T.-L. Zhang

The magnetic barrier at Venus

The magnetic barrier at Venus is a region within which the magnetic pressure dominates all other pressure contributions. The barrier is formed in the inner region of the dayside magnetosheath to transfer solar wind momentum flux to the ionosphere. Passes through the dayside magnetosheath and ionopause with Pioneer Venus have allowed us to probe the magnetic barrier directly. These passes have been used to construct altitude profiles of the barrier. Here we define the ionopause as the lower boundary of the barrier. The upper boundary is defined as the altitude where the magnetosheath magnetic pressure is equal to half of the upstream solar wind dynamic pressure corrected by the boundary normal angle. The magnetic barrier is strongest at the subsolar point and weakens as expected with increasing solar zenith angle. The existence of a north-south asymmetry in the barrier strength is also demonstrated. The magnetic barrier is about 200 km thick at the subsolar point and 800 km thick at the terminator, which is comparable with the so-called “mantle.” We find that the magnetic barrier transfers most of the solar wind dynamic pressure to the ionosphere via the enhanced magnetic pressure. The convected field gasdynamic model is found to predict the correct bow shock location if the magnetic barrier is treated as the obstacle.

Little or no solar wind enters Venus' atmosphere at solar minimum.

Venus has no significant internal magnetic field, which allows the solar wind to interact directly with its atmosphere2,3. A field is induced in this interaction, which partially shields the atmosphere, but we have no knowledge of how effective that shield is at solar minimum. (Our current knowledge of the solar wind interaction with Venus is derived from measurements at solar maximum.) The bow shock is close to the planet, meaning that it is possible that some solar wind could be absorbed by the atmosphere and contribute to the evolution of the atmosphere. Here we report magnetic field measurements from the Venus Express spacecraft in the plasma environment surrounding Venus. The bow shock under low solar activity conditions seems to be in the position that would be expected from a complete deflection by a magnetized ionosphere. Therefore little solar wind enters the Venus ionosphere even at solar minimum.

Double Star/Cluster observation of neutral sheet oscillations on 5 August 2004

Abstract. Previous Cluster observations have shown that the flapping motions of the Earths magnetotail are of internal origin and that kink-like waves are emitted from the central part of the tail and propagate toward the tail flanks. The newly launched Double Star Program (DSP) TC-1 satellite allows us to investigate neutral sheet at 10-13 Re in the tail. Using conjunctions with Cluster we will have simultaneous observations at 10-13 and 16-19 Re of these flapping motions. In this paper, we present the first results of neutral sheet oscillations observed by the Cluster and Double Star satellites on 5 August 2004.

Solar Cycle 21 effects on the interplanetary magnetic field and related parameters at 0.7 and 1.0 AU

Magnetometer data obtained over the course of the previous solar cycle by the Pioneer Venus orbiter (PVO) at ∼ 0.7 AU and IMP 8 at 1.0 AU are used to compare the long-term behavior of the interplanetary magnetic field (IMF) at these two heliocentric distances. Similarities include an enhancement in the typical or median field magnitude during the declining phase of the solar cycle as compared to solar maximum or minimum, slight decreases in the Parker spiral angle from the declining phase through solar minimum, similar trends in the Alfvenic and magnetosonic Mach numbers, and the remarkably consistent sector structure noted previously. Differences include the temporal behavior of the high-field tail of the field distribution, showing that high fields are most frequently observed during solar maximum at the Earth but during the declining phase of activity at Venus. This latter feature suggests that the perceived occurrence history of large fields from transient disturbances such as coronal mass ejections is a sensitive function of position within the heliosphere.

The Martian magnetic field environment: Induced or dominated by an intrinsic magnetic field?

Abstract Even though magnetic field and plasma in-situ measurements near Mars from the 1989 PHOBOS-2 project and from earlier missions are available, the existence of an Martian intrinsic magnetic field is still controversial. In this study we analyze data of the PHOBOS-2 magnetic field experiments MAGMA and FGMM and use the upstream solar wind parameters of the TAUS and ASPERA experiments. Different methods are used to investigate the influence of the interplanetary magnetic field (IMF) and of a possible weak intrinsic field on the solar wind interaction with Mars : The compressibility of plasma boundaries, the correlation between upstream IMF and tail properties and between magnetic field structures and planetary rotation. The study shows that the magnetic field in the tail is strongly correlated with the upstream IMF suggesting that the Martian magnetotail is induced, at least to a large extent. Compressibility studies reveal a weak dependence of the plasma boundaries on the solar wind dynamic pressure but the bow shock location appears to be not affected by the Martian longitude within the accuracy of our measurements. We conclude that an intrinsic planetary field, if it exists, does not play a major role in the interaction between the solar wind and Mars.

Induced magnetosphere and its outer boundary at Venus

The induced magnetosphere at Venus consists of regions near the planet and its wake for which the magnetic pressure dominates all other pressure contributions. Initial Venus Express measurements indicate a well-defined outer boundary, the magnetopause, of the induced magnetosphere. This magnetopause acts as an obstacle to deflect the solar wind. Across this boundary, the magnetic field exhibits abrupt directional changes and pronounced draping. In this paper, we examine the structure of the magnetopause using Venus Express magnetic measurements. We find that the magnetopause is a directional discontinuity resembling either a tangential or a rotational discontinuity depending on the interplanetary magnetic field orientation.

A study of the solar wind deceleration in the Earth's foreshock region

Abstract Previous observations have shown that the solar wind is decelerated and deflected in the earths upstream region populated by long-period waves /1–3/. This deceleration is correlated with the ‘diffuse’ but not with the ‘reflected’ ion population. The speed of the solar wind may decrease tens of km/s in the foreshock region. The solar wind dynamic pressure exerted on the magnetopause may vary due to the fluctuation of the solar wind speed and density in the foreshock region. In this study, we examine this solar wind deceleration and determine how the solar wind deceleration varies in the foreshock region.

Interplanetary magnetic field control of the Mars bow shock: Evidence for Venuslike interaction

The Mars bow shock location and shape have been determined by examining the PHOBOS spacecraft magnetometer data. Observations show that the position of the terminator bow shock varies with interplanetary magnetic field orientation in the same way as at Venus. The shock is farthest from Mars in the direction of the interplanetary electric field, consistent with the idea that mass loading plays an important role in the solar wind interaction with Mars. The authors also find that the shock cross section at the terminator plane is asymmetric and is controlled by the interplanetary magnetic field as expected from the asymmetric propagation velocity of the fast magnetosonic wave. Comparing with earlier mission data, they show that the Mars shock location varies with solar activity. The shock is farther from Mars during solar maximum. Thus the solar wind interaction with Mars appears to be Venuslike, with a magnetic moment too small to affect significantly the solar wind interaction.

Behavior of current sheets at directional magnetic discontinuities in the solar wind at 0.72 AU

[1] Venus Express interplanetary magnetic field measurements have been examined for magnetic ‘‘holes,’’ accompanied by magnetic field directional changes. We examine both the thickness of the current sheet and the depth of the magnetic field depression. We find the thickness of the current sheet is not correlated with the depth of the field depression. The depth of the magnetic holes is related to directional angle change. Since total pressure should balance across these discontinuities, there must be enhanced plasma pressure within the magnetic holes. The dependence of the depth of the hole (i.e., size of the plasma pressure enhancement) on the directional changes suggests that the heating of the plasma associated with the hole formation may be provided by annihilation of the magnetic energy in the current sheet, via slow reconnection. Citation: Zhang, T. L., et al. (2008), Behavior of current sheets at directional magnetic discontinuities in the solar wind at 0.72 AU, Geophys. Res. Lett., 35, L24102, doi:10.1029/2008GL036120.

Venus Express observations of an atypically distant bow shock during the passage of an interplanetary coronal mass ejection

[1] On 10-11 September 2006 the Venus Express magnetometer detected a very strong Interplanetary Coronal Mass Ejection (ICME) event with an average field about 2 times higher than that of a typical ICME at 0.72 AU. While the effective obstacle to the solar wind is compressed to a smaller dimension during this ICME event, the bow shock is located far upstream of its nominal location. The observed shocks are weak and appear very dynamic. The location of the shock crossing can be found all along the Venus Express trajectory, which has an apocenter of 12 R v . We attribute the atypical distant bow shock location as an effect of the extremely low Mach number during the ICME.