or on the opposite flanks of the magnetosheath) we used the INTERBALL-1/GEOTAIL/IMP 8 measurements. In some cases, we observed a remarkable coincidence of the magnetosheath plasma behavior from the spacecraft separated by more than 10RE. It seems that the compression plasma structures move tailward together with the magnetosheath plasma flow.

Analysis of the 3-D shape of the terrestrial bow shock by interball/magion 4 observations

Abstract Location and shape of the terrestrial bow shock are analyzed using MAGION 4 (sub satellite of INTERBALL 1) crossings of this boundary and upstream solar wind parameters measured by the WIND spacecraft. Different crossing points were mapped to the Sun — Earth line and to the terminator plane using an analytical model of the planetary bow shock previously developed for the Martian bow shock investigation. Analysis of the subsolar bow shock position as a function of Alfvenic Mach number ( M a ) revealed fine effect that this boundary tends to approach the Earth when M a is decreasing for field-aligned flow of the solar wind, while for non field-aligned flow the bow shock moves away from the planet. Asymmetry of the terrestrial bow shock in the terminator plane is found for non field-aligned flow with anisotropic Friedrichs diagrams.

Observations of the radial magnetosheath profile and a comparison with gasdynamic model predictions

The gasdynamic model of the magnetosheath flow predicts changes of plasma parameters across the magnetosheath. According to this model, the ion flux peaks at the bow shock and decreases toward the magnetopause. MHD models predict a further plasma depletion near the magnetopause for specific orientations of the interplanetary magnetic field. We present a statistical study of ion flux measurements on the dusk flank of the magnetosheath and its comparison with present magnetosheath models. The study is based on three years of INTERBALL-1 measurements supported by simultaneous WIND solar wind observations. Our results reveal a much flatter magnetosheath radial profile than was expected from the gasdynamic prediction. The profile becomes steeper for higher upstream Mach number or plasma beta. The depletions of the ion flux behind the bow shock and at the magnetopause are attributed to the reflection and acceleration of particles in the bow shock region and magnetopause reconnection, respectively. A comparison of the measured and predicted ion fluxes suggests that the magnetosheath thickness is a rising function of the IMF magnitude.

The Earth's bow shock and magnetopause position as a result of the solar wind-magnetosphere interaction

Abstract The paper deals with a study of the variations of the Earths magnetopause and the bow shock position based on simultaneous measurements by the Prognoz 10 and IMP-8 spacecraft. The real boundary position determined by one spacecraft has been compared with the boundary position calculated from Formisanos model. The solar wind parameters, required for the calculation, have been taken from measurements of the other spacecraft. The differences between the calculated and observed boundary positions are discussed from the point of view of possible influences of different solar wind parameters on these deviations. From our discussion it follows that the magnetopause shape calculated by Olson [(1962), The shape of the tilted magnetopause. J. geophys. Res. 74 , 5642.], Fairfield [(1971), Average and unusual location of the Earths magnetopause and bow shock. J. geophys. Res. 76 , 6700.] or Choe et al . [(1973), Precise calculation of the magnetosphere surface for a tilted dipole. Planet. Space Sci. 21 , 485.] for a dipole magnetic field seems to be a better approximation than Formisanos three-dimensional fit. On the other hand, Formisanos fit to the bow shock shape can be used for determination of the bow shock position if the additional influence of the interplanetary magnetic field strength is taken into account.