Mass and Energy Transfer Between the Solar Photosphere and Corona

Abstract:

The problem of chromospheric and coronal heating is also a problem of mass supply to the corona. On average we see redshifts at transition region temperatures of the order of 10 km/s. If interpreted as downflows, this would quickly empty the corona, and fresh material has to be transported into the corona. Several models have been proposed to understand this mass cycle between the different atmospheric layers. However, as of yet all these proposals have serious shortcomings. On the observational side open questions remain, too. With the new IRIS mission we can observe the transition region at unprecedented spatial and spectral resolution, but the observational results are still puzzling. In particular the finding that the spatial distribution of line widths and Doppler shifts do not change with increasing resolution is against physical intuition. This shows that even with IRIS we still have significant velocity gradients along the line-of-sight, indicating that shocks might play a significant role. Likewise the temporal evolution might be a key for our understanding of the mass cycle. It might well be that the filling and draining of hot plasma occurs on significantly different time scales, which might be part of the difficulty to arrive at a conclusive observational picture. Considering the progress made for the quiet Sun, it seems clear that the processes responsible for the mass exchange are not resolved (yet). Therefore one might wonder to what extent one could use larger and resolved individual events in more active parts of the Sun to understand the details of the mass transport. In particular a common understanding of reconnection events such as Ellerman bombs in the photosphere, explosive events in the transition region and the recently discovered IRIS bombs in-between might provide the key to better understand the mass cycle throughout the atmospheric layers from the photosphere to the corona.