Stefaan Poedts

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%.

MHD seismology of coronal loops using the period and damping of quasi-mode kink oscillations

Aims. We combine the magnetohydrodynamic (MHD) theory of resonantly damped quasi-mode kink oscillations with observational estimates of the period and damping of transverse coronal loop oscillations to extract information on physical parameters in oscillating loops. Methods. A numerical study of the quasi-mode period and damping, in one-dimensional fully non-uniform flux tubes, is used to obtain equilibrium models that reproduce the observed periods and damping rates. This scheme is applied to 11 loop oscillation events. Results. When only the damping rate is used, the valid equilibrium models form a one-dimensional solution curve in the two-dimensional parameter space (density contrast, transverse inhomogeneity length-scale). Lower limits to the transverse inhomogeneity are obtained in the limit of high contrast loops. When both the period and the damping rate are used, the equilibrium Alfven speed (or Alfven travel time) comes into play. The valid equilibrium models then form a one-dimensional solution curve in the three-dimensional parameter space (density contrast, transverse inhomogeneity length-scale, Alfven speed or Alfven travel time). The projection of these solutions onto the Alfven speed axis is found to be constrained to a rather limited interval. Upper limits to the internal Alfven speed are derived for 9 of the 11 analysed events.

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.

Numerical-simulation of coronal heating by resonant absorption of alfven waves

The heating of coronal loops by resonant absorption of Alfvén waves is studied in compressible, resistive magnetohydrodynamics. The loops are approximated by straight cylindrical, axisymmetric plasma columns and the incident waves which excite the coronal loops are modelled by a periodic external driver. The stationary state of this system is determined with a numerical code based on the finite element method. Since the power spectrum of the incident waves is not well known, the intrinsic dissipation is computed. The intrinsic dissipation spectrum is independent of the external driver and reflects the intrinsic ability of the coronal loops to extract energy from incident waves by the mechanism of resonant absorption.The numerical results show that resonant absorption is very efficient for typical parameter values occurring in the loops of the solar corona. A considerable part of the energy supplied by the external driver, is actually dissipated Ohmically and converted into heat. The heating of the plasma is localized in a narrow resonant layer with a width proportional to η1/3. The energy dissipation rate is almost independent of the resistivity for the relevant values of this parameter. The efficiency of the heating mechanism and the localization of the heating strongly depend on the frequency of the external driver. Resonant absorption is extremely efficient when the plasma is excited with a frequency near the frequency of a so-called ‘collective mode’.

On the efficiency of coronal loop heating by resonant absorption

The heating of solar coronal loops by resonant absorption of Alfven waves is investigated in the framework of linearized compressible resistive MHD. The resonant absorption of the waves incident on the coronal loops is numerically simulated in straight cylindrical, axisymmetric loop models externally excited by a periodic source. The stationary state of this driven system and the ohmic dissipation rate in this state are determined by a very accurate code based on the finite element technique. The efficiency of the heating mechanism and the energy deposition profile in this stationary state strongly depend on the characteristics of both the external driver and the equilibrium. It is shown that resonant absorption is very efficient for typical coronal loops as a considerable part of the energy supplied by the external source is actually dissipated ohmically and converted into heat. The heating rate is proportional to the square of the magnitude of the background magnetic field. 19 refs.

THE ROLE OF STREAMERS IN THE DEFLECTION OF CORONAL MASS EJECTIONS: COMPARISON BETWEEN STEREO THREE-DIMENSIONAL RECONSTRUCTIONS AND NUMERICAL SIMULATIONS

On 2009 September 21, a filament eruption and the associated coronal mass ejection (CME) were observed by the Solar Terrestrial Relations Observatory (STEREO) spacecraft. The CME originated from the southern hemisphere and showed a deflection of about 15 ◦ toward the heliospheric current sheet (HCS) during the propagation in the COR1 field of view. The CME source region was near the central meridian, but no on-disk CME signatures could be seen from the Earth. The aim of this paper is to provide a physical explanation for the strong deflection of the CME observed on 2009 September 21. The two-sided view of the STEREO spacecraft allows us to reconstruct the three-dimensional travel path of the CME and the evolution of the CME source region. The observations are combined with a magnetohydrodynamic simulation, starting from a magneticfield configuration closely resembling the extrapolated potential field for that date. By applying localized shearing motions, a CME is initiated in the simulation, showing a similar non-radial evolution, structure, and velocity as the observed event. The CME gets deflected toward the current sheet of the larger northern helmet streamer due to an imbalance in the magnetic pressure and tension forces and finally gets into the streamer. This study shows that during solar minima, even CMEs originating from high latitude can be easily deflected toward the HCS, eventually resulting in geoeffective events. How rapidly they undergo this latitudinal migration depends on the strength of both the large-scale coronal magnetic field and the magnetic flux of the erupting filament.

On waves and instabilities in pair-ion plasma

An analysis of waves and instabilities in the recently created pair-ion plasma is presented. The pair-ions, , have the same mass and opposite charge. In the case of an unmagnetized pair-ion plasma only two wave modes exist, namely the electrostatic Langmuir mode and the electromagnetic transverse mode. In the case of a magnetized pair-ion plasma containing electrons, the linear and nonlinear drift modes are presented. In addition, the instabilities of the two stream and the shear flow gradient type are discussed.

Complex magnetohydrodynamic bow shock topology in field-aligned low-β flow around a perfectly conducting cylinder

Two-dimensional ideal magnetohydrodynamic (MHD) simulations are presented that demonstrate several novel phenomena in MHD shock formation. The stationary symmetrical flow of a uniform, planar, field-aligned, low-β and superfast magnetized plasma around a perfectly conducting cylinder is calculated. The velocity of the incoming flow is chosen such that the formation of fast switch-on shocks is possible. Using a time marching procedure, a stationary bow shock is obtained, composed of two consecutive interacting shock fronts. The leading shock front has a dimpled shape and is composed of fast, intermediate and hydrodynamic shock parts. A second shock front follows the leading front. Additional intermediate shocks and tangential discontinuities are present in the downstream part of the flow. The intermediate shocks are of the 1–3, 1–4, 2–4 and 1=2–3=4 types. This is a confirmation in two dimensions of recent results on the admissibility of these types of shocks. Recently it has also been shown that the 1=2–3=...

Intensity variations in EIT shutterless mode: Waves or flows?

On 11 July 2001 an EIT shutterless campaign was conducted which provided 120 high-cadence (68 s) 304 Angstrom images of the north eastern quarter of the Sun. The most interesting feature seen in the data is an off-limb half loop structure along which systematic intensity variations are seen which appear to propagate from the top of the loop towards its footpoint. We investigate the underlying cause of these propagating disturbances, i.e. whether they are caused by waves or by plasma flows. First we identify 7 blobs with the highest intensities and follow them along the loop. By means of a location-time plot, bulk velocities can be measured at several locations along the loop. The velocity curve found this way is then compared with characteristic wave speeds and with the free-fall speed in order to deduce the nature of the intensity variations. Additional information on density and temperature is derived by measuring the relative intensity enhancements and comparing the EIT 304 A sequence with Big Bear data and 171 Angstrom data (TRACE/EIT). The combination of all these constraints gives us an insight on the nature and origin of these intensity variations. The idea of slow magneto-acoustic waves is rejected, and we find several arguments supporting that these intensity variations are due to flowing/failing plasma blobs.

Electrostatic modes in multi-ion and pair-ion collisional plasmas

The physics of plasmas containing positive and negative ions is discussed with special attention to the recently produced pair-ion plasma containing ions of equal mass and opposite charge. The effects of the density gradient in the direction perpendicular to the ambient magnetic field vector are discussed. The possible presence of electrons is discussed in the context of plasma modes propagating at an angle with respect to the magnetic field vector. It is shown that the electron plasma mode may become a backward mode in the presence of a density gradient, and this behavior may be controlled either by the electron number density or the mode number in the perpendicular direction. In plasmas with hot electrons an instability may develop, driven by the combination of electron collisions and the density gradient, and in the regime of a sound ions’ response. In the case of a pure pair-ion plasma, for lower frequencies and for parameters close to those used in the recent experiments, the perturbed ions may feel ...