Observational Tests of Damping by Resonant Absorption in Coronal Loop Oscillations

One of the proposed damping mechanisms of coronal (transverse) loop oscillations in the kink-mode is resonant absorption as a result of the Alfven speed variation at the outer boundary of coronal loops. Analytical expressions for the period and damping time exist for loop models with thin non-uniform boundaries. Here we measure the thickness of the non-uniform layer in oscillating loops for 11 events, by forward-fitting of the cross-sectional density profile and line-of-sight integration to the cross-sectional fluxes observed with TRACE 171 A. This way we model the internal and external electron density of the coronal plasma in oscillating loops. This allows us to test the theoretically predicted damping rates for thin boundaries as function of the density ratio. We find that the density ratio predicted by the damping time is higher than the density ratio estimated from the background fluxes. The lower densities modeled from the background fluxes are likely to be a consequence of the neglected hotter plasma that is not detected with the TRACE 171 A filter. Taking these correction into account, resonant absorption predicts damping times of kink-mode oscillations that are commensurable with the observed ones and provides a new diagnostic of the density contrast of oscillating loops.Comment: 10 Figure

Extending the dynamic temperature range of Boltzmann thermometers

Lanthanide-doped (nano)crystals are an important class of materials in luminescence thermometry. The working mechanism of these thermometers is diverse but most often relies on variation of the ratio of emission intensities from two thermally coupled excited states with temperature. At low temperatures, nonradiative coupling between the states can be slow compared to radiative decay, but, at higher temperatures, the two states reach thermal equilibrium due to faster nonradiative coupling. In thermal equilibrium, the intensity ratio follows Boltzmann statistics, which gives a convenient model to calibrate the thermometer. Here, we investigate multiple strategies to shift the onset of thermal equilibrium to lower temperatures, which enables Boltzmann thermometry in a wider dynamic range. We use Eu3+-doped microcrystals as a model system and find that the nonradiative coupling rates increase for host lattices with higher vibrational energies and shorter lanthanide–ligand distances, which reduces the onset temperature of thermal equilibrium by more than 400 K. We additionally reveal that thermometers with excited states coupled by electric-dipole transitions have lower onset temperatures than those with magnetic-dipole-coupled states due to selection rules. These insights provide essential guidelines for the optimization of Boltzmann thermometers to operate in an extended temperature range

Waves and oscillations in magnetic fields

This paper gives an overview of the theory of MHD waves in magnetic plasma configurations in the solar atmosphere. The emphasis is on basic properties that are independent of specific equilibrium models but are rather related to the intrinsic structuring and non-uniformity of the plasma. The discussion is confined to MHD waves in uniform and 1-d cylindrical equilibrium models of magnetic flux tubes with a straight magnetic field. These models contain sufficient physics for understanding basic properties of MHD waves and still allow for a relatively straightforward and transparent mathematical analysis.status: publishe

Wiskundige modellering van ruimteweer in the Centrum voor Plasma-Astrofysica van de K.U.Leuven

status: publishe

Applications of resonant wave coupling in the solar corona

edition: IIstatus: publishe

The effect of curvature on quasi-modes in coronal loops

This paper studies quasi-mode oscillations in models of coronal loops that include longitudinal curvature. Using a toroidal coordinate system to incorporate curvature in a basic coronal loop model, the linearized ideal MHD equations are solved for the plasma-beta = 0. As a result of the curvature, quasi-modes with different poloidal wave numbers are coupled resulting in modifications of the frequencies. However, for small curvature, only the coupling of quasi-modes with a neighbouring poloidal wave number remains in first order. In addition, the quasi-mode frequencies are unchanged up to first order in the curvature. The imaginary part of the frequency, however, does change in first order, and quasi-modes are slightly more damped in realistically curved coronal loop configurations.status: publishe

Time dependent simulations of 2D coronal loop models

We use the time dependent code PET in order to perform time evolution simulations of damped oscillations in linetied solar coronal loops. This new numerical code has open boundary conditions implemented and is suitable to perform simulations of transversally driven plasma cylinders. In this paper, we show that a resonance is set up when driving harmonically with a frequency close to the quasimode frequency. In this resonance, all the incoming energy is dissipated. When the driver is shut down, the oscillation in and in the neighbourhood of the resonant layer decays exponentially. Using this code, we hope to show that wave heating is still a viable mechanism to heat coronal loops and to explain the observed damped loop oscillations. We will show this by proving that more energy can be dissipated per oscillation period than is observed.status: publishe

Damping of coronal loop oscillations: Calculation of resonantly damped kink oscillations of one-dimensional nonuniform loops

The analytic study of coronal loop oscillations in equilibrium states with thin nonuniform boundary layers is extended by a numerical investigation for one-dimensional nonuniform equilibrium states. The frequency and the damping time of the ideal kink quasi mode are calculated in fully resistive MHD. In this numerical investigation there is no need to adopt the assumption of a thin nonuniform boundary layer, which is essential for analytic theory. An important realization is that analytical expressions for the damping rate that are equivalent for thin nonuniform layers give results differing by a factor of 2 when they are used for thick nonuniform layers. Analytical theory for thin nonuniform layers does not allow us to discriminate between these analytical expressions. The dependence of the complex frequency of the kink mode on the width of the nonuniform layer, on the length of the loop, and on the density contrast between the internal and the external region is studied and is compared with analytical theory, which is valid only for thin boundaries. Our numerical results enable us to show that there exists an analytical expression for thin nonuniform layers that might be used as a qualitative tool for extrapolation into the regime of thick nonuniform layers. However, when the width of the nonuniform layer is varied, the differences between our numerical results and the results obtained with the version of the analytical approximation that can be extended into the regime of thick nonuniform layers are still as large as 25%.status: publishe

The effect of curvature on quasi-modes in coronal loops

This paper studies quasi-mode oscillations in models of coronal loops that include longitudinal curvature. Using a toroidal coordinate system to incorporate curvature in a basic coronal loop model, the linearized ideal MHD equations are solved for the plasma-β = 0. As a result of the curvature, quasi-modes with different poloidal wave numbers are coupled resulting in modifications of the frequencies. However, for small curvature, only the coupling of quasi-modes with a neighbouring poloidal wave number remains in first order. In addition, the quasi-mode frequencies are unchanged up to first order in the curvature. The imaginary part of the frequency, however, does change in first order, and quasi-modes are slightly more damped in realistically curved coronal loop configurations.status: publishe