Plasma Turbulence
Turbulence
Turbulent fluids are characterized by rapidly fluctuating fluid parameters, such as pressure, flow speed, and density. Two examples of turbulence in fluids are atmospheric turbulence, which may cause bumpy airplane rides, and the water behind the stern of a moving boat.
Whirlpools of water. Image: Leonardo da Vinci, RL 12660, Windsor, Royal Library.
Turbulence in plasmas
Plasma is a state of matter where ions and electrons exist side-by-side in an ionized gas without forming atoms. Plasma is the most abundant state of ordinary matter in the universe, and it is often in a turbulent state. Examples of turbulent astrophysical plasmas include the interstellar medium and the atmospheres of stars. Closer to home, turbulence is prevalent in several regions within our geospace, such as the solar wind, ion foreshock region, magnetosheath, cusps, magnetotail, and ionosphere.
Turbulence acts to transfer energy from large-scale motion into small-scale motion and heat. In ordinary fluids, this energy dissipation is caused by viscosity. However, in collisionless plasmas, energy dissipation occurs through interactions between particles and waves in the plasma. Understanding turbulence is important because it influences particle heating and acceleration, such as solar energetic particles and cosmic rays. It also significantly impacts the structure and dynamics of the Earth's magnetosphere as well as energy and mass inflow to the magnetosphere.
A classic example of turbulence in an astrophysical object: the highly turbulent supernova remnant Crab nebula. Image: NASA, ESA, J. Hester, A. Loll (ASU) Acknowledgement: Davide De Martin (Sky Factory).
Turbulence acts to transfer energy from large-scale motion into small scale motion and heat. In ordinary fluids, this energy dissipation is caused by viscosity in the fluid. In collisionless plasmas, however, energy is dissipated by particles interacting with waves in the plasma. Understanding turbulence is important because it influences particle heating and acceleration, such as solar energetic particles and cosmic rays. It is also important because it heavily influences the structure and dynamics of the Earth's magnetosphere as well as energy and mass inflow to the magnetosphere.
Research
We study plasma turbulence in different regions in space in the solar wind and in the up- and down-stream regions formed at planetary bow shocks. We investigate the turbulence properties by in-situ measurements from single spacecraft missions like Ulysses and Venus Express, and multi-point missions like Cluster and THEMIS. We use the ionosphere for stimulus-response type of experiments to study the natural ionosphere as well as complex plasma turbulence, thus using the near-Earth space environment as the world’s largest laboratory. Powerful radio waves are transmitted into the ionosphere from the ground to excite a multitude of plasma phenomena, which are diagnosed primarily by radars, cameras and other ground-based instruments. The facilities used include that of EISCAT in northern Scandinavia, HAARP and PFISR in Alaska, USA. The latter two facilities have enabled the first experiments with twisted electromagnetic beams interaction with the plasma.
Our research on the dynamics of turbulence in different plasma environments have shown that the Earth's magnetosheath, a turbulent plasma region, exhibits different characteristics depending on the angle between the bow shock normal and the interplanetary magnetic field. In the quasi-parallel magnetosheath, there are strong fluctuations in the magnetic field, leading to complex wave-particle interactions, such as the generation of whistler waves by non-Maxwellian electron velocity distributions. These waves play a crucial role in particle dynamics and energy transfer within the magnetosheath.
Another important aspect of turbulence research involves understanding how turbulence is generated, evolves, and dissipates across different spatial and temporal scales. For instance, Mars' ionosphere presents a unique environment where interactions between the planet's ionospheric plasma and its structured crustal magnetic fields lead to the redistribution of plasma and affect radio wave propagation. These interactions result in phenomena such as plasma turbulence, electron heating, and enhanced plasma densities at lower altitudes, while higher altitudes experience different effects due to the magnetization of both ions and electrons.
Our research is focused on turbulence structure and mechanisms of generation, evolution and dissipation. Some science questions we have are:
– How is turbulence generated in the solar atmosphere, at which scales it is driven?
– How does turbulence evolve spatially and temporally throughout the heliosphere?
– How is energy transferred through different scales?
– How is energy dissipated, at which scale and by what mechanisms?
– What is the source of intermittency and related coherent structures?
– How is turbulence affected by anisotropy?
– How do bow shock parameters affect and modify upstream and downstream turbulence?
The colors represent the magnetic field components – BX (blue), BY (green), and BZ (red), with the Cluster spacecraft symbol for each estimation of the intermittency parameter P; α is the spectral slope. From a study of turbulence evolution downstream quasiparallel bowshock in the terrestrial magnetosheath. It is shown that: there is clear anisotropy of the turbulence with respect to the shock normal. The intermittency of the magnetic field components in the shock plane shows fast development from lower to higher values, while the component along the bow shock normal shows no clear evolution. Image: Yordanova, E. et al., PRL., 100, 205003, 2008.