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Author name code: verth
ADS astronomy entries on 2022-09-14
author:"Verth, Gary"
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Title: Comparison of exact and approximate MHD slow body mode
solutions in photospheric waveguides
Authors: Aldhafeeri, Anwar; Verth, Gary; Fedun, Viktor; Lennard,
Matthew; Ballai, Istvan
2022arXiv220904689A Altcode:
In this study we explore the possibility of simplifying the modeling
of magnetohydrodynamic (MHD) slow body modes observed in photospheric
magnetic structure such as the umbrae of sunspots and pores. The
simplifying approach assumes that the variation of the eigenvalues
of slow body waves can be derived by imposing that the longitudinal
component of velocity with respect to the tube axis is zero at the
boundary of the magnetic flux tube, which is in a good agreement with
observations. To justify our approach we compare the results of our
simplified model for slow body modes in cylindrical flux tubes with
the model prediction obtained by imposing the continuity of the radial
component of the velocity and total pressure at the boundary of the
flux tube. Our results show that, to a high accuracy (less than 1\% for
the considered model), the conditions of continuity of the component
of transversal velocity and pressure at the boundary can be neglected
when modelling slow body modes under photospheric conditions.
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Title: The properties of magnetoacoustic waves in magnetic flux
tubes with background rotational flows
Authors: Skirvin, Samuel; Fedun, Viktor; van Doorsselaere, Tom;
Goossens, Marcel; Verth, Gary; Claes, Niels
2022cosp...44.2547S Altcode:
Rotational flows are commonly observed features in structures within
the lower solar atmosphere, for example in solar vortices and within
intergranular lanes. It is to be expected that the presence of an
equilibrium rotational flow inside a magnetic flux tube will modify
the behaviour of propagating magnetoacoustic (MA) waves inside the
structure. Using a previously developed numerical eigensolver, we obtain
the eigenvalues for both the sausage and kink modes of the magnetic flux
tube in the presence of a background rotational flow under photospheric
conditions. The effect of both the amplitude of the rotational flow and
the flow's radial structure are investigated. It is found that when
the rotational flow is linear, the modified slow continuum, shifted
due to the background flow, reduces to single-point values. However,
in the case when the flow is non linear, the modified slow continuum now
occupies a band of frequencies. The radial structure of the background
rotational flow has important consequences for determining which
wave modes are absorbed into the continua. Furthermore, we present
for the first time 2D velocity field visualisations showing the
resulting wave perturbations alongside incorporation of the additional
background flow for both the sausage and kink modes. For both cases of
linear and nonlinear background rotational flow, it is shown that the
velocity fields with the additional background flow display different
characteristics when compared to the perturbation alone, which may
be useful for observers when interpreting high resolution data from
e.g. DKIST. Finally, we present initial results investigating the
nature of MA waves in rotating magnetic flux tubes with an additional
vertical flow component, a configuration commonly seen in simulations
of solar vortices.
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Title: Small-scale solar jet formation and their associated waves
and instabilities
Authors: Skirvin, Samuel; Verth, Gary; Juan González-Avilés, José;
Shelyag, Sergiy; Sharma, Rahul; Guzmán, Fransisco; Ballai, Istvan;
Scullion, Eamon; Silva, Suzana S. A.; Fedun, Viktor
2022arXiv220509598S Altcode:
Studies on small-scale jets' formation, propagation, evolution,
and role, such as type I and II spicules, mottles, and fibrils in
the lower solar atmosphere's energetic balance, have progressed
tremendously thanks to the combination of detailed observations and
sophisticated mathematical modelling. This review provides a survey of
the current understanding of jets, their formation in the solar lower
atmosphere, and their evolution from observational, numerical, and
theoretical perspectives. First, we review some results to describe
the jet properties, acquired numerically, analytically and through
high-spatial and temporal resolution observations. Further on, we
discuss the role of hydrodynamic and magnetohydrodynamic instabilities,
namely Rayleigh-Taylor and Kelvin-Helmholtz instabilities, in jet
evolution and their role in the energy transport through the solar
atmosphere in fully and partially ionised plasmas. Finally, we discuss
several mechanisms of magnetohydrodynamic wave generation, propagation,
and energy transport in the context of small-scale solar jets in
detail. This review identifies several gaps in the understanding of
small-scale solar jets and some misalignments between the observational
studies and knowledge acquired through theoretical studies and numerical
modelling. It is to be expected that these gaps will be closed with
the advent of high-resolution observational instruments, such as
Daniel K. Inouye Solar Telescope, Solar Orbiter, Parker Solar Probe,
and Solar CubeSats for Linked Imaging Spectropolarimetry, combined
with further theoretical and computational developments.
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Title: Waves in weakly ionized solar plasmas
Authors: Alharbi, A.; Ballai, I.; Fedun, V.; Verth, G.
2022MNRAS.511.5274A Altcode: 2022MNRAS.tmp..445A; 2022arXiv220207387A
Here, we study the nature and characteristics of waves propagating in
partially ionized plasmas in the weakly ionized limit, typical for
the lower part of the solar atmosphere. The framework in which the
properties of waves are discussed depends on the relative magnitude
of collisions between particles, but also on the relative magnitude
of the collisional frequencies compared to the gyro-frequency of
charged particles. Our investigation shows that the weakly ionized
solar atmospheric plasma can be divided into two regions, and this
division occurs, roughly, at the base of the chromosphere. In the solar
photosphere, the plasma is non-magnetized and the dynamics can described
within the three-fluid framework, where acoustic waves associated
to each species can propagate. Due to the very high concentration of
neutrals, the neutral sound waves propagates with no damping, while
for the other two modes the damping rate is determined by collisions
with neutrals. The ion- and electron-related acoustic modes propagate
with a cut-off determined by the collisional frequency of these species
with neutrals. In the weakly ionized chromosphere, only electrons are
magnetized, however, the strong coupling of charged particles reduces
the working framework to a two-fluid model. The disassociation of
charged particles creates electric currents that can influence the
characteristic of waves. The propagation properties of waves with
respect to the angle of propagation are studied with the help of
polar diagrams.
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Title: New Approach for Analyzing Dynamical Processes on the Surface
of Photospheric Vortex Tubes
Authors: Aljohani, Yasir; Fedun, Viktor; Ballai, Istvan; Silva,
Suzana S. A.; Shelyag, Sergiy; Verth, Gary
2022ApJ...928....3A Altcode: 2022arXiv220209332A
The majority of studies on multi-scale vortex motions employ a
two-dimensional geometry by using a variety of observational and
numerical data. This approach limits the understanding the nature of
physical processes responsible for vortex dynamics. Here, we develop
a new methodology to extract essential information from the boundary
surface of vortex tubes. 3D high-resolution magneto-convection MURaM
numerical data has been used to analyze photospheric intergranular
velocity vortices. The Lagrangian averaged vorticity deviation technique
was applied to define the centers of vortex structures and their
boundary surfaces based on the advection of fluid elements. These
surfaces were mapped onto a constructed envelope grid that allows
the study of the key plasma parameters as functions of space and
time. Quantities that help in understanding the dynamics of the
plasma, e.g., Lorentz force, pressure force, and plasma-β were also
determined. Our results suggest that, while density and pressure have
a rather global behavior, the other physical quantities undergo local
changes, with their magnitude and orientation changing in space and
time. At the surface, the mixing in the horizontal direction is not
efficient, leading to appearance of localized regions with higher/colder
temperatures. In addition, the analysis of the MHD Poynting flux
confirms that the majority of the energy is directed in the horizontal
direction. Our findings also indicate that the pressure and magnetic
forces that drive the dynamics of the plasma on vortex surfaces are
unbalanced and therefore the vortices do not rotate as a rigid body.
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Title: Magnetohydrodynamic Wave Mode Identification in Circular and
Elliptical Sunspot Umbrae: Evidence for High-order Modes
Authors: Albidah, A. B.; Fedun, V.; Aldhafeeri, A. A.; Ballai, I.;
Brevis, W.; Jess, D. B.; Higham, J.; Stangalini, M.; Silva, S. S. A.;
Verth, G.
2022ApJ...927..201A Altcode: 2022arXiv220200624A
In this paper, we provide clear direct evidence of multiple concurrent
higher-order magnetohydrodynamic (MHD) modes in circular and elliptical
sunspots by applying both proper orthogonal decomposition (POD) and
dynamic mode decomposition (DMD) techniques on solar observational
data. These techniques are well documented and validated in the areas
of fluid mechanics, hydraulics, and granular flows but are relatively
new to the field of solar physics. While POD identifies modes based
on orthogonality in space and provides a clear ranking of modes
in terms of their contribution to the variance of the signal, DMD
resolves modes that are orthogonal in time. The clear presence of the
fundamental slow sausage and kink body modes, as well as higher-order
slow sausage and kink body modes, have been identified using POD and
DMD analysis of the chromospheric Hα line at 6562.808 Å for both the
circular and elliptical sunspots. Additionally, for the various slow
body modes, evidence for the presence of the fast surface kink mode
was found in the circular sunspot. All of the MHD mode patterns were
cross-correlated with their theoretically predicted counterparts, and
we demonstrated that ellipticity cannot be neglected when interpreting
MHD wave modes. The higher-order MHD wave modes are even more sensitive
to irregularities in umbral cross-sectional shapes; hence, this must be
taken into account for more accurate modeling of the modes in sunspots
and pores.
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Title: The Importance of Horizontal Poynting Flux in the Solar
Photosphere
Authors: Silva, Suzana S. A.; Murabito, Mariarita; Jafarzadeh, Shahin;
Stangalini, Marco; Verth, Gary; Ballai, Istvan; Fedun, Viktor
2022ApJ...927..146S Altcode: 2022arXiv220301221S
The electromagnetic energy flux in the lower atmosphere of the Sun is a
key tool to describe the energy balance of the solar atmosphere. Current
investigations on energy flux in the solar atmosphere focus primarily
on the vertical electromagnetic flux through the photosphere, ignoring
the Poynting flux in other directions and its possible contributions to
local heating. Based on a realistic Bifrost simulation of a quiet-Sun
(coronal hole) atmosphere, we find that the total electromagnetic energy
flux in the photosphere occurs mainly parallel to the photosphere,
concentrating in small regions along intergranular lanes. Thereby,
it was possible to define a proxy for this energy flux based on
only variables that can be promptly retrieved from observations,
namely, horizontal velocities of the small-scale magnetic elements
and their longitudinal magnetic flux. Our proxy accurately describes
the actual Poynting flux distribution in the simulations, with the
electromagnetic energy flux reaching 10<SUP>10</SUP> erg cm<SUP>-2</SUP>
s<SUP>-1</SUP>. To validate our findings, we extended the analysis
to SUNRISE/IMaX data. First, we show that Bifrost realistically
describes photospheric quiet-Sun regions, as the simulation presents
similar distributions for line-of-sight magnetic flux and horizontal
velocity field. Second, we found very similar horizontal Poynting flux
proxy distributions for the simulated photosphere and observational
data. Our results also indicate that the horizontal Poynting flux in the
observations is considerably larger than the vertical electromagnetic
flux from previous observational estimates. Therefore, our analysis
confirms that the electromagnetic energy flux in the photosphere
is mainly horizontal and is most intense in localized regions along
intergranular lanes.
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Title: II. The effect of axisymmetric and spatially varying equilibria
and flow on MHD wave modes: cylindrical geometry
Authors: Skirvin, S. J.; Fedun, V.; Silva, Suzana S. A.; Verth, G.
2022MNRAS.510.2689S Altcode: 2021arXiv211204427S; 2021MNRAS.tmp.3327S
Magnetohydrodynamic (MHD) waves are routinely observed in the solar
atmosphere. These waves are important in the context of solar physics
as it is widely believed they can contribute to the energy budget
of the solar atmosphere and are a prime candidate to contribute
towards coronal heating. Realistic models of these waves are required
representing observed configurations such that plasma properties
can be determined more accurately, since they cannot be measured
directly. This work utilizes a previously developed numerical technique
to find permittable eigenvalues under different non-uniform equilibrium
conditions in a Cartesian magnetic slab geometry. Here, we investigate
the properties of magnetoacoustic waves under non-uniform equilibria
in a cylindrical geometry. Previously obtained analytical results are
retrieved to emphasize the power and applicability of this numerical
technique. Further case studies investigate the effect that a radially
non-uniform plasma density and non-uniform plasma flow, modelled as a
series of Gaussian profiles, have on the properties of different MHD
waves. For all cases the dispersion diagrams are obtained and spatial
eigenfunctions calculated which display the effects of the equilibrium
inhomogeneity. It is shown that as the equilibrium non-uniformity
is increased, the radial spatial eigenfunctions are affected and
extra nodes introduced, similar to the previous investigation of a
magnetic slab. Furthermore, azimuthal perturbations are increased
with increasing inhomogeneity introducing vortical motions inside the
waveguide. Finally, 2D and 3D representations of the velocity fields are
shown which may be useful for observers for wave mode identification
under realistic magnetic waveguides with ever increasing instrument
resolution.
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Title: Large scale coherent magnetohydrodynamic oscillations in
a sunspot
Authors: Stangalini, M.; Verth, G.; Fedun, V.; Aldhafeeri, A. A.;
Jess, D. B.; Jafarzadeh, S.; Keys, P. H.; Fleck, B.; Terradas, J.;
Murabito, M.; Ermolli, I.; Soler, R.; Giorgi, F.; MacBride, C. D.
2022NatCo..13..479S Altcode:
Although theoretically predicted, the simultaneous excitation of
several resonant modes in sunspots has not been observed. Like any
harmonic oscillator, a solar magnetic flux tube can support a variety
of resonances, which constitute the natural response of the system
to external forcing. Apart from a few single low order eigenmodes
in small scale magnetic structures, several simultaneous resonant
modes were not found in extremely large sunspots. Here we report
the detection of the largest-scale coherent oscillations observed
in a sunspot, with a spectrum significantly different from the Sun's
global acoustic oscillations, incorporating a superposition of many
resonant wave modes. Magnetohydrodynamic numerical modeling agrees
with the observations. Our findings not only demonstrate the possible
excitation of coherent oscillations over spatial scales as large as
30-40 Mm in extreme magnetic flux regions in the solar atmosphere,
but also paves the way for their diagnostic applications in other
astrophysical contexts.
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Title: The Solar Magnetic Vortices
Authors: Silva, Suzana; Verth, Gary; Rempel, Erico; Shelyag, Sergiy;
Schiavo, Luiz C. A.; Fedun, Viktor
2021AGUFMSH45B2374S Altcode:
Photospheric flows play a crucial role in the dynamical evolution
of the solar atmosphere as they are inherently coupled to magnetic
fields. This leads to a wealth class of events, like solar tornados
and flares. One key magnetic structure is the twisted flux tubes,
linked to eruptive phenomena, plasma jets and heating. However, the
lack of a universal definition precluded the automatic detection of
twisted flux tubes and, thus, there were no available databases for
significant statistical analysis of their morphology and properties.This
work introduces the magnetic flux tube as a magnetic vortex (M-vortex):
a new typology of a solar vortex. We apply a formal definition based on
a recently developed magnetic coherent structure detection technique,
the Integrated Averaged Current Deviation (IACD) method, to define
the M-vortex boundary. By applying this methodology to the realistic
magnetoconvection simulations of a solar plage it was found that
M-vortices appear in the intergranular regions where shear flows and
high value of plasma beta are present. Their lifetime is, on average,
around a minute and they locally concentrate the vertical magnetic field
and current density. Based on M-vortex magnetic to kinetic energy ratio,
our results indicate two distinct types of them. Type (i): when magnetic
energy dominates, the twisted magnetic flux tube is mainly oriented
perpendicular to the solar surface. Type (ii): when plasma inside of the
M-vortex has more considerable kinetic energy, the magnetic structure
can present various shapes, sizes and magnetic field geometry. It
was also found that magnetic and flow vortices (kinetic vortices,
i.e. K-vortices) are different structures, but they may intersect,
interfering in each other's dynamics. The M-vortices may appear if two
conditions are simultaneously found in the plasma flow: (i) shear and
(ii) plasma-beta>1. Our results show that the presence of a K-vortex
is not a necessary condition for the formation of M-vortices.
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Title: Using Machine Learning Tools To Estimate Photospheric Velocity
Fields Prior To The Formation Of Active Regions.
Authors: Lennard, Matthew; Tremblay, Benoit; Asensio Ramos, Andres;
Hotta, Hideyuki; Iijima, Haruhisa; Park, Sung-Hong; Silva, Suzana;
Verth, Gary; Fedun, Viktor
2021AGUFMSH45B2371L Altcode:
In recent years a number of major advances have been made using
numerical modelling to better our understanding of magnetic structures
and the evolution of active regions (AR, see e.g. Hotta &
Iijima, 2020; Chen et al, 2021). In particular, these high resolution
simulations provide us with the means to study the photospheric flows
associated with the aforementioned magnetic structures. In practice,
plasma flows at the solar surface cannot be directly recovered;
the component transverse to the line-of-sight must be inferred from
observational data. Inferences depend on the method, the observational
data used as input, the spatial resolution of the data and its
cadence. Tracking methods such as local correlation tracking (LCT)
are promising, but the flows they measure are optical and cannot be
used to estimate the flow patterns of an AR until shortly before the
emergence of flux. Another issue with applying LCT to estimate AR
flows is that the recovered velocity field is usually not smooth,
which precludes advanced flow analysis. Besides, depending on the
data, there is a considerable chance of having 'holes' in the velocity
field. Therefore, although LCT methodology can help give a hint on
general flow properties, a more sophisticated technique is necessary
to perform proper analysis on the flow topology. There are also
limitations in the region of the Sun in which we can accurately track
flows as well as problems with accurately extracting longitudinal and
latitudinal velocities. Recently, deep learning has shown promise in
capturing subtleties in Quiet Sun flows at spatial and temporal scales
that typically cannot be recovered by tracking methods (Asensio Ramos
et al, 2017). The DeepVel neural network is trained to infer plasma
flows from surface data using examples from detailed numerical models
(i.e., supervised learning). Using a version of DeepVel that was
trained using a high-resolution numerical simulation of the evolution
of an AR (e.g. Hotta & Iijima, 2020) we developed the algorithm
for predicting flow trajectories from high resolution observational
data. This method was directly compared with previous contenders for
tracking flows and shows more realistic plasma flow field estimation
as well as an increase of reconstruction efficiency.
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Title: On horizontal Poynting flux in the solar photosphere
Authors: Silva, Suzana; Murabito, Mariarita; Jafarzadeh, Shahin;
Stangalini, Marco; Verth, Gary; Ballai, Istvan; Fedun, Viktor
2021AGUFMSH44A..03S Altcode:
Describing the solar atmospheric energy balance and transport is an
essential step to understanding the high temperatures of the upper
atmosphere. This work analyses the 3D electromagnetic energy flux in
the lower atmosphere by combining Bifrost radiative MHD simulations
and Sunrise/IMaX data. Based on a simulated quiet Sun atmosphere, it
was found that only a minor fraction of the Poynting flux propagates
upwards in the photosphere. Most of the total electromagnetic energy
flows parallel to the solar surface, concentrating energy in small
regions along the intergranular lanes. The dominance of the horizontal
component of the electromagnetic energy flux allows an approximation for
the horizontal Poynting flux, which is based solely on the horizontal
velocity and the vertical magnetic field. The proxy to the horizontal
Poynting flux provides a very similar distribution of the total Poynting
flux and can describe the total flux for most of the photosphere with a
small relative error, <30%, in regions with an intense concentration
of electromagnetic energy. The results of the numerical data analysis
were validated by using observational data. First, it was shown that
both Bifrost and IMAX/sunrise data presents similar distributions
for line-of-sight magnetic field and velocity field, indicating
that the simulation realistically describes a quiet Sun region. The
horizontal Poynting flux proxy provided very similar distributions for
the numerical and observational data, which is considerably larger
than previous observational estimates for upwards electromagnetic
flux. Thereby, those findings corroborate that the electromagnetic
energy flux in the photosphere is mainly parallel to the solar surface
and can be properly described by approximated Poynting flux, based only
on the horizontal plasma flow and the vertical magnetic field. The
results also indicate that small scale intergranular motion may be
important to describe the properties of Poynting flux. Therefore,
further investigations based on the high-resolution data from DKIST
will be important for a valuable description of the energy transport
in the lower atmosphere.
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Title: Propagation of Magnetoacoustic Waves in Symmetrically
Structured Non-Uniform Solar Waveguides with Spatially Varying
Equilibria and Flow
Authors: Skirvin, Samuel; Fedun, Viktor; Verth, Gary
2021AGUFMSH45B2366S Altcode:
The modern ground- and space-based instruments (DST, SST, DKIST,
SDO, Hinode, Solar Orbiter) provide solar physicists an ample of
observations of solar plasma processes, i.e. magnetic bright points,
spicules, plasma flows, structure of magnetic fields etc. at different
time and spatial scales. Therefore, advanced theoretical modelling
becomes essential to explain observational results and provide more
accurate information about MHD wave propagation and solar atmospheric
plasma properties. In this work, we discuss a variety of theoretically
constructed 2-3D MHD equilibria obtained by considering different
internal density and flow profiles. To obtain the numerical solution,
the shooting method has been used to match necessary boundary
conditions on continuity of displacement and total pressure of
the waveguide. The analytic dispersion relation is not required,
making this numerical approach a very powerful tool. The proposed
methodology has been successfully tested against well-known analytical
results obtained for uniform slab and cylinder. Considering both a
magnetic slab (Skirvin et al. MNRAS 2021) and cylindrical geometry,
the dispersion diagrams and eigenfunctions were obtained numerically
for the case where the equilibrium plasma density is modelled as a
series of Gaussian profiles. Furthermore, an additional analysis was
conducted investigating a non-uniform background plasma flow. We have
found that under coronal conditions, with increasing inhomogeneity
in the initial equilibria, additional nodes appear in the resulting
eigenfunctions for slow body sausage and kink modes. This could lead to
an alternative interpretation of some existing observational results
and may be used as a new technique for inhomogeneity of density
and flow detection in the solar magnetic structures. Furthermore,
in a cylindrical geometry, we discuss the effects that a non-uniform
equilibrium has on the physical appearance of the perturbed boundary
and resulting velocity and vorticity fields.
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Title: Kink Oscillations of Coronal Loops
Authors: Nakariakov, V. M.; Anfinogentov, S. A.; Antolin, P.; Jain, R.;
Kolotkov, D. Y.; Kupriyanova, E. G.; Li, D.; Magyar, N.; Nisticò, G.;
Pascoe, D. J.; Srivastava, A. K.; Terradas, J.; Vasheghani Farahani,
S.; Verth, G.; Yuan, D.; Zimovets, I. V.
2021SSRv..217...73N Altcode: 2021arXiv210911220N
Kink oscillations of coronal loops, i.e., standing kink waves, is
one of the most studied dynamic phenomena in the solar corona. The
oscillations are excited by impulsive energy releases, such as low
coronal eruptions. Typical periods of the oscillations are from a
few to several minutes, and are found to increase linearly with the
increase in the major radius of the oscillating loops. It clearly
demonstrates that kink oscillations are natural modes of the loops,
and can be described as standing fast magnetoacoustic waves with the
wavelength determined by the length of the loop. Kink oscillations are
observed in two different regimes. In the rapidly decaying regime,
the apparent displacement amplitude reaches several minor radii of
the loop. The damping time which is about several oscillation periods
decreases with the increase in the oscillation amplitude, suggesting a
nonlinear nature of the damping. In the decayless regime, the amplitudes
are smaller than a minor radius, and the driver is still debated. The
review summarises major findings obtained during the last decade,
and covers both observational and theoretical results. Observational
results include creation and analysis of comprehensive catalogues of
the oscillation events, and detection of kink oscillations with imaging
and spectral instruments in the EUV and microwave bands. Theoretical
results include various approaches to modelling in terms of the
magnetohydrodynamic wave theory. Properties of kink oscillations are
found to depend on parameters of the oscillating loop, such as the
magnetic twist, stratification, steady flows, temperature variations
and so on, which make kink oscillations a natural probe of these
parameters by the method of magnetohydrodynamic seismology.
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Title: Solar Vortex Tubes. II. On the Origin of Magnetic Vortices
Authors: Silva, Suzana S. A.; Verth, Gary; Rempel, Erico L.; Shelyag,
Sergiy; Schiavo, Luiz A. C. A.; Fedun, Viktor
2021ApJ...915...24S Altcode:
The solar atmosphere presents a wealth of dynamics due to a constant
interplay between the plasma flows and magnetic fields. Twisted flux
tubes are an essential magnetic structure, believed to be driven by
the rotational surface's motions and linked to plasma heating, jets,
and eruptive phenomena. Despite extensive investigations, twisted
magnetic flux tubes lack a proper mathematical definition, precluding
their automatic detection. This work addresses this issue by defining
them as magnetic vortices and introduces a formal definition that is
based on a recently developed magnetic vortex detection technique, the
integrated averaged current deviation method. We applied this method and
a kinetic vortex identification technique to realistic magnetoconvection
simulations obtained from the MURaM code. The preferential site
for these two types of vortices is the intergranular downflow,
but while the magnetic vortices are found mostly in the small areas
where plasma-β > 1, the rotational flow structures (the kinetic
vortices), were detected in locations where plasma-β < 1. The
magnetic vortices locally concentrate the magnetic field's vertical
components and current, lasting, on average, around a minute. Two types
of magnetic vortices are introduced based on their magnetic-to-kinetic
energy ratio. For the first type, the magnetic energy prevails, and
the magnetic vortices are mostly vertical. The second type of magnetic
vortex presents distinct shapes and a lower magnetic-to-kinetic energy
ratio. We have found that magnetic vortices may appear if two conditions
are simultaneously present: (i) shear flow and (ii) plasma-β >
1. The presence of rotational motion is not necessary.
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Title: I. The effect of symmetric and spatially varying equilibria
and flow on MHD wave modes: slab geometry
Authors: Skirvin, S. J.; Fedun, V.; Verth, G.
2021MNRAS.504.4077S Altcode: 2021MNRAS.tmp.1155S
Realistic theoretical models of magnetohydrodynamic wave propagation
in the different solar magnetic configurations are required to explain
observational results, allowing magnetoseismology to be conducted and
provide more accurate information about local plasma properties. The
numerical approach described in this paper allows a dispersion diagram
to be obtained for any arbitrary symmetric magnetic slab model of
solar atmospheric features. This proposed technique implements the
shooting method to match necessary boundary conditions on continuity
of displacement and total pressure of the waveguide. The algorithm
also implements fundamental physical knowledge of the sausage and kink
modes such that both can be investigated. The dispersion diagrams for
a number of analytic cases that model magnetohydrodynamic waves in a
magnetic slab were successfully reproduced. This work is then extended
by considering density structuring modelled as a series of Gaussian
profiles and a sinc(x) function. A further case study investigates
properties of MHD wave modes in a coronal slab with a non-uniform
background plasma flow, for which the governing equations are
derived. It is found that the dispersive properties of slow body modes
are more greatly altered than those of fast modes when any equilibrium
inhomogeneity is increased, including background flow. The spatial
structure of the eigenfunctions is also modified, introducing extra
nodes and points of inflexion that may be of interest to observers.
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Title: Magnetohydrodynamic Wave Modes of Solar Magnetic Flux Tubes
with an Elliptical Cross Section
Authors: Aldhafeeri, Anwar A.; Verth, Gary; Brevis, Wernher; Jess,
David B.; McMurdo, Max; Fedun, Viktor
2021ApJ...912...50A Altcode:
The purpose of this paper is to study the behavior of
magnetohydrodynamic (MHD) wave modes that propagate in compressible
magnetic flux tubes with an elliptical cross section embedded in
a magnetic environment. The dispersion relation that describes the
behavior of MHD wave modes permitted in an elliptical magnetic flux
tube is solved numerically. Distortion of the spatial structure of the
purely real eigenmodes from the well-known circular flux tube model
has been considered. It has been studied under both photospheric and
coronal conditions. It has been shown that (i) solutions in the form of
even Mathieu functions are more sensitive to the value of eccentricity
than solutions with the form of odd Mathieu functions; (ii) if the
ellipticity of the cross section of the magnetic flux tube increases,
a sausage mode (m = 0) cannot be easily identified; (iii) even solutions
that correspond to the fluting mode (m = 3) can be misinterpreted as a
kink mode (m = 1) due to their similarities. In contrast to the fluting
modes that are polarized along the major axis and strongly depend on
the ellipticity of the magnetic flux tube, the kink and sausage surface
modes are practically unaffected by ellipticity. Several examples of the
spatial structure of the eigenmodes permitted in the pores and sunspots
have been visualized. The solutions obtained in the approximation of
cylindrical symmetry are in agreement with previous studies.
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Title: A novel approach to identify resonant MHD wave modes in solar
pores and sunspot umbrae: B − ω analysis
Authors: Stangalini, M.; Jess, D. B.; Verth, G.; Fedun, V.; Fleck, B.;
Jafarzadeh, S.; Keys, P. H.; Murabito, M.; Calchetti, D.; Aldhafeeri,
A. A.; Berrilli, F.; Del Moro, D.; Jefferies, S. M.; Terradas, J.;
Soler, R.
2021A&A...649A.169S Altcode: 2021arXiv210311639S
The umbral regions of sunspots and pores in the solar photosphere are
generally dominated by 3 mHz oscillations, which are due to p-modes
penetrating the magnetic region. In these locations, wave power is
also significantly reduced with respect to the quiet Sun. However,
here we study a pore where not only is the power of the oscillations
in the umbra comparable to, or even larger than, that of the quiet
Sun, but the main dominant frequency is not 3 mHz as expected, but
instead 5 mHz. By combining Doppler velocities and spectropolarimetry
and analysing the relationship between magnetic field strength and
frequency, the resultant B − ω diagram reveals distinct ridges that
are remarkably clear signatures of resonant magneto-hydrodynamic (MHD)
oscillations confined within the pore umbra. We demonstrate that these
modes, in addition to velocity oscillations, are also accompanied
by magnetic oscillations, as predicted from MHD theory. The novel
technique of B − ω analysis proposed in this article opens up
an exciting new avenue for identifying MHD wave modes in the umbral
regions of both pores and sunspots.
---------------------------------------------------------
Title: Critical Science Plan for the Daniel K. Inouye Solar Telescope
(DKIST)
Authors: Rast, Mark P.; Bello González, Nazaret; Bellot Rubio,
Luis; Cao, Wenda; Cauzzi, Gianna; Deluca, Edward; de Pontieu, Bart;
Fletcher, Lyndsay; Gibson, Sarah E.; Judge, Philip G.; Katsukawa,
Yukio; Kazachenko, Maria D.; Khomenko, Elena; Landi, Enrico; Martínez
Pillet, Valentín; Petrie, Gordon J. D.; Qiu, Jiong; Rachmeler,
Laurel A.; Rempel, Matthias; Schmidt, Wolfgang; Scullion, Eamon; Sun,
Xudong; Welsch, Brian T.; Andretta, Vincenzo; Antolin, Patrick; Ayres,
Thomas R.; Balasubramaniam, K. S.; Ballai, Istvan; Berger, Thomas E.;
Bradshaw, Stephen J.; Campbell, Ryan J.; Carlsson, Mats; Casini,
Roberto; Centeno, Rebecca; Cranmer, Steven R.; Criscuoli, Serena;
Deforest, Craig; Deng, Yuanyong; Erdélyi, Robertus; Fedun, Viktor;
Fischer, Catherine E.; González Manrique, Sergio J.; Hahn, Michael;
Harra, Louise; Henriques, Vasco M. J.; Hurlburt, Neal E.; Jaeggli,
Sarah; Jafarzadeh, Shahin; Jain, Rekha; Jefferies, Stuart M.; Keys,
Peter H.; Kowalski, Adam F.; Kuckein, Christoph; Kuhn, Jeffrey R.;
Kuridze, David; Liu, Jiajia; Liu, Wei; Longcope, Dana; Mathioudakis,
Mihalis; McAteer, R. T. James; McIntosh, Scott W.; McKenzie, David
E.; Miralles, Mari Paz; Morton, Richard J.; Muglach, Karin; Nelson,
Chris J.; Panesar, Navdeep K.; Parenti, Susanna; Parnell, Clare E.;
Poduval, Bala; Reardon, Kevin P.; Reep, Jeffrey W.; Schad, Thomas A.;
Schmit, Donald; Sharma, Rahul; Socas-Navarro, Hector; Srivastava,
Abhishek K.; Sterling, Alphonse C.; Suematsu, Yoshinori; Tarr, Lucas
A.; Tiwari, Sanjiv; Tritschler, Alexandra; Verth, Gary; Vourlidas,
Angelos; Wang, Haimin; Wang, Yi-Ming; NSO and DKIST Project; DKIST
Instrument Scientists; DKIST Science Working Group; DKIST Critical
Science Plan Community
2021SoPh..296...70R Altcode: 2020arXiv200808203R
The National Science Foundation's Daniel K. Inouye Solar Telescope
(DKIST) will revolutionize our ability to measure, understand,
and model the basic physical processes that control the structure
and dynamics of the Sun and its atmosphere. The first-light DKIST
images, released publicly on 29 January 2020, only hint at the
extraordinary capabilities that will accompany full commissioning of
the five facility instruments. With this Critical Science Plan (CSP)
we attempt to anticipate some of what those capabilities will enable,
providing a snapshot of some of the scientific pursuits that the DKIST
hopes to engage as start-of-operations nears. The work builds on the
combined contributions of the DKIST Science Working Group (SWG) and
CSP Community members, who generously shared their experiences, plans,
knowledge, and dreams. Discussion is primarily focused on those issues
to which DKIST will uniquely contribute.
---------------------------------------------------------
Title: Slow magnetoacoustic waves in gravitationally stratified
two-fluid plasmas in strongly ionized limit
Authors: Alharbi, A.; Ballai, I.; Fedun, V.; Verth, G.
2021MNRAS.501.1940A Altcode: 2020MNRAS.tmp.3613A; 2020arXiv201205789A
The plasma dynamics at frequencies comparable with collisional frequency
between various species has to be described in multifluid framework,
where collisional interaction between particles is an important
ingredient. In our study, we will assume that charged particles are
strongly coupled, meaning that they form a single fluid that interacts
with neutrals, therefore we will employ a two-fluid model. Here, we
aim to investigate the evolutionary equation of slow sausage waves
propagating in a gravitationally stratified flux tube in the two-fluid
solar atmosphere in a strongly ionized limit using an initial value
analysis. Due to the collisional interaction between massive particles
(ions and neutrals), the governing equations are coupled. Solutions
are sought in the strongly ionized limit and the density ratio between
neutrals and charged particles is a small parameter. This limit is
relevant to the upper part of the chromosphere. Our results show
that slow sausage waves associated with charged particles propagate
such that their possible frequency is affected by a cut-off due to
the gravitational stratification. In contrast, for neutral acoustic
waves the cut-off value applies on their wavelength and only small
wavelength waves are able to propagate. Slow modes associated with
neutrals are driven by the collisional coupling with ions.
---------------------------------------------------------
Title: Proper orthogonal and dynamic mode decomposition of sunspot
data
Authors: Albidah, A. B.; Brevis, W.; Fedun, V.; Ballai, I.; Jess,
D. B.; Stangalini, M.; Higham, J.; Verth, G.
2021RSPTA.37900181A Altcode: 2020arXiv201008530A
High-resolution solar observations show the complex structure of the
magnetohydrodynamic (MHD) wave motion. We apply the techniques of
proper orthogonal decomposition (POD) and dynamic mode decomposition
(DMD) to identify the dominant MHD wave modes in a sunspot using the
intensity time series. The POD technique was used to find modes that
are spatially orthogonal, whereas the DMD technique identifies temporal
orthogonality. Here, we show that the combined POD and DMD approaches
can successfully identify both sausage and kink modes in a sunspot
umbra with an approximately circular cross-sectional shape. <P />This
article is part of the Theo Murphy meeting issue `High-resolution wave
dynamics in the lower solar atmosphere'.
---------------------------------------------------------
Title: Spicule Jets in the Solar Atmosphere Modeled with Resistive
MHD and Thermal Conduction
Authors: González, J. J. J.; Guzmán, F.; Fedun, V.; Verth, G.
2020AGUFMSH0290007G Altcode:
Using numerical simulations, we study the effects of magnetic
resistivity and thermal conductivity in the dynamics and properties
of solar jets with characteristics of Type II spicules and cool
coronal jets. The resistive MHD equations govern the jets' dynamic
evolution with thermal conduction along the magnetic field lines
on a 2.5D slice. The magnetic field configuration consists of two
symmetric neighboring loops with opposite polarity, used to support
reconnection and followed by the plasma jet formation. In total, ten
simulations were carried out with different values of resistivity and
thermal conductivity that produce jets with different morphological and
thermal properties we quantify. We find that an increase in magnetic
resistivity does not significantly affect the morphology, velocity,
and temperature of the jets. However, thermal conductivity affects
both temperature and morphology of the jets. In particular, thermal
conductivity causes jets to reach greater heights and increases
the temperature of the jet-apex. Also, heat flux maps indicate the
jet-apex and corona interchange energy more efficiently than the jet's
body. These results could potentially open a new avenue for plasma
diagnostics in the Sun's atmosphere.
---------------------------------------------------------
Title: Generation of Low-Frequency Kinetic Waves at the Footpoints
of Pre-Flare Coronal Loops
Authors: Kryshtal, Alexandr; Voitsekhovska, Anna; Cheremnykh, Oleg;
Ballai, Istvan; Verth, Gary; Fedun, Viktor
2020SoPh..295..162K Altcode:
In this study we discuss the excitation of low-frequency plasma waves
in the lower-middle chromosphere region of loop footpoints for the
case when the plasma can be considered to be in a pre-flare state. It
is shown that among the well-known semi-empirical models of the solar
atmosphere, only the VAL (F) model together with a particular set of
basic plasma parameters and amplitudes of the electric and magnetic
fields supports generation of low-frequency wave instability. Our
results show that it is possible to predict the onset of the flare
process in the active region by using the interaction of kinetic
Alfvén and kinetic ion-acoustic waves, which are solutions of the
derived dispersion equation. The VAL (F) model allows situations when
the main source of the aforementioned instability can be a sub-Dreicer
electric field and drift plasma movements due to presence of spatial
inhomogeneities. We also show that the generation of kinetic Alfvén
and kinetic ion-acoustic waves can occur both, in plasma with a purely
Coulomb conductivity and in the presence of small-scale Bernstein
turbulence. The excitation of the small amplitude kinetic waves due to
the development of low threshold instability in plasma with relatively
low values of the magnetic field strength is also discussed.
---------------------------------------------------------
Title: Solar Vortex Tubes: Vortex Dynamics in the Solar Atmosphere
Authors: Silva, Suzana S. A.; Fedun, Viktor; Verth, Gary; Rempel,
Erico L.; Shelyag, Sergiy
2020ApJ...898..137S Altcode: 2020arXiv200704371S
In this work, a state-of-the-art vortex detection method, Instantaneous
Vorticity Deviation, is applied to locate three-dimensional vortex
tube boundaries in numerical simulations of solar photospheric
magnetoconvection performed by the MURaM code. We detected
three-dimensional vortices distributed along intergranular regions and
displaying coned shapes that extend from the photosphere to the low
chromosphere. Based on a well-defined vortex center and boundary, we
were able to determine averaged radial profiles and thereby investigate
the dynamics across the vortical flows at different height levels. The
solar vortex tubes present nonuniform angular rotational velocity,
and, at all height levels, there are eddy viscosity effects within the
vortices, which slow down the plasma as it moves toward the center. The
vortices impact the magnetic field as they help to intensify the
magnetic field at the sinking points, and in turn, the magnetic
field ends up playing an essential role in the vortex dynamics. The
magnetic field was found to be especially important to the vorticity
evolution. On the other hand, it is shown that, in general, kinematic
vortices do not give rise to magnetic vortices unless their tangential
velocities at different height levels are high enough to overcome the
magnetic tension.
---------------------------------------------------------
Title: Spicule Jets in the Solar Atmosphere Modeled with Resistive
MHD and Thermal Conduction
Authors: González-Avilés, J. J.; Guzmán, F. S.; Fedun, V.; Verth, G.
2020ApJ...897..153G Altcode: 2020arXiv200513647G
Using numerical simulations, we study the effects of magnetic
resistivity and thermal conductivity in the dynamics and properties of
solar jets with characteristics of Type II spicules and cool coronal
jets. The dynamic evolution of the jets is governed by the resistive
MHD equations with thermal conduction along the magnetic field lines
on a 2.5D slice. The magnetic field configuration consists of two
symmetric neighboring loops with opposite polarity, used to support
reconnection and followed by the plasma jet formation. In total, 10
simulations were carried out with different values of resistivity and
thermal conductivity that produce jets with different morphological and
thermal properties we quantify. We find that an increase in magnetic
resistivity does not produce significant effects on the morphology,
velocity, and temperature of the jets. However, thermal conductivity
affects both temperature and morphology of the jets. In particular,
thermal conductivity causes jets to reach greater heights and increases
the temperature of the jet-apex. Also, heat flux maps indicate the
jet-apex and corona interchange energy more efficiently than the body
of the jet. These results could potentially open a new avenue for
plasma diagnostics in the Sun's atmosphere.
---------------------------------------------------------
Title: The Effect of the 21 August 2017 Total Solar Eclipse on the
Phase of VLF/LF Signals
Authors: Rozhnoi, A.; Solovieva, M.; Shalimov, S.; Ouzounov, D.;
Gallagher, P.; Verth, G.; McCauley, J.; Shelyag, S.; Fedun, V.
2020E&SS....700839R Altcode:
An experimental study of the phase and amplitude observations
of sub-ionospheric very low and low frequency (VLF/LF) signals is
performed to analyze the response of the lower ionosphere during the 21
August 2017 total solar eclipse in the United States of America. Three
different sub-ionospheric wave paths are investigated. The length of
the paths varies from 2,200 to 6,400 km, and the signal frequencies
are 21.4, 25.2, and 40.75 kHz. The two paths cross the region of
the total eclipse, and the third path is in the region of 40-60%
of obscuration. None of the signals reveal any noticeable amplitude
changes during the eclipse, while negative phase anomalies (from
-33° to -95°) are detected for all three paths. It is shown that
the effective reflection height of the ionosphere in low and middle
latitudes is increased by about 3-5 km during the eclipse. Estimation
of the electron density change in the lower ionosphere caused by
the eclipse, using linear recombination law, shows that the average
decrease is by 2.1 to 4.5 times.
---------------------------------------------------------
Title: New Elliptical Flux Tube Model to Explain Sunspot Oscillations
Authors: Ali Aldhafeeri, A.; Fedun, V.; Jess, D. B.; Brevis, W.;
Verth, G.
2019AGUFMSH41F3322A Altcode:
This work is motivated by elliptical sunspot oscillation data that
cannot be explained by the standard cylinder model. We obtained a
new numerical solution of the dispersion relation for a compressible
magnetic flux tube with an elliptical cross-section embedded in a
magnetic environment. We applied the bisection method to solve this
equation and investigated the behaviour of oscillations which arise
under both photospheric and coronal conditions. From our solutions
we obtain an approximation of a circle like shape which is in perfect
agreement with a previous well known results. Our model also explains
a mode detected in an elliptical sunspot using the ROSA (Rapid
Oscillations in the Solar Atmosphere) instrument based at the Dunn
Solar Telescope.
---------------------------------------------------------
Title: Spicule jets in the chromosphere-corona interface modeled
with resistive MHD with thermal conductivity
Authors: González, J. J.; Guzmán, F.; Fedun, V.; Verth, G.
2019AGUFMSH11D3383G Altcode:
We study the effects of resistivity, thermal conductivity and numerical
methods in the dynamics and properties of solar jets. We focus on the
generation and evolution of type II spicules with a suitable magnetic
field configuration consisting of two loops. The dynamics is assumed
to be governed by the resistive magnetohydrodynamics equations with
thermal conductivity on a 2.5D slice. The equations are solved for
a successfully triggered jet using High Resolution Shock Capturing
methods that use HLLE and HLLC flux formulae, together with first
and second order reconstructors. We quantify differences of height,
thickness, time-life and velocity of the jet excited in terms of solar
atmosphere and numerical parameters applied.
---------------------------------------------------------
Title: Sheffield students win US rocket competition
Authors: Seniuc, Iulius-Vladimir; Kalra, Ankita; Lim, Jae Hyun;
Schiona, Andrea; Narayanankutty, Gopika; Birakasan, Vishan Nair;
Rontogiannis, Georgios; Ioannou, Konstantinos; Verth, Gary; Fedun,
Viktor; Simpson, Charles
2019A&G....60c3.36S Altcode:
Viktor Fedun reports on success for SunrIde, a team of students from
the University of Sheffield who designed and built a rocket.
---------------------------------------------------------
Title: In situ generation of coronal Alfvén waves by jets
Authors: González-Avilés, J. J.; Guzmán, F. S.; Fedun, V.; Verth,
G.; Sharma, R.; Shelyag, S.; Regnier, S.
2019MNRAS.484.1936G Altcode: 2019MNRAS.tmp...67G; 2018arXiv180704224G
Within the framework of 3D resistive magnetohydrodynamic, we simulate
the formation of a plasma jet with the morphology, upward velocity up
to 130 km s<SUP>-1</SUP>, and time-scale formation between 60 and 90
s after beginning of simulation, similar to those expected for type II
spicules. Initial results of this simulation were published in paper by,
e.g. González-Avilés et al. (2018), and present paper is devoted to
the analysis of transverse displacements and rotational-type motion
of the jet. Our results suggest that 3D magnetic reconnection may be
responsible for the formation of the jet in paper by González-Avilés
et al. (2018). In this paper, by calculating times series of the
velocity components v<SUB>x</SUB> and v<SUB>y</SUB> in different points
near to the jet for various heights we find transverse oscillations in
agreement with spicule observations. We also obtain a time-distance
plot of the temperature in a cross-cut at the plane x = 0.1 Mm and
find significant transverse displacements of the jet. By analysing
temperature isosurfaces of 10<SUP>4</SUP> K with the distribution of
v<SUB>x</SUB>, we find that if the line-of-sight (LOS) is approximately
perpendicular to the jet axis then there is both motion towards and
away from the observer across the width of the jet. This red-blue
shift pattern of the jet is caused by rotational motion, initially
clockwise and anti-clockwise afterwards, which could be interpreted
as torsional motion and may generate torsional Alfvén waves in the
corona region. From a nearly vertical perspective of the jet the LOS
velocity component shows a central blue-shift region surrounded by
red-shifted plasma.
---------------------------------------------------------
Title: Vortex Flows in the Solar Atmosphere: Automated Identification
and Statistical Analysis
Authors: Giagkiozis, Ioannis; Fedun, Viktor; Scullion, Eamon; Jess,
David B.; Verth, Gary
2018ApJ...869..169G Altcode:
Vortices on the photosphere are fundamentally important as these
coherent flows have the potential to form coherent magnetic field
structures in the solar atmosphere, e.g., twisted magnetic flux
tubes. These flows have traditionally been identified by tracking
magnetic bright points (BPs) using primarily visual inspection. This
approach has the shortcoming that it introduces bias into the
statistical analyses. In this work we fully automate the process of
vortex identification using an established method from hydrodynamics
for the study of eddies in turbulent flows. For the first time, we apply
this to detect intergranular photospheric intensity vortices. Using this
automated approach, we find that the expected lifetime of intensity
vortices is much shorter (≈17 s) compared with previously observed
magnetic BP swirls. We suggest that at any time there are 1.48 ×
10<SUP>6</SUP> such small-scale intensity vortices covering about 2.8%
of the total surface of the solar photosphere. Lastly, we compare our
results with previous works and speculate what this could imply with
regards to estimating the global energy flux due magnetic tornadoes
in the solar atmosphere with future higher resolution instrumentation.
---------------------------------------------------------
Title: Flows and magnetic field structures in reconnection regions
of simulations of the solar atmosphere: Do flux pile-up models work?
Authors: Shelyag, S.; Litvinenko, Y. E.; Fedun, V.; Verth, G.;
González-Avilés, J. J.; Guzmán, F. S.
2018A&A...620A.159S Altcode: 2018arXiv180900587S
<BR /> Aims: We study the process of magnetic field annihilation
and reconnection in simulations of magnetised solar photosphere
and chromosphere with magnetic fields of opposite polarities and
constant numerical resistivity. <BR /> Methods: Exact analytical
solutions for reconnective annihilations were used to interpret the
features of magnetic reconnection in simulations of flux cancellation
in the solar atmosphere. We used MURaM high-resolution photospheric
radiative magneto-convection simulations to demonstrate the presence of
magnetic field reconnection consistent with the magnetic flux pile-up
models. Also, a simulated data-driven chromospheric magneto-hydrodynamic
simulation is used to demonstrate magnetic field and flow structures,
which are similar to the theoretically predicted ones. <BR /> Results:
Both simulations demonstrate flow and magnetic field structures
roughly consistent with accelerated reconnection with magnetic flux
pile-up. The presence of standard Sweet-Parker type reconnection is
also demonstrated in stronger photospheric magnetic fields.
---------------------------------------------------------
Title: On the Stability of Incompressible MHD Modes in Magnetic
Cylinder with Twisted Magnetic Field and Flow
Authors: Cheremnykh, Oleg; Fedun, Viktor; Ladikov-Roev, Yu.; Verth,
Gary
2018ApJ...866...86C Altcode:
In this work, we studied MHD modes in a magnetically twisted flux
tube with a twisted flow that is embedded in the uniform magnetic
field. We consider when the azimuthal magnetic field and velocity are
linear functions of radius (case i) and also more generally when they
are arbitrary functions of radius (case ii). Under these assumptions,
we obtain the dispersion equation in the incompressible limit. This
solution can also be used to describe the MHD perturbations in
plasma pinches and vortices. The dispersion equation is simplified
by implementing the thin flux tube approximation. It is shown that
sausage modes (m = 0) become unstable for large enough azimuthal
flow speeds. Also, we obtained the unstable modes for m > 0. It
is shown that the stability criterion of the m = 1 mode (for case i)
is independent of the background azimuthal components of the plasma
velocity and magnetic field. These criteria fully coincide with
the result that was previously obtained by Syrovatskiy for a plane
interface. Moreover, this result even remains valid when the azimuthal
magnetic field and velocity have an arbitrary dependence on radius
(case ii). A criterion for the stability of the m ≥ 2 modes is also
obtained. It was found that instability of these modes is determined
by both longitudinal and azimuthal flows. It is shown that if there
is sufficient azimuthal background flow, then all modes with m ≥ 2
will become unstable.
---------------------------------------------------------
Title: Photospheric Observations of Surface and Body Modes in Solar
Magnetic Pores
Authors: Keys, Peter H.; Morton, Richard J.; Jess, David B.; Verth,
Gary; Grant, Samuel D. T.; Mathioudakis, Mihalis; Mackay, Duncan H.;
Doyle, John G.; Christian, Damian J.; Keenan, Francis P.; Erdélyi,
Robertus
2018ApJ...857...28K Altcode: 2018arXiv180301859K
Over the past number of years, great strides have been made in
identifying the various low-order magnetohydrodynamic wave modes
observable in a number of magnetic structures found within the solar
atmosphere. However, one aspect of these modes that has remained
elusive, until now, is their designation as either surface or body
modes. This property has significant implications for how these modes
transfer energy from the waveguide to the surrounding plasma. Here, for
the first time to our knowledge, we present conclusive, direct evidence
of these wave characteristics in numerous pores that were observed to
support sausage modes. As well as outlining methods to detect these
modes in observations, we make estimates of the energies associated
with each mode. We find surface modes more frequently in the data,
as well as that surface modes appear to carry more energy than those
displaying signatures of body modes. We find frequencies in the range
of ∼2-12 mHz, with body modes as high as 11 mHz, but we do not find
surface modes above 10 mHz. It is expected that the techniques we have
applied will help researchers search for surface and body signatures
in other modes and in differing structures from those presented here.
---------------------------------------------------------
Title: Magnetic Shocks and Substructures Excited by Torsional Alfvén
Wave Interactions in Merging Expanding Flux Tubes
Authors: Snow, B.; Fedun, V.; Gent, F. A.; Verth, G.; Erdélyi, R.
2018ApJ...857..125S Altcode: 2018arXiv180306112S
Vortex motions are frequently observed on the solar photosphere. These
motions may play a key role in the transport of energy and momentum from
the lower atmosphere into the upper solar atmosphere, contributing to
coronal heating. The lower solar atmosphere also consists of complex
networks of flux tubes that expand and merge throughout the chromosphere
and upper atmosphere. We perform numerical simulations to investigate
the behavior of vortex-driven waves propagating in a pair of such flux
tubes in a non-force-free equilibrium with a realistically modeled
solar atmosphere. The two flux tubes are independently perturbed
at their footpoints by counter-rotating vortex motions. When the
flux tubes merge, the vortex motions interact both linearly and
nonlinearly. The linear interactions generate many small-scale transient
magnetic substructures due to the magnetic stress imposed by the vortex
motions. Thus, an initially monolithic tube is separated into a complex
multithreaded tube due to the photospheric vortex motions. The wave
interactions also drive a superposition that increases in amplitude
until it exceeds the local Mach number and produces shocks that
propagate upward with speeds of approximately 50 km s<SUP>-1</SUP>. The
shocks act as conduits transporting momentum and energy upward, and
heating the local plasma by more than an order of magnitude, with a
peak temperature of approximately 60,000 K. Therefore, we present a
new mechanism for the generation of magnetic waveguides from the lower
solar atmosphere to the solar corona. This wave guide appears as the
result of interacting perturbations in neighboring flux tubes. Thus, the
interactions of photospheric vortex motions is a potentially significant
mechanism for energy transfer from the lower to upper solar atmosphere.
---------------------------------------------------------
Title: I. Jet Formation and Evolution Due to 3D Magnetic Reconnection
Authors: González-Avilés, J. J.; Guzmán, F. S.; Fedun, V.; Verth,
G.; Shelyag, S.; Regnier, S.
2018ApJ...856..176G Altcode: 2017arXiv170905066G
Using simulated data-driven, 3D resistive MHD simulations of the solar
atmosphere, we show that 3D magnetic reconnection may be responsible for
the formation of jets with the characteristics of Type II spicules. We
numerically model the photosphere-corona region using the C7 equilibrium
atmosphere model. The initial magnetic configuration is a 3D potential
magnetic field, extrapolated up to the solar corona region from a
dynamic realistic simulation of the solar photospheric magnetoconvection
model that mimics the quiet-Sun. In this case, we consider a uniform
and constant value of the magnetic resistivity of 12.56 Ω m. We
have found that the formation of the jet depends on the Lorentz
force, which helps to accelerate the plasma upward. Analyzing various
properties of the jet dynamics, we found that the jet structure shows
a Doppler shift close to regions with high vorticity. The morphology,
the upward velocity covering a range up to 130 km s<SUP>-1</SUP>,
and the timescale formation of the structure between 60 and 90 s,
are similar to those expected for Type II spicules.
---------------------------------------------------------
Title: Evolution of Complex 3D Motions in Spicules
Authors: Sharma, Rahul; Verth, Gary; Erdélyi, Robertus
2018ApJ...853...61S Altcode:
Ubiquitous transverse oscillations observed in spicular waveguides,
identified as the kink wave-mode had previously been reported along
with periodic structural distortions of the flux tubes, observed as
cross-sectional width and associated photometric variations. Previous
studies identified these perturbations as the observed signatures
of concurrent kink and sausage wave-modes. High-resolution Hα
imaging-spectroscopy data from the CRisp Imaging SpectroPolarimeter at
the Swedish Solar Telescope are used to analyze the off-limb spicular
structures. For the first time, the evolution of the resultant
transverse displacement of the flux-tube structure, estimated from
the perpendicular velocity components, is analyzed along with
longitudinal, cross-sectional width, photometric, and azimuthal
shear/torsion variations. The pulse-like nonlinear kink wave-mode
shows strong coupling with these observables, with a period-doubling,
-tripling aspect, supported by mutual phase relations concentrated
around 0° and +/- 180^\circ . The three-dimensional ensemble of the
observed dynamical components revealed complexities pertinent to the
accurate identification and interpretation of, e.g., linear/nonlinear,
coupled/uncoupled magnetohydrodynamical wave-modes in spicules.
---------------------------------------------------------
Title: MHD Wave Modes of Twisted Magnetic Flux Tube
Authors: Fedun, V.; Cheremnykh, O.; Kryshtal, A.; Verth, G.
2017AGUFMSH42B..05F Altcode:
The theoretical study of MHD wave propagation in the solar magnetic
flux tubes is an important problem in modern solar physics. Recent
space- and ground-base observations have shown that that twisted
magnetic fields occur naturally, and indeed are omnipresent in the Sun's
atmosphere. In the framework of ideal MHD, in this talk, we will discuss
small amplitude incompressible wave modes with azimuthal wave number
m=1 and m>1 of twisted magnetic flux tubes. Special attention will
be given to the problem of finding the eigenvalues and eigenfunctions
of modes with m=1 in the long wavelength limit. We will show that the
dispersion relation for kink mode is completely unaffected by the radial
profile of background internal magnetic twist. However, fluting modes
are sensitive to the particular radial profile of magnetic twist chosen.
---------------------------------------------------------
Title: I. Jet Formation and Evolution due to 3D Magnetic Reconnection
Authors: González, J. J.; Guzmán, F.; Fedun, V.; Verth, G.; Shelyag,
S.; Regnier, S.
2017AGUFMSH43A2807G Altcode:
Using simulated data-driven three-dimensional resistive MHD simulations
of the solar atmosphere, we show that magnetic reconnection can be
responsible of the formation of jets with characteristic of Type
II spicules. For this, we numerically model the photosphere-corona
region using the C7 equilibrium atmosphere model. The initial magnetic
configuration is a 3D potential magnetic field, extrapolated up to
the solar corona region from a dynamic realistic simulation of solar
photospheric magnetoconvection model which is mimicking quiet-Sun. In
this case we consider a uniform and constant value of the magnetic
resistivity of 12.56 Ω m. We have found that formation of the jets
depends on the Lorentz force, which helps to accelerate the plasma
upwards. Analyzing various properties of the jet dynamics, we found
that the jet structure shows Doppler shift near to regions with high
vorticity. The morphology, upward velocity, covering a range up to
100 km s-1, and life-time of the estructure, bigger than 100 s, are
similar to those expected for Type II spicules.
---------------------------------------------------------
Title: The Frequency-dependent Damping of Slow Magnetoacoustic Waves
in a Sunspot Umbral Atmosphere
Authors: Krishna Prasad, S.; Jess, D. B.; Van Doorsselaere, T.; Verth,
G.; Morton, R. J.; Fedun, V.; Erdélyi, R.; Christian, D. J.
2017ApJ...847....5K Altcode: 2017ApJ...847....5P; 2017arXiv170804835K
High spatial and temporal resolution images of a sunspot,
obtained simultaneously in multiple optical and UV wavelengths, are
employed to study the propagation and damping characteristics of slow
magnetoacoustic waves up to transition region heights. Power spectra are
generated from intensity oscillations in sunspot umbra, across multiple
atmospheric heights, for frequencies up to a few hundred mHz. It is
observed that the power spectra display a power-law dependence over
the entire frequency range, with a significant enhancement around 5.5
mHz found for the chromospheric channels. The phase difference spectra
reveal a cutoff frequency near 3 mHz, up to which the oscillations
are evanescent, while those with higher frequencies propagate
upward. The power-law index appears to increase with atmospheric
height. Also, shorter damping lengths are observed for oscillations
with higher frequencies suggesting frequency-dependent damping. Using
the relative amplitudes of the 5.5 mHz (3 minute) oscillations, we
estimate the energy flux at different heights, which seems to decay
gradually from the photosphere, in agreement with recent numerical
simulations. Furthermore, a comparison of power spectra across the
umbral radius highlights an enhancement of high-frequency waves near
the umbral center, which does not seem to be related to magnetic field
inclination angle effects.
---------------------------------------------------------
Title: Incompressible magnetohydrodynamic modes in the thin
magnetically twisted flux tube
Authors: Cheremnykh, O. K.; Fedun, V.; Kryshtal, A. N.; Verth, G.
2017A&A...604A..62C Altcode: 2017arXiv170703272C
Context. Observations have shown that twisted magnetic fields naturally
occur, and indeed are omnipresent in the Sun's atmosphere. It is
therefore of great theoretical interest in solar atmospheric waves
research to investigate the types of magnetohydrodynamic (MHD) wave
modes that can propagate along twisted magnetic flux tubes. <BR
/> Aims: Within the framework of ideal MHD, the main aim of this
work is to investigate small amplitude incompressible wave modes
of twisted magnetic flux tubes with m ≥ 1. The axial magnetic
field strength inside and outside the tube will be allowed to vary,
to ensure the results will not be restricted to only cold plasma
equilibria conditions. <BR /> Methods: The dispersion equation for
these incompressible linear MHD wave modes was derived analytically
by implementing the long wavelength approximation. <BR /> Results:
It is shown, in the long wavelength limit, that both the frequency and
radial velocity profile of the m = 1 kink mode are completely unaffected
by the choice of internal background magnetic twist. However, fluting
modes with m ≥ 2 are sensitive to the particular radial profile of
magnetic twist chosen. Furthermore, due to background twist, a low
frequency cut-off is introduced for fluting modes that is not present
for kink modes. From an observational point of view, although magnetic
twist does not affect the propagation of long wavelength kink modes,
for fluting modes it will either work for or against the propagation,
depending on the direction of wave travel relative to the sign of the
background twist.
---------------------------------------------------------
Title: An Inside Look at Sunspot Oscillations with Higher Azimuthal
Wavenumbers
Authors: Jess, David B.; Van Doorsselaere, Tom; Verth, Gary; Fedun,
Viktor; Krishna Prasad, S.; Erdélyi, Robert; Keys, Peter H.; Grant,
Samuel D. T.; Uitenbroek, Han; Christian, Damian J.
2017ApJ...842...59J Altcode: 2017arXiv170506282J
Solar chromospheric observations of sunspot umbrae offer an exceptional
view of magnetohydrodynamic wave phenomena. In recent years, a
wealth of wave signatures related to propagating magneto-acoustic
modes have been presented, which demonstrate complex spatial and
temporal structuring of the wave components. Theoretical modeling has
demonstrated how these ubiquitous waves are consistent with an m = 0
slow magneto-acoustic mode, which is excited by trapped sub-photospheric
acoustic (p-mode) waves. However, the spectrum of umbral waves is broad,
suggesting that the observed signatures represent the superposition
of numerous frequencies and/or modes. We apply Fourier filtering,
in both spatial and temporal domains, to extract chromospheric umbral
wave characteristics consistent with an m = 1 slow magneto-acoustic
mode. This identification has not been described before. Angular
frequencies of 0.037+/- 0.007 {rad} {{{s}}}<SUP>-1</SUP> (2.1+/-
0.4 \deg {{{s}}}<SUP>-1</SUP>, corresponding to a period ≈170 s)
for the m = 1 mode are uncovered for spatial wavenumbers in the range
of 0.45< k< 0.90 arcsec<SUP>-1</SUP> (5000-9000 km). Theoretical
dispersion relations are solved, with corresponding eigenfunctions
computed, which allows the density perturbations to be investigated
and compared with our observations. Such magnetohydrodynamic modeling
confirms our interpretation that the identified wave signatures are
the first direct observations of an m = 1 slow magneto-acoustic mode
in the chromospheric umbra of a sunspot.
---------------------------------------------------------
Title: Vortex Flows in the Solar Atmoshpere: Automated Identification
and Statistical Analysis
Authors: Giagkiozis, Ioannis; Fedun, Viktor; Scullion, Eamon;
Verth, Gary
2017arXiv170605428G Altcode:
Aims. Due to the fundamental importance of vortices on the photosphere,
in this work we aim to fully automate the process of intensity vortex
identification to facilitate a more robust statistical analysis of
their properties. Methods. Using on-disk observational data of the Fe
I continuum, the process of vortex identification is fully automated,
for the first time in solar physics, with the help of an established
method from hydrodynamics initially employed for the study of eddies
in turbulent flows (Graftieaux et al. 2001). Results. We find that
the expected lifetime of intensity vortices is much shorter (~ 17s)
compared with previously observed magnetic bright point swirls. Our
findings suggest that at any time there are 1.4e6 such small-scale
intensity vortices covering about 2.8% of the total surface of the
solar photosphere.
---------------------------------------------------------
Title: Dynamic Behavior of Spicules Inferred from Perpendicular
Velocity Components
Authors: Sharma, Rahul; Verth, Gary; Erdélyi, Robertus
2017ApJ...840...96S Altcode:
Understanding the dynamic behavior of spicules, e.g., in terms of
magnetohydrodynamic (MHD) wave mode(s), is key to unveiling their
role in energy and mass transfer from the photosphere to corona. The
transverse, torsional, and field-aligned motions of spicules have
previously been observed in imaging spectroscopy and analyzed separately
for embedded wave-mode identification. Similarities in the Doppler
signatures of spicular structures for both kink and torsional Alfvén
wave modes have led to the misinterpretation of the dominant wave mode
in these structures and is a subject of debate. Here, we aim to combine
line- of-sight (LOS) and plane-of-sky (POS) velocity components using
the high spatial/temporal resolution Hα imaging-spectroscopy data
from the CRisp Imaging SpectroPolarimeter based at the Swedish Solar
Telescope to achieve better insight into the underlying nature of these
motions as a whole. The resultant three-dimensional velocity vectors
and the other derived quantities (e.g., magnetic pressure perturbations)
are used to identify the MHD wave mode(s) responsible for the observed
spicule motion. We find a number of independent examples where the
bulk transverse motion of the spicule is dominant either in the POS
or along the LOS. It is shown that the counterstreaming action of the
displaced external plasma due to spicular bulk transverse motion has a
similar Doppler profile to that of the m = 0 torsional Alfvén wave when
this motion is predominantly perpendicular to the LOS. Furthermore,
the inferred magnetic pressure perturbations support the kink wave
interpretation of observed spicular bulk transverse motion rather than
any purely incompressible MHD wave mode, e.g., the m = 0 torsional
Alfvén wave.
---------------------------------------------------------
Title: Project SunbYte: solar astronomy on a budget
Authors: Alvarez Gonzalez, F.; Badilita, A. -M.; Baker, A.; Cho,
Y. -H.; Dhot, N.; Fedun, V.; Hare, C.; He, T.; Hobbs, M.; Javed,
M.; Lovesey, H.; Lord, C.; Panoutsos, G.; Permyakov, A.; Pope, S.;
Portnell, M.; Rhodes, L.; Sharma, R.; Taras, P.; Taylor, J.; Tilbrook,
R.; Verth, G.; Wrigley, S. N.; Yaqoob, M.; Cook, R.; McLaughlin, J.;
Morton, R.; Scullion, E.; Shelyag, S.; Hamilton, A.; Zharkov, S.;
Jess, D.; Wrigley, M.
2017A&G....58d2.24A Altcode:
The Sheffield University Nova Balloon Lifted Solar Telescope (SunbYte)
is a high-altitude balloon experiment devised and run largely by
students at the University of Sheffield, and is scheduled for launch
in October 2017. It was the only UK project in 2016 to be selected for
the balloon side of the Swedish-German student programme REXUS/BEXUS
(Rocket and Balloon Experiments for University Students; see box on
p2.25). The success of the SunbYte team in the REXUS/BEXUS selection
process is an unprecedented opportunity for the students to gain
valuable experience working in the space engineering industry, using
their theoretical knowledge and networking with students and technology
companies from all over Europe.
---------------------------------------------------------
Title: Resonant Absorption of Axisymmetric Modes in Twisted Magnetic
Flux Tubes
Authors: Giagkiozis, I.; Goossens, M.; Verth, G.; Fedun, V.; Van
Doorsselaere, T.
2016ApJ...823...71G Altcode: 2017arXiv170609665G
It has been shown recently that magnetic twist and axisymmetric MHD
modes are ubiquitous in the solar atmosphere, and therefore the study of
resonant absorption for these modes has become a pressing issue because
it can have important consequences for heating magnetic flux tubes in
the solar atmosphere and the observed damping. In this investigation,
for the first time, we calculate the damping rate for axisymmetric MHD
waves in weakly twisted magnetic flux tubes. Our aim is to investigate
the impact of resonant damping of these modes for solar atmospheric
conditions. This analytical study is based on an idealized configuration
of a straight magnetic flux tube with a weak magnetic twist inside as
well as outside the tube. By implementing the conservation laws derived
by Sakurai et al. and the analytic solutions for weakly twisted flux
tubes obtained recently by Giagkiozis et al. we derive a dispersion
relation for resonantly damped axisymmetric modes in the spectrum of the
Alfvén continuum. We also obtain an insightful analytical expression
for the damping rate in the long wavelength limit. Furthermore, it
is shown that both the longitudinal magnetic field and the density,
which are allowed to vary continuously in the inhomogeneous layer, have
a significant impact on the damping time. Given the conditions in the
solar atmosphere, resonantly damped axisymmetric modes are highly likely
to be ubiquitous and play an important role in energy dissipation. We
also suggest that, given the character of these waves, it is likely
that they have already been observed in the guise of Alfvén waves.
---------------------------------------------------------
Title: Magnetohydrodynamic Oscillations in the Solar Corona and
Earth's Magnetosphere: Towards Consolidated Understanding
Authors: Nakariakov, V. M.; Pilipenko, V.; Heilig, B.; Jelínek,
P.; Karlický, M.; Klimushkin, D. Y.; Kolotkov, D. Y.; Lee, D. -H.;
Nisticò, G.; Van Doorsselaere, T.; Verth, G.; Zimovets, I. V.
2016SSRv..200...75N Altcode: 2016SSRv..tmp....2N
Magnetohydrodynamic (MHD) oscillatory processes in different
plasma systems, such as the corona of the Sun and the Earth's
magnetosphere, show interesting similarities and differences, which
so far received little attention and remain under-exploited. The
successful commissioning within the past ten years of THEMIS, Hinode,
STEREO and SDO spacecraft, in combination with matured analysis of data
from earlier spacecraft (Wind, SOHO, ACE, Cluster, TRACE and RHESSI)
makes it very timely to survey the breadth of observations giving
evidence for MHD oscillatory processes in solar and space plasmas,
and state-of-the-art theoretical modelling. The paper reviews several
important topics, such as Alfvénic resonances and mode conversion;
MHD waveguides, such as the magnetotail, coronal loops, coronal
streamers; mechanisms for periodicities produced in energy releases
during substorms and solar flares, possibility of Alfvénic resonators
along open field lines; possible drivers of MHD waves; diagnostics of
plasmas with MHD waves; interaction of MHD waves with partly-ionised
boundaries (ionosphere and chromosphere). The review is mainly oriented
to specialists in magnetospheric physics and solar physics, but not
familiar with specifics of the adjacent research fields.
---------------------------------------------------------
Title: Ultra-High-Resolution Observations of MHD Waves in Photospheric
Magnetic Structures
Authors: Jess, D. B.; Verth, G.
2016GMS...216..449J Altcode: 2015arXiv150206960J
This chapter reviews the recent observations of waves and oscillations
manifesting in fine-scale magnetic structures in the solar photosphere,
which are often interpreted as the "building blocks' of the magnetic
Sun. The authors found, through phase relationships between the various
waveforms, that small-scale magnetic bright points (MBPs) in the
photosphere demonstrated signatures of specific magnetoacoustic waves,
in particular the sausage and kink modes. Modern magnetohydrodynamic
(MHD) simulations of the lower solar atmosphere clearly show how
torsional motions can easily be induced in magnetic elements in the
photosphere through the processes of vortical motions and/or buffeting
by neighboring granules. The authors detected significant power
associated with high-frequency horizontal motions, and suggested that
these cases may be especially important in the creation of a turbulent
environment that efficiently promotes Alfvén wave dissipation.
---------------------------------------------------------
Title: MHD Wave Modes Resolved in Fine-Scale Chromospheric Magnetic
Structures
Authors: Verth, G.; Jess, D. B.
2016GMS...216..431V Altcode: 2015arXiv150501155V
Due to its complex and dynamic fine-scale structure, the chromosphere
is a particularly challenging region of the Sun's atmosphere to
understand. It is now widely accepted that to model chromospheric
dynamics, even on a magnetohydrodynamic (MHD) scale, while also
calculating spectral line emission, one must realistically include the
effects of partial ionization and radiative transfer in a multi-fluid
plasma under non-LTE conditions. Accurate quantification of MHD wave
energetics must be founded on a precise identification of the actual
wave mode being observed. This chapter focuses on MHD kink-mode
identification, MHD sausage mode identification, and MHD torsional
Alfvén wave identification. It then reviews progress in determining
more accurate energy flux estimations of specific MHD wave modes
observed in the chromosphere. The chapter finally examines how the
discovery of these MHD wave modes has helped us advance the field of
chromospheric magnetoseismology.
---------------------------------------------------------
Title: Vortex Identification in the Lower Solar Atmosphere
Authors: Fedun, V.; Giagkiozis, I.; Verth, G.; Scullion, E.
2015AGUFMSH53B2484F Altcode:
Vortices in the solar atmosphere present an ideal driving mechanism
for Alfvenic waves that can efficiently carry energy in the upper
layers of the chromosphere and corona. However, the identification
and classification of vortical motions from observations and numerical
simulations is a challenging task. In this work we leverage a number of
methods conventionally employed in turbulence to identify for the fist
time in the solar atmosphere vortices, in an automated fashion. We also
present a statistical analysis of the properties of the identified
motions and relate this with theoretical results for such magnetic
structures.
---------------------------------------------------------
Title: Dissipationless Damping of Compressive MHD Modes in Twisted
Flux Tubes
Authors: Giagkiozis, I.; Fedun, V.; Verth, G.; Goossens, M. L.;
Van Doorsselaere, T.
2015AGUFMSH53B2488G Altcode:
Axisymmetric modes in straight magentic flux tubes exhibit a cutoff
in the long wavelength limit and no damping is predicted. However,
as soon as weak magnetic twist is introduced inside as well as outside
the magnetic flux tube the cutoff recedes. Furthermore, when density
variations are also incomporated within the modelresonant absorption
appears. In this work we explore analytically the expected damping times
for waves within the Alfven continuum for different solar atmospheric
conditions. Based on the results in this work we offer insight on
recent observations of sausage wave damping in the chromosphere.
---------------------------------------------------------
Title: Axisymmetric Modes in Magnetic Flux Tubes with Internal and
External Magnetic Twist
Authors: Giagkiozis, I.; Fedun, V.; Erdélyi, R.; Verth, G.
2015ApJ...810...53G Altcode: 2017arXiv170609669G
Observations suggest that twisted magnetic flux tubes are ubiquitous
in the Sun's atmosphere. The main aim of this work is to advance the
study of axisymmetric modes of magnetic flux tubes by modeling both
twisted internal and external magnetic fields, when the magnetic
twist is weak. In this work, we solve the derived wave equations
numerically assuming that the twist outside the tube is inversely
proportional to the distance from its boundary. We also study the case
of a constant magnetic twist outside the tube and solve these equations
analytically. We show that the solution for a constant twist outside
the tube is a good approximation for the case where the magnetic
twist is proportional to 1/r, namely, the error is in all cases less
than 5.4%. The solution is in excellent agreement with solutions
to simpler models of twisted magnetic flux tubes, i.e., without
external magnetic twist. It is shown that axisymmetric Alfvén waves
are naturally coupled with magnetic twist as the azimuthal component
of the velocity perturbation is nonzero. We compared our theoretical
results with observations and comment on what the Doppler signature
of these modes is expected to be. Lastly, we argue that the character
of axisymmetric waves in twisted magnetic flux tubes can lead to false
positives in identifying observations with axisymmetric Alfvén waves.
---------------------------------------------------------
Title: Observations and mode identification of sausage waves in a
magnetic pore
Authors: Moreels, M. G.; Freij, N.; Erdélyi, R.; Van Doorsselaere,
T.; Verth, G.
2015A&A...579A..73M Altcode:
<BR /> Aims: We aim to determine the phase speed of an oscillation
in a magnetic pore using only intensity images at one height. The
observations were obtained using the CRisp Imaging SpectroPolarimeter
at the Swedisch 1-m Solar Telescope and show variations in both
cross-sectional area and intensity in a magnetic pore. <BR />
Methods: We have designed and tested an observational method to
extract the wave parameters that are important for seismology. We
modelled the magnetic pore as a straight cylinder with a uniform
plasma both inside and outside the flux tube and identify different
wave modes. Using analytic expressions, we are able to distinguish
between fast and slow modes and obtain the phase speed of the
oscillations. <BR /> Results: We found that the observed oscillations
are slow modes with a phase speed around 5 km s<SUP>-1</SUP>. We
also have strong evidence that the oscillations are standing
harmonics. <P />Appendix A is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201425096/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Multiwavelength Studies of MHD Waves in the Solar
Chromosphere. An Overview of Recent Results
Authors: Jess, D. B.; Morton, R. J.; Verth, G.; Fedun, V.; Grant,
S. D. T.; Giagkiozis, I.
2015SSRv..190..103J Altcode: 2015arXiv150301769J; 2015SSRv..tmp...14J
The chromosphere is a thin layer of the solar atmosphere that
bridges the relatively cool photosphere and the intensely heated
transition region and corona. Compressible and incompressible waves
propagating through the chromosphere can supply significant amounts
of energy to the interface region and corona. In recent years an
abundance of high-resolution observations from state-of-the-art
facilities have provided new and exciting ways of disentangling the
characteristics of oscillatory phenomena propagating through the dynamic
chromosphere. Coupled with rapid advancements in magnetohydrodynamic
wave theory, we are now in an ideal position to thoroughly investigate
the role waves play in supplying energy to sustain chromospheric
and coronal heating. Here, we review the recent progress made in
characterising, categorising and interpreting oscillations manifesting
in the solar chromosphere, with an impetus placed on their intrinsic
energetics.
---------------------------------------------------------
Title: Wave Damping Observed in Upwardly Propagating Sausage-mode
Oscillations Contained within a Magnetic Pore
Authors: Grant, S. D. T.; Jess, D. B.; Moreels, M. G.; Morton, R. J.;
Christian, D. J.; Giagkiozis, I.; Verth, G.; Fedun, V.; Keys, P. H.;
Van Doorsselaere, T.; Erdélyi, R.
2015ApJ...806..132G Altcode: 2015arXiv150501484G
We present observational evidence of compressible MHD wave modes
propagating from the solar photosphere through to the base of the
transition region in a solar magnetic pore. High cadence images were
obtained simultaneously across four wavelength bands using the Dunn
Solar Telescope. Employing Fourier and wavelet techniques, sausage-mode
oscillations displaying significant power were detected in both
intensity and area fluctuations. The intensity and area fluctuations
exhibit a range of periods from 181 to 412 s, with an average period
∼290 s, consistent with the global p-mode spectrum. Intensity and
area oscillations present in adjacent bandpasses were found to be
out of phase with one another, displaying phase angles of 6.°12,
5.°82, and 15.°97 between the 4170 Å continuum-G-band, G-band-Na i
D<SUB>1</SUB>, and Na i D<SUB>1</SUB>-Ca ii K heights, respectively,
reiterating the presence of upwardly propagating sausage-mode waves. A
phase relationship of ∼0° between same-bandpass emission and area
perturbations of the pore best categorizes the waves as belonging to
the “slow” regime of a dispersion diagram. Theoretical calculations
reveal that the waves are surface modes, with initial photospheric
energies in excess of 35,000 W m<SUP>-2</SUP>. The wave energetics
indicate a substantial decrease in energy with atmospheric height,
confirming that magnetic pores are able to transport waves that exhibit
appreciable energy damping, which may release considerable energy into
the local chromospheric plasma.
---------------------------------------------------------
Title: Energy Propagation by Transverse Waves in Multiple Flux Tube
Systems Using Filling Factors
Authors: Van Doorsselaere, T.; Gijsen, S. E.; Andries, J.; Verth, G.
2014ApJ...795...18V Altcode:
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.
---------------------------------------------------------
Title: The Transverse and Rotational Motions of Magnetohydrodynamic
Kink Waves in the Solar Atmosphere
Authors: Goossens, M.; Soler, R.; Terradas, J.; Van Doorsselaere,
T.; Verth, G.
2014ApJ...788....9G Altcode:
Magnetohydrodynamic (MHD) kink waves have now been observed to be
ubiquitous throughout the solar atmosphere. With modern instruments,
they have now been detected in the chromosphere, interface region,
and corona. The key purpose of this paper is to show that kink waves
do not only involve purely transverse motions of solar magnetic flux
tubes, but the velocity field is a spatially and temporally varying
sum of both transverse and rotational motion. Taking this fact into
account is particularly important for the accurate interpretation of
varying Doppler velocity profiles across oscillating structures such
as spicules. It has now been shown that, as well as bulk transverse
motions, spicules have omnipresent rotational motions. Here we emphasize
that caution should be used before interpreting the particular MHD
wave mode/s responsible for these rotational motions. The rotational
motions are not necessarily signatures of the classic axisymmetric
torsional Alfvén wave alone, because kink motion itself can also
contribute substantially to varying Doppler velocity profiles observed
across these structures. In this paper, the displacement field of the
kink wave is demonstrated to be a sum of its transverse and rotational
components, both for a flux tube with a discontinuous density profile at
its boundary, and one with a more realistic density continuum between
the internal and external plasma. Furthermore, the Doppler velocity
profile of the kink wave is forward modeled to demonstrate that,
depending on the line of sight, it can either be quite distinct or
very similar to that expected from a torsional Alfvén wave.
---------------------------------------------------------
Title: The Generation and Damping of Propagating MHD Kink Waves in
the Solar Atmosphere
Authors: Morton, R. J.; Verth, G.; Hillier, A.; Erdélyi, R.
2014ApJ...784...29M Altcode: 2013arXiv1310.4650M
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 Alfvén 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.
---------------------------------------------------------
Title: Wave Energy Deposition in the Solar Corona
Authors: Van Doorsselaere, Tom; Goossens, Marcel; Verth, Gary; Soler,
Roberto; Gijsen, Stief; Andries, Jesse
2014cosp...40E3464V Altcode:
Recently, a significant amount of transverse wave energy has been
estimated propagating along solar atmospheric magnetic fields. However,
these estimates have been made with the classic bulk Alfven wave
model which assumes a homogeneous plasma. In this talk, the kinetic,
magnetic, and total energy densities and the flux of energy are first
computed for transverse MHD waves in one-dimensional cylindrical flux
tube models with a piecewise constant density profile. There are
fundamental deviations from the properties for classic bulk Alfven
waves. (1) There is no local equipartition between kinetic and magnetic
energy. (2) The flux of energy and the velocity of energy transfer have,
in addition to a component parallel to the magnetic field, components
in the planes normal to the magnetic field. (3) The energy densities
and the flux of energy vary spatially, contrary to the case of classic
bulk Alfven waves. This last property is then used to connect the
energy flux in such a simple model to the energy flux in multiple flux
tube systems. We use the plasma filling factor f to derive an ad-hoc
formula for estimating the energy that is propagated in bundles of
loops. We find that the energy flux in kink waves is lower than the
energy computed from a bulk Alfven wave interpretation, by a factor
that is (approximately) between f and 2f. We consider some geometric
models to quantify this correction factor.
---------------------------------------------------------
Title: Doppler displacements in kink MHD waves in solar flux tubes
Authors: Goossens, Marcel; Van Doorsselaere, Tom; Terradas, Jaume;
Verth, Gary; Soler, Roberto
2014cosp...40E1045G Altcode:
Doppler displacements in kink MHD waves in solar flux tubes Presenting
author: M. Goossens Co-authors: R. Soler, J. Terradas, T. Van
Doorsselaere, G. Verth The standard interpretation of the transverse MHD
waves observed in the solar atmosphere is that they are non-axisymmetric
kink m=1) waves on magnetic flux tubes. This interpretation is based
on the fact that axisymmetric and non-axisymmetric fluting waves do
not displace the axis of the loop and the loop as a whole while kink
waves indeed do so. A uniform transverse motion produces a Doppler
displacement that is constant across the magnetic flux tube. A recent
development is the observation of Doppler displacements that vary across
the loop. The aim of the present contribution is to show that spatial
variations of the Doppler displacements across the loop can be caused by
kink waves. The motion associated with a kink wave is purely transverse
only when the flux tube is uniform and sufficiently thin. Only in that
case do the radial and azimuthal components of displacement have the
same amplitude and is the azimuthal component a quarter of a period
ahead of the radial component. This results in a unidirectional
or transverse displacement. When the flux tube is non-uniform and
has a non-zero radius the conditions for the generation of a purely
transverse motion are not any longer met. In that case the motion in
a kink wave is the sum of a transverse motion and a non-axisymmetric
rotational motion that depends on the azimuthal angle. It can produce
complicated variations of the Doppler displacement across the loop. I
shall discuss the various cases of possible Doppler displacenents
that can occur depending on the relative sizes of the amplitudes of
the radial and azimuthal components of the displacement in the kink
wave and on the orientation of the line of sight.
---------------------------------------------------------
Title: Characteristics of Transverse Waves in Chromospheric Mottles
Authors: Kuridze, D.; Verth, G.; Mathioudakis, M.; Erdélyi, R.;
Jess, D. B.; Morton, R. J.; Christian, D. J.; Keenan, F. P.
2013ApJ...779...82K Altcode: 2013arXiv1310.3628K
Using data obtained by the high temporal and spatial resolution
Rapid Oscillations in the Solar Atmosphere instrument on the Dunn
Solar Telescope, we investigate at an unprecedented level of detail
transverse oscillations in chromospheric fine structures near the solar
disk center. The oscillations are interpreted in terms of propagating
and standing magnetohydrodynamic kink waves. Wave characteristics
including the maximum transverse velocity amplitude and the phase
speed are measured as a function of distance along the structure's
length. Solar magnetoseismology is applied to these measured parameters
to obtain diagnostic information on key plasma parameters (e.g.,
magnetic field, density, temperature, flow speed) of these localized
waveguides. The magnetic field strength of the mottle along the ~2 Mm
length is found to decrease by a factor of 12, while the local plasma
density scale height is ~280 ± 80 km.
---------------------------------------------------------
Title: Erratum: "Energy Content and Propagation in Transverse Solar
Atmospheric Waves" <A href="/abs/2013ApJ...768..191G">(2013, ApJ,
768, 191)</A>
Authors: Goossens, M.; Van Doorsselaere, T.; Soler, R.; Verth, G.
2013ApJ...771...74G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Evidence for the Photospheric Excitation of Incompressible
Chromospheric Waves
Authors: Morton, R. J.; Verth, G.; Fedun, V.; Shelyag, S.; Erdélyi, R.
2013ApJ...768...17M Altcode: 2013arXiv1303.2356M
Observing the excitation mechanisms of incompressible transverse
waves is vital for determining how energy propagates through the lower
solar atmosphere. We aim to show the connection between convectively
driven photospheric flows and incompressible chromospheric waves. The
observations presented here show the propagation of incompressible
motion through the quiet lower solar atmosphere, from the photosphere
to the chromosphere. We determine photospheric flow vectors to search
for signatures of vortex motion and compare results to photospheric
flows present in convective simulations. Further, we search for the
chromospheric response to vortex motions. Evidence is presented that
suggests incompressible waves can be excited by the vortex motions of a
strong magnetic flux concentration in the photosphere. A chromospheric
counterpart to the photospheric vortex motion is also observed,
presenting itself as a quasi-periodic torsional motion. Fine-scale,
fibril structures that emanate from the chromospheric counterpart
support transverse waves that are driven by the observed torsional
motion. A new technique for obtaining details of transverse waves from
time-distance diagrams is presented and the properties of transverse
waves (e.g., amplitudes and periods) excited by the chromospheric
torsional motion are measured.
---------------------------------------------------------
Title: Energy Content and Propagation in Transverse Solar Atmospheric
Waves
Authors: Goossens, M.; Van Doorsselaere, T.; Soler, R.; Verth, G.
2013ApJ...768..191G Altcode:
Recently, a significant amount of transverse wave energy has been
estimated propagating along solar atmospheric magnetic fields. However,
these estimates have been made with the classic bulk Alfvén wave
model which assumes a homogeneous plasma. In this paper, the kinetic,
magnetic, and total energy densities and the flux of energy are computed
for transverse MHD waves in one-dimensional cylindrical flux tube models
with a piecewise constant or continuous radial density profile. There
are fundamental deviations from the properties for classic bulk Alfvén
waves. (1) There is no local equipartition between kinetic and magnetic
energy. (2) The flux of energy and the velocity of energy transfer have,
in addition to a component parallel to the magnetic field, components
in the planes normal to the magnetic field. (3) The energy densities
and the flux of energy vary spatially, contrary to the case of classic
bulk Alfvén waves. This last property has the important consequence
that the energy flux computed with the well known expression for bulk
Alfvén waves could overestimate the real flux by a factor in the
range 10-50, depending on the flux tube equilibrium properties.
---------------------------------------------------------
Title: SDO/AIA observations of periodic and quasi-periodic phenomenon
associated with an EUV jet
Authors: Morton, Richard; Verth, Gary; Erdelyi, Robertus; Srivastava,
Abhi
2013EGUGA..15...52M Altcode:
It has long been advocated that explosive magnetic activity is
responsible for the mass-balance in the solar atmosphere, supplying
the corona and the solar wind with heated plasma. The explosive
events are thought to be the result of emerging bi-polar (EB) regions
reconnecting with pre-existing, open fields, with the size of the EB's
(i.e., granular, super-granular) being related to size of the resulting
feature (i.e., spicules, EUV/X-ray jets). Recent evidence has suggested
a deeper relationship between spicules and EUV jets (Sterling et al.,
2010). We present here observations of a EUV jet observed with SDO/AIA
close to a southern coronal hole. The jet can be considered as a
'Blowout jet' (using the terminology of Moore et al., 2010), launching
vast amounts of chromospheric plasma into the atmosphere along with
hotter material. The hotter part of the jet appears to be composed
of multiple, (quasi-)periodic ejections that individually resemble
fast moving (>100 km/s) spicules. The multiple ejections appear
crucial for distributing the hotter material high into the corona,
possibly suggesting that larger EUV/X-ray are composed of many smaller
spicule-like events. Although the event is close to the limb, evidence
for reconnection at the chromospheric level is provided. Further,
evidence for helicity (or torsional motion) and the presence of slow and
fast Magnetohydrodynamic waves is given, with the wave mode excitation
likely due to the reconnection process. Exploiting the observed wave
motion, we also use magneto-seismological techniques to determine local
plasma parameters with sub-resolution accuracy along one of the jets
unique features.
---------------------------------------------------------
Title: Observations of ubiquitous compressive waves in the Sun's
chromosphere
Authors: Morton, Richard J.; Verth, Gary; Jess, David B.; Kuridze,
David; Ruderman, Michael S.; Mathioudakis, Mihalis; Erdélyi, Robertus
2012NatCo...3.1315M Altcode: 2012NatCo...3E1315M; 2013arXiv1306.4124M
The details of the mechanism(s) responsible for the observed
heating and dynamics of the solar atmosphere still remain a
mystery. Magnetohydrodynamic waves are thought to have a vital role
in this process. Although it has been shown that incompressible
waves are ubiquitous in off-limb solar atmospheric observations,
their energy cannot be readily dissipated. Here we provide, for the
first time, on-disk observation and identification of concurrent
magnetohydrodynamic wave modes, both compressible and incompressible,
in the solar chromosphere. The observed ubiquity and estimated energy
flux associated with the detected magnetohydrodynamic waves suggest
the chromosphere is a vast reservoir of wave energy with the potential
to meet chromospheric and coronal heating requirements. We are also
able to propose an upper bound on the flux of the observed wave energy
that is able to reach the corona based on observational constraints,
which has important implications for the suggested mechanism(s) for
quiescent coronal heating.
---------------------------------------------------------
Title: Magneto-seismology of solar atmospheric loops by means of
longitudinal oscillations
Authors: Luna-Cardozo, M.; Verth, G.; Erdélyi, R.
2012IAUS..286..437L Altcode: 2012arXiv1204.4197L
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 discussed.
---------------------------------------------------------
Title: Generation of Quasi-periodic Waves and Flows in the Solar
Atmosphere by Oscillatory Reconnection
Authors: McLaughlin, J. A.; Verth, G.; Fedun, V.; Erdélyi, R.
2012ApJ...749...30M Altcode: 2012arXiv1203.6846M
We investigate the long-term evolution of an initially buoyant magnetic
flux tube emerging into a gravitationally stratified coronal hole
environment and report on the resulting oscillations and outflows. We
perform 2.5-dimensional nonlinear numerical simulations, generalizing
the models of McLaughlin et al. and Murray et al. We find that the
physical mechanism of oscillatory reconnection naturally generates
quasi-periodic vertical outflows, with a transverse/swaying aspect. The
vertical outflows consist of both a periodic aspect and evidence
of a positively directed flow. The speed of the vertical outflow
(20-60 km s<SUP>-1</SUP>) is comparable to those reported in the
observational literature. We also perform a parametric study varying
the magnetic strength of the buoyant flux tube and find a range of
associated periodicities: 1.75-3.5 minutes. Thus, the mechanism of
oscillatory reconnection may provide a physical explanation to some
of the high-speed, quasi-periodic, transverse outflows/jets recently
reported by a multitude of authors and instruments.
---------------------------------------------------------
Title: Longitudinal Oscillations in Density Stratified and Expanding
Solar Waveguides
Authors: Luna-Cardozo, M.; Verth, G.; Erdélyi, R.
2012ApJ...748..110L Altcode: 2012arXiv1204.4201L
Waves and oscillations can provide vital information about the internal
structure of waveguides in which they propagate. 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
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.
---------------------------------------------------------
Title: Determination of Sub-resolution Structure of a Jet by Solar
Magnetoseismology
Authors: Morton, R. J.; Verth, G.; McLaughlin, J. A.; Erdélyi, R.
2012ApJ...744....5M Altcode: 2011arXiv1109.4851M
A thin dark thread is observed in a UV/EUV solar jet in the 171 Å,
193 Å, and 211 Å, and partially in 304 Å. The dark thread appears
to originate in the chromosphere but its temperature does not appear
to lie within the passbands of the Atmospheric Imaging Assembly
onboard the Solar Dynamics Observatory. We therefore implement solar
magnetoseismology to estimate the plasma parameters of the dark
thread. A propagating kink (transverse) wave is observed to travel
along the dark thread. The wave is tracked over a range of ~7000 km
by placing multiple slits along the axis of the dark thread. The phase
speed and amplitude of the wave are estimated and magnetoseismological
theory is employed to determine the plasma parameters. We are able
to estimate the plasma temperature, density gradient, magnetic field
gradient, and sub-resolution expansion of the dark thread. The dark
thread is found to be cool, T <~ 3 × 10<SUP>4</SUP>, with both
strong density and magnetic field gradients. The expansion of the flux
tube along its length is ~300-400 km.
---------------------------------------------------------
Title: Frequency Filtering of Torsional Alfvén Waves by Chromospheric
Magnetic Field
Authors: Fedun, V.; Verth, G.; Jess, D. B.; Erdélyi, R.
2011ApJ...740L..46F Altcode:
In this Letter, we demonstrate how the observation of broadband
frequency propagating torsional Alfvén waves in chromospheric
magnetic flux tubes can provide valuable insight into their magnetic
field structure. By implementing a full nonlinear three-dimensional
magnetohydrodynamic numerical simulation with a realistic vortex driver,
we demonstrate how the plasma structure of chromospheric magnetic flux
tubes can act as a spatially dependent frequency filter for torsional
Alfvén waves. Importantly, for solar magnetoseismology applications,
this frequency filtering is found to be strongly dependent on magnetic
field structure. With reference to an observational case study of
propagating torsional Alfvén waves using spectroscopic data from the
Swedish Solar Telescope, we demonstrate how the observed two-dimensional
spatial distribution of maximum power Fourier frequency shows a strong
correlation with our forward model. This opens the possibility of
beginning an era of chromospheric magnetoseismology, to complement
the more traditional methods of mapping the magnetic field structure
of the solar chromosphere.
---------------------------------------------------------
Title: Resonantly Damped Propagating Kink Waves in Longitudinally
Stratified Solar Waveguides
Authors: Soler, R.; Terradas, J.; Verth, G.; Goossens, M.
2011ApJ...736...10S Altcode: 2011arXiv1105.0067S
It has been shown that resonant absorption is a robust physical
mechanism for explaining the observed damping of magnetohydrodynamic
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. More recently, these studies have been extended to explain
the attenuation of propagating coronal kink waves observed by the
Coronal Multi-Channel Polarimeter. 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.
---------------------------------------------------------
Title: Magnetoseismological Determination of Magnetic Field and
Plasma Density Height Variation in a Solar Spicule
Authors: Verth, G.; Goossens, M.; He, J. -S.
2011ApJ...733L..15V Altcode:
The variation of magnetic field strength and plasma density along a
solar spicule is determined by the use of magnetoseismology. From
Solar Optical Telescope observations of a kink wave propagating
along a spicule, by estimating the spatial change in phase speed and
velocity amplitude, a novel approach is demonstrated to determine
the chromospheric height variation of both magnetic field and plasma
density. Furthermore, the magnetoseismological estimate of the plasma
density gradient is combined with electron density estimates from
spectroscopy to determine the changing degree of ionization of hydrogen
along a spicule.
---------------------------------------------------------
Title: Propagating Kink Waves in Stratified Magnetic Waveguides of
the Solar Corona
Authors: Soler, Roberto; Terradas, J.; Verth, G.; Goossens, M.
2011SPD....42.1805S Altcode: 2011BAAS..43S.1805S
Recent observations using the Coronal Multi-Channel Polarimeter (CoMP)
show ubiquitous propagating waves of low amplitude in magnetic loops
of the solar corona. These observations have been interpreted as
magnetohydrodynamic (MHD) resonant kink waves. It has been shown
that resonant absorption is a robust physical mechanism to explain
the observed damping of MHD kink waves in the solar atmosphere due to
naturally occurring plasma inhomogeneity in the direction transverse
to the magnetic field. In the present study, for the first time we
investigate the properties of propagating kink waves in solar magnetic
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.
---------------------------------------------------------
Title: Seismology of Transversely Oscillating Coronal Loops with
Siphon Flows
Authors: Terradas, J.; Arregui, I.; Verth, G.; Goossens, M.
2011ApJ...729L..22T Altcode: 2011arXiv1101.5238T
There are ubiquitous flows observed in the solar atmosphere of
sub-Alfvénic speeds; however, after flaring and coronal mass ejection
events flows can become Alfvénic. In this Letter, we derive an
expression for the standing kink mode frequency due to siphon flow in
coronal loops, valid for both low and high speed regimes. It is found
that siphon flow introduces a linear, spatially dependent phase shift
along coronal loops and asymmetric eigenfunctions. We demonstrate
how this theory can be used to determine the kink and flow speed of
oscillating coronal loops with reference to an observational case
study. It is shown that the presence of siphon flow can cause the
underestimation of magnetic field strength in coronal loops using the
traditional seismological methods.
---------------------------------------------------------
Title: Selective spatial damping of propagating kink waves due to
resonant absorption
Authors: Terradas, J.; Goossens, M.; Verth, G.
2010A&A...524A..23T Altcode: 2010arXiv1004.4468T
Context. There is observational evidence of propagating kink waves
driven by photospheric motions. These disturbances, interpreted
as kink magnetohydrodynamic (MHD) waves are attenuated as they
propagate upwards in the solar corona. <BR /> Aims: We show that
resonant absorption provides a simple explanation to the spatial
damping of these waves. <BR /> Methods: Kink MHD waves are studied
using a cylindrical model of solar magnetic flux tubes, which includes
a non-uniform layer at the tube boundary. Assuming that the frequency
is real and the longitudinal wavenumber complex, the damping length and
damping per wavelength produced by resonant absorption are analytically
calculated in the thin tube (TT) approximation, valid for coronal
waves. This assumption is relaxed in the case of chromospheric tube
waves and filament thread waves. <BR /> Results: The damping length of
propagating kink waves due to resonant absorption is a monotonically
decreasing function of frequency. For kink waves with low frequencies,
the damping length is exactly inversely proportional to frequency, and
we denote this as the TGV relation. When moving to high frequencies,
the TGV relation continues to be an exceptionally good approximation
of the actual dependency of the damping length on frequency. This
dependency means that resonant absorption is selective as it favours
low-frequency waves and can efficiently remove high-frequency waves
from a broad band spectrum of kink waves. The efficiency of the
damping due to resonant absorption depends on the properties of the
equilibrium model, in particular on the width of the non-uniform layer
and the steepness of the variation in the local Alfvén speed. <BR />
Conclusions: Resonant absorption is an effective mechanism for the
spatial damping of propagating kink waves. It is selective because
the damping length is inversely proportional to frequency so that the
damping becomes more severe with increasing frequency. This means that
radial inhomogeneity can cause solar waveguides to be a natural low-pass
filter for broadband disturbances. Kink wave trains travelling along,
e.g., coronal loops, will therefore have a greater proportion of the
high-frequency components dissipated lower down in the atmosphere. This
could have important consequences for the spatial distribution of wave
heating in the solar atmosphere.
---------------------------------------------------------
Title: Observational Evidence of Resonantly Damped Propagating Kink
Waves in the Solar Corona
Authors: Verth, G.; Terradas, J.; Goossens, M.
2010ApJ...718L.102V Altcode: 2010arXiv1007.1080V
In this Letter, we establish clear evidence for the resonant
absorption damping mechanism by analyzing observational data from
the novel Coronal Multi-Channel Polarimeter. This instrument has
established that in the solar corona there are ubiquitous propagating
low-amplitude (≈1 km s<SUP>-1</SUP>) Alfvénic waves with a wide
range of frequencies. Realistically interpreting these waves as the
kink mode from magnetohydrodynamic wave theory, they should exhibit a
frequency-dependent damping length due to resonant absorption, governed
by the Terradas-Goossens-Verth relation showing that transverse plasma
inhomogeneity in coronal magnetic flux tubes causes them to act as
natural low-pass filters. It is found that the observed frequency
dependence on damping length (up to about 8 mHz) can be explained by
the kink wave interpretation; and furthermore, the spatially averaged
equilibrium parameter describing the length scale of transverse plasma
density inhomogeneity over a system of coronal loops is consistent
with the range of values estimated from Transition Region and Coronal
Explorer observations of standing kink modes.
---------------------------------------------------------
Title: Magnetoseismology: Eigenmodes of Torsional Alfvén Waves in
Stratified Solar Waveguides
Authors: Verth, G.; Erdélyi, R.; Goossens, M.
2010ApJ...714.1637V Altcode:
There have recently been significant claims of Alfvén wave observation
in the solar chromosphere and corona. We investigate how the radial and
longitudinal plasma structuring affects the observational properties
of torsional Alfvén waves in magnetic flux tubes for the purposes of
solar magnetoseismology. The governing magnetohydrodynamic equations
of these waves in axisymmetric flux tubes of arbitrary radial and
axial plasma structuring are derived and we study their observable
properties for various equilibria in both thin and finite-width
magnetic flux tubes. For thin flux tubes, it is demonstrated that
observation of the eigenmodes of torsional Alfvén waves can provide
temperature diagnostics of both the internal and surrounding plasma. In
the finite-width flux tube regime, it is shown that these waves are the
ideal magnetoseismological tool for probing radial plasma inhomogeneity
in solar waveguides.
---------------------------------------------------------
Title: Coronal Seismology by Means of Kink Oscillation Overtones
Authors: Andries, J.; van Doorsselaere, T.; Roberts, B.; Verth, G.;
Verwichte, E.; Erdélyi, R.
2009SSRv..149....3A Altcode:
The detection of overtones of coronal loop kink oscillations has been
an important advance in the development of coronal seismology. It has
significantly increased the potential of coronal seismology and has
thus initiated important theoretical and observational improvements. New
detections of overtones have been made and a reduction of the error bars
has been obtained. The efforts of theoreticians to extend eigenmode
studies to more general coronal loop models is no longer a matter
of checking the robustness of the model but now also allows for the
estimation of certain equilibrium parameters. The frequencies of
the detected (longitudinal) overtones are in particular sensitive to
changes in the equilibrium properties along the loop, especially the
density and the magnetic field expansion. Also, attempts have been
made to use the limited longitudinal resolution in combination with
the theoretical eigenmodes as an additional seismological tool.
---------------------------------------------------------
Title: Refined Magnetoseismological Technique for the Solar Corona
Authors: Verth, G.; Erdélyi, R.; Jess, D. B.
2008ApJ...687L..45V Altcode:
We present a step-by-step guide of a refined magnetoseismological
technique for plasma diagnostics in the Sun's corona. Recently developed
MHD wave theory which models a coronal loop as an expanding magnetic
flux tube with an arbitrary longitudinal plasma density profile is
applied to TRACE observations of fast kink oscillations. The theory
predicts that using the observed ratio of the first overtone and
fundamental mode to predict the plasma density scale height and not
taking account of loop expansion will lead to an overestimation of scale
height. For the first time, the size of this correction is quantified
directly from observation and for the particular case study presented
here, it is found that the overestimation is approximately by a factor
of 2.
---------------------------------------------------------
Title: Transverse Oscillations of Longitudinally Stratified Coronal
Loops with Variable Cross Section
Authors: Ruderman, M. S.; Verth, G.; Erdélyi, R.
2008ApJ...686..694R Altcode:
We consider transverse oscillations of coronal loops that have both
variable circular cross-sectional area and plasma density in the
longitudinal direction. The primary focus of this paper is to study the
eigenmodes of these oscillations. Implementing the method of asymptotic
expansions with the ratio of the loop radius to length as a small
parameter, a second-order ordinary differential equation is derived
describing the displacement of the loop axis. Together with the boundary
conditions at the tube ends that follow from the frozen-in condition,
this equation constitutes the Sturm-Liouville problem determining
the eigenfrequencies and eigenmodes. Our results are relevant to the
magnetoseismological method of estimating the coronal density scale
height by using the observed ratio of the fundamental frequency and
first overtone of loop kink oscillations. It is shown that this method
is very sensitive to the tube expansion factor, which is the ratio of
the tube radii at the apex and footpoints. The estimated scale height
is a monotonically decreasing function of the expansion factor.
---------------------------------------------------------
Title: Effect of longitudinal magnetic and density inhomogeneity on
transversal coronal loop oscillations
Authors: Verth, G.; Erdélyi, R.
2008A&A...486.1015V Altcode:
Context: Observations of post-flare transversal coronal loop
oscillations by TRACE have given us an excellent opportunity to
implement magneto-seismological techniques for probing the plasma fine
structure of the Sun's upper atmosphere. <BR />Aims: We investigate
the combined effect of magnetic and density stratification on
transversal coronal loop oscillations. <BR />Methods: A coronal loop
will be modelled as an expanding magnetic flux tube with arbitrary
longitudinal plasma density. The governing equation of the fast
kink body mode is derived and solved by analytical approximation and
numerical methods. <BR />Results: It is found that even a relatively
small coronal loop expansion can have a significant and pronounced
effect on the accuracy of the plasma density scale height measurements
derived from observation of loop oscillations. <BR />Conclusions:
To conduct more accurate and realistic magneto-seismology of coronal
loops, the magnetic field divergence should be taken into account.
---------------------------------------------------------
Title: Discovery of Spatial Periodicities in a Coronal Loop Using
Automated Edge-Tracking Algorithms
Authors: Jess, D. B.; Mathioudakis, M.; Erdélyi, R.; Verth, G.;
McAteer, R. T. J.; Keenan, F. P.
2008ApJ...680.1523J Altcode: 2008arXiv0802.1971J
A new method for automated coronal loop tracking, in both spatial
and temporal domains, is presented. Applying this technique to TRACE
data, obtained using the 171 Å filter on 1998 July 14, we detect a
coronal loop undergoing a 270 s kink-mode oscillation, as previously
found by Aschwanden et al. However, we also detect flare-induced,
and previously unnoticed, spatial periodicities on a scale of 3500 km,
which occur along the coronal loop edge. Furthermore, we establish a
reduction in oscillatory power for these spatial periodicities of 45%
over a 222 s interval. We relate the reduction in detected oscillatory
power to the physical damping of these loop-top oscillations.
---------------------------------------------------------
Title: Magneto-seismology: effect of inhomogeneous magnetic field
on transversal coronal loop oscillations
Authors: Verth, G.
2008IAUS..247..123V Altcode: 2007IAUS..247..123V
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.
---------------------------------------------------------
Title: Solar feature tracking in both spatial and temporal domains
Authors: Jess, D. B.; Mathioudakis, M.; Erdélyi, R.; Verth, G.;
McAteer, R. T. J.; Keenan, F. P.
2008IAUS..247..288J Altcode: 2007IAUS..247..288J
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 171 Å 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.
---------------------------------------------------------
Title: Spatial magneto-seismology: effect of density stratification
on the first harmonic amplitude profile of transversal coronal
loop oscillations
Authors: Verth, G.; Van Doorsselaere, T.; Erdélyi, R.; Goossens, M.
2007A&A...475..341V Altcode:
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. <BR />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. <BR />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. <BR />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. <BR />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.
---------------------------------------------------------
Title: Magneto-seismology of the solar atmosphere
Authors: Verth, G.
2007AN....328..764V Altcode:
Magnetohydrodynamic (MHD) waves in solar coronal loops, which were
previously only predicted by theory have actually been detected with
space-borne instruments. These observations have given an important
and novel tool to measure fundamental parameters in the magnetically
embedded solar corona. This paper will illustrate how information
about the magnetic and density structure along coronal loops can be
determined by measuring the frequency or amplitude profiles of standing
fast kink mode oscillations
---------------------------------------------------------
Title: The effect of density stratification on the amplitude profile
of transversal coronal loop oscillations
Authors: Erdélyi, R.; Verth, G.
2007A&A...462..743E Altcode:
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. <BR />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. <BR />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. <BR />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 Å 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. <BR />Conclusions:
.The effect of density stratification on the amplitude profile of the
fundamental standing kink mode of a TRACE 171 Å 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.
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Title: Solar Coronal Magneto- Seismology With Solar Orbiter
Authors: Verth, G.; Erdéyi, R.
2007ESASP.641E..30V Altcode: 2006ESASP.641E..30V
MHD waves in solar coronal loops, which were previously only predicted
by theory have now actually been detected with instruments such as
TRACE and SUMER on-board SOHO. These observations have given the
solar community an important and novel tool to measure fundamental
parameters in the magnetically embedded solar corona. Theory has been
developed to derive detailed diagnostic information, e.g., density,
magnetic field look structure, geometry, and stratifications. In this
paper we demonstrate through examples of case studies how the EUV
imager on Solar Orbiter can be used for solar atmospheric (coronal)
magneto-seismology. Possible methods will be discussed to determine
(i) if magnetic field divergence or plasma density stratification is
the dominating factor in transversal loop oscillations (ii) important
parameters such as the density scale heigh and magnetic dipole depth
of a loop.
