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The coronal heating problem
Stockholm University, Faculty of Science, Department of Astronomy.
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The heating of the solar corona has been investigated during four of decades and several mechanisms able to produce heating have been proposed. It has until now not been possible to produce quantitative estimates that would establish any of these heating mechanism as the most important in the solar corona. In order to investigate which heating mechanism is the most important, a more detailed approach is needed.

In this thesis, the heating problem is approached ”ab initio”, using well observed facts and including realistic physics in a 3D magneto-hydrodynamic simulation of a small part of the solar atmosphere. The ”engine” of the heating mechanism is the solar photospheric velocity field, that braids the magnetic field into a configuration where energy has to be dissipated. The initial magnetic field is taken from an observation of a typical magnetic active region scaled down to fit inside the computational domain. The driving velocity field is generated by an algorithm that reproduces the statistical and geometrical fingerprints of solar granulation. Using a standard model atmosphere as the thermal initial condition, the simulation goes through a short startup phase, where the initial thermal stratification is quickly forgotten, after which the simulation stabilizes in statistical equilibrium. In this state, the magnetic field is able to dissipate the same amount of energy as is estimated to be lost through radiation, which is the main energy loss mechanism in the solar corona.

The simulation produces heating that is intermittent on the smallest resolved scales and hot loops similar to those observed through narrow band filters in the ultra violet. Other observed characteristics of the heating are reproduced, as well as a coronal temperature of roughly one million K. Because of the ab initio approach, the amount of heating produced in these simulations represents a lower limit to coronal heating and the conclusion is that such heating of the corona is unavoidable.

Place, publisher, year, edition, pages
Stockholm: Institutionen för astronomi , 2004. , 44 p.
Keyword [sv]
Solen, Corona, Magneto hydro dynamik
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:su:diva-32ISBN: 91-7265-798-7 (print)OAI: oai:DiVA.org:su-32DiVA: diva2:192438
Public defence
2004-03-05, sal FB52, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00
Opponent
Supervisors
Available from: 2004-02-04 Created: 2004-02-04Bibliographically approved
List of papers
1. Bulk Heating and Slender Magnetic Loops in the Solar Corona
Open this publication in new window or tab >>Bulk Heating and Slender Magnetic Loops in the Solar Corona
2002 In: Astrophysical Journal, ISSN 0571-7248, Vol. 572, L113-116 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-23511 (URN)
Note
Part of urn:nbn:se:su:diva-32Available from: 2004-02-04 Created: 2004-02-04Bibliographically approved
2. An Ab Initio Approach to the Solar Coronal Heating Problem
Open this publication in new window or tab >>An Ab Initio Approach to the Solar Coronal Heating Problem
2005 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 618, no 2, 1020-1030 p.Article in journal (Refereed) Published
Abstract [en]

We present an ab initio approach to the solar coronal heating problem by modeling a small part of the solar corona in a computational box using a three-dimensional MHD code including realistic physics. The observed solar granular velocity pattern and its amplitude and vorticity power spectra, as reproduced by a weighted Voronoi tessellation method, are used as a boundary condition that generates a Poynting flux in the presence of a magnetic field. The initial magnetic field is a potential extrapolation of a SOHO/MDI high-resolution magnetogram, and a standard stratified atmosphere is used as a thermal initial condition. Except for the chromospheric temperature structure, which is kept nearly fixed, the initial conditions are quickly forgotten because the included Spitzer conductivity and radiative cooling function have typical timescales much shorter than the time span of the simulation. After a short initial start-up period, the magnetic field is able to dissipate (3-4) × 106 ergs cm-2 s-1 in a highly intermittent corona, maintaining an average temperature of ~106 K, at coronal density values for which simulated images of the TRACE 171 and 195 Å passbands reproduce observed photon count rates.

Place, publisher, year, edition, pages
The American Astronomical Society, 2005
Keyword
MHD; Sun: corona; Sun: magnetic fields
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-23512 (URN)10.1086/426063 (DOI)
Note
Part of urn:nbn:se:su:diva-32Available from: 2004-02-04 Created: 2004-02-04 Last updated: 2017-12-13Bibliographically approved
3. An ab Initio Approach to Solar Coronal Loops
Open this publication in new window or tab >>An ab Initio Approach to Solar Coronal Loops
2005 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 618, no 2, 1031-1038 p.Article in journal (Refereed) Published
Abstract [en]

Data from recent numerical simulations of the solar corona and transition region are analyzed, and the magnetic field connections between the low corona and the photosphere are found to be close to those of a potential field. The field line-to-field line displacements follow a power-law distribution with typical displacements of just a few Mm. Three loops visible in simulated TRACE filters are analyzed in detail and found to have significantly different heating rates and distributions thereof, one of them showing a small-scale heating event. The dynamical structure is complicated, even though all the loops are visible in a single filter along most of their lengths. The loops are nonstatic and are in the process of evolving into loops with very different characteristics. Differential emission measure (DEM) curves along one of the loops illustrate that DEM curves have to be treated carefully if physical characteristics are to be extracted.

Place, publisher, year, edition, pages
The American Astronomical Society, 2005
Keyword
MHD; Sun: corona; Sun: magnetic fields
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-23513 (URN)10.1086/426064 (DOI)
Note
Part of urn:nbn:se:su:diva-32Available from: 2004-02-04 Created: 2004-02-04 Last updated: 2017-12-13Bibliographically approved

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