Small-scale magnetic reconnection from an anisotropic turbulent cascade

Small-scale magnetic reconnection from an anisotropic turbulent cascade

The solar wind is an expanding plasma that is formed in the solar corona and spreads across the solar system. As the plasma travels away from the Sun, its electromagnetic and kinetic power spectra exhibit fluctuations over a wide range of temporal and spatial scales. The energy stored in the larger fluctuations cascades to smaller fluctuations. At the end of this cascade, particle heating occurs. It is not clear which mechanisms are responsible for the ultimate heating of the particles. The reason for this lack of knowledge is the general possibility for multiple phenomena to exchange energy between particles and electromagnetic fields depending on the plasma conditions and the nature of the fluctuations. In this context, the outstanding question is whether and how the plasma fluctuations reach the smallest scales to fulfil the conditions for dissipation.

Magnetic reconnection is a fundamental plasma process that modifies the topology of the magnetic field at different scales and increases the kinetic and thermal energy of the particles. The level of turbulence in a plasma before and during magnetic reconnection affects the effectiveness of this process as an energy exchange mechanism. In addition, magnetic reconnection (like the turbulent cascade) transports energy to small scales. Therefore, we ask the science question: “How does magnetic reconnection relate to small-scale plasma phenomena and the turbulent cascade?”

Volume rendering of the electric current density J in one of our DiRAC simulations of anisotropic Alfvénic turbulence. The turbulence forms elongated magnetic structures, which are called “magnetic flux ropes”. The flux ropes are unstable, and magnetic reconnection results from their non-linear interaction. Particles are accelerated during the reconnection process. Artificial spacecraft trajectories through the simulation domain can be compared with in-situ observations in the solar wind. 

To address this problem, the use of peta-scale simulations has proven to be indispensable. The computational power provided by the high-performance computing facilities at DiRAC enables us to perform rigorous studies that resolve the small-scale dynamics, also known as kinetic scales, while preserving the history of the large-scale cascade. In particular, particle-in-cell (PIC) simulations solve the evolution of the plasma based on first principles. This type of simulation accounts for phenomena that only reveal themselves in kinetic theory. With DiRAC’s capabilities, we have come a big step forward to understanding turbulence and magnetic reconnection in plasmas.

We initialise our turbulence 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. This interaction forms, after some time, magnetic and electric current structures. Some of the magnetic structures are so-called “flux ropes” which undergo magnetic reconnection and accelerate particles.  We define a set of criteria to identify where in the simulated plasma reconnection occurs. With these criteria, we find several reconnection sites in our simulation domain. We select one extensive reconnection site and study its properties in great detail. We find that this event is complex and asymmetric unlike the idealised Harris current-sheet system often used to study magnetic reconnection.  One great 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 the solar wind. The measurements from our artificial spacecraft serve as predictions for measurements of reconnection in the solar wind by the latest solar-system spacecraft Solar Orbiter and Parker Solar Probe which have the appropriate instruments onboard to test our predictions.