Project: ULF Waves in Venus' and Mars' magnetosheath

Plasma physics concerns the behaviour of ionised gasses (plasmas, a mixture of positive ions and negative electrons and sometimes neutral atoms). In order to study the behaviour of plasma in the presence of a magnetic field, laboratory experiments and theoretical calculations are performed. However, experiments are limited in size, and not all processes that can take place in nature can be investigated. Therefore, we turn to laboratories that nature provides, and take to space, where the solar wind (a plasma with magnetic field, coming from the Sun) and its interaction with the Earth or other planets can be investigated with satellites.

In this project we are interested in Ultra Low Frequency (ULF) waves, with periods of 1 to 1000 seconds, which have wavelengths much longer than any laboratory can fit. Fortunately, the interaction region around the planets is large enough to study these waves. Of course, this region is very specific in character. In our case, we turn to Venus and Mars, who can be seen in some ways as the siblings of the Earth in terms of size and planetary evolution. Venus is slightly smaller than Earth, whereas Mars is about half its size. Both planets, however, do not have an internal magnetic field, although occasional pockets of weak magnetic fields are trapped within Mars’ surface.

As the solar wind approaches an unmagnetised planet, freshly created ions from the atmosphere will be “picked up” and accelerated by the solar wind electric field, resulting in a slowing down and deflection of the impinging solar wind. At a certain point in its course, a bow shock forms, where the solar wind is decelerated from super- to subsonic speeds: this shock, although collisionless in nature, can be seen as the kinetic plasma analogy of classic hydrodynamics shocks such as those arising from planes exceeding the speed of sound in the air with a characteristic “sonic boom”, or ships moving faster than the water’s surface wave speed, forming a “bow wave”. Behind the planetary bow shock is the magnetosheath, where the magnetic field is draped and stretched like spaghetti strands around the planet. This way, a so-called induced magnetosphere is created.

In the magnetosheath, many ULF waves occur, often created by the presence of the bow shock. One kind is the mirror mode wave, where regions of strong magnetic field/low plasma density and weak field/high density alternate, and move along with the plasma. It was found that these mirror mode waves behave differently for solar minimum and solar maximum conditions.

One of the purposes of this proposal is to find out why these waves behave differently. Is it because the bow shock is different for solar minimum and maximum? If not, what other reason can be found? Similarly, the characteristics of other waves are investigated as well as turbulence. Comparison between Venus and Mars teaches us about the effects of different solar wind conditions, with Mars twice as far from the Sun, and different planetary size and atmospheres on the wave activity in the magnetosheath.