Energy dissipation in coronal loops: statistical analysis of intermittent structures in magnetohydrodynamic turbulence

Abstract:

In recent years, the power law energy distribution observed in dissipation events ranging from flares 
down to nanoflares, has been associated either to intermittent turbulence or to self-organized criticality. 
Even though several studies have been conducted along these lines, it is not even clear whether these 
two paradigms are mutually exclusive or whether they are complementary manifestations of the complexity 
exhibited by the system.

In this work we numericaly integrate the reduced magnetohydrodynamic equations to simulate the dynamics 
of coronal loops driven at their bases by footpoint motions. These simulations show that a stationary 
turbulent regime is reached after a few photospheric turnover times, displaying a broadband power 
spectrum and a dissipation rate consistent with the energy loss rates of the plasma confined in these 
loops. Our main goal is to assess whether the intermittent features observed in this turbulent flow 
can also be regarded as manifestations of self-organized criticality. The energy, area and lifetime 
statistics of the structures detected in these simulated data present robust scaling laws.

We compute the critical exponents linking geometrical properties to the underlying avalanche dynamics. 
We also calculate the spreading exponents of the intermittent structures and investigate if the 
spreading dynamics obeys scaling relations which relate the spreading and avalanche exponents in order 
to test its consistency, as predicted for a self organized critical system.