Austrian Early Career Conference 2024

Contribution:
Talk

Authors:
Johannes Tschernitz; Philippe-A. Bourdin

Affiliations:
Institute of Physics, University of Graz

Title:
Numerical simulations of Ohmic heating of a coronal loop above a sunspot group

Abstract:
The high coronal temperatures of above 1 MK are not fully explained yet. A promising candidate among the various heating models is Ohmic dissipation of direct currents. Advective motions in the photosphere cause disturbances in the magnetic field, which propagate along the field and eventually reach the corona. The Poynting flux indicates the transport of energy into the corona. Due to the stresses, current sheets will form in the corona, where the energy is dissipated and heat the plasma.
We perform an observationally driven simulation of the heating of a coronal loop above a bi-polar active region via Ohmic heating. The simulation uses a grid of 1024×1024×256 grid points and covers a domain of 237×237×156 Mm³. Magnetograms from the Narrowband Filter Imager (NFI) instrument on board of the Hinode satellite serve as boundary conditions and are also used to initialize the magnetic field in the simulation box. A photospheric velocity driver provides the necessary motions and consists of a large-scale velocity field derived from the magnetograms and an artificial granulation driver, which provides the small-scale motions generated by the granulation. The driver and the magnetic field create the vertical Poynting flux, indicating energy transport to the corona. The simulation is allowed to evolve self-consistently for a time period of ~3800 seconds.
After some time, the disturbances in the magnetic field become strong enough so that Ohmic heating sets in and starts to heat up the loop, counteracting the energy losses. The loop reaches coronal temperatures. With the increasing temperature, energy losses get also larger and after ~3600 s the losses balance the heating in the loop. We observe a maximum temperature of around 1.8 MK in the loop. We use the CHIANTI atomic database to calculate synthetic EUV emission and Doppler shifts with the values for temperature, density and velocity obtained from the simulation. The calculated synthetic emission and the Doppler shifts are then compared to co-temporal and co-spatial observations of the active region made with the EUV imaging spectrometer (EIS) on board of the Hinode satellite in the Fe XII spectral line at 195.12 Å. The comparison shows a hot loop at the same time and place above the active region and also the Doppler shifts show agreement with the observations.