demoortel

explanation blue bibcodes open ADS page with paths to full text

Author name code: demoortel
ADS astronomy entries on 2022-09-14
author:"De Moortel, Ineke"

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Title: Propagating Alfvén waves in open structures with random
structuring
Authors: Pascoe, D. J.; De Moortel, I.; Pagano, P.; Howson, T. A.
2022MNRAS.tmp.2248P Altcode:
We consider the behaviour of Alfvén waves propagating in a medium
with random density perturbations. The imposed density perturbations
have a broadband spectrum and their characteristic spatial scale may be
defined according to the peak in the spectrum. The interaction of the
boundary driven Alfvén waves with the medium generates reflections
most efficiently when their wavelength is comparable to the spatial
scale of the density perturbations. For our monotonic driver, this
leads to the generation of quasi-periodic oscillations. The periods
of oscillation of the propagating Alfvén waves is no longer only
associated with the driver. Additional periodicities may be associated
with one or more characteristic spatial scales in the density profile,
or with beating between other spectral components. Multiple wave
reflections cause oscillatory power to be retained at low altitudes,
increasing opportunities to contribute to heating at those locations.

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Title: The effects of driving time scales on coronal heating in a
stratified atmosphere
Authors: Howson, T. A.; De Moortel, I.
2022A&A...661A.144H Altcode: 2022arXiv220412205H
 Aims: We investigate the atmospheric response to coronal
heating driven by random velocity fields with different characteristic
time scales and amplitudes. Methods: We conducted a series
of three-dimensional magnetohydrodynamic simulations of random
driving imposed on a gravitationally stratified model of the solar
atmosphere. In order to understand differences between alternating
current (AC) and direct current (DC) heating, we considered the
effects of changing the characteristic time scales of the imposed
velocities. We also investigated the effects of the magnitude of
the velocity driving. Results: In all cases, complex foot
point motions lead to a proliferation of current sheets and energy
dissipation throughout the coronal volume. For a given driving
amplitude, DC driving typically leads to a greater rate of energy
injection when compared to AC driving. This ultimately leads to the
formation of larger currents, increased heating rates, and higher
coronal temperatures in DC simulations. There is no difference in the
spatial distribution of energy dissipation across simulations; however,
energy release events in AC cases tend to be more frequent and last for
less time than in DC cases. This results in more asymmetric temperature
profiles for field lines heated by AC drivers. Higher velocity driving
is associated with larger currents, higher temperatures, and the
corona occupying a larger fraction of the simulation volume. In all
cases, the majority of heating is associated with small energy release
events, which occur much more frequently than larger events. 
Conclusions: When combined with observational results that highlight
a greater abundance of oscillatory power in lower frequency modes,
these findings suggest that energy release in the corona is more
likely to be driven by longer time scale motions. In the corona,
AC and DC driving occur concurrently and their effects remain
difficult to isolate. The distribution of field line temperatures
and the asymmetry of temperature profiles may reveal the frequency
and longevity of energy release events and therefore the relative
importance of AC and DC heating. Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202142872/olm">https://www.aanda.org</A>

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Title: Coronal Seismology Using Damping of Propagating Kink Waves
Authors: Pascoe, D. J.; Van Doorsselaere, T.; De Moortel, I.
2022ApJ...929..101P Altcode:
We consider the use of propagating kink waves, such as those
observed by the Coronal Multi-channel Polarimeter, as a diagnostic
technique. The transverse structuring of the plasma may be inferred by
the frequency-dependent wave damping, which is attributed to resonant
absorption. We include the effect of reflection of waves at the loop
footpoints, which leads to the asymmetry parameter, describing the ratio
of driven wave power at the footpoints becoming weakly constrained. The
classical model of resonant absorption based on an exponential damping
profile significantly overestimates the damping rate in coronal loops
with low density contrast ratios. The use of the exponential profile in
an analysis of observations therefore leads to underestimates for the
density contrast ratio and associated parameters such as the heating
rate following phase mixing.

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Title: Probing the Physics of the Solar Atmosphere with the Multi-slit
Solar Explorer (MUSE). I. Coronal Heating
Authors: De Pontieu, Bart; Testa, Paola; Martínez-Sykora, Juan;
Antolin, Patrick; Karampelas, Konstantinos; Hansteen, Viggo; Rempel,
Matthias; Cheung, Mark C. M.; Reale, Fabio; Danilovic, Sanja; Pagano,
Paolo; Polito, Vanessa; De Moortel, Ineke; Nóbrega-Siverio, Daniel;
Van Doorsselaere, Tom; Petralia, Antonino; Asgari-Targhi, Mahboubeh;
Boerner, Paul; Carlsson, Mats; Chintzoglou, Georgios; Daw, Adrian;
DeLuca, Edward; Golub, Leon; Matsumoto, Takuma; Ugarte-Urra, Ignacio;
McIntosh, Scott W.; the MUSE Team
2022ApJ...926...52D Altcode: 2021arXiv210615584D
The Multi-slit Solar Explorer (MUSE) is a proposed mission composed of
a multislit extreme ultraviolet (EUV) spectrograph (in three spectral
bands around 171 Å, 284 Å, and 108 Å) and an EUV context imager (in
two passbands around 195 Å and 304 Å). MUSE will provide unprecedented
spectral and imaging diagnostics of the solar corona at high spatial
(≤0.″5) and temporal resolution (down to ~0.5 s for sit-and-stare
observations), thanks to its innovative multislit design. By obtaining
spectra in four bright EUV lines (Fe IX 171 Å, Fe XV 284 Å, Fe XIX-Fe
XXI 108 Å) covering a wide range of transition regions and coronal
temperatures along 37 slits simultaneously, MUSE will, for the first
time, "freeze" (at a cadence as short as 10 s) with a spectroscopic
raster the evolution of the dynamic coronal plasma over a wide range of
scales: from the spatial scales on which energy is released (≤0.″5)
to the large-scale (~170″ × 170″) atmospheric response. We use
numerical modeling to showcase how MUSE will constrain the properties of
the solar atmosphere on spatiotemporal scales (≤0.″5, ≤20 s) and
the large field of view on which state-of-the-art models of the physical
processes that drive coronal heating, flares, and coronal mass ejections
(CMEs) make distinguishing and testable predictions. We describe the
synergy between MUSE, the single-slit, high-resolution Solar-C EUVST
spectrograph, and ground-based observatories (DKIST and others), and
the critical role MUSE plays because of the multiscale nature of the
physical processes involved. In this first paper, we focus on coronal
heating mechanisms. An accompanying paper focuses on flares and CMEs.

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Title: A Fast and Accurate Method to Capture the Solar
Corona/Transition Region Enthalpy Exchange in Multi-dimensional
Magnetohydrodynamic Simulations
Authors: Johnston, Craig; Hood, Alan; De Moortel, Ineke; Daldorff, Lars
2021AGUFMSH12B..03J Altcode:
The brightness of the emission from coronal loops in the solar
atmosphere is strongly dependent on the temperature and density of
the confined plasma. Following an impulsive release of energy, the
coronal plasma undergoes phases of upflow and downflow as it cools,
with significant variations in its properties. In particular, the
sudden increase in coronal temperature leads to an excess downward
heat flux that the transition region (TR) is unable to radiate. This
generates an upflow of mass and enthalpy from the TR to the corona,
increasing the coronal density. The mass and enthalpy exchange is
highly sensitive to the TR resolution in numerical simulations. With
a numerically under-resolved TR, major errors occur in simulating the
coronal density evolution and, thus, the predicted loop emission. We
present a new method that addresses the difficulty of obtaining
the correct interaction between the corona and corona/chromosphere
interface. In the TR, an Adaptive Conduction method is used that
artificially broadens any unresolved parts of the atmosphere, allowing
them to be resolved while maintaining the correct physics. I will
show that this approach, referred to as TRAC, successfully removes the
influence of numerical resolution on the coronal density response to
heating while maintaining high levels of agreement with fully resolved
models. A detailed analytical assessment of the TRAC method will also
be presented to demonstrate why the approach works through all phases
of an impulsive heating event.

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Title: Investigating coronal wave energy estimates using synthetic
non-thermal line widths
Authors: Fyfe, L. E.; Howson, T. A.; De Moortel, I.; Pant, V.; Van
Doorsselaere, T.
2021A&A...656A..56F Altcode: 2021arXiv211000257F
 Aims: Estimates of coronal wave energy remain uncertain as a
large fraction of the energy is likely hidden in the non-thermal line
widths of emission lines. In order to estimate these wave energies,
many previous studies have considered the root mean squared wave
amplitudes to be a factor of \sqrt{2} greater than the non-thermal
line widths. However, other studies have used different factors. To
investigate this problem, we consider the relation between wave
amplitudes and the non-thermal line widths within a variety of 3D
magnetohydrodynamic (MHD) simulations. Methods: We consider
the following 3D numerical models: Alfvén waves in a uniform magnetic
field, transverse waves in a complex braided magnetic field, and two
simulations of coronal heating in an arcade. We applied the forward
modelling code FoMo to generate the synthetic emission data required
to analyse the non-thermal line widths. Results: Determining
a single value for the ratio between the non-thermal line widths and
the root mean squared wave amplitudes is not possible across multiple
simulations. It was found to depend on a variety of factors, including
line-of-sight angles, velocity magnitudes, wave interference, and
exposure time. Indeed, some of our models achieved the values claimed
in recent articles while other more complex models deviated from these
ratios. Conclusions: To estimate wave energies, an appropriate
relation between the non-thermal line widths and root mean squared
wave amplitudes is required. However, evaluating this ratio to be a
singular value, or even providing a lower or upper bound on it, is
not realistically possible given its sensitivity to various MHD models
and factors. As the ratio between wave amplitudes and non-thermal line
widths is not constant across our models, we suggest that this widely
used method for estimating wave energy is not robust.

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Title: Magnetic reconnection and the Kelvin-Helmholtz instability
in the solar corona
Authors: Howson, T. A.; De Moortel, I.; Pontin, D. I.
2021A&A...656A.112H Altcode: 2021arXiv210915019H
Context. The magnetic Kelvin-Helmholtz instability (KHI) has been
proposed as a means of generating magnetohydrodynamic turbulence
and encouraging wave energy dissipation in the solar corona,
particularly within transversely oscillating loops. Aims:
Our goal is to determine whether the KHI encourages magnetic
reconnection in oscillating flux tubes in the solar corona. This will
establish whether the instability enhances the dissipation rate of
energy stored in the magnetic field. Methods: We conducted a
series of three-dimensional magnetohydrodynamic simulations of the
KHI excited by an oscillating velocity shear. We investigated the
effects of numerical resolution, field line length, and background
currents on the growth rate of the KHI and on the subsequent rate
of magnetic reconnection. Results: The KHI is able to trigger
magnetic reconnection in all cases, with the highest rates occurring
during the initial growth phase. Reconnection is found to occur
preferentially along the boundaries of Kelvin-Helmholtz vortices,
where the shear in the velocity and magnetic fields is greatest. The
estimated rate of reconnection is found to be lowest in simulations
where the KHI growth rate is reduced. For example, this is the case
for shorter field lines or due to shear in the background field. 
Conclusions: In non-ideal regimes, the onset of the instability causes
the local reconnection of magnetic field lines and enhances the rate
of coronal wave heating. However, we found that if the equilibrium
magnetic field is sheared across the Kelvin-Helmholtz mixing layer,
the instability does not significantly enhance the rate of reconnection
of the background field, despite the free energy associated with the
non-potential field.

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Title: Forward modelling of heating within a coronal arcade
Authors: Fyfe, L. E.; Howson, T. A.; De Moortel, I.
2021A&A...656A.120F Altcode: 2021arXiv211014257F
 Aims: We investigate the synthetic observational signatures
generated from numerical models of coronal heating in an arcade in
order to determine what features are associated with such heating and
what tools can be used to identify them. Methods: We consider
two simulations of coronal arcades driven by footpoint motions with
different characteristic timescales. Forward modelling is then
conducted, and the synthetic emission data are analysed (e.g.,
intensities, Doppler shifts, line widths and estimated kinetic
energies). Results: The total intensity and Doppler shift
perturbations clearly show the magnetic structure of the coronal
arcade. Contrasts in the local Doppler velocity also highlight
the locations of separatrix surfaces. The distinguishing feature
of the models with short and long timescale photospheric motions
(in comparison to the Alfvén travel time along a loop) is that
of the frequencies. Through fast Fourier transform analysis of the
Doppler velocities, it is clear that when short timescale footpoint
motions are present, higher frequency perturbations are observed. For
longer timescale footpoint motions, the dominant signal is that of
lower frequencies; however, higher (but less powerful) frequencies
were also detected, which matched the natural Alfvén frequency of
the background magnetic field. Signatures of Alfvénic waves were
identified in both models, with fast wave signatures observable when
short timescale driving is present. Finally, we examine the estimates
of the kinetic energy using the Doppler velocities and find it to be
significantly underestimated within these models. Conclusions:
All of the observables within this article behave as one would expect,
given the evolution of the plasma parameters. Such features were,
for example, Alfvén waves, fast waves, the arcade structure and
separatrix surfaces. We were able to differentiate between the two
models by examining the frequencies present. The Doppler velocities
cannot provide accurate estimates of the total kinetic energy or even
the component parallel to the line-of-sight (LOS). This is due to
some of the plasma being outside the formation temperature of the ion,
the multi-directional driver limiting the proportion of the velocity
aligned along the LOS, and cancellation of the velocity along the
LOS. The exact impact each factor has on the estimation is dependent
on the setup of the model and the emission line under investigation.

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Title: Magnetohydrodynamic Waves in Open Coronal Structures
Authors: Banerjee, D.; Krishna Prasad, S.; Pant, V.; McLaughlin, J. A.;
Antolin, P.; Magyar, N.; Ofman, L.; Tian, H.; Van Doorsselaere, T.;
De Moortel, I.; Wang, T. J.
2021SSRv..217...76B Altcode: 2020arXiv201208802B
Modern observatories have revealed the ubiquitous presence of
magnetohydrodynamic waves in the solar corona. The propagating waves
(in contrast to the standing waves) are usually originated in the lower
solar atmosphere which makes them particularly relevant to coronal
heating. Furthermore, open coronal structures are believed to be the
source regions of solar wind, therefore, the detection of MHD waves
in these structures is also pertinent to the acceleration of solar
wind. Besides, the advanced capabilities of the current generation
telescopes have allowed us to extract important coronal properties
through MHD seismology. The recent progress made in the detection,
origin, and damping of both propagating slow magnetoacoustic waves and
kink (Alfvénic) waves is presented in this review article especially
in the context of open coronal structures. Where appropriate, we give
an overview on associated theoretical modelling studies. A few of the
important seismological applications of these waves are discussed. The
possible role of Alfvénic waves in the acceleration of solar wind is
also touched upon.

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Title: A fast multi-dimensional magnetohydrodynamic formulation of
the transition region adaptive conduction (TRAC) method
Authors: Johnston, C. D.; Hood, A. W.; De Moortel, I.; Pagano, P.;
Howson, T. A.
2021A&A...654A...2J Altcode: 2021arXiv210603989J
We have demonstrated that the transition region adaptive conduction
(TRAC) method permits fast and accurate numerical solutions of the
field-aligned hydrodynamic equations, successfully removing the
influence of numerical resolution on the coronal density response
to impulsive heating. This is achieved by adjusting the parallel
thermal conductivity, radiative loss, and heating rates to broaden the
transition region (TR), below a global cutoff temperature, so that
the steep gradients are spatially resolved even when using coarse
numerical grids. Implementing the original 1D formulation of TRAC in
multi-dimensional magnetohydrodynamic (MHD) models would require tracing
a large number of magnetic field lines at every time step in order to
prescribe a global cutoff temperature to each field line. In this paper,
we present a highly efficient formulation of the TRAC method for use
in multi-dimensional MHD simulations, which does not rely on field
line tracing. In the TR, adaptive local cutoff temperatures are used
instead of global cutoff temperatures to broaden any unresolved parts
of the atmosphere. These local cutoff temperatures are calculated using
only local grid cell quantities, enabling the MHD extension of TRAC to
efficiently account for the magnetic field evolution, without tracing
field lines. Consistent with analytical predictions, we show that this
approach successfully preserves the properties of the original TRAC
method. In particular, the total radiative losses and heating remain
conserved under the MHD formulation. Results from 2D MHD simulations
of impulsive heating in unsheared and sheared arcades of coronal
loops are also presented. These simulations benchmark the MHD TRAC
method against a series of 1D models and demonstrate the versatility
and robustness of the method in multi-dimensional magnetic fields. We
show, for the first time, that pressure differences, generated during
the evaporation phase of impulsive heating events, can produce current
layers that are significantly narrower than the transverse energy
deposition. Movies associated to Figs. 4 and 8 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202140987/olm">https://www.aanda.org</A>

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Title: The Heating of the Solar Corona
Authors: Viall, Nicholeen M.; De Moortel, Ineke; Downs, Cooper;
Klimchuk, James A.; Parenti, Susanna; Reale, Fabio
2021GMS...258...35V Altcode:
No abstract at ADS

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Title: Coronal Heating by MHD Waves
Authors: Van Doorsselaere, Tom; Srivastava, Abhishek K.; Antolin,
Patrick; Magyar, Norbert; Vasheghani Farahani, Soheil; Tian, Hui;
Kolotkov, Dmitrii; Ofman, Leon; Guo, Mingzhe; Arregui, Iñigo; De
Moortel, Ineke; Pascoe, David
2020SSRv..216..140V Altcode: 2020arXiv201201371V
The heating of the solar chromosphere and corona to the observed high
temperatures, imply the presence of ongoing heating that balances
the strong radiative and thermal conduction losses expected in the
solar atmosphere. It has been theorized for decades that the required
heating mechanisms of the chromospheric and coronal parts of the active
regions, quiet-Sun, and coronal holes are associated with the solar
magnetic fields. However, the exact physical process that transport
and dissipate the magnetic energy which ultimately leads to the solar
plasma heating are not yet fully understood. The current understanding
of coronal heating relies on two main mechanism: reconnection and MHD
waves that may have various degrees of importance in different coronal
regions. In this review we focus on recent advances in our understanding
of MHD wave heating mechanisms. First, we focus on giving an overview
of observational results, where we show that different wave modes have
been discovered in the corona in the last decade, many of which are
associated with a significant energy flux, either generated in situ
or pumped from the lower solar atmosphere. Afterwards, we summarise
the recent findings of numerical modelling of waves, motivated by the
observational results. Despite the advances, only 3D MHD models with
Alfvén wave heating in an unstructured corona can explain the observed
coronal temperatures compatible with the quiet Sun, while 3D MHD wave
heating models including cross-field density structuring are not yet
able to account for the heating of coronal loops in active regions to
their observed temperature.

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Title: Effect of coronal loop structure on wave heating through
phase mixing
Authors: Pagano, P.; De Moortel, I.; Morton, R. J.
2020A&A...643A..73P Altcode: 2020arXiv200904366P
Context. The mechanism(s) behind coronal heating still elude(s)
direct observation and modelling of viable theoretical processes and
the subsequent effect on coronal structures is one of the key tools
available to assess possible heating mechanisms. Wave heating via the
phase mixing of magnetohydrodynamic (MHD) transverse waves has been
proposed as a possible way to convert magnetic energy into thermal
energy, but MHD models increasingly suggest this is not an efficient
enough mechanism. Aims: We modelled heating by phase mixing
transverse MHD waves in various configurations in order to investigate
whether certain circumstances can enhance the heating sufficiently
to sustain the million degree solar corona and to assess the impact
of the propagation and phase mixing of transverse MHD waves on the
structure of the boundary shell of coronal loops. Methods:
We used 3D MHD simulations of a pre-existing density enhancement in a
magnetised medium and a boundary driver to trigger the propagation of
transverse waves with the same power spectrum as measured by the Coronal
Multi-Channel Polarimeter. We consider different density structures,
boundary conditions at the non-drive footpoint, characteristics of the
driver, and different forms of magnetic resistivity. Results:
We find that different initial density structures significantly
affect the evolution of the boundary shell and that some driver
configurations can enhance the heating generated from the dissipation
of the MHD waves. In particular, drivers coherent on a larger spatial
scale and higher dissipation coefficients can generate significant
heating, although it is still insufficient to balance the radiative
losses in this setup. Conclusions: We conclude that while
phase mixing of transverse MHD waves is unlikely to sustain the
thermal structure of the corona, there are configurations that allow
for an enhanced efficiency of this mechanism. We provide possible
signatures to identify the presence of such configurations, such as
the location of where the heating is deposited along the coronal
loop. Movies associated to Figs. 4 and 8 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202039209/olm">https://www.aanda.org</A>

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Title: Forward modelling of MHD waves in braided magnetic fields
Authors: Fyfe, L. E.; Howson, T. A.; De Moortel, I.
2020A&A...643A..86F Altcode: 2020arXiv200908301F
 Aims: We investigate synthetic observational signatures
generated from numerical models of transverse waves propagating in
complex (braided) magnetic fields. Methods: We consider two
simulations with different levels of magnetic field braiding and impose
periodic, transverse velocity perturbations at the lower boundary. As
the waves reflect off the top boundary, a complex pattern of wave
interference occurs. We applied the forward modelling code FoMo and
analysed the synthetic emission data. We examined the line intensity,
Doppler shifts, and kinetic energy along several line-of-sight (LOS)
angles. Results: The Doppler shift perturbations clearly show
the presence of the transverse (Alfvénic) waves. However, in the
total intensity, and running difference, the waves are less easily
observed for more complex magnetic fields and may be indistinguishable
from background noise. Depending on the LOS angle, the observable
signatures of the waves reflect some of the magnetic field braiding,
particularly when multiple emission lines are available, although it
is not possible to deduce the actual level of complexity. In the more
braided simulation, signatures of phase mixing can be identified. We
highlight possible ambiguities in the interpretation of the wave
modes based on the synthetic emission signatures. Conclusions:
Most of the observables discussed in this article behave in the manner
expected, given knowledge of the evolution of the parameters in the 3D
simulations. Nevertheless, some intriguing observational signatures are
present. Identifying regions of magnetic field complexity is somewhat
possible when waves are present; although, even then, simultaneous
spectroscopic imaging from different lines is important in order to
identify these locations. Care needs to be taken when interpreting
intensity and Doppler velocity signatures as torsional motions, as
is done in our setup. These types of signatures are a consequence of
the complex nature of the magnetic field, rather than real torsional
waves. Finally, we investigate the kinetic energy, which was estimated
from the Doppler velocities and is highly dependent on the polarisation
of the wave, the complexity of the background field, and the LOS angles.

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Title: The effects of driving time scales on heating in a coronal
arcade
Authors: Howson, T. A.; De Moortel, I.; Fyfe, L. E.
2020A&A...643A..85H Altcode: 2020arXiv200907535H
Context. The relative importance of alternating current (AC) and direct
current (DC) heating mechanisms in maintaining the temperature of the
solar corona is not well constrained. Aims: We aim to investigate
the effects of the characteristic time scales of photospheric driving
on the injection and dissipation of magnetic and kinetic energy within
a coronal arcade. Methods: We conducted three-dimensional
magnetohydrodynamic simulations of complex foot point driving imposed
on a potential coronal arcade. We modified the typical time scales
associated with the velocity driver to understand the efficiency of
heating obtained using AC and DC drivers. We considered the implications
for the injected Poynting flux and the spatial and temporal nature
of the energy release in dissipative regimes. Results: For
the same driver amplitude and complexity, long time scale velocity
motions are able to inject a much greater Poynting flux of energy
into the corona. Consequently, in non-ideal regimes, slow stressing
motions result in a greater increase in plasma temperature than for
wave-like driving. In dissipative simulations, Ohmic heating is found
to be much more significant than viscous heating. For all drivers in
our parameter space, energy dissipation is greatest close to the base
of the arcade, where the magnetic field strength is strongest, and at
separatrix surfaces, where the field connectivity changes. Across all
simulations, the background field is stressed with random foot point
motions (in a manner more typical of DC heating studies), and, even
for short time scale driving, the injected Poynting flux is large given
the small amplitude flows considered. For long time scale driving, the
rate of energy injection was comparable to the expected requirements
in active regions. The heating rates were found to scale with the
perturbed magnetic field strength and not the total field strength. Conclusions: Alongside recent studies that show that power within
the corona is dominated by low frequency motions, our results suggest
that, in the closed corona, DC heating is more significant than
AC heating. Movies associated to Fig. 3 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202038869/olm">https://www.aanda.org</A>

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Title: Alfvén on heating by waves
Authors: De Moortel, Ineke; Falconer, Isobel; Stack, Robert
2020A&G....61b2.34D Altcode:
Ineke De Moortel, Isobel Falconer and Robert Stack explore the
achievements and influence of Hannes Alfvén, in particular his seminal
Monthly Notices paper of 1947 on the heating of the solar corona.

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Title: Phase mixing and wave heating in a complex coronal plasma
Authors: Howson, T. A.; De Moortel, I.; Reid, J.
2020A&A...636A..40H Altcode: 2020arXiv200305226H
 Aims: We investigate the formation of small scales and the
related dissipation of magnetohydronamic (MHD) wave energy through
non-linear interactions of counter-propagating, phase-mixed Alfvénic
waves in a complex magnetic field. Methods: We conducted fully
three-dimensional, non-ideal MHD simulations of transverse waves
in complex magnetic field configurations. Continuous wave drivers
were imposed on the foot points of magnetic field lines and the
system was evolved for several Alfvén travel times. Phase-mixed
waves were allowed to reflect off the upper boundary and the
interactions between the resultant counter-streaming wave packets
were analysed. Results: The complex nature of the background
magnetic field encourages the development of phase mixing throughout
the numerical domain, leading to a growth in alternating currents and
vorticities. Counter-propagating phase-mixed MHD wave modes induce a
cascade of energy to small scales and result in more efficient wave
energy dissipation. This effect is enhanced in simulations with more
complex background fields. High-frequency drivers excite localised
field line resonances and produce efficient wave heating. However, this
relies on the formation of large amplitude oscillations on resonant
field lines. Drivers with smaller frequencies than the fundamental
frequencies of field lines are not able to excite resonances and thus
do not inject sufficient Poynting flux to power coronal heating. Even
in the case of high-frequency oscillations, the rate of dissipation
is likely too slow to balance coronal energy losses, even within
the quiet Sun. Conclusions: For the case of the generalised
phase-mixing presented here, complex background field structures
enhance the rate of wave energy dissipation. However, it remains
difficult for realistic wave drivers to inject sufficient Poynting
flux to heat the corona. Indeed, significant heating only occurs in
cases which exhibit oscillation amplitudes that are much larger than
those currently observed in the solar atmosphere.

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Title: Chromospheric evaporation and phase mixing of Alfvén waves
in coronal loops
Authors: Van Damme, H. J.; De Moortel, I.; Pagano, P.; Johnston, C. D.
2020A&A...635A.174V Altcode: 2020arXiv200211695V
Context. Phase mixing of Alfvén waves has been studied extensively as a
possible coronal heating mechanism but without the full thermodynamic
consequences considered self-consistently. It has been argued
that in some cases, the thermodynamic feedback of the heating could
substantially affect the transverse density gradient and even inhibit
the phase mixing process. Aims: In this paper, for the first
time, we use magnetohydrodynamic (MHD) simulations with the appropriate
thermodynamical terms included to quantify the evaporation following
heating by phase mixing of Alfvén waves in a coronal loop and the
effect of this evaporation on the transverse density profile. Methods: The numerical simulations were performed using the
Lagrangian Remap code Lare2D. We set up a 2D loop model consisting
of a field-aligned thermodynamic equilibrium and a cross-field
(background) heating profile. A continuous, sinusoidal, high-frequency
Alfvén wave driver was implemented. As the Alfvén waves propagate
along the field, they undergo phase mixing due to the cross-field
density gradient in the coronal part of the loop. We investigated the
presence of field-aligned flows, heating from the dissipation of the
phase-mixed Alfvén waves, and the subsequent evaporation from the lower
atmosphere. Results: We find that phase mixing of Alfvén waves
leads to modest heating in the shell regions of the loop and evaporation
of chromospheric material into the corona with upflows of the order of
only 5-20 m s-1. Although the evaporation leads to a mass
increase in the shell regions of the loop, the effect on the density
gradient and, hence, on the phase mixing process, is insignificant. Conclusions: This paper self-consistently investigates the effect
of chromospheric evaporation on the cross-field density gradient and
the phase mixing process in a coronal loop. We found that the effects
in our particular setup (small amplitude, high frequency waves) are
too small to significantly change the density gradient.

---------------------------------------------------------
Title: Modelling the solar transition region using an adaptive
conduction method
Authors: Johnston, C. D.; Cargill, P. J.; Hood, A. W.; De Moortel,
I.; Bradshaw, S. J.; Vaseekar, A. C.
2020A&A...635A.168J Altcode: 2020arXiv200201887J
Modelling the solar Transition Region with the use of an Adaptive
Conduction (TRAC) method permits fast and accurate numerical solutions
of the field-aligned hydrodynamic equations, capturing the enthalpy
exchange between the corona and transition region, when the corona
undergoes impulsive heating. The TRAC method eliminates the need
for highly resolved numerical grids in the transition region and the
commensurate very short time steps that are required for numerical
stability. When employed with coarse spatial resolutions, typically
achieved in multi-dimensional magnetohydrodynamic codes, the errors at
peak density are less than 5% and the computation time is three orders
of magnitude faster than fully resolved field-aligned models. This
paper presents further examples that demonstrate the versatility and
robustness of the method over a range of heating events, including
impulsive and quasi-steady footpoint heating. A detailed analytical
assessment of the TRAC method is also presented, showing that the
approach works through all phases of an impulsive heating event
because (i) the total radiative losses and (ii) the total heating
when integrated over the transition region are both preserved at all
temperatures under the broadening modifications of the method. The
results from the numerical simulations complement this conclusion.

---------------------------------------------------------
Title: Resonant absorption in expanding coronal magnetic flux tubes
with uniform density
Authors: Howson, T. A.; De Moortel, I.; Antolin, P.; Van Doorsselaere,
T.; Wright, A. N.
2019A&A...631A.105H Altcode: 2019arXiv190910781H
 Aims: We investigate the transfer of energy between a fundamental
standing kink mode and azimuthal Alfvén waves within an expanding
coronal magnetic flux tube. We consider the process of resonant
absorption in a loop with a non-uniform Alfvén frequency profile but
in the absence of a radial density gradient. Methods: Using the
three dimensional magnetohydrodynamic (MHD) code, Lare3d, we modelled a
transversely oscillating magnetic flux tube that expands radially with
height. An initially straight loop structure with a magnetic field
enhancement was allowed to relax numerically towards a force-free
state before a standing kink mode was introduced. The subsequent
dynamics, rate of wave damping and formation of small length scales are
considered. Results: We demonstrate that the transverse gradient
in Alfvén frequency required for the existence of resonant field lines
can be associated with the expansion of a high field-strength flux tube
from concentrated flux patches in the lower solar atmosphere. This
allows for the conversion of energy between wave modes even in the
absence of the transverse density profile typically assumed in wave
heating models. As with standing modes in straight flux tubes, small
scales are dominated by the vorticity at the loop apex and by currents
close to the loop foot points. The azimuthal Alfvén wave exhibits the
structure of the expanded flux tube and is therefore associated with
smaller length scales close to the foot points of the flux tube than
at the loop apex. Conclusions: Resonant absorption can proceed
throughout the coronal volume, even in the absence of visible, dense,
loop structures. The flux tube and MHD waves considered are difficult
to observe and our model highlights how estimating hidden wave power
within the Sun's atmosphere can be problematic. We highlight that,
for standing modes, the global properties of field lines are important
for resonant absorption and coronal conditions at a single altitude
will not fully determine the nature of MHD resonances. In addition,
we provide a new model in partial response to the criticism that wave
heating models cannot self-consistently generate or sustain the density
profile upon which they typically rely.

---------------------------------------------------------
Title: Magnetohydrodynamic waves in braided magnetic fields
Authors: Howson, T. A.; De Moortel, I.; Reid, J.; Hood, A. W.
2019A&A...629A..60H Altcode: 2019arXiv190803089H
 Aims: We investigate the propagation of transverse
magnetohydrodynamic (MHD) wave fronts through a coronal plasma
containing a braided magnetic field. Methods: We performed
a series of three dimensional MHD simulations in which a small
amplitude, transverse velocity perturbation is introduced into a
complex magnetic field. We analysed the deformation of the wave
fronts as the perturbation propagates through the braided magnetic
structures and explore the nature of Alfvénic wave phase mixing in
this regime. We considered the effects of viscous dissipation in a
weakly non-ideal plasma and evaluate the effects of field complexity
on wave energy dissipation. Results: Spatial gradients in the
local Alfvén speed and variations in the length of magnetic field
lines ensure that small scales form throughout the propagating wave
front due to phase mixing. Additionally, the presence of complex,
intricate current sheets associated with the background field locally
modifies the polarisation of the wave front. The combination of these
two effects enhances the rate of viscous dissipation, particularly in
more complex field configurations. Unlike in classical phase mixing
configurations, the greater spatial extent of Alfvén speed gradients
ensures that wave energy is deposited over a larger cross-section
of the magnetic structure. Further, the complexity of the background
magnetic field ensures that small gradients in a wave driver can map
to large gradients within the coronal plasma. Conclusions:
The phase mixing of transverse MHD waves in a complex magnetic
field will progress throughout the braided volume. As a result,
in a non-ideal regime wave energy will be dissipated over a greater
cross-section than in classical phase mixing models. The formation
rate of small spatial scales in a propagating wave front is a function
of the complexity of the background magnetic field. As such, if the
coronal field is sufficiently complex it remains plausible that phase
mixing induced wave heating can contribute to maintaining the observed
temperatures. Furthermore, the weak compressibility of the transverse
wave and the observed phase mixing pattern may provide seismological
information about the nature of the background plasma.

---------------------------------------------------------
Title: MHD simulations of the in situ generation of kink and sausage
waves in the solar corona by collision of dense plasma clumps
Authors: Pagano, P.; Van Damme, H. J.; Antolin, P.; De Moortel, I.
2019A&A...626A..53P Altcode: 2019arXiv190503749P
Context. Magnetohydrodynamic (MHD) waves are ubiquitous in the solar
corona where the highly structured magnetic fields provide efficient
wave guides for their propagation. While MHD waves have been observed
originating from lower layers of the solar atmosphere, recent studies
have shown that some can be generated in situ by the collision of dense
counter-propagating flows. Aims: In this theoretical study, we
analyse the mechanism that triggers the propagation of kink and sausage
modes in the solar corona following the collision of counter-propagating
flows, and how the properties of the flows affect the properties of
the generated waves. Methods: To study in detail this mechanism
we ran a series of ideal 2D and 3D MHD simulations where we varied
the properties of the counter-propagating flows; by means of a simple
technique to estimate the amplitudes of the kink and sausage modes,
we investigated their role in the generation and propagation of the
MHD waves. Results: We find that the amplitude of the waves is
largely dependent on the kinetic energy of the flows, and that the
onset of kink or sausage modes depends on the asymmetries between
the colliding blobs. Moreover, the initial wavelength of the MHD
waves is associated with the magnetic configuration resulting from
the collision of the flows. We also find that genuine 3D systems
respond with smaller wave amplitudes. Conclusions: In this
study, we present a parameter space description of the mechanism that
leads to the generation of MHD waves from the collision of flows
in the corona. Future observations of these waves can be used to
understand the properties of the plasma and magnetic field of the solar
corona. The movies associated to Figs. 2 and 21 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201935539/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: The effects of numerical resolution, heating timescales and
background heating on thermal non-equilibrium in coronal loops
Authors: Johnston, C. D.; Cargill, P. J.; Antolin, P.; Hood, A. W.;
De Moortel, I.; Bradshaw, S. J.
2019A&A...625A.149J Altcode: 2019arXiv190407287J
Thermal non-equilibrium (TNE) is believed to be a potentially important
process in understanding some properties of the magnetically closed
solar corona. Through one-dimensional hydrodynamic models, this paper
addresses the importance of the numerical spatial resolution, footpoint
heating timescales and background heating on TNE. Inadequate transition
region (TR) resolution can lead to significant discrepancies in TNE
cycle behaviour, with TNE being suppressed in under-resolved loops. A
convergence on the periodicity and plasma properties associated with
TNE required spatial resolutions of less than 2 km for a loop of length
180 Mm. These numerical problems can be resolved using an approximate
method that models the TR as a discontinuity using a jump condition, as
proposed by Johnston et al. (2017a, A&amp;A, 597, A81; 2017b, A&amp;A,
605, A8). The resolution requirements (and so computational cost)
are greatly reduced while retaining good agreement with fully resolved
results. Using this approximate method we (i) identify different regimes
for the response of coronal loops to time-dependent footpoint heating
including one where TNE does not arise and (ii) demonstrate that TNE
in a loop with footpoint heating is suppressed unless the background
heating is sufficiently small. The implications for the generality of
TNE are discussed.

---------------------------------------------------------
Title: Phase mixing of nonlinear Alfvén waves
Authors: Prokopyszyn, A. P. K.; Hood, A. W.; De Moortel, I.
2019A&A...624A..90P Altcode: 2019arXiv190308093P
 Aims: This paper presents 2.5D numerical experiments of Alfvén
wave phase mixing and aims to assess the effects of nonlinearities on
wave behaviour and dissipation. In addition, this paper aims to quantify
how effective the model presented in this work is at providing energy to
the coronal volume. Methods: The model is presented and explored
through the use of several numerical experiments which were carried out
using the Lare2D code. The experiments study footpoint driven Alfvén
waves in the neighbourhood of a two-dimensional x-type null point with
initially uniform density and plasma pressure. A continuous sinusoidal
driver with a constant frequency is used. Each experiment uses different
driver amplitudes to compare weakly nonlinear experiments with linear
experiments. Results: We find that the wave trains phase-mix
owing to variations in the length of each field line and variations
in the field strength. The nonlinearities reduce the amount of energy
entering the domain, as they reduce the effectiveness of the driver,
but they have relatively little effect on the damping rate (for the
range of amplitudes studied). The nonlinearities produce density
structures which change the natural frequencies of the field lines
and hence cause the resonant locations to move. The shifting of the
resonant location causes the Poynting flux associated with the driver
to decrease. Reducing the magnetic diffusivity increases the energy
build-up on the resonant field lines, however, it has little effect
on the total amount of energy entering the system. From an order of
magnitude estimate, we show that the Poynting flux in our experiments is
comparable to the energy requirements of the quiet Sun corona. However
a (possibly unphysically) large amount of magnetic diffusion was used
however and it remains unclear if the model is able to provide enough
energy under actual coronal conditions.

---------------------------------------------------------
Title: Contribution of observed multi frequency spectrum of Alfvén
waves to coronal heating
Authors: Pagano, P.; De Moortel, I.
2019A&A...623A..37P Altcode: 2019arXiv190102310P
Context. Whilst there are observational indications that transverse
magnetohydrodynamic (MHD) waves carry enough energy to maintain
the thermal structure of the solar corona, it is not clear whether
such energy can be efficiently and effectively converted into
heating. Phase-mixing of Alfvén waves is considered a candidate
mechanism, as it can develop transverse gradient where magnetic energy
can be converted into thermal energy. However, phase-mixing is a process
that crucially depends on the amplitude and period of the transverse
oscillations, and only recently have we obtained a complete measurement
of the power spectrum for transverse oscillations in the corona. 
Aims: We aim to investigate the heating generated by phase-mixing of
transverse oscillations triggered by buffeting of a coronal loop that
follows from the observed coronal power spectrum as well as the impact
of these persistent oscillations on the structure of coronal loops. Methods: We considered a 3D MHD model of an active region coronal
loop and we perturbed its footpoints with a 2D horizontal driver that
represents a random buffeting motion of the loop footpoints. Our driver
was composed of 1000 pulses superimposed to generate the observed
power spectrum. Results: We find that the heating supply from
the observed power spectrum in the solar corona through phase-mixing
is not sufficient to maintain the million-degree active region
solar corona. We also find that the development of Kelvin-Helmholtz
instabilities could be a common phenomenon in coronal loops that
could affect their apparent life time. Conclusions: This study
concludes that is unlikely that phase-mixing of Alfvén waves resulting
from an observed power spectrum of transverse coronal loop oscillations
can heat the active region solar corona. However, transverse waves could
play an important role in the development of small scale structures. Movies associated to Figs. 12, 13, 15, 18, and 19 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201834158/olm">https://www.aanda.org</A>

---------------------------------------------------------
Title: Heating Effects from Driven Transverse and Alfvén Waves in
Coronal Loops
Authors: Guo, Mingzhe; Van Doorsselaere, Tom; Karampelas, Konstantinos;
Li, Bo; Antolin, Patrick; De Moortel, Ineke
2019ApJ...870...55G Altcode: 2018arXiv181107608G
Recent numerical studies revealed that transverse motions of coronal
loops can induce the Kelvin-Helmholtz instability (KHI). This process
could be important in coronal heating because it leads to dissipation
of energy at small spatial scale plasma interactions. Meanwhile,
small-amplitude decayless oscillations in coronal loops have
been discovered recently in observations of SDO/AIA. We model
such oscillations in coronal loops and study wave heating effects,
considering a kink and Alfvén driver separately and a mixed driver at
the bottom of flux tubes. Both the transverse and Alfvén oscillations
can lead to the KHI. Meanwhile, the Alfvén oscillations established
in loops will experience phase mixing. Both processes will generate
small spatial scale structures, which can help the dissipation of
wave energy. Indeed, we observe the increase of internal energy and
temperature in loop regions. The heating is more pronounced for the
simulation containing the mixed kink and Alfvén driver. This means that
the mixed wave modes can lead to a more efficient energy dissipation
in the turbulent state of the plasma and that the KHI eddies act as an
agent to dissipate energy in other wave modes. Furthermore, we also
obtained forward-modeling results using the FoMo code. We obtained
forward models that are very similar to the observations of decayless
oscillations. Due to the limited resolution of instruments, neither
Alfvén modes nor the fine structures are observable. Therefore,
this numerical study shows that Alfvén modes probably can coexist
with kink modes, leading to enhanced heating.

---------------------------------------------------------
Title: Contribution of phase-mixing of Alfvén waves to coronal
heating in multi-harmonic loop oscillations
Authors: Pagano, P.; Pascoe, D. J.; De Moortel, I.
2018A&A...616A.125P Altcode: 2018arXiv180410562P
Context. Kink oscillations of a coronal loop are observed and studied in
detail because they provide a unique probe into the structure of coronal
loops through magnetohydrodynamics (MHD) seismology and a potential
test of coronal heating through the phase mixing of Alfvén waves. In
particular, recent observations show that standing oscillations of loops
often involve higher harmonics in addition to the fundamental mode. The
damping of these kink oscillations is explained by mode coupling
with Alfvén waves. Aims: We investigate the consequences
for wave-based coronal heating of higher harmonics and which coronal
heating observational signatures we may use to infer the presence of
higher harmonic kink oscillations. Methods: We performed a set
of non-ideal MHD simulations in which we modelled the damping of the
kink oscillation of a flux tube via mode coupling. We based our MHD
simulation parameters on the seismological inversion of an observation
for which the first three harmonics are detected. We studied the phase
mixing of Alfvén waves, which leads to the deposition of heat in the
system, and we applied seismological inversion techniques to the MHD
simulation output. Results: We find that the heating due to phase
mixing of Alfvén waves triggered by the damping of kink oscillation
is relatively small. We can however illustrate how the heating location
drifts from subsequent damping of lower order harmonics. We also address
the role of higher order harmonics and the width of the boundary shell
in the energy deposition. Conclusions: We conclude that the
coronal heating due to phase mixing does not seem to provide enough
energy to maintain the thermal structure of the solar corona even when
multi-harmonic oscillations are included; these oscillations play an
inhibiting role in the development of smaller scale structures.

---------------------------------------------------------
Title: Reconnection Microjets in the Solar Corona
Authors: Antolin, Patrick; Pagano, Paolo; De Moortel, Ineke
2018cosp...42E..96A Altcode:
Coronal rain is one of the highest resolution tracers of the coronal
magnetic field. In this work the dynamics of a prominence/coronal rain
complex are analysed based on spectroscopic and imaging observations
with IRIS, Hinode/SOT and SDO/AIA. The loop-like magnetic field arcade
hosting the rain is observed to slowly expand in height. Prior and
especially during this movement, several ( 100) small ( 1 arcsec)
and short (&lt;20 sec) bursts of plasma perpendicular to the loop
arcade are captured in the Si IV and Mg II lines. The line profiles are
broad and asymmetric with long tails above 100 km/s. These microjets
are accompanied with strong intensity enhancements co-spatially and
along the loop in most of the AIA channels, indicating significant
energy release increasing the temperature to several MK. Some events
generate transverse MHD waves and the strongest events are accompanied
by ejection of plasmoid-like structures. We interpret these microjets as
magnetic reconnection outflows, produced by component reconnection in a
strong guide field. The originally cold conditions of the rain allows,
in this case, a unique high resolution glance into the reconnection
dynamics in low beta plasmas, and marks the X-target in the Sun for
next-generation telescopes.

---------------------------------------------------------
Title: In Situ Generation of Transverse Magnetohydrodynamic Waves
from Colliding Flows in the Solar Corona
Authors: Antolin, Patrick; Pagano, Paolo; De Moortel, Ineke;
Nakariakov, Valery M.
2018ApJ...861L..15A Altcode: 2018arXiv180700395A
Transverse magnetohydrodynamic (MHD) waves permeate the solar
atmosphere and are a candidate for coronal heating. However, the
origin of these waves is still unclear. In this Letter, we analyze
coordinated observations from Hinode/Solar Optical Telescope (SOT) and
Interface Region Imaging Spectrograph ( IRIS) of a prominence/coronal
rain loop-like structure at the limb of the Sun. Cool and dense
downflows and upflows are observed along the structure. A collision
between a downward and an upward flow with an estimated energy flux
of 107-108 erg cm-2 s-1
is observed to generate oscillatory transverse perturbations of the
strands with an estimated ≈40 km s-1 total amplitude, and
a short-lived brightening event with the plasma temperature increasing
to at least 105 K. We interpret this response as sausage
and kink transverse MHD waves based on 2D MHD simulations of plasma
flow collision. The lengths, density, and velocity differences between
the colliding clumps and the strength of the magnetic field are major
parameters defining the response to the collision. The presence of
asymmetry between the clumps (angle of impact surface and/or offset
of flowing axis) is crucial for generating a kink mode. Using the
observed values, we successfully reproduce the observed transverse
perturbations and brightening, and show adiabatic heating to coronal
temperatures. The numerical modeling indicates that the plasma β
in this loop-like structure is confined between 0.09 and 0.36. These
results suggest that such collisions from counter-streaming flows can
be a source of in situ transverse MHD waves, and that for cool and dense
prominence conditions such waves could have significant amplitudes.

---------------------------------------------------------
Title: Impact of Type II Spicules in the Corona: Simulations and
Synthetic Observables
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; De Moortel, Ineke;
Hansteen, Viggo H.; Carlsson, Mats
2018ApJ...860..116M Altcode: 2018arXiv180506475M
The role of type II spicules in the corona has been a much debated topic
in recent years. This paper aims to shed light on the impact of type
II spicules in the corona using novel 2.5D radiative MHD simulations,
including ion-neutral interaction effects with the Bifrost code. We
find that the formation of simulated type II spicules, driven by
the release of magnetic tension, impacts the corona in various
manners. Associated with the formation of spicules, the corona
exhibits (1) magneto-acoustic shocks and flows, which supply mass
to coronal loops, and (2) transversal magnetic waves and electric
currents that propagate at Alfvén speeds. The transversal waves and
electric currents, generated by the spicule’s driver and lasting
for many minutes, are dissipated and heat the associated loop. These
complex interactions in the corona can be connected with blueshifted
secondary components in coronal spectral lines (red-blue asymmetries)
observed with Hinode/EIS and SOHO/SUMER, as well as the EUV counterpart
of type II spicules and propagating coronal disturbances observed with
the 171 Å and 193 Å SDO/AIA channels.

---------------------------------------------------------
Title: Transverse Wave Induced Kelvin-Helmholtz Rolls in Spicules
Authors: Antolin, P.; Schmit, D.; Pereira, T. M. D.; De Pontieu, B.;
De Moortel, I.
2018ApJ...856...44A Altcode: 2018arXiv180300821A
In addition to their jet-like dynamic behavior, spicules usually exhibit
strong transverse speeds, multi-stranded structure, and heating from
chromospheric to transition region temperatures. In this work we first
analyze Hinode and IRIS observations of spicules and find different
behaviors in terms of their Doppler velocity evolution and collective
motion of their sub-structure. Some have a Doppler shift sign change
that is rather fixed along the spicule axis, and lack coherence in
the oscillatory motion of strand-like structure, matching rotation
models, or long-wavelength torsional Alfvén waves. Others exhibit a
Doppler shift sign change at maximum displacement and coherent motion
of their strands, suggesting a collective magnetohydrodynamic (MHD)
wave. By comparing with an idealized 3D MHD simulation combined with
radiative transfer modeling, we analyze the role of transverse MHD
waves and associated instabilities in spicule-like features. We find
that transverse wave induced Kelvin-Helmholtz (TWIKH) rolls lead to
coherence of strand-like structure in imaging and spectral maps, as seen
in some observations. The rapid transverse dynamics and the density
and temperature gradients at the spicule boundary lead to ring-shaped
Mg II k and Ca II H source functions in the transverse cross-section,
potentially allowing IRIS to capture the Kelvin-Helmholtz instability
dynamics. Twists and currents propagate along the spicule at Alfvénic
speeds, and the temperature variations within TWIKH rolls, produce the
sudden appearance/disappearance of strands seen in Doppler velocity
and in Ca II H intensity. However, only a mild intensity increase in
higher-temperature lines is obtained, suggesting there is an additional
heating mechanism at work in spicules.

---------------------------------------------------------
Title: Observations and Modeling of Transition Region and Coronal
Heating Associated with Spicules
Authors: De Pontieu, B.; Martinez-Sykora, J.; De Moortel, I.;
Chintzoglou, G.; McIntosh, S. W.
2017AGUFMSH43A2793D Altcode:
Spicules have been proposed as significant contributorsto the coronal
energy and mass balance. While previous observationshave provided
a glimpse of short-lived transient brightenings in thecorona that
are associated with spicules, these observations have beencontested
and are the subject of a vigorous debate both on the modelingand
the observational side so that it remains unclear whether plasmais
heated to coronal temperatures in association with spicules. We use
high-resolution observations of the chromosphere and transition region
with the Interface Region Imaging Spectrograph (IRIS) and ofthe corona
with the Atmospheric Imaging Assembly (AIA) onboard theSolar Dynamics
Observatory (SDO) to show evidence of the formation of coronal
structures as a result of spicular mass ejections andheating of
plasma to transition region and coronaltemperatures. Our observations
suggest that a significant fraction of the highly dynamic loop fan
environment associated with plage regions may be the result of the
formation of such new coronal strands, a process that previously had
been interpreted as the propagation of transient propagating coronal
disturbances (PCD)s. Our observationsare supported by 2.5D radiative
MHD simulations that show heating tocoronal temperatures in association
with spicules. Our results suggest that heating and strong flows play
an important role in maintaining the substructure of loop fans, in
addition to the waves that permeate this low coronal environment. Our
models also matches observations ofTR counterparts of spicules and
provides an elegant explanation forthe high apparent speeds of these
"network jets".

---------------------------------------------------------
Title: Energetics of the Kelvin-Helmholtz instability induced by
transverse waves in twisted coronal loops
Authors: Howson, T. A.; De Moortel, I.; Antolin, P.
2017A&A...607A..77H Altcode: 2017arXiv170804124H
 Aims: We quantify the effects of twisted magnetic fields on
the development of the magnetic Kelvin-Helmholtz instability (KHI) in
transversely oscillating coronal loops. Methods: We modelled a
fundamental standing kink mode in a straight, density-enhanced magnetic
flux tube using the magnetohydrodynamics code, Lare3d. In order to
evaluate the impact of an azimuthal component of the magnetic field,
various degrees of twist were included within the flux tube's magnetic
field. Results: The process of resonant absorption is only
weakly affected by the presence of a twisted magnetic field. However,
the subsequent evolution of the KHI is sensitive to the strength of the
azimuthal component of the field. Increased twist values inhibit the
deformation of the loop's density profile, which is associated with
the growth of the instability. Despite this, much smaller scales in
the magnetic field are generated when there is a non-zero azimuthal
component present. Hence, the instability is more energetic in cases
with (even weakly) twisted fields. Field aligned flows at the loop
apex are established in a twisted regime once the instability has
formed. Further, in the straight field case, there is no net vertical
component of vorticity when integrated across the loop. However, the
inclusion of azimuthal magnetic field generates a preferred direction
for the vorticity which oscillates during the kink mode. 
Conclusions: The KHI may have implications for wave heating in the
solar atmosphere due to the creation of small length scales and the
generation of a turbulent regime. Whilst magnetic twist does suppress
the development of the vortices associated with the instability, the
formation of the KHI in a twisted regime will be accompanied by greater
Ohmic dissipation due to the larger currents that are produced, even if
only weak twist is present. The presence of magnetic twist will likely
make the instability more difficult to detect in the corona, but will
enhance its contribution to heating the solar atmosphere. Further,
the development of velocities along the loop may have observational
applications for inferring the presence of magnetic twist within
coronal structures.

---------------------------------------------------------
Title: Above the Noise: The Search for Periodicities in the Inner
Heliosphere
Authors: Threlfall, James; De Moortel, Ineke; Conlon, Thomas
2017SoPh..292..165T Altcode:
Remote sensing of coronal and heliospheric periodicities can provide
vital insight into the local conditions and dynamics of the solar
atmosphere. We seek to trace long (one hour or longer) periodic
oscillatory signatures (previously identified above the limb in
the corona by, e.g., Telloni et al. in Astrophys. J.767, 138, 2013)
from their origin at the solar surface out into the heliosphere. To
do this, we combined on-disk measurements taken by the Atmospheric
Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO)
and concurrent extreme ultra-violet (EUV) and coronagraph data
from one of the Solar Terrestrial Relations Observatory (STEREO)
spacecraft to study the evolution of two active regions in the
vicinity of an equatorial coronal hole over several days in early
2011. Fourier and wavelet analysis of signals were performed. Applying
white-noise-based confidence levels to the power spectra associated
with detrended intensity time series yields detections of oscillatory
signatures with periods from 6 - 13 hours in both AIA and STEREO
data. As was found by Telloni et al. (2013), these signatures are
aligned with local magnetic structures. However, typical spectral
power densities all vary substantially as a function of period,
indicating spectra dominated by red (rather than white) noise. Contrary
to the white-noise-based results, applying global confidence levels
based on a generic background-noise model (allowing a combination of
white noise, red noise, and transients following Auchère et al. in
Astrophys. J.825, 110, 2016) without detrending the time series uncovers
only sporadic, spatially uncorrelated evidence of periodic signatures
in either instrument. Automating this method to individual pixels in
the STEREO/COR coronagraph field of view is non-trivial. Efforts to
identify and implement a more robust automatic background noise model
fitting procedure are needed.

---------------------------------------------------------
Title: Observations and Numerical Models of Solar Coronal Heating
Associated with Spicules
Authors: De Pontieu, B.; De Moortel, I.; Martinez-Sykora, J.; McIntosh,
S. W.
2017ApJ...845L..18D Altcode: 2017arXiv171006790D
Spicules have been proposed as significant contributors to the mass
and energy balance of the corona. While previous observations have
provided a glimpse of short-lived transient brightenings in the
corona that are associated with spicules, these observations have
been contested and are the subject of a vigorous debate both on the
modeling and the observational side. Therefore, it remains unclear
whether plasma is heated to coronal temperatures in association with
spicules. We use high-resolution observations of the chromosphere and
transition region (TR) with the Interface Region Imaging Spectrograph
and of the corona with the Atmospheric Imaging Assembly on board
the Solar Dynamics Observatory to show evidence of the formation
of coronal structures associated with spicular mass ejections and
heating of plasma to TR and coronal temperatures. Our observations
suggest that a significant fraction of the highly dynamic loop fan
environment associated with plage regions may be the result of the
formation of such new coronal strands, a process that previously had
been interpreted as the propagation of transient propagating coronal
disturbances. Our observations are supported by 2.5D radiative MHD
simulations that show heating to coronal temperatures in association
with spicules. Our results suggest that heating and strong flows
play an important role in maintaining the substructure of loop fans,
in addition to the waves that permeate this low coronal environment.

---------------------------------------------------------
Title: A new approach for modelling chromospheric evaporation in
response to enhanced coronal heating. II. Non-uniform heating
Authors: Johnston, C. D.; Hood, A. W.; Cargill, P. J.; De Moortel, I.
2017A&A...605A...8J Altcode: 2017arXiv170504054J
We proposed that the use of an approximate "jump condition" at the
solar transition region permits fast and accurate numerical solutions
of the one dimensional hydrodynamic equations when the corona undergoes
impulsive heating. In particular, it eliminates the need for the very
short timesteps imposed by a highly resolved numerical grid. This
paper presents further examples of the applicability of the method
for cases of non-uniform heating, in particular, nanoflare trains
(uniform in space but non-uniform in time) and spatially localised
impulsive heating, including at the loop apex and base of the transition
region. In all cases the overall behaviour of the coronal density and
temperature shows good agreement with a fully resolved one dimensional
model and is significantly better than the equivalent results from a
1D code run without using the jump condition but with the same coarse
grid. A detailed assessment of the errors introduced by the jump
condition is presented showing that the causes of discrepancy with the
fully resolved code are (I) the neglect of the terms corresponding
to the rate of change of total energy in the unresolved atmosphere;
(II) mass motions at the base of the transition region and (III) for
some cases with footpoint heating, an over-estimation of the radiative
losses in the transition region.

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Title: The effects of resistivity and viscosity on the Kelvin-
Helmholtz instability in oscillating coronal loops
Authors: Howson, T. A.; De Moortel, I.; Antolin, P.
2017A&A...602A..74H Altcode: 2017arXiv170302423H
 Aims: We investigate the effects of resistivity and viscosity
on the onset and growth of the Kelvin-Helmholtz instability (KHI) in
an oscillating coronal loop. Methods: We modelled a standing
kink wave in a density-enhanced loop with the three dimensional (3D),
resistive magnetohydrodynamics code, Lare3d. We conducted a parameter
study on the viscosity and resistivity coefficients to examine the
effects of dissipation on the KHI. Results: Enhancing the
viscosity (ν) and resistivity (η) acts to suppress the KHI. Larger
values of η and ν delay the formation of the instability and, in some
cases, prevent the onset completely. This leads to the earlier onset
of heating for smaller values of the transport coefficients. We note
that viscosity has a greater effect on the development of the KHI than
resistivity. Furthermore, when using anomalous resistivity, the Ohmic
heating rate associated with the KHI may be greater than that associated
with the phase mixing that occurs in an instability-suppressed regime
(using uniform resistivity). Conclusions: From our study, it is
clear that the heating rate crucially depends on the formation of small
length scales (influenced by the numerical resolution) as well as the
values of resistivity and viscosity. As larger values of the transport
coefficients suppress the KHI, the onset of heating is delayed but
the heating rate is larger. As increased numerical resolution allows
smaller length scales to develop, the heating rate will be higher even
for the same values of η and ν.

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Title: Contribution of mode-coupling and phase-mixing of Alfvén
waves to coronal heating
Authors: Pagano, P.; De Moortel, I.
2017A&A...601A.107P Altcode:
Context. Phase-mixing of Alfvén waves in the solar corona has been
identified as one possible candidate to explain coronal heating. While
this scenario is supported by observations of ubiquitous oscillations
in the corona carrying sufficient wave energy and by theoretical
models that have described the concentration of energy in small-scale
structures, it is still unclear whether this wave energy can be
converted into thermal energy in order to maintain the million-degree
hot solar corona. Aims: The aim of this work is to assess how
much energy can be converted into thermal energy by a phase-mixing
process triggered by the propagation of Alfvénic waves in a cylindric
coronal structure, such as a coronal loop, and to estimate the impact
of this conversion on the coronal heating and thermal structure
of the solar corona. Methods: To this end, we ran 3D MHD
simulations of a magnetised cylinder where the Alfvén speed varies
through a boundary shell, and a footpoint driver is set to trigger
kink modes that mode couple to torsional Alfvén modes in the boundary
shell. These Alfvén waves are expected to phase-mix, and the system
allows us to study the subsequent thermal energy deposition. We ran a
reference simulation to explain the main process and then we varied
the simulation parameters, such as the size of the boundary shell,
its structure, and the persistence of the driver. Results:
When we take high values of magnetic resistivity and strong footpoint
drivers into consideration, we find that I) phase-mixing leads to
a temperature increase of the order of 105 K or less,
depending on the structure of the boundary shell; II) this energy
is able to balance the radiative losses only in the localised region
involved in the heating; and III) we can determine the influence of
the boundary layer and the persistence of the driver on the thermal
structure of the system. Conclusions: Our conclusion is that as
a result of the extreme physical parameters we adopted and the moderate
impact on the heating of the system, it is unlikely that phase-mixing
can contribute on a global scale to the heating of the solar corona.

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Title: Contribution of mode coupling and phase-mixing of Alfvén
waves to coronal heating
Authors: Pagano, Paolo; De Moortel, Ineke
2017EGUGA..19.9255P Altcode: 2017arXiv170305707P
The solar corona is a million degree plasma that has been investigated
for long time to understand the cause of this high heating rate. In
particular, phase-mixing of Alfvén waves in the solar corona has
been identified as one possible candidate to explain coronal heating
by observations of ubiquitous oscillations in the corona carrying
sufficient wave energy and by theoretical models that have described
the concentration of energy in small scale structures. The aim of
this work is to assess how much energy can be converted into thermal
energy by a phase-mixing process triggered by the propagation of
Alfvénic waves in a cylindric coronal structure, such as a coronal
loop, and to estimate the impact of this conversion on the coronal
heating and thermal structure of the solar corona plasma. We run 3D
MHD simulations of a magnetised cylinder where the Alfvén speed varies
through a boundary shell and a footpoint driver is set to trigger kink
modes which mode couple to torsional Alfvén modes in the boundary
shell. These Alfvén waves are expected to phase-mix and the system
allows us to study the following thermal energy deposition on the
plasma. We run a reference simulation to explain the main process and
then we vary simulation parameters, such as the size of the boundary
shell, its structure and the persistence of the driver. Taking into
consideration high values of magnetic resistivity and strong footpoint
drivers, we find i) that phase-mixing leads to a temperature increase,
ii) that this energy is able to balance the radiative losses only in
the localised region involved in the heating.

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Title: Observational Signatures of Transverse Magnetohydrodynamic
Waves and Associated Dynamic Instabilities in Coronal Flux Tubes
Authors: Antolin, P.; De Moortel, I.; Van Doorsselaere, T.; Yokoyama,
T.
2017ApJ...836..219A Altcode:
Magnetohydrodynamic (MHD) waves permeate the solar atmosphere
and constitute potential coronal heating agents. Yet, the waves
detected so far may be but a small subset of the true existing wave
power. Detection is limited by instrumental constraints but also by
wave processes that localize the wave power in undetectable spatial
scales. In this study, we conduct 3D MHD simulations and forward
modeling of standing transverse MHD waves in coronal loops with
uniform and non-uniform temperature variation in the perpendicular
cross-section. The observed signatures are largely dominated by the
combination of the Kelvin-Helmholtz instability (KHI), resonant
absorption, and phase mixing. In the presence of a cross-loop
temperature gradient, we find that emission lines sensitive to the
loop core catch different signatures compared to those that are more
sensitive to the loop boundary and the surrounding corona, leading to
an out-of-phase intensity and Doppler velocity modulation produced by
KHI mixing. In all of the considered models, common signatures include
an intensity and loop width modulation at half the kink period, a fine
strand-like structure, a characteristic arrow-shaped structure in the
Doppler maps, and overall line broadening in time but particularly at
the loop edges. For our model, most of these features can be captured
with a spatial resolution of 0.″33 and a spectral resolution of 25
km s-1, although we do obtain severe over-estimation of
the line width. Resonant absorption leads to a significant decrease
of the observed kinetic energy from Doppler motions over time, which
is not recovered by a corresponding increase in the line width from
phase mixing and KHI motions. We estimate this hidden wave energy to
be a factor of 5-10 of the observed value.

---------------------------------------------------------
Title: Observational signatures of transverse MHD waves and associated
dynamic instabilities
Authors: Antolin, Patrick; De Moortel, Ineke; Van Doorsselaere, Tom;
Yokoyama, Takaaki
2017arXiv170200775A Altcode:
MHD waves permeate the solar atmosphere and constitute potential
coronal heating agents. Yet, the waves detected so far may be but a
small subset of the true existing wave power. Detection is limited by
instrumental constraints, but also by wave processes that localise the
wave power in undetectable spatial scales. In this study we conduct 3D
MHD simulations and forward modelling of standing transverse MHD waves
in coronal loops with uniform and non-uniform temperature variation in
the perpendicular cross-section. The observed signatures are largely
dominated by the combination of the Kelvin-Helmholtz instability (KHI),
resonant absorption and phase mixing. In the presence of a cross-loop
temperature gradient we find that emission lines sensitive to the
loop core catch different signatures than those more sensitive to the
loop boundary and the surrounding corona, leading to an out-of-phase
intensity modulation produced by the KHI mixing. Common signatures to
all considered models include an intensity and loop width modulation
at half the kink period, fine strand-like structure, a characteristic
arrow-shaped structure in the Doppler maps, overall line broadening in
time but particularly at the loop edges. For our model, most of these
features can be captured with a spatial resolution of $0.33\arcsec$ and
spectral resolution of 25~km~s$^{-1}$, although severe over-estimation
of the line width is obtained. Resonant absorption leads to a
significant decrease of the observed kinetic energy from Doppler
motions over time, which is not recovered by a corresponding increase
in the line width from phase mixing and the KHI motions. We estimate
this hidden wave energy to be a factor of $5-10$ of the observed value.

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Title: JPEG2000 Image Compression on Solar EUV Images
Authors: Fischer, Catherine E.; Müller, Daniel; De Moortel, Ineke
2017SoPh..292...16F Altcode: 2017arXiv170201946F
For future solar missions as well as ground-based telescopes,
efficient ways to return and process data have become increasingly
important. Solar Orbiter, which is the next ESA/NASA mission to explore
the Sun and the heliosphere, is a deep-space mission, which implies a
limited telemetry rate that makes efficient onboard data compression
a necessity to achieve the mission science goals. Missions like the
Solar Dynamics Observatory (SDO) and future ground-based telescopes
such as the Daniel K. Inouye Solar Telescope, on the other hand, face
the challenge of making petabyte-sized solar data archives accessible
to the solar community. New image compression standards address
these challenges by implementing efficient and flexible compression
algorithms that can be tailored to user requirements. We analyse solar
images from the Atmospheric Imaging Assembly (AIA) instrument onboard
SDO to study the effect of lossy JPEG2000 (from the Joint Photographic
Experts Group 2000) image compression at different bitrates. To assess
the quality of compressed images, we use the mean structural similarity
(MSSIM) index as well as the widely used peak signal-to-noise ratio
(PSNR) as metrics and compare the two in the context of solar EUV
images. In addition, we perform tests to validate the scientific use
of the lossily compressed images by analysing examples of an on-disc
and off-limb coronal-loop oscillation time-series observed by AIA/SDO.

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Title: A new approach for modelling chromospheric evaporation in
response to enhanced coronal heating. I. The method
Authors: Johnston, C. D.; Hood, A. W.; Cargill, P. J.; De Moortel, I.
2017A&A...597A..81J Altcode: 2016arXiv160905075J
We present a new computational approach that addresses the difficulty
of obtaining the correct interaction between the solar corona and
the transition region, in response to rapid heating events. In
the coupled corona, transition region, and chromosphere system, an
enhanced downward conductive flux results in an upflow (chromospheric
evaporation). However, obtaining the correct upflow generally requires
high spatial resolution in order to resolve the transition region. With
an unresolved transition region, artificially low coronal densities are
obtained because the downward heat flux "jumps" across the unresolved
region to the chromosphere, underestimating the upflows. Here, we treat
the lower transition region as a discontinuity that responds to changing
coronal conditions through the imposition of a jump condition that is
derived from an integrated form of energy conservation. To illustrate
and benchmark this approach against a fully resolved one-dimensional
model, we present field-aligned simulations of coronal loops in response
to a range of impulsive (spatially uniform) heating events. We show that
our approach leads to a significant improvement in the coronal density
evolution than just when using coarse spatial resolutions insufficient
to resolve the lower transition region. Our approach compensates for the
jumping of the heat flux by imposing a velocity correction that ensures
that the energy from the heat flux goes into driving the transition
region dynamics, rather than being lost through radiation. Hence,
it is possible to obtain improved coronal densities. The advantages
of using this approach in both one-dimensional hydrodynamic and
three-dimensional magnetohydrodynamic simulations are discussed.

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Title: IRIS and SDO observations of coronal heating associated
with spicules
Authors: De Moortel, Ineke
2017psio.confE..55D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Reconnection Microjets in the Pre-eruption Phase of a
Prominence/Coronal Rain Complex
Authors: Antolin, P.; Mehta, T.; Conlon, T.; De Moortel, I.
2016AGUFMSH43C2582A Altcode:
Coronal rain is known to be one of the highest resolution tracers
of the coronal magnetic field. In this work the dynamics of a
prominence/coronal rain complex are analysed based on imaging and
spectroscopic observations with IRIS. Prior to eruption, the loop-like
magnetic field arcade hosting the rain is observed to slowly expand
in height. This movement is accompanied by several small ( 1 arsec)
and short (&lt;20 sec) bursts of plasma perpendicular to the field,
captured in the Si IV and Mg II lines. The line profiles are broad
and asymmetric with long tails above 100 km/s. These microjets are
accompanied with strong intensity enhancements along the loop in most of
the AIA channels, indicating significant energy release. We interpret
these microjets as reconnection outflows, produced by component
reconnection as the magnetic structure expands transversely. The
originally cold conditions of the rain allows in this case a unique
high resolution glance at the reconnection dynamics in low beta plasmas.

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Title: IRIS and SDO/AIA observations of coronal heating associated
with spicules
Authors: De Pontieu, B.; De Moortel, I.; Mcintosh, S. W.
2016AGUFMSH42B..07D Altcode:
Chromospheric spicules have been proposed as significant contributors
to the coronal energy and mass balance. While previous observations
have provided a glimpse of short-lived transient brightenings in
the corona that are associated with spicules, these observations
have been contested and the subject of a vigorous debate both on the
modeling and the observational side so that it remains unclear whether
plasma associated with spicules is heated to coronal temperatures. We
use high-resolution observations of the chromosphere and transition
region with the Interface Region Imaging Spectrograph (IRIS) and of
the corona with the Atmospheric Imaging Assembly (AIA) onboard the
Solar Dynamics Observatory (SDO) to show evidence of the formation of
coronal structures as a result of spicular mass ejections and subsequent
heating of plasma first to transition region and later to coronal
temperatures. Our observations suggest that much of the highly dynamic
loop fan environment associated with plage regions may be the result
of the formation of such new coronal strands, a process that previously
had been interpreted as the propagation of transient propagating coronal
disturbances (PCD)s. Our results suggest that heating and strong flows
play an important role in maintaining the substructure of loop fans,
in addition to the waves that permeate this low coronal environment.

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Title: Impact of flux distribution on elementary heating events
Authors: O'Hara, J. P.; De Moortel, I.
2016A&A...594A..67O Altcode:
Context. The complex magnetic field on the solar surface has been
shown to contain a range of sizes and distributions of magnetic
flux structures. The dynamic evolution of this magnetic carpet by
photospheric flows provides a continual source of free magnetic
energy into the solar atmosphere, which can subsequently be released
by magnetic reconnection. Aims: We investigate how the
distribution and number of magnetic flux sources impact the energy
release and locations of heating through magnetic reconnection driven
by slow footpoint motions. Methods: 3D magnetohydrodynamic
(MHD) simulations using Lare3D are carried out, where flux tubes are
formed between positive and negative sources placed symmetrically on
the lower and upper boundaries of the domain, respectively. The flux
tubes are subjected to rotational driving velocities on the boundaries
and are forced to interact and reconnect. Results: Initially,
simple flux distributions with two and four sources are compared. In
both cases, central current concentrations are formed between the flux
tubes and Ohmic heating occurs. The reconnection and subsequent energy
release is delayed in the four-source case and is shown to produce
more locations of heating, but with smaller magnitudes. Increasing the
values of the background field between the flux tubes is shown to delay
the onset of reconnection and increases the efficiency of heating in
both the two- and four-source cases. The cases with two flux tubes are
always more energetic than the corresponding four flux tube cases,
however the addition of the background field makes this disparity
less significant. A final experiment with a larger number of smaller
flux sources is considered and the field evolution and energetics are
shown to be remarkably similar to the two-source case, indicating the
importance of the size and separation of the flux sources relative to
the spatial scales of the velocity driver.

---------------------------------------------------------
Title: Modeling Observed Decay-less Oscillations as Resonantly
Enhanced Kelvin-Helmholtz Vortices from Transverse MHD Waves and
Their Seismological Application
Authors: Antolin, P.; De Moortel, I.; Van Doorsselaere, T.; Yokoyama,
T.
2016ApJ...830L..22A Altcode: 2016arXiv160909716A
In the highly structured solar corona, resonant absorption is an
unavoidable mechanism of energy transfer from global transverse MHD
waves to local azimuthal Alfvén waves. Due to its localized nature,
direct detection of this mechanism is extremely difficult. Yet, it is
the leading theory explaining the observed fast damping of the global
transverse waves. However, at odds with this theoretical prediction
are recent observations that indicate that in the low-amplitude regime
such transverse MHD waves can also appear decay-less, a still unsolved
phenomenon. Recent numerical work has shown that Kelvin-Helmholtz
instabilities (KHI) often accompany transverse MHD waves. In this work,
we combine 3D MHD simulations and forward modeling to show that for
currently achieved spatial resolution and observed small amplitudes,
an apparent decay-less oscillation is obtained. This effect results
from the combination of periodic brightenings produced by the KHI
and the coherent motion of the KHI vortices amplified by resonant
absorption. Such an effect is especially clear in emission lines forming
at temperatures that capture the boundary dynamics rather than the core,
and reflects the low damping character of the local azimuthal Alfvén
waves resonantly coupled to the kink mode. Due to phase mixing, the
detected period can vary depending on the emission line, with those
sensitive to the boundary having shorter periods than those sensitive
to the loop core. This allows us to estimate the density contrast at
the boundary.

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Title: On the Connection between Propagating Solar Coronal
Disturbances and Chromospheric Footpoints
Authors: Bryans, P.; McIntosh, S. W.; De Moortel, I.; De Pontieu, B.
2016ApJ...829L..18B Altcode:
The Interface Region Imaging Spectrograph (IRIS) provides an
unparalleled opportunity to explore the (thermal) interface between the
chromosphere, transition region, and the coronal plasma observed by the
Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory
(SDO). The SDO/AIA observations of coronal loop footpoints show
strong recurring upward propagating signals—“propagating coronal
disturbances” (PCDs) with apparent speeds of the order of 100-120 km
s-1. That signal has a clear signature in the slit-jaw images
of IRIS in addition to identifiable spectral signatures and diagnostics
in the Mg iih (2803 Å) line. In analyzing the Mg iih line, we are able
to observe the presence of magnetoacoustic shock waves that are also
present in the vicinity of the coronal loop footpoints. We see there is
enough of a correspondence between the shock propagation in Mg iih, the
evolution of the Si IV line profiles, and the PCD evolution to indicate
that these waves are an important ingredient for PCDs. In addition, the
strong flows in the jet-like features in the IRIS Si IV slit-jaw images
are also associated with PCDs, such that waves and flows both appear
to be contributing to the signals observed at the footpoints of PCDs.

---------------------------------------------------------
Title: Coronal Density Structure and its Role in Wave Damping in Loops
Authors: Cargill, P. J.; De Moortel, I.; Kiddie, G.
2016ApJ...823...31C Altcode:
It has long been established that gradients in the Alfvén speed, and
in particular the plasma density, are an essential part of the damping
of waves in the magnetically closed solar corona by mechanisms such as
resonant absorption and phase mixing. While models of wave damping often
assume a fixed density gradient, in this paper the self-consistency
of such calculations is assessed by examining the temporal evolution
of the coronal density. It is shown conceptually that for some coronal
structures, density gradients can evolve in a way that the wave-damping
processes are inhibited. For the case of phase mixing we argue that
(a) wave heating cannot sustain the assumed density structure and (b)
inclusion of feedback of the heating on the density gradient can lead to
a highly structured density, although on long timescales. In addition,
transport coefficients well in excess of classical are required to
maintain the observed coronal density. Hence, the heating of closed
coronal structures by global oscillations may face problems arising from
the assumption of a fixed density gradient, and the rapid damping of
oscillations may have to be accompanied by a separate (non-wave-based)
heating mechanism to sustain the required density structuring.

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Title: Transverse, propagating velocity perturbations in solar
coronal loops
Authors: De Moortel, I.; Pascoe, D. J.; Wright, A. N.; Hood, A. W.
2016PPCF...58a4001D Altcode: 2015arXiv151000976D
As waves and oscillations carry both energy and information, they
have enormous potential as a plasma heating mechanism and, through
seismology, to provide estimates of local plasma properties which
are hard to obtain from direct measurements. Being sufficiently near
to allow high-resolution observations, the atmosphere of the Sun
forms a natural plasma laboratory. Recent observations have revealed
that an abundance of waves and oscillations is present in the solar
atmosphere, leading to a renewed interest in wave heating mechanisms. This short review paper gives an overview of recently observed
transverse, propagating velocity perturbations in coronal loops. These
ubiquitous perturbations are observed to undergo strong damping as
they propagate. Using 3D numerical simulations of footpoint-driven
transverse waves propagating in a coronal plasma with a cylindrical
density structure, in combination with analytical modelling, it is
demonstrated that the observed velocity perturbations can be understood
in terms of coupling of different wave modes in the inhomogeneous
boundaries of the loops. Mode coupling in the inhomogeneous boundary
layers of the loops leads to the coupling of the transversal (kink) mode
to the azimuthal (Alfvén) mode, observed as the decay of the transverse
kink oscillations. Both the numerical and analytical results show the
spatial profile of the damped wave has a Gaussian shape to begin with,
before switching to exponential decay at large heights. In addition,
recent analysis of CoMP (Coronal Multi-channel Polarimeter) Doppler
shift observations of large, off-limb, trans-equatorial loops shows that
Fourier power at the apex appears to be higher in the high-frequency
part of the spectrum than expected from theoretical models. This excess
high-frequency FFT power could be tentative evidence for the onset of
a cascade of the low-to-mid frequency waves into (Alfvénic) turbulence.

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Title: Sub-photosphere to Solar Atmosphere Connection
Authors: Komm, Rudolf; De Moortel, Ineke; Fan, Yuhong; Ilonidis,
Stathis; Steiner, Oskar
2015SSRv..196..167K Altcode: 2013SSRv..tmp...93K
Magnetic fields extend from the solar interior through the
atmosphere. The formation and evolution of active regions can be studied
by measuring subsurface flows with local helioseismology. The emergence
of magnetic flux from the solar convection zone is associated with
acoustic perturbation signatures. In near-surface layers, the average
dynamics can be determined for emerging regions. MHD simulations
of the emergence of a twisted flux tube show how magnetic twist
and free energy are transported from the interior into the corona
and the dynamic signatures associated with such transport in the
photospheric and sub-photospheric layers. The subsurface twisted flux
tube does not emerge into the corona as a whole in emerging active
regions. Shear flows at the polarity inversion line and coherent
vortical motions in the subsurface flux tubes are the major means by
which twist is transported into the corona, leading to the formation
of sigmoid-shaped coronal magnetic fields capable of driving solar
eruptions. The transport of twist can be followed from the interior
by using the kinetic helicity of subsurface flows as a proxy of
magnetic helicity; this quantity holds great promise for improving
the understanding of eruptive phenomena. Waves are not only vital for
studying the link between the solar interior and the surface but for
linking the photosphere with the corona as well. Acoustic waves that
propagate from the surface into the magnetically structured, dynamic
atmosphere undergo mode conversion and refraction. These effects
enable atmospheric seismology to determine the topography of magnetic
canopies in the solar atmosphere. Inclined magnetic fields lower
the cut-off frequency so that low frequency waves can leak into the
outer atmosphere. Recent high resolution, high cadence observations of
waves and oscillations in the solar atmosphere, have lead to a renewed
interest in the potential role of waves as a heating mechanism. In light
of their potential contribution to the heating of the solar atmosphere,
some of the recent observations of waves and oscillations and ongoing
modelling efforts are reviewed.

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Title: Impact of Flux Distribution on Elementary Heating Events
Authors: O'Hara, Jennifer; De Moortel, Ineke
2015IAUGA..2254861O Altcode:
We present the results of numerical simulations of reconnection between
flux tubes driven by rotational footpoint motions using the 3D MHD
code, Lare3d. The basic model consists of two, initially aligned, flux
tubes that are forced to interact by rotational driving velocities
on the flux concentrations on the boundaries. We extend this model
by altering the number, distribution and strength of the sources,
while maintaining the same total magnetic flux on the boundaries. In
all cases, the magnetic field is stressed by the boundary motions and
a current grows within the volume. We examine the dynamical evolution
and the resultant magnitude, distribution and timing of the heating
events for the different flux distributions.

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Title: On the Parallel and Perpendicular Propagating Motions Visible
inPolar Plumes: An Incubator For (Fast) Solar Wind Acceleration?
Authors: Liu, Jiajia; McIntosh, Scott W.; De Moortel, Ineke; Wang,
Yuming
2015ApJ...806..273L Altcode: 2015arXiv150700143L
We combine observations of the Coronal Multi-channel Polarimeter and the
Atmospheric Imaging Assembly on board the Solar Dynamics Observatory
to study the characteristic properties of (propagating) Alfvénic
motions and quasi-periodic intensity disturbances in polar plumes. This
unique combination of instruments highlights the physical richness of
the processes taking place at the base of the (fast) solar wind. The
(parallel) intensity perturbations with intensity enhancements around
1% have an apparent speed of 120 km s-1 (in both the 171
and 193 Å passbands) and a periodicity of 15 minutes, while the
(perpendicular) Alfvénic wave motions have a velocity amplitude
of 0.5 km s-1, a phase speed of 830 km s-1,
and a shorter period of 5 minutes on the same structures. These
observations illustrate a scenario where the excited Alfvénic
motions are propagating along an inhomogeneously loaded magnetic field
structure such that the combination could be a potential progenitor
of the magnetohydrodynamic turbulence required to accelerate the fast
solar wind.

---------------------------------------------------------
Title: Excitation and damping of broadband kink waves in the solar
corona
Authors: Pascoe, D. J.; Wright, A. N.; De Moortel, I.; Hood, A. W.
2015A&A...578A..99P Altcode:
Context. Observations such as those by the Coronal Multi-Channel
Polarimeter (CoMP) have revealed that broadband kink oscillations
are ubiquitous in the solar corona. Aims: We consider
footpoint-driven kink waves propagating in a low β coronal plasma with
a cylindrical density structure. We investigate the excitation and
damping of propagating kink waves by a broadband driver, including
the effects of different spatial profiles for the driver. 
Methods: We employ a general spatial damping profile in which the
initial stage of the damping envelope is approximated by a Gaussian
profile and the asymptotic state by an exponential one. We develop
a method of accounting for the presence of these different damping
regimes and test it using data from numerical simulations. 
Results: Strongly damped oscillations in low density coronal loops
are more accurately described by a Gaussian spatial damping profile
than an exponential profile. The consequences for coronal seismology
are investigated and applied to observational data for the ubiquitous
broadband waves observed by CoMP. Current data cannot distinguish
between the exponential and Gaussian profiles because of the levels
of noise. We demonstrate the importance of the spatial profile of the
driver on the resulting damping profile. Furthermore, we show that a
small-scale turbulent driver is inefficient at exciting propagating
kink waves.

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Title: Recent advances in coronal heating
Authors: De Moortel, Ineke; Browning, Philippa
2015RSPTA.37340269D Altcode: 2015arXiv151000977D
The solar corona, the tenuous outer atmosphere of the Sun, is orders of
magnitude hotter than the solar surface. This 'coronal heating problem'
requires the identification of a heat source to balance losses due to
thermal conduction, radiation and (in some locations) convection. The
review papers in this Theo Murphy meeting issue present an overview
of recent observational findings, large- and small-scale numerical
modelling of physical processes occurring in the solar atmosphere
and other aspects which may affect our understanding of the proposed
heating mechanisms. At the same time, they also set out the directions
and challenges which must be tackled by future research. In this brief
introduction, we summarize some of the issues and themes which reoccur
throughout this issue.

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Title: Observational Signatures of Waves and Flows in the Solar Corona
Authors: De Moortel, I.; Antolin, P.; Van Doorsselaere, T.
2015SoPh..290..399D Altcode: 2014SoPh..tmp..133D; 2015arXiv151001030D
Propagating perturbations have been observed in extended coronal loop
structures for a number of years, but the interpretation in terms of
slow (propagating) magneto-acoustic waves and/or as quasi-periodic
upflows remains unresolved. We used forward-modelling to construct
observational signatures associated with a simple slow magneto-acoustic
wave or periodic flow model. Observational signatures were computed
for the 171 Å Fe IX and the 193 Å Fe XII spectral lines. Although
there are many differences between the flow and wave models, we did
not find any clear, robust observational characteristics that can be
used in isolation (i.e. that do not rely on a comparison between the
models). For the waves model, a relatively rapid change of the average
line widths as a function of (shallow) line-of-sight angles was found,
whereas the ratio of the line width amplitudes to the Doppler velocity
amplitudes is relatively high for the flow model. The most robust
observational signature found is that the ratio of the mean to the
amplitudes of the Doppler velocity is always higher than one for the
flow model. This ratio is substantially higher for flows than for
waves, and for the flows model used in the study is exactly the same
in the 171 Å Fe IX and the 193 Å Fe XII spectral lines. However,
these potential observational signatures need to be treated cautiously
because they are likely to be model-dependent.

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Title: Statistical Evidence for the Existence of Alfvénic Turbulence
in Solar Coronal Loops
Authors: Liu, Jiajia; McIntosh, Scott W.; De Moortel, Ineke; Threlfall,
James; Bethge, Christian
2014ApJ...797....7L Altcode: 2014arXiv1411.5094L
Recent observations have demonstrated that waves capable of
carrying large amounts of energy are ubiquitous throughout the solar
corona. However, the question of how this wave energy is dissipated
(on which timescales and length scales) and released into the plasma
remains largely unanswered. Both analytic and numerical models have
previously shown that Alfvénic turbulence may play a key role not
only in the generation of the fast solar wind, but in the heating
of coronal loops. In an effort to bridge the gap between theory and
observations, we expand on a recent study by analyzing 37 clearly
isolated coronal loops using data from the Coronal Multi-channel
Polarimeter instrument. We observe Alfvénic perturbations with phase
speeds which range from 250 to 750 km s-1 and periods from
140 to 270 s for the chosen loops. While excesses of high-frequency wave
power are observed near the apex of some loops (tentatively supporting
the onset of Alfvénic turbulence), we show that this excess depends on
loop length and the wavelength of the observed oscillations. In deriving
a proportional relationship between the loop length/wavelength ratio
and the enhanced wave power at the loop apex, and from the analysis
of the line widths associated with these loops, our findings are
supportive of the existence of Alfvénic turbulence in coronal loops.

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Title: Standing Kink Modes in Three-dimensional Coronal Loops
Authors: Pascoe, D. J.; De Moortel, I.
2014ApJ...784..101P Altcode:
So far, the straight flux tube model proposed by Edwin &amp; Roberts
is the most commonly used tool in practical coronal seismology,
in particular, to infer values of the (coronal) magnetic field from
observed, standing kink mode oscillations. In this paper, we compare
the period predicted by this basic model with three-dimensional (3D)
numerical simulations of standing kink mode oscillations, as the period
is a crucial parameter in the seismological inversion to determine the
magnetic field. We perform numerical simulations of standing kink modes
in both straight and curved 3D coronal loops and consider excitation
by internal and external drivers. The period of oscillation for the
displacement of dense coronal loops is determined by the loop length
and the kink speed, in agreement with the estimate based on analytical
theory for straight flux tubes. For curved coronal loops embedded in
a magnetic arcade and excited by an external driver, a secondary mode
with a period determined by the loop length and external Alfvén speed
is also present. When a low number of oscillations is considered, these
two periods can result in a single, non-resolved (broad) peak in the
power spectrum, particularly for low values of the density contrast
for which the two periods will be relatively similar. In that case
(and for this particular geometry), the presence of this additional
mode would lead to ambiguous seismological estimates of the magnetic
field strength.

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Title: Potential Evidence for the Onset of Alfvénic Turbulence in
Trans-equatorial Coronal Loops
Authors: De Moortel, I.; McIntosh, S. W.; Threlfall, J.; Bethge, C.;
Liu, J.
2014ApJ...782L..34D Altcode:
This study investigates Coronal Multi-channel Polarimeter Doppler-shift
observations of a large, off-limb, trans-equatorial loop system observed
on 2012 April 10-11. Doppler-shift oscillations with a broad range of
frequencies are found to propagate along the loop with a speed of about
500 km s-1. The power spectrum of perturbations travelling
up from both loop footpoints is remarkably symmetric, probably due to
the almost perfect north-south alignment of the loop system. Compared
to the power spectrum at the footpoints of the loop, the Fourier power
at the apex appears to be higher in the high-frequency part of the
spectrum than expected from theoretical models. We suggest this excess
high-frequency power could be tentative evidence for the onset of a
cascade of the low-to-mid frequency waves into (Alfvénic) turbulence.

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Title: The Evolving Magnetic Scales of the Outer Solar Atmosphere
and Their Potential Impact on Heliospheric Turbulence
Authors: McIntosh, Scott W.; Bethge, Christian; Threlfall, James;
De Moortel, Ineke; Leamon, Robert J.; Tian, Hui
2013arXiv1311.2538M Altcode:
The presence of turbulent phenomena in the outer solar atmosphere
is a given. However, because we are reduced to remotely sensing the
atmosphere of a star with instruments of limited spatial and/or spectral
resolution, we can only infer the physical progression from macroscopic
to microscopic phenomena. Even so, we know that many, if not all,
of the turbulent phenomena that pervade interplanetary space have
physical origins at the Sun and so in this brief article we consider
some recent measurements which point to sustained potential source(s)
of heliospheric turbulence in the magnetic and thermal domains. In
particular, we look at the scales of magnetism that are imprinted on
the outer solar atmosphere by the relentless magneto-convection of the
solar interior and combine state-of-the-art observations from the Solar
Dynamics Observatory (SDO) and the Coronal Multi-channel Polarimeter
(CoMP) which are beginning to hint at the origins of the wave/plasma
interplay prevalent closer to the Earth. While linking these disparate
scales of observation and understanding of their connection is near
to impossible, it is clear that the constant evolution of subsurface
magnetism on a host of scales guides and governs the flow of mass
and energy at the smallest scales. In the near future significant
progress in this area will be made by linking observations from high
resolution platforms like the Interface Region Imaging Spectrograph
(IRIS) and Advanced Technology Solar Telescope (ATST) with full-disk
synoptic observations such as those presented herein.

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Title: First comparison of wave observations from CoMP and AIA/SDO
Authors: Threlfall, J.; De Moortel, I.; McIntosh, S. W.; Bethge, C.
2013A&A...556A.124T Altcode: 2013arXiv1306.3354T
Context. Waves have long been thought to contribute to the heating
of the solar corona and the generation of the solar wind. Recent
observations have demonstrated evidence of quasi-periodic longitudinal
disturbances and ubiquitous transverse wave propagation in many
different coronal environments. Aims: This paper investigates
signatures of different types of oscillatory behaviour, both above
the solar limb and on-disk, by comparing findings from the Coronal
Multi-channel Polarimeter (CoMP) and the Atmospheric Imaging
Assembly (AIA) on-board the Solar Dynamics Observatory (SDO) for
the same active region. Methods: We study both transverse and
longitudinal motion by comparing and contrasting time-distance images
of parallel and perpendicular cuts along/across active region fan
loops. Comparisons between parallel space-time diagram features in
CoMP Doppler velocity and transverse oscillations in AIA images are
made, together with space-time analysis of propagating quasi-periodic
intensity features seen near the base of loops in AIA. Results:
Signatures of transverse motions are observed along the same magnetic
structure using CoMP Doppler velocity (vphase = 600 → 750
km s-1, P = 3 → 6 min) and in AIA/SDO above the limb (P =
3 → 8 min). Quasi-periodic intensity features (vphase =
100 → 200 km s-1, P = 6 → 11 min) also travel along the
base of the same structure. On the disk, signatures of both transverse
and longitudinal intensity features were observed by AIA, and both show
similar properties to signatures found along structures anchored in
the same active region three days earlier above the limb. Correlated
features are recovered by space-time analysis of neighbouring tracks
over perpendicular distances of ≲2.6 Mm.

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Title: Damping of kink waves by mode coupling. I. Analytical treatment
Authors: Hood, A. W.; Ruderman, M.; Pascoe, D. J.; De Moortel, I.;
Terradas, J.; Wright, A. N.
2013A&A...551A..39H Altcode:
 Aims: We investigate the spatial damping of propagating
kink waves in an inhomogeneous plasma. In the limit of a thin tube
surrounded by a thin transition layer, an analytical formulation
for kink waves driven in from the bottom boundary of the corona
is presented. Methods: The spatial form for the damping
of the kink mode was investigated using various analytical
approximations. When the density ratio between the internal
density and the external density is not too large, a simple
differential-integral equation was used. Approximate analytical
solutions to this equation are presented. Results: For the
first time, the form of the spatial damping of the kink mode is shown
analytically to be Gaussian in nature near the driven boundary. For
several wavelengths, the amplitude of the kink mode is proportional
to (1 + exp(-z2/Lg2))/2, where
Lg2 = 16/ɛκ2k2. Although
the actual value of 16 in Lg depends on the particular
form of the driver, this form is very general and its dependence on
the other parameters does not change. For large distances, the damping
profile appears to be roughly linear exponential decay. This is shown
analytically by a series expansion when the inhomogeneous layer width
is small enough. Appendix A is available in electronic form at
<A href="http://www.aanda.org">http://www.aanda.org</A>

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Title: Damping of kink waves by mode coupling. II. Parametric study
and seismology
Authors: Pascoe, D. J.; Hood, A. W.; De Moortel, I.; Wright, A. N.
2013A&A...551A..40P Altcode:
Context. Recent observations of the corona reveal ubiquitous
transverse velocity perturbations that undergo strong damping as
they propagate. These can be understood in terms of propagating kink
waves that undergo mode coupling in inhomogeneous regions. 
Aims: The use of these propagating waves as a seismological tool
for the investigation of the solar corona depends upon an accurate
understanding of how the mode coupling behaviour is determined by local
plasma parameters. Our previous work suggests the exponential spatial
damping profile provides a poor description of the behaviour of strongly
damped kink waves. We aim to investigate the spatial damping profile
in detail and provide a guide to the approximations most suitable
for performing seismological inversions. Methods: We propose
a general spatial damping profile based on analytical results that
accounts for the initial Gaussian stage of damped kink waves as well
as the asymptotic exponential stage considered by previous authors. The
applicability of this profile is demonstrated by a full parametric study
of the relevant physical parameters. The implication of this profile
for seismological inversions is investigated. Results: The
Gaussian damping profile is found to be most suitable for application
as a seismological tool for observations of oscillations in loops with
a low density contrast. This profile also provides accurate estimates
for data in which only a few wavelengths or periods are observed.

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Title: The Source of 3 Minute Magnetoacoustic Oscillations in
Coronal Fans
Authors: Jess, D. B.; De Moortel, I.; Mathioudakis, M.; Christian,
D. J.; Reardon, K. P.; Keys, P. H.; Keenan, F. P.
2012ApJ...757..160J Altcode: 2012arXiv1208.3194J
We use images of high spatial, spectral, and temporal resolution,
obtained using both ground- and space-based instrumentation, to
investigate the coupling between wave phenomena observed at numerous
heights in the solar atmosphere. Analysis of 4170 Å continuum images
reveals small-scale umbral intensity enhancements, with diameters
~0farcs6, lasting in excess of 30 minutes. Intensity oscillations
of ≈3 minutes are observed to encompass these photospheric
structures, with power at least three orders of magnitude higher
than the surrounding umbra. Simultaneous chromospheric velocity and
intensity time series reveal an 87° ± 8° out-of-phase behavior,
implying the presence of standing modes created as a result of
partial wave reflection at the transition region boundary. We find a
maximum waveguide inclination angle of ≈40° between photospheric
and chromospheric heights, combined with a radial expansion factor
of &lt;76%. An average blueshifted Doppler velocity of ≈1.5 km
s-1, in addition to a time lag between photospheric and
chromospheric oscillatory phenomena, confirms the presence of upwardly
propagating slow-mode waves in the lower solar atmosphere. Propagating
oscillations in EUV intensity are detected in simultaneous coronal
fan structures, with a periodicity of 172 ± 17 s and a propagation
velocity of 45 ± 7 km s-1. Numerical simulations reveal that
the damping of the magnetoacoustic wave trains is dominated by thermal
conduction. The coronal fans are seen to anchor into the photosphere
in locations where large-amplitude umbral dot (UD) oscillations
manifest. Derived kinetic temperature and emission measure time series
display prominent out-of-phase characteristics, and when combined with
the previously established sub-sonic wave speeds, we conclude that
the observed EUV waves are the coronal counterparts of the upwardly
propagating magnetoacoustic slow modes detected in the lower solar
atmosphere. Thus, for the first time, we reveal how the propagation
of 3 minute magnetoacoustic waves in solar coronal structures is a
direct result of amplitude enhancements occurring in photospheric UDs.

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Title: Propagating Disturbances in Coronal Loops: A Detailed Analysis
of Propagation Speeds
Authors: Kiddie, G.; De Moortel, I.; Del Zanna, G.; McIntosh, S. W.;
Whittaker, I.
2012SoPh..279..427K Altcode: 2012arXiv1205.0891K
Quasi-periodic disturbances have been observed in the outer solar
atmosphere for many years. Although first interpreted as upflows
(Schrijver et al., Solar Phys.187, 261, 1999), they have been widely
regarded as slow magneto-acoustic waves, due to their observed
velocities and periods. However, recent observations have questioned
this interpretation, as periodic disturbances in Doppler velocity,
line width, and profile asymmetry were found to be in phase with the
intensity oscillations (De Pontieu and McIntosh, Astrophys. J.722,
1013, 2010; Tian, McIntosh, and De Pontieu, Astrophys. J. Lett.727,
L37, 2011), suggesting that the disturbances could be quasi-periodic
upflows. Here we conduct a detailed analysis of the velocities of
these disturbances across several wavelengths using the Atmospheric
Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). We
analysed 41 examples, including both sunspot and non-sunspot regions
of the Sun. We found that the velocities of propagating disturbances
(PDs) located at sunspots are more likely to be temperature dependent,
whereas the velocities of PDs at non-sunspot locations do not show a
clear temperature dependence. This suggests an interpretation in terms
of slow magneto-acoustic waves in sunspots but the nature of PDs in
non-sunspot (plage) regions remains unclear. We also considered on
what scale the underlying driver is affecting the properties of the
PDs. Finally, we found that removing the contribution due to the cooler
ions in the 193 Å wavelength suggests that a substantial part of the
193 Å emission of sunspot PDs can be attributed to the cool component
of 193 Å.

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Title: Nonlinear wave propagation and reconnection at magnetic
X-points in the Hall MHD regime
Authors: Threlfall, J.; Parnell, C. E.; De Moortel, I.; McClements,
K. G.; Arber, T. D.
2012A&A...544A..24T Altcode: 2012arXiv1202.3648T
Context. The highly dynamical, complex nature of the solar atmosphere
naturally implies the presence of waves in a topologically varied
magnetic environment. Here, the interaction of waves with topological
features such as null points is inevitable and potentially important
for energetics. The low resistivity of the solar coronal plasma implies
that non-magnetohydrodynamic (MHD) effects should be considered in
studies of magnetic energy release in this environment. Aims:
This paper investigates the role of the Hall term in the propagation and
dissipation of waves, their interaction with 2D magnetic X-points and
the nature of the resulting reconnection. Methods: A Lagrangian
remap shock-capturing code (Lare2d) was used to study the evolution of
an initial fast magnetoacoustic wave annulus for a range of values of
the ion skin depth (δi) in resistive Hall MHD. A magnetic
null-point finding algorithm was also used to locate and track the
evolution of the multiple null-points that are formed in the system. Results: Depending on the ratio of ion skin depth to system size,
our model demonstrates that Hall effects can play a key role in the
wave-null interaction. In particular, the initial fast-wave pulse now
consists of whistler and ion-cyclotron components; the dispersive nature
of the whistler wave leads to (i) earlier interaction with the null;
(ii) the creation of multiple additional, transient nulls and, hence,
an increased number of energy release sites. In the Hall regime, the
relevant timescales (such as the onset of reconnection and the period
of the oscillatory relaxation) of the system are reduced significantly,
and the reconnection rate is enhanced.

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Title: A contemporary view of coronal heating
Authors: Parnell, C. E.; De Moortel, I.
2012RSPTA.370.3217P Altcode: 2012arXiv1206.6097P
Determining the heating mechanism (or mechanisms) that causes the outer
atmosphere of the Sun, and many other stars, to reach temperatures
orders of magnitude higher than their surface temperatures has long
been a key problem. For decades the problem has been known as the
coronal heating problem, but it is now clear that `coronal heating'
cannot be treated or explained in isolation and that the heating of the
whole solar atmosphere must be studied as a highly coupled system. The
magnetic field of the star is known to play a key role, but, despite
significant advancements in solar telescopes, computing power and
much greater understanding of theoretical mechanisms, the question
of which mechanism or mechanisms are the dominant supplier of energy
to the chromosphere and corona is still open. Following substantial
recent progress, we consider the most likely contenders and discuss
the key factors that have made, and still make, determining the actual
(coronal) heating mechanism (or mechanisms) so difficult.

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Title: Magnetohydrodynamic waves and coronal seismology: an overview
of recent results
Authors: De Moortel, I.; Nakariakov, V. M.
2012RSPTA.370.3193D Altcode: 2012arXiv1202.1944D
Recent observations have revealed that MHD waves and oscillations are
ubiquitous in the solar atmosphere, with a wide range of periods. We
give a brief review of some aspects of MHD waves and coronal seismology
which have recently been the focus of intense debate or are newly
emerging. In particular, we focus on four topics: (i) the current
controversy surrounding propagating intensity perturbations along
coronal loops, (ii) the interpretation of propagating transverse
loop oscillations, (iii) the ongoing search for coronal (torsional)
Alfven waves and (iv) the rapidly developing topic of quasi-periodic
pulsations (QPP) in solar flares.

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Title: The Fifth Hinode Science Meeting
Authors: Golub, L.; De Moortel, I.; Shimizu, T.
2012ASPC..456.....G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Spatial damping of propagating kink waves due to mode coupling
Authors: Pascoe, D. J.; Hood, A. W.; de Moortel, I.; Wright, A. N.
2012A&A...539A..37P Altcode:
 Aims: We investigate the damping process for propagating
transverse velocity oscillations, observed to be ubiquitous in the
solar corona, due to mode coupling. Methods: We perform
3D numerical simulations of footpoint-driven transverse waves
propagating in a low β coronal plasma with a cylindrical density
structure. Mode coupling in an inhomogeneous layer leads to the
coupling of the kink mode to the Alfvén mode, observed as the decay
of the transverse kink oscillations. Results: We consider the
spatial damping profile and find a Gaussian damping profile of the form
exp(-z2/Lg2) to be the most congruent
with our numerical data, rather than the exponential damping profile
of the form exp(- z/Ld) used in normal mode analysis. Our
results highlight that the nature of the driver itself will have
a substantial influence on observed propagating kink waves. 
Conclusions: Our study suggests that this modified damping profile
should be taken into account when using coronal seismology to infer
local plasma properties from observed damped oscillations.

---------------------------------------------------------
Title: Coupling, damping and dissipation of magnetic waves in the
chromosphere and corona
Authors: De Moortel, I.
2012decs.confE..46D Altcode:
In this talk I will give an overview of current (numerical) modelling
of MHD waves and oscillations, emphasising in particular the process
of mode coupling. Can models predict the observed damping rates
and energy flux? How reliable are the comparisons between theory
and observations? As observations of waves and oscillations become
increasingly more detailed, it has become clear that the role of wave
heating of the solar atmosphere has to be reassessed. I will highlight
some of the recent modelling results as well as try to outline where
future efforts are needed.

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Title: What we can and cannot learn from seismology of the solar
atmosphere
Authors: De Moortel, I.
2012decs.confE..45D Altcode:
During the last decade or so, new instruments have revealed a
surprisingly large number of observations of oscillatory behaviour
in the solar atmosphere. Both standing and propagating waves have now
been detected in a variety of different structures with a wide range
of instruments. After the initial euphoria in coronal seismology
applications, the subject now needs to go through a period of
consolidating and verifying results. So, what can we actually learn
from coronal seismology? This talk will invite the community to debate
future directions for both theoretical modelling and observational
campaigns. How robust are the basic MHD waves models? Do they apply
in the highly dynamical and structured solar atmosphere? What can we
learn from numerical modelling? And what exactly can we deduce from
the observations?

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Title: What can we learn from propagating Alfvenic waves?
Authors: Pascoe, D. J.; De Moortel, I.; Hood, A. W.; Wright, A. N.
2012decs.confE..24P Altcode:
Observations have revealed ubiquitous transverse velocity perturbation
waves propagating in the solar corona. We perform 3D numerical
simulations of footpoint-driven transverse waves propagating in a
low beta plasma. When density structuring is present, mode coupling
in inhomogeneous regions leads to the coupling of the kink mode to the
Alfvén mode. The frequency-dependent decay of the propagating kink wave
is observed as energy is transferred to the local Alfvén mode. Modest
changes in density are capable of efficiently converting energy from the
driving footpoint motion to localised Alfvén modes. Thus, realistic
transverse footpoint motions will deposit energy to (azimuthal)
Alfvén modes in the corona. Mode coupling is investigated in detail
for propagating kink modes as an explanation for the observed wave
damping and as a possible seismological tool. The observed strong
damping of the Doppler shift oscillations indicates the presence of
wide inhomogeneous layers at the edges of the loops. Our simulations
(backed up by analytical calculations) show that in this regime, the
traditional exp(-z/L) damping rate no longer applies. Hence, care has
to be taken when seismologically inferring damping lengths from the
observed oscillations. In addition, taking into account line-of-sight
integration of multiple loops supporting transverse oscillations, we
show that the energy budget present in the 3D coronal volume could be
substantially higher than the energy budget derived from the observed
Doppler shift oscillations.

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Title: The Effects of Line-of-sight Integration on Multistrand
Coronal Loop Oscillations
Authors: De Moortel, I.; Pascoe, D. J.
2012ApJ...746...31D Altcode:
Observations have shown that transverse oscillations are present
in a multitude of coronal structures. It is generally assumed that
these oscillations are driven by (sub)surface footpoint motions. Using
fully three-dimensional MHD simulations, we show that these footpoint
perturbations generate propagating kink (Alfvénic) modes which couple
very efficiently into (azimuthal) Alfvén waves. Using an ensemble of
randomly distributed loops, driven by footpoint motions with random
periods and directions, we compare the absolute energy in the numerical
domain with the energy that is "visible" when integrating along the
line of sight (LOS). We show that the kinetic energy derived from the
LOS Doppler velocities is only a small fraction of the actual energy
provided by the footpoint motions. Additionally, the superposition of
loop structures along the LOS makes it nearly impossible to identify
which structure the observed oscillations are actually associated with
and could impact the identification of the mode of oscillation.

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Title: Computer Vision for the Solar Dynamics Observatory (SDO)
Authors: Martens, P. C. H.; Attrill, G. D. R.; Davey, A. R.; Engell,
A.; Farid, S.; Grigis, P. C.; Kasper, J.; Korreck, K.; Saar, S. H.;
Savcheva, A.; Su, Y.; Testa, P.; Wills-Davey, M.; Bernasconi, P. N.;
Raouafi, N. -E.; Delouille, V. A.; Hochedez, J. F.; Cirtain, J. W.;
DeForest, C. E.; Angryk, R. A.; De Moortel, I.; Wiegelmann, T.;
Georgoulis, M. K.; McAteer, R. T. J.; Timmons, R. P.
2012SoPh..275...79M Altcode: 2011SoPh..tmp..144M; 2011SoPh..tmp..213M; 2011SoPh..tmp....8M
In Fall 2008 NASA selected a large international consortium to produce
a comprehensive automated feature-recognition system for the Solar
Dynamics Observatory (SDO). The SDO data that we consider are all of the
Atmospheric Imaging Assembly (AIA) images plus surface magnetic-field
images from the Helioseismic and Magnetic Imager (HMI). We produce
robust, very efficient, professionally coded software modules that
can keep up with the SDO data stream and detect, trace, and analyze
numerous phenomena, including flares, sigmoids, filaments, coronal
dimmings, polarity inversion lines, sunspots, X-ray bright points,
active regions, coronal holes, EIT waves, coronal mass ejections
(CMEs), coronal oscillations, and jets. We also track the emergence and
evolution of magnetic elements down to the smallest detectable features
and will provide at least four full-disk, nonlinear, force-free magnetic
field extrapolations per day. The detection of CMEs and filaments is
accomplished with Solar and Heliospheric Observatory (SOHO)/Large
Angle and Spectrometric Coronagraph (LASCO) and ground-based Hα
data, respectively. A completely new software element is a trainable
feature-detection module based on a generalized image-classification
algorithm. Such a trainable module can be used to find features that
have not yet been discovered (as, for example, sigmoids were in the
pre-Yohkoh era). Our codes will produce entries in the Heliophysics
Events Knowledgebase (HEK) as well as produce complete catalogs for
results that are too numerous for inclusion in the HEK, such as the
X-ray bright-point metadata. This will permit users to locate data on
individual events as well as carry out statistical studies on large
numbers of events, using the interface provided by the Virtual Solar
Observatory. The operations concept for our computer vision system is
that the data will be analyzed in near real time as soon as they arrive
at the SDO Joint Science Operations Center and have undergone basic
processing. This will allow the system to produce timely space-weather
alerts and to guide the selection and production of quicklook images and
movies, in addition to its prime mission of enabling solar science. We
briefly describe the complex and unique data-processing pipeline,
consisting of the hardware and control software required to handle
the SDO data stream and accommodate the computer-vision modules, which
has been set up at the Lockheed-Martin Space Astrophysics Laboratory
(LMSAL), with an identical copy at the Smithsonian Astrophysical
Observatory (SAO).

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Title: Review Article: MHD Wave Propagation Near Coronal Null Points
of Magnetic Fields
Authors: McLaughlin, J. A.; Hood, A. W.; de Moortel, I.
2011SSRv..158..205M Altcode: 2010SSRv..tmp..174M; 2010arXiv1004.5568M; 2010SSRv..tmp..157M
We present a comprehensive review of MHD wave behaviour in the
neighbourhood of coronal null points: locations where the magnetic
field, and hence the local Alfvén speed, is zero. The behaviour of
all three MHD wave modes, i.e. the Alfvén wave and the fast and slow
magnetoacoustic waves, has been investigated in the neighbourhood
of 2D, 2.5D and (to a certain extent) 3D magnetic null points, for
a variety of assumptions, configurations and geometries. In general,
it is found that the fast magnetoacoustic wave behaviour is dictated
by the Alfvén-speed profile. In a β=0 plasma, the fast wave is
focused towards the null point by a refraction effect and all the
wave energy, and thus current density, accumulates close to the null
point. Thus, null points will be locations for preferential heating
by fast waves. Independently, the Alfvén wave is found to propagate
along magnetic fieldlines and is confined to the fieldlines it is
generated on. As the wave approaches the null point, it spreads out due
to the diverging fieldlines. Eventually, the Alfvén wave accumulates
along the separatrices (in 2D) or along the spine or fan-plane (in
3D). Hence, Alfvén wave energy will be preferentially dissipated at
these locations. It is clear that the magnetic field plays a fundamental
role in the propagation and properties of MHD waves in the neighbourhood
of coronal null points. This topic is a fundamental plasma process and
results so far have also lead to critical insights into reconnection,
mode-coupling, quasi-periodic pulsations and phase-mixing.

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Title: Solar physics: Waves galore
Authors: Cargill, Peter; de Moortel, Ineke
2011Natur.475..463C Altcode:
Wave energy has long been proposed to be a source of the hot solar
corona and fast solar wind. Direct measurements made by spacecraft
have finally established that coronal waves are ubiquitous and can
have the required energy. See Letter p.477

---------------------------------------------------------
Title: Observed Damping of the Slow Magnetoacoustic Mode
Authors: Marsh, M. S.; De Moortel, I.; Walsh, R. W.
2011ApJ...734...81M Altcode: 2011arXiv1104.1100M
Spectroscopic and stereoscopic imaging observations of slow
magnetoacoustic wave propagation within a coronal loop are investigated
to determine the decay length scale of the slow magnetoacoustic
mode in three dimensions and the density profile within the loop
system. The slow wave is found to have an e-folding decay length scale
of 20,000+4000 - 3000 km with a uniform density
profile along the loop base. These observations place quantitative
constraints on the modeling of wave propagation within coronal
loops. Theoretical forward modeling suggests that magnetic field line
divergence is the dominant damping factor and thermal conduction
is insufficient, given the observed parameters of the coronal loop
temperature, density, and wave mode period.

---------------------------------------------------------
Title: Coupled Alfvén and kink oscillations in an inhomogeneous
corona
Authors: Pascoe, David J.; Wright, Andrew N.; De Moortel, Ineke
2011IAUS..274..129P Altcode:
We perform 3D numerical simulations of footpoint-driven transverse
waves propagating in a low β plasma. The presence of inhomogeneities
in the density profile leads to the coupling of the driven kink mode
to Alfvén modes by resonant absorption. The decay of the propagating
kink wave as energy is transferred to the local Alfvén mode is in good
agreement with a modified interpretation of the analytical expression
derived for standing kink modes. This coupling may account for the
damping of transverse velocity perturbation waves which have recently
been observed to be ubiquitous in the solar corona.

---------------------------------------------------------
Title: Propagating Coupled Alfvén and Kink Oscillations in an
Arbitrary Inhomogeneous Corona
Authors: Pascoe, D. J.; Wright, A. N.; De Moortel, I.
2011ApJ...731...73P Altcode:
Observations have revealed ubiquitous transverse velocity perturbation
waves propagating in the solar corona. We perform three-dimensional
numerical simulations of footpoint-driven transverse waves propagating
in a low β plasma. We consider the cases of distorted cylindrical
flux tubes and a randomly generated inhomogeneous medium. When density
structuring is present, mode coupling in inhomogeneous regions leads
to the coupling of the kink mode to the Alfvén mode. The decay of the
propagating kink wave is observed as energy is transferred to the local
Alfvén mode. In all cases considered, modest changes in density were
capable of efficiently converting energy from the driving footpoint
motion to localized Alfvén modes. We have demonstrated that mode
coupling efficiently couples propagating kink perturbations to Alfvén
modes in an arbitrary inhomogeneous medium. This has the consequence
that transverse footpoint motions at the base of the corona will
deposit energy to Alfvén modes in the corona.

---------------------------------------------------------
Title: Phase mixing of nonlinear visco-resistive Alfvén waves
Authors: McLaughlin, J. A.; de Moortel, I.; Hood, A. W.
2011A&A...527A.149M Altcode: 2011arXiv1101.5945M
 Aims: We investigate the behaviour of nonlinear, nonideal Alfvén
wave propagation within an inhomogeneous magnetic environment. 
Methods: The governing MHD equations are solved in 1D and 2D using
both analytical techniques and numerical simulations. Results:
We find clear evidence for the ponderomotive effect and visco-resistive
heating. The ponderomotive effect generates a longitudinal component
to the transverse Alfvén wave, with a frequency twice that of the
driving frequency. Analytical work shows the addition of resistive
heating. This leads to a substantial increase in the local temperature
and thus gas pressure of the plasma, resulting in material being pushed
along the magnetic field. In 2D, our system exhibits phase mixing and
we observe an evolution in the location of the maximum heating, i.e. we
find a drifting of the heating layer. Conclusions: Considering
Alfvén wave propagation in 2D with an inhomogeneous density gradient,
we find that the equilibrium density profile is significantly modified
by both the flow of density due to visco-resistive heating and the
nonlinear response to the localised heating through phase mixing.

---------------------------------------------------------
Title: Alfvén wave phase-mixing and damping in the ion cyclotron
range of frequencies
Authors: Threlfall, J.; McClements, K. G.; De Moortel, I.
2011A&A...525A.155T Altcode: 2010arXiv1007.4752T
 Aims: We determine the effect of the Hall term in the generalised
Ohm's law on the damping and phase mixing of Alfvén waves in the ion
cyclotron range of frequencies in uniform and non-uniform equilibrium
plasmas. Methods: Wave damping in a uniform plasma is treated
analytically, whilst a Lagrangian remap code (Lare2d) is used to
study Hall effects on damping and phase mixing in the presence of
an equilibrium density gradient. Results: The magnetic energy
associated with an initially Gaussian field perturbation in a uniform
resistive plasma is shown to decay algebraically at a rate that is
unaffected by the Hall term to leading order in k2δ_i^2
where k is wavenumber and δ_i is ion skin depth. A similar
algebraic decay law applies to whistler perturbations in the limit
k2δ_i^2 ≫ 1. In a non-uniform plasma it is found that
the spatially-integrated damping rate due to phase mixing is lower in
Hall MHD than it is in MHD, but the reduction in the damping rate,
which can be attributed to the effects of wave dispersion, tends to
zero in both the weak and strong phase mixing limits.

---------------------------------------------------------
Title: Periodic Spectral Line Asymmetries in Solar Coronal Structures
from Slow Magnetoacoustic Waves
Authors: Verwichte, E.; Marsh, M.; Foullon, C.; Van Doorsselaere,
T.; De Moortel, I.; Hood, A. W.; Nakariakov, V. M.
2010ApJ...724L.194V Altcode:
Recent spectral observations of upward moving quasi-periodic intensity
perturbations in solar coronal structures have shown evidence of
periodic line asymmetries near their footpoints. These observations
challenge the established interpretation of the intensity perturbations
in terms of propagating slow magnetoacoustic waves. We show that slow
waves inherently have a bias toward enhancement of emission in the
blue wing of the emission line due to in-phase behavior of velocity
and density perturbations. We demonstrate that slow waves cause line
asymmetries when the emission line is averaged over an oscillation
period or when a quasi-static plasma component in the line of sight
is included. Therefore, we conclude that slow magnetoacoustic waves
remain a valid explanation for the observed quasi-periodic intensity
perturbations.

---------------------------------------------------------
Title: Magnetic reconnection in the solar atmosphere: from proposal
to paradigm
Authors: Cargill, Peter; Parnell, Clare; Browning, Philippa; de
Moortel, Ineke; Hood, Alan
2010A&G....51c..31C Altcode:
MEETING REPORT On 13 November 2009, the RAS hosted a discussion meeting
to commemorate the formal retirement of Prof. Eric Priest. Here Peter
Cargill, Clare Parnell, Philippa Browning, Ineke de Moortel and Alan
Hood examine how magnetic reconnection has evolved over the past
50 years from an important but controversial proposal, to a general
paradigm.

---------------------------------------------------------
Title: Coupled Alfvén and Kink Oscillations in Coronal Loops
Authors: Pascoe, D. J.; Wright, A. N.; De Moortel, I.
2010ApJ...711..990P Altcode:
Observations have revealed ubiquitous transverse velocity perturbation
waves propagating in the solar corona. However, there is ongoing
discussion regarding their interpretation as kink or Alfvén waves. To
investigate the nature of transverse waves propagating in the solar
corona and their potential for use as a coronal diagnostic in MHD
seismology, we perform three-dimensional numerical simulations of
footpoint-driven transverse waves propagating in a low β plasma. We
consider the cases of both a uniform medium and one with loop-like
density structure and perform a parametric study for our structuring
parameters. When density structuring is present, resonant absorption
in inhomogeneous layers leads to the coupling of the kink mode to
the Alfvén mode. The decay of the propagating kink wave as energy
is transferred to the local Alfvén mode is in good agreement with a
modified interpretation of the analysis of Ruderman &amp; Roberts for
standing kink modes. Numerical simulations support the most general
interpretation of the observed loop oscillations as a coupling of the
kink and Alfvén modes. This coupling may account for the observed
predominance of outward wave power in longer coronal loops since the
observed damping length is comparable to our estimate based on an
assumption of resonant absorption as the damping mechanism.

---------------------------------------------------------
Title: Automated Feature and Event Detection with SDO AIA and HMI Data
Authors: Davey, Alisdair; Martens, P. C. H.; Attrill, G. D. R.;
Engell, A.; Farid, S.; Grigis, P. C.; Kasper, J.; Korreck, K.; Saar,
S. H.; Su, Y.; Testa, P.; Wills-Davey, M.; Savcheva, A.; Bernasconi,
P. N.; Raouafi, N. -E.; Delouille, V. A.; Hochedez, J. F. .; Cirtain,
J. W.; Deforest, C. E.; Angryk, R. A.; de Moortel, I.; Wiegelmann,
T.; Georgouli, M. K.; McAteer, R. T. J.; Hurlburt, N.; Timmons, R.
2010cosp...38.2878D Altcode: 2010cosp.meet.2878D
The Solar Dynamics Observatory (SDO) represents a new frontier in
quantity and quality of solar data. At about 1.5 TB/day, the data will
not be easily digestible by solar physicists using the same methods
that have been employed for images from previous missions. In order for
solar scientists to use the SDO data effectively they need meta-data
that will allow them to identify and retrieve data sets that address
their particular science questions. We are building a comprehensive
computer vision pipeline for SDO, abstracting complete metadata
on many of the features and events detectable on the Sun without
human intervention. Our project unites more than a dozen individual,
existing codes into a systematic tool that can be used by the entire
solar community. The feature finding codes will run as part of the SDO
Event Detection System (EDS) at the Joint Science Operations Center
(JSOC; joint between Stanford and LMSAL). The metadata produced will
be stored in the Heliophysics Event Knowledgebase (HEK), which will be
accessible on-line for the rest of the world directly or via the Virtual
Solar Observatory (VSO) . Solar scientists will be able to use the
HEK to select event and feature data to download for science studies.

---------------------------------------------------------
Title: Longitudinal Waves in Coronal Loops
Authors: de Moortel, I.
2009SSRv..149...65D Altcode:
Outwardly propagating intensity disturbances are a common feature in
large, quiescent coronal loop structures. In this paper, an overview
is given of the observed properties and the theoretical modelling. As a
large number of events have been observed and analysed, good statistical
results on the estimated parameters have now been obtained. The
theoretical modelling mainly focuses on two distinct aspects, namely
the observed rapid damping of the perturbations, thought to be due
to thermal conduction and the origin of the driver. Leakage of the
solar surface p-modes is the main candidate to explain the observed
periodicity, due to the strong correlation between loop position and
period and the filamentary nature of the observed coronal intensity
perturbations. Recent observational results appear to confirm the
leakage and subsequent upward propagation of the solar surface 5 minute
oscillations into the overlying atmospheric layers.

---------------------------------------------------------
Title: Impulsively generated oscillations in a 3D coronal loop
Authors: Pascoe, D. J.; de Moortel, I.; McLaughlin, J. A.
2009A&A...505..319P Altcode:
Aims: The effect of changing the attack angle for the interaction of
a fast MHD wave with a 3D coronal loop is studied, to investigate
to what extent the properties of the excited transverse kink mode
oscillations of the loop depend on this angle. Methods: 3D
numerical simulations are performed of the interaction of a fast
MHD wave, generated by a pressure pulse, with a 3D coronal loop. The
loop itself is modelled as a density enhancement (with a finite plasma
beta) within a magnetic arcade. The initial pressure pulse has a width
comparable to the loop diameter and is situated outside of the loop,
at the same height as the loop apex. This height is kept fixed but
the (horizontal) angle between the pressure pulse and the loop is
varied. Results: We find that the global, transverse kink mode is
efficiently excited for a range of attack angles and qualitatively in
agreement with theoretical expectations. The period and damping time are
found to be independent of the attack angle. For larger values of the
attack angle, the global (longitudinal) slow wave is excited, whereas
for intermediate values the second harmonic kink mode is also present.

---------------------------------------------------------
Title: Putting Coronal Seismology Estimates of the Magnetic Field
Strength to the Test
Authors: De Moortel, I.; Pascoe, D. J.
2009ApJ...699L..72D Altcode:
The magnetic field strength inside a model coronal loop is
"estimated" using coronal seismology, to examine the reliability of
magnetic field strengths derived from observed, transverse coronal
loop oscillations. Three-dimensional numerical simulations of the
interaction of an external pressure pulse with a coronal loop (modeled
as a three-dimensional density enhancement inside a two-dimensional
magnetic arcade) are analyzed and the "observed" properties of the
excited transverse loop oscillations are used to derive the value of the
local magnetic field strength, following the method of Nakariakov &amp;
Ofman. Due to the (unexpected) change in periodicity, the magnetic field
derived from our "observed" oscillation is substantially different from
the actual (input) magnetic field value (approximately 50%). Coronal
seismology can derive useful information about the local magnetic field,
but the combined effect of the loop curvature, the density ratio, and
aspect ratio of the loop appears to be more important than previously
expected.

---------------------------------------------------------
Title: Computer Vision for The Solar Dynamics Observatory
Authors: Martens, Petrus C.; Angryk, R. A.; Bernasconi, P. N.; Cirtain,
J. W.; Davey, A. R.; DeForest, C. E.; Delouille, V. A.; De Moortel,
I.; Georgoulis, M. K.; Grigis, P. C.; Hochedez, J. E.; Kasper, J.;
Korreck, K. E.; Reeves, K. K.; Saar, S. H.; Savcheva, A.; Su, Y.;
Testa, P.; Wiegelmann, T.; Wills-Davey, M.
2009SPD....40.1711M Altcode:
NASA funded a large international consortium last year to produce
a comprehensive system for automated feature recognition in SDO
images. The data we consider are all AIA and EVE data plus surface
magnetic field images from HMI. Helioseismology is addressed by another
group. We will produce robust and very efficient software modules
that can keep up with the relentless SDO data stream and detect, trace,
and analyze a large number of phenomena, including: flares, sigmoids,
filaments, coronal dimmings, polarity inversion lines, sunspots,
X-ray bright points, active regions, coronal holes, EIT waves, CME's,
coronal oscillations, and jets. In addition we will track the emergence
and evolution of magnetic elements down to the smallest features
that are detectable, and we will also provide at least four full
disk nonlinear force-free magnetic field extrapolations per day. A completely new software element that rounds out this suite is a
trainable feature detection module, which employs a generalized image
classification algorithm to produce the texture features of the images
analyzed. A user can introduce a number of examples of the phenomenon
looked and the software will return images with similar features. We
have tested a proto-type on TRACE data, and were able to "train" the
algorithm to detect sunspots, active regions, and loops. Such a module
can be used to find features that have not even been discovered yet,
as, for example, sigmoids were in the pre-Yohkoh era. Our codes
will produce entries in the Helio Events Knowledge base, and that will
permit users to locate data on individual events as well as carry out
statistical studies on large numbers of events, using the interface
provided by the Virtual Solar Observatory.

---------------------------------------------------------
Title: Nonlinear fast magnetoacoustic wave propagation in the
neighbourhood of a 2D magnetic X-point: oscillatory reconnection
Authors: McLaughlin, J. A.; De Moortel, I.; Hood, A. W.; Brady, C. S.
2009A&A...493..227M Altcode: 2009arXiv0901.1781M
Context: This paper extends the models of Craig &amp; McClymont
(1991, ApJ, 371, L41) and McLaughlin &amp; Hood (2004, A&amp;A,
420, 1129) to include finite β and nonlinear effects. Aims:
We investigate the nature of nonlinear fast magnetoacoustic waves
about a 2D magnetic X-point. Methods: We solve the compressible
and resistive MHD equations using a Lagrangian remap, shock capturing
code (Arber et al. 2001, J. Comp. Phys., 171, 151) and consider an
initial condition in {v}×{B} \cdot {hat{z}} (a natural variable of
the system). Results: We observe the formation of both fast and
slow oblique magnetic shocks. The nonlinear wave deforms the X-point
into a “cusp-like” point which in turn collapses to a current
sheet. The system then evolves through a series of horizontal and
vertical current sheets, with associated changes in connectivity,
i.e. the system exhibits oscillatory reconnection. Our final state is
non-potential (but in force balance) due to asymmetric heating from
the shocks. Larger amplitudes in our initial condition correspond to
larger values of the final current density left in the system. Conclusions: The inclusion of nonlinear terms introduces several
new features to the system that were absent from the linear regime. A movie is available in electronic form at http://www.aanda.org

---------------------------------------------------------
Title: Forward modelling to determine the observational signatures
of propagating slow waves for TRACE, SoHO/CDS, and Hinode/EIS
Authors: Owen, N. R.; De Moortel, I.; Hood, A. W.
2009A&A...494..339O Altcode:
Context: The propagation and damping of slow MHD waves in the solar
atmosphere are investigated by numerical simulations and forward
modelling, with particular emphasis placed on waves with periodicities
of the order of five minutes. Aims: We extend a coronal model
by adding an equilibrium temperature gradient allowing study of wave
propagation from the transition region to the corona. Methods:
A 1D model is used that includes gravitational stratification
and damping by thermal conduction, optically thin radiation, and
compressive viscosity. Forward modelling of the simulation results,
for both uniform and non-uniform equilibrium temperature profiles, is
undertaken to establish the observational consequences of the physical
processes involved for TRACE, SoHO/CDS, and Hinode/EIS. Results:
The presence of thermal conduction causes a phase shift between the
wave velocity, energy, and density. This shift may be observable by
comparing Doppler velocity and intensity observations. Phase shifts are
also seen between intensity observations by different instruments and
between different spectral lines. This is an observational effect that
arises due to the forward modelling process in which observations are
synthesised, but it is not seen in the simulation results. Oscillations
from the transition region are found to dominate the coronal emission
for TRACE 171 Å by nearly two orders of magnitude.

---------------------------------------------------------
Title: Forward Modelling of Coronal Intensity Perturbations
Authors: De Moortel, I.; Bradshaw, S. J.
2008SoPh..252..101D Altcode: 2008SoPh..tmp..139D
In this paper, forward modelling is used to investigate the relation
between given temperature and density perturbations and the resulting
(synthesised) intensity perturbations, as would be observed by,
e.g., TRACE and EIS (onboard Hinode). Complex and highly non-linear
interactions between the components which make up the intensity
(density, ionisation balance and emissivity) mean that it is non-trivial
to reverse this process, i.e., obtain the density and temperature
perturbations associated with observed intensity oscillations. In
particular, it is found that the damping rate does not often `survive'
the forward modelling process, highlighting the need for a very
careful interpretation of observed (intensity) damping rates. With a
few examples, it is demonstrated that in some cases even the period
of the oscillations can be altered and that it is possible for two
different sets of input temperature and density to lead to very similar
intensities (the well-known `ill-posed' inversion process).

---------------------------------------------------------
Title: Coronal Seismology
Authors: De Moortel, I.
2008ESPM...12.3.26D Altcode:
The idea of exploiting observed oscillations as a diagnostic tool
for determining the physical conditions of the coronal plasma was
first suggested several decades ago. During the last few years, high
quality space-based observations have shown evidence for waves and
oscillations in a wide variety of solar structures, such as coronal
loops, polar plumes and prominences. I will (very) briefly summarise
MHD wave theory, which forms the basis for coronal seismology, as well
as present some of the recent theoretical models. In particular, I will
focus on how observations and theoretical modelling are intrinsically
linked, both guided and constrained by each other.

---------------------------------------------------------
Title: The way forward for coronal heating
Authors: De Moortel, Ineke; Browning, Philippa; Bradshaw, Stephen J.;
Pintér, Balázs; Kontar, Eduard P.
2008A&G....49c..21D Altcode:
Ineke De Moortel, Philippa K Browning, Stephen J Bradshaw, Balázs
Pintér and Eduard P Kontar consider approaches to the longstanding
and enigmatic problem of coronal heating, as presented at the RAS
discussion meeting on 11 January 2008.

---------------------------------------------------------
Title: Coronal Loop Seismology: Selective Examples
Authors: De Moortel, I.
2008ASPC..383..266D Altcode:
The idea of exploiting observed oscillations as a diagnostic tool for
determining the physical conditions of the coronal plasma was first
suggested several decades ago. During the last few years, high quality
space-based observations have shown evidence for waves and oscillations
in a wide variety of solar structures, such as coronal loops, polar
plumes and prominences. In this review, observations of propagating,
slow waves and standing, fast modes in the solar corona are summarized
and examples are given of how these observations can be used to deduce
information about different physical properties of the solar corona. A
few suggestions are made as to how the relatively large uncertainties
in the derived parameters could be reduced.

---------------------------------------------------------
Title: An Estimate of P-Mode Damping by Wave Leakage
Authors: De Moortel, I.; Rosner, R.
2007SoPh..246...53D Altcode:
High-cadence TRACE observations show that outward-propagating
intensity disturbances are a common feature in large, quiescent coronal
loops. Analysis of the frequency distribution of these modes shows
peaks at both three- and five-minute periods, indicating that they may
be driven by the solar surface oscillations (p modes). The energy flux
contained within the coronal intensity disturbances is of the order of
(1.1±0.4)×103 ergs cm−2 s−1. A
simple order-of-magnitude estimate of the damping rate of the relevant
p modes allows us to put an observational constraint on the damping of
p modes and shows that leakage into the overlying coronal atmosphere
might be able to account for a significant fraction of p-mode damping.

---------------------------------------------------------
Title: Magnetic reconnection in flux-tubes undergoing spinning
footpoint motions
Authors: Wilmot-Smith, A. L.; De Moortel, I.
2007A&A...473..615W Altcode:
Aims:Photospheric motions acting on the coronal magnetic field have
the potential to build up huge amounts of magnetic energy. The energy
may be released through magnetic reconnection, and so a detailed
understanding of the 3D process is crucial if its implications for
coronal heating are to be fully addressed. Methods: A 3D MHD
experiment is described in which misaligned magnetic flux tubes are
subjected to simple spinning boundary motions. Results: The
resulting shear between adjacent flux systems generates a twisted
central separator current sheet that extends vertically throughout
the domain. Current density is amplified to a sufficient extent that
reconnection begins, and occurs everywhere along the separator current
sheet, while the separatrix current sheets that exist in the early
stages of the experiment are found to be unimportant in the systems
dynamical evolution. In 2D cross-sections, the reconnection process
exhibits many similarities to the regime of flux pile-up reconnection.

---------------------------------------------------------
Title: Observation of Higher Harmonic Coronal Loop Oscillations
Authors: De Moortel, I.; Brady, C. S.
2007ApJ...664.1210D Altcode:
A sequence of TRACE 171 Å observations taken on 2001 May 13 shows
evidence of flare-induced, transverse coronal loop oscillations. We
revisit this particular data set and present evidence of the
presence of spatially resolved higher harmonics in the transverse loop
displacements. The oscillations are identified as the second-harmonic,
fast MHD kink waves (periods of 577-672 s), with higher harmonics
(250-346 s) also present. The apparent absence of the fundamental
mode and the fact that it is the second harmonic (P2) that
dominates the oscillatory behavior of this particular loop may shed
more light on either the excitation and/or the damping mechanism(s)
of flare-induced, transverse loop oscillations.

---------------------------------------------------------
Title: Magnetic Field Extrapolations And Current Sheets
Authors: Welsch, Brian; De Moortel, I.; McTiernan, J. M.
2007AAS...210.9101W Altcode: 2007BAAS...39Q.204W
Solar flares and coronal mass ejections (CMEs) --- phenomena which
impact our society, but are scientifically interesting in themselves ---
are driven by free magnetic energy in the coronal magnetic field. Since
the coronal magnetic field cannot be directly measured, modelers often
extrapolate the coronal field from the photospheric magnetograms ---
the only field measurements routinely available. The best extrapolation
techniques assume that the field is force free (coronal currents
parallel the magnetic field), but that currents are not simply a linear
function of the magnetic field. Recent tests, however, suggest that
such non-linear force-free field (NLFFF) extrapolation techniques
often underestimate free magnetic energy. We hypothesize that, since
relaxation-based NLFFF techniques tend to smooth field discontinuities,
such approaches will fail when current sheets are present. Here,
we test this hypothesis by applying the Optimization NLFFF method to
two configurations from an MHD simulation --- one with strong current
concentrations, and one with weak concentrations. This work is supported
by a NASA Sun-Earth Connections Theory grant to UC-Berkeley.

---------------------------------------------------------
Title: Numerical modelling of 3D reconnection. II. Comparison between
rotational and spinning footpoint motions
Authors: De Moortel, I.; Galsgaard, K.
2006A&A...459..627D Altcode:
The coronal magnetic field is constantly subjected to a variety of
photospheric, footpoint motions, leading to the build up, and subsequent
release, of magnetic energy. Two different types of footpoint motions
are considered here, namely (large scale) rotating and (small scale)
spinning, using 3D numerical MHD simulations. The initial model consists
of two aligned, thin flux tubes, which are forced to interact due to
the boundary driving of the footpoints. Two variations of this setup
are studied, namely with and without an additional, constant, background
magnetic field. The nature of the boundary motions determines the shape
of the central current sheet, the driving force of the reconnection
process, as well as the efficiency of the build up of quasi-separatrix
layers (when B_bg ≠ 0). The reconnection process is more efficient for
the rotating of the flux sources and when a background magnetic field
is added. In general, heating due to large and small scale motions is
of comparable magnitude when no background field is present. However,
with an additional background magnetic field, heating due to small
scale footpoint motions seems substantially more efficient.

---------------------------------------------------------
Title: Understanding Magnetic Structures in the Solar Corona Through
Topological Analysis
Authors: Maclean, R. C.; Parnell, C. E.; De Moortel, I.; Büchner,
J.; Priest, E. R.
2006ESASP.617E.156M Altcode: 2006soho...17E.156M
No abstract at ADS

---------------------------------------------------------
Title: Numerical modelling of 3D reconnection due to rotational
footpoint motions
Authors: De Moortel, I.; Galsgaard, K.
2006A&A...451.1101D Altcode:
The rapid dynamical evolution of the photospheric magnetic carpet
provides a large energy source for the solar corona. In this context,
the role of 3D magnetic reconnection is crucial in releasing the free
magnetic energy, build up due to the continuous footpoint motions. To
understand the processes by which this can take place, we have to obtain
a better understanding of the basic reconnection process that can take
place in 3D magnetic field configurations. In this paper, we investigate
magnetic reconnection, driven by rotational footpoint motions, using 3D
numerical MHD simulations. The model consists of two positive and two
negative sources, which are placed symmetrically on opposite boundaries
of the cubic domain. The initially potential fluxtubes are forced to
interact by the rotational driving of the flux concentrations on the
boundaries. We consider two variations of this setup, namely with
and without an additional, constant, background magnetic field. In
the no-background case, the magnetic connectivity is divided into
independent regions by separatrix surfaces, while the case with a
background field is represented by one global connectivity region. The
dynamical evolution is followed and found to differ significantly from
the comparable potential evolution. Strong currents are concentrated
along separatrix surfaces or rapidly developing quasi-separatrix
layers (QSLs). Investigating the reconnection rates of the systems
shows that the stronger the background field is, the more efficient
the reconnection process of the flux in the respective fluxtubes.

---------------------------------------------------------
Title: Longitudinal intensity oscillations observed with TRACE:
evidence of fine-scale structure
Authors: McEwan, M. P.; de Moortel, I.
2006A&A...448..763M Altcode:
The aim of this paper is two-fold: to increase the number of
examples of observed longitudinal oscillations in coronal loops and
to find evidence of the small temporal and spatial scales of these
loop oscillations. Increasing the number of observed longitudinal
oscillations allows for improvement in the statistics of the measured
parameters, providing more accurate values for numerical and theoretical
models. Furthermore, the small temporal and spatial scales of these
loop oscillations could give indication of a driving force, symptomatic
of coupling with the global p-modes. We found evidence that individual
loop strands of wide coronal loop footpoints oscillate independently
for short time periods. These strands have a diameter of the order
of a few Mm, and the timescales on which the oscillations exist are
typically less than an hour. We suggest that this is indicative of the
oscillating strands being driven by the leakage of the global 5 min
p-modes up into the corona, as simulated by De Pontieu et al. (2005,
ApJ, 624, L61). Additionally, we find 25 further examples, added to
those of De Moortel et al. (2002a, Sol. Phys., 209, 89), of outwardly
propagating slow MHD waves in coronal loop footpoints. The datasets are
taken from JOP83, observed between April 21st 2003 and May 3rd 2003, in
the TRACE 171 Å bandpass. The intensity oscillations travel outwards
with a propagation speed of order v = 99.7 ± 3.9 km s-1
and they are of small amplitude, with variations of approximately
3.7 ± 0.2 % of the background intensity. These disturbances are only
detected for short distances, around 8.3 ± 0.6 Mm along the loops,
and the main period of oscillation is around 300 s. A second peak of
period was found at around 200 s, however no correlation with the
presence of a sunspot was observed within this study. Using these
measured parameters we have estimated the energy flux to be of order
313 ± 26 erg cm-2 s-1.

---------------------------------------------------------
Title: Propagating magnetohydrodynamics waves in coronal loops
Authors: De Moortel, I.
2006RSPTA.364..461D Altcode:
No abstract at ADS

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Title: 3D Numerical Simulations of Coronal Tectonics
Authors: De Moortel, I.; Galsgaard, K.
2006IAUS..233..149D Altcode:
We present the results of numerical simulations of 3D magnetic
reconnection driven by photospheric footpoint motions. The model
consists of two positive and two negative sources, which are placed
on opposite boundaries of the cubic domain. Two different types
of photospheric motions are then considered, namely rotating and
twisting of the sources. These different footpoint motions result in a
difference in the evolution of the magnetic skeleton and the location
and efficiency of the energy build up. Both the dynamical evolution and
the corresponding potential evolution of each system is investigated
and a comparison is made between the energy storage and release that
occurs at separators and separatrix surfaces.

---------------------------------------------------------
Title: An overview of coronal seismology
Authors: De Moortel, I.
2005RSPTA.363.2743D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: D Numerical Simulations of Magnetic Reconnection Driven by
Rotational Footpoint Motions
Authors: De Moortel, I.; Galsgaard, K.
2005ESASP.600E..22D Altcode: 2005dysu.confE..22D; 2005ESPM...11...22D
No abstract at ADS

---------------------------------------------------------
Title: Numerical Simulations of 3d Magnetic Reconnection due to
Rotational Driving
Authors: De Moortel, I.; Galsgaard, K.
2005ESASP.596E..31D Altcode: 2005ccmf.confE..31D
No abstract at ADS

---------------------------------------------------------
Title: Tracing Coronal Waves Back to the Photosphere
Authors: De Pontieu, B.; Erdelyi, R.; De Moortel, I.; Metcalf, T.
2005AGUSMSH11C..03D Altcode:
There are now many observations of waves with periods around 5 minutes
in the outer atmosphere of the Sun. We provide an observational
overview of 5 minute periodicity in chromospheric spicules in active
region plage, upper transition region moss and the low legs of coronal
loops. Using a numerical model, we show that all of these phenomena
are connected: normally evanescent photospheric oscillations can
propagate into the low atmosphere as long as they are guided along
magnetic field lines that are inclined away from the vertical. The
leaked photospheric oscillations develop into shocks and lead to
periodic upward chromospheric flows, which we have identified as
active region spicules. These shocks continue upwards and enter
into the corona. We suggest that TRACE observations of propagating
acoustic waves in the corona are shocked and tunneled photospheric
oscillations. Using SOHO/MDI, TRACE and Imaging Vector Magnetograph
(Hawaii) data we explore how these coronal waves can be exploited to
determine the connectivity between photosphere and corona,and thus
allow seismology of the lower solar atmosphere.

---------------------------------------------------------
Title: How to Channel Photospheric Oscillations into the Corona
Authors: De Pontieu, B.; Erdélyi, R.; De Moortel, I.
2005ApJ...624L..61D Altcode:
There are now many observations of waves in the solar corona with
periods around 5 minutes. The source of these waves is uncertain,
although global p-modes in the photosphere are an obvious candidate,
given the similarity of the dominant periods. However, p-modes are
traditionally considered evanescent in the upper photosphere, and it
has been unclear how they could propagate through the chromosphere
into the corona. Using a numerical model, we show that photospheric
oscillations with periods around 5 minutes can actually propagate into
the corona so long as they are guided along an inclined magnetic flux
tube. The nonverticality of the flux tube increases the acoustic cutoff
period to values closer to the dominant periods of the photospheric
oscillations, thus allowing tunneling or even direct propagation into
the outer atmosphere. The photospheric oscillations develop into shocks,
which drive chromospheric spicules and reach the corona. We suggest
that Transition Region and Coronal Explorer (TRACE) observations of
propagating magnetoacoustic waves in the corona represent these shocked
and tunneled photospheric oscillations. We also explore how seismology
of these waves could be exploited to determine the connectivity between
photosphere and corona.

---------------------------------------------------------
Title: Coronal Seismology and the Propagation of Acoustic Waves
along Coronal Loops
Authors: Klimchuk, J. A.; Tanner, S. E. M.; De Moortel, I.
2004ApJ...616.1232K Altcode: 2004astro.ph.12085K
We use a combination of analytical theory, numerical simulation, and
data analysis to study the propagation of acoustic waves along coronal
loops. We show that the intensity perturbation of a wave depends
on a number of factors, including dissipation of the wave energy,
pressure and temperature gradients in the loop atmosphere, work action
between the wave and a flow, and the sensitivity properties of the
observing instrument. In particular, the scale length of the intensity
perturbation varies directly with the dissipation scale length (i.e.,
damping length) and the scale lengths of pressure, temperature, and
velocity. We simulate wave propagation in three different equilibrium
loop models and find that dissipation and pressure and temperature
stratification are the most important effects in the low corona where
the waves are most easily detected. Velocity effects are small and
cross-sectional area variations play no direct role for lines of
sight that are normal to the loop axis. The intensity perturbation
scale lengths in our simulations agree very well with the scale
lengths we measure in a sample of loops observed by TRACE. The median
observed value is 4.35×109 cm. In some cases the intensity
perturbation increases with height, which is likely an indication of
a temperature inversion in the loop (i.e., temperature that decreases
with height). Our most important conclusion is that thermal conduction,
the primary damping mechanism, is accurately described by classical
transport theory. There is no need to invoke anomalous processes to
explain the observations.

---------------------------------------------------------
Title: Photospheric Oscillations in the Solar Atmosphere: Driving
Chromospheric Spicules and Coronal Waves
Authors: De Pontieu, B.; Erdelyi, R.; De Moortel, I.; Metcalf, T.
2004AGUFMSH13A1142D Altcode:
There are now many observations of oscillations and waves with periods
around 5 minutes in the solar transition region and corona. We provide
an observational overview of 5 minute periodicity in upper transition
region moss, the low legs of coronal loops, and chromospheric spicules
in active region plage. The source of the 5 minute periodicity is
unclear, since photospheric p-modes are evanescent in the upper
photosphere which should prevent them from propagating into the
chromosphere, transition region and corona. Using a numerical model
we show that photospheric oscillations can propagate into the low
atmosphere as long as they are guided along a magnetic flux tube that is
inclined away from the vertical. The leaked photospheric oscillations
develop non-linearly into shocks at low chromospheric heights because
of the density decrease with height. The upward traveling shocks and
resulting rebound shocks of the chromosphere lead to periodic upward
chromospheric flows, which in a recent paper we have identified as
the periodic spicules that we observe in active region plage. After
passage through the spicule, these shocked photospheric oscillations
propagate into the corona. We suggest that TRACE observations of
propagating acoustic waves in the corona are shocked and tunneled
photospheric oscillations. We also explore whether these coronal waves
can be exploited to determine the connectivity between photosphere
and corona, and thus perform seismology of the lower solar atmosphere.

---------------------------------------------------------
Title: Coronal Seismology and the Propagation of Acoustic Waves
Along Coronal Loops
Authors: Klimchuk, J. A.; Tanner, S. E.; De Moortel, I.
2004AGUFMSH24A..06K Altcode:
We use a combination of analytical theory, numerical simulation, and
data analysis to study the propagation of acoustic waves along coronal
loops. We show that the intensity perturbation of a wave depends
on a number of factors, including dissipation of the wave energy,
pressure and temperature gradients in the loop atmosphere, work action
between the wave and a flow, and the sensitivity properties of the
observing instrument. In particular, the scale length of the intensity
perturbation varies directly with the dissipation scale length (i.e.,
damping length) and the scale lengths of pressure, temperature, and
velocity. We simulate wave propagation in three different equilibrium
loop models and find that dissipation and pressure and temperature
stratification are the most important effects in the low corona where
the waves are most easily detected. Velocity effects are small, and
cross-sectional area variations play no direct role for lines-of-sight
that are normal to the loop axis. The intensity perturbation scale
lengths in our simulations agree very well with the scale lengths we
measure in a sample of loops observed by TRACE. The median observed
value is 4.35×109 cm. In some cases the intensity
perturbation increases with height, which is likely an indication of
a temperature inversion in the loop (i.e., temperature that decreases
with height). Our most important conclusion is that thermal conduction,
the primary damping mechanism, is accurately described by classical
transport theory. There is no need to invoke anomalous processes to
explain the observations.

---------------------------------------------------------
Title: The damping of slow MHD waves in solar coronal magnetic
fields. III. The effect of mode coupling
Authors: De Moortel, I.; Hood, A. W.; Gerrard, C. L.; Brooks, S. J.
2004A&A...425..741D Altcode:
The properties of slow MHD waves in a two dimensional model are
investigated, in a low-beta plasma. Including a horizontal density
variation causes “phase mixing” and coupling between slow and fast
MHD waves. The effects of different density profiles, different driving
frequencies, different values for the viscosity coefficient and plasma
beta (&lt;1) are studied. Using numerical simulations, it was found
that the behaviour of the perturbed velocity was strongly dependent
on the values of the parameters. From analytical approximations, a
strong interaction with the fundamental, normal modes of the system
was found to play an important role. The coupling to the fast wave
proved to be an inefficient way to extract energy from the driven
slow wave and is unlikely to be responsible for the rapid damping of
propagating slow MHD waves, observed by TRACE. The “phase mixing”
of the slow waves due to the (horizontal) density inhomogeneity does
cause a significant amount of damping, but is again unlikely to be
sufficiently strong to explain the rapid observed damping.

---------------------------------------------------------
Title: Waves and wavelets: An automated detection technique for
solar oscillations
Authors: De Moortel, I.; McAteer, R. T. J.
2004SoPh..223....1D Altcode: 2004SoPh..223....1M
This paper investigates the possibility of automating the detection
of propagating intensity perturbations in coronal loops using
wavelet analysis. Two different sets of TRACE 171 Å images are
studied using the automated wavelet routine presented by McAteer et
al. (2004). Both localised, short-lived periodicities and sustained,
periodic, oscillations are picked up by the routine, with the results
dependent to a large extent on the signal-to-noise ratio of the
dataset. At present, the automation is only partial; the relevance
of the detected periodicity and the identification of the coronal
structure supporting it still have to be determined by the user, as
does the judging of the accuracy of the results. Care has to be taken
when interpreting the results of the wavelet analysis, and a good
knowledge of all possible factors that might influence or distort the
results is a necessity. Despite these limitations, wavelet analysis
can play an important role in automatically identifying a variety of
phenomena and in the analysis of the ever-growing (observational or
simulated) datasets.

---------------------------------------------------------
Title: Wavelet Analysis: the effect of varying basic wavelet
parameters
Authors: De Moortel, I.; Munday, S. A.; Hood, A. W.
2004SoPh..222..203D Altcode:
The most commonly used methods to analyse (observed) quasi-periodic
signals are standard techniques such as Fourier and wavelet
analysis. Whereas a Fourier transform provides information on the
dominant frequencies, wavelet analysis has the added advantage of
providing the time localisation of the various frequency components. The
usefulness and robustness of wavelet analysis is investigated by varying
the different parameters which characterise the `mother' wavelet. We
examine the effect of varying these parameters on the temporal and
frequency resolution and the damping profile, which can be obtained
from the wavelet transform. Additionally, the effect of a changing
periodicity on the wavelet transform is investigated. Both simple
harmonic functions and intensity oscillations observed by TRACE are
used to demonstrate the various advantages and disadvantages of the
different methods. In general, using the Paul wavelet or a smaller
value of the wavelet parameter k provides a better time resolution,
whereas the Morlet wavelet or a larger value of k improves the frequency
resolution. Overall, our results indicate that great care is needed
when using a wavelet analysis and that all the possible factors that
could affect the transform should be taken into consideration.

---------------------------------------------------------
Title: Acoustic Wave Interpretation of Propagating Intensity
Disturbances in Coronal Loops
Authors: Klimchuk, J. A.; Tanner, S. E. M.; De Moortel, I.
2004AAS...204.9503K Altcode: 2004BAAS...36..826K
Intensity disturbances have been observed by TRACE and EIT to propagate
upward along the legs of long active region coronal loops. The
periodic nature and speed of these disturbances suggest that they
are traveling acoustic waves. It is being debated, however, whether
the damping of the perturbations is consistent with the acoustic
wave interpretation. We here examine this issue in detail with a
combination of numerical simulation, analytical theory, and improved
analysis of the observations. Using our state-of-the-art 1D hydro code,
we simulate the propagation of waves generated at the base of model
coronal loops. We consider static equilibrium loops having constant and
expanding cross-section, and an equilibrium loop with steady flow. We
show that the amplitude of the intensity perturbation is affected by
a number of factors: wave dissipation (direct plasma heating), work
done by the wave on the flow, pressure stratification, nonuniform
temperature, and temperature-dependent sensitivity of the observing
instrument. We compare our theoretical results with intensity scale
lengths measured in a sample of loops observed by TRACE. Research
supported by NASA and ONR.

---------------------------------------------------------
Title: Observations and theory of slow waves in coronal loops
Authors: De Moortel, I.; Hood, A. W.
2004AAS...204.9502D Altcode: 2004BAAS...36..826D
High cadence TRACE observations show that outward propagating intensity
disturbances are a common feature in large, quiescent coronal loops,
close to active regions. An overview is given of measured parameters
of such longitudinal oscillations in coronal loops. The observed
oscillations are interpreted as propagating slow magneto-acoustic
waves and are unlikely to be flare-driven. A theoretical model of slow
magneto-acoustic waves, incorporating the effects of gravitational
stratification, the magnetic field geometry, thermal conduction and
compressive viscosity is presented to explain the very short observed
damping lengths. The results of these numerical simulations are compared
with the TRACE observations. Preliminary results indicate that thermal
conduction and the magnetic field geometry play an important role.

---------------------------------------------------------
Title: The damping of slow MHD waves in solar coronal magnetic
fields. II. The effect of gravitational stratification and field
line divergence
Authors: De Moortel, I.; Hood, A. W.
2004A&A...415..705D Altcode:
This paper continues the study of De Moortel &amp; Hood
(\cite{Moortelh03}) into the propagation of slow MHD waves in the
solar corona. Firstly, the damping due to optically thin radiation
is investigated and compared to the effect of thermal conduction. In
a second stage, gravitational stratification is included in the
model and it is found that this increases the damping length
significantly. Finally, the effect of different magnetic field
geometries on the damping of the slow waves is investigated. As a
first approximation, a purely radial magnetic field is considered
and although the amplitudes of the perturbations decrease due to the
divergence of the field, the effect is small compared to the effect of
thermal conduction. A more realistic local geometry, estimated from the
observations, is investigated and it is demonstrated that a general
area divergence can cause a significant, additional, decrease of the
amplitudes of the perturbations. The results of numerical simulations,
incorporating the effects of gravitational stratification, the
magnetic field geometry and thermal conduction are compared with TRACE
observations of propagating waves in coronal loops. It is found that a
combination of thermal conduction and (general) area divergence yields
detection lengths that are in good agreement with observed values.

---------------------------------------------------------
Title: Time-Frequency Analysis of Quasi-Periodic Signals
Authors: De Moortel, I.; Munday, S.; Hood, A. W.
2004ESASP.547..501D Altcode: 2004soho...13..501D
In recent years, the analysis of quasi-periodic signals observed by
satellites such as SOHO and TRACE has become increasingly important. So
far, mostly standard methods have been used, such as Fourier analysis
to identify the dominant frequencies and wavelet analysis to provide
the time localisation of the various frequency components. We compare
the temporal and frequency resolution of different `time-frequency'
methods. In particular, the usefulness and robustness of wavelet
analysis is investigated by varying the different parameters which
characterise the `mother' wavelet. Both simple harmonic functions and
intensity oscillations observed by TRACE are used to demonstrate the
various advantages and disadvantages of the different methods.

---------------------------------------------------------
Title: A Search for Photospheric Sources of Coronal Longitudinal
Oscillations
Authors: Ireland, J.; De Moortel, I.; Walsh, R. W.; Moretti, P. F.
2004ESASP.547...57I Altcode: 2004soho...13...57I
It has recently been shown that longitudinal intensity fluctuations
observed in TRACE 171 Å loops come in what appear to be two separate
populations. These populations are differentiated by their period, and
are clustered distinctly around 3 and 5 minute periods. The 3 minute
fluctuations appear to be rooted in sunspots, whereas the 5 minute
sunspots are not. This study presents two test cases in the search
for a photospheric source to these oscillations. A wavelet analysis
is presented in the search for a fluctuating magnetic component since
previous studies show that a magnetic fluctuation may be intermittent. A
Fourier analysis is used to look in the intensity and Doppler regions of
the same area of interest. Some comments are made on the photosphere in
relation to the search for the driver of the as yet unobserved driver
of longitudinal coronal oscillations.

---------------------------------------------------------
Title: Longitudinal Oscillations in Coronal Loops - Joint Observations
with SOHO/CDS and TRACE
Authors: Marsh, M. S.; Walsh, R. W.; De Moortel, I.; Ireland, J.
2004ESASP.547..519M Altcode: 2004soho...13..519M
Joint Observing Program (JOP) 83 Solar and Heliospheric
Observatory/Coronal Diagnostic Spectrometer (SOHO/CDS) and Transition
Region and Coronal Explorer (TRACE) data is analysed for evidence of
propagating intensity oscillations along loop structures in the solar
corona. A propagating intensity oscillation with a minimum estimated
speed of 50-195 km s is observed within a TRACE 171 Å coronal loop
using a running difference method. Co-spatial and co-temporal CDS
and TRACE observations of this loop are analysed using a wavelet
analysis method. The TRACE data shows a propagating oscillation with
a period of 300 s. This period is also observed with CDS suggesting
propagating oscillations at chromospheric, transition region and
coronal temperatures in the He I, O V and Mg IX lines.

---------------------------------------------------------
Title: Observations and Theory of Longitudinal Waves in Coronal Loops
Authors: De Moortel, I.; Hood, A. W.; De Pontieu, B.
2004ESASP.547..427D Altcode: 2004soho...13..427D
High cadence TRACE observations show that outward propagating
intensity disturbances are a common feature in large, quiescent
coronal loops, close to active regions. An overview is given of
measured parameters of such longitudinal oscillations in coronal
loops. The observed oscillations are interpreted as propagating slow
magnetoacoustic waves and are unlikely to be flare-driven. A basic
magnetic field extrapolation is used to estimate the local geometry
of the magnetic field. A theoretical model of slow magneto-acoustic
waves, incorporating the effects of gravitational stratification, the
magnetic field geometry, thermal conduction and compressive viscosity
is presented to explain the very short observed damping lengths. The
results of these numerical simulations are compared with the TRACE
observations. Preliminary results indicate that the magnetic field
geometry plays an important role.

---------------------------------------------------------
Title: The damping of slow MHD waves in solar coronal magnetic fields
Authors: De Moortel, I.; Hood, A. W.
2003A&A...408..755D Altcode:
A theoretical description of slow MHD wave propagation in the solar
corona is presented. Two different damping mechanisms, namely thermal
conduction and compressive viscosity, are included and discussed in
detail. We revise the properties of the “thermal” mode, which is
excited when thermal conduction is included. The thermal mode is
purely decaying in the case of standing waves, but is oscillatory
and decaying in the case of driven waves. When thermal conduction
is dominant, the waves propagate largely undamped, at the slower,
isothermal sound speed. This implies that there is a minimum damping
time (or length) that can be obtained by thermal conduction alone. The
results of numerical simulations are compared with TRACE observations
of propagating waves, driven by boundary motions, and standing waves
observed by SUMER/SOHO, excited by an initial impulse. For typical
coronal conditions, thermal conduction appears to be the dominant
damping mechanism.

---------------------------------------------------------
Title: Determination of coronal loop properties from trace
observations
Authors: De Moortel, I.; Parnell, C. E.; Hood, A. W.
2003SoPh..215...69D Altcode:
In this paper, we determine the temperature profile along the footpoints
of large coronal loops observed by TRACE in both the 171 Å and 195
Å passbands. The temperature along the lower part of these coronal
loops only shows small variations and can probably be considered to
be isothermal. Using the obtained temperature profile T(s) and an
estimate of the column depth along the loop, we then determine the
pressure along the lower part of the observed coronal loops and hence
the value of the pressure scale length. The obtained scale lengths
correspond in order-of-magnitude with the theoretically predicted
gravitational scale height. We show that the differences between
the observed and predicted scale heights are unlikely to be caused by
(significant) flows along the loops but could possibly be a consequence
of the inclination of the loops. This implies that the quasi-periodic
intensity oscillations observed in the loops are most probably caused
by compressive waves propagating upward at the coronal sound speed.

---------------------------------------------------------
Title: Joint observations of propagating oscillations with SOHO/CDS
and TRACE
Authors: Marsh, M. S.; Walsh, R. W.; De Moortel, I.; Ireland, J.
2003A&A...404L..37M Altcode:
Joint Observing Program (JOP) 83 Solar and Heliospheric
Observatory/Coronal Diagnostic Spectrometer (SOHO/CDS) and Transition
Region and Coronal Explorer (TRACE) data is analysed for evidence of
propagating intensity oscillations along loop structures in the solar
corona. A propagating intensity oscillation with a minimum estimated
speed of 50-195 km s-1 is observed within a TRACE 171
Å coronal loop using a running difference method. Co-spatial and
co-temporal CDS and TRACE observations of this loop are analysed
using a wavelet analysis method. The TRACE data shows a propagating
oscillation with a period of ~300 s. This period is also observed with
CDS suggesting propagating oscillations at chromospheric, transition
region and coronal temperatures in the He I, O V and Mg Ix lines.

---------------------------------------------------------
Title: Hydrodynamic Simulations of Longitudinal Intensity Oscillations
Observed in Coronal Loops by TRACE
Authors: Tanner, S. E.; Klimchuk, J. A.; Hood, A. W.; De Moortel, I.
2003SPD....34.0406T Altcode: 2003BAAS...35..811T
Propagating intensity disturbances are often observed by TRACE in
large coronal loops located at the perimeters of active regions
(e.g., De Moortel et al., 2002, Solar Phys., 209, 61). On average,
the disturbances have periods of 280 s, propagation speeds of 120
km s-1, intensity amplitudes of 4%, and surprisingly
small damping (detection) lengths of 9000 km. In addition, there
is a positive correlation between damping length and period. The
preliminary interpretation of these disturbances is that they are
rapidly dissipating slow magneto-acoustic waves. To investigate
this interpretation more rigorously, we have performed a series of
detailed coronal loop simulations using our 1D hydrodynamic code,
ARGOS. We generate waves in the loop by imposing a spatially localized
oscillating force at the loop footpoint, using a range of different
oscillation periods. We here report on the results of our study and,
in particular, whether the damping lengths have the properties observed
by TRACE. This work was supported by NASA and ONR.

---------------------------------------------------------
Title: Thermal conduction damping of longitudinal waves in coronal
loops
Authors: De Moortel, I.; Hood, A. W.
2003PADEU..13..127D Altcode:
High cadence TRACE observations show that outward propagating intensity
disturbances are a common feature in large coronal loops. An overview
is given of measured parameters of such longitudinal waves in coronal
loops. We found that loops that are situated above sunspot regions
display intensity oscillations with periods centred around 3 minutes,
whereas oscillations in `non-sunspot' loops show periods centred
around 5 minutes. The observed longitudinal waves are interpreted
as propagating slow magneto-acoustic waves and we show that the
disturbances are not flare-driven but are most likely caused by
an underlying driver exciting the loop footpoints. We found that
(slightly enhanced) thermal conduction could account for the observed
damping lengths.

---------------------------------------------------------
Title: An overview of longitudinal oscillations in coronal loops
Authors: De Moortel, I.; Hood, A. W.; Ireland, J.; Walsh, R. W.
2002ESASP.506..509D Altcode: 2002svco.conf..509D; 2002ESPM...10..509D
High cadence TRACE observations show that outward propagating
intensity disturbances are a common feature in large, quiescent coronal
loops. An overview is given of geometric and physical parameters of
such propagating disturbances observed in 38 coronal loops. We found
that loops that are situated above sunspot regions display intensity
oscillations with periods centred around 3 minutes, whereas oscillations
in 'non-sunspot' loops show periods centred around 5 minutes. The
observed longitudinal oscillations are interpreted as propagating
slow magneto-acoustic waves and we show that the disturbances are
not flare-driven but are most likely caused by an underlying driver
exciting the loop footpoints. We present a simple theoretical model
to explain the observed features.

---------------------------------------------------------
Title: Observational evidence of underlying driving of longitudinal
oscillations in coronal loops
Authors: De Moortel, I.; Ireland, J.; Hood, A. W.; Walsh, R. W.
2002ESASP.505..211D Altcode: 2002IAUCo.188..211D; 2002solm.conf..211D
We give an overview of both geometric and physical parameters of
propagating disturbances in coronal loops, using high cadence TRACE
data (JOP83 &amp; JOP144). The majority of these outward propagating
oscillations are found in the footpoints of large diffuse coronal loop
structures, close to active regions. The disturbances travel outward
with a propagation speed v = 122±43 km s-1. The variations
in intensity are estimated to be of the order of 4.1±1.5%, compared
to the background brightness and are found to be damped very quickly,
within 8.9±4.4 Mm along the loop. Using a wavelet analysis, periods in
the 282±93 seconds range are obtained. However, it was found that loops
that are situated above sunspot regions display intensity oscillations
with a period smaller than 200 seconds, whereas oscillations in
'non-sunspot' loops show periods larger than 200 seconds. This result
provides evidence that the underlying oscillations can propagate
through the transition region and into the corona. We conclude that
the observed longitudinal oscillations are not flare-driven but are
most likely caused by an underlying driver exciting the loop footpoints.

---------------------------------------------------------
Title: Examination of the photospheric magnetic field underlying
longitudinally oscillating coronal loops
Authors: Ireland, J.; Walsh, R. W.; De Moortel, I.; Moretti, P. F.
2002ESASP.505..429I Altcode: 2002IAUCo.188..429I; 2002solm.conf..429I
Longitudinally oscillating coronal loops have been seen in TRACE 171
Å data in many different quiescent active regions. The oscillation
is thought to be an example of an outwardly propagating slow
magneto-acoustic wave. However, the source of these waves is as
yet unknown. In the context of SOHO Joint Observing Program 144, we
search for a possible photospheric driver to these waves. We examine
the photospheric longitudinal magnetic flux underlying an oscillating
loop observed between 1200-1300 UT on June 7th 2001. The field was
imaged using the Kanzelhöhe Magneto-Optical Filter instrument and the
SOHO Michelson Doppler Imager (MDI). The dynamics of the photospheric
magnetic field underlying these loops is discussed in the context of
possible mechanisms causing the observed coronal oscillations.

---------------------------------------------------------
Title: Longitudinal intensity oscillations in coronal loops observed
with TRACE II. Discussion of Measured Parameters
Authors: De Moortel, I.; Hood, A. W.; Ireland, J.; Walsh, R. W.
2002SoPh..209...89D Altcode:
In this paper, we give a detailed discussion of the parameters of
longitudinal oscillations in coronal loops, described in Paper I. We
found a surprising absence of correlations between the measured
variables, with the exception of a relation between the estimated
damping length and the period of the intensity variations. Only for
2 out of the 38 cases presented in Paper I did we find a significant
perturbation in the 195 Å TRACE data. The loops supporting the
propagating disturbances were typically stable, quiescent loops and
the total luminosity of the analyzed structures generally varied by
no more than 10%. The observed density oscillations are unlikely to be
flare-driven and are probably caused by an underlying driver exciting
the loop footpoints. It was demonstrated that the rapid damping of
the perturbations could not simply be explained as a consequence
of the decreasing intensity along the loops. However, we found that
(slightly enhanced) thermal conduction alone could account for the
observed damping lengths and wavelengths, and, additionally, explain
the correlation between propagation period and damping length.

---------------------------------------------------------
Title: Longitudinal intensity oscillations in coronal loops observed
with TRACE I. Overview of Measured Parameters
Authors: De Moortel, I.; Ireland, J.; Walsh, R. W.; Hood, A. W.
2002SoPh..209...61D Altcode:
In this paper we aim to give a comprehensive overview of geometric
and physical properties of longitudinal oscillations in large coronal
loops. The 38 examples of propagating disturbances were obtained
from the analysis of high cadence, 171 Å TRACE data (JOP 83 and JOP
144). The majority of these outward propagating oscillations are found
in the footpoints of large diffuse coronal loop structures, close to
active regions. The disturbances travel outward with a propagation
speed of the order of v≈122±43 km s−1. The variations
in intensity are estimated to be roughly 4.1±1.5% of the background
loop brightness. The propagating disturbances are found to be damped
very quickly and are typically only detected in the first 8.9±4.4
Mm along the loop. Using a wavelet analysis, periods of the order
of 282±93 s are found and the energy flux was estimated as 342±126
erg cm−2 s−1. We found highly filamentary
behavior in the lower part of the coronal loops and showed that the
intensity oscillations can be present for several consecutive hours,
with a more or less constant period. It is evident that the longitudinal
oscillations are a widespread, regularly occurring coronal phenomena. A
companion paper is devoted to the interpretation and discussion of
the results.

---------------------------------------------------------
Title: Application of wavelet analysis to transversal coronal loop
oscillations
Authors: Ireland, J.; De Moortel, I.
2002A&A...391..339I Altcode:
There as yet remain few examples of well observed, transversal
oscillations in coronal loops. Such oscillations have the potential
to yield much information on the nature of the solar corona, as
demonstrated by the analysis of Nakariakov et al. (\cite{nak})
of a transversely oscillating loop observed in the TRACE 171 Å
passband on 14th July, 1998. Their analysis extracts a decaying loop
oscillation signal from the data which is then considered in the light
of the substantial body of theoretically and computationally derived
knowledge of the dynamics of coronal loops. The analysis presented in
this paper approaches the reduction of the same dataset using wavelet
techniques described by De Moortel &amp; Hood (\cite{demhood}) and De
Moortel et al. (\cite{dhi}). The authors show that the value of the
decay exponent N in a decaying oscillating time series of the form exp
(-ktN) is measurable from a wavelet transform of the time
series (for some decay constant k and time t). The application of
these techniques shows that the value of the decay exponent in the
14th July, 1998 event is not well determined by the data, i.e., the
associated error is very large. Since the value of the decay exponent
implies the presence of particular decay mechanisms and not others,
the large error associated with the exponent value implies that a wide
range of mechanisms should be considered when discussing the physics
behind this event. Comments are also made on the time dependence of
the oscillation wavelet scale. Two additional examples of transversal
coronal loop oscillations are also analysed.

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Title: Preliminary description of Kanzelhöhe/MDI magnetograms and
the search for sources of coronal oscillations
Authors: Ireland, J.; Walsh, R. W.; De Moortel, I.; Moretti, P. F.
2002ESASP.508..299I Altcode: 2002soho...11..299I
Many examples of transverse (Schrijver et al., 2002; Aschwanden
et al., 2002) and longitudinal coronal loop oscillations have now
been observed in TRACE 171 Å data (see De Moortel et al., 2002
at this meeting for examples of longitudinal oscillations). These
oscillations hold the promise of telling us much about the physics of
the corona. However, the mechanisms describing these distinct phenomena
are as yet unclear. Magnetogram data from MDI and Kanzelhöhe taken
as part of SOHO Joint Observing 144 allows us to use the spatial
resolution of MDI and temporal resolution of Kanzelhöhe to probe the
photospheric magnetic field at likely footpoint sources of coronal
loop oscillations at length and time scales not available to either
instrument separately. Variations in the photospheric magnetic field
are analysed in conjunction with co-temporally observed TRACE 171 Å
derived time series.

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Title: Trace observations of propagating slow magneto-acoustic
disturbances in coronal loops
Authors: De Moortel, I.; Ireland, J.; Walsh, R. W.
2002ESASP.508..275D Altcode: 2002soho...11..275D
We study propagating disturbances in 38 coronal loops and give an
overview of their properties using high cadence, 171 Å, TRACE data
(JOP 83 &amp; JOP 144). The majority of these outward propagating
oscillations are found in the footpoints of large diffuse coronal loop
structures, close to active regions. The disturbances travel outward
with a propagation speed of the order of v ≍ 119+/-39 km/s. The
variations in intensity are estimated to be roughly 4.1+/-1.6% of the
background brightness and the propagating disturbances are found to
be damped very quickly, within 8.6+/-3.8 Mm along the loop. Using a
wavelet analysis, periods of the order of 282+/-93 seconds are found
and the energy flux was estimated as 346+/-132 ergs/cm2s. It
is suggested that these oscillations are slow magneto-acoustic waves
propagating along the lower part of large, quiescent, coronal loops.

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Title: The detection of 3 &amp; 5 min period oscillations in
coronal loops
Authors: De Moortel, I.; Ireland, J.; Hood, A. W.; Walsh, R. W.
2002A&A...387L..13D Altcode:
High cadence, 171 Alfvén A, TRACE observations show that outward
propagating intensity disturbances are a common feature in large,
quiescent coronal loops. These oscillations are interpreted as
propagating slow magneto-acoustic waves. Using a wavelet analysis, we
found periods of the order of 282 +/- 93 s. However, a careful study of
the location of the footpoints revealed a distinct separation between
those loops that support oscillations with periods smaller than 200 s
and periods larger than 200 s. It was found that loops that are situated
above sunspot regions display intensity oscillations with a period
of the order of 172 +/- 32 s, whereas oscillations in “non-sunspot”
loops show periods of the order of 321 +/- 74 s. We conclude that the
observed longitudinal oscillations are not flare-driven but are most
likely caused by an underlying driver exciting the loop footpoints. This
result suggests that the underlying oscillations can propagate through
the transition region and into the corona.

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Title: Coronal seismology through wavelet analysis
Authors: De Moortel, I.; Hood, A. W.; Ireland, J.
2002A&A...381..311D Altcode:
This paper expands on the suggestion of De Moortel &amp; Hood
(\cite{DeMoortel00}) that it will be possible to infer coronal plasma
properties by making a detailed study of the wavelet transform of
observed oscillations. TRACE observations, taken on 14 July 1998, of a
flare-excited, decaying coronal loop oscillation are used to illustrate
the possible applications of wavelet analysis. It is found that a decay
exponent n ~ 2 gives the best fit to the double logarithm of the wavelet
power, thus suggesting an e-varepsilon t^2 damping profile
for the observed oscillation. Additional examples of transversal loop
oscillations, observed by TRACE on 25 October 1999 and 21 March 2001,
are analysed and a damping profile of the form e-varepsilon
t^n, with n ~ 0.5 and n ~ 3 respectively, is suggested. It is
demonstrated that an e-varepsilon t^n damping profile of
a decaying oscillation survives the wavelet transform, and that the
value of both the decay coefficient varepsilon and the exponent n can
be extracted by taking a double logarithm of the normalised wavelet
power at a given scale. By calculating the wavelet power analytically,
it is shown that a sufficient number of oscillations have to be present
in the analysed time series to be able to extract the period of the
time series and to determine correct values for both the damping
coefficient and the decay exponent from the wavelet transform.

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Title: Wavelet analysis and the determination of coronal plasma
properties
Authors: De Moortel, I.; Hood, A. W.
2000A&A...363..269D Altcode:
The usefulness of wavelet analysis is demonstrated by considering
analytical expressions for phase mixed Alfvén waves in different
physical circumstances. The wavelet analysis is briefly introduced,
using the complex-valued Morlet wavelet, consisting of a plane wave
modulated by a Gaussian, as the basic wavelet. The time and scale
resolution of the wavelet transform are then discussed in more
detail, by working out the transform of simple harmonic functions
analytically. As an illustration of the power of wavelet analysis, phase
mixed Alfvén waves are investigated. A comparison is made between a
truly finite harmonic wave and an Alfvén wave, dissipated by phase
mixing and, using the wavelet transform, it is demonstrated that it
is possible to distinguish between these two `finite' signals. It
is also possible to extract the value of the dissipation coefficient
from the wavelet transform. When considering phase mixing of Alfvén
waves in a gravitationally stratified atmosphere, the lengthening of
the wavelengths is clearly evident in the transform, which provides
an independent estimate of the value of the pressure scale height. In
a radially diverging atmosphere, the shortening of the wavelengths is
also apparent in the wavelet transform, showing how the Alfvén speed
varies along the loop and thus providing information on the coronal
density and magnetic field. When applying wavelet analysis to observed
wave-like oscillations, it should be possible to infer properties of
the coronal plasma by making a detailed study of the wavelet transform.

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Title: Observation of oscillations in coronal loops
Authors: De Moortel, I.; Walsh, R. W.; Ireland, J.
2000AIPC..537..216D Altcode: 2000wdss.conf..216D
High cadence TRACE data (JOP 83) in the 171 Å bandpass are used to
report on several examples of outward propagating oscillations in the
footpoints of large diffuse coronal loop structures close to active
regions. The disturbances travel outward with a propagation speed
between 70 and 160 km s-1. The variations in intensity are
of the order of 2%-4%, compared to the background brightness and these
get weaker as the disturbance propagates along the structure. From a
wavelet analysis at different positions along the structures, periods
in the 200-400 seconds range are found. It is suggested that these
oscillations are slow magneto-acoustic waves propagating along the
loop, carrying an estimated energy flux of 4×102 ergs
cm-2 s-1. .

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Title: Phase mixing of Alfvén waves in an open and stratified
atmosphere
Authors: De Moortel, I.; Hood, A. W.; Arber, T. D.
2000AIPC..537..224D Altcode: 2000wdss.conf..224D
Phase mixing was introduced by Heyvaerts and Priest [1] as a mechanism
for heating plasma in open magnetic field regions. Here we include a
stratified density and a diverging background magnetic field. We present
numerical and WKB solutions to describe the effect of stratification
and divergence on phase mixing of Alfvén waves. It is shown that
the decrease in density lengthens the oscillation wavelengths and
thereby reduces the generation of transverse gradients. However,
the divergence of the field lines shortens the wavelengths and thus
enhances the generation of gradients. Furthermore we found that in
a stratified atmosphere, ohmic heating is spread out over a greater
height range whereas viscous heating is not strongly influenced by
the stratification. A wavelet analysis indicated that the wavelet
transform could provide us with information about the medium the waves
are traveling through. .

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Title: Observation of oscillations in coronal loops
Authors: De Moortel, I.; Ireland, J.; Walsh, R. W.
2000A&A...355L..23D Altcode:
On March 23rd 1999, a set of TRACE observations in the 171 Alfvén A
(Fe Ix) bandpass was made of active region AR 8496. A wavelet analysis
of a bright loop-footpoint to the south west of this active region
displays outward propagating perturbations with periods 180-420
seconds at approximately 70-165 km s-1. We suggest that
these oscillations are slow magneto-acoustic waves propagating along
the loop, carrying an estimated energy flux of 4 x 102
ergs cm-2 s-1.

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Title: Phase mixing of Alfvén waves in a stratified and radially
diverging, open atmosphere
Authors: De Moortel, I.; Hood, A. W.; Arber, T. D.
2000A&A...354..334D Altcode:
Phase mixing was proposed by Heyvaerts and Priest (1983) as a mechanism
for heating the plasma in open magnetic field regions of coronal
holes. Here the basic model is modified to include a gravitationally
stratified density and a diverging background magnetic field. We
present WKB solutions and use a numerical code to describe the effect
of dissipation, stratification and divergence on phase mixing of
Alfvén\ waves. It is shown that the wavelengths of an Alfvén\ wave is
shortened as it propagates outwards which enhances the generation of
gradients. Therefore, the convection of wave energy into heating the
plasma occurs at lower heights than in a uniform model. The combined
effect of a stratified density and a radially diverging background
magnetic field on phase mixing of Alfvén\ waves depends strongly on
the particular geometry of the configuration. Depending on the value
of the pressure scale height, phase mixing can either be more or less
efficient than in the uniform case.

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Title: Phase Mixing of Alfvén Waves in an Open and Stratified
Atmosphere
Authors: De Moortel, I.; Hood, A. W.; Arber, T. D.
1999ESASP.448..257D Altcode: 1999ESPM....9..257D; 1999mfsp.conf..257D
No abstract at ADS

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Title: Phase mixing of Alfvén waves in a stratified and open
atmosphere
Authors: De Moortel, I.; Hood, A. W.; Ireland, J.; Arber, T. D.
1999A&A...346..641D Altcode:
Phase mixing was introduced by Heyvaerts and Priest (1983) as a
mechanism for heating the plasma in the open magnetic field regions
of coronal holes. Here the basic process is modified to include a
stratified atmosphere in which the density decreases with height. We
present an analytical solution in the case of zero dissipation and
use a numerical code in the non-zero dissipation case to describe the
effect of stratification on phase mixing. The exponential damping
behaviour derived by Heyvaerts and Priest is largely confirmed in
the non stratified limit. However, it is shown that the decrease in
density lengthens the oscillation wavelengths and thereby reduces the
generation of transverse gradients. Furthermore we found that in a
stratified atmosphere the perturbed magnetic field and velocity behave
quite differently depending on whether we consider resistivity or
viscosity. Ohmic heating is spread out over a greater height range in
a stratified medium whereas viscous heating is not strongly influenced
by the stratification.