Catastrophic cooling in solar coronal loops : thermal instability as a road to complex evolution

Satellite observations with high spatial and temporal resolution have revealed the highly dynamic nature of the solar corona and enabled us to study physical processes in the outer atmosphere of our mother star in great detail. This thesis deals with dynamic processes in coronal loops, i.e. magnetic structures which may be thought of as the elementary building blocks of the solar corona. Using computational fluid dynamics as a tool, I calculate time-dependent models of solar coronal loops in order to address the following questions: Are dynamic processes in coronal loops, such as flows and transient brightenings, necessarily the result of a time-dependent driving mechanism? Can various observations of fast downflows be explained by a common mechanism? Which parameters determine the dynamics of coronal loops? <br> <br>It is found that coronal loops which are predominantly heated around their footpoints can develop a thermal instability in the upper part of the loop. This instability results in a self-amplifying “catastrophic cooling” process and leads to the formation of dense, cool plasma condensations. The first part of the work focuses on plasma condensations in short cool loops, which presumably constitute the solar transition region, and describes how the catastrophic cooling process leads to transient brightenings in spectral lines formed in the transition region. In the second part of the work, the model is extended to larger coronal loops, and the conclusion is reached that the damping length of the energy dissipation acts as a control parameter of this non-linear system. While a long damping length results in stable, static loops, damping lengths below a critical value give rise to a dynamic evolution. The dynamic evolution can be understood on the basis of an evaporation-condensation cycle: Plasma is first evaporated by coronal heating from the cool and dense chromosphere into the corona, then condenses in the coronal part of the loop as a result of thermal instability, drains towards the footpoints of the loop and finally evaporates again. When applying the catastrophic cooling scenario to long active region loops, I find that the draining process is accompanied by fast downflows which can reach flow speeds of up to several 100 km/s. Also the proper motions of the dense plasma blobs themselves are of the order of 100 km/s, which offers an explanation for the recent observations of moving bright blobs in coronal loops. <br> <br>In contrast to earlier models it is suggested that the process of catastrophic cooling does not have to be initiated by a drastic decrease of the total loop heating but rather results from a loss of equilibrium at the loop apex as a natural consequence of footpoint-centered heating which can be constant in time. To obtain a broader picture, a parameter study is carried out which describes the evolution of coronal loops as a function of different lengths, heating rates and damping lengths. A connection between the thermal instability in coronal loops and global relaxation oscillations of stellar coronae is pointed out.

Author dc:creator

• Müller, Daniel

Contributors dc:contributor

• Peter, Hardi

• sun
• corona
• physics
• astrophysics
• UV radiation

Repository record source_url

https://freidok.uni-freiburg.de/data/1537

OAI identifier oai:identifier

oai:freidok.uni-freiburg.de:1537