Low-frequency magnetic field fluctuations in Venus' solar wind interaction region: Venus Express observations

L Guicking,M Delva,K.-H Glassmeier,T L Zhang,U Motschmann,Y Narita,H.-U Auster

doi:10.5194/angeo-28-951-2010

Abstract

Abstract. We investigate wave properties of low-frequency magnetic field fluctuations in Venus' solar wind interaction region based on the measurements made on board the Venus Express spacecraft. The orbit geometry is very suitable to investigate the fluctuations in Venus' low-altitude magnetosheath and mid-magnetotail and provides an opportunity for a comparative study of low-frequency waves at Venus and Mars. The spatial distributions of the wave properties, in particular in the dayside and nightside magnetosheath as well as in the tail and mantle region, are similar to observations at Mars. As both planets do not have a global magnetic field, the interaction process of the solar wind with both planets is similar and leads to similar instabilities and wave structures. We focus on the spatial distribution of the wave intensity of the fluctuating magnetic field and detect an enhancement of the intensity in the dayside magnetosheath and a strong decrease towards the terminator. For a detailed investigation of the intensity distribution we adopt an analytical streamline model to describe the plasma flow around Venus. This allows displaying the evolution of the intensity along different streamlines. It is assumed that the waves are generated in the vicinity of the bow shock and are convected downstream with the turbulent magnetosheath flow. However, neither the different Mach numbers upstream and downstream of the bow shock, nor the variation of the cross sectional area and the flow velocity along the streamlines play probably an important role in order to explain the observed concentration of wave intensity in the dayside magnetosheath and the decay towards the nightside magnetosheath. But, the concept of freely evolving or decaying turbulence is in good qualitative agreement with the observations, as we observe a power law decay of the intensity along the streamlines. The observations support the assumption of wave convection through the magnetosheath, but reveal at the same time that wave sources may not only exist at the bow shock, but also in the magnetosheath.

Highlights

Waves in plasmas, which are generally considered as fluctuations in the electric field, the magnetic field, the density, and the temperature, play an important role in the interaction processes of the solar wind with planets and other solar system bodies
We investigate the spatial distribution of the intensity in more detail and discuss possible mechanisms and processes which may lead to the made observations
We performed a statistical analysis of low-frequency magnetic field fluctuations in the frequency range 30 to 300 mHz in the Venusian solar wind interaction region

Summary

Introduction

Waves in plasmas, which are generally considered as fluctuations in the electric field, the magnetic field, the density, and the temperature, play an important role in the interaction processes of the solar wind with planets and other solar system bodies. It is important to study wave characteristics, their origins, and generation mechanisms in order to improve our understanding of the complex interaction processes. It is interesting to study the plasma environment. L. Guicking et al.: Statistical analysis of wave properties at Venus of Venus, because it does not possess an intrinsic magnetic field and the solar wind interaction is similar to that at Mars A statistical study of low-frequency magnetic field oscillations in the Martian plasma environment is presented by Espley et al (2004)

Results

Discussion

Conclusion