Modelling the energetic Sun
The input parameters to simulations are, roughly speaking, the entropy flux needed in the convection zone to maintain a given effective temperature, and the initial magnetic field configuration and/or the magnetic field to be injected at the bottom boundary. Simulations are run over one to several hours solar time, typically producing a snapshot containing all the magnetohydrodynamic variables every 10 seconds.
These snapshots can be the basis for computing detailed synthetic diagnostics of optically thick or optically thin spectral lines that can, at least statistically, be compared with actual observations from the Solar atmosphere. In addition, the Bifrost code includes the possibility of inserting “corks” that follow the follow individual packets of fluid as they move within or through various layers of the atmosphere.
Hitherto these simulations have been concentrated on sections of the solar atmosphere best characterized as “quiet Sun” with relatively weak magnetic fields compared to sites of emerging flux or fully developed active regions. However, the premise of RoCS is to study “the active Sun” and the simulations carried out have focused on the dynamics and energetics that ensue when the magnetic field becomes stronger and approaches values similar to that encountered in sunspots. We have explored the importance of spatial resolution in determining the dynamics of the chromosphere and corona; previous years simulations have shown that the solar atmosphere may be more dynamic and more magnetically dominated, even in the quiet or semi-quiet Sun, than thought earlier.
Importance of resolution
For example, there are indications that increased numerical resolution would increase the dynamics found in the chromosphere in the simulation. Since one of the major shortcomings of the current generation of simulations is too narrow spectral lines compared with the observations, increased numerical resolution could be important for the creation of more realistic simulations. To investigate this further, a number of simulations have been started - a series of simulations with hydrogen ionisation in equilibrium, a restricted height range and a small box (to have short wall clock times for the runs): 6 Mm x 6 Mm horizontal box and [-2.5,7.5] Mm vertically with horizontal resolutions of 40, 20, 10 and 5 km. The results are under investigation. A large simulation corresponding to the published hydrogen non-equilibrium simulation (Carlsson et al 2016) but with double the resolution (24 km horizontally) has also been started.
In the same vein, but also to better understand the corrugation of the transition region, 2D numerical experiments have been computed of a coronal-hole-like atmosphere in a box with 16 Mm in the horizontal direction and [-2.6, 14] Mm vertically. There are four experiments of this kind with increasing numerical resolution; from horizontal resolution of 31 km to 4 km and vertical resolutions from 20 km to 2.5 km. The results are not yet fully analyzed, but a first look at the experiments shows potentially interesting features in terms of the generation of spicules. These snapshots have a cadence of 2 seconds each, which increases the possibilities of analyzing the more dynamical structures and to carry out Lagrangian tracing to study them in detail.
Flux emergence
The interaction of magnetic flux recently emerged from the solar interior with the preexisting coronal magnetic field is of special interest: many prominent features in the solar atmosphere are related to this fundamental process. A number of simulations have been run both in 3D and in 2D in which fields are injected at the lower boundary and are allowed to propagate up through the convection zone. The field breaks through the photosphere and emerges into the outer atmosphere interacting with the pre-existing ambient field as large-scale magnetic structures are built up in the chromosphere and corona.
A number of studies with horizontal resolution of 31 km and vertical resolution of 12 km have been run. These simulations have several goals: To see whether the quiet Sun magnetic field is maintained by a local shallow dynamo or requires the injection of field from the deeper convection zone. To understand the formation of strong chromospheric fields in a nascent active region. To study the observational consequences of the interaction of emerging fields as they form the active chromosphere and corona. These simulations show synthetic diagnostics that are remarkably similar to what is observed. Amongst other results we find that Ellerman bombs and UV-bursts are formed naturally and sometimes co-located and co-temporally as a result of large angle reconnection in emerging flux regions.
Nevertheless, there are still open questions concerning the flux emergence process itself, also because of the challenge posed by out of equilibrium ionization throughout the outer atmosphere. Therefore, 2D flux emergence experiments that combine the non-equilibrium ionization of hydrogen and including the effects of ambipolar diffusion have been run.
Deep models, large models
Finally, a number of “deep” models, extending to 8 Mm or more below and up to 50 Mm above the photosphere and 72 Mm wide horizontally are being run. These models should allow the study of the formation and evolution of the chromospheric network and most ambitiously the formation of strong photospheric fields such as sunspots and the study of the chromosphere and corona above these. These models have so far been run with a relatively coarse horizontal resolution of 100 km, but it is planned to refine this to 50 km now as the models have achieved close to steady-state conditions where the dynamics of the deep convection zone are mirrored in the topology of the chromosphere and corona.
Currently we are also running similar large scale models with a slight magnetic flux imbalance: to achieve an open magnetic field at the top boundary. This gives us the possibility of seeing how much energy such models can supply to the acceleration of the solar wind (which occurs at even greater heights than 50 Mm above the surface) in the form of Alfvén- or Alfvénic waves.
