Self-consistent equilibrium models of prominence thin threads heated by Alfvén waves propagating from the photosphere

Abstract

The fine structure of solar prominences is composed of thin threads that outline the prominence magnetic field lines. Observations have shown that transverse waves of Alfvénic nature are ubiquitous in prominence threads. These waves are driven at the photosphere and propagate to prominences suspended in the corona. Heating due to Alfvén wave dissipation could be a relevant mechanism in the cool and partially ionised prominence plasma. In this work, we explore the construction of 1D equilibrium models of prominence thin threads that satisfy an energy balance condition between radiative losses, thermal conduction, and Alfvén wave heating. We assumed the presence of a broadband driver at the photosphere that launches Alfvén waves towards the prominence. An iterative method was implemented in which the energy balance equation and the Alfvén wave equation are consecutively solved. From the energy balance equation and considering no wave heating initially, we computed the equilibrium profiles along the thread of the temperature, density, ionisation fraction, and other relevant parameters. On these equilibrium profiles, we used the Alfvén wave equation to compute the wave heating rate, which was then put back in the energy balance equation to obtain new equilibrium profiles, and so on. The process was repeated until convergence to a self-consistent thread model heated by Alfvén waves was achieved. We obtained equilibrium models composed of a cold and dense thread, an extremely thin prominence-corona transition region, and an extended coronal region. We found that the length of the cold thread decreases with the temperature at the prominence core and increases with the Alfvén wave energy flux injected at the photosphere. However, computed equilibrium models for large wave energy fluxes are not possible when the wave heating rate inside the cold thread becomes larger than the radiative losses. The maximum value of the wave energy flux that allows for an equilibrium depends on the prominence core temperature. This constrains the existence of thread equilibria in realistic conditions.