All planets in our solar system, as well as exoplanets (i.e. planets circling around other stars than our Sun), have interactions with the solar/stellar wind coming from their host star. The solar wind is comprised of plasma moving away from the Sun at velocities around 400 km/s and densities around a few per cubic cm around 1 Astronomical Unit (AU, distance Sun-Earth). Embedded and moving along with the plasma is the Sun’s magnetic field with a strength of several nT, also called the interplanetary magnetic field (IMF). These values can strongly vary when eruptions on the Sun, so-called solar flares and coronal mass ejections (CMEs) occur.  

Naturally, for each planet the interaction with the solar wind and the IMF is different, caused e.g. by the presence of a planet-internally generated magnetic field and/or by the distance of the planet from the Sun. However, the regions around the planet are basically similar for all planets: the super-sonic solar wind gets braked at the bow shock to sub-sonic speeds. The slowed down magnetoplasma flows in the so-called magnetosheath, where it interacts with the (magnetised) planet. It forms a boundary at the location where the magnetic pressure becomes equal to the ram pressure of the solar wind. This boundary is called the magnetopause/ionopause/pile-up boundary, depending on the magnetisation of the planet. In the region between the bow shock and the magnetopause, the magnetosheath, many plasma phenomena take place, see for example Song and Russell (1997).

In this project, the magnetosheaths of two terrestrial planets, Venus and Mars, will be investigated and compared from the point of view of ultra-low frequency (ULF) waves, especially mirror modes.

Mirror modes are large magnetic bottles imprisoning relatively dense plasma, with a characteristic anti-correlation between magnetic field intensity and ion/electron density. They arise from an imbalance in the ion and electron temperatures, called anisotropy, which is at the heart of an instability that favourably grows in the plasma, producing a collection of wave modes. These wave modes include the Alfvén ion cyclotron mode and the mirror mode, which are often seen in the magnetosheath of any planet (Gary 1993).

The plasma is unstable to the generation of these modes when (Hasegawa, 1969):

1+β⊥(1−T⊥T||)<0

With β is the plasma beta, that is, the ratio of the thermal plasma pressure to the total magnetic pressure, and with ⊥ and || denoting the directions perpendicular and parallel to the ambient magnetic field direction. For high plasma β, the mirror mode dominates: this is typically what happens in the wake of a quasi-perpendicular shock crossing (that is, when the solar wind impinges on the shock in such a way that the IMF direction is perpendicular to the normal to the shock’s surface). Other ways of creating the anisotropy at the origin of these modes, include pickup ion effects.

References

  • Gary, S. P., Fuselier, S. A. and Anderson, B. J. (1993), Ion anisotropy instabilities in the magnetosheath, J. Geophys. Res.,98, 1481–1488.
  • Hasegawa, A. (1969), Drift mirror instability in the magnetosphere, Phys. Fluids, 12, 2642–2650.
  • Song, P. and Russell, C. T. (1997), What do we really know about the magnetosheath?, Avd. Space Res.,20, 474–765.