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Quasi-periodic pulsations in solar and stellar flares: an overview of recent results  

Tom Van Doorsselaere   Submitted: 2016-09-09 02:33

Quasi-periodic pulsations (or QPPs) are periodic intensity variations in the flare emission, across all wavelength bands. In this paper, we review the observational and modelling achievements since the previous review on this topic by Nakariakov & Melnikov (2009). In recent years, it has become clear that QPPs are an inherent feature of solar flares, because almost all flares exhibit QPPs. Moreover, it is now firmly established that QPPs often show multiple periods. We also review possible mechanisms for generating QPPs. Up to now, it has not been possible to conclusively identify the triggering mechanism or cause of QPPs. The lack of this identification currently hampers possible seismological inferences of flare plasma parameters. QPPs in stellar flares have been detected for a long time, and the high quality data of the Kepler mission allows to study the QPP more systematically. However, it has not been conclusively shown whether the time scales of stellar QPPs are different or the same as those in solar flares.

Authors: Tom Van Doorsselaere, Elena G. Kupriyanova, Ding Yuan
Projects: None

Publication Status: Solar Physics, accepted
Last Modified: 2016-09-10 13:40
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Perturbations of gyrosynchrotron emission polarization from solar flares by sausage modes: forward modelling  

Tom Van Doorsselaere   Submitted: 2015-01-27 08:57

We examined the polarization of the microwave flaring emission and its modulation by the fast sausage standing wave using a linear 3D magnetohydrodynamic model of a plasma cylinder. We analized the effects of the line-of-sight angle on the perturbations of the gyrosynchrotron intensity for two models: a base model with strong Razin suppression and a low-density model in which the Razin effect was negligible. The circular polarization (Stokes V) oscillation is in phase with the intensity oscillation, and the polarization degree (Stokes V/I) oscillates in phase with the magnetic field at the examined frequencies in both models. The two quantities experience a periodical reversal of their signs with period equal to a half of the sausage wave period when seen at a 90 degree viewing angle, in this case, their modulation depth reaches 100%.

Authors: Reznikova V.E., Van Doorsselaere T., and Kuznetsov A.A.
Projects: STEREO

Publication Status: accepted
Last Modified: 2015-02-25 12:40
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Perturbations of gyrosynchrotron emission polarization from solar flares by sausage modes: forward modelling  

Tom Van Doorsselaere   Submitted: 2015-01-27 08:57

We examined the polarization of the microwave flaring emission and its modulation by the fast sausage standing wave using a linear 3D magnetohydrodynamic model of a plasma cylinder. We analized the effects of the line-of-sight angle on the perturbations of the gyrosynchrotron intensity for two models: a base model with strong Razin suppression and a low-density model in which the Razin effect was negligible. The circular polarization (Stokes V) oscillation is in phase with the intensity oscillation, and the polarization degree (Stokes V/I) oscillates in phase with the magnetic field at the examined frequencies in both models. The two quantities experience a periodical reversal of their signs with period equal to a half of the sausage wave period when seen at a 90 degree viewing angle, in this case, their modulation depth reaches 100%.

Authors: Reznikova V.E., Van Doorsselaere T., and Kuznetsov A.A.
Projects: None

Publication Status: accepted
Last Modified: 2015-01-28 12:48
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Energy propagation by transverse waves in multiple flux tube systems using filling factors  

Tom Van Doorsselaere   Submitted: 2014-10-10 02:25

In the last few years, it has been found that transverse waves are present at all times in coronal loops or spicules. Their energy has been estimated with an expression derived for bulk Alfvén waves in homogeneous media, with correspondingly uniform wave energy density and flux. The kink mode, however, is localized in space with the energy density and flux dependent on the position in the cross-sectional plane. The more relevant quantities for the kink mode are the integrals of the energy density and flux over the cross-sectional plane. The present paper provides an approximation to the energy propagated by kink modes in an ensemble of flux tubes by means of combining the analysis of single flux tube kink oscillations with a filling factor for the tube cross-sectional area. This finally allows one to compare the expressions for energy flux of Alfvén waves with an ensemble of kink waves. We find that the correction factor for the energy in kink waves, compared to the bulk Alfvén waves, is between f and 2f, where f is the density filling factor of the ensemble of flux tubes.

Authors: Van Doorsselaere T., Gijsen S.E., Andries J., and Verth G.
Projects: None

Publication Status: Published
Last Modified: 2014-10-10 10:24
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LYRA observations of two oscillation modes in a single flare  

Tom Van Doorsselaere   Submitted: 2011-08-23 06:43

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Van Doorsselaere T., De Groof A., Zender J., Berghmans D., and Goossens M.
Projects: Hinode/XRT, PROBA2/LYRA

Publication Status: ApJ (accepted)
Last Modified: 2011-08-23 08:55
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The first measurement of the adiabatic index in the solar corona using time-dependent spectroscopy of Hinode/EIS observations  

Tom Van Doorsselaere   Submitted: 2011-06-24 07:53

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Van Doorsselaere T., Wardle N., Del Zanna G., Jansari K., Verwichte E., and Nakariakov V.M.
Projects: Hinode/EIS

Publication Status: published
Last Modified: 2011-06-24 11:54
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Detection of three periodicities in a single oscillating coronal loop  

Tom Van Doorsselaere   Submitted: 2010-06-28 04:21

Context. TRACE 171 Å observations of transverse loop oscillations on May 13, 2001 have been studied by De Moortel & Brady (2007). They found hints of 3 periodicities present in this event. Aims. The aim of this article is to improve the analysis, and measure the periodicities more accurately in order to do coronal seismology. Methods. We create multiple (x, t)-slices across the loop, and determine the oscillatory parameters within those slices. We then use statistical methods to reduce the errors on the measured oscillations parameters. Results. We find two populations with a clearly distinct period. The population at the loop top has a period of 895 ? 2 s, while the population in the loop leg has a period of 452 ? 1 s. The fact that the ratio of these two periods is roughly 2 and the geometry of the problem suggests that two harmonics are observed. In the residues, we confirm the presence of a 3rd harmonic with period 309 ? 15 s using two separate data analysis techniques. The three periods are then used to do a seismological estimate of the loop expansion and the density scale height.

Authors: Van Doorsselaere T., Birtill D.C.C., and Evans G.R.
Projects: TRACE

Publication Status: A&A, 2009, vol. 508, pp. 1485
Last Modified: 2010-06-28 09:15
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The effect of loop curvature on coronal loop kink oscillations  

Tom Van Doorsselaere   Submitted: 2009-05-08 08:12

We will review analytical and numerical efforts in modelling the influence of curvature on coronal loop oscillations. We will mainly focus our attention on fast kink mode oscillations.<br> A curved slab model will be presented, where it becomes clear that curvature introduces wave leakage into the system, because of changes in the equilibrium. The importance of leakage will be assessed through the use of a slab and cylindrical model where lateral leakage is allowed. A full analytical model for a semi-toroidal loop will be constructed for a system with no leaking waves but with an inhomogeneous layer that introduces damping due to the process of resonant absorption. The model for a semi-toroidal loop will be extended to also include leakage, and will be studied numerically. The numerical results will be compared to the analytical model.

Authors: T. Van Doorsselaere, E. Verwichte, J. Terradas
Projects: None

Publication Status: Spac. Sci. Reviews, accepted
Last Modified: 2009-05-08 08:43
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Seismological demonstration of perpendicular density structuringin the solar corona  

Tom Van Doorsselaere   Submitted: 2008-09-22 07:19

The peculiarities of the propagating transverse waves observed in the solar corona with the Coronal Multi-channel Polarimeter (CoMP) indicates the existence of fine field structuring in coronal density. We present results of numerical simulations studying the evolution of a localised transverse magneto-hydrodynamic wave in a uniform magnetic field. We consider two initial low plasma-beta equilibria: one with a homogeneous density, and one with a field-aligned dense structure (such as a loop or a plume). The perpendicular localisation of the wave determines strongly the angular distribution of the energy propagation. If the perpendicular scale of the wave is significantly smaller than the parallel scale (e.g. wavelength), as established by CoMP, the wave develops as an oblique fast magneto-acoustic wave. In an unstructured medium, the energy of such a wave is transferred mainly across the magnetic field. However, it is possible to channel energy of the transverse wave along the magnetic field in the presence of a field aligned density enhancement. We conclude that the CoMP results provide an independent seismological proof that the corona is structured in density in the perpendicular direction.

Authors: Van Doorsselaere, T., Brady, C.S., Verwichte, E., Nakariakov, V.M.
Projects: None

Publication Status: A&A Letters, accepted
Last Modified: 2008-09-23 18:09
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Coronal magnetic field measurement using loop oscillations observed by Hinode/EIS  

Tom Van Doorsselaere   Submitted: 2008-06-12 09:29

We report the first spectroscopic detection of a kink MHD oscillation of a solar coronal structure, made with the Extreme-Ultraviolet Imaging Spectrometer (EIS) on the recently launched Japanese Hinode satellite. The detected oscillation has an amplitude of 1 km s-1 in the Doppler shift of the FeXII 195Å spectral line (1.3 MK), and a period of 296 s. The unique combination of EIS's spectroscopic and imaging abilities allows us to simultaneously measure the mass density and length of oscillating loop. It allows us to directly measure the magnitude of the local magnetic field, the fundamental coronal plasma parameter, as 39±8 G, with unprecedented accuracy. This proof-of-concept makes EIS an exclusive instrument for the full scale implementation of the MHD coronal seismological technique.

Authors: T. Van Doorsselaere, V.M. Nakariakov, P.R. Young, E. Verwichte
Projects: Hinode/EIS

Publication Status: A&A Letters, accepted
Last Modified: 2008-09-23 20:59
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Subject will be restored when possible  

Tom Van Doorsselaere   Submitted: 2008-05-16 08:58

Context. Collective oscillations of two or more coronal magnetic loops are observed very often.<br> Aims. We study the eigenmodes of oscillations of a system consisting of two parallel magnetic loops.<br> Methods. The linearised MHD equations for a cold plasma are solved analytically in bicylindrical coordinates using the long-wavelength approximation. A dispersion equation determining the frequencies of eigenmodes is derived and solved analytically.<br> Results. Two solutions of the dispersion relation were found. The higher frequency corresponds to the antisymmetric mode polarised in the direction parallel to the line connecting the loop centres, and the symmetric mode polarised in the perpendicular direction. Depending on the polarisation of modes corresponding to the lower frequency, the systems of two parallel loops are classified as standard and anomalous. In standard systems the lower frequency corresponds to the symmetric mode polarised in the direction parallel to the line connecting the loop centres, and the antisymmetric mode polarised in the perpendicular direction. In anomalous systems the lower frequency corresponds to the antisymmetric mode polarised in the direction parallel to the line connecting the loop centres, and the symmetric mode polarised in the perpendicular direction. The limiting case of two identical loops is studied. The results for this case are compared with recent numerical results.

Authors: T. Van Doorsselaere, M.S. Ruderman, D. Robertson
Projects: None

Publication Status: accepted
Last Modified: 2008-09-23 21:01
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Subject will be restored when possible  

Tom Van Doorsselaere   Submitted: 2008-05-16 08:58

Collective oscillations of two or more coronal magnetic loops are observed very often. We study the eigenmodes of oscillations of a system consisting of two parallel magnetic loops. The linearised MHD equations for a cold plasma are solved analytically in bicylindrical coordinates using the long-wavelength approximation. A dispersion equation determining the frequencies of eigenmodes is derived and solved analytically. Two solutions of the dispersion relation were found. The higher frequency corresponds to the antisymmetric mode polarised in the direction parallel to the line connecting the loop centres, and the symmetric mode polarised in the perpendicular direction. Depending on the polarisation of modes corresponding to the lower frequency, the systems of two parallel loops are classified as standard and anomalous. In standard systems the lower frequency corresponds to the symmetric mode polarised in the direction parallel to the line connecting the loop centres, and the antisymmetric mode polarised in the perpendicular direction. In anomalous systems the lower frequency corresponds to the antisymmetric mode polarised in the direction parallel to the line connecting the loop centres, and the symmetric mode polarised in the perpendicular direction. The limiting case of two identical loops is studied. The results for this case are compared with recent numerical results.

Authors: T. Van Doorsselaere, M.S. Ruderman, D. Robertson
Projects: None

Publication Status: accepted
Last Modified: 2008-09-23 21:01
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Subject will be restored when possible  

Tom Van Doorsselaere   Submitted: 2008-05-16 08:57

Context. Collective oscillations of two or more coronal magnetic loops are observed very often. Aims. We study the eigenmodes of oscillations of a system consisting of two parallel magnetic loops. Methods. The linearised MHD equations for a cold plasma are solved analytically in bicylindrical coordinates using the long-wavelength approximation. A dispersion equation determining the frequencies of eigenmodes is derived and solved analytically. Results. Two solutions of the dispersion relation were found. The higher frequency corresponds to the antisymmetric mode polarised in the direction parallel to the line connecting the loop centres, and the symmetric mode polarised in the perpendicular direction. Depending on the polarisation of modes corresponding to the lower frequency, the systems of two parallel loops are classified as standard and anomalous. In standard systems the lower frequency corresponds to the symmetric mode polarised in the direction parallel to the line connecting the loop centres, and the antisymmetric mode polarised in the perpendicular direction. In anomalous systems the lower frequency corresponds to the antisymmetric mode polarised in the direction parallel to the line connecting the loop centres, and the symmetric mode polarised in the perpendicular direction. The limiting case of two identical loops is studied. The results for this case are compared with recent numerical results.

Authors: T. Van Doorsselaere, M.S. Ruderman, D. Robertson
Projects: None

Publication Status: accepted
Last Modified: 2008-09-23 21:01
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Subject will be restored when possible  

Tom Van Doorsselaere   Submitted: 2008-02-20 10:38

Recently, the omnipresence of waves has been discovered in the corona using the CoMP instrument. We demonstrate that the observational findings can be explained in terms of guided kink magnetoacoustic modes. The interpretation of the observations in terms of Alfvén waves is shown to be inconsistent with MHD wave theory. The implications of the interpretation in terms of kink waves are discussed.

Authors: Van Doorsselaere, T., Nakariakov, V. M., Verwichte E.
Projects: None

Publication Status: ApJ Letters (accepted)
Last Modified: 2008-02-20 10:40
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Coronal loop seismology using multiple transverse loop oscillation harmonics  

Tom Van Doorsselaere   Submitted: 2007-08-16 07:02

Context. TRACE observations (23/11/1998 06:35:57-06:48:43UT) in the 171 Å bandpass of an active region are studied. Coronal loop oscillations are observed after a violent disruption of the equilibrium. Aims. The oscillation properties are studied to give seismological estimates of physical quantities, such as the density scale height. Methods. A loop segment is traced during the oscillation, and the resulting time series is analysed for periodicities. Results. In the loop segment displacement, two periods are found: 435.6 ? 4.5 s and 242.7 ? 6.4 s, consistent with the periods of the fundamental and 2nd harmonic fast kink oscillation. The small uncertainties allow us to estimate the density scale height in the loop to be 109 Mm, which is about double the estimated hydrostatical value of 50 Mm. Because a loop segment is traced, the amplitude dependence along the loop is found for each of these oscillations. The obtained spatial information is used as a seismological tool to give details about the geometry of the observed loop.

Authors: T. Van Doorsselaere, V.M. Nakariakov, and E. Verwichte
Projects: TRACE

Publication Status: A&A (accepted)
Last Modified: 2007-08-16 08:36
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Observational evidence favors a resistive wave heating mechanism for coronal loops over a viscous phenomenon  

Tom Van Doorsselaere   Submitted: 2007-08-16 06:54

Context. How coronal loops are heated to their observed temperatures is the subject of a long standing debate. Aims. Observational evidence exists that the heating in coronal loops mainly occurs near the loop footpoints. In this article, analytically and numerically obtained heating profiles produced by resonantly damped waves are compared to the observationally estimated profiles. Methods. To do that, the predicted heating profiles are fitted with an exponential heating function, which was also used to fit the observations. The results of both fits, the estimated heating scale heights, are compared to determine the viability of resonant absorption as a heating mechanism for coronal loops. Results. Two results are obtained. It is shown that any wave heating mechanism (i.e. not just resonant absorption) should be dominated by a resistive (and not a viscous) phenomenon in order to accomodate the constraint of footpoint heating. Additionally it is demonstrated that the analytically and numerically estimated heating scale heights for the resonant absorption damping mechanism fit the observations very well.

Authors: T. Van Doorsselaere, J. Andries, and S. Poedts
Projects: None

Publication Status: A&A (published)
Last Modified: 2007-08-16 08:37
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Abstracts by Author
Quasi-periodic pulsations in solar and stellar flares: an overview of recent results
Perturbations of gyrosynchrotron emission polarization from solar flares by sausage modes: forward modelling
Perturbations of gyrosynchrotron emission polarization from solar flares by sausage modes: forward modelling
Energy propagation by transverse waves in multiple flux tube systems using filling factors
LYRA observations of two oscillation modes in a single flare
The first measurement of the adiabatic index in the solar corona using time-dependent spectroscopy of Hinode/EIS observations
Detection of three periodicities in a single oscillating coronal loop
The effect of loop curvature on coronal loop kink oscillations
Seismological demonstration of perpendicular density structuring in the solar corona
Coronal magnetic field measurement using loop oscillations observed by Hinode/EIS
Subject will be restored when possible
Subject will be restored when possible
Subject will be restored when possible
Subject will be restored when possible
Coronal loop seismology using multiple transverse loop oscillation harmonics
Observational evidence favors a resistive wave heating mechanism for coronal loops over a viscous phenomenon

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