Dusty Plasmas in Space

The presence of a significant component of ionized dust in a plasma can strongly influence its properties, and drive it to behave in a very different manner to a ‘pure’ ion-electron plasma. Dust grains can range in size over multiple orders of magnitude, from diameters of a few nm up to macroscopic sizes.

These grains, when exposed to sunlight, or embedded in a pre-existing plasma, will accumulate their own charge (positive or negative, depending on the surrounding environment). The extremely high mass of these grains, relative to an isolated ion, leads them to respond much more slowly to applied electric and magnetic fields. If the amount of charged dust present is sufficiently large, it can even completely dominate the behaviour of the whole plasma.

Dusty plasma in space covers many different research topics that have expanded interest in recent years. At IRF-Uppsala, we study dusty plasmas in both the Saturnian system, and at Comet 67P/Churyumov–Gerasimenko.

Dusty plasmas at Enceladus

Enceladus is situated in the dense part of Saturn's E-ring, and its south polar plume is the primary source of water gas and ice grains (dust). The dust is possibly an important part of the collective plasma and therefore capable of influencing the behavior of the plasma as a whole.

Using multiple instruments on the Cassini spacecraft, we can accurately measure and compare the densities of ions, electrons, and larger dust grains. These measurements reveal the structure of the plasma regions around the small moon and how they interact with and feed Saturn's vast magnetosphere and E-ring. Notably, the Langmuir probe data from five close flybys of Enceladus, during which Cassini crossed the plume over the south pole at an altitude of ~100 km, have been re-analyzed, revealing fascinating details about the plasma environment.

The data indicate that Enceladus' plume contains an electronegative plasma, characterized by a significant presence of negative ions and a depletion of free electrons. This plasma region coincides with areas where negatively charged nanograins have been detected by the Cassini Plasma Spectrometer (CAPS/ELS). In this region, the observed current is dominated by ions rather than free electrons, and some measurements suggest the presence of doubly charged molecules. The electronegative nature of the plasma is particularly pronounced near the "tiger stripes," where the water vapor jets and ice grains originate.

The interaction between Enceladus' plume and Saturn's magnetosphere is a key aspect to understand the dynamics of dust, ionization processes, and particle coagulation in such environments. The evolution of dust grain sizes at different altitudes within the plume suggests that particle coagulation, influenced by mutual electrostatic attraction, plays a significant role in the plume's development. These findings enhance our understanding of the unique plasma environment around Enceladus and its broader implications for planetary science and plasma physics.

Dusty plasmas at Titan

Titan's cold, dense atmosphere and high-altitude ionosphere provide a unique laboratory for studying dust-plasma interactions. The interactions within Titan's ionosphere play a crucial role in the formation of organic-rich aerosols and pre-biotic compounds, which have significant implications for the study of astrobiology and the origins of life on Earth.

The magnetotail of Titan, formed as a result of the interaction between Saturn's magnetospheric flow and Titan's ionosphere, is a key area of interest. Titan's ionosphere is primarily created by EUV radiation and the impact of magnetospheric particles on the atmosphere, while the magnetotail is shaped by plasma outflow processes, the properties of the upstream magnetospheric flow, and the upstream magnetic field direction. Previous studies have shown the existence of a highly dynamic tail structure influenced by various factors.

Recent research involving Cassini's multiple flybys of Titan, particularly during the T122-T126 flybys, has provided detailed insights into the magnetotail's structure and behavior. Measurements from the Langmuir probe (RPWS/LP) and the Cassini Plasma Spectrometer (CAPS) revealed spatial constraints on the tail's geometry and its orientation based on electron and ion density distributions. These studies also revisited the estimation of escape rates and explored the variability sources impacting the magnetotail structure.

Researching Titan's environment also has broader implications for other space environments. The research into Titan's dusty plasma interactions contributes to our knowledge of similar processes in protoplanetary accretion disks around newborn stars, dusty exospheres around moons like Ganymede and Europa, and the broader study of planetary space physics, which is crucial for understanding exoplanets and the potential for life beyond Earth.

Dusty plasmas at Comet 67P/Churyumov–Gerasimenko

With the recent arrival of ESA's flagship Rosetta mission to the comet 67P, we hope to use our understanding developed chiefly in the Saturnian system to inform our studies of the dusty plasma environment of the comet. Once again, IRF-Uppsala built and operated hardware is central to this, through our involvement in the Langmuir Probe, as part of the Rosetta Plasma Consortium. As the comet approaches perihelion, and its activity increases, we expect the amount of dusty material ejected from its surface to increase. We aim to study how this process develops through the mission, and its effect on the cometary plasma environment and solar wind interaction.

IRF-Uppsala's involvement in the Langmuir Probe, as part of the Rosetta Plasma Consortium, is central to this study. As the comet approached perihelion and its activity increased, the amount of dusty material ejected from its surface also rose, significantly altering the plasma environment. This increase in activity allowed us to observe and analyze how the dusty plasma evolved over time and how it interacted with the solar wind.

One of the key findings from the Rosetta mission was the discovery of a diamagnetic cavity near the comet, where the magnetic field strength dropped to zero. This phenomenon, similar to what was observed at comet 1P/Halley in 1986, was detected close to perihelion in July 2015. The cavity observed at comet 67P was larger than previously predicted by models and exhibited unusual magnetic field configurations. The presence of this diamagnetic cavity suggests complex interactions between the outgassing cometary material and the solar wind, including possible instabilities along the cavity boundary and a low magnetic pressure in the surrounding solar wind. These observations help characterize the plasma within different regions surrounding the comet and how large-scale structures such as the diamagnetic cavity depend on factors like outgassing rate and solar wind conditions.

Within IRF-Uppsala

• Jan-Erik Wahlund is PI on the Cassini Langmuir Probe
• Anders Eriksson is PI on the Langmuir probe on Rosetta