Space Physics, Planetary Science, and Moon-Magnetosphere Interaction

I am a PhD student in the Department of Climate and Space Sciences and Engineering at the University of Michigan, where I study Jupiter’s moon Europa with my advisors Xianzhe Jia and James A. Slavin. Interaction between Jupiter’s magnetospheric plasma and Europa’s ionospheric plasma generates electric current systems and magnetic fields near the moon. With multi-fluid magnetohydrodynamics (MHD) we can represent the most important plasma populations in the interaction and better understand the role they play in producing these magnetic signatures. The goal for my dissertation research is to develop a comprehensive model for Europa’s variable plasma interaction which can be used to study Europa’s space environment.

Europa: a small moon in a big magnetosphere

Jupiter orbits the Sun from 490 million miles away, 5 times the distance between the Sun and the Earth, and completes an orbit once every 12 Earth-years. Just as the planets orbit the Sun, a host of moons orbits Jupiter, the largest of which are the Galilean moons Io, Europa, Ganymede, and Callisto. Europa orbits Jupiter at a distance of 419 thousand miles, or 9.4 Jupiter Radii (RJ).

Jupiter is the biggest and the strongest planet in the solar system, especially with respect to plasma and magnetic fields. Jupiter produces a magnetic moment 20,000 times stronger than the Earth’s geomagnetic moment. The influence of this magnetic field extends outward from Jupiter, enveloping the planet in a roughly tear-drop shaped region called the magnetosphere. The shape of the magnetosphere is governed by the pressure balance between the solar wind and Jupiter’s magnetic field and plasma. Jupiter’s magnetosphere can be 100s of RJ wide and can extend 7,000 RJ long, enough to stretch out to the orbit of Saturn.

The Galilean moons orbit Jupiter within the magnetosphere. Io, the innermost moon orbiting at 5.9 RJ, is pock-marked with volcanoes that release 1 ton of material into the magnetosphere every second. Europa, orbiting at 9.4 RJ, is bathed in the plasma generated from this volcanic ejecta. Europa’s icy surface is scarred by whorls and linear features, as well as regions of more and less reflective ice. Plumes of liquid water may intermittedly erupt from Europa’s surface as suggested by analysis of remote observations by the Hubble Space Telescope and most recently the Keck Observatory, as well as older datasets from the Galileo mission.

Europa likely possesses a subsurface global ocean beneath its outer layer of ice. Because the ocean has not been directly observed the thickness of the icy shell and the depth and salinity of the ocean are not yet known. Evidence of the ocean is derived from observations of the magnetic fields near Europa by the Galileo Mission. To understand these magnetic signatures of the ocean, however, requires modeling of Europa’s local electromagnetic fields.

Modeling Europa’s space environment

Space plasmas are composed of electrically charged particles suspended in the magnetic fields that fill the solar system. At Europa these charged particles are typically electrons and ions of oxygen and sulfur, with other species present in much smaller quantities. In addition to the energetic plasma from Jupiter’s magnetosphere, cooler plasma is generated at Europa from Europa’s atmosphere. Magnetospheric plasma washes over Europa to swirl and break against this cooler plasma. The interaction between these different plasma populations generates electrical currents which perturb the local magnetic fields. The aim of my dissertation research is to develop an adaptable computational model to represent this complex plasma interaction and all its variability in response to the changing conditions of the magnetosphere as well as variations in Europa’s atmosphere.

Many space plasmas are effectively modeled by approximating this collection of discrete charged particles as a continuous charged fluid. This mathematical approximation is known as magnetohydrodynamics (MHD). MHD modeling of space plasmas is an area of rich, active research spanning many research groups across the world. At the University of Michigan, the Center for Space Environment Modeling develops the MHD code BATS-R-US as a component of the Space Weather Modeling Framework. With the advanced computing capabilities provided by modern supercomputers, BATS-R-US has been used to effectively model large, complex space plasma systems such as the magnetospheres of Jupiter and Saturn, as well as the interaction between comets and the solar wind. We have used BATS-R-US to develop our model for Europa’s plasma interaction.

The predecessor to our model for Europa’s plasma interaction has already helped to find evidence for water plumes at Europa. The new model that I am developing incorporates general improvements to the BATS-R-US code and focuses on discerning the roles of different plasma populations in the interaction.

What’s Next?

This model will help determine the properties of Europa’s subsurface ocean. When the Galileo spacecraft observed the magnetic fields near Europa it observed an induced magnetic field generated by electrical currents in Europa’s conducting ocean. The properties of the ocean which produce this induced field can be determined by characterizing that magnetic field more precisely. However, the induced magnetic field was measured in the presence of the interfering magnetic signatures of the plasma interaction. Because the plasma interaction magnetic fields are variable, untangling them from the induced field is a complicated task.

NASA’s upcoming Europa Clipper Mission seeks to use Europa’s induced field to characterize the subsurface ocean. By developing a sophisticated MHD model for the plasma interaction we will be able to predict how the plasma interaction magnetic fields will interfere with the induced magnetic fields, enabling the induced field to be identified in Europa Clipper’s magnetometer data. With high-quality measurements of the induced field scientists will be able to constrain the thickness, depth, and salinity of Europa’s subsurface ocean.

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