ULF Waves in Venus´ and Mars´ Magnetosheath

Plasma physics concerns the behaviour of ionized 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 this specific case we turn to Venus and Mars, who are the sister and brother of the Earth. Venus is slightly smaller than the Earth, whereas Mars is about half the size of the Earth. Both planets, however, do not have an internal magnetic field. As the solar wind approaches an unmagnetized planet, freshly created ions from the atmosphere will be picked up by the solar wind, thereby slowing it down. At a certain point this results in the creation of a bow shock, where the solar wind is decelerated from super- to subsonic speeds. Behind the bow shock is the so called magnetosheath, where the magnetic field is draped like spaghetti 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? And if not, what other reason can be found. Similarly, the characteristics of other waves will be investigated as well as turbulence. Comparison between Venus and Mars will teach us about the effects of different solar wind conditions, with Mars twice as far from the Sun, and different planetary size on the wave activity in the magnetosheath.

The interaction between the solar wind, a stream of charge particles and magnetic field (plasma) emitted by the Sun, and a planet creates a bow shock. At this location the supersonic solar wind is braked, where both the density and the temperature of the particles increases. Behind the bow shock instabilities can occur, one of which is the mirror mode instability. This one occurs when the temperature perpendicular to the magnetic field is higher than that along the magnetic field. This creates "waves" in the magnetic field, in which the magnetic field strength and the particle density variations are in anti-phase. These waves are transported with the plasma flow through the so-called magnetosheath. Using the instruments on the spacecraft Venus Express and MAVEN, these waves could be measured at Venus and Mars. First of all, the occurrence rate of these waves was determined, depending on the location of the spacecraft in the magnetosheath. Just behind the bow shock and close to the planet an increase in the occurrence rate was found. This increase is caused by the strong temperature increase when the particles cross the bow shock, and when the particle density is compressed close to the planet. Interestingly, the lowest occurrence rate is there where the instability criterion is still fulfilled, which means that indeed the creation of these waves are working to equalize the temperatures perpendicular and along the magnetic field. Athough Venus is much larger than Mars, the distribution of the occurrence rate over the magnetosheath is quite the same. However, for Venus the location of the bow shock is dependent on the solar activity, which is not the case for Mars.