Kink-unstable flux ropes in three-dimensional, multi-fluid, partially ionised plasma

Kink-unstable flux ropes in three-dimensional, multi-fluid, partially ionised plasma

Andrew Hillier, Ben Snow, Giulia Murtas

The solar atmosphere is a complex and chaotic plasma environment, rife with explosive phenomena

driven by the complex magnetic structure undergoing topological changes and releasing this

stored energy through a process known as magnetic reconnection. This process readily occurs in

the twisted (kinked) magnetic flux ropes that exist throughout the solar atmosphere, however the behaviour

of this process has not been well studied in the lower solar atmosphere where the medium is

partially ionised, i.e., consists of both ionised and neutral species. Partial ionisation can accelerate

the rate at which energy is released through magnetic reconnection and lead to additional heating.

In this project, we performed on the DiRAC COSMA7 system 3D multi-fluid, partially ionised simulations

of the kink-instability to understand how magnetic reconnection occurs in the lower solar

atmosphere. Numerical simulations are performed using the (PIP) code that evolves two-fluid (neutral,

ion+electron) equations using the non-dimensional form. The species are coupled through both

thermal collisions and ionisation/recombination. The energy required to ionise the system exists as

an energy sink in the plasma species and is balanced initially by a heating term

Figure 1: (left) snapshots of the three-dimensional structure of the magnetic field through time,
with selected cuts showing the drift velocity. (right) Detailed analysis of a single reconnection cite
embedded in this complex three dimensional structure showing the thermal imbalance, velocity drift,
and the current density.

Our initial conditions are a 3D force-free magnetic flux rope that is unstable to the kink instability.

As the system evolves, the magnetic field twists, creating current sheets that undergo magnetic reconnection,

converting magnetic energy to kinetic and thermal energy. Since magnetic reconnection

is a plasma process, it does not directly affect the neutral species. The plasma couples to the neutral

species through thermal collisions and ionisation/recombination. We find that in a partially ionised

plasma, the instability is accelerated increasing the explosive nature of the dynamics.

Three-dimensional partially ionised simulations become numerically taxing due to the small timestep

(determined by the interactions between ionised and neutral fluids) and the complex physics

involved (thermal collisions, ionisation, recombination, radiative losses). As such, 3D simulations

of two-fluid plasma become computationally expensive. This project was only possible due to the

generous computational time provided on the COSMA7 system (DiRAC@Durham) as part of the

STFC DiRAC HPC Facility (ST/P002293/1, ST/R002371/1 and ST/S002502/1, ST/R000832/1).