Author name code: cally ADS astronomy entries on 2022-09-14 author:"Cally, Paul S." ------------------------------------------------------------------------ Title: On the fragility of Alfvén waves in a stratified atmosphere Authors: Cally, Paul S. Bibcode: 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. Title: Benchmarking hall-induced magnetoacoustic to Alfvén mode conversion in the solar chromosphere Authors: Raboonik, Abbas; Cally, Paul S. Bibcode: 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 zeq; and (ii) Hall-rotation of the fast wave in z > zeq. Thus, for the two-stage process to yield upgoing Alfvén waves, zeq 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 kx 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. Title: Benchmarking Hall-Induced Magnetoacoustic to Alfvén Mode Conversion in the Solar Chromosphere Authors: Raboonik, Abbas; Cally, Paul Bibcode: 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. Title: Second bounces of seismic signals from sunquakes: modelling and data Authors: Donea, A.; Cally, P. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. Title: Study of Acoustic Halos in NOAA Active Region 12683 Authors: Tripathy, S. C.; Jain, K.; Kholikov, S.; Hill, F.; Cally, P. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. Title: Fast-to-Alfvén Mode Conversion and Ambipolar Heating in Structured Media. I. Simplified Cold Plasma Model Authors: Cally, Paul S.; Khomenko, Elena Bibcode: 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. Title: Hall-coupling of Slow and Alfvén Waves at Low Frequencies in the Lower Solar Atmosphere Authors: Raboonik, Abbas; Cally, Paul S. Bibcode: 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. Title: Fast-to-Alfvén Mode Conversion and Ambipolar Heating in Structured Media. II. Numerical Simulation Authors: Khomenko, Elena; Cally, Paul S. Bibcode: 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. Title: Smoothing of MHD Shocks in Mode Conversion Authors: Pennicott, Jamon D.; Cally, Paul S. Bibcode: 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. 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. Bibcode: 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 ∼103 W m-2, based on an amplitude of 500 m s-1 at β = 1, which is sufficient to play a major role in both quiet and active region coronal heating. Title: Investigation of Acoustic Halos using Multi-Height SDO Observations Authors: Tripathy, S. C.; Jain, Kiran; Kholikov, S.; Burtseva, O.; Hill, F.; Cally, P. Bibcode: 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. Title: Probing sunspots with two-skip time-distance helioseismology Authors: Duvall, Thomas L., Jr.; Cally, Paul S.; Przybylski, Damien; Nagashima, Kaori; Gizon, Laurent Bibcode: 2018A&A...613A..73D Altcode: 2018arXiv180601032D Context. Previous helioseismology of sunspots has been sensitive to both the structural and magnetic aspects of sunspot structure.
Aims: We aim to develop a technique that is insensitive to the magnetic component so the two aspects can be more readily separated.
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.
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.
Conclusions: With sufficient modeling effort, these should lead to better understanding of sunspot structure. Title: Fast-to-Alfvén Mode Conversion in the Presence of Ambipolar Diffusion Authors: Cally, Paul S.; Khomenko, Elena Bibcode: 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.” 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. Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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. Title: The Origin of the "Seasons" in Space Weather Authors: Dikpati, Mausumi; Cally, Paul S.; McIntosh, Scott W.; Heifetz, Eyal Bibcode: 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. Title: Alfvén waves in the structured solar corona Authors: Cally, Paul S. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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∘ (up to 34∘ 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 -1/2 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∘ 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 Bibcode: 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 10-4. In galactic star-forming regions, f can be 10-10 or lower. Under these circumstances, the Hall current can couple low-frequency Alfvén and magnetoacoustic waves via the dimensionless Hall parameter ɛ =ω /{{{Ω }}}{{i}}f, where ω is the wave frequency and {{{Ω }}}{{i}} 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 ωccosθ 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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.

Eclipse photograph

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.

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.

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.

Paul Cally, Robert Erdélyi and Aimee Norton

Conference photograph Title: Modelling Fast-Alfvén Mode Conversion Using SPARC Authors: Moradi, H.; Cally, P. S. Bibcode: 2013JPhCS.440a2047M Altcode: 2013arXiv1302.6301M We successfully utilise the SPARC code to model fast-Alfvén mode conversion in the region cA >> cs 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 1 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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.
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.
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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 × 1023 J, approximately 500 times the seismic emission of ~4 × 1020 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 1, 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. Bibcode: 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

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?

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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 2005RoAJ...15...33B Altcode: No abstract at ADS Title: Observations and Interpretation of Subsurface Magnetic Structures Authors: Cally, P. S.; Crouch, A. D. Bibcode: 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. Bibcode: 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. Bibcode: 2004ESASP.559..392C Altcode: 2004soho...14..392C No abstract at ADS Title: Acoustics of Surface Magnetic Fields Authors: Cally, P. S. Bibcode: 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. Bibcode: 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 5×1011 and 3×1010 cm2 s-1 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 τℓ≈4π−4(L/R)2P, 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, τℓ=Rae/a2, where a and ae 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 αp exceeds the rotational angular velocity ωp, with maximum growth rate (α2p- ω2p)1/2. 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 α2p < ω2p, 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. Bibcode: 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. Bibcode: 2003ASPC..305..152C Altcode: 2003mfob.conf..152C No abstract at ADS Title: Observations and Interpretation of Sub-Surface Structures Authors: Cally, Paul Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 2F3 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. Bibcode: 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. Bibcode: 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≫ωc, N, where ωc is the acoustic cut-off and N the Brunt-Väisälä frequency. Although ωc 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 wn, 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. Bibcode: 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

Γ ∝ fωM-½ ∝fωm+2,

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 ∝ ω4.2) than previous predictions based on the excitation and damping of solar p-modes by turbulent convection (which yields Γ ∝ γ2 M-1 ∝ω2m+4), or the scattering of p-modes by convective eddies (which yields Γ ∝ ω(4/3)m+3), 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 ωf=ωfr-iωfi, 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 ΔωA=2ωfi develops on a timescale of τfi=ωfi-1. On timescales longer than τfi, 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 Lph(t)=2π(tdωA/dx)-1. 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.

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 τI in terms of the height-integrated Pedersen conductivity ΣP, and hence derive a limiting ionospheric perturbation scale LI=2π(τIdωA/dx)-1, in agreement with previous steady state analyses. For sufficiently high ΣP, FLR might be able to evolve so that their radial scales reach a kinetic scale length Lk. For this to occur, we require the pulsations to live for longer than τk=2π(LkdωA/dx)-1. For t<τk,τI, 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. Bibcode: 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 p1-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 P1-P3 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1991mcch.conf..103C Altcode: No abstract at ADS Title: Turbulent Thermal Conduction in the Solar Transition Region Authors: Cally, P. S. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 (≡108 erg cm-2 s-1) 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. Bibcode: 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. Bibcode: 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 «δv2», is shown to be generally incorrect in the presence of field spread, and must be replaced by F = N−2/3(4π)−1ϱa «δv2», 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. Bibcode: 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 10−1 km s−1 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. Bibcode: 1983BAAS...15Q.706A Altcode: No abstract at ADS Title: Complex eigenvalue bounds in magnetoatmospheric shear flow. II Authors: Cally, P. S.; Adam, J. A. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 1979PhDT.......233C Altcode: No abstract at ADS