Identifying and Quantifying the Role of Magnetic Reconnection in Space Plasma Turbulence

Identifying and Quantifying the Role of Magnetic Reconnection in Space Plasma Turbulence

Plasma is the fourth state of matter after the solid, liquid, and gaseous states. Almost all of the visible matter in the Universe is in the plasma state. This includes all the stars, the material between the stars, the intergalactic medium, and also the gas in the Earth’s cosmic neighbourhood: the solar wind and the geospace environment in Earth’s magnetosphere. Very often, these plasmas have a low viscosity. Therefore, we expect that these plasmas are highly turbulent. The goal of our project is to understand the role of a process called “magnetic reconnection” in the omnipresent plasma turbulence in space.

Magnetic reconnection is a multiscale plasma phenomenon in which the magnetic field re-organises and transfers energy into the plasma particles. It occurs in laboratory plasmas, such as fusion or confinement experiments, and in astrophysical plasmas, such as the solar wind, coronal mass ejections, solar flares, explosive events in planetary magnetospheres, accretion discs, and star-formation regions. Although reconnection has been studied for over 60 years, there is still no consensus about a complete theory to describe magnetic reconnection at all scales involved.

Figure 1. Volume rendering of the electric current density J in one of our DiRAC simulations. The turbulence forms elongated structures, which are called “magnetic flux ropes”. The flux ropes are unstable and magnetic reconnection results from their non-linear interaction. Figure from Agudelo Rueda et al. (2020, in preparation).

Current theories and numerical simulations suggest that turbulence and magnetic reconnection are closely related. Turbulence transfers energy across the scales of the plasma. Energy is injected at large spatial scales and then transported to the very small scales where it dissipates as heat. This transfer is known as the turbulent energy cascade. It is proposed that the ultimate dissipation of energy at small scales strongly depends on a link between turbulence and reconnection.

We study this link between turbulence and reconnection with large computer simulations of plasma turbulence. The computational power provided by high-performance computing facilities at DiRAC enables us to study plasma processes from first principles using three-dimensional cutting-edge particle-in-cell (PIC) simulations. This type of simulation resolves the smallest plasma scales and accounts for phenomena that only reveal themselves in kinetic theory. Even with the enormous power of DiRAC, full PIC simulations are unable to cover the whole range of scales involved in plasma turbulence and reconnection in a real plasma, since these simulations are very expensive in terms of computing memory and time. With DiRAC’s capabilities, however, we have taken a big step forward to understanding turbulence and magnetic reconnection in plasmas.

Figure 2. Side view of our three-dimensional simulation box. The different colours represent various markers for magnetic reconnection. We use these markers to identify reconnection regions and to analyse the details of magnetic reconnection in plasma turbulence. Figure from Agudelo Rueda et al. (2020, in preparation).

We initialise our computations through the collision of counter-propagating plasma waves in our simulation box. These waves interact with each other and create turbulence, which is consistent with the observed turbulence in the solar wind. We find that, after some time, the turbulence itself generates current sheets and forms regions of magnetic reconnection. We define a set of criteria to identify where in the simulated plasma reconnection occurs. With these criteria, we find one extensive reconnection site in our simulation and study its properties in great detail.

One huge advantage of our simulations is that we can fly an artificial spacecraft through our simulation box. This spacecraft records data in the same way as a real spacecraft would record data in space plasmas like the solar wind. The measurements from our artificial spacecraft will serve as predictions for measurements of reconnection in the turbulent solar wind by ESA’s and NASA’s latest solar-system spacecraft Solar Orbiter and Parker Solar Probe which have the appropriate instruments onboard to test our predictions.