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Solar system bodies and their plasma environments

The planets, satellites, comets and asteroids within our solar system are embedded in a moving magnetized plasma environment. The plasma is in direct contact with the atmospheres and the surfaces of the solar system bodies causing a complex plasma interaction. The interaction is dependent on the nature of the body, especially the presence of an atmosphere, a conductive subsurface layer and a magnetic field. Among other problems our research group is particularly interested in the understanding of the plasma interaction of the Galilean moons that are surrounded by Jupiter's strong magnetic field. The interaction between Jupiter's magnetospheric plasma and its moons is mostly sub-Alfvénic, i.e., the magnetic field energy density dominates the bulk flow energy and the thermal energy density. The flow of magnetized plasma past a moon's tenuous atmosphere creates various interesting features in the local environment and also far away from the moon. Due to collisions between the atmospheric and the charged magnetospheric particles and mass loading processes a strong electrodynamic interaction is produced which modifies the plasma density, momentum and the magnetic field. An important feature of this interaction is the propagation of Alfvén waves and the associated development of Alfvén wings far away from the moon. The moon’s physical properties control the plasma interaction, which we describe with appropriate models.

The Search for Liquid Water

Liquid water is generally considered an essential building block for the evolution and maintances of life at least as we know it. Environments which can possibly maintain life are called habitable. The large icy moon of the outer solar system are long known to possess outer layers of frozen water. Under or within these icy crusts, layers of liquid water can possibly be maintained by internal energy sources.

In our group we gather observational evidence and work on characteriziation of the possible liquid oceans in the outer solar system. Evidence for the ocean comes from particular magnetic field signatues around the moons. Oceans, which are saline, i.e. salty, like the Earth’s oceans, generate induced magnetic fields in response to Jupiter’s time variable magnetic field. These magnetic field can be measured insitu by magnetometers on board of spacecraft pasing the moons or by its effects on the auroral structures of the moons.

Our studies to search and characeterize subsurface oceans are currently being funded through an ERC-Advanced grant (EXO-OCEANS).

We monitored Ganymede’s auroral oval with Hubble Space Telescope observations and found based on the movements of the aurora that Ganymede possesses a subsurface ocean. For more details see here or here.

We discovered water vapor plumes on Europa using the Hubble Space Telescope. Water vapor plumes might originate from the subsurface ocean and thus would provide an easy way to probe the content of the water. For more details see here and here.

We characterized Europa’s ocean by numerical modeling of magnetic field measurements (see here).

We were involved in the discovery of the plumes on Saturn’s moon Encleadus (see here).

Low Frequency Turbulence in the Solar Wind

The study of turbulence in the solar wind and other space plasmas (e.g. magnetospherical plasmas) has become the subject of active research in the last few decades. Thanks to in-situ data from various spacecraft (e.g. Helios, Wind, ACE, Ulysses, THEMIS etc.), thorough investigations of different turbulent properties have become feasible. Interestingly, solar wind turbulence consists of a large range of fluctuation length scales which correspond to frequencies ranging from 10-5 to 102 Hz. Low frequency turbulence in the solar wind is often modelized in the framework of magnetohydrodynamics where the plasma is considered as a fluid of electrons and ions. For the sake of simplicity, very often we neglect the compressibility of the solar wind. Incompressible models are usually reasonable as the density fluctuations in the free solar wind are as small as 10-15. Nevertheless, recent researches show that the compressible effects can be important in understanding the nature of energy cascade and the universality in solar wind turbulence. I derived an exact scaling law for isothermal MHD turbulence which I try to verify for low frequency fluctuations in the fast solar wind using THEMIS spacecraft data.

Exoplanets and Brown Dwarfs

Extrasolar systems provide a unique opportunity to explore the richness of planetary systems beyond the small number of the eight planets in the solar system. In our group, we are particulary interested in Hot Jupiters and their plasma environments. Hot Jupiters are gas giant exoplanets orbiting very close to their host star (< 0.1 AU). They are expexted to be surrounded by a flow of dense and hot magnetized plasma originating from the central star. The planets are obstacles to the magnetized flow, which leads to a generation of various wave modes. The Alfvén wave is of special interest, because it can transport momentum and energy along the local background magnetic field without much dissipation over large distances. Exoplanets, which orbit its host star at close distances, can be surrounded by a sub-Alfvénic stellar wind and so establish a magnetic coupling between themselves and the star. The extreme ultraviolet (EUV) radiation, which the exoplanet receives from its star, leads to heating of the atmospheres by photoionization, reaching temperatures of 104 K and more. This forces the atmosphere to a steady expansion and thus forms a planetary wind leading to high atmospheric mass losses. The interaction of the stellar wind and the huge planetary atmosphere leads to the production of energetic neutral particles and a planetary tail similar to that of a comet.

Brown Dwarfs are objects in the mass range between very massive planets and low mass stars, i.e., they lie in the mass range of roughly 14 to 80 Jovian masses. We are observing such objects with the Hubble Space Telescope to detect and characterize auroral emission.

Planetary Atmospheres

Most planets and some moons of the Solar System have some kind of an atmosphere. However, there is a large diversity among the planetary atmospheres with respect to chemical composition, atmospheric density or aeronomy/meteorology. Our group is particularly interested in the atmospheres of the moons of the outer planets.

One main focus of our research is Saturn’s moon Titan, which has the densest atmosphere among all moons in the Solar System. Titan’s neutral atmosphere has many analogies with the terrestrial atmosphere and is subdivided into troposphere, stratosphere, mesosphere and thermosphere. We address scientific questions related to meteorology and climatology of the present era as well as of past eras, in particular interactions with the surface that contains liquid hydrocarbon lakes, dunes and mountains. These topics are mainly investigated by 3-dimensional global climate models. Aside from the atmosphere itself, the oceanography and thermodynamics of hydrocarbon seas/lakes are also investigated since their influence on the climate can be substantial under certain circumstances.

Another main focus of our research are the atmospheres of the Galilean moons of Jupiter Ganymede, Callisto and Europa. These atmospheres are tenuous and cannot be described by the law of fluid dynamics. Furthermore, they are characterized by complex interactions with the plasma environment of the host planet as well as the solar wind, but also by interactions with moon’s surface and interior. Topics of our research include aurora of Ganymede and Jupiter, neutral atmosphere of Callisto or water vapor plumes on Europa.