| dc.description.abstracteng | Explaining the existence of the million degree corona on top of the much cooler Solar
surface has provided scientist with a challenge for a several decades. It is not possible for a cooler object to heat something that is hotter by conduction, which implies that there is another mean of energy transport into the corona. The general consensus is that this role is taken by the magnetic fields that are ever present at the solar surface.
The focus of today’s research is on the actual mechanism that thermalizes the energy
transported by the magnetic field. Several mechanisms responsible for this conversion
into thermal energy are being put forward. These suggestions fall often in the Alternating
Current (AC) or Direct Current (DC) category. The first involves rapid changes of the
magnetic field relative to the Alfvén crossing time of a coronal loop, while the second
category involves slow changes. While a convincing case can be made for each suggested heating mechanism from modeling alone, the observational confirmation is lacking. The theoretical estimates on which scales the energy conversion happens in these models are on the order of centimetres to metres. Observations, however, reach a resolution of 100 km, at best, in the relevant wavelengths, and as such no direct observational confirmation of one heating mechanism over the other is possible. Synthetic observations derived from self-consistent 3D MHD models can provide the link
between theory and observation. Investigation of the emission structures and distribution
of Doppler shifts of emission lines can provide insight on which of these mechanisms is
dominant. Fully self consistent 3D MHD models have already shown the feasibility of
this method. In this work we will expand this approach in two ways.
First we investigate the effect of the strength of the magnetic field at the photospheric
layer. We find that the behaviour of the Doppler shifts is strongly depended on the the
magnetic field strength. When interpreting the stronger photospheric magnetic fields as
higher magnetic activity, the patterns seen in the Doppler shifts as a function of formation temperature are consistent with observation of magnetically active stars. Also comparing the C IV emission with a proxy for the X-ray flux is roughly consistent with observations. Next we explore the observational consequences of different heating mechanisms using 3D MHD numerical experiments. This provides some insight on which of these mechanisms is dominant. For this we replace the Ohmic heating term in the energy equation with parametrized forms of the heating, which are derived from reduced MHD models. These models involve heating through Alfvén wave dissipation and MHD-turbulence. We find that the different heating parametrizations give similar coronae in terms of synthesized emission as it would be observed e.g. by EUV imaging. Thus EUV imaging alone are not sufficient to distinguish between these parametrizations. However, Doppler shift observations acquired by e.g. Hinode/EIS can provide the pivotal information. In our numerical experiments the different parametrizations of the heating leads to significantly different distributions of the Doppler shifts of the synthesized emission lines in the transition region and corona. In particular, this applies to the average redshifts seen in the transition region and the average blueshifts in the coronal lines. Based on this, our results favour the turbulent cascade over the Alfvén wave heating, at least when considering an active region. Future observational and numerical studies will have to show to what extend this will hold in general.
Combining the results from the two investigations we conclude that different heating distributions produce different observables. But it is not trivial to conclude which distribution is the most likely. | de |