Comparative investigation of the terrestrial and Venusian magnetopause: Kinetic modeling and experimental observations by Cluster and Venus Express

https://doi.org/10.1016/j.pss.2010.04.019Get rights and content

Abstract

In June 2006 Venus Express crossed several times the outer boundary of Venus induced magnetosphere, its magnetosheath and its bow shock. During the same interval the Cluster spacecraft surveyed the dawn flank of the terrestrial magnetosphere, intersected the Earth's magnetopause and spent long time intervals in the magnetosheath. This configuration offers the opportunity to perform a joint investigation of the interface between Venus and Earth's outer plasma layers and the shocked solar wind. We discuss the kinetic structure of the magnetopause of both planets, its global characteristics and the effects on the interaction between the planetary plasma and the solar wind. A Vlasov equilibrium model is constructed for both planetary magnetopauses and provides good estimates of the magnetic field profile across the interface. The model is also in agreement with plasma data and evidence the role of planetary and solar wind ions on the spatial scale of the equilibrium magnetopause of the two planets. The main characteristics of the two magnetopauses are discussed and compared.

Introduction

The existence of a magnetopause downstream the Venusian bow shock at the interface with the shocked solar wind required solid scientific proof since Venus does not have an intrinsic magnetic field and thus it does not have a terrestrial-like magnetosphere. Nevertheless, laboratory experiments by Podgorny et al. (1980) and in situ data from Venus and Mars showed that even the planets with no intrinsic magnetic field are surrounded by an induced magnetosphere (Whang and Gringauz, 1982, Russell, 2001). The concept has been defined more precisely by Luhmann et al. (2004) and updated by Zhang et al. (2008) as “the regions near the planet and its wake in which magnetic pressure dominates the other pressure contributions”. First observations of the Venusian induced magnetosphere were reported by Russell et al. (1979) and Zhang et al. (1991) from Pioneer Venus Orbiter data.

The terrestrial magnetopause is a key-region for the transfer of energy, mass and momentum from the solar wind to the magnetosphere. It has been studied more extensively than any other magnetopause of the solar system. The in situ discovery of the terrestrial magnetopause dates back to 1963 when Cahill and Amazeen (1963) reported magnetic field observations from Explorer 12. Since then the structure and substructure of the terrestrial magnetopause, its dynamics and stability were investigated by a large number of satellites. The four Cluster spacecraft (Escoubet et al., 1997) provide for the first time a multi-point, three-dimensional, in situ probing of the magnetospheric plasma interfaces. New methods for data analysis developed for multi-point measurements, like, for instance, the discontinuity analyzer (Dunlop and Woodward, 1999), or the least squares gradient analyzer (De Keyser et al., 2007, Hamrin et al., 2008) provide powerful tools to improve our understanding of the physical processes at the terrestrial magnetopause. A review of Cluster results at the magnetopause has been published by De Keyser et al. (2005).

In June 2006 Venus Express (Svedhem et al., 2007) crossed several times the interface between the induced magnetosphere of Venus and its magnetosheath. The magnetic field observations by the fluxgate magnetometer MAG-VEX (Zhang et al., 2006) have been analyzed by Zhang et al. (2008). Hybrid simulations have been carried out for one entire orbit by Kallio et al. (2008). The moments of the electron and ion velocity distribution functions (density, bulk velocity, temperature) are provided by the plasma instrument ASPERA-4 (Barabash et al., 2007). Plasma data reveal the composition of the magnetosheath and give insight about the dynamics of the bow shock and plasma interfaces in the Venus plasma environment (Martinecz et al., 2008). During the same time period, the Cluster spacecraft crossed the terrestrial magnetopause in the dawn flank of the magnetosphere. Particle data from the Cluster ion spectrometer (Rème et al., 2001) and magnetic field data from the FGM magnetometer (Balogh et al., 2001) give a multi-point description of the terrestrial magnetopause. Joint observations by Venus Express and Cluster during the same time intervals enable a comparative study between the solar wind interaction with the outer magnetospheric layers of the Earth and Venus.

In this paper we also compare the experimental results from Venus Express and Cluster with the results of kinetic models of plasma interfaces, adapted for the magnetopause of the two planets. The kinetic models are based on the Vlasov equilibrium solutions developed for 1D tangential discontinuities (Roth et al., 1996) and 2D directional discontinuities (Echim and Lemaire, 2005). They provide the self-consistent profile of transition between asymptotic states by solving the coupled Vlasov–Maxwell system of equations.

The paper is organized as follows: in Section 2 we discuss briefly the main characteristics of the kinetic model adapted for the plasma interface between the planetary magnetospheres of Venus and the Earth and the solar wind. In Section 3 we discuss the magnetopause observations by Venus Express and Cluster and then compare them with the results of the kinetic model. A summary, the conclusions and the perspectives to continue this study are included in the last section.

Section snippets

Kinetic models of plasma interfaces

A self-consistent description of a plasma interface can be achieved starting from first principles of kinetic theory. In the kinetic approach the solution of the Vlasov equation (Eq. 7 in Appendix A) specifies the velocity distribution function of electrons and positive ions and is coupled to the solution of Maxwell's equations for the electromagnetic field (Eqs. (8), (9) in Appendix A). The solution of this system of coupled equations is also called a “Vlasov equilibrium”. This type of

Comparative observations and numerical simulations of the magnetopause of Venus and Earth

In this section we discuss the experimental observations of the Venusian and terrestrial magnetopause and compare these data with results of the kinetic model of TDs outlined in the previous section. The heliospheric positions of Venus and Earth on June 27, 2006, are illustrated by Fig. 2. Although the two planets were not connected by the same solar wind streamline, the comparison between their interface with the solar wind is of interest. Indeed, we can understand and compare the physical

Summary and conclusions

In this paper we discuss observations and modeling of the magnetopause of Venus and Earth, the interface between the outer planetary plasma layers and the shocked solar wind. We analyze in situ data from Venus Express and Cluster on June 27, 2006, during a period of magnetic quietness on Earth. Venus Express detects along the same orbit a magnetopause that has properties typical for a tangential discontinuity at the dawn magnetospheric flank and a rotational discontinuity at the dusk side.

Acknowledgments

We thank Iannis Dandouras, the PI of CIS instrument, and ESA's Cluster Active Archive for providing Cluster data. M. Echim wishes to thank Joseph Lemaire and Michel Roth for long term collaboration in kinetic modeling of plasma discontinuities and acknowledges support from the Belgian Solar-Terrestrial Center of Excellence in Brussels, and from ESA (PECS Contract 98049) and the Romanian Agency for Science (Parteneriate-D8, Contract 81-009). R.M. and J.D.K. are supported by a PRODEX/CLUSTER

References (41)

  • L.H. Brace et al.

    The dynamic behavior of the Venus ionosphere in response to solar wind interactions

    Journal of Geophysical Research

    (1980)
  • L.J. Cahill et al.

    The boundary of the geomagnetic field

    Journal of Geophysical Research

    (1963)
  • J. De Keyser et al.

    Least-squares gradient calculation from multi-point observations of scalar and vector fields: methodology and applications with Cluster in the plasmasphere

    Annales Geophysicae

    (2007)
  • J. De Keyser et al.

    Magnetopause and boundary layer

    Space Science Reviews

    (2005)
  • J. De Keyser et al.

    Equilibrium conditions for the tangential discontinuity magnetopause

    Journal of Geophysical Research

    (1997)
  • J. De Keyser et al.

    Equilibrium conditions and magnetic field rotation at the tangential discontinuity magnetopause

    Journal of Geophysical Research

    (1998)
  • M.W. Dunlop et al.

    Analysis of thick, non-planar boundaries using the discontinuity analyser

    Annales Geophysicae

    (1999)
  • Echim, M., 2004. Kinetic investigation of the impulsive penetration of 2D plasma elements into the Earth's...
  • M. Echim et al.

    Two-dimensional solution for a collisionless plasma jet across transverse magnetic field lines with a sheared bulk velocity

    Physical Review E

    (2005)
  • M. Echim et al.

    Sheared magnetospheric plasma flows and discrete auroral arcs: a quasi-static coupling model

    Annales Geophysicae

    (2007)
  • View full text