Statistical seismology of transverse waves in the solar corona 

Erwin Verwichte Submitted: 20130228 01:37
Observations show that transverse oscillations occur commonly in solar coronal loops. The rapid damping of these waves has been attributed to resonant absorption. The oscillation characteristics carries information of the structuring of the corona. However, selfconsistent seismological methods to extract information from individual oscillations is limited because there are less observables than model unknown parameters and the problem is underdetermined. Furthermore, it has been shown that onetoone comparisons of the observed scaling of period and damping times with wave damping theories is misleading. We aim to investigate if seismological information can be gained from the observed scaling laws in a statistical sense. A statistical approach is used whereby scalinglaws are produced by forward modelling using distributions of values for key loop crosssectional structuring parameters. We study two types of observations: 1) transverse loops oscillations as seen mainly with TRACE and SDO and 2) running transverse waves seen with CoMP. We demonstrate that the observed perioddamping time scaling law does provide information about the physical damping mechanism, if observations are collected from as wide as possible range of periods and a comparison with theory is performed in a statistical sense. The distribution of the ratio of damping time over period, i.e. the quality factor, has been derived analytically and fitted to the observations. A minimum value for the quality factor of 0.65 has been found. From this, a constraint linking the ranges of possible values for the density contrast and inhomogeneity layer thickness is obtained for transverse loop oscillations. If the layer thickness is not constrained, then the density contrast is maximally equal to 3. For transverse waves seen by CoMP, it is found that the ratio of maximum to minimum values for these two parameters has to be less than 2.06. i.e. the sampled values for the layer thickness and Alfvén travel time comes from a relatively narrow distribution. Now that more and more transverse loop oscillations have been analysed, a statistical approach to coronal seismology becomes possible. Using the observed data cloud restrictions in the loop parameter space of density contrast and inhomogeneity layer thickness are found and surprisingly for the running waves narrow distributions for loop parameters have been found.
Authors: Verwichte, E., Van Doorsselaere, T., White, R.S. & Antolin, P.
Projects: SDOAIA

Publication Status: accepted for publication in A&A
Last Modified: 20130228 12:33



Periodic spectral line asymmetries in solar coronal structures from slow magnetoacoustic waves 

Erwin Verwichte Submitted: 20100917 11:24
Recent spectral observations of upward moving quasiperiodic intensity perturbations in solar coronal structures have shown evidence of periodic line asymmetries near their footpoints. These observations challenge the established interpretation of the intensity perturbations in terms of propagating slow magnetoacoustic waves. We show that slow waves inherently have a bias towards enhancement of emission in the blue wing of the emission line due to inphase behaviour of velocity and density perturbations. We demonstrate that slow waves
cause line asymmetries when the emission line is averaged over an oscillation period or when a quasistatic plasma component in the lineofsight is included. Therefore, we conclude that slow magnetoacoustic waves remain a valid explanation for the observed quasiperiodic intensity perturbations.
Authors: Verwichte, E., Marsh, M., Foullon, C., Van Doorsselaere, T., De Moortel, I., Hood, A.W. and Nakariakov, V.M.
Projects: Hinode/EIS

Publication Status: accepted for publication in ApJL
Last Modified: 20100918 13:46



Spatial seismology of a large coronal loop arcade from TRACE & EIT observations of its transverse oscillations 

Erwin Verwichte Submitted: 20100510 12:54
We present a study of transverse loop oscillations in a large coronal loop arcade, using observations from the Transition Region And Coronal Explorer (TRACE) and Extremeultraviolet Imaging Telescope (EIT). For the first time we reveal the presence of longperiod transverse oscillations with periods between 24 minutes and 3 hours. One loop bundle, 690 Mm long and with an oscillation period of 40 min., is analysed in detail and its oscillation characteristics are determined in an automated manner. The oscillation quality factor is similar to what has been found earlier for oscillations in much shorter loops. This indicates that the damping mechanism of transverse loop oscillations is independent of loop length or period. The displacement profile along the whole length of the oscillating loop is determined for the first time
and consistently between TRACE and EIT. By comparing the observed profile with models of the threedimensional geometry of the equilibrium and perturbed loop, we test the effect of longitudinal structuring (spatial seismology) and find that the observations cannot unambigiously
distinguish between structuring and nonplanarity of the equilibrium loop. Associated intensity variations with a similar periodicity are explained in terms of variations in the line of sight column depth. Also, we report intensity oscillations at the loop foot point, which are in antiphase
with respect to the intensity oscillations in the loop body. Lastly, this observation offers the first opportunity to use the transverse oscillations of the arcade to model the Alfvén speed profile in
the global corona.
Authors: E. Verwichte, C. Foullon & T. Van Doorsselaere
Projects: SoHOEIT,TRACE

Publication Status: ApJ, in press
Last Modified: 20100511 07:11



Seismology of a large solar coronal loop from EUVI/STEREO observations of its transverse oscillation 

Erwin Verwichte Submitted: 20090324 13:50
The first analysis of a transverse loop oscillation observed by both STEREO spacecraft is presented, for an event on the 27th of June, 2007 as seen by the Extreme UltraViolet Imager (EUVI). The threedimensional loop geometry is determined using a threedimensional reconstruction with a semicircular loop model, which allows for an accurate measurement of the loop length. The plane of wave polarisation is found from comparison with a simulated loop model and shows that the oscillation is a fundamental horizontally polarised fast magnetoacoustic kink mode. The oscillation is characterised using an automated method and the results from both spacecraft are found to match closely. The oscillation period is 630+30 s and the damping time is 1000+300 s. Also, clear intensity variations associated with the transverse loop oscillations are reported for the first time. They are shown to be caused by the effect of lineofsight integration. The Alfvén speed and coronal magnetic field derived using coronal seismology are discussed. This study shows that EUVI/STEREO observations achieve an adequate accuracy for studying longperiod, large amplitude transverse loop oscillations.
Authors: Verwichte, E., Aschwanden, M.J., Van Doorsselaere, T., Foullon, C. and Nakariakov, V.M.
Projects: STEREO

Publication Status: accepted for publication in ApJ
Last Modified: 20090325 10:04



Subject will be restored when possible 

Erwin Verwichte Submitted: 20080607 08:36
he damping of slow magnetoacoustic coronal loop oscillations by shock dissipation is investigated.
Observations of large amplitude slow mode observations by SUMER show a clear dependency of the damping rate on the oscillation amplitude. Fully nonlinear MHD simulations of slow mode oscillations in the presence of thermal conduction are performed that show that shock dissipation is an important damping mechanism at large amplitudes, which enhances the damping rate by up to 50% above the rate given by thermal conduction alone. A comparison between the numerical simulations and the SUMER observations shows that, although the shock dissipation model can indeed produce an enhanced damping rate that is function of the oscillation amplitude, the found dependency is not as strong as that for the observations, even after considering observational corrections and the inclusion of enhanced linear dissipation.
Authors: Verwichte, E., Haynes, M., Arber, T.D. and Brady, C.S
Projects: SoHOSUMER

Publication Status: Accepted for publication in ApJ
Last Modified: 20080923 21:00



Subject will be restored when possible 

Erwin Verwichte Submitted: 20080607 08:36
The damping of slow magnetoacoustic coronal loop oscillations by shock dissipation is investigated.
Observations of large amplitude slow mode observations by SUMER show a clear dependency of the damping rate on the oscillation amplitude. Fully nonlinear MHD simulations of slow mode oscillations in the presence of thermal conduction are performed that show that shock dissipation is an important damping mechanism at large amplitudes, which enhances the damping rate by up to 50% above the rate given by thermal conduction alone. A comparison between the numerical simulations and the SUMER observations shows that, although the shock dissipation model can indeed produce an enhanced damping rate that is function of the oscillation amplitude, the found dependency is not as strong as that for the observations, even after considering observational corrections and the inclusion of enhanced linear dissipation.
Authors: Verwichte, E., Haynes, M., Arber, T.D. and Brady, C.S
Projects: SoHOSUMER

Publication Status: Accepted for publication in ApJ
Last Modified: 20080923 21:00



Damping of slow MHD coronal loop oscillations by shocks 

Erwin Verwichte Submitted: 20080607 08:36
The damping of slow magnetoacoustic coronal loop oscillations by shock dissipation is investigated.
Observations of large amplitude slow mode observations by SUMER show a clear dependency of the damping rate on the oscillation amplitude. Fully nonlinear MHD simulations of slow mode oscillations in the presence of thermal conduction are performed that show that shock dissipation is an important damping mechanism at large amplitudes, which enhances the damping rate by up to 50% above the rate given by thermal conduction alone. A comparison between the numerical simulations and the SUMER observations shows that, although the shock dissipation model can indeed produce an enhanced damping rate that is function of the oscillation amplitude, the found dependency is not as strong as that for the observations, even after considering observational corrections and the inclusion of enhanced linear dissipation.
Authors: Verwichte, E., Haynes, M., Arber, T.D. and Brady, C.S
Projects: SoHOSUMER

Publication Status: Accepted for publication in ApJ
Last Modified: 20080923 21:00



Fast magnetoacoustic waves in curved coronal loops I. Trapped and leaky modes 

Erwin Verwichte Submitted: 20051002 08:24
A study of vertically polarised fast magnetoacoustic waves in a curved coronal loop is presented. The loop is modeled as a semicircular magnetic slab in the zero plasmabeta limit. The governing equations for linear waves are derived. We show that the wave mode behaviour depends on the slope of the equilibrium density profile, which is modeled as a piecewise continuous power law curve of index α For all profiles, except for α 4, wave modes are not trapped in the loop and leak out into the external medium through wave tunneling. The behaviour of the density perturbation depends on α which provides a seismological tool to infer information about the equilibrium density profile from observational signatures of vertically polarised fast magnetoacoustic kink oscillations.
The particular case of α 4, which corresponds to a linearly increasing Alfvén speed profile, is examined in more detail as this is the only model that can support trapped wave modes. We compare the results with a straight slab model and find similar behaviour. Coupling between sausage and kink wave modes has not been found in the model.
Authors: Verwichte, E., Foullon, C. and Nakariakov, V.M.
Projects:

Publication Status: A&A 446, 11391149
Last Modified: 20060301 10:17




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