{"id":{"repo_id":"freiburg-diss","oai_identifier":"oai:freidok.uni-freiburg.de:1537"},"canonical_url":"https://search.dev.ndltd.org/etd/freiburg-diss/oai:freidok.uni-freiburg.de:1537","repository":{"repo_id":"freiburg-diss","name":"University of Freiburg","base_url":"https://freidok.uni-freiburg.de/oai/oai2.php"},"display":{"title":"Catastrophic cooling in solar coronal loops : thermal instability as a road to complex evolution","abstract":"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.","abstract_html":"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? &lt;br&gt; &lt;br&gt;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. &lt;br&gt; &lt;br&gt;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.","abstract_has_math":false,"creators":["Müller, Daniel"],"institution":null,"degree_name":null,"degree_level":null,"degree_discipline":null,"degree_department":null,"school":null,"contributors":["Peter, Hardi"],"advisors":[],"committee_chairs":[],"committee_members":[],"year":null,"date_issued":"","date_published":null,"updated_at":"2026-07-24T02:22:22Z","subjects":["sun","corona","physics","astrophysics","UV radiation"],"languages":[],"rights":[],"rights_urls":[],"identifier_entries":[]},"links":{"outbound_url":"https://freidok.uni-freiburg.de/data/1537","outbound_label":"Repository record","outbound_source":"source_url"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor","label":"Contributor","values":["Peter, Hardi"]},{"key":"dc:creator","label":"Author","values":["Müller, Daniel"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:type","label":"Dc Type","values":["DoctoralThesis"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["sun","corona","physics","astrophysics","UV radiation"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description.abstract","label":"Abstract","values":["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.","Satellitenbeobachtungen mit hoher räumlicher und zeitlicher Auflösung zeigen die dynamische Natur der Sonnenkorona und ermöglichen uns, physikalische Prozesse in der äußeren Atmosphäre der Sonne mit großer Genauigkeit zu untersuchen. Die vorliegende Arbeit beschäftigt sich mit dynamischen Prozessen in koronalen Bögen, d.h. magnetischen Strukturen, die einen wichtigen Bestandteil der Sonnenkorona bilden. Zeitabhängige numerische Modelle koronaler Bögen werden berechnet, um die folgenden Fragen zu beantworten: Verlangen dynamische Prozesse in koronalen Bögen, wie z.B. Strömungen und kurzzeitige Helligkeitsvariationen, notwendigerweise nach einem externen Antriebsmechanismus? Können unterschiedliche Beobachtungen von schnellen Abströmungen in koronalen Bögen mit einem einzigen Mechanismus erklärt werden? Welche Parameter bestimmen die Dynamik koronaler Bögen? <br> <br>Es zeigt sich, dass koronale Bögen, die vorwiegend in der Nähe der Fußpunkte geheizt werden (worauf sowohl Beobachtungen als auch theoretische Überlegungen hindeuten) eine thermische Instabilität im oberen Teil des Bogens entwickeln können. Diese Instabilität hat einen selbstverstärkenden Kühlungsprozess (\"catastrophic cooling\") zur Folge, der zur Bildung dichter, kühler Plasmakondensationen führt. Der erste Teil dieser Arbeit beschäftigt sich mit Plasmakondensationen in kleinen magnetischen Bögen relativ niedriger Temperatur, aus denen sich vermutlich die solare Übergangsregion zusammensetzt, und beschreibt, wie der drastische Kühlungsprozess zu starken Helligkeitsvariationen in Spektrallinien der Übergangsregion führt. Im zweiten Teil der Arbeit werden Modelle für längere koronale Bögen vorgestellt, die zeigen, dass die Dämpfungslänge der Energiedissipation im Bogen einen Kontrollparameter dieses nichtlinearen Systems darstellt. Während eine große Dämpfungslänge stabile, statische Bögen liefert, führen Dämpfungslängen unterhalb einer kritischen Schwelle zu einer dynamischen Entwicklung. Diese kann auf der Basis eines Evaporations-Kondensations-Zyklus verstanden werden: Plasma wird zuerst durch einen koronalen Heizungsmechanismus von der kühlen, dichten Chromosphäre in die Korona verdampft, kondensiert dann im koronalen Teil des Bogens in Folge thermischer Instabilität, fließt zu den Fußpunkten des Bogens ab und verdampft schließich erneut. Eine Anwendung des \"catastrophic cooling\"-Modells auf lange Bögen aktiver Regionen zeigt, dass beim Abströmen des Plasmas Geschwindigkeiten von mehreren 100 km/s erreicht werden und auch die dichten Plasmawolken selbst sich mit Geschwindigkeiten von bis zu 100 km/s bewegen. Dies liefert eine Erklärung für Beobachtungen von wandernden hellen Strukturen und schnellen Abströmungen in koronalen Bögen. <br> <br>Im Gegensatz zu früheren Modellen legt diese Arbeit nahe, dass der selbstverstärkende Kühlungsprozess nicht durch eine drastische Verringerung der Heizung eines koronalen Bogens angetrieben werden muss, sondern vielmehr aus einer thermischen Instabilität des Bogens resultiert. Dies ist die natürliche Konsequenz eines vorrangig im unteren Teil des Bogens wirkenden Heizungsmechanismus, der nicht zeitabhängig sein muss. Um die verschiedenen Lösungsregime zu lokalisieren, wurde eine Parameterstudie durchgeführt, die die Entwicklung koronaler Bögen in Abhängigkeit ihrer Länge, Heizungsrate und Dämpfungslänge beschreibt. Dabei wird eine Verbindung zwischen der thermischen Instabilität in koronalen Bögen und globalen Relaxationsoszillationen in stellaren Koronen aufgezeigt."]},{"key":"dc:format.medium","label":"Dc Format Medium","values":["application/pdf"]},{"key":"dc:title","label":"Title","values":["Catastrophic cooling in solar coronal loops : thermal instability as a road to complex evolution","Katastrophale Kühlung in solaren koronalen Bögen : thermische Instabilität als Weg zu komplexer Entwicklung"]}]}],"canonical_facts":{"dc:contributor":["Peter, Hardi"],"dc:creator":["Müller, Daniel"],"dc:description.abstract":["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.","Satellitenbeobachtungen mit hoher räumlicher und zeitlicher Auflösung zeigen die dynamische Natur der Sonnenkorona und ermöglichen uns, physikalische Prozesse in der äußeren Atmosphäre der Sonne mit großer Genauigkeit zu untersuchen. Die vorliegende Arbeit beschäftigt sich mit dynamischen Prozessen in koronalen Bögen, d.h. magnetischen Strukturen, die einen wichtigen Bestandteil der Sonnenkorona bilden. Zeitabhängige numerische Modelle koronaler Bögen werden berechnet, um die folgenden Fragen zu beantworten: Verlangen dynamische Prozesse in koronalen Bögen, wie z.B. Strömungen und kurzzeitige Helligkeitsvariationen, notwendigerweise nach einem externen Antriebsmechanismus? Können unterschiedliche Beobachtungen von schnellen Abströmungen in koronalen Bögen mit einem einzigen Mechanismus erklärt werden? Welche Parameter bestimmen die Dynamik koronaler Bögen? <br> <br>Es zeigt sich, dass koronale Bögen, die vorwiegend in der Nähe der Fußpunkte geheizt werden (worauf sowohl Beobachtungen als auch theoretische Überlegungen hindeuten) eine thermische Instabilität im oberen Teil des Bogens entwickeln können. Diese Instabilität hat einen selbstverstärkenden Kühlungsprozess (\"catastrophic cooling\") zur Folge, der zur Bildung dichter, kühler Plasmakondensationen führt. Der erste Teil dieser Arbeit beschäftigt sich mit Plasmakondensationen in kleinen magnetischen Bögen relativ niedriger Temperatur, aus denen sich vermutlich die solare Übergangsregion zusammensetzt, und beschreibt, wie der drastische Kühlungsprozess zu starken Helligkeitsvariationen in Spektrallinien der Übergangsregion führt. Im zweiten Teil der Arbeit werden Modelle für längere koronale Bögen vorgestellt, die zeigen, dass die Dämpfungslänge der Energiedissipation im Bogen einen Kontrollparameter dieses nichtlinearen Systems darstellt. Während eine große Dämpfungslänge stabile, statische Bögen liefert, führen Dämpfungslängen unterhalb einer kritischen Schwelle zu einer dynamischen Entwicklung. Diese kann auf der Basis eines Evaporations-Kondensations-Zyklus verstanden werden: Plasma wird zuerst durch einen koronalen Heizungsmechanismus von der kühlen, dichten Chromosphäre in die Korona verdampft, kondensiert dann im koronalen Teil des Bogens in Folge thermischer Instabilität, fließt zu den Fußpunkten des Bogens ab und verdampft schließich erneut. Eine Anwendung des \"catastrophic cooling\"-Modells auf lange Bögen aktiver Regionen zeigt, dass beim Abströmen des Plasmas Geschwindigkeiten von mehreren 100 km/s erreicht werden und auch die dichten Plasmawolken selbst sich mit Geschwindigkeiten von bis zu 100 km/s bewegen. Dies liefert eine Erklärung für Beobachtungen von wandernden hellen Strukturen und schnellen Abströmungen in koronalen Bögen. <br> <br>Im Gegensatz zu früheren Modellen legt diese Arbeit nahe, dass der selbstverstärkende Kühlungsprozess nicht durch eine drastische Verringerung der Heizung eines koronalen Bogens angetrieben werden muss, sondern vielmehr aus einer thermischen Instabilität des Bogens resultiert. Dies ist die natürliche Konsequenz eines vorrangig im unteren Teil des Bogens wirkenden Heizungsmechanismus, der nicht zeitabhängig sein muss. Um die verschiedenen Lösungsregime zu lokalisieren, wurde eine Parameterstudie durchgeführt, die die Entwicklung koronaler Bögen in Abhängigkeit ihrer Länge, Heizungsrate und Dämpfungslänge beschreibt. Dabei wird eine Verbindung zwischen der thermischen Instabilität in koronalen Bögen und globalen Relaxationsoszillationen in stellaren Koronen aufgezeigt."],"dc:format.medium":["application/pdf"],"dc:subject":["sun","corona","physics","astrophysics","UV radiation"],"dc:title":["Catastrophic cooling in solar coronal loops : thermal instability as a road to complex evolution","Katastrophale Kühlung in solaren koronalen Bögen : thermische Instabilität als Weg zu komplexer Entwicklung"],"dc:type":["DoctoralThesis"]},"updated_at":"2026-07-24T02:22:22Z"}