324 research outputs found
Magneto-seismology of solar atmospheric loops by means of longitudinal oscillations
There is increasingly strong observational evidence that slow magnetoacoustic
modes arise in the solar atmosphere. Solar magneto-seismology is a novel tool
to derive otherwise directly un-measurable properties of the solar atmosphere
when magnetohydrodynamic (MHD) wave theory is compared to wave observations.
Here, MHD wave theory is further developed illustrating how information about
the magnetic and density structure along coronal loops can be determined by
measuring the frequencies of the slow MHD oscillations. The application to
observations of slow magnetoacoustic waves in coronal loops is discused.Comment: 4 pages, 2 figures, to appear in Proceedings of IAU Symp 286,
Comparative Magnetic Minima, C. H. Mandrini, ed
Magneto-seismology: effect of inhomogeneous magnetic field on transversal coronal loop oscillations
The extreme-ultraviolet (EUV) imagers onboard the planned Solar Dynamics Observatory (SDO) and Solar Orbiter (SO) will offer us the best chance yet of using observations of post-flare loop oscillations to probe the fine structure of the corona. Recently developed magnetohydrodynamic (MHD) wave theory has shown that the properties of loop oscillations depend on their plasma fine structure. Up to this point, many studies have concentrated solely on the effect of plasma density stratification on coronal loop oscillations. In this paper we develop MHD wave theory which models the effect of an inhomogeneous magnetic field on coronal loop oscillations. The results have the potential to be used in testing the efficacy of photospheric magnetic field extrapolations and have important implications regarding magneto-seismology of the corona
MHD wave modes resolved in fine-scale chromospheric magnetic structures
Within the last decade, due to significant improvements in the spatial and
temporal resolution of chromospheric data, magnetohydrodynamic (MHD) wave
studies in this fascinating region of the Sun's atmosphere have risen to the
forefront of solar physics research. In this review we begin by reviewing the
challenges and debates that have manifested in relation to MHD wave mode
identification in fine-scale chromospheric magnetic structures, including
spicules, fibrils and mottles. Next we go on to discuss how the process of
accurately identifying MHD wave modes also has a crucial role to play in
estimating their wave energy flux. This is of cardinal importance for
estimating what the possible contribution of MHD waves is to solar atmospheric
heating. Finally, we detail how such advances in chromospheric MHD wave studies
have also allowed us, for the first time, to implement cutting-edge
magnetoseismological techniques that provide new insight into the
sub-resolution plasma structuring of the lower solar atmosphere.Comment: 16 pages, 6 figures, to appear as a chapter in the upcoming AGU/Wiley
book "Low-frequency Waves in Space Plasmas
Resonantly Damped Propagating Kink Waves in Longitudinally Stratified Solar Waveguides
It has been shown that resonant absorption is a robust physical mechanism to
explain the observed damping of magnetohydrodynamic (MHD) kink waves in the
solar atmosphere due to naturally occurring plasma inhomogeneity in the
direction transverse to the direction of the magnetic field. Theoretical
studies of this damping mechanism were greatly inspired by the first
observations of post-flare standing kink modes in coronal loops using the
Transition Region And Coronal Explorer (TRACE). More recently, these studies
have been extended to explain the attenuation of propagating coronal kink waves
observed by the Coronal Multi-Channel Polarimeter (CoMP). In the present study,
for the first time we investigate the properties of propagating kink waves in
solar waveguides including the effects of both longitudinal and transverse
plasma inhomogeneity. Importantly, it is found that the wavelength is only
dependent on the longitudinal stratification and the amplitude is simply a
product of the two effects. In light of these results the advancement of solar
atmospheric magnetoseismology by exploiting high spatial/temporal resolution
observations of propagating kink waves in magnetic waveguides to determine the
length scales of the plasma inhomogeneity along and transverse to the direction
of the magnetic field is discussed.Comment: Accepted for publication in Ap
Longitudinal oscillations in density stratified and expanding solar waveguides
Waves and oscillations can provide vital information about the internal
structure of waveguides they propagate in. Here, we analytically investigate
the effects of density and magnetic stratification on linear longitudinal
magnetohydrodynamic (MHD) waves. The focus of this paper is to study the
eigenmodes of these oscillations. It is our specific aim is to understand what
happens to these MHD waves generated in flux tubes with non-constant (e.g.,
expanding or magnetic bottle) cross-sectional area and density variations. The
governing equation of the longitudinal mode is derived and solved analytically
and numerically. In particular, the limit of the thin flux tube approximation
is examined. The general solution describing the slow longitudinal MHD waves in
an expanding magnetic flux tube with constant density is found. Longitudinal
MHD waves in density stratified loops with constant magnetic field are also
analyzed. From analytical solutions, the frequency ratio of the first overtone
and fundamental mode is investigated in stratified waveguides. For small
expansion, a linear dependence between the frequency ratio and the expansion
factor is found. From numerical calculations it was found that the frequency
ratio strongly depends on the density profile chosen and, in general, the
numerical results are in agreement with the analytical results. The relevance
of these results for solar magneto-seismology is discussed.Comment: 10 pages, 5 figures, published in ApJ, uses emulateap
The generation and damping of propagating MHD kink waves in the solar atmosphere
The source of the non-thermal energy required for the heating of the upper solar atmosphere to temperatures in excess of a million degrees and the acceleration of the solar wind to hundreds of kilometers per second is still unclear. One such mechanism for providing the required energy flux is incompressible torsional Alfven and kink magnetohydrodynamic (MHD) waves, which are magnetically dominated waves supported by the Sun's pervasive and complex magnetic field. In particular, propagating MHD kink waves have recently been observed to be ubiquitous throughout the solar atmosphere, but, until now, critical details of the transport of the kink wave energy throughout the Sun's atmosphere were lacking. Here, the ubiquity of the waves is exploited for statistical studies in the highly dynamic solar chromosphere. This large-scale investigation allows for the determination of the chromospheric kink wave velocity power spectra, a missing link necessary for determining the energy transport between the photosphere and corona. Crucially, the power spectra contain evidence for horizontal photospheric motions being an important mechanism for kink wave generation in the quiescent Sun. In addition, a comparison with measured coronal power spectra is provided for the first time, revealing frequency-dependent transmission profiles, suggesting that there is enhanced damping of kink waves in the lower corona
Spatial magneto-seismology : effect of density stratification on the first harmonic amplitude profile of transversal coronal loop oscillations
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 5.6Mm. 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
Solar feature tracking in both spatial and temporal domains
A new method for automated coronal loop tracking, in both spatial and temporal
domains, is presented. The reliability of this technique was tested with TRACE 171A observations.
The application of this technique to a flare-induced kink-mode oscillation, revealed a
3500 km spatial periodicity which occur along the loop edge. We establish a reduction in oscillatory
power, for these spatial periodicities, of 45% over a 322 s interval. We relate the reduction
in oscillatory power to the physical damping of these loop-top oscillations
Identication of linear slow sausage waves in magnetic pores
The analysis of an 11-hour series of high resolution white light observations of a large pore in the sunspot group NOAA 7519, observed on 5 June 1993 with the Swedish Vacuum Solar Telescope at La Palma on Canary Islands, has been recently described by Dorotovič et al. (2002). Special attention was paid to the evolution of a filamentary region attached to the pore, to horizontal motions around the pore, and to small-scale morphological changes. One of the results, relevant to out work here, was the determination of temporal area evolution of the studied pore where the area itself showed a linear trend of decrease with time at an average rate of −0.23 Mm2h−1 during the entire observing period. Analysing the time series of the are of the pore, there is strong evidence that coupling between the solar interior and magnetic atmosphere can occur at various scales and that the referred decrease of the area may be connected with a decrease of the magnetic field strength according to the magnetic field-to-size relation. Periods of global acoustic, e.g. p-mode, driven waves are usually in the range of 5–10 minutes, and are favourite candidates for the coupling of interior oscillations with atmospheric dynamics. However, by assuming that magneto-acoustic gravity waves may be there too, and may act as drivers, the observed periodicities (frequencies) are expected to be much longer (smaller), falling well within the mMHz domain. In this work we determine typical periods of such range in the area evolution of the pore using wavelet analysis. The resulted periods are in the range of 20–70 minutes, suggesting that periodic elements of the temporal evolution of the area of this studied pore could be linked to, and considered as, observational evidence of linear low-frequency slow sausage (magneto-acoustic gravity) waves in magnetic pores. This would give us further evidence on the coupling of global solar oscillations to the overlaying magnetic atmosphere
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