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Solar system bodies and their plasma environments

Ganymede's Magnetosphere

The planets, satellites, comets and asteroids within our solar system are embedded in a moving magnetized plasma environment. The plasmas are in direct contact with the atmospheres or the surfaces of the solar system bodies. How the plasma flows and interacts with the planets and moons depends on the nature of the bodies, especially the presence of an atmosphere, a conductive subsurface layer or a magnetic field. Among other problems our research group is interested in is the understanding of the plasma interaction of the Galilean moons which are surrounded by Jupiter's dense magnetospheric plasma and 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. Collisions between the atmospheric and the charged magnetospheric particles modify the plasma density, momentum and the magnetic field around the moons. An other 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. In our group we work on a better understanding of the flows of plasma around planetary bodies and use this understanding to also derive properties of the planetary bodies.

The search for liquid water

Sketch of the interior of Jupiter’s moon Ganymede (Image Credit: NASA and STScI)

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. However, 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.

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

Turbulence is a basic property of the flow of liquids, gases and plasmas. In our group we particularly study turbulence in the magnetospheres of the outer planets. The turbulence is different compared to the turbulent plasma flow in the solar wind. The magnetospheric turbulence occurs within a strong background magnetic field, which acts as an ordering field for the evolution of the turbulence and often causes wave-dominated turbulence. In our group, we started investigating these type of turbulence in the outer planets and showed that the dissipation of the turbulence can be responsible for heating the magnetosphere to very large temperatures. We also demonstrated that the turbulence can play a key role in powering Jupiter's and Saturn's aurora.

Searching UV aurora and characterizing the space environment of Brown Dwarfs and Exoplanets

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.

Brown dwarfs are objects within the mass range of very massive gaseous planets and low mass stars. They usually possess very strong magnetic fields and are fast rotators. They are in this respect "super".  In our group we investigate the plasma, magnetic fields and auroral properties of these objects. We demonstrated that brown dwarfs due to their fast rotation and extremely strong magnetic fields, are ideal objects to search and characterize aurora outside the solar system.

Planetary Atmospheres

Artist impression of water vapor plume on Jupiter’s moon Europa (Image Credit: SwRI, Kurt Retherford)

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.