Z. Němeček

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.

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.

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.

High and low frequency large amplitude variations of plasma and magnetic field in the magnetosheath: Radial profile and some features

Abstract Variations in magnetosheath parameters, such as the ion flux and the magnitude of the magnetic field, over a wide range of frequencies are much larger than the variations in the undisturbed solar wind. We present the results of a statistical investigation of magnetosheath variations using a large database of high time resolution (ls) INTERBALL-1 measurements and 1 min resolution IMP 8 data. Radial profiles of the ion flux and the standard deviations of the ion flux on 1-minute intervals were obtained for dusk and dawn flanks of the magnetosheath. These profiles are similar on the dawn and dusk flanks of the magnetosheath and for high and low frequency variations. The amplitude of the high frequency variations depends on the angle between the bow shock normal and interplanetary magnetic field direction; variations are much larger behind quasi-parallel bow shocks.

INTERMITTENCY OF SOLAR WIND DENSITY FLUCTUATIONS FROM ION TO ELECTRON SCALES

The intermittency of density fluctuations in the solar wind at kinetic scales has been examined using high time resolution Faraday cup measurements from the Spektr-R spacecraft. It was found that the probability density functions (PDFs) of the fluctuations are highly non-Gaussian over this range, but do not show large changes in shape with scale. These properties are statistically similar to those of the magnetic fluctuations and are important to understanding the dynamics of small scale turbulence in the solar wind. Possible explanations for the behavior of the density and magnetic fluctuations are discussed.

Two point observation of high‐latitude reconnection

Reconnection on the high-latitude magnetopause can produce sunward plasma flow in a portion of the lobes. This plasma should precipitate into the polar cap, poleward of the locations where magnetosheath plasma is normally found. We present two point INTERBALL observations of reconnection which is qualitatively consistent with the antiparallel merging model. Reconnection creates a plasma layer about 1R E thick adjacent to the magnetopause (MP). Timing considerations suggest tailward motion of the reconnection site with a velocity ∼ 20 km/s and a reconnection site with dimensions on the order of 1 R E .

Surface potential of small particles charged by the medium-energy electron beam

Abstract The present paper deals with the charging of glass particles of micron sizes by the monoenergetic electron beam with the energy in the range from 400 eV–5 keV. The particles are trapped in an electrodynamic quadrupole placed in an ultra-high vacuum chamber and bombarded by the electron beam. The surface of a trapped grain can be modified by the ion beam. The charge of the particles as well as their dimensions are determined from the dynamics of their motion inside the quadrupole and the corresponding surface potential is determined. The equilibrium surface potential is measured for different beam energies and the influence of the secondary electrons and background electrons on this potential is discussed for the grains of different sizes. A simple computer model of the charging process which has been developed is able to predict the equilibrium surface potential.