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Author name code: cally
ADS astronomy entries on 2022-09-14
author:"Cally, Paul S."
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Title: On the fragility of Alfvén waves in a stratified atmosphere
Authors: Cally, Paul S.
2022MNRAS.510.1093C Altcode: 2021MNRAS.tmp.3141C; 2021arXiv211113435C
Complete asymptotic expansions are developed for slow, Alfvén, and
fast magnetohydrodynamic waves at the base of an isothermal 3D plane
stratified atmosphere. Together with existing convergent Frobenius
series solutions about z = ∞, matchings are numerically calculated
that illuminate the fates of slow and Alfvén waves injected from
below. An Alfvén wave in a two-dimensional model is 2.5D in the sense
that the wave propagates in the plane of the magnetic field but its
polarization is normal to it in an ignorable horizontal direction, and
the wave remains an Alfvén wave throughout. The rotation of the plane
of wave propagation away from the vertical plane of the magnetic field
pushes the plasma displacement vector away from horizontal, thereby
coupling it to stratification. It is shown that potent slow-Alfvén
coupling occurs in such 3D models. It is found that about 50 per cent
of direction-averaged Alfvén wave flux generated in the low atmosphere
at frequencies comparable to or greater than the acoustic cut-off can
reach the top as Alfvén flux for small magnetic field inclinations
θ, and this increases to 80 per cent or more with increasing θ. On
the other hand, direction-averaged slow waves can be 40 per cent
effective in converting to Alfvén waves at small inclination, but
this reduces sharply with increasing θ and wave frequency. Together
with previously explored fast-slow and fast-Alfvén couplings, this
provides valuable insights into which injected transverse waves can
reach the upper atmosphere as Alfvén waves, with implications for
solar and stellar coronal heating and solar/stellar wind acceleration.
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Title: Benchmarking hall-induced magnetoacoustic to Alfvén mode
conversion in the solar chromosphere
Authors: Raboonik, Abbas; Cally, Paul S.
2021MNRAS.507.2671R Altcode: 2021MNRAS.tmp.2106R
A 2.5D numerical model of magnetoacoustic-Alfvén linear mode
conversions in the partially ionized low solar atmosphere induced
by the Hall effect is surveyed, varying magnetic field strength and
inclination, and wave frequency and horizontal wavenumber. It is
found that only the magnetic component of wave energy is subject to
Hall-mediated conversions to Alfvén wave-energy via a process of
polarization rotation. This strongly boosts direct mode conversion
between slow magnetoacoustic and Alfvén waves in the quiet low
chromosphere, even at mHz frequencies. However, fast waves there,
which are predominantly acoustic in nature, are only subject to
Hall-induced conversion via an indirect two-step process: (i) a
geometry-induced fast-slow transformation near the Alfvén-acoustic
equipartition height z<SUB>eq</SUB>; and (ii) Hall-rotation of the
fast wave in z > z<SUB>eq</SUB>. Thus, for the two-stage process to
yield upgoing Alfvén waves, z<SUB>eq</SUB> must lie below or within
the Hall-effective window 0 ≲ z ≲ 700 km. Magnetic field strengths
over 100 G are required to achieve this. Since the potency of this
Hall effect varies inversely with the field strength but directly
with the wave frequency, only frequencies above about 100 mHz are
significantly affected by the two-stage process. Increasing magnetic
field inclination θ generally strengthens the Hall convertibility,
but the horizontal wavenumber k<SUB>x</SUB> has little effect. The
direct and indirect Hall mechanisms both have implications for the
ability of MHD waves excited at the photosphere to reach the upper
chromosphere, and by implication the corona.
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Title: Benchmarking Hall-Induced Magnetoacoustic to Alfvén Mode
Conversion in the Solar Chromosphere
Authors: Raboonik, Abbas; Cally, Paul
2021arXiv210802396R Altcode:
A 2.5D numerical model of magnetoacoustic-Alfvén linear mode
conversions in the partially ionised low solar atmosphere induced
by the Hall effect is surveyed, varying magnetic field strength and
inclination, and wave frequency and horizontal wave number. It is
found that only the magnetic component of wave energy is subject to
Hall-mediated conversions to Alfvén wave-energy via a process of
polarisation rotation. This strongly boosts direct mode conversion
between slow magneto\-acoustic and Alfvén waves in the quiet low
chromosphere, even at mHz frequencies. However, fast waves there, which
are predominantly acoustic in nature, are only subject to Hall- induced
conversion via an indirect two-step process: (i) a geometry-induced
fast-slow transformation near the Alfvén-acoustic equipartition height
$z_{\rm eq}$; and (ii) Hall-rotation of the fast wave in $z>z_{\rm
eq}$. Thus, for the two-stage process to yield upgoing Alfvén waves,
$z_{\rm eq}$ must lie below or within the Hall-effective window
$0\lesssim z\lesssim700$ km. Magnetic field strengths over 100 G are
required to achieve this. Since the potency of this Hall effect varies
inversely with the field strength but directly with the wave frequency,
only frequencies above about 100 mHz are significantly affected by
the two-stage process. Increasing magnetic field inclination $\theta$
generally strengthens the Hall convertibility, but the horizontal
wavenumber $k_x$ has little effect. The direct and indirect Hall
mechanisms both have implications for the ability of MHD waves excited
at the photosphere to reach the upper chromosphere, and by implication
the corona.
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Title: Second bounces of seismic signals from sunquakes: modelling
and data
Authors: Donea, A.; Cally, P.
2021AAS...23811318D Altcode:
The 6 September 2017 X9.3 solar flare produced interesting magnetic
field transients and a few seismic sources generating visible acoustic
ripples in the quiet magnetic bay of the hosting active region.Zharkov
et al (2020) for the first time presented observations of the second
bounce of acoustic waves generated in the solar photosphere by one of
the seismic sources. In this work we model the second skips using the
ray-theory and simple models of magnetic fields of a sunspot. We want
to understand how various parametrisations related to the local magnetic
field, source locations and source movements can influence the behaviour
of the first and second skips in time-distance plots. This work will
help us to understand why the majority of sunquakes do not present
second bounces; it may give us clues on how to look at observations
in search for ripples and will help with backtracking information on
seismic sources.
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Title: Conversion and Smoothing of MHD Shocks in Atmospheres with
Open and Closed Magnetic Field and Neutral Points
Authors: Pennicott, Jamon D.; Cally, Paul S.
2021SoPh..296...97P Altcode: 2021arXiv210502329P
Planar acoustically dominated magnetohydrodynamic waves are initiated
at the high-β base of a simulated 2D isothermal stratified atmosphere
with potential magnetic field exhibiting both open and closed field
regions as well as neutral points. They shock on their way upward toward
the Alfvén-acoustic equipartition surface a =c , where a and c are
the Alfvén and sound speeds, respectively. Expanding on recent 1.5D
findings that such shocks mode-convert to fast shocks and slow smoothed
waves on passing through a =c , we explore the implications for these
more complex magnetic geometries. It is found that the 1.5D behaviour
carries over to the more complex case, with the fast shocks strongly
attracted to neutral points, which are disrupted producing extensive
fine structure. It is also observed that shocks moving in the opposite
direction, from a >c to a <c , split into fast and slow components
too, and that again it is the slow component that is smoothed.
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Title: Chromospheric Heating by Magnetohydrodynamic Waves and
Instabilities
Authors: Srivastava, A. K.; Ballester, J. L.; Cally, P. S.; Carlsson,
M.; Goossens, M.; Jess, D. B.; Khomenko, E.; Mathioudakis, M.;
Murawski, K.; Zaqarashvili, T. V.
2021JGRA..12629097S Altcode: 2021arXiv210402010S
The importance of the chromosphere in the mass and energy transport
within the solar atmosphere is now widely recognized. This review
discusses the physics of magnetohydrodynamic waves and instabilities
in large-scale chromospheric structures as well as in magnetic flux
tubes. We highlight a number of key observational aspects that have
helped our understanding of the role of the solar chromosphere
in various dynamic processes and wave phenomena, and the heating
scenario of the solar chromosphere is also discussed. The review
focuses on the physics of waves and invokes the basics of plasma
instabilities in the context of this important layer of the solar
atmosphere. Potential implications, future trends and outstanding
questions are also delineated.
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Title: Study of Acoustic Halos in NOAA Active Region 12683
Authors: Tripathy, S. C.; Jain, K.; Kholikov, S.; Hill, F.; Cally, P.
2020ASSP...57..121T Altcode:
We characterize the spatio-temporal power distribution around the
active region 12683 as a function of height in the solar atmosphere,
wave frequencies, magnetic field strength and inclination of the
magnetic field.
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Title: An introductory guide to fluid models with anisotropic
temperatures. Part 1. CGL description and collisionless fluid
hierarchy
Authors: Hunana, P.; Tenerani, A.; Zank, G. P.; Khomenko, E.;
Goldstein, M. L.; Webb, G. M.; Cally, P. S.; Collados, M.; Velli,
M.; Adhikari, L.
2019JPlPh..85f2002H Altcode: 2019arXiv190109354H
We present a detailed guide to advanced collisionless fluid models
that incorporate kinetic effects into the fluid framework, and that are
much closer to the collisionless kinetic description than traditional
magnetohydrodynamics. Such fluid models are directly applicable to
modelling the turbulent evolution of a vast array of astrophysical
plasmas, such as the solar corona and the solar wind, the interstellar
medium, as well as accretion disks and galaxy clusters. The text
can be viewed as a detailed guide to Landau fluid models and it is
divided into two parts. Part 1 is dedicated to fluid models that
are obtained by closing the fluid hierarchy with simple (non-Landau
fluid) closures. Part 2 is dedicated to Landau fluid closures. Here
in Part 1, we discuss the fluid model of Chew-Goldberger-Low (CGL)
in great detail, together with fluid models that contain dispersive
effects introduced by the Hall term and by the finite Larmor radius
corrections to the pressure tensor. We consider dispersive effects
introduced by the non-gyrotropic heat flux vectors. We investigate
the parallel and oblique firehose instability, and show that the
non-gyrotropic heat flux strongly influences the maximum growth rate of
these instabilities. Furthermore, we discuss fluid models that contain
evolution equations for the gyrotropic heat flux fluctuations and that
are closed at the fourth-moment level by prescribing a specific form
for the distribution function. For the bi-Maxwellian distribution,
such a closure is known as the `normal' closure. We also discuss a
fluid closure for the bi-kappa distribution. Finally, by considering
one-dimensional Maxwellian fluid closures at higher-order moments,
we show that such fluid models are always unstable. The last possible
non Landau fluid closure is therefore the `normal' closure, and beyond
the fourth-order moment, Landau fluid closures are required.
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Title: An introductory guide to fluid models with anisotropic
temperatures. Part 2. Kinetic theory, Padé approximants and Landau
fluid closures
Authors: Hunana, P.; Tenerani, A.; Zank, G. P.; Goldstein, M. L.;
Webb, G. M.; Khomenko, E.; Collados, M.; Cally, P. S.; Adhikari, L.;
Velli, M.
2019JPlPh..85f2003H Altcode: 2019arXiv190109360H
In Part 2 of our guide to collisionless fluid models, we concentrate
on Landau fluid closures. These closures were pioneered by Hammett
and Perkins and allow for the rigorous incorporation of collisionless
Landau damping into a fluid framework. It is Landau damping that sharply
separates traditional fluid models and collisionless kinetic theory,
and is the main reason why the usual fluid models do not converge to the
kinetic description, even in the long-wavelength low-frequency limit. We
start with a brief introduction to kinetic theory, where we discuss in
detail the plasma dispersion function Z(ζ), and the associated plasma
response function R(ζ)=1+ζZ(ζ)=-Z^' }(ζ)/2. We then consider a
one-dimensional (1-D) (electrostatic) geometry and make a significant
effort to map all possible Landau fluid closures that can be constructed
at the fourth-order moment level. These closures for parallel moments
have general validity from the largest astrophysical scales down to
the Debye length, and we verify their validity by considering examples
of the (proton and electron) Landau damping of the ion-acoustic mode,
and the electron Landau damping of the Langmuir mode. We proceed by
considering 1-D closures at higher-order moments than the fourth order,
and as was concluded in Part 1, this is not possible without Landau
fluid closures. We show that it is possible to reproduce linear
Landau damping in the fluid framework to any desired precision,
thus showing the convergence of the fluid and collisionless kinetic
descriptions. We then consider a 3-D (electromagnetic) geometry in the
gyrotropic (long-wavelength low-frequency) limit and map all closures
that are available at the fourth-order moment level. In appendix Ae
provide comprehensive tables with Padé approximants of R(ζ) up to
the eighth-pole order, with many given in an analytic form.
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Title: Fast-to-Alfvén Mode Conversion and Ambipolar Heating in
Structured Media. I. Simplified Cold Plasma Model
Authors: Cally, Paul S.; Khomenko, Elena
2019ApJ...885...58C Altcode:
Linear fast magnetoacoustic waves are introduced into a cold
stratified plasma model made up of a doubly periodic ensemble of
straight diminished-Alfvén-speed tubes. Coupling between fast and
Alfvén waves is produced by stratification and the tube structures,
and scattering is strong for wavelengths comparable to the inter-tube
separation. Ambipolar diffusion is found to be enhanced by the
structuring and is potentially significant at high frequencies. The
production of kink waves is discussed and modeled. It is found that
the tube structure significantly alters the wave energy reaching the
corona and the form that it takes, even for moderate fast-slow tube
contrast, with Alfvén waves becoming prominent.
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Title: Hall-coupling of Slow and Alfvén Waves at Low Frequencies
in the Lower Solar Atmosphere
Authors: Raboonik, Abbas; Cally, Paul S.
2019SoPh..294..147R Altcode:
The Hall effect due to weak ionization in the lower solar atmosphere
is shown to produce significant coupling between slow magneto-acoustic
and Alfvén waves, especially in highly inclined magnetic fields,
and even at low frequencies (≈5 mHz and above). Based on the exact
magneto-acoustic linear wave solutions in a 2D isothermal model
atmosphere, a perturbation approach is used to calculate the coupling
to Alfvén waves polarized in the third dimension. First, a fast
wave is injected at the bottom and is partially and often strongly
reflected/converted to a down-going slow wave at the Alfvén-acoustic
equipartition height, depending on magnetic field inclination,
frequency, and wave number. This slow wave then couples strongly to
the down-going Alfvén wave via the Hall effect for realistic Hall
parameters. The coupling is strongest for horizontal wavenumbers
oriented opposite to the field inclination, and magnetic fields around
100 G, for which large values of the Hall parameter are co-spatial
with the region where slow and Alfvén waves have almost identical
wave forms. Second, a slow wave is injected at the bottom, and found
to couple even more strongly to up-going Alfvén waves in certain
regions of the wavenumber-frequency plane where acoustic-gravity waves
are evanescent. These results contrast with those for Hall-mediated
fast-Alfvén coupling, which occurs higher in the atmosphere and is
evident only at much higher frequencies.
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Title: Fast-to-Alfvén Mode Conversion and Ambipolar Heating in
Structured Media. II. Numerical Simulation
Authors: Khomenko, Elena; Cally, Paul S.
2019ApJ...883..179K Altcode:
This paper studies the effectiveness of production of Alfvén waves
in the solar atmosphere through the processes of mode conversion,
taking into account several new effects that have not been considered
before. We perform simulations of wave propagation and conversion from
the photosphere to the corona. Both magnetic field and plasma parameters
are structured in the form of small-scale flux tubes with characteristic
scale significantly below the wavelength of the waves. The waves are
allowed to dissipate through the ambipolar diffusion (AD) mechanism. We
use an analytical magneto-static equilibrium model, which provides
the AD coefficient values at the lower end of what is expected for
the quiet solar regions. This work extends the simplified study of
mode conversion by Cally and Cally & Khomenko to the case of warm,
partially ionized, and structured plasma. We conclude that interaction
of waves with the flux tube ensemble produces a discrete spectrum of
high-order harmonics. The scattering is a linear process: however,
the nonlinear effects have considerable influence upon the amplitudes
of the harmonics. The magnetic Poynting flux reaching the corona is
enhanced by about 35% and the reflection of waves at the transition
region is decreased by about 50% when the flux tubes structure is
present, relative to the horizontally homogeneous case. The energy
flux of Alfvén waves exceeds that of acoustic waves at coronal
heights. Ambipolar diffusion decreases the magnetic Poynting flux in
the corona because the fast waves entering the transformation region
at chromospheric heights are degraded and have lower amplitudes. The
effect of the enhancement of Alfvén wave production due to interaction
with flux tubes is independent of the numerical resolution, while the
effect of the AD is resolution-dependent and is not converged at the
10 km resolution of our best simulations.
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Title: Smoothing of MHD Shocks in Mode Conversion
Authors: Pennicott, Jamon D.; Cally, Paul S.
2019ApJ...881L..21P Altcode: 2019arXiv190710954P
Shock waves are simulated passing through the Alfvén-acoustic
equipartition layer in a stratified isothermal magneto-atmosphere. The
recent ray-theoretic calculations of Núñez predicted smoothing of the
shock through this layer, causing both the fast and slow components to
emerge as continuous waves. However, it is found that the partial mode
conversion expected from linear theory for oblique incidence of the
shock on the magnetic field is accompanied by a smoothing of the slow
shock only, while the fast shock persists. Explanations are presented
based on magnetohydrodynamic mode conversion and shock theory.
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Title: Fast-to-Alfvén Mode Conversion Mediated by Hall
Current. II. Application to the Solar Atmosphere
Authors: González-Morales, P. A.; Khomenko, E.; Cally, P. S.
2019ApJ...870...94G Altcode: 2018arXiv181106565G
Coupling between fast magnetoacoustic and Alfvén waves can be observed
in fully ionized plasmas mediated by stratification and 3D geometrical
effects. In Paper I, Cally & Khomenko have shown that in a weakly
ionized plasma, such as the solar photosphere and chromosphere, the
Hall current introduces a new coupling mechanism. The present study
extends the results from Paper I to the case of warm plasma. We report
on numerical experiments where mode transformation is studied using
quasi-realistic stratification in thermodynamic parameters resembling
the solar atmosphere. This redresses the limitation of the cold plasma
approximation assumed in Paper I, in particular allowing the complete
process of coupling between fast and slow magnetoacoustic modes and
subsequent coupling of the fast mode to the Alfvén mode through
the Hall current. Our results confirm the efficacy of the mechanism
proposed in Paper I for the solar case. We observe that the efficiency
of the transformation is a sensitive function of the angle between
the wave propagation direction and the magnetic field, and of the
wave frequency. The efficiency increases when the field direction and
the wave direction are aligned for increasing wave frequencies. After
scaling our results to typical solar values, the maximum amplitude of
the transformed Alfvén waves, for a frequency of 1 Hz, corresponds
to an energy flux (measured above the height of peak Hall coupling)
of ∼10<SUP>3</SUP> W m<SUP>-2</SUP>, based on an amplitude of 500
m s<SUP>-1</SUP> at β = 1, which is sufficient to play a major role
in both quiet and active region coronal heating.
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Title: Investigation of Acoustic Halos using Multi-Height SDO
Observations
Authors: Tripathy, S. C.; Jain, Kiran; Kholikov, S.; Burtseva, O.;
Hill, F.; Cally, P.
2018csc..confE.130T Altcode:
The interpretation of acoustic waves surrounding active regions has
been a challenging task since the influence of magnetic field on the
incident waves is not fully understood. As a result, structure and
dynamics of active regions beneath the surface show significant
uncertainties. Recent numerical simulations and helioseismic
measurements in active regions have demonstrated that the key to the
understanding of these complex processes requires a synergy between
models and helioseismic inferences from observations. In this context,
using data from Helioseismic Magnetic Imager and Atmospheric Imaging
Assembly instruments on board the Solar Dynamics Observatory, we
characterize the spatio-temporal power distribution around active
regions as a function of the height in the solar atmosphere. We
find power enhancements (acoustic halos) occur above the acoustic
cutoff frequency and extends up to 10 mHz in HMI Doppler and AIA
170 nm observations and are strong functions of magnetic field and
their inclination angle. We also examine the relative phases and
cross-coherence spectra and find different wave characteristics at
different heights.
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Title: Probing sunspots with two-skip time-distance helioseismology
Authors: Duvall, Thomas L., Jr.; Cally, Paul S.; Przybylski, Damien;
Nagashima, Kaori; Gizon, Laurent
2018A&A...613A..73D Altcode: 2018arXiv180601032D
Context. Previous helioseismology of sunspots has been sensitive to
both the structural and magnetic aspects of sunspot structure. <BR />
Aims: We aim to develop a technique that is insensitive to the magnetic
component so the two aspects can be more readily separated. <BR />
Methods: We study waves reflected almost vertically from the underside
of a sunspot. Time-distance helioseismology was used to measure travel
times for the waves. Ray theory and a detailed sunspot model were used
to calculate travel times for comparison. <BR /> Results: It is shown
that these large distance waves are insensitive to the magnetic field
in the sunspot. The largest travel time differences for any solar
phenomena are observed. <BR /> Conclusions: With sufficient modeling
effort, these should lead to better understanding of sunspot structure.
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Title: Fast-to-Alfvén Mode Conversion in the Presence of Ambipolar
Diffusion
Authors: Cally, Paul S.; Khomenko, Elena
2018ApJ...856...20C Altcode:
It is known that fast magnetohydrodynamic waves partially convert to
upward and/or downward propagating Alfvén waves in a stratified
atmosphere where Alfvén speed increases with height. This
happens around the fast wave reflection height, where the fast
wave’s horizontal phase speed equals the Alfvén speed (in a
low-β plasma). Typically, this takes place in the mid to upper
solar chromosphere for low-frequency waves in the few-millihertz
band. However, this region is weakly ionized and thus susceptible to
nonideal MHD processes. In this article, we explore how ambipolar
diffusion in a zero-β plasma affects fast waves injected from
below. Classical ambipolar diffusion is far too weak to have any
significant influence at these low frequencies, but if enhanced
by turbulence (in the quiet-Sun chromosphere but not in sunspot
umbrae) or the production of sufficiently small-scale structure,
can substantially absorb waves for turbulent ambipolar Reynolds
numbers of around 20 or less. In that case, it is found that the mode
conversion process is not qualitatively altered from the ideal case,
though conversion to Alfvén waves is reduced because the fast wave
flux reaching the conversion region is degraded. It is also found
that any upward propagating Alfvén waves generated in this process
are almost immune to further ambipolar attenuation, thereby reducing
local ambipolar heating compared to cases without mode conversion. In
that sense, mode conversion provides a form of “Alfvén cooling.”
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Title: Role of Interaction between Magnetic Rossby Waves and
Tachocline Differential Rotation in Producing Solar Seasons
Authors: Dikpati, Mausumi; McIntosh, Scott W.; Bothun, Gregory; Cally,
Paul S.; Ghosh, Siddhartha S.; Gilman, Peter A.; Umurhan, Orkan M.
2018ApJ...853..144D Altcode:
We present a nonlinear magnetohydrodynamic shallow-water model
for the solar tachocline (MHD-SWT) that generates quasi-periodic
tachocline nonlinear oscillations (TNOs) that can be identified with
the recently discovered solar “seasons.” We discuss the properties
of the hydrodynamic and magnetohydrodynamic Rossby waves that interact
with the differential rotation and toroidal fields to sustain these
oscillations, which occur due to back-and-forth energy exchanges among
potential, kinetic, and magnetic energies. We perform model simulations
for a few years, for selected example cases, in both hydrodynamic and
magnetohydrodynamic regimes and show that the TNOs are robust features
of the MHD-SWT model, occurring with periods of 2-20 months. We find
that in certain cases multiple unstable shallow-water modes govern
the dynamics, and TNO periods vary with time. In hydrodynamically
governed TNOs, the energy exchange mechanism is simple, occurring
between the Rossby waves and differential rotation. But in MHD cases,
energy exchange becomes much more complex, involving energy flow among
six energy reservoirs by means of eight different energy conversion
processes. For toroidal magnetic bands of 5 and 35 kG peak amplitudes,
both placed at 45° latitude and oppositely directed in north and south
hemispheres, we show that the energy transfers responsible for TNO, as
well as westward phase propagation, are evident in synoptic maps of the
flow, magnetic field, and tachocline top-surface deformations. Nonlinear
mode-mode interaction is particularly dramatic in the strong-field
case. We also find that the TNO period increases with a decrease in
rotation rate, implying that the younger Sun had more frequent seasons.
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Title: Sensitivity of coronal loop sausage mode frequencies and
decay rates to radial and longitudinal density inhomogeneities:
a spectral approach
Authors: Cally, Paul S.; Xiong, Ming
2018JPhA...51b5501C Altcode: 2017arXiv171100256C
Fast sausage modes in solar magnetic coronal loops are only fully
contained in unrealistically short dense loops. Otherwise they are
leaky, losing energy to their surrounds as outgoing waves. This
causes any oscillation to decay exponentially in time. Simultaneous
observations of both period and decay rate therefore reveal the
eigenfrequency of the observed mode, and potentially insight into the
tubes’ nonuniform internal structure. In this article, a global
spectral description of the oscillations is presented that results
in an implicit matrix eigenvalue equation where the eigenvalues are
associated predominantly with the diagonal terms of the matrix. The
off-diagonal terms vanish identically if the tube is uniform. A
linearized perturbation approach, applied with respect to a uniform
reference model, is developed that makes the eigenvalues explicit. The
implicit eigenvalue problem is easily solved numerically though,
and it is shown that knowledge of the real and imaginary parts of the
eigenfrequency is sufficient to determine the width and density contrast
of a boundary layer over which the tubes’ enhanced internal densities
drop to ambient values. Linearized density kernels are developed that
show sensitivity only to the extreme outside of the loops for radial
fundamental modes, especially for small density enhancements, with no
sensitivity to the core. Higher radial harmonics do show some internal
sensitivity, but these will be more difficult to observe. Only kink
modes are sensitive to the tube centres. Variation in internal and
external Alfvén speed along the loop is shown to have little effect
on the fundamental dimensionless eigenfrequency, though the associated
eigenfunction becomes more compact at the loop apex as stratification
increases, or may even displace from the apex.
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Title: A study of acoustic halos in active region NOAA 11330 using
multi-height SDO observations
Authors: Tripathy, S. C.; Jain, K.; Kholikov, S.; Hill, F.; Rajaguru,
S. P.; Cally, P. S.
2018AdSpR..61..691T Altcode: 2017arXiv171101259T
We analyze data from the Helioseismic Magnetic Imager (HMI) and the
Atmospheric Imaging Assembly (AIA) instruments on board the Solar
Dynamics Observatory (SDO) to characterize the spatio-temporal acoustic
power distribution in active regions as a function of the height in
the solar atmosphere. For this, we use Doppler velocity and continuum
intensity observed using the magnetically sensitive line at 6173 Å
as well as intensity at 1600 Å and 1700 Å. We focus on the power
enhancements seen around AR 11330 as a function of wave frequency,
magnetic field strength, field inclination and observation height. We
find that acoustic halos occur above the acoustic cutoff frequency and
extends up to 10 mHz in HMI Doppler and AIA 1700 Å observations. Halos
are also found to be strong functions of magnetic field and their
inclination angle. We further calculate and examine the spatially
averaged relative phases and cross-coherence spectra and find different
wave characteristics at different heights.
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Title: The Origin of the "Seasons" in Space Weather
Authors: Dikpati, Mausumi; Cally, Paul S.; McIntosh, Scott W.;
Heifetz, Eyal
2017NatSR...714750D Altcode:
Powerful `space weather' events caused by solar activity pose
serious risks to human health, safety, economic activity and national
security. Spikes in deaths due to heart attacks, strokes and other
diseases occurred during prolonged power outages. Currently it is
hard to prepare for and mitigate the impact of space weather because
it is impossible to forecast the solar eruptions that can cause these
terrestrial events until they are seen on the Sun. However, as recently
reported in Nature, eruptive events like coronal mass ejections and
solar flares, are organized into quasi-periodic "seasons", which
include enhanced bursts of eruptions for several months, followed by
quiet periods. We explored the dynamics of sunspot-producing magnetic
fields and discovered for the first time that bursty and quiet
seasons, manifested in surface magnetic structures, can be caused by
quasi-periodic energy-exchange among magnetic fields, Rossby waves
and differential rotation of the solar interior shear-layer (called
tachocline). Our results for the first time provide a quantitative
physical mechanism for forecasting the strength and duration of
bursty seasons several months in advance, which can greatly enhance
our ability to warn humans about dangerous solar bursts and prevent
damage to satellites and power stations from space weather events.
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Title: Alfvén waves in the structured solar corona
Authors: Cally, Paul S.
2017MNRAS.466..413C Altcode: 2016arXiv161202064C
A simple model of a periodic ensemble of closely packed flux tubes,
sitting atop a vertically stratified layer, reveals that an incident
fast wave from below preferentially converts almost immediately to
Alfvén waves in the flux tubes, with kink waves restricted to at most
a very few Fourier modes. This suggests that observations of coronal
kink modes in such structured systems may greatly underestimate the net
wave-energy flux being transported into and through the corona, much
of which may reside in harder-to-observe Alfvén waves. The processes
of mode conversion/resonant absorption and Alfvén phase mixing are
implicated. It is suggested that the Sun's internal p-mode field -
the 5-min oscillations - may contribute substantially to the process
by supplying incident fast waves in the chromosphere that scatter and
mode-convert in the tube ensemble.
---------------------------------------------------------
Title: Current State of Seismic Emission Associated with Solar Flares
Authors: Besliu-Ionescu, D.; Donea, A.; Cally, P.
2017SunGe..12...59B Altcode:
Certain solar flares are followed by photospheric seismic emission,
also known as sunquakes. Sunquakes were predicted more than 40 years
ago, but observed for the first time 20 years ago. A valid scenario
that would fit all discoveries made so far is still missing. This paper
summarises the current state of the literature concerning sunquakes. It
describes all published reports of known seismic sources to date and
presents possible triggering mechanisms.
---------------------------------------------------------
Title: Helioseismology with Solar Orbiter
Authors: Löptien, Björn; Birch, Aaron C.; Gizon, Laurent; Schou,
Jesper; Appourchaux, Thierry; Blanco Rodríguez, Julián; Cally,
Paul S.; Dominguez-Tagle, Carlos; Gandorfer, Achim; Hill, Frank;
Hirzberger, Johann; Scherrer, Philip H.; Solanki, Sami K.
2017hdsi.book..257L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetoseismology of Active Regions using Multi-wavelength
Observations from SDO
Authors: Tripathy Sushanta C.; Jain, Kiran; Kholikov, Shukur; Hill,
Frank; Cally, Paul S.
2016usc..confE..85T Altcode:
The structure and dynamics of active regions beneath the surface show
significant uncertainties due to our limited understanding of the
wave interaction with magnetic field. Recent numerical simulations
further demonstrate that the atmosphere above the photospheric
levels also modifies the seismic observables at the surface. Thus
the key to improve helioseismic interpretation beneath the active
regions requires a synergy between models and helioseismic inferences
from observations. In this context, using data from Helioseismic
Magnetic Imager and Atmospheric Imaging Assembly onboard Solar
Dynamics Observatory, we characterize the spatio-temporal power
distribution in and around active regions. Specifically, we focus on
the power enhancements seen around active regions as a function of wave
frequencies, strength, inclination of magnetic field and observation
height as well as the relative phases of the observables and their
cross-coherence functions. It is expected that these effects will help
us to comprehend the interaction of acoustic waves with fast and slow
MHD waves in the solar photosphere.
---------------------------------------------------------
Title: Magnetoseismology of Active Regions using Multi-wavelength
Observations from GONG and SDO
Authors: Tripathy, Sushanta; Jain, Kiran; Kholikov, Shukur; Hill,
Frank; Cally, Paul
2016SPD....47.0721T Altcode:
The structure and dynamics of active regions beneath the surface show
significant uncertainties due to our limited understanding of the wave
interaction with magnetic field. Recent numerical simulations further
demonstrate that the atmosphere above the photospheric levels also
modifies the seismic observables at the surface. Thus the key to improve
helioseismic interpretation beneath the active regions requires a
synergy between models and helioseismic inferences from observations. In
this context, using data from Global Oscillation Network Group and from
Helioseismic Magnetic Imager and Atmospheric Imaging Assembly onboard
Solar Dynamics Observatory, we characterize the spatio-temporal power
distribution in and around active regions. Specifically, we focus on
the power enhancements seen around active regions as a function of wave
frequencies, strength, inclination of magnetic field and observation
height as well as the relative phases of the observables and their
cross-coherence functions. It is expected that these effects will help
us to comprehend the interaction of acoustic waves with magnetic field
in the solar photosphere.
---------------------------------------------------------
Title: Seismology and the Wounded Sun
Authors: Cally, Paul
2016SPD....47.0704C Altcode:
Active regions provide an opening in the Sun's surface that allow
seismic waves to penetrate the overlying atmosphere. Some proportion
then return due to reflection, with implications for "internal"
seismology. This is illustrated using simulations with particular
reference to "travel times" and acoustic halos.
---------------------------------------------------------
Title: Amplitudes of MHD Waves in Sunspots
Authors: Norton, Aimee Ann; Cally, Paul; Baldner, Charles; Kleint,
Lucia; Tarbell, Theodore D.; De Pontieu, Bart; Scherrer, Philip H.;
Rajaguru, Paul
2016SPD....47.1009N Altcode:
The conversion of p-modes into MHD waves by strong magnetic fields
occurs mainly in the sub-photospheric layers. The photospheric
signatures of MHD waves are weak due to low amplitudes at the beta=1
equipartion level where mode-conversion occurs. We report on small
amplitude oscillations observed in the photosphere with Hinode SOT/SP
in which we analyze time series for sunspots ARs 12186 (11.10.2014)
and 12434 (17.10.2015). No significant magnetic field oscillations
are recovered in the umbra or penumbra in the ME inversion. However,
periodicities in the inclination angle are found at the umbral/penumbral
boundary with 5 minute periods. Upward propagating waves are indicated
in the intensity signals correlated between HMI and AIA at different
heights. We compare SP results with the oscillations observed in HMI
data. Simultaneous IRIS data shows transition region brightening above
the umbral core.
---------------------------------------------------------
Title: Division E Commission 10: Solar Activity
Authors: Schrijver, Carolus J.; Fletcher, Lyndsay; van Driel-Gesztelyi,
Lidia; Asai, Ayumi; Cally, Paul S.; Charbonneau, Paul; Gibson, Sarah
E.; Gomez, Daniel; Hasan, Siraj S.; Veronig, Astrid M.; Yan, Yihua
2016IAUTA..29..245S Altcode: 2015arXiv151003348S
After more than half a century of community support related to the
science of “solar activity”, IAU's Commission 10 was formally
discontinued in 2015, to be succeeded by C.E2 with the same area
of responsibility. On this occasion, we look back at the growth of
the scientific disciplines involved around the world over almost a
full century. Solar activity and fields of research looking into the
related physics of the heliosphere continue to be vibrant and growing,
with currently over 2,000 refereed publications appearing per year from
over 4,000 unique authors, publishing in dozens of distinct journals
and meeting in dozens of workshops and conferences each year. The
size of the rapidly growing community and of the observational and
computational data volumes, along with the multitude of connections
into other branches of astrophysics, pose significant challenges;
aspects of these challenges are beginning to be addressed through,
among others, the development of new systems of literature reviews,
machine-searchable archives for data and publications, and virtual
observatories. As customary in these reports, we highlight some
of the research topics that have seen particular interest over the
most recent triennium, specifically active-region magnetic fields,
coronal thermal structure, coronal seismology, flares and eruptions,
and the variability of solar activity on long time scales. We close
with a collection of developments, discoveries, and surprises that
illustrate the range and dynamics of the discipline.
---------------------------------------------------------
Title: On mode conversion, reflection, and transmission of
magnetoacoustic waves from above in an isothermal stratified
atmosphere
Authors: Hansen, Shelley C.; Cally, Paul S.; Donea, Alina-C.
2016MNRAS.456.1826H Altcode: 2015arXiv151107364H
We use the exact solutions for magnetoacoustic waves in a
two-dimensional (2D) isothermal atmosphere with uniform inclined
magnetic field to calculate the wave reflection, transmission, and
conversion of slow and fast waves incident from above (z = ∞). This is
relevant to the question of whether waves excited by flares in the solar
atmosphere can penetrate the Alfvén/acoustic equipartition layer (which
we identify as the canopy) to reach the photosphere with sufficient
energy to create sunquakes. It is found that slow waves above the
acoustic cutoff frequency efficiently penetrate (transmit) as acoustic
(fast) waves if directed at a small attack angle to the magnetic field,
with the rest converting to magnetic (slow) waves, in accord with
Generalized Ray Theory. This may help explain the compact nature of
seismic sources of sunquakes identified using seismic holography. The
incident slow waves can also efficiently transmit at low frequency
in inclined field due to the reduction in acoustic cutoff frequency
(ramp effect). Incident fast (magnetic) `waves' from infinity with
specified non-zero horizontal wavenumber are necessarily evanescent,
but can carry energy to the equipartition level by tunnelling. It is
found that this can then efficiently convert to acoustic (fast) energy
that can again reach the photosphere as a travelling wave. Overall,
there appear to be ample avenues for substantial compressive wave
energy to penetrate the canopy and impact the photosphere.
---------------------------------------------------------
Title: Erratum: Erratum to: Multiple Scattering of Seismic Waves
from Ensembles of Upwardly Lossy Thin Flux Tubes
Authors: Hanson, Chris S.; Cally, Paul S.
2016SoPh..291..727H Altcode: 2016SoPh..tmp....7H
No abstract at ADS
---------------------------------------------------------
Title: p-Mode Interaction with Sunspots
Authors: Cally, P. S.; Moradi, H.; Rajaguru, S. P.
2016GMS...216..489C Altcode:
This chapter brings together two of the prominent features of our star
the Sun: its well-developed p-mode spectrum of global oscillations
excited by near-surface convection, and its magnetic activity famously
represented by sunspots. Modern developments in observations of p-mode
and sunspot magnetic field interactions have helped bridge the gap
between theory and observations: both the oscillations within a sunspot
and in the immediate surroundings of the sunspot appear to be due to
magnetohydrodynamics waves driven by p-modes with characteristic spatial
patterns of frequencies intimately related to magnetic inclination
and the height variation of the plasma p = 1 layer. The chapter makes
the case that the behavior of waves in sunspots is dominated by four
processes: the ramp effect; fast-to-slow mode conversion; fast-wave
reflection; and fast-to-Alfvén mode conversion. The chapter then
discusses some of the major developments in helioseismic forward and
inverse modeling that have occurred over the last decade.
---------------------------------------------------------
Title: 3D Simulations of Realistic Power Halos in Magnetohydrostatic
Sunspot Atmospheres: Linking Theory and Observation
Authors: Rijs, Carlos; Rajaguru, S. P.; Przybylski, Damien; Moradi,
Hamed; Cally, Paul S.; Shelyag, Sergiy
2016ApJ...817...45R Altcode: 2015arXiv151201297R
The well-observed acoustic halo is an enhancement in time-averaged
Doppler velocity and intensity power with respect to quiet-Sun
values that is prominent for the weak and highly inclined field
around the penumbra of sunspots and active regions. We perform 3D
linear wave modeling with realistic distributed acoustic sources in
a magnetohydrostatic sunspot atmosphere and compare the resultant
simulation enhancements with multiheight SDO observations of the
phenomenon. We find that simulated halos are in good qualitative
agreement with observations. We also provide further proof that the
underlying process responsible for the halo is the refraction and
return of fast magnetic waves that have undergone mode conversion at
the critical a = c atmospheric layer. In addition, we also find strong
evidence that fast Alfvén mode conversion plays a significant role
in the structure of the halo, taking energy away from photospheric and
chromospheric heights in the form of field-aligned Alfvén waves. This
conversion process may explain the observed “dual-ring” halo
structure at higher (>8 mHz) frequencies.
---------------------------------------------------------
Title: Helioseismology with Solar Orbiter
Authors: Löptien, Björn; Birch, Aaron C.; Gizon, Laurent; Schou,
Jesper; Appourchaux, Thierry; Blanco Rodríguez, Julián; Cally,
Paul S.; Dominguez-Tagle, Carlos; Gandorfer, Achim; Hill, Frank;
Hirzberger, Johann; Scherrer, Philip H.; Solanki, Sami K.
2015SSRv..196..251L Altcode: 2014arXiv1406.5435L; 2014SSRv..tmp...31L
The Solar Orbiter mission, to be launched in July 2017, will
carry a suite of remote sensing and in-situ instruments, including
the Polarimetric and Helioseismic Imager (PHI). PHI will deliver
high-cadence images of the Sun in intensity and Doppler velocity
suitable for carrying out novel helioseismic studies. The orbit of
the Solar Orbiter spacecraft will reach a solar latitude of up to
21<SUP>∘</SUP> (up to 34<SUP>∘</SUP> by the end of the extended
mission) and thus will enable the first local helioseismology studies of
the polar regions. Here we consider an array of science objectives to be
addressed by helioseismology within the baseline telemetry allocation
(51 Gbit per orbit, current baseline) and within the science observing
windows (baseline 3×10 days per orbit). A particularly important
objective is the measurement of large-scale flows at high latitudes
(rotation and meridional flow), which are largely unknown but play an
important role in flux transport dynamos. For both helioseismology
and feature tracking methods convection is a source of noise in
the measurement of longitudinally averaged large-scale flows, which
decreases as T <SUP>-1/2</SUP> where T is the total duration of the
observations. Therefore, the detection of small amplitude signals (e.g.,
meridional circulation, flows in the deep solar interior) requires long
observation times. As an example, one hundred days of observations at
lower spatial resolution would provide a noise level of about three m/s
on the meridional flow at 80<SUP>∘</SUP> latitude. Longer time-series
are also needed to study temporal variations with the solar cycle. The
full range of Earth-Sun-spacecraft angles provided by the orbit will
enable helioseismology from two vantage points by combining PHI with
another instrument: stereoscopic helioseismology will allow the study
of the deep solar interior and a better understanding of the physics
of solar oscillations in both quiet Sun and sunspots. We have used a
model of the PHI instrument to study its performance for helioseismology
applications. As input we used a 6 hr time-series of realistic solar
magneto-convection simulation (Stagger code) and the SPINOR radiative
transfer code to synthesize the observables. The simulated power
spectra of solar oscillations show that the instrument is suitable for
helioseismology. In particular, the specified point spread function,
image jitter, and photon noise are no obstacle to a successful mission.
---------------------------------------------------------
Title: Fast-to-Alfvén Mode Conversion Mediated by the Hall
Current. I. Cold Plasma Model
Authors: Cally, Paul S.; Khomenko, Elena
2015ApJ...814..106C Altcode: 2015arXiv151003927C
The photospheric temperature minimum in the Sun and solar-like
stars is very weakly ionized, with an ionization fraction f as
low as 10<SUP>-4</SUP>. In galactic star-forming regions, f can be
10<SUP>-10</SUP> or lower. Under these circumstances, the Hall current
can couple low-frequency Alfvén and magnetoacoustic waves via the
dimensionless Hall parameter ɛ =ω /{{{Ω }}}<SUB>{{i</SUB>}}f,
where ω is the wave frequency and {{{Ω }}}<SUB>{{i</SUB>}} is the
mean ion gyrofrequency. This is analyzed in the context of a cold
(zero-β) plasma and in less detail for a warm plasma. It is found
that Hall coupling preferentially occurs where the wavevector is nearly
field-aligned. In these circumstances, Hall coupling in theory produces
a continual oscillation between fast and Alfvén modes as the wave
passes through the weakly ionized region. At low frequencies (mHz),
characteristic of solar and stellar normal modes, ɛ is probably too
small for more than a fraction of one oscillation to occur. On the
other hand, the effect may be significant at the far higher frequencies
(Hz) associated with magnetic reconnection events. In another context,
characteristic parameters for star-forming gas clouds suggest that
{O}(1) or more full oscillations may occur in one cloud crossing. This
mechanism is not expected to be effective in sunspots, due to their high
ion gyrofrequencies and Alfvén speeds, since the net effect depends
inversely on both and therefore inverse quadratically on field strength.
---------------------------------------------------------
Title: Spectropolarimetrically Accurate Magnetohydrostatic Sunspot
Model for Forward Modeling in Helioseismology
Authors: Przybylski, D.; Shelyag, S.; Cally, P. S.
2015ApJ...807...20P Altcode: 2015arXiv150402189P
We present a technique to construct a spectropolarimetrically
accurate magnetohydrostatic model of a large-scale solar magnetic
field concentration, mimicking a sunspot. Using the constructed model
we perform a simulation of acoustic wave propagation, conversion,
and absorption in the solar interior and photosphere with the sunspot
embedded into it. With the 6173 Å magnetically sensitive photospheric
absorption line of neutral iron, we calculate observable quantities
such as continuum intensities, Doppler velocities, as well as the full
Stokes vector for the simulation at various positions at the solar disk,
and analyze the influence of non-locality of radiative transport in
the solar photosphere on helioseismic measurements. Bisector shapes
were used to perform multi-height observations. The differences
in acoustic power at different heights within the line formation
region at different positions at the solar disk were simulated
and characterized. An increase in acoustic power in the simulated
observations of the sunspot umbra away from the solar disk center was
confirmed as the slow magnetoacoustic wave.
---------------------------------------------------------
Title: Multiple Scattering of Seismic Waves from Ensembles of Upwardly
Lossy Thin Flux Tubes
Authors: Hanson, Chris S.; Cally, Paul S.
2015SoPh..290.1889H Altcode: 2015SoPh..tmp...86H; 2015arXiv150607638H
Our previous semi-analytic treatment of f - and p -mode multiple
scattering from ensembles of thin flux tubes (Hanson and Cally,
Astrophys. J.781, 125, 2014a; 791, 129, 2014b) is extended by
allowing both sausage and kink waves to freely escape at the top of
the model using a radiative boundary condition there. As expected,
this additional avenue of escape, supplementing downward loss into the
deep solar interior, results in substantially greater absorption of
incident f - and p -modes. However, less intuitively, it also yields
mildly to substantially smaller phase shifts in waves emerging from
the ensemble. This may have implications for the interpretation of
seismic data for solar plage regions, and in particular their small
measured phase shifts.
---------------------------------------------------------
Title: Directional time-distance probing of model sunspot atmospheres
Authors: Moradi, H.; Cally, P. S.; Przybylski, D.; Shelyag, S.
2015MNRAS.449.3074M Altcode: 2015arXiv150304270M
A crucial feature not widely accounted for in local helioseismology is
that surface magnetic regions actually open a window from the interior
into the solar atmosphere, and that the seismic waves leak through this
window, reflect high in the atmosphere, and then re-enter the interior
to rejoin the seismic wave field normally confined there. In a series
of recent numerical studies using translation invariant atmospheres,
we utilized a `directional time-distance helioseismology' measurement
scheme to study the implications of the returning fast and Alfvén waves
higher up in the solar atmosphere on the seismology at the photosphere
(Cally & Moradi 2013; Moradi & Cally 2014). In this study,
we extend our directional time-distance analysis to more realistic
sunspot-like atmospheres to better understand the direct effects
of the magnetic field on helioseismic travel-time measurements
in sunspots. In line with our previous findings, we uncover a
distinct frequency-dependent directional behaviour in the travel-time
measurements, consistent with the signatures of magnetohydrodynamic
mode conversion. We found this to be the case regardless of the sunspot
field strength or depth of its Wilson depression. We also isolated and
analysed the direct contribution from purely thermal perturbations
to the measured travel times, finding that waves propagating in the
umbra are much more sensitive to the underlying thermal effects of
the sunspot.
---------------------------------------------------------
Title: MHD Wave Refraction and the Acoustic Halo Effect around Solar
Active Regions: A 3D Study
Authors: Rijs, Carlos; Moradi, Hamed; Przybylski, Damien; Cally,
Paul S.
2015ApJ...801...27R Altcode: 2015arXiv150101074R
An enhancement in high-frequency acoustic power is commonly observed
in the solar photosphere and chromosphere surrounding magnetic active
regions. We perform three-dimensional linear forward wave modeling
with a simple wavelet pulse acoustic source to ascertain whether the
formation of the acoustic halo is caused by MHD mode conversion through
regions of moderate and inclined magnetic fields. This conversion
type is most efficient when high frequency waves from below intersect
magnetic field lines at a large angle. We find a strong relationship
between halo formation and the equipartition surface at which the
Alfvén speed a matches the sound speed c, lending support to the
theory that photospheric and chromospheric halo enhancement is due
to the creation and subsequent reflection of magnetically dominated
fast waves from essentially acoustic waves as they cross a = c. In
simulations where we have capped a such that waves are not permitted
to refract after reaching the a = c height, halos are non-existent,
which suggests that the power enhancement is wholly dependent on
returning fast waves. We also reproduce some of the observed halo
properties, such as a dual 6 and 8 mHz enhancement structure and a
spatial spreading of the halo with height.
---------------------------------------------------------
Title: Time-Distance Seismology and the Solar Transition Region
Authors: Hansen, Shelley C.; Cally, Paul S.
2014SoPh..289.4425H Altcode: 2014SoPh..tmp..128H
Time-Distance `travel time' perturbations (as inferred from wave
phase) are calculated relative to the quiet-Sun as a function of wave
orientation and field inclination in a uniform inclined magnetic
field. Modelling indicates that the chromosphere-corona Transition
Region (TR) profoundly alters travel times at inclinations from the
vertical θ for which the ramp-reduced acoustic cutoff frequency
ω<SUB>c</SUB>cosθ is similar to the wave frequency ω. At smaller
inclinations phase shifts are much smaller as the waves are largely
reflected before reaching the TR. At larger inclinations, the shifts
resume their quiet-Sun values, although with some resonant oscillatory
behaviour. Changing the height of the TR in the model atmosphere has
some effect, but the thickness and temperature jump do not change
the results substantially. There is a strong correspondence between
travel-time shifts and the Alfvén flux that emerges at the top of the
modelled region as a result of fast/Alfvén mode conversion. We confirm
that the TR transmission coefficient for Alfvén waves generated by
mode conversion in the chromosphere is far larger (typically 30 %
or more) than for Alfvén waves injected from the photosphere.
---------------------------------------------------------
Title: The Scattering of f- and p-modes from Ensembles of Thin
Magnetic Flux Tubes: An Analytical Approach
Authors: Hanson, Chris S.; Cally, Paul S.
2014ApJ...791..129H Altcode: 2014arXiv1407.0780H
Motivated by the observational results of Braun, we extend the model
of Hanson & Cally to address the effect of multiple scattering of
f and p modes by an ensemble of thin vertical magnetic flux tubes in
the surface layers of the Sun. As in the observational Hankel analysis,
we measure the scatter and phase shift from an incident cylindrical wave
in a coordinate system roughly centered in the core of the ensemble. It
is demonstrated that although thin flux tubes are unable to interact
with high-order fluting modes individually, they can indirectly absorb
energy from these waves through the scatters of kink and sausage
components. It is also shown how the distribution of absorption and
phase shift across the azimuthal order m depends strongly on the tube
position as well as on the individual tube characteristics. This is
the first analytical study into an ensembles multiple-scattering regime
that is embedded within a stratified atmosphere.
---------------------------------------------------------
Title: An Analytical Approach to Scattering Regimes Between Thin
Magnetic Flux Tubes Within an Ensemble
Authors: Hanson, Chris S; Cally, Paul S
2014AAS...22411204H Altcode:
Motivated by the inability to directly observe the small scale structure
of solar magnetic features. We present an analytical method to model
the multiple scattering regime within ensembles of random thin magnetic
flux tubes, embedded in a stratified medium. Results demonstrate that
the near-field interactions play an important role in the resultant
scattered wave field. As such the ensemble no longer behaves as a
bunch of individual tubes, rather as a larger collective. It is also
shown that the scattering between azimuth orders (m) is as significant
as scattering between p-modes. We present a comparison between this
analytical model and observations, as well as recent numerical studies.
---------------------------------------------------------
Title: An Analytical Approach to Scattering between Two thin Magnetic
Flux Tubes in a Stratified Atmosphere
Authors: Hanson, Chris S.; Cally, Paul S.
2014ApJ...781..125H Altcode: 2014arXiv1401.0027H
We expand on recent studies to analytically model the behavior of
two thin flux tubes interacting through the near- and acoustic
far-field. The multiple scattering that occurs between the pair
alters the absorption and phase of the outgoing wave when compared
to non-interacting tubes. We have included both the sausage and kink
scatter produced by the pair. It is shown that the sausage mode's
contribution to the scattered wave field is significant, and plays an
equally important role in the multiple scattering regime. A disparity
between recent numerical results and analytical studies, in particular
the lack of symmetry between the two kink modes, is addressed. This
symmetry break is found to be caused by an incorrect solution for the
near-field modes.
---------------------------------------------------------
Title: Sensitivity of Helioseismic Travel Times to the Imposition
of a Lorentz Force Limiter in Computational Helioseismology
Authors: Moradi, Hamed; Cally, Paul S.
2014ApJ...782L..26M Altcode: 2014arXiv1401.5518M
The rapid exponential increase in the Alfvén wave speed with height
above the solar surface presents a serious challenge to physical
modeling of the effects of magnetic fields on solar oscillations,
as it introduces a significant Courant-Friedrichs-Lewy time-step
constraint for explicit numerical codes. A common approach adopted in
computational helioseismology, where long simulations in excess of
10 hr (hundreds of wave periods) are often required, is to cap the
Alfvén wave speed by artificially modifying the momentum equation
when the ratio between the Lorentz and hydrodynamic forces becomes
too large. However, recent studies have demonstrated that the Alfvén
wave speed plays a critical role in the MHD mode conversion process,
particularly in determining the reflection height of the upwardly
propagating helioseismic fast wave. Using numerical simulations of
helioseismic wave propagation in constant inclined (relative to the
vertical) magnetic fields we demonstrate that the imposition of such
artificial limiters significantly affects time-distance travel times
unless the Alfvén wave-speed cap is chosen comfortably in excess of
the horizontal phase speeds under investigation.
---------------------------------------------------------
Title: Helioseismic Implications of Mode Conversion
Authors: Moradi, H.; Cally, P. S.
2013ASPC..478..263M Altcode:
The Sun leaks waves through its active regions. The leakage of acoustic
waves into the atmosphere through these ‘magnetoacoustic portals’
is well known, but magnetic (fast) waves also enter the atmosphere
there. Fast waves ultimately reflect because of the increase in Alfvén
speed with height, but when they do so they can partially convert to
Alfvén waves. The weakened fast waves then re-enter the interior, to
rejoin the seismic p-mode field. But how has the Alfvénic loss they
suffered affected the seismology? We present results from simulations
that compare Alfvénic losses with travel-time shifts, and draw
general conclusions about the role of active region atmospheres in
local helioseismology.
---------------------------------------------------------
Title: Seismology of the wounded Sun
Authors: Cally, Paul S.; Moradi, Hamed
2013MNRAS.435.2589C Altcode: 2013MNRAS.tmp.2129C; 2013arXiv1308.1184C
Active regions are open wounds in the Sun's surface. Seismic
oscillations from the interior pass through them into the atmosphere,
changing their nature in the process to fast and slow magneto-acoustic
waves. The fast waves then partially reflect and partially mode convert
to upgoing and downgoing Alfvén waves. The reflected fast and downgoing
Alfvén waves then re-enter the interior through the active regions that
spawned them, infecting the surface seismology with signatures of the
atmosphere. Using numerical simulations of waves in uniform magnetic
fields, we calculate the upward acoustic and Alfvénic losses in the
atmosphere as functions of field inclination and wave orientation as
well as the time-distance `travel time' perturbations, and show that
they are related. Travel time perturbations relative to quiet Sun
can exceed 40 s in 1 kG magnetic field. It is concluded that active
region seismology is indeed significantly infected by waves leaving
and re-entering the interior through magnetic wounds, with differing
travel times depending on the orientation of the wave vector relative
to the magnetic field. This presages a new directional-time-distance
seismology.
---------------------------------------------------------
Title: Alfvén Waves in Simulations of Solar Photospheric Vortices
Authors: Shelyag, S.; Cally, P. S.; Reid, A.; Mathioudakis, M.
2013ApJ...776L...4S Altcode: 2013arXiv1309.2019S
Using advanced numerical magneto-hydrodynamic simulations of the
magnetized solar photosphere, including non-gray radiative transport and
a non-ideal equation of state, we analyze plasma motions in photospheric
magnetic vortices. We demonstrate that apparent vortex-like motions in
photospheric magnetic field concentrations do not exhibit "tornado"-like
behavior or a "bath-tub" effect. While at each time instance the
velocity field lines in the upper layers of the solar photosphere show
swirls, the test particles moving with the time-dependent velocity
field do not demonstrate such structures. Instead, they move in a
wave-like fashion with rapidly changing and oscillating velocity field,
determined mainly by magnetic tension in the magnetized intergranular
downflows. Using time-distance diagrams, we identify horizontal
motions in the magnetic flux tubes as torsional Alfvén perturbations
propagating along the nearly vertical magnetic field lines with local
Alfvén speed.
---------------------------------------------------------
Title: PREFACE: Eclipse on the Coral Sea: Cycle 24 Ascending
Authors: Cally, Paul; Erdélyi, Robert; Norton
2013JPhCS.440a1001C Altcode:
A total solar eclipse is the most spectacular and awe-inspiring
astronomical phenomenon most people will ever see in their
lifetimes. Even hardened solar scientists draw inspiration from it. The
eclipse with 2 minutes totality in the early morning of 14 November 2012
(local time) drew over 120 solar researchers (and untold thousands of
the general public) to the small and picturesque resort town of Palm
Cove just north of Cairns in tropical north Queensland, Australia,
and they were rewarded when the clouds parted just before totality
to reveal a stunning solar display. <P />Eclipse photograph <P />The
eclipse was also the catalyst for an unusually broad and exciting
conference held in Palm Cove over the week 12--16 November. Eclipse on
the Coral Sea: Cycle 24 Ascending served as GONG 2012, LWS/SDO-5, and
SOHO 27, indicating how widely it drew on the various sub-communities
within solar physics. Indeed, as we neared the end of the ascending
phase of the peculiar Solar Cycle 24, it was the perfect time to
bring the whole community together to discuss our Sun's errant recent
behaviour, especially as Cycle 24 is the first to be fully observed by
the Solar Dynamics Observatory (SDO). The whole-Sun perspective was a
driving theme of the conference, with the cycle probed from interior
(helioseismology), to atmosphere (the various lines observed by the
Atmospheric Imaging Assemble (AIA) aboard SDO, the several instruments
on Hinode, and other modern observatories), and beyond (CMEs etc). The
quality of the presentations was exceptional, and the many speakers
are to be commended for pitching their talks to the broad community
present. <P />These proceedings draw from the invited and contributed
oral presentations and the posters exhibited in Palm Cove. They give an
(incomplete) snapshot of the meeting, illustrating its broad vistas. The
published contributions are organized along the lines of the conference
sessions, as set out in the Contents, leading off with a provocative
view of Cycle 24 thus far from Sarbani Basu. Other invited papers
presented here include an appreciation of Hinode's view of solar
activity as the cycle rises by Toshifumi Shimizu; a first taxonomy of
magnetic tornadoes and chromospheric swirls by Sven Wedemeyer {\it et
al}; an analysis of Hinode/EIS observations of transient heating events;
a timely re-examination of solar dynamo theory by Paul Charbonneau;
an exciting teaser for the solar potential of the Murchison Widefield
Array now operating in Western Australia by Steven Tingay {\it et al};
an overview and critique of the state of nonlinear force-free magnetic
field extrapolation theory and practice by Mike Wheatland and Stuart
Gilchrist; and a masterful review of atmospheric MHD wave coupling to
the Sun's internal p-mode oscillations by Elena Khomenko and Irantzu
Calvo Santamaria. The many contributed papers published here are no less
exciting. <P />All papers have been refereed to a high standard. The
editors thank all the referees, drawn both from conference attendees
and the wider community, who have taken their tasks very seriously and
provided very detailed and helpful reports. Nearly all contributions
have been substantially improved by the process. We must also thank
our financial sponsors. Both the Global Oscillations Network Group
(GONG) and LWS/SDO were generous in their support, as were the School
of Mathematical Sciences and the Monash Centre for Astrophysics (MoCA)
at Monash University, Melbourne, and the Centre for Astronomy at James
Cook University, Townsville. The Local Organizing Committee and the
many students who assisted before and during the conference also deserve
high praise for facilitating such a memorable meeting. <P />Paul Cally,
Robert Erdélyi and Aimee Norton <P />Conference photograph
---------------------------------------------------------
Title: Modelling Fast-Alfvén Mode Conversion Using SPARC
Authors: Moradi, H.; Cally, P. S.
2013JPhCS.440a2047M Altcode: 2013arXiv1302.6301M
We successfully utilise the SPARC code to model fast-Alfvén mode
conversion in the region c<SUB>A</SUB> >> c<SUB>s</SUB> via 3-D
MHD numerical simulations of helioseismic waves within constant inclined
magnetic field configurations. This was achieved only after empirically
modifying the background density and gravitational stratifications in
the upper layers of our computational box, as opposed to imposing a
traditional Lorentz Force limiter, to ensure a manageable timestep. We
found that the latter approach inhibits the fast-Alfvén mode conversion
process by severely damping the magnetic flux above the surface.
---------------------------------------------------------
Title: MHD wave propagation in the solar network
Authors: Calvo Santamaria, I.; Khomenko, E.; Cally, P. S.; Collados, M.
2013hsa7.conf..806C Altcode:
Magneto-acoustic and Alfvénic waves are ubiquitous in solar coronal
loops, possibly being excited by photospheric motions. It is not
clear, though, how these waves get so high, having obstacles such as
the acoustic cut-off frequency, reflection and refraction of fast
MHD waves and also the strongly reflecting transition region. In
this contribution we report on 2D numerical modelling of waves in
magnetic arcade structures extending from photospheric layers through
the transition region to the corona. Waves in the arcade are excited
by sub-photospheric p-modes. We discuss the behaviour of waves, their
conversion and propagation properties and possible mechanisms allowing
their escape through the transition region.
---------------------------------------------------------
Title: Near- and Far-field Response to Compact Acoustic Sources in
Stratified Convection Zones
Authors: Cally, Paul S.
2013ApJ...768...35C Altcode: 2013arXiv1303.4027C
The role of the acoustic continuum associated with compact sources
in the Sun's interior wave field is explored for a simple polytropic
model. The continuum produces a near-field acoustic structure—the
so-called acoustic jacket—that cannot be represented by a
superposition of discrete normal modes. Particular attention is paid
to monochromatic point sources of various frequency and depth, and
to the surface velocity power that results, both in the discrete f-
and p-mode spectrum and in the continuum. It is shown that a major
effect of the continuum is to heal the surface wave field produced by
compact sources, and therefore to hide them from view. It is found that
the continuous spectrum is not a significant contributor to observable
inter-ridge seismic power.
---------------------------------------------------------
Title: Alfvén Reflection and Reverberation in the Solar Atmosphere
Authors: Cally, P. S.
2012SoPh..280...33C Altcode: 2012SoPh..tmp..157C; 2012arXiv1206.2114C
Magneto-atmospheres with Alfvén speed [a] that increases monotonically
with height are often used to model the solar atmosphere, at least out
to several solar radii. A common example involves a uniform vertical
or inclined magnetic field in an isothermal atmosphere, for which
the Alfvén speed is exponential. We address the issue of internal
reflection in such atmospheres, both for time-harmonic and for transient
waves. It is found that a mathematical boundary condition may be devised
that corresponds to perfect absorption at infinity, and, using this,
that many atmospheres where a(x) is analytic and unbounded present
no internal reflection of harmonic Alfvén waves. However, except
for certain special cases, such solutions are accompanied by a wake,
which may be thought of as a kind of reflection. For the initial-value
problem where a harmonic source is suddenly switched on (and optionally
off), there is also an associated transient that normally decays with
time as O(t^{-1}) or O(t^{-1}\ln t), depending on the phase of the
driver. Unlike the steady-state harmonic solutions, the transient
does reflect weakly. Alfvén waves in the solar corona driven by a
finite-duration train of p-modes are expected to leave such transients.
---------------------------------------------------------
Title: Web-based Comprehensive Data Archive of Seismically Active
Solar Flares
Authors: Besliu-Ionescu, Diana; Donea, Alina; Cally, Paul; Lindsey,
Charles
2012asst.book...31B Altcode:
Some solar flares can release acoustic transients into the solar
subsurface of the active regions that host them. Most of the acoustic
power in these transients propagates something like 10-30 Mm beneath
the photosphere before it is refracted back to the surface, where it
raises a significant disturbance. In the strongest of these "sunquakes",
the manifestation of this transient in helio-seismic movies is an
outwardly expanding surface ripple that becomes conspicuous about
20 minutes after the impulsive phase of the flare. These "sunquakes"
offer a powerful diagnostic of wave propagation in the active region
photosphere and of the structure and dynamics of the subphotosphere. We
will present here a detailed description of our comprehensive survey
of the SOHO-MDI database for acoustic signatures from flares and the
technique used in this process. The results of the survey are presented
in a database of seismic sources generated by X and M class solar flares
during 1996-2007. It is based on a table format showing the general
characteristics of the acoustically active flares, and the times of the
solar quakes (beginning, maximum and end). The database is linked to
a composite of images of the seismic sources in different wavelengths.
---------------------------------------------------------
Title: Benchmarking Fast-to-Alfvén Mode Conversion in a Cold MHD
Plasma. II. How to Get Alfvén Waves through the Solar Transition
Region
Authors: Hansen, Shelley C.; Cally, Paul S.
2012ApJ...751...31H Altcode: 2012arXiv1203.3822H
Alfvén waves may be difficult to excite at the photosphere due
to low-ionization fraction and suffer near-total reflection at the
transition region (TR). Yet they are ubiquitous in the corona and
heliosphere. To overcome these difficulties, we show that they may
instead be generated high in the chromosphere by conversion from
reflecting fast magnetohydrodynamic waves, and that Alfvénic TR
reflection is greatly reduced if the fast reflection point is within
a few scale heights of the TR. The influence of mode conversion on
the phase of the reflected fast wave is also explored. This phase
can potentially be misinterpreted as a travel speed perturbation with
implications for the practical seismic probing of active regions.
---------------------------------------------------------
Title: Numerical Simulations of Conversion to Alfvén Waves in
Sunspots
Authors: Khomenko, E.; Cally, P. S.
2012ApJ...746...68K Altcode: 2011arXiv1111.2851K
We study the conversion of fast magnetoacoustic waves to Alfvén waves
by means of 2.5D numerical simulations in a sunspot-like magnetic
configuration. A fast, essentially acoustic, wave of a given frequency
and wave number is generated below the surface and propagates upward
through the Alfvén/acoustic equipartition layer where it splits
into upgoing slow (acoustic) and fast (magnetic) waves. The fast wave
quickly reflects off the steep Alfvén speed gradient, but around and
above this reflection height it partially converts to Alfvén waves,
depending on the local relative inclinations of the background magnetic
field and the wavevector. To measure the efficiency of this conversion
to Alfvén waves we calculate acoustic and magnetic energy fluxes. The
particular amplitude and phase relations between the magnetic field
and velocity oscillations help us to demonstrate that the waves
produced are indeed Alfvén waves. We find that the conversion to
Alfvén waves is particularly important for strongly inclined fields
like those existing in sunspot penumbrae. Equally important is the
magnetic field orientation with respect to the vertical plane of
wave propagation, which we refer to as "field azimuth." For a field
azimuth less than 90° the generated Alfvén waves continue upward, but
above 90° downgoing Alfvén waves are preferentially produced. This
yields negative Alfvén energy flux for azimuths between 90° and
180°. Alfvén energy fluxes may be comparable to or exceed acoustic
fluxes, depending upon geometry, though computational exigencies limit
their magnitude in our simulations.
---------------------------------------------------------
Title: On the Dispersion and Scattering of Magnetohydrodynamic Waves
by Longitudinally Stratified Flux Tubes
Authors: Andries, J.; Cally, P. S.
2011ApJ...743..164A Altcode:
We provide a fairly general analytic theory for the dispersion and
scattering of magnetohydrodynamic waves by longitudinally stratified
flux tubes. The theory provides a common framework for, and synthesis
of, many previous studies of flux tube oscillations that were
carried out under various simplifying assumptions. The present theory
focuses on making only a minimal number of assumptions. As a result
it thus provides an analytical treatment of several generalizations of
existing tube oscillation models. The most important practical cases are
inclusion of plasma pressure and possibly buoyancy effects in models
of straight non-diverging tubes as applied in coronal seismology, and
relaxation of the "thin tube" approximation in oscillation models of
diverging tubes as applied both in the context of p-mode scattering
and coronal seismology. In particular, it illustrates the unifying
theoretical framework underlying both the description of waves scattered
by flux tubes and the dispersion of waves carried along flux tubes.
---------------------------------------------------------
Title: Mode conversion of radiatively damped magnetogravity waves
in the solar chromosphere
Authors: Newington, Marie E.; Cally, Paul S.
2011MNRAS.417.1162N Altcode: 2011arXiv1107.2208N; 2011MNRAS.tmp.1413N
Modelling of adiabatic gravity wave propagation in the solar atmosphere
showed that mode conversion to field guided acoustic waves or Alfvén
waves was possible in the presence of highly inclined magnetic
fields. This work aims to extend the previous adiabatic study,
exploring the consequences of radiative damping on the propagation
and mode conversion of gravity waves in the solar atmosphere. We
model gravity waves in a VAL-C atmosphere, subject to a uniform,
and arbitrarily orientated magnetic field, using the Newton cooling
approximation for radiatively damped propagation. The results indicate
that the mode conversion pathways identified in the adiabatic study
are maintained in the presence of damping. The wave energy fluxes are
highly sensitive to the form of the height dependence of the radiative
damping time. While simulations starting from 0.2 Mm result in modest
flux attenuation compared to the adiabatic results, short damping times
expected in the low photosphere effectively suppress gravity waves in
simulations starting at the base of the photosphere. It is difficult
to reconcile our results and observations of propagating gravity waves
with significant energy flux at photospheric heights unless they are
generated in situ.
---------------------------------------------------------
Title: Benchmarking Fast-to-Alfvén Mode Conversion in a Cold
Magnetohydrodynamic Plasma
Authors: Cally, Paul S.; Hansen, Shelley C.
2011ApJ...738..119C Altcode: 2011arXiv1105.5754C
Alfvén waves may be generated via mode conversion from fast
magnetoacoustic waves near their reflection level in the solar
atmosphere, with implications both for coronal oscillations and for
active region helioseismology. In active regions this reflection
typically occurs high enough that the Alfvén speed a greatly
exceeds the sound speed c, well above the a = c level where the
fast and slow modes interact. In order to focus on the fundamental
characteristics of fast/Alfvén conversion, stripped of unnecessary
detail, it is therefore useful to freeze out the slow mode by adopting
the gravitationally stratified cold magnetohydrodynamic model c →
0. This provides a benchmark for fast-to-Alfvén mode conversion in
more complex atmospheres. Assuming a uniform inclined magnetic field
and an exponential Alfvén speed profile with density scale height h,
the Alfvén conversion coefficient depends on three variables only:
the dimensionless transverse-to-the-stratification wavenumber κ = kh,
the magnetic field inclination from the stratification direction θ,
and the polarization angle phi of the wavevector relative to the plane
containing the stratification and magnetic field directions. We present
an extensive exploration of mode conversion in this parameter space
and conclude that near-total conversion to outward-propagating Alfvén
waves typically occurs for small θ and large phi (80°-90°), though
it is absent entirely when θ is exactly zero (vertical field). For
wavenumbers of helioseismic interest, the conversion region is broad
enough to encompass the whole chromosphere.
---------------------------------------------------------
Title: Analyses Of Three-dimensional Magnetohydrodynamic Instability
of Antisolar Latitudinal Differential Rotation in F, G, and K Stars
Authors: Dikpati, Mausumi; Cally, Paul S.
2011ApJ...739....4D Altcode: 2011arXiv1107.2434D
Motivated by observations that only a very few stars have been found
to have antisolar differential rotation, much weaker in amplitude than
that of the Sun, we analyze the stability of antisolar and solar-type
latitudinal differential rotations in the tachoclines of typical F,
G, and K stars. We employ two three-dimensional thin-shell models,
one for a Boussinesq but nonhydrostatic system and the other for
a hydrostatic but non-Boussinesq system. We find that, in general,
the combination of toroidal field band and differential rotation is
more unstable, and unstable for lower toroidal fields, for antisolar
than for solar-type differential rotation. In the antisolar case,
the instability is always found to weaken the differential rotation,
even if the primary energy source for the instability is the magnetic
field. This favors surface antisolar differential rotations in stars
being weaker than solar types, if the instability in the tachocline
is felt at the surface of the star. This is most likely to happen in F
stars, whose convection zones are much thinner than they are in G and
K stars. This effect could help explain why the antisolar differential
rotations that have been found are very weak compared with the rotation
of the Sun.
---------------------------------------------------------
Title: Resonant Absorption as Mode Conversion? II. Temporal Ray Bundle
Authors: Hanson, Chris S.; Cally, Paul S.
2011SoPh..269..105H Altcode: 2010arXiv1011.3808H; 2010SoPh..tmp..250H
A fast-wave pulse in a simple, cold, inhomogeneous MHD model plasma
is constructed by Fourier superposition over frequency of harmonic
waves that are singular at their respective Alfvén resonances. The
pulse partially reflects before reaching the resonance layer, but also
partially tunnels through to it to convert to an Alfvén wave. The exact
absorption/conversion coefficient for the pulse is shown to be given
precisely by a function of transverse wavenumber tabulated in Paper
I of this sequence, and to be independent of frequency and pulse width.
---------------------------------------------------------
Title: Alfvén waves are easy: mode conversion in magnetic regions
Authors: Cally, P. S.
2011ASInC...2..221C Altcode:
Alfvén waves are shown to be readily generated by mode conversion
from fast MHD waves reflecting off the steep atmospheric Alfvén speed
gradient in active region atmospheres. A simple analytic description
of this process in terms of an `interaction integral' indicates that
it is spread over many vertical scale heights, and indeed fills the
whole active region chromosphere for waves of moderate helioseismic
degree ℓ, even up to ℓ=1000 or more. This suggests that active
region chromospheres are Alfvén wave factories.
---------------------------------------------------------
Title: Numerical simulations of conversion to Alfvén waves in solar
active regions
Authors: Khomenko, E.; Cally, P. S.
2011JPhCS.271a2042K Altcode: 2010arXiv1009.4575K
We study the coupling of magneto-acoustic waves to Alvén waves using
2.5D numerical simulations. In our experiment, a fast magnetoacoustic
wave of a given frequency and wavenumber is generated below the
surface. The magnetic field in the domain is assumed homogeneous and
inclined. The efficiency of the conversion to Alfvén waves near the
layer of equal acoustic and Alfven speeds is measured calculating
their energy flux. The particular amplitude and phase relations
between the oscillations of magnetic field and velocity help us to
demonstrate that the waves produced after the transformation and
reaching upper atmosphere are indeed Alfvén waves. We find that
the conversion from fast magneto-acoustic waves to Alfvén waves is
particularly important for the inclination θ and azimuth phi angles of
the magnetic field between 55 and 65 degrees, with the maximum shifted
to larger inclinations for lower frequency waves. The maximum Alfvén
flux transmitted to the upper atmosphere is about 2-3 times lower than
the corresponding acoustic flux.
---------------------------------------------------------
Title: Solar physics research in Australia
Authors: Cally, P. S.; Wheatland, M. S.; Cairns, I. H.; Melrose, D. B.
2011ASInC...2..397C Altcode:
Australia has a small but world-class solar physics research community,
with strong international ties, working in areas of particular
strength defined by the research interests of individuals and small
groups. Most research occurs at the major universities, and a small
number of Ph.D. students are trained in the field each year. This paper
surveys Australia's current contribution to solar physics research,
and the prospects for future development of the field.
---------------------------------------------------------
Title: How to turn gravity waves into Alfvén waves and other
such tricks
Authors: Newington, Marie E.; Cally, Paul S.
2011JPhCS.271a2037N Altcode:
Recent observations of travelling gravity waves at the base of the
chromosphere suggest an interplay between gravity wave propagation and
magnetic field. Our aims are: to explain the observation that gravity
wave flux is suppressed in magnetic regions; to understand why we see
travelling waves instead of standing waves; and to see if gravity waves
can undergo mode conversion and couple to Alfvén waves in regions where
the plasma beta is of order unity. We model gravity waves in a VAL C
atmosphere, subject to a uniform magnetic field of various orientations,
considering both adiabatic and radiatively damped propagation. Results
indicate that in the presence of a magnetic field, the gravity wave can
propagate as a travelling wave, with the magnetic field orientation
playing a crucial role in determining the wave character. For
the majority of magnetic field orientations, the gravity wave is
reflected at low heights as a slow magneto-acoustic wave, explaining
the observation of reduced flux in magnetic regions. In a highly
inclined magnetic field, the gravity wave undergoes mode conversion
to either field guided acoustic waves or Alfvén waves. The primary
effect of incorporating radiative damping is a reduction in acoustic
and magnetic fluxes measured at the top of the integration region. By
demonstrating the mode conversion of gravity waves to Alfvén waves,
this work identifies a possible pathway for energy transport from the
solar surface to the upper atmosphere.
---------------------------------------------------------
Title: How surface magnetism affects helioseismic waves
Authors: Cally, Paul S.
2010HiA....15..349C Altcode:
It has been known for two decades that sunspots both absorb and advance
the phase of solar f and p-modes. More recently, Time-Distance and
other local helioseismic techniques have been used to probe active
regions by exploring phase shifts which are interpreted as travel-time
perturbations. Although absorption is an intrinsically magnetic effect,
phase shifts may be produced by both thermal and magnetic effects
(and of course flows, though these can be factored out by averaging
travel times in opposite directions). We will show how these two
effects alter wave phase, and conclude that phase shifts in umbrae
are predominantly thermal, whilst those in highly inclined field
characteristic of penumbrae are essentially magnetic. The two effects
are generally not additive.
---------------------------------------------------------
Title: Resonant Absorption as Mode Conversion?
Authors: Cally, P. S.; Andries, J.
2010SoPh..266...17C Altcode: 2010arXiv1007.1808C; 2010SoPh..tmp..151C
Resonant absorption and mode conversion are both extensively studied
mechanisms for wave "absorption" in solar magnetohydrodynamics
(MHD). But are they really distinct? We re-examine a well-known simple
resonant absorption model in a cold MHD plasma that places the resonance
inside an evanescent region. The normal mode solutions display the
standard singular resonant features. However, these same normal modes
may be used to construct a ray bundle which very clearly undergoes
mode conversion to an Alfvén wave with no singularities. We therefore
conclude that resonant absorption and mode conversion are in fact the
same thing, at least for this model problem. The prime distinguishing
characteristic that determines which of the two descriptions is
most natural in a given circumstance is whether the converted wave
can provide a net escape of energy from the conversion/absorption
region of physical space. If it cannot, it is forced to run away in
wavenumber space instead, thereby generating the arbitrarily small
scales in situ that we recognize as fundamental to resonant absorption
and phase mixing. On the other hand, if the converted wave takes net
energy away, singularities do not develop, though phase mixing may
still develop with distance as the wave recedes.
---------------------------------------------------------
Title: Seismic Discrimination of Thermal and Magnetic Anomalies in
Sunspot Umbrae
Authors: Lindsey, C.; Cally, P. S.; Rempel, M.
2010ApJ...719.1144L Altcode:
Efforts to model sunspots based on helioseismic signatures need to
discriminate between the effects of (1) a strong magnetic field that
introduces time-irreversible, vantage-dependent phase shifts, apparently
connected to fast- and slow-mode coupling and wave absorption and (2)
a thermal anomaly that includes cool gas extending an indefinite depth
beneath the photosphere. Helioseismic observations of sunspots show
travel times considerably reduced with respect to equivalent quiet-Sun
signatures. Simulations by Moradi & Cally of waves skipping
across sunspots with photospheric magnetic fields of order 3 kG show
travel times that respond strongly to the magnetic field and relatively
weakly to the thermal anomaly by itself. We note that waves propagating
vertically in a vertical magnetic field are relatively insensitive to
the magnetic field, while remaining highly responsive to the attendant
thermal anomaly. Travel-time measurements for waves with large skip
distances into the centers of axially symmetric sunspots are therefore
a crucial resource for discrimination of the thermal anomaly beneath
sunspot umbrae from the magnetic anomaly. One-dimensional models of
sunspot umbrae based on compressible-radiative-magnetic-convective
simulations such as by Rempel et al. can be fashioned to fit
observed helioseismic travel-time spectra in the centers of sunspot
umbrae. These models are based on cooling of the upper 2-4 Mm of the
umbral subphotosphere with no significant anomaly beneath 4.5 Mm. The
travel-time reductions characteristic of these models are primarily
a consequence of a Wilson depression resulting from a strong downward
buoyancy of the cooled umbral medium.
---------------------------------------------------------
Title: Reflection and conversion of magnetogravity waves in the
solar chromosphere: windows to the upper atmosphere
Authors: Newington, Marie E.; Cally, Paul S.
2010MNRAS.402..386N Altcode: 2009arXiv0910.3233N; 2009MNRAS.tmp.1790N
The detection of upward propagating internal gravity waves at the base
of the Sun's chromosphere has recently been reported by Straus et al.,
who postulated that these may efficiently couple to Alfvén waves in
magnetic regions. This may be important in transporting energy to higher
levels. Here we explore the propagation, reflection and mode conversion
of linear gravity waves in a model atmosphere and find that even weak
magnetic fields usually reflect gravity waves back downwards as slow
magnetoacoustic waves well before they reach the Alfvén/acoustic
equipartition height at which mode conversion might occur. However,
for certain highly inclined magnetic field orientations in which the
gravity waves manage to penetrate near or through the equipartition
level, there can be substantial conversion to either or both up-going
Alfvén and acoustic waves. Wave-energy fluxes comparable to the
chromospheric radiative losses are expected.
---------------------------------------------------------
Title: Nonlinear Evolution of Axisymmetric Twisted Flux Tubes in
the Solar Tachocline
Authors: Hollerbach, R.; Cally, P. S.
2009SoPh..260..251H Altcode: 2009arXiv0909.3809H
We numerically study the evolution of magnetic fields and fluid flows in
a thin spherical shell. We take the initial field to be a latitudinally
confined, predominantly toroidal flux tube. For purely toroidal,
untwisted flux tubes, we recover previously known radial-shredding
instabilities, and show further that in the nonlinear regime these
instabilities can very effectively destroy the original field. For
twisted flux tubes, also including a poloidal component, there are
several possibilities, including the suppression of the radial-shredding
instability, but also a more directly induced evolution, brought about
because twisted flux tubes in general are not equilibrium solutions
of the governing equations.
---------------------------------------------------------
Title: Investigating The Role of Strong Magnetic Fields on
Helioseismic Wave Propagation and the Consequences for Time-Distance
Helioseismology
Authors: Moradi, H.; Hanasoge, S. M.; Cally, P. S.
2009ASPC..416..143M Altcode:
We investigate the direct contribution of strong, sunspot-like magnetic
fields to helioseismic wave travel-time shifts via two numerical
forward models, a 3D ideal MHD solver and MHD ray theory. We confirm
some existing ideas and bring forth new ones: (i) that the observed
travel-time shifts in the vicinity of sunspots are largely governed by
MHD physics, (ii) the travel-time shifts are sensitively dependent on
frequency and phase speed filter parameters and the background power
below the p_1 mode, and finally, (iii) despite its seeming limitations,
MHD ray theory appears to succeed in capturing the essence of the
travel-time variations as derived from the MHD simulations.
---------------------------------------------------------
Title: Seismology and the Dynamo: History and Prospects
Authors: Cally, P. S.
2009ASPC..416....3C Altcode:
The rapid "advances" in our knowledge and understanding of solar
and stellar dynamos that resulted from the development of mean-field
dynamo theory were sent into even more rapid retreat in the 1980s by
difficulties in explaining observed features of the solar cycle, and
especially by the new helioseismically derived differential rotation
profiles. The climb back out of the mire has been slow and uncertain. A
promising—but by no means universally accepted—route is afforded
by the resurrection of Babcock-Leighton (flux transport) models of the
dynamo, where meridional circulation plays a dominant "conveyor belt"
role, aided and abetted by other near-surface flow patterns which
disperse old flux poleward. In this brief overview, we review what
helioseismology has told us about meridional and near-surface flows,
and muse on the prospects for improved flow measurements in the future.
---------------------------------------------------------
Title: Axisymmetric MHD Instabilities in Solar/Stellar Tachoclines
Authors: Dikpati, M.; Cally, P. S.; Gilman, P. A.; Miesch, M. S.
2009ASPC..416..525D Altcode:
We show that banded toroidal fields in the tachoclines of the Sun and
other stars should be unstable to 3-D axisymmetric overturning modes if
the peak toroidal field is ∼100 kG or more. This instability should
fragment and limit the amplitude of toroidal fields in tachoclines.
---------------------------------------------------------
Title: Magnetic and thermal phase shifts in the local helioseismology
of sunspots
Authors: Cally, Paul S.
2009MNRAS.395.1309C Altcode: 2009arXiv0902.4727C; 2009MNRAS.tmp..493C
Phase perturbations due to inclined surface magnetic field of active
region strength are calculated numerically in quiet Sun and simple
sunspot models in order to estimate and compare the direct and
indirect (thermal) effects of the fields on helioseismic waves. It
is found that the largest direct effects occur in highly inclined
field characteristic of penumbrae, and scale roughly linearly with
magnetic field strength. The combined effects of sunspot magnetic and
thermal anomalies typically yield negative travel-time perturbations
in penumbrae. Travel-time shifts in umbrae depend on details of how the
thermal and density structure differs from the quiet Sun. The combined
shifts are generally not well approximated by the sum of the thermal
and magnetic effects applied separately, except at low field strengths
of around 1 kG or less, or if the thermal shift is small. A useful
rule-of-thumb appears to be that travel-time perturbations in umbrae are
predominantly thermal, whereas in penumbrae they are mostly magnetic.
---------------------------------------------------------
Title: Multiple Scattering of Waves by a Pair of Gravitationally
Stratified Flux Tubes
Authors: Hanasoge, Shravan M.; Cally, Paul S.
2009ApJ...697..651H Altcode: 2008arXiv0812.1284H
We study the near-field coupling of a pair of flux tubes embedded in
a gravitationally stratified environment. The mutual induction of the
near-field jackets of the two flux tubes can considerably alter the
scattering properties of the system, resulting in sizable changes in
the magnitudes of scattering coefficients and bizarre trends in the
phases. The dominant length scale governing the induction zone turns
out to be approximately half the horizontal wavelength of the incident
mode, a result that fits in quite pleasantly with extant theories
of scattering. Higher-β flux tubes are more strongly coupled than
weaker ones, a consequence of the greater role that the near-field
jacket modes play in such tubes. We also comment on the importance
of incorporating the effects of multiple scattering when studying the
effects of mode absorption in plage and interpreting related scattering
measurements. That the near field plays such an important role in
the scattering process lends encouragement to the eventual goal of
observationally resolving subwavelength features of flux tubes using
techniques of helioseismology.
---------------------------------------------------------
Title: Discriminating Thermal and Magnetic Seismic Anomalies in
Sunspot Umbrae
Authors: Lindsey, Charles A.; Cally, P. S.
2009SPD....40.0702L Altcode:
Efforts to model sunspots based on helioseismic signatures are
confronted by the need to discriminate between two significantly
separate anomalies: (1) a strong magnetic field that introduces
time-irreversible, vantage-dependent phase shifts apparently connected
to fast- and slow-mode coupling and wave absorption, and (2) a thermal
anomaly that includes cool gas extending an unknown depth beneath the
photosphere. Simulations by Moradi & Cally of waves skipping across
sunspots with photospheric magnetic fields of order 3 kG show travel
times that respond strongly to the magnetic field and relatively
weakly to the thermal anomaly by itself. We understand that waves
propagating vertically in a vertical magnetic field are insensitive
to the magnetic field and highly responsive to an attendant thermal
anomaly. We therefore recognize travel-time measurements for waves with
large skip distances into the centers of axially symmetric sunspots
as an important resource for discrimination of the thermal anomaly
beneath sunspot umbrae. Helioseismic observations of the response of
sunspot umbrae to low-degree waves impinging into them from beneath
their photospheres invariably show strongly reduced travel times,
the reduction increasing sharply with frequency. These profiles
agree nicely by 1-D simulations of the acoustics of a strong thermal
deficit in the upper few hundred km beneath the sunspot photosphere
with no significant anomaly below 2.2 Mm. This thermal structure is
characteristic of sunspot simulations by Rempel, Schuessler &
Knoelker. We understand the reduced travel times for these models
to be substantially the result of a 450-km Wilson depression caused
by the foregoing thermal deficit. According to this understanding,
the travel-time reduction due to the Wilson depression significantly
outweighs the effect of a reduced sound speed in the cool gas.
---------------------------------------------------------
Title: An Exact Test of Generalised Ray Theory in Local
Helioseismology
Authors: Hansen, Shelley C.; Cally, Paul S.
2009SoPh..255..193H Altcode:
Generalised ray theory provides a simple description of MHD mode
transmission and conversion between magnetoacoustic fast and slow waves
and is directly applicable to solar active regions. Here it is tested
in a simple two-dimensional, isothermal, gravitationally stratified
model with inclined magnetic field using previously published exact
solutions and found to perform very well.
---------------------------------------------------------
Title: Phase Jumps in Local Helioseismology
Authors: Cally, P. S.
2009SoPh..254..241C Altcode:
Helioseismic rays trapped in a nonmagnetic acoustic cavity suffer
a +90° phase jump at their lower (Lamb) turning point and −90°
at the upper (acoustic cutoff) reflection point. That the two cancel
allows helioseismologists to effectively assume that phase is locally
continuous along a ray path joining two surface points. However, in
strong surface magnetic field, as found in sunspots, it is shown - for
an isothermal model with uniform magnetic field - that the phase jump
for fast magnetoacoustic rays that penetrate the acoustic/Alfvénic
equipartition level (c=a) is around −120°. Moreover, there are
further negative phase jumps on the upgoing and downgoing legs at c=a
that add to the net phase change. Neglecting these effects can lead to
a misinterpretation of helioseismic data in terms of travel-time shifts.
---------------------------------------------------------
Title: An Exact Test of Generalized Ray Theory in Local
Helioseismology
Authors: Hansen, Shelley C.; Cally, Paul S.
2009arXiv0902.1581H Altcode:
Generalized Ray Theory (GRT) provides a simple description of MHD mode
transmission and conversion between magnetoacoustic fast and slow waves
and is directly applicable to solar active regions. Here it is tested
in a simple two-dimensional, isothermal, gravitationally-stratified
model with inclined magnetic field using previously published exact
solutions and found to perform very well.
---------------------------------------------------------
Title: Axisymmetric MHD Instabilities in Solar/Stellar Tachoclines
Authors: Dikpati, Mausumi; Gilman, Peter A.; Cally, Paul S.; Miesch,
Mark S.
2009ApJ...692.1421D Altcode:
Extensive studies over the past decade showed that HD and MHD
nonaxisymmetric instabilities exist in the solar tachocline for
a wide range of toroidal field profiles, amplitudes, and latitude
locations. Axisymmetric instabilities (m = 0) do not exist in two
dimensions, and are excited in quasi-three-dimensional shallow-water
systems only for very high field strengths (2 mG). We investigate here
MHD axisymmetric instabilities in a three-dimensional thin-shell model
of the solar/stellar tachocline, employing a hydrostatic, non-Boussinesq
system of equations. We deduce a number of general properties of the
instability by use of an integral theorem, as well as finding detailed
numerical solutions for unstable modes. Toroidal bands become unstable
to axisymmetric perturbations for solar-like field strengths (100
kG). The e-folding time can be months down to a few hours if the field
strength is 1 mG or higher, which might occur in the solar core, white
dwarfs, or neutron stars. These instabilities exist without rotation,
with rotation, and with differential rotation, although both rotation
and differential rotation have stabilizing effects. Broad toroidal
fields are stable. The instability for modes with m = 0 is driven from
the poleward shoulder of banded profiles by a perturbation magnetic
curvature stress that overcomes the stabilizing Coriolis force. The
nonaxisymmetric instability tips or deforms a band; with axisymmetric
instability, the fluid can roll in latitude and radius, and can convert
bands into tubes stacked in radius. The velocity produced by this
instability in the case of low-latitude bands crosses the equator,
and hence can provide a mechanism for interhemispheric coupling.
---------------------------------------------------------
Title: Numerical Models of Travel-Time Inhomogeneities in Sunspots
Authors: Moradi, H.; Hanasoge, S. M.; Cally, P. S.
2009ApJ...690L..72M Altcode: 2008arXiv0808.3628M
We investigate the direct contribution of strong, sunspot-like magnetic
fields to helioseismic wave travel-time shifts via two numerical forward
models, a three-dimensional ideal MHD solver and MHD ray theory. The
simulated data cubes are analyzed using the traditional time-distance
center-to-annulus measurement technique. We also isolate and analyze the
direct contribution from purely thermal perturbations to the observed
travel-time shifts, confirming some existing ideas and bringing forth
new ones: (i) that the observed travel-time shifts in the vicinity
of sunspots are largely governed by MHD physics, (ii) the travel-time
shifts are sensitively dependent on frequency and phase-speed filter
parameters and the background power below the p <SUB>1</SUB> ridge,
and finally, (iii) despite its seeming limitations, ray theory succeeds
in capturing the essence of the travel-time variations as derived from
the MHD simulations.
---------------------------------------------------------
Title: Three-dimensional magneto-shear instabilities in the solar
tachocline - II. Axisymmetric case
Authors: Cally, Paul S.; Dikpati, Mausumi; Gilman, Peter A.
2008MNRAS.391..891C Altcode: 2008MNRAS.tmp.1248C
A Boussinesq model of the development of non-axisymmetric (in particular
m = 1) three-dimensional magneto-shear instabilities in the solar
tachocline was presented in Paper I. However, there it was erroneously
concluded that the axisymmetric (m = 0) modes are stable, and they were
not discussed further. Here it is shown that, although m = 0 modes are
indeed stable for broad magnetic profiles, they are strongly unstable to
radial shredding (high radial wavenumber) instabilities on the poleward
shoulders of toroidal magnetic bands at high field strengths (roughly
40-100kG depending on bandwidth and latitude). These instabilities
have growth rates comparable to or greater than those for tipping
instabilities (m = 1) in many cases, but both are strongly stabilized
by gravitational stratification characteristic of the upper radiative
core. Weaker fields are m = 0 stable (though weakly m = 1 unstable),
even in neutral gravitational stratification (convection zone).
---------------------------------------------------------
Title: Helioseismic analysis of the solar flare-induced sunquake of
2005 January 15 - II. A magnetoseismic study
Authors: Martínez-Oliveros, J. C.; Donea, A. -C.; Cally, P. S.;
Moradi, H.
2008MNRAS.389.1905M Altcode: 2008arXiv0807.3783M; 2008MNRAS.tmp.1032M; 2008MNRAS.tmp..971M
On 2005 January 15, the active region AR10720 produced an X1.2 solar
flare that induced high levels of seismicity in the photospheric
layers. The seismic source was detected using helioseismic holography
and analysed in detail in Paper I. Egression power maps at 6 mHz, with a
2 mHz bandwidth, revealed a compact acoustic source, strongly correlated
with the footpoints of the coronal loop that hosted the flare. We
present a magnetosiesmic study of this active region to understand,
for the first time, the magnetic topological structure of a coronal
field that hosts an acoustically active solar flare. The accompanying
analysis attempts to answer questions such as: can the magnetic field
act as a barrier and prevent seismic waves from spreading away from the
focus of the sunquake? What is the most efficient magnetic structure
that would facilitate the development of a strong seismic source in
the photosphere?
---------------------------------------------------------
Title: Three-dimensional ray propagation in a toy sunspot
Authors: Moradi, H.; Cally, P. S.
2008JPhCS.118a2037M Altcode:
In time-distance helioseismology wave travel times are measured from
the cross-correlation between Doppler velocities recorded at any two
locations on the solar surface. However, one of the main uncertainties
associated with such measurements is how to interpret observations made
in regions of strong magnetic field. Isolating the effects of wave
anisotropies produced by the magnetic field from those thought to be
associated with temperature and flow perturbations has proved to be
quite complex and has yet to yield results when extracting acoustic
travel times from the cross-correlation function. One possible way to
decouple these effects is by using a three-dimensional toy sunspot with
a surrounding stratified field-free Model S atmosphere to model the
magneto-acoustic ray propagation and produce artificial travel time
perturbation maps that directly account for wave speed anisotropies
produced by the magnetic field.
---------------------------------------------------------
Title: Three-Dimensional MHD Wave Propagation and Conversion to
Alfvén Waves near the Solar Surface. I. Direct Numerical Solution
Authors: Cally, P. S.; Goossens, M.
2008SoPh..251..251C Altcode: 2007arXiv0711.0498C
The efficacy of fast - slow MHD mode conversion in the surface
layers of sunspots has been demonstrated over recent years
using a number of modelling techniques, including ray theory,
perturbation theory, differential eigensystem analysis, and direct
numerical simulation. These show that significant energy may be
transferred between the fast and slow modes in the neighbourhood
of the equipartition layer where the Alfvén and sound speeds
coincide. However, most of the models so far have been two
dimensional. In three dimensions the Alfvén wave may couple to the
magnetoacoustic waves with important implications for energy loss from
helioseismic modes and for oscillations in the atmosphere above the
spot. In this paper, we carry out a numerical "scattering experiment,"
placing an acoustic driver 4 Mm below the solar surface and monitoring
the acoustic and Alfvénic wave energy flux high in an isothermal
atmosphere placed above it. These calculations indeed show that energy
conversion to upward travelling Alfvén waves can be substantial,
in many cases exceeding loss to slow (acoustic) waves. Typically,
at penumbral magnetic field strengths, the strongest Alfvén fluxes
are produced when the field is inclined 30° - 40° from the vertical,
with the vertical plane of wave propagation offset from the vertical
plane containing field lines by some 60° - 80°.
---------------------------------------------------------
Title: Physical Properties of Wave Motion in Inclined Magnetic Fields
within Sunspot Penumbrae
Authors: Schunker, H.; Braun, D. C.; Lindsey, C.; Cally, P. S.
2008SoPh..251..341S Altcode: 2008arXiv0801.4448S; 2008SoPh..tmp...41S
At the surface of the Sun, acoustic waves appear to be affected by the
presence of strong magnetic fields in active regions. We explore the
possibility that the inclined magnetic field in sunspot penumbrae may
convert primarily vertically-propagating acoustic waves into elliptical
motion. We use helioseismic holography to measure the modulus and phase
of the correlation between incoming acoustic waves and the local surface
motion within two sunspots. These correlations are modeled by assuming
the surface motion to be elliptical, and we explore the properties
of the elliptical motion on the magnetic-field inclination. We also
demonstrate that the phase shift of the outward-propagating waves is
opposite to the phase shift of the inward-propagating waves in stronger,
more vertical fields, but similar to the inward phase shifts in weaker,
more-inclined fields.
---------------------------------------------------------
Title: A Comparison of the Acoustic Hardness of Acoustically Active
and Non-Active Solar Flares
Authors: Beşliu-Ionescu, Diana; Donea, Alina; Cally, Paul
2008AIPC.1043..252B Altcode:
Recent corrections to some of the GONG+intensity images of flares allow
us to image the acoustic power of white light flare signatures. The
images clearly show compact regions of white light power at 6 mHz,
which are well correlated spatially with the seismic signatures of the
flares, when the flare proved to be acoustically active. It has been a
puzzle why some of the white light flares, mainly very strong flares,
did not induced any seismic waves into the photosphere. We believe
that a comparison of the white light spectral hardness of two flares
(one seismically active and another one seismically quiet) is the clue
to understand the physics of the sun quakes.
---------------------------------------------------------
Title: Time - Distance Modelling in a Simulated Sunspot Atmosphere
Authors: Moradi, H.; Cally, P. S.
2008SoPh..251..309M Altcode: 2008SoPh..tmp...91M; 2008arXiv0804.2716M
In time - distance helioseismology, wave travel times are measured
from the cross-correlation between Doppler velocities recorded at
any two locations on the solar surface. However, one of the main
uncertainties associated with such measurements is how to interpret
observations made in regions of strong magnetic field. Isolating the
effects of the magnetic field from thermal or sound-speed perturbations
has proved to be quite complex and has yet to yield reliable results
when extracting travel times from the cross-correlation function. One
possible way to decouple these effects is by using a 3D sunspot model
based on observed surface magnetic-field profiles, with a surrounding
stratified, quiet-Sun atmosphere to model the magneto-acoustic ray
propagation, and analyse the resulting ray travel-time perturbations
that will directly account for wave-speed variations produced by the
magnetic field. These artificial travel-time perturbation profiles
provide us with several related but distinct observations: i) that
strong surface magnetic fields have a dual effect on helioseismic rays
- increasing their skip distance while at the same time speeding them
up considerably compared to their quiet-Sun counterparts, ii) there
is a clear and significant frequency dependence of both skip-distance
and travel-time perturbations across the simulated sunspot radius,
iii) the negative sign and magnitude of these perturbations appears
to be directly related to the sunspot magnetic-field strength and
inclination, iv) by "switching off" the magnetic field inside the
sunspot, we are able to completely isolate the thermal component
of the travel-time perturbations observed, which is seen to be both
opposite in sign and much smaller in magnitude than those measured
when the magnetic field is present. These results tend to suggest that
purely thermal perturbations are unlikely to be the main effect seen in
travel times through sunspots, and that strong, near-surface magnetic
fields may be directly and significantly altering the magnitude and
lateral extent of sound-speed inversions of sunspots made by time -
distance helioseismology.
---------------------------------------------------------
Title: Preface
Authors: Gizon, Laurent; Cally, Paul; Leibacher, John
2008SoPh..251....1G Altcode: 2008SoPh..tmp..148G
No abstract at ADS
---------------------------------------------------------
Title: HXR photospheric footprints
Authors: Martínez-Oliveros, J. C.; Donea, A. -C.; Cally, P. S.
2008IAUS..247..110M Altcode: 2007IAUS..247..110M
We have analysed the 6 mHz egression power signatures of some
accoustically active X-class solar flares. During the impulsive
phase these flares produced conspicuous seismic signatures which
have kernel-like structures, mostly aligned with the neutral line of
the host active region. The kernel-like structures show the effect
of constructive interference of the acoustic waves emanating from
the complex sources, suggesting motion of the acoustic sources. The
co-aligment between the seismic signatures and the hard X-ray emission
observed by RHESSI from the footpoints of the coronal loops suggests
a direct link between relativistic particles accelerated during the
flare and the hydrodynamic response of the photosphere during flares.
---------------------------------------------------------
Title: Correlative study of the emission from flares associated with
Sun quakes
Authors: Martínez-Oliveros, J. C.; Donea, A. -C.; Cally, P. S.
2008IAUS..247...99M Altcode: 2007IAUS..247...99M
Multi-wavelength studies of energetic solar flares with seismic
emissions have revealed interesting common features that may help us
to identify the correlations of flare signatures from the inner to
the outer solar atmosphere and, to develop diagnostic techniques to
aid in the sun quake detection. In our study, we make use the relation
between the microwave and the hard X-ray emissions associated with such
flares to propose a scenario for the ignition of seismic transients
from flares. We explore the mechanisms of energy transport to the
photosphere, such us back-warming or direct particle impacts.
---------------------------------------------------------
Title: Recent Developments in Solar Quakes Studies
Authors: Bešliu-Ionescu, D.; Donea, A. -C.; Cally, P.; Lindsey, C.
2008ASPC..383..297B Altcode:
Observations in hard and soft X-rays, chromospheric lines, and the
visible continuum, together with helioseismic observations, make
it possible to model the 3-dimensional profile of a sunquake from
the corona into the subphotosphere of the active region that hosts
the flare. Chromospheric line observations show us the part of the
solar atmosphere where high-energy electrons are thought to cause
thick-target heating that causes intense white-light emission and
drives seismic waves into the active region subphotosphere. We have
made a preliminary analysis of observations for some of the strongest
acoustically noisy flares, including spectral observations in line
NaD1 (586.9 nm) and line-center observations in Hα. Hα line-center
observations will be shown for other sunquakes in Solar Cycle 23. Hinode
will give us especially high-resolution chromospheric-line observations
of acoustically active flares in Solar Cycle 24.
---------------------------------------------------------
Title: Signatures of Seismic Absorption in Magnetic Regions
Authors: Lindsey, C.; Cally, P. S.
2008ASPC..383..141L Altcode:
One of the major developments in local helioseismology of the late
1980s was the discovery by Braun, Duvall, & LaBonte that magnetic
regions strongly absorb p modes that the quiet Sun itself efficiently
reflects. A second major development, in the mid 1990s with the advent
of the {Solar and Heliospheric Observatory}, was the discovery by
Duvall et al. that phase travel times for waves propagating into sunspot
photospheres are significantly longer than for waves propagating away
from them along identical paths, a phenomenon to which we refer in
this review as “the phase asymmetry.” Theoretical work by Cally et
al. proposes to explain absorption of p modes by coupling of p modes to
Alfvén modes. Recent work by Schunker et al. shows compelling evidence
that this coupling contributes strongly to the phase asymmetry. More
recent work by Rajaguru et al. suggests that radiative transfer effects
in magnetic photospheres with upward-propagating waves contribute
significantly to the phase asymmetry. Both of these contributions
depend on strong absorption of p modes in magnetic photospheres. We
will comment on the physics that relates phase shifts that underlie
phenomena such as the phase asymmetry to irreversible processes such
as p-mode absorption in magnetic regions. Magnetic contributions to
the phase asymmetry have significant implications respecting seismic
diagnostics of flows in active region subphotospheres.
---------------------------------------------------------
Title: From Gigahertz to Millihertz: A Multiwavelength Study of the
Acoustically Active 14 August 2004 M7.4 Solar Flare
Authors: Martínez-Oliveros, J. C.; Moradi, H.; Besliu-Ionescu, D.;
Donea, A. -C.; Cally, P. S.; Lindsey, C.
2007SoPh..245..121M Altcode: 2007arXiv0707.2019M
We carried out an electromagnetic acoustic analysis of the solar
flare of 14 August 2004 in active region AR10656 from the radio to the
hard X-ray spectrum. The flare was a GOES soft X-ray class M7.4 and
produced a detectable sun quake, confirming earlier inferences that
relatively low energy flares may be able to generate sun quakes. We
introduce the hypothesis that the seismicity of the active region is
closely related to the heights of coronal magnetic loops that conduct
high-energy particles from the flare. In the case of relatively short
magnetic loops, chromospheric evaporation populates the loop interior
with ionised gas relatively rapidly, expediting the scattering of
remaining trapped high-energy electrons into the magnetic loss cone and
their rapid precipitation into the chromosphere. This increases both
the intensity and suddenness of the chromospheric heating, satisfying
the basic conditions for an acoustic emission that penetrates into
the solar interior.
---------------------------------------------------------
Title: Chromospheric Line Emission Analysis of the July 16, 2004
Sun Quake
Authors: Beşliu-Ionescu, Diana; Donea, Alina; Cally, Paul; Lindsey,
Charles
2007AIPC..934...38B Altcode:
Observations in chromospheric lines and the visible continuum together
with photospheric helioseismic measurements make possible to image a
3-dimensional profile of a sun quake in a flaring region. Chromospheric
line observations show us the part of the solar atmosphere where
high-energy electrons are thought to cause thick target heating that
then could also cause intense white-light emission and could drive
seismic waves into the active region subphotosphere, we present here
the preliminary results of the sun quake of July 16, 2004.
---------------------------------------------------------
Title: Modified p-modes in penumbral filaments?
Authors: Bloomfield, D. S.; Solanki, S. K.; Lagg, A.; Borrero, J. M.;
Cally, P. S.
2007A&A...469.1155B Altcode: 2007arXiv0705.0481B
Aims:The primary objective of this study is to search for and identify
wave modes within a sunspot penumbra. <BR />Methods: Infrared
spectropolarimetric time series data are inverted using a model
comprising two atmospheric components in each spatial pixel. Fourier
phase difference analysis is performed on the line-of-sight velocities
retrieved from both components to determine time delays between the
velocity signals. In addition, the vertical separation between the
signals in the two components is calculated from the Stokes velocity
response functions. <BR />Results: The inversion yields two atmospheric
components, one permeated by a nearly horizontal magnetic field, the
other with a less-inclined magnetic field. Time delays between the
oscillations in the two components in the frequency range 2.5-4.5 mHz
are combined with speeds of atmospheric wave modes to determine wave
travel distances. These are compared to expected path lengths obtained
from response functions of the observed spectral lines in the different
atmospheric components. Fast-mode (i.e., modified p-mode) waves exhibit
the best agreement with the observations when propagating toward the
sunspot at an angle ~50° to the vertical.
---------------------------------------------------------
Title: Surface magnetic field effects in local helioseismology
Authors: Schunker, H.; Braun, D. C.; Cally, P. S.
2007AN....328..292S Altcode: 2010arXiv1002.2379S
Using helioseismic holography strong evidence is presented that
the phase (or equivalent travel-time) of helioseismic signatures in
Dopplergrams within sunspots depend upon the line-of-sight angle in
the plane containing the magnetic field and vertical directions. This
is shown for the velocity signal in the penumbrae of two sunspots
at 3, 4 and 5 mHz. Phase-sensitive holography demonstrates that they
are significantly affected in a strong, moderately inclined magnetic
field. This research indicates that the effects of the surface magnetic
field are potentially very significant for local helioseismic analysis
of active regions.
---------------------------------------------------------
Title: Magnetoseismic signatures and flow diagnostics beneath
magnetic regions
Authors: Lindsey, C.; Schunker, H.; Cally, P. S.
2007AN....328..298L Altcode:
% One of the major, important developments in local helioseismology was
the discovery by Duvall et al. (1996) that the travel times of seismic
waves into sunspots from the surrounding quiet Sun significantly exceed
the same in the reverse direction, a behavior they suggested was the
result of rapid downflows directly beneath the sunspot photosphere. This
led to the need for rapid near-surface horizontal inflows to replace the
mass evacuated from the sunspot subphotosphere by such downflows. The
lack of independent evidence for such inflows led to the suggestion
that the travel-time asymmetry could be explained by a relative phase
delay in the response of the sunspot photosphere to incoming waves with
respect to that of the quiet Sun. In the succeeding ten years major
progress has been made in our understanding of how magnetic photospheres
respond to incoming waves, at the instigation of theoretical work by
Spruit, Cally and Bogdan. This has led to the recognition of inclined
penumbral magnetic fields as a major avenue for control work on the
subject of the travel-time asymmetry and its relation to the absorption
of p-modes by magnetic regions. A major recent development has been
the discovery by Schunker et al. (2005) that the phase of this response
in Doppler observations of penumbral photospheres depends strongly on
the vantage of the Doppler measurements projected into the vertical
plane of the magnetic field. This discovery heavily reinforces the
proposition that the travel-time asymmetry is largely the signature of
the same irreversible damping processes that are responsible for the
strong absorption of p-modes in magnetic regions. We will elaborate
on the implications of the foregoing developments respecting the
diagnostics of subphotospheric flows based on seismic observations in
which magnetic regions cannot be avoided.
---------------------------------------------------------
Title: What to look for in the seismology of solar active regions
Authors: Cally, P. S.
2007AN....328..286C Altcode:
Using a newly developed extension of ray theory which accounts for
mode transmission and conversion between fast and slow magnetoacoustic
waves, as well as simple wave mechanical calculations, we find that
strong surface magnetic fields, as may be found in solar and stellar
active regions, have several related but distinct effects on seismic
waves: transmission/conversion, shortened travel times, a directional
filtering of acoustic waves entering the overlying atmosphere, and
a tendency to more closely align velocities with the field as height
increases in the atmosphere. Magnetic field inclination is particularly
relevant to these effects. Here, we briefly review these findings,
and present some new results on ray travel times and magnetic filtering.
---------------------------------------------------------
Title: Helioseismic analysis of the solar flare-induced sunquake of
2005 January 15
Authors: Moradi, H.; Donea, A. -C.; Lindsey, C.; Besliu-Ionescu, D.;
Cally, P. S.
2007MNRAS.374.1155M Altcode: 2007arXiv0704.3472M; 2006MNRAS.tmp.1369M
We report the discovery of one of the most powerful sunquakes
detected to date, produced by an X1.2-class solar flare in active
region AR10720 on 2005 January 15. We used helioseismic holography
to image the source of seismic waves emitted into the solar interior
from the site of the flare. Acoustic egression power maps at 3 and 6
mHz with a 2-mHz bandpass reveal a compact acoustic source strongly
correlated with impulsive hard X-ray and visible-continuum emission
along the penumbral neutral line separating the two major opposing
umbrae in the δ-configuration sunspot that predominates AR10720. At 6
mHz the seismic source has two components, an intense, compact kernel
located on the penumbral neutral line of the δ-configuration sunspot
that predominates AR10720, and a significantly more diffuse signature
distributed along the neutral line up to ~15 Mm east and ~30 Mm west
of the kernel. The acoustic emission signatures were directly aligned
with both hard X-ray and visible continuum emission that emanated
during the flare. The visible continuum emission is estimated at 2.0
× 10<SUP>23</SUP> J, approximately 500 times the seismic emission
of ~4 × 10<SUP>20</SUP> J. The flare of 2005 January 15 exhibits
the same close spatial alignment between the sources of the seismic
emission and impulsive visible continuum emission as previous flares,
reinforcing the hypothesis that the acoustic emission may be driven
by heating of the low photosphere. However, it is a major exception
in that there was no signature to indicate the inclusion of protons
in the particle beams thought to supply the energy radiated by the
flare. The continued strong coincidence between the sources of seismic
emission and impulsive visible continuum emission in the case of a
proton-deficient white-lightflare lends substantial support to the
`back-warming' hypothesis, that the low photosphere is significantly
heated by intense Balmer and Paschen continuum-edge radiation from
the overlying chromosphere in white-light flares.
---------------------------------------------------------
Title: Study of the Seismically Active Flare of July 16, 2004
Authors: Besliu-Ionescu, D.; Donea, A. C.; Cally, P.; Lindsey, C.
2007RoAJ...17S..83B Altcode:
Sunquakes have proven to be the most powerful events occurring at the
solar surface. They are triggered by the impulsive flares produced
in the corona, just above the acoustically active regions. Not
every impulsive flare produces seismic waves emanating from the
highly depressed photosphere, just beneath the flare. So far, we
have identified a few mechanisms which can deliver acoustic energy
into the photosphere: 1) the back-warming radiation suddenly heating
the photosphere; 2) a strong shock-like compression wave propagating
downwards into the chromosphere; 3) relativistic particles delivering
directly the energy and momentum into the photosphere; and, 4)
probably the magnetic tension at the feet of the loops. In order to
discriminate which of these is the most efficient or dominated during a
particular acoustically active flare, we have analysed the coronal and
chromospheric emission of the regions just above the seismic source. We
have performed a multiwavelength analysis of the active region 10649
that hosted the acoustically active solar flare of July 16, 2004. The
spatial coincidence between the emissions at different layers of the
sun, from the photosphere to the corona, suggests that high-energy
particles travel through the coronal layers from the reconnection
site, hit the solar chromosphere warming it up, which then, responds
by sending further into the photosphere enough energy (carried either
by the shock wave or by the Balmer and Pachen radiation) to produce
a seismic event.
---------------------------------------------------------
Title: Global MHD instabilities of the tachocline
Authors: Gilman, Peter A.; Cally, Paul S.
2007sota.conf..243G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Chromospheric line emission in seismically active flares
Authors: Beşliu-Ionescu, D.; Donea, A. -C.; Lindsey, C.; Cally, P.;
Mariş, G.
2007AdSpR..40.1921B Altcode:
Some flares are known to drive seismic transients into the solar
interior. The effects of these seismic transients are seen in
helioseismic observations of the Sun's surface thousands of km
from their sources in the hour succeeding the impulsive phase of
the flare. Energetic particles impinging from the corona into the
chromosphere are known to drive strong, downward-propagating shocks in
active region chromospheres during the impulsive phases of flares. Hα
observations have served as an important diagnostic of these shocks,
showing intense emission with characteristic transient redshifts. In
most flares no detectable transients penetrate beneath the active region
photosphere. In those that do, there is a strong correlation between
compact white-light emission and the signature of seismic emission. This
study introduces the first known Hα observations of acoustically active
flares, centered in the core of the line. The morphology of line-core
emission Hα in the impulsive phase of the flare is similar to that
of co-spatial line-core emission in NaD <SUB>1</SUB>, encompassing
the site of seismic emission but more extended. The latter shows a
compact red shift in the region of seismic emission, but a similar
feature is known to appear in a conjugate magnetic footpoint from
which no seismic emission emanates. Radiative MHD modelling based
on the profiles of chromospheric line emission during the impulsive
phases of flares can contribute significantly to our understanding of
the mechanics of flare acoustic emission penetrating into the solar
interior and the conditions under which it occurs.
---------------------------------------------------------
Title: New Detection of Acoustic Signatures from Solar Flares
Authors: Donea, A. C.; Besliu-Ionescu, D.; Cally, P.; Lindsey, C.
2006ASPC..354..204D Altcode:
With the advancement of local helioseismic techniques such as
helioseismic holography we have now detected numerous seismic sources
of varying size and intensity produced by solar flares. We have
performed a systematic survey of the SOHO-MDI database in search for
seismic waves from X-class flares produced during 1996 -- 2005. The
detection of acoustically active flares <P />has opened a new and
promising connection between helioseismology and flare physics. The
main question we ask is: why are some large flares acoustically active
while most are acoustically inactive? <P />We also address questions
such as: Is photospheric heating by high-energy protons a major factor
in seismic emission from flares? What is the effect of magnetic fields
in the acoustics of a flare?
---------------------------------------------------------
Title: Seismic Emission from A M9.5-Class Solar Flare
Authors: Donea, A. -C.; Besliu-Ionescu, D.; Cally, P. S.; Lindsey,
C.; Zharkova, V. V.
2006SoPh..239..113D Altcode: 2006SoPh..tmp...65D
Following the discovery of a few significant seismic sources at
6.0 mHz from the large solar flares of October 28 and 29, 2003, we
have extended SOHO/MDI helioseismic observations to moderate M-class
flares. We report the detection of seismic waves emitted from the β
γ δ active region NOAA 9608 on September 9, 2001. A quite impulsive
solar flare of type M9.5 occurred from 20:40 to 20:48 UT. We used
helioseismic holography to image seismic emission from this flare into
the solar interior and computed time series of egression power maps
in 2.0 mHz bands centered at 3.0 and 6.0 mHz. The 6.0 mHz images show
an acoustic source associated with the flare some 30 Mm across in the
East - West direction and 15 Mm in the North - South direction nestled
in the southern penumbra of the main sunspot of AR 9608. This coincides
closely with three white-light flare kernels that appear in the sunspot
penumbra. The close spatial correspondence between white-light and
acoustic emission adds considerable weight to the hypothesis that the
acoustic emission is driven by heating of the lower photosphere. This
is further supported by a rough hydromechanical model of an acoustic
transient driven by sudden heating of the low photosphere. Where direct
heating of the low photosphere by protons or high-energy electrons is
unrealistic, the strong association between the acoustic source and
co-spatial continuum emission can be regarded as evidence supporting
the back-warming hypothesis, in which the low photosphere is heated
by radiation from the overlying chromosphere. This is to say that a
seismic source coincident with strong, sudden radiative emission in the
visible continuum spectrum indicates a photosphere sufficiently heated
so as to contribute significantly to the continuum emission observed.
---------------------------------------------------------
Title: The Role of MHD Mode Conversion in Sunspot Seismology
Authors: Crouch, A. P.; Cally, P. S.; Charbonneau, P.; Braun, D. C.;
Desjardins, M.
2006ASPC..354..161C Altcode:
Sunspots absorb energy from and shift the phase of f and p modes
incident upon them. Understanding the mechanism causing each of these
effects is vital to the local helioseismology of sunspots (and magnetic
flux concentrations in general). Because the beta-equals-unity layer
typically lies in the near surface layers below the photospheres
of sunspot umbrae, MHD mode conversion can occur. Mode conversion
provides a promising absorption mechanism because the slow
magnetoacoustic-gravity waves and Alfvén waves guide energy along
the magnetic field away from the acoustic cavity. Our previous mode
conversion calculations have shown that simple sunspot models with
non-vertical magnetic fields can produce ample absorption to explain the
Hankel analysis measurements, along with phase shift predictions that
agree well with the observations. Those calculations only considered
the possibility of MHD waves propagating down the magnetic field into
the interior. In this contribution, we consider a second additional
possibility -- waves propagating up into the atmosphere overlying
sunspots.
---------------------------------------------------------
Title: Behaviour of Acoustic Waves in Sunspots
Authors: Schunker, H.; Braun, D. C.; Cally, P. S.; Lindsey, C.
2006ASPC..354..244S Altcode:
Because helioseismology uses surface signals to calculate subsurface
characteristics the behaviour of surface acoustic oscillations in
sunspots is important in interpreting helioseismic results. SOHO-MDI
Doppler velocity analysis of AR9026 and AR9033 at 3, 4, and 5 mHz,
using helioseismic holography, show that the amplitude and the phase
in the correlation of the acoustic ingression with the observed surface
velocity are found to be sensitive to the relative line-of-sight angle
in the penumbra of both sunspots. This is consistent with a conversion
of vertically propagating acoustic waves into magneto-acoustic waves
with motion described by ellipses.
---------------------------------------------------------
Title: Magnetohelioseismic Analysis of AR10720 Using Helioseismic
Holography
Authors: Moradi, H.; Donea, A.; Besliu-Ionescu, D.; Cally, P.; Lindsey,
C.; Leka, K.
2006ASPC..354..168M Altcode:
We report on the recent discovery of one of the most powerful sunquakes
detected to date produced by the January 15, 2005 X1.2 solar flare
in active region 10720. We used helioseismic holography to image the
acoustic source of the seismic waves produced by the flare. Egression
power maps at 6 mHz with a 2 mHz bandwidth reveal a strong, compact
acoustic source correlated with the footpoints of a coronal loop
that hosted the flare. Using data from various solar observatories,
we present a comprehensive analysis of the acoustic properties of
the sunquake and investigate the role played by the configuration of
the photospehric magnetic field in the production of flare generated
sunquakes.
---------------------------------------------------------
Title: Seismic emission from M-class solar flares
Authors: Besliu-Ionescu, D.; Donea, A. -C.; Cally, P.; Lindsey, C.
2006ESASP.624E..67B Altcode: 2006soho...18E..67B
No abstract at ADS
---------------------------------------------------------
Title: Magnetic field inclination and atmospheric oscillations above
solar active regions: theory
Authors: Cally, P. S.; Schunker, H.
2006ESASP.624E..64C Altcode: 2006soho...18E..64C
No abstract at ADS
---------------------------------------------------------
Title: Observed and simulated photospheric velocities within inclined
magnetic fields
Authors: Schunker, H.; Cally, P.
2006ESASP.624E...5S Altcode: 2006soho...18E...5S
No abstract at ADS
---------------------------------------------------------
Title: Magnetic field inclination and atmospheric oscillations above
solar active regions
Authors: Schunker, H.; Cally, P. S.
2006MNRAS.372..551S Altcode: 2006MNRAS.tmp.1003S
Recent observational evidence for magnetic field direction effects
on helioseismic signals in sunspot penumbrae is suggestive of
magnetohydrodynamic (MHD) mode conversion occurring at lower
levels. This possibility is explored using wave mechanical and ray
theory in a model of the Sun's surface layers permeated by uniform
inclined magnetic field. It is found that fast-to-slow conversion
near the equipartition depth at which the sound and Alfvén speeds
coincide can indeed greatly enhance the atmospheric acoustic signal
at heights observed by Solar and Heliospheric Observatory/Michelson
Doppler Imager and other helioseismic instruments, but that this effect
depends crucially on the wave attack angle, i.e. the angle between
the wavevector and the magnetic field at the conversion/transmission
depth. A major consequence of this insight is that the magnetic field
acts as a filter, preferentially allowing through acoustic signal from
a narrow range of incident directions. This is potentially testable
by observation.
---------------------------------------------------------
Title: The acoustically active solar flare of 2005 January 15
Authors: Moradi, H.; Donea, A. -C.; Lindsey, C.; Besliu-Ionescu, D.;
Cally, P. S.
2006ESASP.624E..66M Altcode: 2006soho...18E..66M
No abstract at ADS
---------------------------------------------------------
Title: The interaction between acoustic waves and inclined magnetic
fields near the β~1 layer
Authors: Schunker, H.; Braun, D. C.; Cally, P. S.; Lindsey, C.
2006IAUJD..17E..16S Altcode:
The acoustic showerglass effect may be hindering our helioseismic
renditions of the solar subsurface. We present the results of near
-surface wave conversion of acoustic waves in a model polytropic
atmosphere by a uniform, inclined magnetic field. The upcoming fast,
acoustic wave undergoes conversion to a slow, magnetic wave at the β
~ 1 layer where the sound speed and Alfven speed are comparable. The
conversion is dependent upon the " attack angle" between the ray path
and the magnetic field. The angle of the wave vectors at the polytropic
" surface" is compared to observations of surface velocity vectors in
sunspot penumbrae. AR9026 and AR9057 both have well- defined, static
penumbrae and their Doppler velocities are viewed from different
angles by SOHO-MDI as they cross the solar disk. The phase of the
correlation between the ingression and surface velocity, determined by
helioseismic holography, is used to assess the effect the penumbral
magnetic field has on incoming acoustic waves. The phase is found to
be dependent upon the line-of-sight of observation indicating that
this is a surface phenomenon, which could otherwise be interpreted as
subsurface travel-time perturbations of up to one minute. Furthermore,
using vector magnetograms from the IVM at the Mees Observatory, the
phase of the correlation is found to be dependent on the magnetic
field tilt from vertical, and the dependence is consistent across the
two sunspots. Comparing the results from the polytropic model with the
observations show strong support for the near-surface wave conversion
theory, although many questions still remain.
---------------------------------------------------------
Title: Dispersion relations, rays and ray splitting in
magnetohelioseismology
Authors: Cally, P. S.
2006RSPTA.364..333C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Seismic Radiation from M-class Solar Flares
Authors: Besliu-Ionescu, Diana; Donea, Alina-C.; Cally, Paul; Lindsey,
Charles
2006IAUS..233..385B Altcode:
Helioseismic holography is a technique used to image the sources
of seismic disturbances observed at the solar surface. It has been
used to detect acoustic emission, known as sun quakes, radiated from
X-class solar flares. Since the seismic power emitted by the X-class
flares has proved to be independent of the strength of the flare,
we have undertaking a systematic search for seismic signatures from
M-class solar flares, observed by SOHO-MDI.We have detected significant
acoustic emission from a few M-class solar flares. Preliminary results
of the survey of M-type solar flares studied so far is available at:
aira.astro.ro/~deanna/M.html.
---------------------------------------------------------
Title: Note on the Initial Value Problem for Coronal Loop Kink Waves
Authors: Cally, P. S.
2006SoPh..233...79C Altcode:
Simple fluxtube models of coronal loops have previously been found
to support leaky oscillations, where energy escapes from the tube
laterally, thereby damping the mode. Of particular interest is the
Principal Fast Leaky Kink mode (PFLK), which may be implicated in
the decay of loop oscillations observed by TRACE. However, recently,
M.S. Ruderman and B. Roberts, J. Plarma Phys. (in press), claimed
that the PFLK mode, and many other leaky modes of oscillation, are
unphysical, based on an initial value calculation. In this note,
their arguments are shown to be incorrect.
---------------------------------------------------------
Title: Significant Acoustic Activity in AR10720 on January 15, 2005
Authors: Beşliu-Ionescu, D.; Donea, A. -C.; Cally, P.; Lindsey, C.
2006RoAJ...16S.203B Altcode: 2006RoAJS..16..203B
We report the recent discovery of one of the most acoustically powerful
flare detected to date produced by the January 2005 2005, X1.2 solar
flare in AR10720. We used helioseismic holography to image the acoustic
source of the seismic waves produced by the flare. Egression Power maps
at 6 mHz show a strong, extended acoustic signature which is the focus
of the solar quake. At approximately 20 minutes after the appearance
of the flare signature, we could also see the seismic response of
the photosphere to the energy deposited by the flare in the form of
"ripples" on the solar surface.
---------------------------------------------------------
Title: a Survey of X-Class Solar Flares during 2001 and 2002 IN
Search for Seismic Radiation
Authors: Besliu-Ionescu, D.; Donea, A. -C.; Cally, P.; Lindsey, C.
2005ESASP.600E.111B Altcode: 2005ESPM...11..111B; 2005dysu.confE.111B
No abstract at ADS
---------------------------------------------------------
Title: Genetic magnetohelioseismology with Hankel analysis data
Authors: Crouch, A. D.; Cally, P. S.; Charbonneau, P.; Braun, D. C.;
Desjardins, M.
2005MNRAS.363.1188C Altcode: 2005MNRAS.tmp..856C
Hankel analysis determined that sunspots absorb energy from and
shift the phase of f- and p-modes incident upon them. One promising
mechanism that can explain the absorption is partial conversion
to slow magnetoacoustic-gravity (MAG) waves and Alfvén waves,
which guide energy along the magnetic field away from the acoustic
cavity. Our recent mode conversion calculations demonstrated that
simple sunspot models, which roughly account for the radial variation
of the magnetic field strength and inclination, can produce ample
absorption to explain the observations, along with phase shifts that
agree remarkably well with the Hankel analysis data. In this paper,
we follow the same approach, but adopt a more realistic model for the
solar convection zone that includes the thermal perturbation associated
with a sunspot's magnetic field. Consistent with our earlier findings,
we show that a moderately inclined, uniform magnetic field exhibits
significantly enhanced absorption (mode conversion) in comparison
to a vertical field (depending on the frequency and radial order of
the mode). A genetic algorithm is employed to adjust the parameters
that control the radial structure of our sunspot models, in order
to minimize the discrepancy between the theoretical predictions and
the Hankel analysis measurements. For models that best fit the phase
shifts, the agreement with the Hankel analysis data is excellent, and
the corresponding absorption coefficients are generally in excess of the
observed levels. On the other hand, for models that best fit the phase
shift and absorption data simultaneously, the overall agreement is very
good but the phase shifts agree less well. This is most likely caused
by the different sizes of the regions responsible for the absorption and
phase shift. Typically, the field strengths required by such models lie
in the range 1-3kG, compatible with observations for sunspots and active
regions. While there remain some uncertainties, our results provide
further evidence that mode conversion is the predominant mechanism
responsible for the observed absorption in sunspots; and that field
inclination away from vertical is a necessary ingredient for any model
that aims to simultaneously explain the phase shift and absorption data.
---------------------------------------------------------
Title: Modelling the Interaction of p-modes With Sunspots
Authors: Crouch, A. D.; Cally, P. S.; Charbonneau, P.; Desjardins, M.
2005AGUSMSP23C..04C Altcode:
Sunspots absorb energy from and shift the phase of f- and p-modes
incident upon them. One promising absorption mechanism is partial
conversion to slow magnetoacoustic-gravity waves (and Alfvén waves),
which guide energy along the magnetic field away from the acoustic
cavity. Recent mode conversion calculations by Cally, Crouch, and
Braun have shown that simple sunspot models with non-vertical magnetic
fields can produce ample absorption to explain the observations,
along with phase shift predictions that agree remarkably well with the
Hankel analysis data. In this investigation, we further test the mode
conversion hypothesis. We use a realistic solar model that accounts for
both the magnetic and thermal influences associated with a sunspot. Our
model has several adjustable parameters - the field strength and
inclination can vary (crudely) across the spot. We employ a genetic
algorithm to adjust these parameters to optimize the agreement between
the model predictions and the observations. At this stage, our model
is too simple to perform quantitative forward modelling. However,
the genetic algorithm allows us to rigorously test the model. We
will discuss the results of this testing in detail. Broadly speaking,
our findings are consistent with those of Cally, Crouch, and Braun:
the predicted phase shifts are in excellent agreement with the
Hankel analysis data, and the corresponding absorption coefficients
are generally ample to explain the observations. While there remain
several uncertainties, our results further verify that mode conversion
is a significant process in sunspot acoustics.
---------------------------------------------------------
Title: Towards Assessing, Understanding, and Correcting the Influence
of Surface Magnetism in Local Helioseismology
Authors: Braun, D. C.; Schunker, H.; Lindsey, C.; Cally, P. S.
2005AGUSMSP23C..03B Altcode:
Efforts to probe subsurface wave-speed variations and mass flows
near and under solar active regions are complicated and potentially
compromised by strong phase and amplitude perturbations introduced in
the photosphere by magnetic fields. Recently it has been shown that
the phase distortions correlate with surface magnetic field strength
and may be corrected to image wave-speed variations underlying active
regions. A strong phase asymmetry between waves arriving into and
departing from a magnetic region is also shown to produce spurious
signatures of horizontal outflows below active regions. Further
evidence of the photospheric origin of these phase distortions, as
well as a key to their physical origin, is also presented. Using MDI
observations, from the SOHO spacecraft, we perform ingression control
correlations in the inclined magnetic fields of sunspot penumbra and
demonstrate that incoming acoustic waves produce photospheric motion
that describes an ellipse tilted towards the inclination of the magnetic
field. A consequence is that the phase of the correlation varies with
the viewing angle with respect to the field direction. Observations
of the vector components of the photospheric fields could be used to
correct these phase perturbations analogous to the procedures already
developed using line-of-sight magnetograms. A physical understanding
of surface distortions will come through MHD modeling, including
simulations of the interaction of acoustic and magneto-acoustic-gravity
waves with prescribed magnetic and sound-speed perturbations and flows
(artificial data). The development of appropriate tools for assessing
and correcting the effects of the surface magnetism will be vital for
the interpretation of helioseismic data from the upcoming HMI mission
on SDO. DCB and CL gratefully acknowledge support from the NSF (SAA/AST)
and NASA (LWS, SR&T).
---------------------------------------------------------
Title: Local magnetohelioseismology of active regions
Authors: Cally, P. S.
2005MNRAS.358..353C Altcode: 2005MNRAS.tmp..156C
Solar active regions are distinguished by their strong magnetic
fields. Modern local helioseismology seeks to probe them by observing
waves which emerge at the solar surface having passed through their
interiors. We address the question of how an acoustic wave from
below is partially converted to magnetic waves as it passes through
a vertical magnetic field layer where the sound and Alfvén speeds
coincide (the equipartition level), and find that (i) there is no
associated reflection at this depth, either acoustic or magnetic,
only transmission and conversion to an ongoing magnetic wave; and (ii)
conversion in active regions is likely to be strong, though not total,
at frequencies typically used in local helioseismology, with lower
frequencies less strongly converted. A simple analytical formula is
presented for the acoustic-to-magnetic conversion coefficient.
---------------------------------------------------------
Title: The Local Helioseismology of Inclined Magnetic Fields and
the Showerglass Effect
Authors: Schunker, H.; Braun, D. C.; Cally, P. S.; Lindsey, C.
2005ApJ...621L.149S Altcode:
We present evidence for the dependence of helioseismic Doppler
signatures in active regions on the line-of-sight angle in inclined
magnetic fields. Using data from the Michelson Doppler Imager
(MDI) on board the Solar and Heliospheric Observatory, we performed
phase-sensitive holography in the penumbrae of sunspots over the
course of several days as the spots traversed the solar disk. Control
correlations, which comprise a correlation of the surface wave amplitude
with the incoming acoustic wave amplitude from a surrounding region,
were mapped. There is a direct dependence of control-correlation phase
signatures on the line-of-sight angle in the plane defined by the
vertical and magnetic field vectors. The phase shift of waves observed
along directions close to the orientation of the magnetic field is
smaller than the phase shift observed when the line of sight is at a
significant angle with respect to the field orientation. These findings
have important implications for local helioseismology. The variation
in phase shift (or the equivalent acoustic travel-time perturbations)
with line-of-sight direction suggests that a substantial portion of the
phase shift occurs in the photospheric magnetic field. Observations of
the vector components of the field may be used to develop a proxy to
correct these phase perturbations (known as the acoustic showerglass)
that introduce uncertainties in the signatures of acoustic perturbations
below the surface.
---------------------------------------------------------
Title: Mode Conversion of Solar p-Modes in Non-Vertical Magnetic
Fields
Authors: Crouch, A. D.; Cally, P. S.
2005SoPh..227....1C Altcode:
Sunspots absorb and scatter incident f- and p-modes. Until
recently, the responsible absorption mechanism was uncertain. The
most promising explanation appears to be mode conversion to slow
magnetoacoustic-gravity waves, which carry energy down the magnetic
field lines into the interior. In vertical magnetic field, mode
conversion can adequately explain the observed f-mode absorption,
but is too inefficient to account for the absorption of p-modes. In
the first paper of the present series we calculated the efficiency
of fast-to-slow magnetoacoustic-gravity wave conversion in uniform
non-vertical magnetic fields. We assumed two-dimensional propagation,
where the Alfvén waves decouple. In comparison to vertical field, it
was found that mode conversion is significantly enhanced in moderately
inclined fields, especially at higher frequencies. Using those results,
Cally, Crouch, and Braun showed that the resultant p-mode absorption
produced by simple sunspot models with non-vertical magnetic fields is
ample to explain the observations. In this paper, we further examine
mode conversion in non-vertical magnetic fields. In particular,
we consider three-dimensional propagation, where the fast and slow
magnetoacoustic-gravity waves and the Alfvén waves are coupled. Broadly
speaking, the p-mode damping rates are not substantially different to
the two-dimensional case. However, we do find that the Alfvén waves
can remove similar quantities of energy to the slow MAG waves.
---------------------------------------------------------
Title: Simultaneous Magnetic Field Time Series in AR10486 and AR10488
During the Period October 29--31, 2003
Authors: Beşliu, Diana; Donea, Alina-Catalina; Cally, Paul; Maris,
Georgeta
2005RoAJ...15...33B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observations and Interpretation of Subsurface Magnetic
Structures
Authors: Cally, P. S.; Crouch, A. D.
2005HiA....13..435C Altcode:
Immediate subsurface local structures - flow magnetic and thermal -
especially those associated with active regions are of considerable
interest and are potentially accessible using high l helioseismology. In
recent years though classical modal seismology has been supplemented
with new approaches such as time-distance helioseismology and acoustic
holography. In this talk the various methods and their results
are briefly reviewed and implications discussed. Of the structure
types subsurface magnetic fields are the most difficult to identify
observationally so an overview is also given of how solar oscillations
are expected to interact with them.
---------------------------------------------------------
Title: Local Helioseismology of Inclined Magnetic Fields and the
Showerglass Effect
Authors: Schunker, H.; Braun, D. C.; Lindsey, C.; Cally, P. S.
2004ESASP.559..227S Altcode: 2004soho...14..227S
No abstract at ADS
---------------------------------------------------------
Title: What can P-Mode Absorption and Phase Shift Data Currently Tell
us about the Subsurface Structure of Sunspots? Preliminary Results
Authors: Crouch, A. D.; Cally, P. S.
2004ESASP.559..392C Altcode: 2004soho...14..392C
No abstract at ADS
---------------------------------------------------------
Title: Acoustics of Surface Magnetic Fields
Authors: Cally, P. S.
2004ESASP.559..213C Altcode: 2004soho...14..213C
No abstract at ADS
---------------------------------------------------------
Title: Linear Analysis and Nonlinear Evolution of Two-Dimensional
Global Magnetohydrodynamic Instabilities in a Diffusive Tachocline
Authors: Dikpati, Mausumi; Cally, Paul S.; Gilman, Peter A.
2004ApJ...610..597D Altcode:
We develop a more realistic two-dimensional model for global MHD
instabilities in the solar tachocline, by including diffusion in
the form of kinetic and magnetic drag (following Newton's cooling
law formulation). This instability has previously been studied by us
and others for an idealized tachocline with no kinematic viscosity
and magnetic diffusivity. Since radial diffusion is more important
than latitudinal diffusion in the thin solar tachocline, diffusive
decay of flow and magnetic fields can be considered as proportional
to those variables. We find that, for solar-like toroidal magnetic
fields of ~100 kG, instability exists for a wide range of kinetic
and magnetic drag parameters, providing a mechanism for enhanced
angular momentum transport in latitudes, which could explain how thin
the solar tachocline is. From a detailed parameter space survey,
we set upper limits of 5×10<SUP>11</SUP> and 3×10<SUP>10</SUP>
cm<SUP>2</SUP> s<SUP>-1</SUP> for kinematic viscosity and magnetic
diffusivity, respectively, such that this instability occurs in the
solar tachocline on a timescale shorter than a sunspot cycle. We
find that magnetic drag has much more influence than kinetic drag
in damping this instability. This happens because the sink due to
magnetic drag dissipates perturbation magnetic energy faster than
the vorticity sink from kinetic drag dissipates perturbation kinetic
energy. Consequently, in the presence of a large enough magnetic drag,
the nonsolar-like clamshell pattern, found by Cally to be an inevitable
final state of a broad profile undergoing an ideal MHD tachocline
instability, is suppressed, whereas a banded profile still tips with
no reduction in tip angle. We discuss how tipping may affect various
surface manifestations of magnetic features, such as the latitudes
and orientations of bipolar active regions.
---------------------------------------------------------
Title: Absorption of Waves in Sunspots
Authors: Cally, P. S.
2004ESASP.547...15C Altcode: 2004soho...13...15C
Sunspots absorb and scatter the sun's global modes, the fand
p-modes. Initial hopes were that this would allow us to probe the
subsurface structure of spots, as helioseismology has been probing
the sun on a global scale. However, it has turned out to be more
difficult than first imagined. At the INTAS Workshop on MHD Waves in
Mallorca in 2001, I explained the supposed mechanism, coupling to slow
magnetoacoustic waves, but gave a rather gloomy picture of its ability
to quantitatively match observations. However, recent advances in
modelling in CSPA at Monash have for the first time produced results
which seem to explain much of the data. In this talk, a simple model
is presented, and its predictions compared with the best available
observational Hankel data. Discrepancies concerning near-surface
wave speeds between Hankel and holographic analyses on the one
hand and time-distance inversions on the other are discussed. It
is suggested that helioseismic inversions of active regions should
address magneto-acoustic mode coupling if they are to fully account
for absorption and wave speed variations.
---------------------------------------------------------
Title: The Solar Tachocline: Limiting Magneto-Tipping Instabilities
Authors: Cally, P. S.; Dikpati, M.; Gilman, P. A.
2004IAUS..219..541C Altcode: 2003IAUS..219E.172C
Two dimensional magneto-shear instabilities in the solar tachocline
have been extensively explored in recent years. One of their most
notable traits over a wide range of shear and magnetic profiles is
a propensity for the magnetic field to tip substantially from its
initial axisymmetric configuration with possible implications for
patterns of flux emergence. However it is found that modifications
of the standard models to include either kinetic and magnetic drag
or prograde toroidal velocity jets associated with magnetic bands
can suppress the instabilities or considerably reduce their nonlinear
development. In the case of tip reduction by jets for a toroidal field
of ~100kG in the tachocline (required for sunspots to emerge in sunspot
latitudes) simulations indicate that jets capable of reducing tipping
below the limits of detection from sunspot patterns at the surface are
potentially detectable by helioseismic methods and should be looked
for. Establishing an upper limit to the jet may result in a lower
limit for the amount of tipping to be expected.
---------------------------------------------------------
Title: Mode Conversion of Solar p-Modes In Non-Vertical Magnetic
Fields
Authors: Crouch, A. D.; Cally, P. S.
2004ESASP.547...81C Altcode: 2004soho...13...81C
Sunspots absorb and scatter incident - and -modes. One suggested
absorption mechanism is mode conversion to slow magnetoacoustic-gravity
waves, which carry some of the energy down magnetic field lines
into the interior, or, less preferentially, up into the overlying
atmosphere. Assuming uniform vertical magnetic field, this mechanism
easily explains -mode absorption, but cannot fully account for the
observed absorption of -modes, especially beyond the first few radial
orders. Recently, we have calculated the efficiency of mode conversion
in uniform non-vertical magnetic fields assuming two dimensional
propagation, where the Alfvén waves decouple. In moderately inclined
field ( ) at higher frequencies ( ), we found significant absorption
enhancements - up to an order of magnitude. Using these two dimensional
calculations, Cally, Crouch, and Braun constructed a simplified model
for the interaction between a sunspot and its surroundings. They found
excellent agreement with the observational -mode absorption and phase
shift data. In this investigation, we determine the efficiency of
mode conversion in non-vertical magnetic field with three dimensional
propagation, where fast and slow magnetoacoustic-gravity waves and
Alfvén waves are all coupled.
---------------------------------------------------------
Title: Probing sunspot magnetic fields with p-mode absorption and
phase shift data
Authors: Cally, P. S.; Crouch, A. D.; Braun, D. C.
2003MNRAS.346..381C Altcode:
Long-standing observations of incoming and outgoing f- and p-modes
in annuli around sunspots reveal that the spots partially absorb
and substantially shift the phase of waves incident upon them. The
commonly favoured absorption mechanism is partial conversion to
slow magneto-acoustic waves that disappear into the solar interior
channelled by the magnetic field of the sunspot. However, up until
now, only f-mode absorption could be accounted for quantitatively by
this means. Based on vertical magnetic field models, the absorption of
p-modes was insufficient. In this paper, we use the new calculations
of Crouch & Cally for inclined fields, and a simplified model of
the interaction between spot interior and exterior. We find excellent
agreement with phase shift data assuming field angles from the
vertical in excess of 30° and Alfvén/acoustic equipartition depths
of around 600-800 km. The absorption of f-modes produced by such
models is considerably larger than is observed, but consistent with
numerical simulations. On the other hand, p-mode absorption is generally
consistent with observed values, up to some moderate frequency dependent
on radial order. Thereafter, it is too large, assuming absorbing regions
comparable in size to the inferred phase-shifting region. The excess
absorption produced by the models is in stark contrast with previous
calculations based on a vertical magnetic field, and is probably due
to finite mode lifetimes and excess emission in acoustic glories. The
excellent agreement of phase shift predictions with observational
data allows some degree of probing of subsurface field strengths, and
opens up the possibility of more accurate inversions using improved
models. Most importantly, though, we have confirmed that slow mode
conversion is a viable, and indeed the likely, cause of the observed
absorption and phase shifts.
---------------------------------------------------------
Title: A comparison between model calculations and observations of
sunspot oscillations
Authors: Rüedi, I.; Cally, P. S.
2003A&A...410.1023R Altcode:
We investigate the signal which is expected to be produced by magnetic
field oscillations in sunspots umbrae due to the combination of the
oscillation model, radiative transfer and observing procedure. For
this purpose we investigate the signal expected to be produced by
theoretical models of sunspot oscillations and compare them with the
signal seen in observed power spectra of sunspot magnetograms. We show
that the amplitudes of the observed oscillations are compatible with
the predictions of the theoretical model of magnetoacoustic oscillations
for the 5-min as well as for the 3-min band. For the particular sunspot
umbral oscillation models used, our analysis suggests that most of
the expected observed power in the magnetogram signal oscillations is
actually due to cross-talk from the temperature and density oscillations
associated with the magnetoacoustic wave. A detailed modelling of
the observing procedure turns out to be of central importance for the
assignment of the observed oscillations to a specific wave type.
---------------------------------------------------------
Title: Coronal Leaky Tube Waves and Oscillations Observed with Trace
Authors: Cally, P. S.
2003SoPh..217...95C Altcode:
Leaky tube waves are examined in the context of kink oscillations
in coronal loops, observed in recent years using TRACE. It
is pointed out that the standard (non-leaky) principal
kink mode has a leaky bifurcated counterpart with decay time
τ<SUB>ℓ</SUB>≈4π<SUP>−4</SUP>(L/R)<SUP>2</SUP>P, where
R and L are the loop radius and length, and P is the oscillation
period. This is somewhat too long to explain the observed decays,
except for very short or thick loops, but may be implicated in the
initial excitation. Higher harmonics decay much more rapidly. The
external solution takes the form of a wave running nearly parallel to
the tube, but with a small outward component. In addition, a number of
other leaky modes are described which decay on timescales of seconds,
τ<SUB>ℓ</SUB>=Ra<SUB>e</SUB>/a<SUP>2</SUP>, where a and a<SUB>e</SUB>
are the loop and external Alfvén speeds respectively, and which can
be identified as being almost radially propagating fast magnetoacoustic
waves. These are outside the currently observable range, but are likely
to be important energetically.
---------------------------------------------------------
Title: Mode Conversion of Solar p Modes in non-Vertical Magnetic
Fields - i. two-Dimensional Model
Authors: Crouch, A. D.; Cally, P. S.
2003SoPh..214..201C Altcode:
Sunspots absorb incident p modes. The responsible mechanism
is uncertain. One possibility is mode conversion to slow
magnetoacoustic-gravity waves. In vertical field mode conversion
can adequately explain the observed f-mode absorption, but is too
inefficient to explain the absorption of p modes. In this investigation
we calculate the efficiency of fast-to-slow magnetoacoustic-gravity wave
conversion in non-vertical field. We assume two-dimensional propagation
where the Alfvén waves decouple. It is found that resultant p-mode
absorption is significantly enhanced for moderate inclinations at
higher frequencies, whereas for p modes at lower frequencies, and
the f mode in general, there is no useful enhancement. However, the
enhancement is insufficient to explain the observed p-mode absorption
by sunspots. Paper II considers the efficiency of mode conversion in
non-vertical field with three-dimensional propagation, where fast and
slow magnetoacoustic-gravity waves and Alfvén waves are coupled.
---------------------------------------------------------
Title: The Search for a Tipped Toroidal Field
Authors: Norton, A. A.; Gilman, P. A.; Henney, C. J.; Cally, P. S.
2003SPD....34.1903N Altcode: 2003BAAS...35..842N
A magnetic tipping instability of the tachocline toroidal field has
been predicted (Cally et al., 2003) that could produce a toroidal
field tipped with respect to the equatorial axis. One result of a
tipped toroidal band is the eruption of magnetic flux over a range of
latitudes from the same toroidal ring. The longitudinal dependence
of this flux emergence would contribute to non-axisymmetry of the
whole flux emergence pattern by giving it a longitudinal wavenumber
m=1 dependence. We search for evidence (or lack thereof) of a tipped
toroidal field for some phases of the solar cycle by examining the
dominant latitude of emerging flux as a function of longitude. We use
the existing observational data from Kitt Peak synoptic Carrington
Rotation magnetograms to identify the location of strong flux,
independently in each hemisphere, and test whether the location of
the flux reveals a pattern compatible with the tipping instability m=1.
---------------------------------------------------------
Title: Three-dimensional magneto-shear instabilities in the solar
tachocline
Authors: Cally, P. S.
2003MNRAS.339..957C Altcode:
The solar tachocline straddles the base of the convection zone. In the
radiative part, global scale latitudinal magneto-shear instabilities
have been thought to be constrained to two dimensions by strongly
stable stratification, and so two-dimensional (2D) instabilities have
been examined in great detail recently. However, it is shown here that
three-dimensional (3D) effects can be important. We generalize the
linear 2D analysis to 3D in the Boussinesq thin layer approximation
for models where the magnetic field is wrapped toroidally around the
Sun, and the equilibrium field and flow are independent of depth. It
is found that a very rapid `polar kink instability' dominates the
dynamics of broad magnetic field distributions if the polar Alfvén
angular speed α<SUB>p</SUB> exceeds the rotational angular velocity
ω<SUB>p</SUB>, with maximum growth rate (α<SUP>2</SUP><SUB>p</SUB>-
ω<SUP>2</SUP><SUB>p</SUB>)<SUP>1/2</SUP>. This might typically
produce an e-folding time as short as a few months. Interestingly,
the instability only affects the m= 1`tipping modes', twisting
polar loops towards a vertical orientation. On the other hand,
for α<SUP>2</SUP><SUB>p</SUB> < ω<SUP>2</SUP><SUB>p</SUB>,
3D instabilities are restricted to radial length scales in which
perhaps just a few wavelengths could fit across the tachocline. These
could supplement, or even dominate, the shallow-water modes examined
recently by Gilman and Dikpati. An analysis of the role of a large
Brunt-Väisälä frequency, as found in the radiative part of the
tachocline, suggests that its main effect is to flatten the motions in
the instabilities rather than to suppress them. Strong banded magnetic
profiles are found to be susceptible to an instability similar to but
distinct from the polar kink.
---------------------------------------------------------
Title: Clamshell and Tipping Instabilities in a Two-dimensional
Magnetohydrodynamic Tachocline
Authors: Cally, Paul S.; Dikpati, Mausumi; Gilman, Peter A.
2003ApJ...582.1190C Altcode:
Building on Cally's nonlinear model of two-dimensional MHD tachocline
instability, we further explore the evolution of a wide variety of
toroidal field profiles due to this instability. Cally showed in a
recent study that an initially broad toroidal field opens up into a
“clamshell” pattern because of nonlinear evolution of MHD tachocline
instability. Various other toroidal field profiles-single toroidal
bands, double bands, and mixed profiles with a band in addition to
broad profiles-may also occur in the Sun during various phases of the
solar cycle. Detailed study of the evolution of banded profiles shows
no occurrence of clamshell instability, but the bands commonly tip
relative to the axis of rotation. The higher the latitude location of
the band, the more it tips. Extreme tipping results when the band is
at 60° latitude or higher-the magnetic ring hangs from the pole on one
side of the Sun. For bands of 10° latitude width at sunspot latitudes
(<=40°), the band tip is within +/-10° about the mean latitude
of the band. This tipping could either enhance or reduce the observed
tilt in bipolar active regions. Double bands, or profiles consisting
of a band and a broad profile, may exist at certain phases of the
solar cycle. We find that double-band systems with two oppositely
directed bands separated widely (>15°) in latitude, as well
as two close bands of same polarities, do not interact in the same
hemisphere-the higher latitude band tips, while the lower latitude
band hardly responds. A significant interaction between two individual
bands in one hemisphere takes place only when the band separation is
<=15° and the bands are oppositely directed, which is a nonsolar
case. In this case, the band system either tips or forms the clamshell
pattern depending on the dominant mode symmetry. We also show that
a mixed profile with oppositely directed narrow fields close to the
equator in addition to the broad fields evolve in such a way as to
oppose the reconnection of the broad fields across the equator, and
thus inhibiting the clamshell formation, at least at certain phases
of the solar cycle. Finally, we note that the tipping and clamshell
instabilities strongly inhibit differential rotation.
---------------------------------------------------------
Title: Interaction of Solar p-modes with Magnetic Field
Authors: Cally, P. S.
2003ASPC..305..152C Altcode: 2003mfob.conf..152C
No abstract at ADS
---------------------------------------------------------
Title: Observations and Interpretation of Sub-Surface Structures
Authors: Cally, Paul
2003IAUJD..12E..43C Altcode:
Immediate subsurface local structures - flow magnetic and thermal -
especially those associated with active regions are of considerable
interest and are potentially accessible using high l helioseismology. In
recent years though classical modal seismology has been supplemented
with new approaches such as time-distance helioseismology and acoustic
holography. In this talk the various methods and their results
are briefly reviewed and implications discussed. Of the structure
types subsurface magnetic fields are the most difficult to identify
observationally so an overview is also given of how solar oscillations
are expected to interact with them.
---------------------------------------------------------
Title: Numerical Solutions of Three-Dimensional Pressure-Bounded
Magnetohydrostatic Flux Tubes
Authors: Hennig, B. S.; Cally, P. S.
2001SoPh..201..289H Altcode:
We present a three-dimensional technique for the solution of the
magnetohydrostatic equations when we are modeling structures bounded by
a current sheet that is free to move to satisfy pressure balance. The
magnetic field is expressed in terms of Euler potentials and the
equations are transformed to flux coordinates, greatly simplifying the
problem of locating the free boundary. Multi-grid techniques are used
to rapidly solve the resulting nonlinear elliptic partial differential
equations. The method is tested against Low's (1982) exact solution
of a bipolar plasma loop. It is shown that fast, accurate solutions
can be found.
---------------------------------------------------------
Title: Nonlinear Evolution of 2d Tachocline Instabilities
Authors: Cally, P. S.
2001SoPh..199..231C Altcode:
A spectral method is used to explore the nonlinear evolution
of known linear instabilities in a 2D differentially rotating
magneto-hydrodynamic shell, representing the solar tachocline. Several
simulations are presented, with a range of outcomes for the magnetic
field configuration. Most spectacularly, the `clam instability', which
occurs for solar differential rotation and a strong broad toroidal
magnetic field structure, results in the field tipping over by 90°
and reconnecting. A common characteristic of all the simulations though
is that the nonlinear instabilities produce a strong angular momentum
mixing effect which pushes the rotation towards a solid body form. It
is argued that this may be the mechanism required by the model of
Spiegel and Zahn to limit the tachocline's thickness.
---------------------------------------------------------
Title: Note on an Exact Solution for Magnetoatmospheric Waves
Authors: Cally, P. S.
2001ApJ...548..473C Altcode:
Solutions for magnetoatmospheric waves in an isothermal plane
stratified atmosphere with uniform vertical magnetic field have
long been known in terms of Meijer G-functions. It is pointed out
that they may alternatively be expressed using the more familiar
hypergeometric <SUB>2</SUB>F<SUB>3</SUB> functions, with significant
advantages for ease of use and physical interpretation. The nature
of these solutions in different regions of the frequency-wavenumber
plane is fully discussed, with particular reference to reflection,
transmission, and mode conversion. Reflection, transmission, and mode
conversion coefficients for slow and fast waves incident from below,
including the effects of tunnelling, are calculated exactly. The exact
solutions are useful in interpreting observational results and numerical
simulations of more complex magnetoatmospheric waves.
---------------------------------------------------------
Title: An Observational Manifestation of Magnetoatmospheric Waves
in Internetwork Regions of the Chromosphere and Transition Region
Authors: McIntosh, S. W.; Bogdan, T. J.; Cally, P. S.; Carlsson, M.;
Hansteen, V. H.; Judge, P. G.; Lites, B. W.; Peter, H.; Rosenthal,
C. S.; Tarbell, T. D.
2001ApJ...548L.237M Altcode:
We discuss an observational signature of magnetoatmospheric waves in
the chromosphere and transition region away from network magnetic
fields. We demonstrate that when the observed quantity, line or
continuum emission, is formed under high-β conditions, where β is
the ratio of the plasma and magnetic pressures, we see fluctuations in
intensity and line-of-sight (LOS) Doppler velocity consistent with the
passage of the magnetoatmospheric waves. Conversely, if the observations
form under low-β conditions, the intensity fluctuation is suppressed,
but we retain the LOS Doppler velocity fluctuations. We speculate that
mode conversion in the β~1 region is responsible for this change in
the observational manifestation of the magnetoatmospheric waves.
---------------------------------------------------------
Title: Frequency Dependent Ray Paths in Local Helioseismology
Authors: Barnes, G.; Cally, P. S.
2001PASA...18..243B Altcode:
The surface of the Sun is continually oscillating due to sound waves
encroaching on it from the interior. Measurements of the surface
velocity are used to infer some of the properties of the regions
through which the sound waves have propagated. Traditionally,
this has been done by using a modal decomposition of the surface
disturbances. However, the use of ray descriptions, in the form
of acoustic holography or time-distance helioseismology, provides
an alternative approach which may reveal more detailed information
about the properties of local phenomena such as sunspots and active
regions. Fundamental to any such treatment is determining the correct
ray paths in a given atmosphere. In the simplest approach, the ray
paths are constructed to minimise the travel time between two points
(Fermat's principle). However, such an approach is only valid in the
high frequency limit, ω≫ω<SUP>c</SUP>, N, where ω<SUP>c</SUP>
is the acoustic cut-off and N the Brunt-Väisälä frequency. Although
ω<SUP>c</SUP> is often included in time-distance calculations, and N
occasionally, the same is not true of acoustic holography. We argue
that this raises concerns about image sharpness. As illustrations,
representative ray paths are integrated in a realistic solar model to
show that the Fermat approximation performs poorly for frequencies of
helioseismic interest. We also briefly discuss the importance of the
Brunt-Väisälä frequency to the time-distance diagram.
---------------------------------------------------------
Title: A Sufficient Condition for Instability in a Sheared
Incompressible Magnetofluid
Authors: Cally, P. S.
2000SoPh..194..189C Altcode:
It is shown that a sufficient condition for the stability of an
incompressible sheared gravitationally stratified ideal magnetofluid
with flow-aligned horizontal magnetic field is that there exists a
Galilean frame in which the flow is nowhere super-Alfvénic (similarly,
stability is assured in a compressible shear flow without gravity
if there exists a frame in which the flow nowhere exceeds the cusp
speed). Complex eigenvalue bounds are presented for unstable flows. The
stability condition is applied to the solar tachocline; it suggests
that any shear instabilities associated with radial gradients in flow
speed should be stabilized by fields of above about 7 kG.
---------------------------------------------------------
Title: Mode Mixing by a Shallow Sunspot
Authors: Barnes, G.; Cally, P. S.
2000SoPh..193..373B Altcode:
Sunspots are strong absorbers of f and p modes. A possible
absorption mechanism is direct conversion to slow magnetoacoustic
waves. Calculations based on vertical magnetic field models show that
this works well for f modes, but is inadequate for p modes. Using a very
simple `shallow spot' model, in which the effects of the magnetic field
are accounted for solely by a surface condition, we investigate the
possibility that p modes first scatter into f modes inside the spot,
which are then more susceptible to conversion to slow modes. We find
that the coupling between an incident p mode and the internal f mode
is unlikely to be strong enough to account for the observed absorption,
but that the incident modes do couple strongly to the acoustic jacket in
some cases, leading to a region immediately around the sunspot where a
significant fraction of the surface velocity is due to the jacket modes.
---------------------------------------------------------
Title: Modelling p-Mode Interaction with a Spreading Sunspot Field
Authors: Cally, P. S.
2000SoPh..192..395C Altcode:
Sunspots absorb and scatter incident p modes. The dominant mechanism
is still uncertain. One possibility, mode conversion to slow
magneto-acoustic waves, has been shown to yield results in agreement
with observations for the f mode only. Absorption of p modes in simple
vertical magnetic field models is too weak by an order of magnitude or
more. Here we report on numerical calculations of p modes encountering
a simple sunspot model with field which spreads with height. It is
found that p-mode absorption is greatly enhanced by field spread, to
a level consistent with observations, and it appears that it occurs
preferentially in the outer regions of the spot, in line with recent
results from acoustic holography.
---------------------------------------------------------
Title: The Contribution by Thin Magnetic Flux Tubes to p-Mode
Line Widths
Authors: Crouch, A. D.; Cally, P. S.
1999ApJ...521..878C Altcode:
The contribution to p-mode line widths from the excitation of tube mode
oscillations on an individual magnetic fibril is computed. An idealized
model of the fibril within the photosphere is implemented, consisting
of a vertical, thin magnetic flux tube embedded in a plane-parallel
isentropic polytrope of index m. Bogdan et al. considered a similar
model but imposed a stress-free boundary condition at the top of
the photosphere, which acts to reflect any upward-propagating tube
waves completely back down into the tube. The stress-free boundary
condition neglects a possibly important physical process: the loss of
energy to the upper solar atmosphere by the excitation of waves in the
chromosphere and corona. Using simple models of the solar chromosphere
and corona, we explore the consequences of applying various boundary
conditions. The resultant upward energy fluxes are not large, but
surprisingly the more realistic upper boundary conditions lead to a
significant increase in kink mode flux out the bottom. Nevertheless, the
sausage mode remains dominant in cases of interest and is essentially
unaffected by the new boundary conditions. Consequently, the resultant
total p-mode line width computed here can account for only a few
percent of the observed line width.
---------------------------------------------------------
Title: Interaction of P-Modes with Sunspots
Authors: Bogdan, T. J.; Barnes, G. K.; Cally, P. S.; Crouch, A. D.
1999AAS...194.5607B Altcode: 1999BAAS...31R.912B
We report on our ongoing efforts to model the interaction of
the solar acoustic oscillations with solar surface magnetic flux
concentrations. The simulation code employs a finite difference
discretization of the linearized MHD wave equations written in
conservative form. A staggered grid is used to ensure strict numerical
conservation, and the time-stepping is based on a Lax-Wendroff-type
two-step method that yields negligible numerical diffusion. Analysis
of the results from these computations indicates that a significant
fraction of the incident acoustic wave flux is converted into MHD waves
which propagate along the magnetic lines of force. The efficiency
of this coupling increases as the magnetic flux concentration is
endowed with a more pronounced penumbra, wherein the magnetic field
is highly inclined with respect to the local surface gravity. Intense
small-scale fluid motions accompany this enhancement, particularly in
the super-penumbral canopy that surrounds the flux concentration. Such
a wave-coupling process provides an excellent qualitative explanation
of the observed absorption of solar p-modes by sunspots, and is in
basic accord with the excess in the penumbral Doppler signal relative
to that recorded in the sunspot umbra. The versatility of our numerical
simulations permits a sensible quantitative comparison between the model
predictions and these observations, opening the potential to diagnose
certain aspects of the hidden subsurface structure of sunspots. The
National Center for Atmospheric Research is sponsored by the National
Science Foundation.
---------------------------------------------------------
Title: Mode Mixing by a Shallow Sunspot
Authors: Barnes, G.; Cally, P. S.
1999soho....9E..35B Altcode:
In a polytropic atmosphere, the oscillation modes are described by
well-known special functions. However, the presence of a magnetic
field inside a sunspot makes the mode structure much more complex,
so that analytic expressions are not available. Recent observations
of sunspots suggest that most of the scattering and absorption due
to the spots occurs in a layer immediately below the surface of the
sun. We have therefore modelled the acoustic modes inside a sunspot by
assuming that the effect of the magnetic field is concentrated right
at the surface. Instead of imposing the conventional upper boundary
condition, that the divergence of the velocity vanish, we require
that the horizontal component of the velocity vanish, which allows
us to write down analytic expressions for the acoustic modes inside
the spot. This may be justified by arguing that a vertical magnetic
field will tend to inhibit horizontal fluid motions. In effect, we are
introducing a purely scattering disk to the surface of the sun. More
realistic models, in which the disk both scatters and absorbs energy are
also possible. We consider the scattering of an incident p-mode off our
"sunspot," matching the pressure and horizontal velocity across the
boundary. The result is a mixing of the incident mode into outgoing
external p-modes and internal p-modes, as well as jacket modes both
inside and outside the spot. We find that the inclusion of the jacket
modes is crucial to satisfying the matching conditions, and we present
results indicating the spectrum of outgoing and internal modes that
are present.
---------------------------------------------------------
Title: Modelling P-Mode Interaction with a Spreading Sunspot Field
Authors: Cally, Paul; Bogdan, Tom
1999soho....9E..47C Altcode:
Sunspots absorb and scatter incident p-modes. The dominant mechanism
is still uncertain. One possibility, mode conversion to slow
magneto-acoustic waves, has been shown to yield results in agreement
with observations for the f-mode only. Absorption of p-modes in simple
vertical magnetic field models is too weak by an order of magnitude or
more. Here we report on numerical calculations of p-modes encountering
a simple sunspot model with field which spreads with height. It is
found that interesting interactions take place in the near horizontal
"canopy" which may greatly enhance absorption. We present a video
which aids in visualizing the interactions.
---------------------------------------------------------
Title: Sunspot magnetic oscillations: Comparison between observations
and models
Authors: Rüedi, I.; Solanki, S. K.; Bogdan, T.; Cally, P.
1999ASSL..243..337R Altcode: 1999sopo.conf..337R
No abstract at ADS
---------------------------------------------------------
Title: Velocity and Magnetic Field Fluctuations in the Photosphere
of a Sunspot
Authors: Lites, Bruce W.; Thomas, John H.; Bogdan, Thomas J.; Cally,
Paul S.
1998ApJ...497..464L Altcode:
We use a data set of exceptionally high quality to measure oscillations
of Doppler velocity, intensity, and the vector magnetic field at
photospheric heights in a sunspot. Based on the full Stokes inversion
of the line profiles of Fe I 630.15 and 630.25 nm, in the sunspot umbra
we find upper limits of 4 G (root mean square [rms]) for the amplitude
of 5 minute oscillations in magnetic field strength and 0.09d (rms)
for the corresponding oscillations of the inclination of the magnetic
field to the line of sight. Our measured magnitude of the oscillation
in magnetic field strength is considerably lower than that found in
1997 by Horn, Staude, & Landgraf. Moreover, we find it likely
that our measured magnetic field oscillation is at least partly due to
instrumental and inversion cross talk between the velocity and magnetic
signals, so that the actual magnetic field strength fluctuations are
even weaker than 4 G. In support of this we show, on the basis of the
eigenmodes of oscillation in a theoretical model of the sunspot umbra,
that magnetic field variations of at most 0.5 G are all that is to
be expected. The theoretical model also provides an explanation of
the shift of power peaks in Doppler velocity to the 3 minute band in
chromospheric umbral oscillations, as a natural consequence of the
drastic change in character of the eigenmodes of oscillation between
frequencies of about 4.5 and 5.0 mHz due to increased tunneling through
the acoustic cutoff-frequency barrier. Using measurements of the
phase of velocity oscillations above the acoustic cutoff frequency,
we determine the relative velocity response height in the umbra of
four different photospheric spectral lines from the phase differences
between velocities in these lines, assuming that the oscillations
propagate vertically at the local sound speed. In spacetime maps of
fluctuations in continuum intensity, Doppler velocity, magnetic field
strength, and field inclination, we see distinct features that migrate
radially inward from the inner penumbra all the way to the center of
the umbra, at speeds of a few tenths of a kilometer per second. These
moving features are probably a signature of the convective interchange
of magnetic flux tubes in the sunspot, although we failed to find any
strong correlation among the features in the different quantities,
indicating that these features have not been fully resolved.
---------------------------------------------------------
Title: Simulation of f- and p-Mode Interactions with a Stratified
Magnetic Field Concentration
Authors: Cally, P. S.; Bogdan, T. J.
1997ApJ...486L..67C Altcode:
The interaction of f- and p-modes with a slab of vertical magnetic
field of sunspot strength is simulated numerically in two spatial
dimensions. Both f-modes and p-modes are partially converted to
slow magnetoatmospheric gravity (MAG) waves within the magnetic slab
because of the strong gravitational stratification of the plasma along
the magnetic lines of force. The slow MAG waves propagate away from
the conversion layer guided by the magnetic field lines, and the
energy they extract from the incident f- and p-modes results in a
reduced amplitude for these modes as they exit from the back side of
the slab. In addition, the incident p-modes are partially mixed into
f-modes of comparable frequency, and therefore larger spherical harmonic
degree, when they exit the magnetic flux concentration. These findings
have important implications for the interpretation of observations
of p-mode absorption by sunspots, both in terms of the successes and
failures of this simple numerical simulation viewed in the sunspot
seismology context.
---------------------------------------------------------
Title: Waves in magnetized polytropes.
Authors: Bogdan, T. J.; Cally, P. S.
1997RSPSA.453..943B Altcode: 1997RSPSA.453..943J
The authors consider the linear oscillations of a plane-parallel
semi-infinite electrically conducting atmosphere with a constant
temperature gradient, subjected to an imposed uniform gravitational
acceleration and uniform magnetic field. The oscillations are treated
in the ideal (dissipationless) limit and the uniform gravitational
acceleration and magnetic field are taken to be co-aligned with the
prevailing temperature gradient. It is demonstrated that atmospheric
motions with prescribed horizontal variations of the form exp(ikx),
with k real, possess both a discrete set of complex eigenfrequencies
w<SUB>n</SUB>, n=0, 1, 2,..., and a continuous spectrum. These
two behaviours derive from a particular fourth-order ordinary
differential equation that arises in the solution of the initial value
problem via an integral transform and describes the coupled fast-
and slow-magnetoatmospheric waves. The authors devote considerable
efforts to document how the discrete spectrum varies in response to
incremental changes in the horizontal wavenumber k and they compare
and contrast this behaviour with that found by Lamb for the same
atmosphere, but with the magnetic field being absent. Implications
for the helioseismology of sunspots are discussed.
---------------------------------------------------------
Title: Absorption of p-Modes by Slender Magnetic Flux Tubes and
p-Mode Lifetimes
Authors: Bogdan, T. J.; Hindman, B. W.; Cally, P. S.; Charbonneau, P.
1996ApJ...465..406B Altcode:
The presence of a fibril magnetic field in the solar envelope not only
induces shifts in the p-mode resonant frequencies, but also contributes
to the line width of the modes. The augmentation of the line widths
results from two related physical processes: the excitation of tube
mode oscillations on the individual magnetic fibrils and the attendant
mode mixing between p-modes with identical oscillation frequencies. We
assay the magnitude of the contribution from the former physical
process based upon an idealized model consisting of vertical, slender,
magnetic flux tubes embedded in a plane-parallel isentropic polytrope
of index m. We restrict our attention to axisymmetric flux tubes
that are in mechanical and thermal equilibrium with their immediate
nonmagnetic surroundings. For low p-mode oscillation frequencies,
ω, this model predicts that the line width, F, varies as <P />Γ
∝ fωM<SUP>-½</SUP> ∝fω<SUP>m+2</SUP>, <P />where M the mode
mass, and f is the magnetic filling factor reckoned at the surface
of the polytrope. This scaling is in better overall agreement with
the observations (Γ ∝ ω<SUP>4.2</SUP>) than previous predictions
based on the excitation and damping of solar p-modes by turbulent
convection (which yields Γ ∝ γ<SUP>2</SUP> M<SUP>-1</SUP>
∝ω<SUP>2m+4</SUP>), or the scattering of p-modes by convective
eddies (which yields Γ ∝ ω<SUP>(4/3)m+3)</SUP>, and it suggests
that tube mode excitation on fibril magnetic fields may be a dominant
and detectable (through its solar cycle variation) component of the
low-frequency p-mode line widths.
---------------------------------------------------------
Title: Driven Acoustic Oscillations within a Vertical Magnetic Field
Authors: Hindman, Bradley W.; Zweibel, Ellen G.; Cally, P. S.
1996ApJ...459..760H Altcode:
We examine the effects of a vertical magnetic field on p-mode
frequencies, line widths, and eigenfunctions. We employ a simple solar
model consisting of a neutrally stable polytropic interior matched to
an isothermal chromosphere. The p-modes are produced by a spatially
distributed driver. The atmosphere is threaded by a constant vertical
magnetic field. The frequency shifts due to the vertical magnetic
field are much smaller than the shifts caused by horizontal fields
of similar strength. A large vertical field of 2000 G produces shifts
on the order of 1 muHz while a weak field of 50 G produces very small
shifts of several nanohertz. We find that the frequency shifts decrease
with increasing frequency and increase with field strength. The shifts
are positive, except at high frequency and low field strength, where
small negative shifts are possible. Coupling of the acoustic fast mode
to escaping slow modes is extremely inefficient. Constant vertical
magnetic field models are therefore incapable of explaining the high
level of absorption observed in sunspots and plage The damping due
to this mode conversion process produces very narrow line widths. For
a 2000 G field the line widths are several microhertz and for a 50 G
field the line widths are several nanohertz.
---------------------------------------------------------
Title: Jacket Modes: Solar Acoustic Oscillations Confined to Regions
Surrounding Sunspots and Plage
Authors: Bogdan, T. J.; Cally, P. S.
1995ApJ...453..919B Altcode:
It is demonstrated by the application of classical eigenfunction
expansion techniques that the interaction of an incident solar acoustic
oscillation, or p-mode, with a scattering obstacle-such as a sunspot
involves not only the scattering of the incident p-mode into other
p-modes of like frequency (mode mixing), but also the generation of an
acoustic halo localized like a "jacket" about the scatterer. This halo
oscillates at the same frequency as the incident p-mode and is composed
of a superposition of a continuous spectrum of modes referred to as
jacket modes. It is pointed out that these jacket modes should appear
as a broad enhancement to the background power underlying the p-mode
ridges in κ-ω diagrams for regions in close proximity to sunspots and
the boundaries of plage The basic results draw upon a remarkable analogy
between the oscillations of an adiabatically stratified plane-parallel
polytrope and the wave functions for the hydrogen atom: the laterally
propagating p-modes that participate in the mode mixing correspond
to the negative-energy bounds states of the hydrogen atom, while the
continuous spectrum of jacket modes corresponds to the continuum of
positive-energy states in which the electron is not bound to the proton.
---------------------------------------------------------
Title: Coupling of magnetospheric cavity modes to field line
resonances: A study of resonance widths
Authors: Mann, Ian R.; Wright, Andrew N.; Cally, Paul S.
1995JGR...10019441M Altcode:
By using a box model for the magnetosphere and by using a matrix
eigenvalue method to solve the cold linearized ideal MHD equations,
we examine the temporal evolution of the irreversible coupling
between fast magnetospheric cavity modes and field line resonances
(FLRs). By considering the fast mode frequency to be of the form
ω<SUB>f</SUB>=ω<SUB>fr</SUB>-iω<SUB>fi</SUB>, and using a Fourier
transform approach, we have determined the full time-dependent evolution
of resonance energy widths. We find that at short times the resonances
are broad, and narrower widths continue to develop in time. Ultimately,
an asymptotic resonance Alfvén frequency full width at half maximum
(FWHM) of Δω<SUB>A</SUB>=2ω<SUB>fi</SUB> develops on a timescale of
τ<SUB>fi</SUB>=ω<SUB>fi</SUB><SUP>-1</SUP>. On timescales longer than
τ<SUB>fi</SUB>, we find that the resonance perturbations can continue
to develop even finer scales by phase mixing. Thus, at any time, the
finest scales within the resonance are governed by the phase mixing
length L<SUB>ph</SUB>(t)=2π(tdω<SUB>A</SUB>/dx)<SUP>-1</SUP>. The
combination of these two effects naturally explains the localisation of
pulsations in L shells observed in data, and the finer perturbation
scales which may exist within them. <P />During their evolution,
FLRs may have their finest perturbation scales limited by either
ionospheric dissipation or by kinetic effects (including the
breakdown of single fluid MHD). For a continually driven resonance,
we define an ionospheric limiting timescale τ<SUB>I</SUB> in terms
of the height-integrated Pedersen conductivity Σ<SUB>P</SUB>,
and hence derive a limiting ionospheric perturbation scale
L<SUB>I</SUB>=2π(τ<SUB>I</SUB>dω<SUB>A</SUB>/dx)<SUP>-1</SUP>,
in agreement with previous steady state analyses. For sufficiently
high Σ<SUB>P</SUB>, FLR might be able to evolve so that their
radial scales reach a kinetic scale length L<SUB>k</SUB>. For
this to occur, we require the pulsations to live for longer than
τ<SUB>k</SUB>=2π(L<SUB>k</SUB>dω<SUB>A</SUB>/dx)<SUP>-1</SUP>. For
t<τ<SUB>k</SUB>,τ<SUB>I</SUB>, kinetic effects and ionospheric
dissipation are not dominant, and the ideal MHD results presented
here may be expected to model realistically the growth phase of ULF
pulsations. .
---------------------------------------------------------
Title: Effects of Weak-to-Moderate Vertical Magnetic Fields on Solar
f- and p-Modes
Authors: Cally, P. S.
1995ApJ...451..372C Altcode:
Observations indicate that solar p-modes encountering regions of
enhanced magnetic field such as active region plage are partially
absorbed. We adopt a simple vertical field model and investigate
the possibility that the responsible mechanism is slow mode leakage,
which has been at least partially successful in explaining absorption
by sunspots. It is found that there are two frequency ranges with
very different behavior: (1) below the chromospheric acoustic cutoff
frequency, and (2) above it. In the first case, which includes the 5
minute band, the f- and p<SUB>1</SUB>-modes would be expected to suffer
substantial absorption, but p-modes of higher radial order would not
be damped sufficiently to account for observations. On the other hand,
in the higher frequency range (≥ 5 mHz), damping lengths of f- and
P<SUB>1</SUB>-P<SUB>3</SUB> modes are smaller than or comparable to
typical plage sizes, indicating that substantial absorption should
occur.
---------------------------------------------------------
Title: Driven Acoustic Oscillations Within a Vertical Magnetic Field
Authors: Hindman, B. W.; Zweibel, E. G.; Cally, P. S.
1995ESASP.376b..77H Altcode: 1995help.confP..77H; 1995soho....2...77H
Examines the effects of a vertical magnetic field on p-mode frequencies,
line widths, and eigenfunctions. The authors employ a simple solar
model consisting of a neutrally stable polytropic interior matched to
an isothermal chromosphere. The p-modes are produced by a spatially
distributed driver. The atmosphere is threaded by a constant vertical
magnetic field. The frequency shifts due to the vertical magnetic
field are much smaller than the shifts caused by horizontal fields of
similar strength. A large vertical field of 2000 G produces shifts
on the order of a μHz while a weak field of 50 G produces very
small shifts of several nHz. The authors find that the frequency
shifts decrease with increasing frequency and increase with field
strength. Coupling of the acoustic fast mode to escaping slow modes
is extremely inefficient. Constant vertical magnetic field models are
therefore incapable of explaining the high level of absorption observed
in sunspots and plages.
---------------------------------------------------------
Title: Umbral Oscillations in Sunspots: Absorption of p-Modes and
Active Region Heating by Mode Conversion
Authors: Cally, P. S.; Bogdan, T. J.; Zweibel, E. G.
1994ApJ...437..505C Altcode:
The linear adiabatic oscillations of an infinite, stratified,
perfectly conducting atmosphere pervaded by a uniform vertical magnetic
field are computed. The stratification is chosen to approximate the
conditions appropriate for a sunspot umbra where strong reflection
of the fast magnetoacoustic-gravity waves from the rapid increase
of the Alfven speed with height and the sound speed with depth is
anticipated. Since the umbral oscillations are presumably excited by
external p-mode forcing, the angular frequency omega is prescribed --
being set by the p-modes -- and it is required to solve for the allowed
eigenvalues assumed by the horizontal wavenumber k and the corresponding
displacement eigenfunctions. We term these allowed solutions pi-modes
in recognition of the crucial influence of the imposed magnetic
field and to distinguish them from their p-mode cousins present in
the surrounding nonmagnetic quiet Sun. The wavenumber eigenvalues are
complex and are consistent with the spatial decay of the pi-modes inward
toward the center of the sunspot from the umbral boundary. This spatial
attenuation is a consequence of the slow magnetoacoustic-gravity waves
that propagate along the magnetic field lines and extract energy from
the trapped fast waves through mode coupling in the layers where the
sound and Alfven speeds are comparable. Through the consideration of
several ancillary computations we argue that this salient attribute
of the pi-modes may be essential both in explaining the observed
absorption of (the forcing) p-modes by sunspots and in providing a
source of mechanical energy for the overlying active regions. However,
more realistic calculations are clearly called for before these notions
may be confirmed.
---------------------------------------------------------
Title: A Fourier-space description of oscillations in an inhomogeneous
plasma. Part 1. Continuous Fourier transformation
Authors: Sedláček, Z.; Cally, P. S.
1994JPlPh..52..245S Altcode:
Oscillations in inhomogeneous cold plasmas or inhomogeneous
magnetofluids are interpreted in terms of the dynamics of their
spectra in wavenumber space. By Fourier transforming the basic
integro-differential equation of the problem, a generalized
wave equation in wavenumber space is derived, thus converting the
oscillation and phase-mixing processes in the original χ space into
processes of dispersive propagation and scattering of the spectrum
in wavenumber space. The Barston singular continuum eigenmodes
correspond to stationary scattering states of a monochromatic wave in
wavenumber space, whereas the damping phenomena in χ space correspond
to transient ‘leaking’ phenomena accompanying scattering and
dispersive propagation of a wave packet in wavenumber space.
---------------------------------------------------------
Title: A Fourier-space description of oscillations in an inhomogeneous
plasma. Part 2. Discrete approach
Authors: Cally, P. S.; Sedláček, Z.
1994JPlPh..52..265C Altcode:
The process of phase mixing in inhomogeneous MHD or cold plasmas
is interpreted as one of energy propagation in discrete Fourier
space. Three instructive scenarios are examined: (i) an isolated
inhomogeneity with zero boundary conditions; (ii) a periodic
inhomogeneity; and (iii) a monotonic inhomogeneity sandwiched between
two semi-infinite uniform regions. In each case the coefficients of the
associated wave equation in Fourier space for an appropriately chosen
dependent variable are very nearly constant almost everywhere, so the
propagation is like that of a free unreflected wave. An exception
may arise in the coupling of the lowest modes, which can be highly
reflective. It is argued that Fourier space is the simplest and most
natural context in which to discuss the development of fine-scale
oscillations.
---------------------------------------------------------
Title: Modelling magnetoacoustic oscillations in sunspots: a
progress report
Authors: Charbonneau, P.; Cally, P. S.; Bogdan, T. J.
1994smf..conf..251C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar p-Modes in a Vertical Magnetic Field: Trapped and Damped
pi -Modes
Authors: Cally, P. S.; Bogdan, T. J.
1993ApJ...402..721C Altcode:
The study addresses the question of whether the polytropic atmosphere
with an imposed uniform vertical magnetic field possesses normal
modes of oscillation despite the potential for the s-modes to drain
energy from the resonant cavity created by the favorable vertical
stratification. This question is answered in the affirmative via
direct numerical construction of these eigenfunctions for the complete
uniformly magnetized polytrope. The basic equilibrium atmosphere
is discussed, and the relevant linearized equations and boundary
conditions are provided. The properties of the trapped pi-modes are
considered. It is suggested that they play a role in the magnetized
sunspot atmosphere analogous to that played by the p-modes in the
unmagnetized surrounding quiet sun. Oscillations in sunspots and sunspot
seismology are reassessed in light of the discovery of the natural modes
of oscillation of unbounded atmospheres with vertical magnetic fields.
---------------------------------------------------------
Title: Steady and Nonsteady Siphon Flow in Hot Coronal Loops
Authors: Robb, T. D.; Cally, P. S.
1992ApJ...397..329R Altcode:
Siphon flow in hot coronal loops is examined, in both its steady
and dynamic states, in the latter case using a flux-corrected
transport simulation. We find that such flows are inhibited by (1)
low heating rates, (2) high pressures, (3) short loop lengths, and
(4) turbulence. In accordance with expectations, we find that small
footpoint pressure asymmetries produce steady subsonic flow. However,
the standard picture that larger values yield standing shocks is
shown to be valid only for sufficiently high heating, long loops, or
low pressure. Values of these parameters more characteristic of active
regions produce instead a quasi-periodic 'surge' flow when the pressure
asymmetry exceeds a critical value at which the temperature gradient
at the inflow end reverses sign. These flows are normally subsonic,
though examples can be found where the surge is supersonic for a part
of each period. The difficulty of driving substantial siphon flows
for realistic hot loop models is in accordance with the comparative
rarity of observations of these flows.
---------------------------------------------------------
Title: Phase mixing and surface-wave decay in an inhomogeneous plasma
Authors: Cally, P. S.; Sedláček, Z.
1992JPlPh..48..145C Altcode:
The decay rate of an Alfvén or plasma surface wave propagating along
an inhomogeneous layer of plasma is calculated. The inhomogeneous
profile is thin and odd, but otherwise arbitrary. The wave's decay
rate is determined using two fundamentally different methods,
the integro-differential equation approach of Sedl´ček and the
Sturm-Liouville expansion technique of Cally, and found by both to
depend only on the slope of the Alfvén or plasma frequency profile at
the ‘resonant point’, and not on other details of its shape. The
result is verified numerically. This problem represents a good example
with which to compare and contrast the two methods.
---------------------------------------------------------
Title: Resonant structures within incompressible ideal MHD
Authors: Zorzan, C.; Cally, P. S.
1992JPlPh..47..321Z Altcode:
The resonant characteristics of an incompressible ideal MHD fluid
are highly structured. To help expose this structure, an equivalent
electrical analogue of the MHD system is developed. The model, in
the form of a transmission line, makes it possible to identify a
number of new and important concepts, one of which is the effective
impedance. This in turn enables entire regions of MHD fluid to be
replaced with equivalent impedances. When fully exploited, the model
also provides a more consistent interpretation of the spectrum of ideal
MHD. The discrete Alfvén modes are found to be highly degenerate,
while the transition to a discontinuous profile is accompanied by
a redistribution of an uncountably infinite number of ‘poles’
from the continuous spectrum and onto the Alfvén modes. In addition,
the electrical analogue shows that within a continuously structured
fluid the characteristic behaviour is not necessarily dominated by the
‘surface mode’ alone. This view is also supported by the results of
a numerical simulation of the linear MHD equations. Depending on the
initial conditions, the collective behaviour can have any frequency
within the range spanned by the transition zone. The energy itself
is monitored using a new pair of energy and flux expressions derived
from a variational (Lagrangian) description of the MHD system. Again
the electrical model is used to provide a physical interpretation
of the individual terms within these expressions. In particular,
it allows a partition of the total energy into separate kinetic-
and potential-energy terms.
---------------------------------------------------------
Title: Phase-mixing and surface waves: a new interpretation
Authors: Cally, P. S.
1991JPlPh..45..453C Altcode:
The classical incompressible MHD or cold plasma phase mixing problem,
which involves Alfvén or plasma waves in inhornogeneous media,
is re-examined using a spatial Fourier series rather than the usual
temporal Fourier or Laplace transform approach. A number of exact and
near-exact analytic and numerical results are derived which reveal
an attractive picture of energy cascading to smaller length-scales in
a manner reminiscent of turbulence. Furthermore, we present a simple
and unambiguous description of how a surface wave arises in the limit
in which the inhomogeneity becomes a discontinuity.
---------------------------------------------------------
Title: Stability, Structure, and Evolution of Cool Loops
Authors: Cally, P. S.; Robb, T. D.
1991ApJ...372..329C Altcode:
The criteria for the existence and stability of cool loops are
reexamined. It is found that the stability of the loops strongly depends
on the form of the heating and radiative loss functions and that if the
Ly-alpha peak which appears in most calculations of the radiative loss
function is real, cool loops are almost certainly unstable. Removing
the hydrogen contribution from the recent loss function Q(T) by Cook
et al. (1989) does not produce the much-used result, Q proportional to
T-cubed, which is so favorable to cool loop stability. Even using the
probably unrealistically favorable loss function Q1 of Cook et al. with
the hydrogen contribution removed, the maximum temperature attainable
in stable cool loops is a factor of 2-3 too small to account for the
excess emission observed in lower transition region lines. Dynamical
simulations of cool loop instabilities reveal that the final state of
such a model is the hot loop equilibrium.
---------------------------------------------------------
Title: Why Heating is Not Necessary in the Transition Region or
Upper Chromosphere (With 3 Figures)
Authors: Cally, P. S.
1991mcch.conf..103C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Turbulent Thermal Conduction in the Solar Transition Region
Authors: Cally, P. S.
1990ApJ...355..693C Altcode:
Emission measures E(T) derived from EUV observations of the lower
solar transition region indicate that there is far more material in
that temperature range than can be explained within the steady single
flux-tube scenario if the classical energy transfer and loss mechanisms
are adopted. This suggests that some mechanism may be operating which
reduces the temperature gradient. The inclusion of a turbulent thermal
conductivity kappa(T) = phi rho c(p)ul in a one-dimensional model
is investigated, where u is the rms microvelocity in one direction,
l is the mixing length, and phi is a constant of order unity. If it
is assumed that that l is proportional to T exp-alpha, it is shown
that near-perfect agreement with the observed emission measures is
obtained for alpha = 3/2, both as regards the slope d log E/d log T
equal to about -3 in the LTR, and the upturn at the top of this range.
---------------------------------------------------------
Title: Force and Energy Balance in the Transition Region Network
Authors: Fiedler, R. A. S.; Cally, P. S.
1990SoPh..126...69F Altcode:
Two-dimensional numerical models of the solar transition region
are calculated using an inverse coordinates method which attains
pressure equilibrium between the network magnetic field and the
external comparatively field-free gas. If A(y, z) is the magnetic
potential (a scalar in 2D), which is constant on field lines, the
method involves interchanging dependent and independent variables to
obtain a quasi-linear PDE for y(A, z), which is solved iteratively. The
advantage of this approach is that magnetic field lines, including any
magnetic interface, become coordinate lines, thereby simplifying the
energy equation and free boundary problem. In order to examine the
effects of self-consistent geometry on the thermal structure of the
transition region network, we calculate four models. The energy balance
includes the effects of radiation, conduction, and enthalpy flux. It
is confirmed that the lower branch of the emission measure curve cannot
be explained within the single fluxtube model if the classical Spitzer
thermal conductivity is used. However, by including a turbulent thermal
conductivity as proposed by Cally (1990a), transition region models
are obtained for which the resulting emission measure curves exhibit
the correct behaviour, including the observed turn-up below about 200
000 K. In summary, the broad conclusions of previous non-turbulent 2D
models are confirmed, but most importantly, the turbulent conductivity
hypothesis tested in 1D by Cally is shown to produce excellent agreement
with observations in the more realistic geometry.
---------------------------------------------------------
Title: Magnetohydrodynamic Tube Waves and High Speed Solar Wind
Streams
Authors: Cally, P. S.
1987SoPh..108..183C Altcode:
It has been widely conjectured that magnetohydrodynamic (MHD) waves
may provide the extra momentum or energy required to explain the high
speed solar wind streams that originate in coronal holes. Although the
magnetic structuring inherent in this problem has been incorporated into
models of the bulk flow, this is not generally true of the associated
treatments of wave propagation. In particular, as pointed out by
Davila (1985), we might generally expect the magnetic geometry to
substantially modify those waves whose wavelength is comparable to the
hole width. Using both a geometrical optics and an eigenmode approach,
Davila addressed the question of wave propagation in a simple uniform
width flux slab model of a coronal hole and concluded the hole may act
as a `leaky wave guide', i.e., waves travelling along it may leak into
the surrounding corona, but
---------------------------------------------------------
Title: Leaky and Non-Leaky Oscillations in Magnetic Flux Tubes
Authors: Cally, P. S.
1986SoPh..103..277C Altcode:
An extensive analysis, both analytic and numerical, of waves in flux
tubes imbedded in (possibly) magnetic surroundings is given. It is
shown that any wave confined to the tube and its neighbourhood can
be put into one of seven categories. Simple criteria for deciding the
existence of each type in any particular case are derived. Many other
(leaky) modes are found which excite waves in the external medium
and thereby lose energy to the surroundings. A number of asymptotic
analyses allow much information to be gained about these without the
need for numerical solution of the complicated equations involved. Three
particular cases, pertaining to photospheric flux tubes, Hα fibrils,
and coronal loops, are considered in detail.
---------------------------------------------------------
Title: Magnetohydrodynamic tube waves - Waves in fibrils
Authors: Cally, P. S.
1985AuJPh..38..825C Altcode:
A discussion of waves in magnetic flux tubes imbedded in magnetic
unstratified surroundings is given. Seven types of non-leaky wave
are identified. Many more leaky waves, tube oscillations which
drive waves in the external medium and thereby lose energy to it,
are also found. The particular example of longitudinal and transverse
oscillations in chromospheric fibrils is examined in detail.
---------------------------------------------------------
Title: Magnetohydrodynamic critical levels and radiative damping
Authors: Cally, P. S.
1984A&A...136..121C Altcode:
The full MHD wave equation for radiatively damped wave propagation in
a stratified magnetoatmosphere permeated by a horizontal magnetic field
is derived. In the adiabatic case a discussion of the various types of
singularity (critical level) is given. For an isothermal atmosphere
with uniform magnetic field the damped wave equation is solved in
terms of hypergeometric functions, and it is concluded that for many
cases of interest the erstwhile discontinuity in the wave energy flux
is considerably "smoothed out" by the damping. Substantial fluxes
(≡10<SUP>8</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP>) are found to
be carried by some modes which, in the absence of critical levels,
would carry no energy.
---------------------------------------------------------
Title: Chromospheric and coronal Alfvénic oscillations in
non-vertical magnetic fields
Authors: Schwartz, S. J.; Cally, P. S.; Bel, N.
1984SoPh...92...81S Altcode:
We generalize previous studies of Alfvénic oscillations in the solar
atmosphere to geometries in which the background magnetic field is
not parallel to the gravitational acceleration. A uniform but inclined
field produces only subtle changes in the mathematics, and virtually
no changes to the coronal energy flux, over previous vertical field
studies. We show that simple, two-layer models agree remarkably
well with more sophisticated, multi-layer calculations. In addition,
we derive several useful and accurate analytic results with which we
highlight many features and parameter dependences. We also study a model
with a spreading field geometry. For low magnetic fields (∼- 10 G)
the corona still appears WKB to the oscillations and we do not find
any significant deviations from the uniform field calculations. This
is not the case for higher magnetic fields in active regions (∼-
3000 G) where we confirm previous results which suggest an order
of magnitude increase in the coronal flux. Again, we derive useful
analytic approximations.
---------------------------------------------------------
Title: Umbral Oscillations in the Presence of a Spreading Magnetic
Field
Authors: Cally, P. S.
1983SoPh...88...77C Altcode:
A simple umbral model incorporating spreading magnetic field lines is
introduced, and the equations governing Magneto-Acoustic-Gravity (MAG)
and Alfvén modes are derived. In the absence of stratification analytic
solutions may be found. For the MAG modes, under coronal conditions,
these consist of a travelling (slow) wave and an evanescent (fast)
mode, whilst the Alfvén solution may be expressed in terms of a
Hankel function. These are matched onto numerical solutions in a
stratified model photosphere-chromosphere, and the vertical energy
fluxes are calculated. Resonant frequencies are generally found to be
shifted downwards compared with the vertical field case, though the
MAG flux is virtually unaltered otherwise; however, the Alfvén flux is
increased by up to two orders of magnitude. In an unstratified medium,
the usual formula for the Alfvén flux, F = ϱa «δv<SUP>2</SUP>»,
is shown to be generally incorrect in the presence of field spread,
and must be replaced by F = N<SUP>−2/3</SUP>(4π)<SUP>−1</SUP>ϱa
«δv<SUP>2</SUP>», where N is a spread parameter which usually
satisfies N ≪ 1.
---------------------------------------------------------
Title: On photospheric and chromospheric penumbral waves.
Authors: Cally, P. S.; Adam, J. A.
1983SoPh...85...97C Altcode:
The full magnetoatmospheric wave equation is numerically solved for the
particular penumbral model of Nye and Thomas (1974). For chromospheric
running penumbral waves it is found that the maximum vertical velocity
occurs at the base of the Hα region (z ≈ 300 km) much lower than
previous WKB estimates have suggested. The maximum `vertical' kinetic
energy occurs at z ≈ - 130 km. Very significant horizontal velocities
are also found for these waves, and, in the absence of shear flow it
appears that previous estimates of photospheric vertical velocities
of order 10<SUP>−1</SUP> km s<SUP>−1</SUP> could be substantial
underestimates. For the photospheric events of Musman et al. (1976),
a high vertical velocity maximum is found in the corona, and the
modes appear highly dispersive for periods ≥ 220 s. The effects of a
sinusoidal shear flow profile on running penumbral waves are examined,
and it is found that the eigenvalues (horizontal wavenumber k) are
changed little, but the eigenmodes become significantly distorted;
the position of the vertical velocity peak rises compared to the zero
flow case, and the velocity below that peak drops significantly. This
effect may well cancel the increased estimates based on zero flow.
---------------------------------------------------------
Title: Photospheric and Chromospheric Penumbral Waves
Authors: Adam, J. A.; Cally, P. S.
1983BAAS...15Q.706A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Complex eigenvalue bounds in magnetoatmospheric shear flow. II
Authors: Cally, P. S.; Adam, J. A.
1983GApFD..23...57C Altcode:
A method discussed in Paper I (Cally, 1983) for determining complex
eigenvalue bounds for second-order ordinary differential systems
is applied to an isothermal, perfectly conducting, compressible,
stratified medium permeated by a horizontal magnetic field and aligned
shear flow. The magnetic field is chosen such that the Alfvén velocity
is everywhere constant. For this particular system, the eigenvalue
bounds obtained are generally tighter than those obtained in Paper I,
and an additional feature is that the bound on the maximum growth
rate peaks at a finite value of horizontal wavenumber k, not as k ,
as is found in Paper I.
---------------------------------------------------------
Title: Complex eigenvalue bounds in magnetoatmospheric shear flow. I
Authors: Cally, P. S.
1983GApFD..23...43C Altcode:
A general method is presented for deriving complex eigenvalue bounds for
linear second order systems. It is applied to the problem of stratified
horizontal magnetohydrodynamic shear flow, and it is found that, if the
system is convectively stable, the imaginary part of the eigenfrequency
is bounded by ½ú'0max, where u0 is the flow velocity. In the absence
of a magnetic field, the classical Miles-Howard-Chimonas result is
recovered, but with more detail about the growth rates of individual
modes. These results are shown to hold for free as well as rigid
boundaries.
---------------------------------------------------------
Title: The equilibrium statistical mechanics of self-gravitating
systems
Authors: Cally, P. S.
1981AuJPh..34..267C Altcode:
The investigation of the exact statistical mechanics of self-gravitating
systems presents certain difficulties, because of the long-range forces
involved. The reported investigation is concerned with the equilibrium
structure of a spherically confined system consisting of a large
number of identical particles. One objective of the investigation
is to verify that in the large N limit the thermodynamics and the
distribution functions are as predicted by the continuum theory. This
is of interest in connection with some doubt which may arise concerning
the applicability of the conventional thermodynamic ideas to systems
with long-range forces. The investigation shows that, at least for
the system in a heat bath, the thermodynamics derived from statistical
mechanics is equivalent to classical thermodynamics for the spherically
symmetric self-gravitating system.
---------------------------------------------------------
Title: Statistical mechanics and the gravothermal catastrophe.
Authors: Cally, P. S.; Monaghan, J. J.
1981JMP....22..348C Altcode: 1981JMaPh..22..348C
Jensen's inequality is applied to the canonical partition function of
a self-gravitating system to determine the best independent particle
potential. The inequality allows the stability to be analyzed very
easily. We recover the results of Lynden-Bell and Wood for the onset of
an instability in an isothermal sphere in a heat bath. Our eigenvalue
analysis leads to results very similar to those of Horwitz and Katz,
but we differ in the description of the l=1 perturbation.
---------------------------------------------------------
Title: Many body problems in astrophysics
Authors: Cally, Paul Stuart
1979PhDT.......233C Altcode:
No abstract at ADS