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Marcel Goossens - One of the best experts on this subject based on the ideXlab platform.
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spatial magneto seismology effect of Density Stratification on the first harmonic amplitude profile of transversal coronal loop oscillations
Astronomy and Astrophysics, 2007Co-Authors: G Verth, R Erdelyi, T Van Doorsselaere, Marcel GoossensAbstract:Context. The new generation of extreme-ultraviolet (EUV) imagers onboard missions such as the Solar Dynamics Observatory (SDO) and Solar Orbiter (SO) will provide the most accurate spatial measurements of post-flare coronal loop oscillations yet. The amplitude profiles of these loop oscillations contain important information about plasma fine structure in the corona. Aims. We show that the position of the anti-nodes of the amplitude profile of the first harmonic of the standing fast kink wave of a coronal loop relate to the plasma Density Stratification of that loop. Methods. The MHD kink transversal waves of coronal loops are modelled both numerically and analytically. The numerical model implements the implicit finite element code pollux. Dispersion relations are derived and solved analytically. The results of the two methods are compared and verified. Results. Density Stratification causes the anti-nodes of the first harmonic to shift towards the loop footpoints. The greater the Density Stratification, the larger the shift. The anti-node shift of the first harmonic of a semi-circular coronal loop with a Density scale height H = 50 Mm and loop half length L = 100 Mm is approximately 5.6 Mm. Shifts in the Mm range are measureable quantities providing valuable information about the subresolution structure of coronal loops. Conclusions. The measurement of the anti-node shift of the first harmonic of the standing fast kink wave of coronal loops is potentially a new tool in the field of solar magneto-seismology, providing a novel complementary method of probing plasma fine structure in the corona.
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determination of the coronal Density Stratification from the observation of harmonic coronal loop oscillations
The Astrophysical Journal, 2005Co-Authors: I Arregui, Jesse Andries, Marcel GoossensAbstract:The recent detection of multiple harmonic standing transverse oscillations in coronal loops by Verwichte et al. is of special importance, as it allows one to obtain information on the longitudinal Density variation in loops. Verwichte et al. detected the simultaneous presence of both the fundamental and the first-overtone mode in two coronal loops. Here we point out that the ratio of the period of the fundamental mode to the period of the overtone mode differs from 2 in loops with longitudinal Density Stratification. Conversely, the difference between this ratio and 2 can be used as a seismological tool to obtain information about the Density scale height in loops.
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DYNAMICS OF CORONAL LOOP OSCILLATIONS
2005Co-Authors: T. Van Doorsselaere, Marcel Goossens, Jesse Andries, I Arregui, Stefaan PoedtsAbstract:We will discuss the observed, heavily damped transversal oscillations of coronal loops. These oscillations are often modeled as transversal kink oscillations in a cylinder. Several features are added to the classical cylindrical model. In our models we include loop curvature, longitudinal Density Stratification and highly inhomogeneous radial Density profiles. We investigate the effect of vertical Density Stratification both numerically and analytically. We can conclude that longitudinal Stratification establishes a coupling between different longitudinal mode numbers. On the other hand, the observational parameter damping =Period is not influenced. However, since the ratio of periods of different longitudinal numbers is different from 2, we can estimate the Density Stratification in a loop if two longitudinal mode numbers are observed. We can conclude that the inclusion of vertical Density Stratification provides an excellent tool for coronal seismology. Additionally, we analyze a theoretical model for curved coronal loops. Because of the curvature, poloidal mode numbers are coupled. In the small curvature limit, however, only coupling to the two neighbouring poloidal mode numbers occurs. Expressions for the (complex) frequency are obtained.
G Verth - One of the best experts on this subject based on the ideXlab platform.
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spatial magneto seismology effect of Density Stratification on the first harmonic amplitude profile of transversal coronal loop oscillations
Astronomy and Astrophysics, 2007Co-Authors: G Verth, R Erdelyi, T Van Doorsselaere, Marcel GoossensAbstract:Context. The new generation of extreme-ultraviolet (EUV) imagers onboard missions such as the Solar Dynamics Observatory (SDO) and Solar Orbiter (SO) will provide the most accurate spatial measurements of post-flare coronal loop oscillations yet. The amplitude profiles of these loop oscillations contain important information about plasma fine structure in the corona. Aims. We show that the position of the anti-nodes of the amplitude profile of the first harmonic of the standing fast kink wave of a coronal loop relate to the plasma Density Stratification of that loop. Methods. The MHD kink transversal waves of coronal loops are modelled both numerically and analytically. The numerical model implements the implicit finite element code pollux. Dispersion relations are derived and solved analytically. The results of the two methods are compared and verified. Results. Density Stratification causes the anti-nodes of the first harmonic to shift towards the loop footpoints. The greater the Density Stratification, the larger the shift. The anti-node shift of the first harmonic of a semi-circular coronal loop with a Density scale height H = 50 Mm and loop half length L = 100 Mm is approximately 5.6 Mm. Shifts in the Mm range are measureable quantities providing valuable information about the subresolution structure of coronal loops. Conclusions. The measurement of the anti-node shift of the first harmonic of the standing fast kink wave of coronal loops is potentially a new tool in the field of solar magneto-seismology, providing a novel complementary method of probing plasma fine structure in the corona.
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the effect of Density Stratification on the amplitude profile of transversal coronal loop oscillations
Astronomy and Astrophysics, 2007Co-Authors: R Erdelyi, G VerthAbstract:Context. Measuring how the Density of the plasma in the Sun's corona varies along fine structures such as coronal loops using emission measure is very difficult as the exact value of the line of sight filling factor and the correct amount of background subtraction are unknown. Aims. To investigate if magnetohydrodynamic (MHD) wave theory can be used to complement existing observational methods to obtain diagnostic information about the Density structure of coronal loops by measuring the amplitude profile of the fundamental standing mode of the fast kink wave. Methods. Analytical and numerical approaches are used to show how the amplitude profile of the fundamental fast kink mode changes by varying the amount of Density Stratification in a coronal loop. Results. Increasing the amount of Density Stratification in a coronal loop will increase the difference in amplitude profile from one of constant longitudinal Density. For a semi-circular TRACE 171 A temperature loop of length 200 Mm and Density scale height, H = 50 Mm, the maximum change in amplitude profile is of the order of 50 km. It is also found that the amplitudes of these oscillations are effectively negligible at altitudes lower than the transition region. Conclusions. The effect of Density Stratification on the amplitude profile of the fundamental standing kink mode of a TRACE 171 A temperature loop may be very subtle. Unfortunately, the observational signatures of this effect are likely to be well below the resolution of the best currently available extreme-ultraviolet (EUV) imagers. Only loops with homogeneous magnetic fields have been addressed here, but inhomogeneous magnetic fields may well dominate over the effect of Density Stratification on the amplitude profile.
Anirban Guha - One of the best experts on this subject based on the ideXlab platform.
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an inverse technique for reconstructing ocean s Density Stratification from surface data
Ocean Modelling, 2020Co-Authors: Subhajit Kar, Anirban GuhaAbstract:Abstract In this article, we propose an inverse technique that accurately reconstructs the ocean’s Density Stratification profile simply from free surface elevation data. Satellite observations suggest that ocean surface contains the signature of internal tides, which are internal gravity waves generated by the barotropic tides. Since internal tides contain the information of ocean’s Density Stratification, the latter can in principle be reconstructed from the free surface signature. We consider a simple theoretical model that approximates a continuously stratified ocean as discrete layers of constant buoyancy frequency; this facilitates the derivation of a closed-form dispersion relation. First, we numerically simulate internal tide generation for toy ocean scenarios and subsequently perform Space–Time Fourier Transform (STFT) of the free surface, which yields internal tide spectra with wavenumbers corresponding to the tidal frequency. The Density profile is reconstructed by substituting these wavenumbers into the dispersion relation. Finally, we consider a more realistic situation with rotation, bottom topography, shear and Density profiles representative of the Strait of Gibraltar. Density reconstruction in the presence and absence of shear is respectively found to be 90.2% and 94.2% accurate. The proposed method can be used to reconstruct climatological mean ocean Density field of uniform spatial resolution using only surface elevation data obtained via satellite altimetry.
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An inverse technique for reconstructing ocean’s Density Stratification from surface data
Ocean Modelling, 2020Co-Authors: Subhajit Kar, Anirban GuhaAbstract:Abstract In this article, we propose an inverse technique that accurately reconstructs the ocean’s Density Stratification profile simply from free surface elevation data. Satellite observations suggest that ocean surface contains the signature of internal tides, which are internal gravity waves generated by the barotropic tides. Since internal tides contain the information of ocean’s Density Stratification, the latter can in principle be reconstructed from the free surface signature. We consider a simple theoretical model that approximates a continuously stratified ocean as discrete layers of constant buoyancy frequency; this facilitates the derivation of a closed-form dispersion relation. First, we numerically simulate internal tide generation for toy ocean scenarios and subsequently perform Space–Time Fourier Transform (STFT) of the free surface, which yields internal tide spectra with wavenumbers corresponding to the tidal frequency. The Density profile is reconstructed by substituting these wavenumbers into the dispersion relation. Finally, we consider a more realistic situation with rotation, bottom topography, shear and Density profiles representative of the Strait of Gibraltar. Density reconstruction in the presence and absence of shear is respectively found to be 90.2% and 94.2% accurate. The proposed method can be used to reconstruct climatological mean ocean Density field of uniform spatial resolution using only surface elevation data obtained via satellite altimetry.
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Estimating ocean's Density Stratification from surface data
arXiv: Fluid Dynamics, 2018Co-Authors: Subhajit Kar, Anirban GuhaAbstract:Here we propose a semi-analytical technique that accurately reconstructs ocean's Density Stratification profile, and hence, the pycnocline depth, simply from the free surface elevation data. The ocean surface contains the signature of internal gravity waves (IGWs), which are generated when stably stratified ocean water is forced to move back and forth over submarine topography by barotropic tides. Since IGWs, in turn, contain the information of ocean's Density Stratification, the latter can in principle be reconstructed from the free surface signature. We consider a simple theoretical model that approximates a continuously stratified ocean as discrete layers of constant buoyancy frequency; this facilitates the derivation of a closed-form dispersion relation. First, we numerically simulate IGW generation for toy ocean scenarios and subsequently perform space-time Fourier transform (STFT) of the free surface. Free surface STFT yields IGW spectra which has wavenumbers corresponding to the tidal frequency. Density profile reconstruction has been performed by substituting these IGW wavenumbers into the dispersion relation. Next we consider a more realistic situation in which rotation is included; moreover the bottom topography, Density and shear profiles are representative of the Strait of Gibraltar. While Density reconstruction is 94.4% accurate in the absence of shear, the accuracy decreases to 90.2% when moderate shear is present. Since shear causes loss of coherence in IGW beams, Density reconstruction can be adversely affected in those regions of the global ocean where shear is very strong. Finally, using wavenumbers obtained from multi-satellite altimetry data near the Hawaiian Ridge, we have reconstructed actual Density profile with 94.4% accuracy. Since shear is either low or moderate in most parts of the global ocean, we expect our proposed technique to be broadly applicable.
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An inverse technique for reconstructing ocean's Density Stratification from surface data.
arXiv: Fluid Dynamics, 2018Co-Authors: Subhajit Kar, Anirban GuhaAbstract:In this article, we propose an inverse technique that accurately reconstructs the ocean's Density Stratification profile simply from free surface elevation data. Satellite observations suggest that ocean surface contains the signature of internal tides, which are internal gravity waves generated by the barotropic tides. Since internal tides contain the information of ocean's Density Stratification, the latter can in principle be reconstructed from the free surface signature. We consider a simple theoretical model that approximates a continuously stratified ocean as discrete layers of constant buoyancy frequency; this facilitates the derivation of a closed-form dispersion relation. First, we numerically simulate internal tide generation for toy ocean scenarios and subsequently perform Space-Time Fourier Transform (STFT) of the free surface, which yields internal tide spectra with wavenumbers corresponding to the tidal frequency. The Density profile is reconstructed by substituting these wavenumbers into the dispersion relation. Finally, we consider a more realistic situation with rotation, bottom topography, shear and Density profiles representative of the Strait of Gibraltar. Density reconstruction in the presence and absence of shear are respectively found to be $90.2\%$ and $94.2\%$ accurate.
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Large Density Stratification stabilizes Rayleigh-Taylor instability in presence of shear
arXiv: Fluid Dynamics, 2017Co-Authors: Raunak Raj, Anirban GuhaAbstract:This letter investigates the effect of shear on Rayleigh-Taylor instability (RTI). Even simple uniform shear strongly influences the instability; longer waves are completely stabilized when Density Stratification is large (higher Atwood numbers). This apparently counter-intuitive result is due to the presence of Atwood number in the shear term. When the unstable Density interface is embedded in a shear layer, shear is again found to stabilize the RTI. However, this configuration introduces additional unstable shear instability modes in the lower wavenumber regime. A new type of shear instability, whose growth rate increases with Atwood number, plays a dominant role, while Kelvin-Helmholtz instability (KHI), which was previously understood to be the only possible shear instability in this context, has little significance. Hence the billows observed in the nonlinear stages of RTI, which are usually attributed to KHI, may actually be the nonlinear manifestation of this new instability.
Jesse Andries - One of the best experts on this subject based on the ideXlab platform.
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determination of the coronal Density Stratification from the observation of harmonic coronal loop oscillations
The Astrophysical Journal, 2005Co-Authors: I Arregui, Jesse Andries, Marcel GoossensAbstract:The recent detection of multiple harmonic standing transverse oscillations in coronal loops by Verwichte et al. is of special importance, as it allows one to obtain information on the longitudinal Density variation in loops. Verwichte et al. detected the simultaneous presence of both the fundamental and the first-overtone mode in two coronal loops. Here we point out that the ratio of the period of the fundamental mode to the period of the overtone mode differs from 2 in loops with longitudinal Density Stratification. Conversely, the difference between this ratio and 2 can be used as a seismological tool to obtain information about the Density scale height in loops.
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DYNAMICS OF CORONAL LOOP OSCILLATIONS
2005Co-Authors: T. Van Doorsselaere, Marcel Goossens, Jesse Andries, I Arregui, Stefaan PoedtsAbstract:We will discuss the observed, heavily damped transversal oscillations of coronal loops. These oscillations are often modeled as transversal kink oscillations in a cylinder. Several features are added to the classical cylindrical model. In our models we include loop curvature, longitudinal Density Stratification and highly inhomogeneous radial Density profiles. We investigate the effect of vertical Density Stratification both numerically and analytically. We can conclude that longitudinal Stratification establishes a coupling between different longitudinal mode numbers. On the other hand, the observational parameter damping =Period is not influenced. However, since the ratio of periods of different longitudinal numbers is different from 2, we can estimate the Density Stratification in a loop if two longitudinal mode numbers are observed. We can conclude that the inclusion of vertical Density Stratification provides an excellent tool for coronal seismology. Additionally, we analyze a theoretical model for curved coronal loops. Because of the curvature, poloidal mode numbers are coupled. In the small curvature limit, however, only coupling to the two neighbouring poloidal mode numbers occurs. Expressions for the (complex) frequency are obtained.
R Erdelyi - One of the best experts on this subject based on the ideXlab platform.
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spatial magneto seismology effect of Density Stratification on the first harmonic amplitude profile of transversal coronal loop oscillations
Astronomy and Astrophysics, 2007Co-Authors: G Verth, R Erdelyi, T Van Doorsselaere, Marcel GoossensAbstract:Context. The new generation of extreme-ultraviolet (EUV) imagers onboard missions such as the Solar Dynamics Observatory (SDO) and Solar Orbiter (SO) will provide the most accurate spatial measurements of post-flare coronal loop oscillations yet. The amplitude profiles of these loop oscillations contain important information about plasma fine structure in the corona. Aims. We show that the position of the anti-nodes of the amplitude profile of the first harmonic of the standing fast kink wave of a coronal loop relate to the plasma Density Stratification of that loop. Methods. The MHD kink transversal waves of coronal loops are modelled both numerically and analytically. The numerical model implements the implicit finite element code pollux. Dispersion relations are derived and solved analytically. The results of the two methods are compared and verified. Results. Density Stratification causes the anti-nodes of the first harmonic to shift towards the loop footpoints. The greater the Density Stratification, the larger the shift. The anti-node shift of the first harmonic of a semi-circular coronal loop with a Density scale height H = 50 Mm and loop half length L = 100 Mm is approximately 5.6 Mm. Shifts in the Mm range are measureable quantities providing valuable information about the subresolution structure of coronal loops. Conclusions. The measurement of the anti-node shift of the first harmonic of the standing fast kink wave of coronal loops is potentially a new tool in the field of solar magneto-seismology, providing a novel complementary method of probing plasma fine structure in the corona.
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the effect of Density Stratification on the amplitude profile of transversal coronal loop oscillations
Astronomy and Astrophysics, 2007Co-Authors: R Erdelyi, G VerthAbstract:Context. Measuring how the Density of the plasma in the Sun's corona varies along fine structures such as coronal loops using emission measure is very difficult as the exact value of the line of sight filling factor and the correct amount of background subtraction are unknown. Aims. To investigate if magnetohydrodynamic (MHD) wave theory can be used to complement existing observational methods to obtain diagnostic information about the Density structure of coronal loops by measuring the amplitude profile of the fundamental standing mode of the fast kink wave. Methods. Analytical and numerical approaches are used to show how the amplitude profile of the fundamental fast kink mode changes by varying the amount of Density Stratification in a coronal loop. Results. Increasing the amount of Density Stratification in a coronal loop will increase the difference in amplitude profile from one of constant longitudinal Density. For a semi-circular TRACE 171 A temperature loop of length 200 Mm and Density scale height, H = 50 Mm, the maximum change in amplitude profile is of the order of 50 km. It is also found that the amplitudes of these oscillations are effectively negligible at altitudes lower than the transition region. Conclusions. The effect of Density Stratification on the amplitude profile of the fundamental standing kink mode of a TRACE 171 A temperature loop may be very subtle. Unfortunately, the observational signatures of this effect are likely to be well below the resolution of the best currently available extreme-ultraviolet (EUV) imagers. Only loops with homogeneous magnetic fields have been addressed here, but inhomogeneous magnetic fields may well dominate over the effect of Density Stratification on the amplitude profile.