Author name code: roberts ADS astronomy entries on 2022-09-14 =author:"Roberts, B." ------------------------------------------------------------------------ Title: UCGretina GEANT4 simulation of the GRETINA Gamma-Ray Energy Tracking Array Authors: Riley, L. A.; Weisshaar, D.; Crawford, H. L.; Agiorgousis, M. L.; Campbell, C. M.; Cromaz, M.; Fallon, P.; Gade, A.; Gregory, S. D.; Haldeman, E. B.; Jarvis, L. R.; Lawson-John, E. D.; Roberts, B.; Sadler, B. V.; Stine, C. G. Bibcode: 2021NIMPA100365305R Altcode: 2021arXiv210409986R UCGretina, a GEANT4 simulation of the GRETINA gamma-ray tracking array of highly-segmented high-purity germanium detectors is described. We have developed a model of the array, in particular of the Quad Module and the capsules, that gives good agreement between simulated and measured photopeak efficiencies over a broad range of gamma-ray energies and reproduces the shape of the measured Compton continuum. Both of these features are needed in order to accurately extract gamma-ray yields from spectra collected in in-beam gamma-ray spectroscopy measurements with beams traveling at v ∕ c ≳ 0 . 3 at the National Superconducting Cyclotron Laboratory and the Facility for Rare Isotope Beams. In the process of developing the model, we determined that millimeter-scale layers of passive germanium surrounding the active volumes of the simulated crystals must be included in order to reproduce measured photopeak efficiencies. We adopted a simple model of effective passive layers and developed heuristic methods of determining passive-layer thicknesses by comparison of simulations and measurements for a single crystal and for the full array. Prospects for future development of the model are discussed. Title: Violation of the equivalence principle from light scalar fields: from Dark Matter candidates to scalarized black holes Authors: Hees, A.; Minazzoli, O.; Savalle, E.; Stadnik, Y. V.; Wolf, P.; Roberts, B. Bibcode: 2019arXiv190508524H Altcode: Tensor-scalar theory is a wide class of alternative theory of gravitation that can be motivated by higher dimensional theories, by models of dark matter or dark ernergy. In the general case, the scalar field will couple non-universally to matter producing a violation of the equivalence principle. In this communication, we review a microscopic model of scalar/matter coupling and its observable consequences in terms of universality of free fall, of frequencies comparison and of redshifts tests. We then focus on two models: (i) a model of ultralight scalar dark matter and (ii) a model of scalarized black hole in our Galactic Center. For both these models, we present constraints using recent measurements: atomic clocks comparisons, universality of free fall measurements, measurement of the relativistic redshift with the short period star S0-2 orbiting the supermassive black hole in our Galactic Center. Title: Simulation Studies for a Surface Veto Array to Identify Astrophysical Neutrinos at the South Pole Authors: Euler, S.; Gonzalez, J.; Roberts, B. Bibcode: 2015ICRC...34.1070E Altcode: 2015PoS...236.1070E No abstract at ADS Title: NuSTAR observatory operations and data analysis Authors: Forster, Karl; Harrison, F.; Grefenstette, B.; Madsen, K.; Miyasaka, H.; Rana, V.; Hubbard, M.; Davis, A.; Perri, M.; Puccetti, S.; Spagnuolo, N.; Giommi, P.; Bester, M.; Lewis, M.; Roberts, B.; Craig, W. W.; Marchant, W.; Markwardt, C.; Marshall, F. E.; Zonak, S.; Dodd, S. R.; Stern, D. Bibcode: 2014AAS...22343805F Altcode: We present an overview of the observatory operations and science data analysis for the Nuclear Spectroscopic Telescope Array (NuSTAR) mission. Launched in June 2012, NuSTAR is the first focusing high energy (3-79 keV) X-ray observatory. The NuSTAR project is led by the California Institute of Technology (Caltech) with mission operations managed by the Space Sciences Laboratory (SSL) at the University of California, Berkeley. NuSTAR science data are processed automatically at the Science Operations Center (SOC) at Caltech, making use of the NuSTAR Data Analysis Software package (NuSTARDAS), jointly developed by the ASI Science Data Center (ASDC, Italy) and Caltech. Calibrated data from completed observations are made publicly available at NASA's High Energy Astrophysics Science Archive Center (HEASARC) at the Goddard Space Flight Center and at ASDC. The NuSTARDAS package is integrated into the multi-mission HEASoft X-ray data analysis software package, and NuSTAR calibration data are now part of the HEASARC Calibration Database. NuSTAR will complete its primary mission phase in 2014 and opportunities for the community to propose for observing time will become available through a guest investigator program as well as joint observing proposals with the XMM-Newton and Chandra observatories. Title: Simultaneous and Common-Volume Lidar Observations of Mesospheric Fe and Na Layers at Boulder: Main Layers, Sporadic Layers, and Meteor Trails Authors: Huang, W.; Chu, X.; Wang, Z.; Fong, W.; Yu, Z.; Smith, J. A.; Roberts, B. Bibcode: 2011AGUFMSA51B1946H Altcode: The rich features of the mesospheric metal layers are produced by the complicated chemical and dynamical processes in the Upper Mesosphere/Lower Thermosphere region. Current chemical models with parameterized inputs are able to reproduce some large-scale structures but challenged in small-scale features. Simultaneous and common-volume observations of multiple species provide crucial data to help reveal the determining mechanisms in small scales and validate the models. Such simultaneous observations of Fe and Na layers are still rare, despite several reports based on one or a few nights of data, and one extensive study at Wuhan (30°N), China. In summer and fall 2010, we made over 16 nights of simultaneous Fe and Na observations at Boulder (40°N, 105°W) with two resonance lidars: an Fe Boltzmann temperature lidar and a 3-frequency Na Doppler lidar. The data show the primary characteristics of Fe vs. Na layers, e.g., higher Fe density and column abundance, lower Fe peak and centroid altitude, larger density gradient at the bottom of Fe layer, similar to previous reports at mid-latitude. The altitudes of lower boundaries for both species are similar and closely follow each other, while the upper boundary of Na layer normally extends to higher altitude. Moreover, we introduce a method of using the relative density perturbations to analyze the simultaneous measurements, which is powerful to reveal the correlated density variations for both species in the main and sporadic layers. Differences in the amplitude and phase are noticed in a few cases. On 11 August 2010, two lidars caught meteor trails of Perseids meteor shower, which may have increased the column abundance of both Fe and Na layers. At last, the temperature profiles obtained simultaneously by the lidars for most of the nights will provide unique and critical information for future modeling efforts. Title: The period ratio for kink and sausage modes in a magnetic slab Authors: Macnamara, C. K.; Roberts, B. Bibcode: 2011A&A...526A..75M Altcode:
Aims: Increasing observational evidence of wave modes in the solar corona brings us to a closer understanding of that medium. Coronal seismology allows us to combine wave observations and theory to determine otherwise unknown parameters. The period ratio, P1/2P2, between the period P1 of the fundamental mode and twice the period P2 of its first overtone, is one such tool of coronal seismology and its departure from unity provides information about the structure of the corona.
Methods: We consider analytically the period ratio for the fast kink and sausage modes of a magnetic slab, discussing both an Epstein density profile and a simple step function profile.
Results: Transverse density structuring in the form of an Epstein profile or a step function profile may contribute to the shift of the period ratio for long thin slab-like structures. Title: Initial Results of Na Density and Temperature Measurements by a STAR Na Lidar at Boulder Authors: Fong, W.; Dahlke, I.; Roberts, B.; Smith, J. A.; Yu, Z.; Huang, W.; Chu, X. Bibcode: 2010AGUFMSA43A1741F Altcode: A new Na resonance fluorescence Doppler lidar, Student Atmospheric Resonance Lidar (STAR Lidar), has been deployed at the Table Mountain Lidar Observatory in Boulder (40°N) in the summer of 2010. With the implementation of the acoustic-optic modulators in this August, the two additional shifted frequencies of the output lidar pulses enable the capability of temperature and wind measurement in the mesosphere and lower thermosphere (MLT) region. Several nights of Na density and temperature observation results will be presented and discussed, as well as sporadic Na layers at high altitude. Except to serve as a powerful tool for scientific study of temperature and wind structures, the STAR Lidar is also expected to support lidar education in Boulder providing students with hands-on experience by operating the lidar system in the future. Title: Turbulence in the Solar Atmosphere and Solar Wind Authors: Petrosyan, A.; Balogh, A.; Goldstein, M. L.; Léorat, J.; Marsch, E.; Petrovay, K.; Roberts, B.; von Steiger, R.; Vial, J. C. Bibcode: 2010SSRv..156..135P Altcode: 2010SSRv..tmp..117P The objective of this review article is to critically analyze turbulence and its role in the solar atmosphere and solar wind, as well as to provide a tutorial overview of topics worth clarification. Although turbulence is a ubiquitous phenomenon in the sun and its heliosphere, many open questions exist concerning the physical mechanisms of turbulence generation in solar environment. Also, the spatial and temporal evolution of the turbulence in the solar atmosphere and solar wind are still poorly understood. We limit the scope of this paper (leaving out the solar interior and convection zone) to the magnetized plasma that reaches from the photosphere and chromosphere upwards to the corona and inner heliosphere, and place particular emphasis on the magnetic field structures and fluctuations and their role in the dynamics and radiation of the coronal plasma. To attract the attention of scientists from both the fluid-dynamics and space-science communities we give in the first two sections a phenomenological overview of turbulence-related processes, in the context of solar and heliospheric physics and with emphasis on the photosphere-corona connection and the coupling between the solar corona and solar wind. We also discuss the basic tools and standard concepts for the empirical analysis and theoretical description of turbulence. The last two sections of this paper give a concise review of selected aspects of oscillations and waves in the solar atmosphere and related fluctuations in the solar wind. We conclude with some recommendations and suggest topics for future research. Title: Effects of thermal conduction and compressive viscosity on the period ratio of the slow mode Authors: Macnamara, C. K.; Roberts, B. Bibcode: 2010A&A...515A..41M Altcode:
Aims: Increasing observational evidence of wave modes brings us to a closer understanding of the solar corona. Coronal seismology allows us to combine wave observations and theory to determine otherwise unknown parameters. The period ratio, P1/2P2, between the period P1 of the fundamental mode and the period P2 of its first overtone, is one such tool of coronal seismology and its departure from unity provides information about the structure of the corona.
Methods: We consider analytically the effects of thermal conduction and compressive viscosity on the period ratio for a longitudinally propagating sound wave.
Results: For coronal values of thermal conduction the effect on the period ratio is negligible. For compressive viscosity the effect on the period ratio may become important for some short hot loops.
Conclusions: Damping typically has a small effect on the period ratio, suggesting that longitudinal structuring remains the most significant effect. Title: Coronal Seismology by Means of Kink Oscillation Overtones Authors: Andries, J.; van Doorsselaere, T.; Roberts, B.; Verth, G.; Verwichte, E.; Erdélyi, R. Bibcode: 2009SSRv..149....3A Altcode: The detection of overtones of coronal loop kink oscillations has been an important advance in the development of coronal seismology. It has significantly increased the potential of coronal seismology and has thus initiated important theoretical and observational improvements. New detections of overtones have been made and a reduction of the error bars has been obtained. The efforts of theoreticians to extend eigenmode studies to more general coronal loop models is no longer a matter of checking the robustness of the model but now also allows for the estimation of certain equilibrium parameters. The frequencies of the detected (longitudinal) overtones are in particular sensitive to changes in the equilibrium properties along the loop, especially the density and the magnetic field expansion. Also, attempts have been made to use the limited longitudinal resolution in combination with the theoretical eigenmodes as an additional seismological tool. Title: THEMIS Operations Authors: Bester, M.; Lewis, M.; Roberts, B.; McDonald, J.; Pease, D.; Thorsness, J.; Frey, S.; Cosgrove, D.; Rummel, D. Bibcode: 2008SSRv..141...91B Altcode: 2008SSRv..tmp..175B THEMIS—a five-spacecraft constellation to study magnetospheric events leading to auroral outbursts—launched on February 17, 2007. All aspects of operations are conducted at the Mission Operations Center at the University of California at Berkeley. Activities of the multi-mission operations team include mission and science operations, flight dynamics and ground station operations. Communications with the constellation are primarily established via the Berkeley Ground Station, while NASA’s Ground Network provides secondary pass coverage. In addition, NASA’s Space Network supports maneuver operations near perigee. Following a successful launch campaign, the operations team performed on-orbit probe bus and instrument check-out and commissioning tasks, and placed the constellation initially into a coast phase orbit configuration to control orbit dispersion and conduct initial science operations during the summer of 2007. Mission orbit placement was completed in the fall of 2007, in time for the first winter observing season in the Earth’s magnetospheric tail. Over the course of the first 18 months of on-orbit constellation operations, procedures for instrument configuration, science data acquisition and navigation were refined, and software systems were enhanced. Overall, the implemented ground systems at the Mission Operations Center proved to be very successful and completely adequate to support reliable and efficient constellation operations. A high degree of systems automation is employed to support lights-out operations during off-hours. Title: Progress in coronal seismology Authors: Roberts, B. Bibcode: 2008IAUS..247....3R Altcode: 2007IAUS..247....3R Coronal seismology is now a well developed area of solar physics, even though many questions remain for resolution. Here we take stock of the progress made since the first direct imaging of oscillating loops was achieved through TRACE spacecraft observations in 1999. Title: Analytical determination of coronal parameters using the period ratio P1/2P2 Authors: McEwan, M. P.; Díaz, A. J.; Roberts, B. Bibcode: 2008A&A...481..819M Altcode: Context: In transverse coronal loop oscillations, two periodicities have been measured simultaneously and are interpreted as the fundamental kink mode (with period P_1) and the first harmonic (with period P_2). Deviations of the period ratio P1/2P2 from unity provide information about the extent of longitudinal structuring within the loop.
Aims: Here we develop an analytical approximation that describes the shift in P1/2P2 in terms of the ratio L/Λc of the length 2L of a coronal loop and the density scale height Λ_c.
Methods: We study the MHD wave equations in a low β plasma using the thin tube approximation. Disturbances are described by a differential equation which may be solved for various equilibrium density profiles, obtaining dispersion relations in terms of Bessel functions. These dispersion relations may be used to obtain analytical approximations to the periods P1 and P2. We also present a variational approach to determining the period ratio and show how the WKB method may be used.
Results: Analytical approximations to the period ratio P1/2P2 are used to shed light on the magnitude of longitudinal structuring in a loop, leading to a determination of the density scale height. We apply our formula to the observations in Verwichte et al. (2004) and Van Doorsselaere et al. (2007), obtaining the coronal density scale height.
Conclusions: Our simple formula and approximate approaches highlight a useful analytical tool for coronal seismology. We demonstrate that P1/2P2 is linked to the density scale height, with no need for estimates of other external parameters. Given the accuracy of current observations, our formula provides a convenient means of determining density scale heights. Title: Effect of coronal structure on loop oscillations: exponential profiles Authors: Díaz, A. J.; Donnelly, G. R.; Roberts, B. Bibcode: 2007A&A...476..359D Altcode: Aims:The role of longitudinal structuring of the surrounding corona on the modes of oscillation of a coronal magnetic flux tube was studied in Donnelly et al. (2006) for a piecewise uniform profile. Here we investigate whether a more realistic continuous exponential profile changes the conclusions drawn from that paper.
Methods: A partial differential equation is derived for the total pressure perturbation of the fast modes, which is then decomposed by separation of variables. The longitudinal part is solved numerically, obtaining a dispersion relation. These results are supported by an analytical investigation in terms of Bessel functions of purely imaginary order.
Results: Structure in the interior of the loop shifts the frequencies of the modes (and may trap higher harmonics), an effect which can be understood by taking an averaged profile with a suitable weight. Structure in the environment modifies only slightly the frequencies, but displaces the cutoff frequency. The shift due to the structure in the fundamental period is small, but the ratio between the periods of the fundamental mode and its harmonics can be used to probe the structure.
Conclusions: The results support our previous study in a more realistic, continuously varying profile and provide limits to the conclusions drawn in coronal seismology if an unstructured loop is used. Also, the ratio between the period of the fundamental kink (even) mode and its first (odd) harmonic is proven as an extra seismological tool for coronal loops. Title: Effect of longitudinal density structure on a straight magnetic field modelling coronal arcade oscillations Authors: Donnelly, G. R.; Díaz, A. J.; Roberts, B. Bibcode: 2007A&A...471..999D Altcode: Aims:Motivated by recent observations of oscillations in coronal arcades, we investigate analytically the influence of longitudinal structuring on the modes of oscillation of a straight coronal loop arcade. As a first step towards more complicated models, we use a simple structure to obtain analytical solutions.
Methods: A partial differential equation is derived for the total pressure perturbation of the fast modes in a zero beta plasma and it is solved analytically. We first recover the results for a homogeneous structure, and then study an equilibrium with an exponentially structured density profile, solving it in terms of Bessel functions of non-integer order and exponential argument, thus obtaining a dispersion relation. The properties of this dispersion relation are discussed and some limits studied, leading to analytical approximations to the eigenfrequencies.
Results: The introduction of longitudinal structuring results in a modification to the oscillatory frequencies of the modes of oscillation in such structures when compared with the uniform case. Regarding the oscillatory periods P_n, n=1,2, dots, the period ratios P_1/2P2 and P_1/3P3 are both shifted from unity. Other properties described in structured coronal loops are also found in an arcade: the occurrence of avoided crossings in the dispersion diagram and the displacement of the extrema towards the footpoints in the spatial structure of the eigenmodes.
Conclusions: We show analytically for simple arcade modes that the shift in the fundamental period proves to be small, but the ratio P_1/2P2 depends strongly on the density structure. Moreover, transversal propagation also shifts the ratio P_1/2P2 from unity, so it can be used in the coronal seismology of arcades in which transversal propagation is present. We use the currently available observational data to illustrate this application.

Appendix A is only available in electronic form at http://www.aanda.org Title: On the period ratio P1/2P2 in the oscillations of coronal loops Authors: McEwan, M. P.; Donnelly, G. R.; Díaz, A. J.; Roberts, B. Bibcode: 2006A&A...460..893M Altcode: Aims.With strong evidence of fast and slow magnetoacoustic modes arising in the solar atmosphere there is scope for improved determinations of coronal parameters through coronal seismology. Of particular interest is the ratio P1/2P2 between the period P1 of the fundamental mode and the period P2 of its first harmonic; in an homogeneous medium this ratio is one, but in a more complex configuration it is shifted to lower values.
Methods: .We consider analytically the effects on the different magnetohydrodynamic modes of structuring and stratification, pointing out that transverse or longitudinal structuring or gravitational stratification modifies the ratio P1/2P2.
Results: .The deviations caused by gravity and structure are studied for the fast and slow modes. Structure along the loop is found to be the dominant effect.
Conclusions: .The departure of P1/2P2 from unity can be used as a seismological tool in the corona. We apply our technique to the observations by Verwichte et al. (2004), deducing the density scale height in a coronal loop. Title: Slow MHD oscillations in density structured coronal loops Authors: Díaz, A. J.; Roberts, B. Bibcode: 2006A&A...458..975D Altcode: Aims.Signals of stationary slow modes have been detected in observational data and modelled through numerical computations, comparing these results with the modes of a homogeneous tube. Here we explore the effect of structure along the magnetic field on the modes of oscillation of a coronal loop.
Methods: .We present a limit in which the slow mode is decoupled from the other magnetohydrodynamic modes, describing its behaviour in terms of a relatively simple partial differential equation. This equation is solved analytically and numerically for various longitudinal profiles.
Results: .For low density contrast between footpoints and apex, the modes of the structured tube are similar to the modes of the homogeneous tube, evolving regularly from them, with small modifications in frequency and spatial structure. As the density contrast is increased, the extrema are displaced towards the dense layers and the frequencies of the higher harmonics are strongly modified. Finally, as the ratio is increased further, two types of modes appear: modes approximately line-tied in the dense layer and modes with high amplitude in them (with avoided crossings between them in the dispersion diagrams).
Conclusions: .Different regimes can be identified, depending on the density contrast between the loop footpoints and its apex. This allows us to compare apparently different numerical results and understand their various features. Our analytical results are in accordance with current numerical simulations. Title: Effect of coronal structure on loop oscillations Authors: Donnelly, G. R.; Díaz, A. J.; Roberts, B. Bibcode: 2006A&A...457..707D Altcode: Aims.We investigate the influence of longitudinal structuring of the surrounding corona on the modes of oscillation of a coronal magnetic flux tube.
Methods: .A partial differential equation is derived for the total pressure perturbation of the fast modes and it is solved analytically in terms of Bessel functions, obtaining a dispersion relation.
Results: .The introduction of coronal structuring changes the cutoff frequency, enhancing coronal leakage, so even the fundamental kink mode may become leaky. Structure also modifies the loop's oscillatory frequencies and may result in higher harmonics being trapped.
Conclusions: .Depending on the structuring, two competing effects take place: environmental structuring enhances leakage, while loop structuring helps confine the modes. This has important consequences for coronal seismology, leading to the absence of trapped modes for certain parameters and shifts in frequencies. Title: Fast magnetohydrodynamic oscillations in a force-free line-tied coronal arcade Authors: Díaz, A. J.; Zaqarashvili, T.; Roberts, B. Bibcode: 2006A&A...455..709D Altcode: Aims.We discuss a simple model of a line-tied coronal arcade with piecewise constant density to explore the effects of curvature on radially polarised fast modes.
Methods.A partial differential equation is derived for the velocity perturbation of the fast modes and it is solved analytically in terms of Bessel functions of half integer order, obtaining a dispersion relation.
Results.The properties of the modes are studied in terms of the parameters. All the modes are leaky under these conditions. Besides the usual kink and sausage modes, new families are described: the vertical, swaying (longitudinal), and rocking modes arise.
Conclusions.The damping rates are similar to observed rates. For thin arcades the modes are markedly different from those of a straight slab and resemble more the modes of a circular membrane. Title: Comment on ``Note on the Initial Value Problem for Coronal Loop Kink Waves'' By P. S. Cally Authors: Ruderman, M. S.; Roberts, B. Bibcode: 2006SoPh..237..119R Altcode: 2006SoPh..tmp....5R In our comment we argue that the results obtained by Cally (2006) are based on a wrong solution of the initial value problem. Title: Java Message Service (JMS) use in the Telescope Automation and Remote Observing System (TAROS) Authors: Czezowski, A.; Green, A.; Hovey, G.; Jarnyk, M.; Nielsen, J.; Roberts, B.; Sebo, K.; Smith, D.; Vaccarella, A.; Wilson, G.; Young, P. Bibcode: 2006ASPC..351..208C Altcode: 2006adass..15..208C JMS is an enterprise messaging system, part of the Java 2 Platform, Enterprise Edition (J2EE). It provides distributed applications a method for asynchronously sending and receiveing critical data and events. It also decouples the message delivery management subsystem from the application itself. TAROS is a distributed system that will allow the Australian National University telescopes at Siding Spring Observatory to be operated automatically or interactively over the Internet. JMS technology is used throughout TAROS, providing a communication path between the TAROS back-end software components as well as between TAROS and its external clients. Title: Asteroseismology of sdB stars with FUSE. Authors: Kuassivi; Bonanno, A.; Ferlet, R.; Roberts, B.; Caplinger, J. Bibcode: 2006ASPC..348..215K Altcode: The hot subdwarfs B (sdB) stars form an homogeneous group populating an extension of the horizontal branch (HB) in the (Teff--log g) diagram towards temperatures up to 40,000 K. The recent discovery that many of them are multimode pulsators has triggered a large observational and theoretical effort. We discuss the possibility to perform space--based asteroseismology with FUSE and we demonstrate that periodic luminosity variations are already detected in archival TTAG data of sdB stars. In particular, we report on the FUSE observation of the pulsating PG 1219+534 which shows the presence of periodic variations consistent with those reported from ground--based observations at 6.9 mHz and 7.8 mHz. Title: Two-wave interaction in ideal magnetohydrodynamics Authors: Zaqarashvili, T. V.; Roberts, B. Bibcode: 2006A&A...452.1053Z Altcode: 2006astro.ph..2535Z The weakly nonlinear interaction of sound and linearly polarised Alfvén waves propagating in the same direction along an applied magnetic field is studied. It is found that a sound wave is coupled to the Alfvén wave with double period and wavelength when the sound and Alfvén speeds are equal. The Alfvén wave drives the sound wave through the ponderomotive force, while the sound wave returns energy back to the Alfvén wave through the parametric (swing) influence. As a result, the two waves alternately exchange their energy during propagation. The process of energy exchange is faster for waves with stronger amplitudes. The phenomenon can be of importance in astrophysical plasmas, including the solar atmosphere and solar wind. Title: Fast Magnetohydrodynamic Oscillations in a Fibril Prominence Model Authors: Díaz, A. J.; Roberts, B. Bibcode: 2006SoPh..236..111D Altcode: Observations of quiescent filaments show fine-scale structures, suggesting that they can be composed of small-scale threads or fibrils. Here, we study the oscillations of group of periodic fibrils in a Cartesian geometry using Bloch's theorem. The periodic array of fibrils provides a bridge between the seemingly different oscillations of a finite number of fibrils and the oscillations of an homogeneous prominence. Our results show that fine structure plays an important role and cannot be neglected. The filling factor can be used to obtain reliable estimates of periods. Also, the presence of the fibrils introduces a fine-scale modulation in the spatial structure of the global oscillations. Title: Slow magnetohydrodynamic waves in the solar atmosphere Authors: Roberts, B. Bibcode: 2006RSPTA.364..447R Altcode: No abstract at ADS Title: Resonant Conversion of Standing Acoustic Oscillations Into ALFVÉN Waves in the β~1 Region of the Solar Atmosphere Authors: Kuridze, D.; Zaqarashvili, T. V.; Roberts, B. Bibcode: 2005ESASP.600E..89K Altcode: 2005dysu.confE..89K; 2005ESPM...11...89K; 2005astro.ph.10437K We show that 5-minute acoustic oscillations may resonantly convert into Alfv{é}n waves in the $\beta{\sim}1$ region of the solar atmosphere. Considering the 5-minute oscillations as pumping standing acoustic waves oscillating along unperturbed vertical magnetic field, we find on solving the ideal MHD equations that amplitudes of Alfv{é}n waves with twice the period and wavelength of acoustic waves exponentially grow in time when the sound and Alfv{é}n speeds are equal, i.e. $c_s \approx v_A$. The region of the solar atmosphere where this equality takes place we call a {\it swing layer}. The amplified Alfv{é}n waves may easily pass through the chromosphere and transition region carrying the energy of p-modes into the corona. Title: Telescope Automation and Remote Observing System (TAROS) Authors: Wilson, G.; Czezowski, A.; Hovey, G. R.; Jarnyk, M. A.; Nielsen, J.; Roberts, B.; Sebo, K.; Smith, D.; Vaccarella, A.; Young, P. Bibcode: 2005ASPC..347..563W Altcode: 2005adass..14..563W TAROS is a system that will allow for the Australian National University telescopes at a remote location to be operated automatically or interactively with authenticated control via the internet. TAROS is operated by a Java front-end GUI and employs the use of several Java technologies - such as Java Message Service (JMS) for communication between the telescope and the remote observer, Java Native Interface to integrate existing data acquisition software written in C++ (CICADA) with new Java programs and the JSky collection of Java GUI components for parts of the remote observer client. In this poster the design and implementation of TAROS is described. Title: The influence of internal magnetic layers on the frequencies of solar p-modes Authors: Foullon, C.; Roberts, B. Bibcode: 2005A&A...439..713F Altcode: Buried magnetic fields at the base of the convection zone and in the anchoring zones of sunspots are presumed to vary over the solar activity cycle. Their effect on p-mode oscillations is explored in detail, through theoretical modelling. The helioseismic signature from a "tachocline" or "shallow" horizontal layer of magnetic field, buried in a plane-stratified model of the Sun, is explored by examining frequency shifts of various order and degree. p-modes propagating perpendicular to the magnetic field lines are found to yield the largest frequency shifts. However, frequency shifts due to buried magnetic fields are considerably smaller than observationally determined shifts over the solar cycle. Nonetheless, an analytical approach to the problem provides useful insight for solar and stellar applications. Title: ``Swing Absorption'' of fast magnetosonic waves in inhomogeneous media Authors: Shergelashvili, B. M.; Zaqarashvili, T. V.; Poedts, S.; Roberts, B. Bibcode: 2005A&A...433...15S Altcode: A&A, 429, 767-777 (2005), DOI: 10.1051/0004-6361:20041494 Title: ``Swing Absorption'' of fast magnetosonic waves in inhomogeneous media Authors: Shergelashvili, B. M.; Zaqarashvili, T. V.; Poedts, S.; Roberts, B. Bibcode: 2005A&A...429..767S Altcode: 2004astro.ph..8114S The recently suggested swing interaction between fast magnetosonic and Alfvén waves (Zaqarashvili & Roberts \cite{paper1}) is generalized to inhomogeneous media. We show that the fast magnetosonic waves propagating across an applied non-uniform magnetic field can parametrically amplify the Alfvén waves propagating along the field through the periodical variation of the Alfvén speed. The resonant Alfvén waves have half the frequency and the perpendicular velocity polarization of the fast waves. The wavelengths of the resonant waves have different values across the magnetic field, due to the inhomogeneity in the Alfvén speed. Therefore, if the medium is bounded along the magnetic field, then the harmonics of the Alfvén waves, which satisfy the condition for onset of a standing pattern, have stronger growth rates. In these regions the fast magnetosonic waves can be strongly ``absorbed'', their energy going in transversal Alfvén waves. We refer to this phenomenon as ``Swing Absorption''. This mechanism can be of importance in various astrophysical situations. Title: The Mechanism of Swing Absorption of Fast Magnetosonic Waves in Inhomogeneous Media Authors: Shergelashvili, B. M.; Zaqarashvili, T. V.; Poedts, S.; Roberts, B. Bibcode: 2004ESASP.575..431S Altcode: 2004astro.ph.10277S; 2004soho...15..431S The recently suggested swing interaction between fast magnetosonic and Alfvén waves (2002) is generalized to inhomogeneous media. We show that the fast magnetosonic waves propagating across an applied non-uniform magnetic field can parametrically amplify the Alfvén waves propagating along the field through the periodical variation of the Alfvén speed. The resonant Alfvén waves have half the frequency and the perpendicular velocity polarization of the fast waves. The wavelengths of the resonant waves have different values across the magnetic field, due to the inhomogeneity in the Alfvén speed. Therefore, if the medium is bounded along the magnetic field, then the harmonics of the Alfvén waves, which satisfy the condition for onset of a standing pattern, have stronger growth rates. In these regions the fast magnetosonic waves can be strongly 'absorbed', their energy going in transversal Alfvén waves. We refer to this phenomenon as 'Swing Absorption'. This mechanism can be of importance in various astrophysical situations. Title: Asteroseismology of sdB stars with FUSE Authors: Kuassivi; Bonanno, A.; Ferlet, R.; Roberts, B.; Caplinger, J. Bibcode: 2004astro.ph.11371K Altcode: The hot subdwarf B (sdB) stars form an homogeneous group populating an extension of the horizontal branch (HB) in the ($T_{\rm eff}$--$\log g$) diagram towards temperatures up to 40,000 K. The recent discovery that many of them are multimode pulsators has triggered a large observational and theoretical effort. We discuss the possibility of performing space-based asteroseismology with {\em FUSE}, and we demonstrate that periodic luminosity variations are already detectable in archival TTAG data of sdB stars. In particular, we report on the {\em FUSE} observation of the pulsator PF 1219+534, which shows the presence of periodic variations at 6.9 mHz and 7.8 mHz, consistent with those reported from ground--based observations. Title: Meeting report: Waves in solar and magnetospheric plasmas Authors: Nakariakov, V. M.; Roberts, B.; Wright, A. N. Bibcode: 2004A&G....45e..32N Altcode: This RAS discussion meeting was arranged with the idea of bringing together solar physicists and magnetospheric physicists, so as to understand better how the two disciplines approach waves, and ultimately to learn from each other. V M Nakariakov, B Roberts and A N Wright report. Title: Fast MHD oscillations in line-tied homogeneous coronal loops Authors: Díaz, A. J.; Oliver, R.; Ballester, J. L.; Roberts, B. Bibcode: 2004A&A...424.1055D Altcode: Loop oscillations have been abundantly reported in recent years. Earlier analytical studies of loop oscillations consider freely propagating waves, allowing for line-tying by a quantization of the wavenumber. Here we consider the rich spectrum of fast MHD modes (both standing and leaky) in coronal loops, allowing for line-tying and performing some comparisons with observational data. We point out that in a straight and homogeneous cylindrical flux tube there should be observational signatures of the excitation of higher order harmonics. Our results indicate that these modes become leaky with the addition of the chromospheric structure at the base of the loop. Leakage can be quite efficient in damping the oscillations for many of these high frequency (compared to fundamental) modes.

Appendix A is only available in electronic form at http://www.edpsciences.org Title: Coronal Loop Oscillations Authors: Donnelly, G. R.; Roberts, B. Bibcode: 2004ESASP.547..531D Altcode: 2004soho...13..531D Following Edwin and Roberts (1983), we carry out a study of the trapped modes of oscillation of a uniform flux tube embedded in a uniform environment. A magnetic flux tube which has a density enhancement is a wave guide for many modes of oscillation. Observations suggest that sausage and kink modes are commonly occurring, both as propagating and as standing waves. Therefore we study the principal modes of oscillation, determining the eigenfunctions for each mode, with the aim of matching these results with observational data. Title: MHD Waves in the Solar Atmosphere Authors: Roberts, B. Bibcode: 2004ESASP.547....1R Altcode: 2004soho...13....1R No abstract at ADS Title: Turbulence, Waves and Instabilities in the Solar Plasma Authors: Erdélyi, R.; Petrovay, K.; Roberts, B.; Aschwanden, M. Bibcode: 2003twis.book.....E Altcode: Significant advances have been made recently in both the theoretical understanding and observation of small-scale turbulence in different layers of the Sun, and in the instabilities that give rise to them. The general development of solar physics, however, has led to such a degree of specialization as to hinder interaction between workers in the field. This book therefore presents studies of different layers and regions of the Sun, but from the same aspect, concentrating on the study of small-scale motions. The main emphasis is on the common theoretical roots of these phenomena, but the book also contains an extensive treatment of the observational aspects.

Link: http://www.springer.com/east/home?SGWID=5-102-22-3362=5696-0&changeHeader=true Title: ION-Neutral Collisions Effect on MHD Surface Waves Authors: Kumar, Nagendra; Roberts, B. Bibcode: 2003SoPh..214..241K Altcode: The effect of ion-neutral collisions on the propagation of MHD waves and surface waves at a single magnetic interface is investigated. The dispersion equations for MHD waves in a partially ionized medium are derived. There are three damped propagating modes in a uniform unbounded medium: an Alfvén mode, and fast and slow modes. The damping of waves depends on both the collisional frequency and the ionization fraction. Wave damping increases as ionization fraction decreases. Surface waves are discussed in three cases: (a) the incompressible limit, (b) the low β plasma, and (c) for parallel propagation. The incompressible limit leads to Alfvén surface waves in a partially ionized medium and the dispersion characteristics are similar to those obtained by Uberoi and Datta. In the low β plasma of the Earth's auroral F region there are two damped propagating magnetoacoustic surface waves for θ=π/3. There is only one damped surface mode for θ=π/2, but no surface wave is able to propagate for θ=0°. For the case of parallel propagation (θ=0°) the results obtained in the absence of ion-neutral collisions are consistent with the results of Jain and Roberts. It is found that a three-mode structure of damped propagating waves occurs owing to ion-neutral collisions for a comparatively high ionization fraction. For the case of the solar photosphere, where the ionization fraction is low, two weakly damped surface waves are found, though the damping is almost negligible. The pattern of propagation is similar to that found in the case discussed by Jain and Roberts, but the wave speeds are lower due to ion-neutral collisions. The strong collisions tie the ion-neutral species together and reduce the damping. Title: Solitary waves in a Hall solar wind plasma Authors: Ballai, I.; Thelen, J. C.; Roberts, B. Bibcode: 2003A&A...404..701B Altcode: In the solar wind, the effect of the Hall current generated perpendicular to the ambient magnetic field can influence the plasma behaviour. In particular, the Hall current introduces wave dispersion which may compensate the nonlinear steepening of waves. In the presence of viscosity, these effects lead to a slowly decaying KdV soliton. Here we investigate a solitary wave propagating obliquely to an ambient magnetic field. Our results show that the nonlinear slow magnetoacoustic wave, that arises from nonlinear steepening, propagates faster than the linear slow mode speed; by contrast, the nonlinear fast magnetoacoustic wave propagates slower than the linear fast wave speed. We apply our calculations to the observed non-thermal Doppler broadening of the ion emission line, measured by the SoHO UVCS telescope in the acceleration region of the solar wind. The observed line broadening is recovered in our theory by the nonlinear slow magnetoacoustic waves; by contrast, fast magnetoacoustic produce too narrow broadening. Title: Swing coupling of sound and Alfvén waves Authors: Zaqarashvili, T. V.; Roberts, B. Bibcode: 2002ESASP.506...79Z Altcode: 2002svco.conf...79Z; 2002ESPM...10...79Z The weakly nonlinear interaction of sound and linearly polarized Alfvén waves propagating along an applied magnetic field is studied in the case when the sound and Alfvén speeds are equal. The sound wave is coupled to an Alfvén wave with double period and wavelength. The Alfvén wave drives the sound wave due to the ponderomotive force, while the sound wave returns the energy back to the Alfvén wave through the swing wave-wave interaction (Zaqarashvili, 2001). As a result, the waves alternately exchange their energy during the propagation. The process of energy exchange is faster for waves with stronger amplitudes. The phenomenon can be of importance in the solar atmosphere, solar wind and other astrophysical situations. Title: Waves and oscillations in the corona: theory Authors: Roberts, B. Bibcode: 2002ESASP.506..481R Altcode: 2002ESPM...10..481R; 2002svco.conf..481R Recent observations of oscillatory phenomena in the corona have provided considerable impetus to the theoretical development of magnetohydrodynamic wave theory in magnetic structures. This review aims to lay out some of the basic ideas and unifying principles that underpin many of the theoretical models of waves and oscillations in magnetic structures. The combination of such theories and the observations leads towards the development of a coronal seismology and moreover promises to provide a greater understanding of coronal heating. Title: Damping of coronal loop oscillations: resonant absorption at work Authors: Ruderman, M. S.; Roberts, B. Bibcode: 2002ESASP.506..745R Altcode: 2002ESPM...10..745R; 2002svco.conf..745R Motivated by recent TRACE observations of damped oscillations in coronal loops, we consider analytically the motion of an inhomogeneous coronal magnetic tube of radius α in a zero β plasma. An initially perturbed tube may vibrate in its kink mode of oscillation with the frequency ωk but those vibrations are damped. The damping is due to resonant absorption, acting in the inhomogeneous regions of the tube, which leads to a transfer of energy from the kink mode to azimuthal oscillations within the inhomogeneous layer. We determine explicitly the decay decrement γ for a coronal flux tube whose plasma density varies in a thin layer of thickness l<<a on the tube boundary. We apply our results to the observations, suggesting that loop oscillations decay principally because of inhomogeneities in the loop. It follows from our theory that only those loops with density inhomogeneities on a small scale (confined to within a thin layer of order αγ/ωk in thickness) are able to support coherent oscillations for any length of time, and so be observable. Loops with a more gradual density variation, on a scale comparable with the tube radius α, do not exhibit pronounced oscillations. Title: Ducted compressional waves in the magnetosphere in the double-polytropic approximation Authors: Ballai, I.; Erdélyi, R.; Roberts, B. Bibcode: 2002AnGeo..20.1553B Altcode: Small-amplitude compressional magnetohydrodynamic-type waves are studied in the magnetosphere. The magnetosphere is treated as a rarefied plasma with anisotropy in the kinetic pressure distribution. The parallel and perpendicular pressures are defined by general polytropic pressure laws. This double-polytropic model can be considered as a natural extension of the magnetohydrodynamic (MHD) model when the plasma is collisionless. Title: Swing coupling between fast magnetosonic and Alfvén waves Authors: Zaqarashvili, T. V.; Roberts, B. Bibcode: 2002ESASP.505..623Z Altcode: 2002solm.conf..623Z; 2002IAUCo.188..623Z We suggest a mechanism of energy transformation from fast magnetosonic waves propagating in homogeneous medium across a uniform magnetic field to Alfvén waves propagating along the field. The mechanism is based on swing wave-wave interaction [T.V. Zaqarashvili, Astrophys. J. Lett. 552, 107 (2001)]. The standing fast magnetosonic waves cause a periodical variation in the Alfvén speed, with the amplitude of an Alfvén wave being governed by Mathieu's equation. Consequently, sub-harmonics of Alfvén waves with a frequency half that of magnetosonic waves grow exponentially in time. It is suggested that the energy of nonelectromagnetic forces, which are able to support the magnetosonic oscillations, may be transmitted into the energy of purely magnetic oscillations. Possible astrophysical applications of the mechanism are briefly discussed. Title: Wave damping in Hall plasmas: application to the solar wind Authors: Ballai, I.; Thelen, J. C.; Roberts, B. Bibcode: 2002ESASP.505..345B Altcode: 2002IAUCo.188..345B; 2002solm.conf..345B The present study focusses on the damping of nonlinear magnetoacoustic waves in Hall viscous plasmas. Special attention is paid to solitary waves and their effect in the solar wind. In particular, the plasma acceleration by an obliquely propagating soliton is investigated. The results show that the solitons originating from the nonlinear steepening of slow waves can accelerate the plasma; the fast solitons obtained from fast waves can decelerate the plasma. For certain propagation angles, the solitons evolve into shock waves. The results are consistent with the observed non-thermal Doppler broadening of the ion emission line, observed by SoHO UVCS telescope in the acceleration region of the solar wind. Title: The Damping of Coronal Loop Oscillations Authors: Ruderman, M. S.; Roberts, B. Bibcode: 2002ApJ...577..475R Altcode: Motivated by recent Transition Region and Coronal Explorer (TRACE) observations of damped oscillations in coronal loops, we consider analytically the motion of an inhomogeneous coronal magnetic tube of radius a in a zero-β plasma. An initially perturbed tube may vibrate in its kink mode of oscillation, but those vibrations are damped. The damping is due to resonant absorption, acting in the inhomogeneous regions of the tube, which leads to a transfer of energy from the kink mode to Alfvén (azimuthal) oscillations within the inhomogeneous layer. We determine explicitly the decrement γ (decay time γ-1) for a coronal flux tube whose plasma density varies only in a thin layer of thickness l on the tube boundary. The effect of viscosity is also considered. We show that, in general, the problem involves two distinct timescales, γ-1 and ω-1kR1/3, where R is the Reynolds number and ωk is the frequency of the kink mode. Under coronal conditions (when γ-1<<ω- 1kR1/3), the characteristic damping time of global oscillations is γ-1. During this time, most of the energy in the initial perturbation is transferred into a resonant absorption layer of thickness of order l2/a, with motions in this layer having an amplitude of order a/l times the initial amplitude. We apply our results to the observations, suggesting that loop oscillations decay principally because of inhomogeneities in the loop. Our theory suggests that only those loops with density inhomogeneities on a small scale (confined to within a thin layer of order aγ/ωk in thickness) are able to support coherent oscillations for any length of time and so be observable. Loops with a more gradual density variation, on the scale of the tube radius a, do not exhibit pronounced oscillations. Title: Swing wave-wave interaction: Coupling between fast magnetosonic and Alfvén waves Authors: Zaqarashvili, T. V.; Roberts, B. Bibcode: 2002PhRvE..66b6401Z Altcode: 2002astro.ph.10151Z We suggest a mechanism of energy transformation from fast magnetosonic waves propagating across a magnetic field to Alfvén waves propagating along the field. The mechanism is based on swing wave-wave interaction [T. V. Zaqarashvili, Astrophys. J. Lett. 552, 107 (2001)]. The standing fast magnetosonic waves cause a periodical variation in the Alfvén speed, with the amplitude of an Alfvén wave being governed by Mathieu's equation. Consequently, subharmonics of Alfvén waves with a frequency half that of magnetosonic waves grow exponentially in time. It is suggested that the energy of nonelectromagnetic forces, which are able to support the magnetosonic oscillations, may be transmitted into the energy of purely magnetic oscillations. Possible astrophysical applications of the mechanism are briefly discussed. Title: Swing Coupling between Stellar Pulsations and Torsional Alfvén Oscillations Authors: Zaqarashvili, T.; Roberts, B. Bibcode: 2002ASPC..259..484Z Altcode: 2002rnpp.conf..484Z; 2002IAUCo.185..484Z No abstract at ADS Title: Nonlinear Waves In The Solar Wind Authors: Ballai, I.; Thelen, J. -C.; Roberts, B. Bibcode: 2002EGSGA..27..724B Altcode: One of the most puzzling effect of the solar physics is the mechanism responsible for acceleration of the high-speed solar wind. In the solar wind, the effect of the Hall current generated perpendicular to the ambient magnetic field can influence the plasma behaviour. In particular, the presence of this current introduces a dispersion in the linear waves which may compensate the nonlinear steepening of waves leading to a slowly decaying KdV soliton due to the viscous character of the plasma. In the present paper, we investigate the possibility of plasma acceleration by this solitary wave. The obtained results reveal that the plasma is accelerated only by solitons which in the linear limit propagate as slow waves; solitons obtained from nonlinear steepening of fast magnetoacoustic waves brakes the plasma. Title: Magnetoacoustic Surface Waves at the Base of the Convection Zone Authors: Foullon, C.; Roberts, B. Bibcode: 2001ASSL..259..291F Altcode: 2001dysu.conf..291F Magnetoacoustic surface waves on the magnetic field at the base of the convection zone are explored. The influence of this field on p- and f-modes is also considered. Title: EUVE J0854+390: a New Cataclysmic Variable Authors: Christian, D. J.; Craig, N.; Dupuis, J.; Roberts, B. Bibcode: 2001IBVS.5032....1C Altcode: We report the identification of a new CV in the field of EUVE J0854+390. The optical spectra show Balmer, He I and He II lines strongly in emission and indicates the source is a CV, probably magnetic. Title: Solar Interior: Influence of Magnetic Fields Authors: Roberts, B. Bibcode: 2000eaa..bookE2251R Altcode: The p-modes that propagate in the solar interior are sound waves in a gravitationally stratified medium (see SOLAR INTERIOR; HELIOSEISMOLOGY: THEORY). Sound waves involve compressions and rarefactions of the medium and as such are influenced by other effects that cause pressure variations. Magnetic fields impart an extra degree of elasticity to a medium. In the plasma that models the Sun and its ... Title: Solar Photospheric Magnetic Flux Tubes: Theory Authors: Roberts, B. Bibcode: 2000eaa..bookE2255R Altcode: The magnetic field in the photospheric layers of the Sun is found to occur not in a homogeneous form but in discrete concentrations of intense field. The most obvious form of magnetic flux (magnetic field strength times surface area occupied by the field) is seen in SUNSPOTS but it turns out that much smaller arrangements of magnetic field are to be found in the lanes between granules where downd... Title: Magnetohydrodynamic Waves Authors: Roberts, B. Bibcode: 2000eaa..bookE2225R Altcode: Magnetohydrodynamic waves are found in a wide variety of astrophysical plasmas. They have been measured in plasma fusion devices and detected in the MAGNETOSPHERE OF EARTH, the SOLAR WIND and a number of magnetic structures seen in the Sun's atmosphere. In space plasmas their detection is often indirect, by matching measured properties (such as propagation speed or pressure variation) with theore... Title: Convective collapse in a thin flux tube Authors: Boddie, David; Roberts, B. Bibcode: 2000AIPC..537..176B Altcode: 2000wdss.conf..176B The magnetic field in the photospheric layers of the Sun outside sunspots is found to be in the form of concentrated magnetic flux tubes of 1-2 kG field strength. The model of convective collapse in a thin flux tube, where an instability causes ``draining'' of the tube and concentration of the magnetic field, has proved useful in attempting to explain the inferred field concentrations. Here we explore the mechanism of convective instability allowing for the inflow of matter in the upper reaches of the tube. . Title: The influence of a buried magnetic field on solar p-modes Authors: Foullon, C.; Roberts, B. Bibcode: 2000AIPC..537..240F Altcode: 2000wdss.conf..240F The magnetic field considered to reside at the base of the convection zone is presumed to vary over the solar activity cycle. We examine the effect of such a buried magnetic layer on the properties of solar p-modes. Analytical and numerical solutions to the dispersion relation for these modes are presented. Frequency changes due to the stored magnetic field are found to be negligible in comparison with the low- and intermediate-degree frequency shifts reported over the solar activity cycle. Nonetheless, there are grounds for inferring the signature of such a buried field through examining shifts of various degree. The p-mode frequencies are increased proportionally to the square of the field strength at the base of the convection zone and depend upon the thickness of the magnetic layer. . Title: Waves and Oscillations in the Corona - (Invited Review) Authors: Roberts, B. Bibcode: 2000SoPh..193..139R Altcode: It has long been suggested on theoretical grounds that MHD waves must occur in the solar corona, and have important implications for coronal physics. An unequivocal identification of such waves has however proved elusive, though a number of events were consistent with an interpretation in terms of MHD waves. Recent detailed observations of waves in events observed by SOHO and TRACE removes that uncertainty, and raises the importance of MHD waves in the corona to a higher level. Here we review theoretical aspects of how MHD waves and oscillations may occur in a coronal medium. Detailed observations of waves and oscillations in coronal loops, plumes and prominences make feasible the development of coronal seismology, whereby parameters of the coronal plasma (notably the Alfvén speed and through this the magnetic field strength) may be determined from properties of the oscillations. MHD fast waves are refracted by regions of low Alfvén speed and slow waves are closely field-guided, making regions of dense coronal plasma (such as coronal loops and plumes) natural wave guides for MHD waves. There are analogies with sound waves in ocean layers and with elastic waves in the Earth's crust. Recent observations also indicate that coronal oscillations are damped. We consider the various ways this may be brought about, and its implications for coronal heating. Title: Damping and frequency reduction of the f-mode due to turbulent motion in the solar convection zone Authors: Mȩdrek, M.; Murawski, K.; Roberts, B. Bibcode: 1999A&A...349..312M Altcode: Solar f-mode properties were observed recently with high accuracy using high-resolution Michelson Doppler Imager (MDI) data from SOHO (Duvall et al. 1998). According to these observations, linewidths increase with wavenumber k and the f-mode frequency omega is significantly lower than the frequency omega_0 given by the simple dispersion relation omega_ {0}(2) = gk. This paper provides a possible explanation of these observations on the basis of the turbulent flow that is in the convection zone. The f-mode spends more time propagating against the flow than with the flow. As a result, its effective speed and consequently frequency are reduced. This reduction is revealed by the real part of omega . A negative imaginary part of the frequency omega represents the damping of the coherent f-mode field due to scattering by the turbulent flow. The f-mode damping is a result of the generation of the turbulent field at the expense of the coherent field. Title: Shear wave splitting observations in the Archean Craton of western Superior Authors: Kay, I.; Sol, S.; Kendall, J. -M.; Thomson, C.; White, D.; Asudeh, I.; Roberts, B.; Francis, D. Bibcode: 1999GeoRL..26.2669K Altcode: Shear wave splitting observations in the Archean Superior Province of the Canadian Shield show moderate to large delay times (1.1-2.1 s) with azimuths suggesting upper-mantle anisotropy is subparallel to the structural grain of the craton. Two regions with uniform anisotropy azimuth (62°±7° and 87°±3°) are separated by a transitional zone showing strong dependence of observed azimuth on source direction, suggesting that lateral structure is being observed. No splitting was observed at stations near the coast of Hudson Bay, continuing a pattern of weak splitting in surrounding Proterozoic orogenic belts. Current absolute plate motion is consistent with the direction of anisotropy, but would not explain the regional contrasts. Instead, the anisotropy appears to be related to Archean structure and tectonic history. Title: MHD Waves in Open Magnetic Structures Authors: Nakariakov, V. M.; Roberts, B.; Murawski, K. Bibcode: 1999ASPC..184..243N Altcode: The dynamics of Alfvén and magnetoacoustic waves of finite amplitude in open magnetic structures is considered. A transversal inhomogeneity in the magnetic structure in the density and/or in a field-aligned steady flow gives rise to Alfvén wave phase mixing, which, in the almost collisionless and dissipationless plasma of the corona, nonlinearly generates obliquely propagating fast magnetoacoustic waves. The generated fast waves are more effectively damped than the Alfvén waves and so the process of heating by phase mixing is enhanced. This "indirect" heating leads to the spreading of the heated plasma across the magnetic field. We point out also that an inhomogeneous flow gives rise to the secular generation of longitudinal motions through Alfvén wave phase mixing, accelerating the plasma along the field. Title: Imaging the moho and Vp/Vs ratio in the Western Superior Archean Craton with wide angle reflections Authors: Kay, I.; Musacchio, G.; White, D.; Asudeh, I.; Roberts, B.; Forsyth, D.; Hajnal, Z.; Koperwhats, B.; Farrell, D. Bibcode: 1999GeoRL..26.2585K Altcode: A refraction/wide-angle reflection survey in northwestern Ontario investigated the Archean Superior Province of the Canadian Shield. Images of apparent Moho topography were produced from both the PmP and SmS Moho wide angle reflections. The average Moho depth is 41 km, and average P- and S-wave velocities 6.6 and 3.8 (±0.05) km/s respectively. The Moho dips southward from 39.5 to 42 km depth over roughly 400 km. From ratios of S and P travel times to the Moho, profiles of Vp/Vs were constructed. The average Vp/Vs ratios are 1.74 and 1.76 for on the E-W and N-S lines (Poisson's ratio σ = 0.253 and 0.262 respectively). Vp/Vs profiles show a sharp increase from west to east and a gradual increase from north to south, correlating with the distribution of more felsic continental crust and composite crust in the Western Superior. Title: TRACE observation of damped coronal loop oscillations: Implications for coronal heating Authors: Nakariakov, V. M.; Ofman, L.; Deluca, E. E.; Roberts, B.; Davila, J. M. Bibcode: 1999Sci...285..862N Altcode: The imaging telescope on board the Transition Region and Coronal Explorer (TRACE) spacecraft observed the decaying transversal oscillations of a long [(130 ± 6) × 106 meters], thin [diameter (2.0 ± 0.36) × 106 meters], bright coronal loop in the 171 angstrom FeIX emission line. The oscillations were excited by a solar flare in the adjacent active region. The decay time of the oscillations is 14.5 ± 2.7 minutes for an oscillation with a frequency 3.90 ± 0.13 millihertz. The coronal dissipation coefficient is estimated to be eight to nine orders of magnitude larger than the theoretically predicted classical value. The larger dissipation coefficient may solve existing difficulties with wave heating and reconnection theories. Title: Waves and instabilities in a stratified isothermal atmosphere with constant Alfvén speed - revisited Authors: Pintér, B.; Čadež , V. M.; Roberts, B. Bibcode: 1999A&A...346..190P Altcode: Harmonic perturbations are studied for a plane parallel model for the solar magnetic atmosphere. In the present model, the magnetic field lines are horizontal, the sound speed and the Alfvén speed are constants. It is known that in this model, analytical solutions can be obtained for linearized perturbations. The harmonic perturbations can either propagate as MHD modes modified by the action of gravity or be unstable due to magnetic buoyancy. Dispersion properties and stability limits of these waves as well as the growthrates of the instability are displayed for various combinations of parameters: directional angles phi and theta of wave propagation, wave number k and the plasma parameter begin {equation}ta. There are three modes propagating in the considered atmosphere: the fast, the slow and the Alfvén mode, all modified by gravity. Only the modified slow MHD mode can become unstable; the modified Alfvén and fast MHD modes are always stable. In the limit of a vanishing magnetic field the modified fast mode becomes an acoustic gravity mode, the modified Alfvén mode becomes a mode oscillating with the Brunt-Väisälä frequency at large k, while the modified slow mode disappears. Title: Determination of the Reynolds number from TRACE Observation of Damped Coronal Loop Oscillations Induced by a Flare Authors: Ofman, L.; Nakariakov, V. M.; Deluca, E.; Roberts, B.; Davila, J. M. Bibcode: 1999AAS...194.7909O Altcode: 1999BAAS...31..964O The Transition Region and Coronal Expolorer (TRACE) observes the solar corona with unprecedented spatial and temporal resolution. We analyzed active region loop observation in the 171 Angstroms Fe IX emission line, and report the direct observations of damped transverse oscillations of a long (130+/-6 Mm) thin (diameter 2+/-0.36 Mm) bright active region loop. The oscillations were detected following a flare in the adjacent active region. We determined the oscillation frequency and the decay time by the least-square fit of an exponentially decaying sinusoidal function. Using the dispersion relation for the transverse oscillations, and the observed loop geometry we estimated the Alfven crossing time in the loop. The Alfven time can be used to determine the magnetic field strength in the loop if the density is known. All parts of the loop were observed to oscillate transversly in-phase, implying that the ocillation is a global mode of the loop. Using dissipative MHD model for resonant absorption of global mode oscillations for the coronal loop we determined the Reynolds number that produces the observed damping rate of the observed global mode. The value of the Reynolds number is in the 10(5-10^6) range, which is eight to nine orders of magnitude smaller than the classical coronal value. We discuss the important implication of the small Reynolds number on coronal heating theories. Title: The Second Extreme Ultraviolet Explorer Right Angle Program Catalog Authors: Christian, D. J.; Craig, N.; Cahill, W.; Roberts, B.; Malina, R. F. Bibcode: 1999AJ....117.2466C Altcode: We present the detection of 235 extreme ultraviolet sources, of which 169 are new detections, using the Extreme Ultraviolet Explorer's (EUVE) Right Angle Program (RAP) data. This catalog includes observations since the first EUVE RAP catalog (1994 January) and covers 17% of the sky. The EUVE RAP uses the all-sky survey telescopes (also known as ``scanners''), mounted at right angles to the Deep Survey and spectrometer instruments, to obtain photometric data in four wavelength bands centered at ~100 Å (Lexan/B), ~200 Å (Al/Ti/C), ~400 Å (Ti/Sb/Al), and ~550 Å (Sn/SiO). This allows the RAP to accumulate data serendipitously during pointed spectroscopic observations. The long exposure times possible with RAP observations provide much greater sensitivity than the all-sky survey. We present EUVE source count rates and probable source identifications from the available catalogs and literature. The source distribution is similar to previous extreme ultraviolet (EUV) catalogs with 2% early-type stars, 45% late-type stars, 8% white dwarfs, 6% extragalactic, 24% with no firm classification, and 15% with no optical identification. We also present 36 detections of early-type stars that are probably the result of non-EUV radiation. We have detected stellar flares from approximately 12 sources, including: EUVE J0008+208, M4 star G32-6 (EUVE J0016+198), a new source EUVE J0202+105, EUVE J0213+368, RS CVn V711 Tau (EUVE J0336+005), BY Draconis type variable V837 Tau (EUVE J0336+259), the new K5 binary EUVE J0725-004, EUVE J1147+050, EUVE J1148-374, EUVE J1334-083 (EQ Vir), EUVE J1438-432 (WT 486/487), EUVE J1808+297, and the M5.5e star G208-45 (EUVE J1953+444). We present sample light curves for the brighter sources. Title: Waves in Twisted Magnetic Flux Tubes Authors: Bennett, K.; Roberts, B.; Narain, U. Bibcode: 1999SoPh..185...41B Altcode: The modes of oscillation of a twisted magnetic flux tube in an incompressible medium are investigated analytically. An exact dispersion relation for the case of uniform twist is obtained. In contrast to the case of an untwisted incompressible tube, body, surface, and hybrid (surface-body) modes arise. Title: Alfvén wave phase mixing in two-dimensional open magnetic configurations. Authors: Ruderman, M. S.; Nakariakov, V. M.; Roberts, B. Bibcode: 1999joso.proc..143R Altcode: The phase mixing of Alfvén waves in planar two-dimensional open magnetic configurations is considered. It is assumed that the characteristic vertical spatial scale of the configuration is much larger than the horizontal scale, and that the latter is of the order of a wavelength. The WKB method is used to derive the governing equation for the wave amplitude, which in appropriate coordinates is the diffusion equation with the diffusion coefficient being spatially dependent. The dependency of the energy flux on the vertical coordinate is obtained for monochromatic waves, and illustrated for three particular cases. The theory is applied to Alfvén wave damping in coronal holes. Title: On the magnetoacoustic waves in a current sheet. Authors: Boddie, D.; Roberts, B.; Hood, A. W.; van der Linden, R. Bibcode: 1999joso.proc..139B Altcode: Magnetoacoustic waves in a structured medium can exist as hybrid waves, exhibiting the properties of both surface and body waves (Smith et al., 1997). Such hybrid waves may occur within a structured medium in which the parameters describing the physical properties of the medium are defined to be continuous functions. The dispersive behaviour of hybrid waves in a current sheet with this property is explored and compared to that in a slab model. Title: Nonlinear coupling of MHD waves in inhomogeneous steady flows Authors: Nakariakov, V. M.; Roberts, B.; Murawski, K. Bibcode: 1998A&A...332..795N Altcode: The nonlinear coupling of MHD waves in a cold (beta =3D 0) compressible plasma with a smoothly inhomogeneous low-speed steady flow directed along the magnetic field is considered. The effect is similar to Alfven wave phase mixing in a static, inhomogeneous medium and leads to the production of steep transversal gradients in the plasma parameters, which increases dissipation. Transversal gradients in the total pressure, produced by phase mixing, lead to the secular generation of obliquely propagating fast magnetosonic waves, at double the frequency and the wavenumber of the source Alfven waves. The efficiency of the generation is defined by the Alfven wave amplitude and the transversal spatial scale of the flow inhomogeneity. The secular growth of density perturbations, connected with fast waves, takes place for flow speeds that are considerably below the thresholds of the Kelvin - Helmholtz and negative energy wave instabilities. The initial stage of the nonlinear generation of the fast waves is considered analytically and illustrated by numerical simulations. Title: A Manifestation of Negative Energy Waves in the Solar Atmosphere Authors: Joarder, P. S.; Nakariakov, V. M.; Roberts, B. Bibcode: 1997SoPh..176..285J Altcode: Magnetosonic modes of magnetic structures of the solar atmosphere in the presence of inhomogeneous steady flows are considered. It is shown that, when the speed of the steady flow exceeds the phase speed of one of the modes, the mode has negative energy, and can be subject to an over-stability due to the negative energy wave instabilities. It is shown that registered steady flows in the solar atmosphere, with speeds below the threshold of the Kelvin-Helmholtz instability, can provide the existence of the magnetosonic negative energy wave phenomena. In particular, in isolated photospheric magnetic flux tubes, there are kink surface modes with negative energy, produced by the external granulation downflows. Dissipative instability of these modes due to finite thermal conductivity and explosive instability due to nonlinear coupling of these modes with Alfvén waves are discussed. For coronal loops, it is found that only very high-speed flows (>300 km s-1) can produce negative energy slow body modes. In solar wind flow structures, both slow and fast body modes have negative energy and are unstable. Title: Preliminary Results from the Second EUVE Right Angle Program Catalog Authors: Christian, D. J.; Craig, N.; Cahill, W. D.; Roberts, B.; Malina, R. F. Bibcode: 1997AAS...191.4112C Altcode: 1997BAAS...29.1271C We present preliminary results of our search for new sources in the EUVE Right Angle Program (RAP) data. The EUVE all-sky survey telescopes (also known as ``scanners'') are mounted at right angles to the Deep Survey and spectrometer instruments, and obtain photometric data in four bands centered at ~ 100 Angstroms (Lexan/B), ~ 200 Angstroms (Al/Ti/C), ~ 400 Angstroms (Ti/Sb/Al), and ~ 550 Angstroms (Sn/SiO). This allows the Right Angle Program to accumulate data serendipitously during pointed spectroscopic observations. The long exposure times possible with RAP observations provide much greater sensitivity than the all-sky survey. One-half of the scanner data since January 1994 has been analyzed, yielding approximately 200 new source detections. We present light-curves and variability measurements for the brighter sources. We have detected stellar flares from several yet to be identified sources. Source identifications and distribution by type will be also be presented. Title: Magnetoacoustic wave propagation in current sheets. Authors: Smith, J. M.; Roberts, B.; Oliver, R. Bibcode: 1997A&A...327..377S Altcode: Magnetoacoustic waves in a Harris neutral current sheet are investigated. The magnetic field strength, Alfven speed, sound speed and plasma density vary continuously across the structure. The sheet supports body, surface and hybrid modes. Two surface modes exist (a kink and a sausage). In addition, fundamental and harmonic body waves are found when their phase speeds approach the maximum Alfven speed in the sheet. As the phase speeds of the modes decrease, the waves have properties of both body and surface modes - a hybrid wave. In the long wavelength limit the phase speed of the fundamental kink body mode and sausage surface mode approach the maximum Alfven and tube speeds respectively. At short wavelengths, the phase speeds tend to constant values, each pair of kink and sausage modes merging after passing through the maximum value of the sound speed, c_s_^max^. For all modes, in the inner part of the current sheet, the gas pressure is out of phase with the magnetic pressure, with the gas pressure dominating (a property of a slow mode). In the outer part of the sheet the magnetic pressure is in phase with the total pressure (a fast mode characteristic). No purely fast or slow mode exists. For uniform density a minimum in the group velocity occurs only for the fundamental kink mode. For the Epstein density profile, minima in the group velocity are found for the other modes also. Impulsively generated waves are expected to show temporal variations that are similar to observations of oscillations of radio and X-ray emission of the solar corona and neutral sheet oscillations in the Earth's magnetotail. Calculated periods of oscillation are in good agreement with observations. Title: Alfvén Wave Phase Mixing as a Source of Fast Magnetosonic Waves Authors: Nakariakov, V. M.; Roberts, B.; Murawski, K. Bibcode: 1997SoPh..175...93N Altcode: The nonlinear excitation of fast magnetosonic waves by phase mixing Alfvén waves in a cold plasma with a smooth inhomogeneity of density across a uniform magnetic field is considered. If initially fast waves are absent from the system, then nonlinearity leads to their excitation by transversal gradients in the Alfvén wave. The efficiency of the nonlinear Alfvén-fast magnetosonic wave coupling is strongly increased by the inhomogeneity of the medium. The fast waves, permanently generated by Alfvén wave phase mixing, are refracted from the region with transversal gradients of the Alfvén speed. This nonlinear process suggests a mechanism of indirect plasma heating by phase mixing through the excitation of obliquely propagating fast waves. Title: Oscillations in Prominence Fine-Structures Authors: Joarder, P. S.; Nakariakov, V. M.; Roberts, B. Bibcode: 1997SoPh..173...81J Altcode: Oscillatory spectra of solar quiescent prominences highlight the importance of incorporating the effect of prominence fine-structure in the theory of prominence oscillations. We determine the magnetohydrodynamic modes of oscillation of an elementary, zero-β model of a prominence fibril, arguing that the fast body kink modes, namely, the string and the internal magnetic Love modes, produce the observed short periodicities in prominence fine-structures. Estimates for the periods of these modes are presented: the modes are subject to testing in future high-resolution observations. Title: Ducted fast waves in coronal loops: curvature effects. Authors: Smith, J. M.; Roberts, B.; Oliver, R. Bibcode: 1997A&A...317..752S Altcode: We examine the effect of curvature on fast magnetoacoustic waves in dense coronal loops situated in a potential coronal arcade. The wave equation governing the velocity perturbations has been solved numerically under the zero-β approximation and in the absence of gravity. Due to the curvature of the structure, leaky waves arise. The extent of the leakage depends upon the length, width and density enhancement of the loop, as well as the frequency of oscillation and its geometrical nature. The sausage mode is more affected by curvature and is more leaky than the kink mode. An increase in length, width or gas density ratio reduces the leakage, whilst higher frequencies result in greater energy leakage. Odd modes of oscillation (zero velocity at the loop summit) are more strongly confined than even modes. Modes of oscillation may be destroyed by curvature and above a critical frequency ducted waves cease to exist. Title: Negative energy wave instabilities in magnetic flux tubes. Authors: Nakariakov, V. M.; Roberts, B.; Joarder, P. S. Bibcode: 1997joso.proc...64N Altcode: No abstract at ADS Title: Dynamics of Flux Tubes in the Solar Atmosphere: Theory Authors: Roberts, B.; Ulmschneider, P. Bibcode: 1997LNP...489...75R Altcode: 1997shpp.conf...75R The modes of oscillation of a photospheric magnetic flux tube are reviewed, taking into account both linear and nonlinear aspects. Analytical and computational developments are discussed, beginning with the basic characteristics of linear wave propagation and progressing to a consideration of nonlinearity and the question of the generation of tube waves and the energy flux they transport. Title: Nonlinearly Selected Frequencies in Coronal Loops Authors: Nakariakov, V. M.; Roberts, B. Bibcode: 1996SoPh..168..273N Altcode: A nonlinear process for the resonant generation of low-frequency fast magnetosonic kink waves in coronal loops is discussed. The efficiency of the process is strongly enhanced due to the existence of a nonlinearly selected frequency produced by a constant frequency difference in the dispersion curves in the short wavelength limit. The kink wave with the selected frequency interacts with high-frequency kink and sausage waves. The efficiency of such interaction does not require coherence in the interactive waves. In a loop of width 2 × 103 km, field strength 50 G and number density 5 × 1015 m−3, the nonlinearly selected frequency is of order 46 mHz (period 21.8 s), but this may range through 11 mHz to 184 mHz (periods 86.5 s to 5.4 s) for typical coronal conditions. Title: Time-Resolved Optical Observations of the Magnetic CV EUVE_J1429-38.0 Authors: Roberts, B.; Craig, N.; Howell, S.; McGee, P. Bibcode: 1996AAS...188.7602R Altcode: 1996BAAS...28Q.952R We present time-resolved spectroscopic and photometric observations of the AM Her type magnetic variable EUVE_J1429-38.0. Previous work resulted in some ambiguity as to its period, which we can now clarify using new 8 km s(-1) time-resolved spectra obtained with the 1.5 m telescope at CTIO in December 1995 and February 1996, covering 3600--7600 Angstroms. In addition, unfiltered CCD time-series photometry, and VRI photometry were obtained in February 1996 from Woomera, Australia. Our preliminary analysis indicates that the spectroscopic radial velocities and the photometric light curve both show a dominant modulation consistent with a period of 142 min, in agreement with the previously published EUV photometry. EUVE_J1429-38.0 was fainter during the February observations, possibly indicating that the CV was in a low state at the time, and the photometric light may have been dominated by the secondary star. This work is supported by NASA contract NAS5-29298 and NASA grant NAG5-2902. Title: MHD modes of solar wind flow tubes. Authors: Nakariakov, V. M.; Roberts, B.; Mann, G. Bibcode: 1996A&A...311..311N Altcode: The properties of linear magnetosonic waves trapped within solar wind flow tubes are considered. The trapping is due to reflection of the waves from the tube boundary, which may correspond to either a jump in plasma density or magnetic field, or a jump in the steady flow velocity. The structure of the waves transverse to the magnetic field is defined by the boundary conditions. The phase and group speeds of the waves depend upon the wave frequency and wavelength. For realistic conditions in a flow tube, corresponding to a "spaghetti structure", there are two types of trapped waves propagating away from the Sun, fast and slow waves. These waves are "backward", in the sense that they would propagate towards the Sun in the absence of any difference in the steady flow velocity inside and outside the tube. The "forward" magnetosonic waves are not trapped under solar wind conditions. Both slow and fast backward waves have phase velocities that are sub-Alfvenic. There are no MHD waves propagating towards the Sun. Title: The influence of chromospheric magnetism on oscillation frequencies Authors: Roberts, B. Bibcode: 1996BASI...24..199R Altcode: No abstract at ADS Title: Optical Identifications of Fainter Late-type Stars from EUVE and ROSAT Detections Authors: Christian, D. J.; Craig, N.; Roberts, B.; Lampton, M. L.; Mathioudakis, M.; Drake, J. J. Bibcode: 1996AAS...188.7102C Altcode: 1996BAAS...28..941C We present results of the continuing optical campaign to identify sources detected in the 100 Angstroms Extreme Ultraviolet Explorer (EUVE ) all-sky survey that are also detected in the ROSAT 0.25 keV band. This paper concentrates on a dozen sources for which the optical counterparts are identified as Me stars. Data were obtained in 1995 December and 1996 February using the CTIO 1.5 m Cassegrain with resolution of 8 km s(-1) and covering a bandpass of 3500--6700 Angstroms. All spectra show the Balmer series and Ca H & K lines in emission. We present source identifications, spectral classes, distances, visual magnitudes, estimated hydrogen column densities, and the EUVE and ROSAT count rates. Implications for the fainter end of the EUV log N-log S distribution will be discussed. This work was supported by NASA contract NAS5-29298. Title: A New Magnetic CV: EUVE J2115-58.6 Authors: Sirk, M. M.; Craig, N.; Christian, D.; Roberts, B.; Howell, S.; McGee, P. Bibcode: 1996AAS...188.7601S Altcode: 1996BAAS...28T.951S We report on a newly identified magnetic cataclysmic variable, EUVE J2115-58.6, observed as part of the EUVE Optical ID Campaign on 1995 December 10 with the 1.5 m telescope at CTIO\@. An optical counterpart for EUVE J2115-58.6 has been identified and has the following position: R.A. = 21(h15^m41^s) , Decl. = -58d40arcmin 54arcsec , well within the source error circle. The newly identified star has an estimated m_V = 16.0 and is probably an AM Her-type cataclysmic variable. We present an optical spectrum that clearly shows the characteristic signatures of a magnetic cataclysmic variable: A blue continuum with Balmer, He i, and Ca ii emission lines, plus strong He ii emission. We provide an estimate of the orbital period based on optical photometry. This newly discovered cataclysmic variable was detected during the EUVE all-sky survey in the Lexan/B band (5.8--17.4 nm, 0.07--0.21 keV) on 1992 October 20, during the gap-filling on 1993 April 19, and during a 1000 s EUVE guest observer pointing. The EUV source showed a count rate of 0.056 counts s(-1) during the all-sky survey, and 0.112 counts s(-1) in the Deep Survey observations. This work is supported by NASA contract NAS5-29298 and NASA grant NAG5-2902. Title: The distance to the Chain A high-velocity cloud complex. Authors: Welsh, B. Y.; Craig, N.; Roberts, B. Bibcode: 1996A&A...308..428W Altcode: We present high-resolution (R~5km/s) NaI and CaII interstellar absorption line spectra observed toward five B stars in the line of sight to the high-velocity cloud complex Chain A. We have failed to detect the presence of any high-velocity (V<-100km/s) interstellar absorption toward these stars. This null detection enables us to place a new lower limit on the distance to the Chain A complex of z>=837+/-200pc, assuming normal gas-phase abundances of NaI and CaII. If this distance determination is typical for other high-velocity clouds, it poses problems for theories that currently argue these clouds interact with the galactic disk. Title: Discrete High-Frequency p-Modes Authors: Jain, Rekha; Roberts, B. Bibcode: 1996ApJ...456..399J Altcode: Observations report that the p-mode frequencies change with the solar activity cycle. Over the frequency range 0 to 8.5 mHz, shifts can be either positive or negative, appearing to exhibit a wavelike behavior with downturn occurring at ∼4.0 mHz and upturn at ∼5.5 mHz. A simple polytrope convection zone model overlaid with an isothermal chromospheric atmosphere threaded by a uniform horizontal magnetic field demonstrates frequency shifts that are qualitatively similar to those observed for both high and low frequencies, with shifts being a consequence of simultaneous increases in chromospheric field strength and temperature. Title: Magnetosonic Waves in Structured Atmospheres with Steady Flows, I Authors: Nakariakov, V. M.; Roberts, B. Bibcode: 1995SoPh..159..213N Altcode: The magnetosonic modes of magnetic plasma structures in the solar atmosphere are considered taking into account steady flows of plasma in the internal and external media and using a slab geometry. The investigation brings nearer the theory of magnetosonic waveguides, in such structures as coronal loops and photospheric flux tubes, to realistic conditions of the solar atmosphere. The general dispersion relation for the magnetosonic modes of a magnetic slab in magnetic surroundings is derived, allowing for field-aligned steady flows in either region. It is shown that flows change both qualitatively and quantitatively the characteristics of magnetosonic modes. The flow may lead to the appearance of a new type of trapped mode, namelybackward waves. These waves are the usual slab modes propagating in the direction opposite to the internal flow, but advected with the flow. The disappearance of some modes due to the flow is also demonstrated. Title: On Fast Magnetosonic Coronal Pulsations Authors: Nakariakov, V. M.; Roberts, B. Bibcode: 1995SoPh..159..399N Altcode: The linear properties of the fast magnetosonic modes of a coronal loop modelled as a smooth density inhomogeneity in a uniform magnetic field are compared with the case of a step function slab. It is shown that the group velocityCgof the modes, important in determining the structure of impulsively excited wave packets, possesses a minimum for a wide class of profile including the slab, with the exception of the Epstein profile for which the minimum inCgmoves out to infinity. Results for the simple step profile are thus of wider validity, and likely to be applicable to coronal loops. Title: Book-Review - Solar Magnetic Fields Authors: Schussler, M.; Schmidt, W.; Roberts, B. Bibcode: 1995Obs...115...97S Altcode: No abstract at ADS Title: High Frequency Waves and Chromospheric Magnetism Authors: Johnston, A.; Roberts, B.; Wright, A. N. Bibcode: 1995ASPC...76..264J Altcode: 1995gong.conf..264J No abstract at ADS Title: The World Wide Web: Cornerstone of the EUVE Science Archive Authors: McDonald, K.; Stroozas, B.; Antia, B.; Roberts, B.; Chen, K.; Craig, N.; Christian, C. Bibcode: 1995ASPC...77...44M Altcode: 1995adass...4...44M The Science Archive for the Extreme Ultraviolet Explorer (EUVE) satellite is using the World Wide Web (WWW) client/server software model as the cornerstone for the dissemination of EUVE archival material. A number of on-line services are available for accessing the large amounts of EUVE data that have been released publicly in the past year; additional services provide access to EUVE-related software and information. This paper outlines the current Archive WWW services and discusses plans for the future. Title: Book Review: The cosmic dynamo / Kluwer, 1993 Authors: Roberts, B. Bibcode: 1994Obs...114..183R Altcode: 1994Obs...114..183K No abstract at ADS Title: Solar Cycle Variations in P-Modes and Chromospheric Magnetism Authors: Jain, Rekha; Roberts, B. Bibcode: 1994SoPh..152..261J Altcode: 1994IAUCo.143..261J; 1994svs..coll..261J The effect onp-mode frequencies of a horizontal chromospheric canopy field is studied theoretically and the results compared with Libbrecht and Woodard's observations of frequency changes. Combined changes in field strength and chromospheric temperature cause frequency shifts that are similar in form to those observed. Frequency shifts inp-modes offer the possibility of signatures of solar activity cycles distinct from sunspot numbers and butterfly diagrams. Title: Surface effects of a magnetic field on p-modes: two layer atmosphere Authors: Jain, R.; Roberts, B. Bibcode: 1994A&A...286..254J Altcode: The effects of chromospheric canopy fields on p-mode frequencies are evaluated for a two layer model atmosphere. The convection zone is field-free, while the chromosphere contains a magnetic field which is uniform up to a certain height after which it decreases with height. Frequency shifts due to simultaneous changes in chromospheric field strengths and temperatures are calculated numerically. The frequency shifts increase or decrease as a function of frequency, depending upon the thickness of the middle region. Title: Effects of non-parallel propagation on p- and f-modes Authors: Jain, R.; Roberts, B. Bibcode: 1994A&A...286..243J Altcode: The frequencies of p- and f-modes are calculated for a model atmosphere consisting of an isothermal chromospheric gas, within which is embedded a non-uniform horizontal magnetic field, resting upon a field-free medium whose temperature increases linearly with depth. The Alfven speed within the chromosphere is assumed to be independent of height. The modes may propagate at an angle θ to the magnetic field. The p-mode frequencies decrease with an increase in either the magnetic field strength or the propagation angle θ. The frequency of the f-mode is increased by the presence of a magnetic field; the frequency decreases with increasing angle θ. Title: Time Signatures of Impulsively Generated Waves in a Coronal Plasma Authors: Murawski, K.; Roberts, B. Bibcode: 1994SoPh..151..305M Altcode: Impulsively generated waves in solar coronal loops are numerically simulated in the frame-work of cold magnetohydrodynamics. Coronal inhomogeneities are approximated by gas density slabs embedded in a uniform magnetic field. The simulations show that an initially excited pulse results in the propagation of wave packets which correspond to both trapped and leaky waves. Whereas the leaky waves propagate outside the slab, the trapped waves occur as a result of a total reflection from the slab walls. Time signatures of these waves are made by a detection of the trapped waves at a fixed spatial location. For waves excited within the slab, time signatures exhibit periodic, quasi-periodic and decay phases. The time signatures for waves excited outside the slab, or for a multi-series of variously shaped impulses generated at different places and times, can possess extended quasi-periodic phases. The case of parallel slabs, when the presence of a second slab influences the character of wave propagation in the first slab, exhibits complex time signatures as a result of solitary waves interaction. Title: Modulations of slow sausage surface waves travelling along a magnetized slab Authors: Zhelyazko, I.; Murawski, K.; Goossens, M.; Nenovaki, P.; Roberts, B. Bibcode: 1994JPlPh..51..291Z Altcode: In this paper we consider a set of nonlinear MHD equations that admits in a linear approximation a solution in the form of a slow sausage surface wave travelling along an isolated magnetic slab. For a wave of small but finite amplitude, we investigate how a slowly varying amplitude is modulated by nonlinear self-interactions. A stretching transformation shows that, at the lowest order of an asymptotic expansion, the original set of equations with appropriate boundary conditions (free interfaces) can be reduced to the cubic nonlinear Schrödinger equation, which determines the amplitude modulation. We study analytically and numerically the evolution of impulsively generated waves, showing a transition of the initial states into a train of solitons and periodic waves. The possibility of the existence of solitary waves in the solar atmosphere is also briefly discussed. Title: Oscillations in quiescent prominences Authors: Roberts, B.; Joarder, P. S. Bibcode: 1994LNP...432..173R Altcode: 1994LNPM...11..173R Theoretical aspects of prominence oscillations are discussed in terms of the basic modes of oscillation of a simple slab in the long wavelength limit. Oscillations with a period of the order of one hour are produced by the Alfvén string mode. Intermediate period oscillations, of ten to twenty minutes, arise from the fast string mode or possibly an internal Alfvén mode or an internal slow mode. Short period oscillations, of the order of a few minutes, are produced either by internal fast modes or by an internal Alfvén mode. Sub-minute oscillations are generated by guided fast magnetoacoustic waves in fibril structures embedded within the prominence. Title: The modes of oscillation of a prominence. III. The slab in a skewed magnetic field. Authors: Joarder, P. S.; Roberts, B. Bibcode: 1993A&A...277..225J Altcode: We consider theoretically the possible modes of oscillation of a solar quiescent prominence, treating the prominence as a plasma slab embedded in a uniform, line-tied magnetic field that is inclined at a small angle (≍2O°) to the long axis of the slab. Particularly interesting is the appearance of a string Alfvén mode that is analogous to the fundamental mode of vibration of a stretched string loaded with a heavy mass at its centre. This mode, along with a slow internal mode of the slab, may correspond to the observed long period (40-90 min) oscillation of quiescent prominences. Intermediate periodicities, in the range 8-20 min, may be associated with an internal Alfvén mode and a fast string mode. Short periodicities, in the range 2-5 min, may be due to an internal fast mode in the prominence slab. Title: Do p-Mode Frequency Shifts Suggest a Hotter Chromosphere at Solar Maximum? Authors: Jain, Rekha; Roberts, B. Bibcode: 1993ApJ...414..898J Altcode: The frequencies of p-modes are calculated for a model atmosphere consisting of an isothermal gas, within which is embedded a uniform horizontal magnetic field, resting upon a field-free medium whose temperature increases linearly with depth. We compare our results with Libbrecht & Woodard's recent observations of frequency changes. It is found that if simultaneously the chromospheric magnetic field strength and chromospheric temperature are increased, then the frequencies of the p-modes are shifted in a qualitatively similar fashion to that found in the observations, i.e., the frequency shift increases with frequency until about 3.9 mHz, when the shift decreases dramatically. We suggest that the combined influence of an increase in magnetic field strength and an increase in chromospheric temperature plays an important role in determining the observed downturn in frequency shift. Although the resulting frequency shift curve resembles the observed shift, with both possessing a gradual rise phase followed by a turnover, the calculated shift fails to match the steep downturn. Refinements in our basic model and/or new physics are thus indicated. Title: The modes of oscillation of a Menzel prominence Authors: Joarder, P. S.; Roberts, B. Bibcode: 1993A&A...273..642J Altcode: We investigate numerically the normal modes of oscillation of Menzel's (1951) two-dimensional, isothermal magneto static equilibrium model of a prominence, comparing its periods with those predicted from the magnetoacoustic modes of a non-gravitating, uniform, bounded medium. The periods of the fundamental magnetoacoustic modes are changed only slightly in the presence of stratification and field-line curvature. An order of magnitude estimate for these periods is thus provided by a simple slab-type model of a prominence. Title: Numerical Simulations of Fast Magnetohydrodynamic Waves in a Coronal Plasma - Part Four Authors: Murawski, K.; Roberts, B. Bibcode: 1993SoPh..145...65M Altcode: The temporal evolution of ducted waves in coronal loops (represented by smoothed slabs of enhanced gas density embedded within a uniform magnetic field) is studied in the framework of cold magnetohydrodynamics by means of numerical simulations. The numerical results show that there is an energy leakage from the slab, associated with the propagation of wave packets which exhibit periodic, quasi-periodic and decay phases. Even for weak slabs the nonlinearity can play a significant role, leading to wave breaking and shifted time signatures in comparison to the corresponding signatures of linear waves. The quasi-periodic phase possesses the strongest amplitudes in an event, making this phase the most significant for observations. Title: Random velocity field corrections of the f-mode. I. Horizontal flows Authors: Murawski, K.; Roberts, B. Bibcode: 1993A&A...272..595M Altcode: The influence of granulation (modelled as a random flow) on f-mode frequencies is evaluated theoretically for a simple model of the solar plasma. Frequency changes due to the presence of a stochastic flow are calculated analytically in the long wavelength limit, and numerically for arbitrary wavelength. Frequencies ω are close to the parabola ω2 = qk at low and intermediate wavenumbers k; but at high k they fall below the parabola. Such a behaviour is in agreement with recent solar observations. Title: Random velocity field corrections of the f-mode. II. Vertical and horizontal flow Authors: Murawski, K.; Roberts, B. Bibcode: 1993A&A...272..601M Altcode: The influence of a random flow on f-mode frequencies is assessed using a model in which the convection zone is represented by an incompressible fluid within which is a horizontal and vertical random flow. We derive an exact dispersion relation which is solved numerically. We find that the frequency ω of the f-mode is close to the parabola ω2 = gk(ρ1-ρ2)/(P1+P2) at low and intermediate horizontal wavenumbers k; but at high k it departs from the parabola, falling below it. Such a behaviour is in general agreement with recent observations. Title: Numerical Simulations of Fast Magnetohydrodynamic Waves in a Coronal Plasma - Part Three Authors: Murawski, K.; Roberts, B. Bibcode: 1993SoPh..144..255M Altcode: A method is presented for the numerical study of the temporal evolution of nonlinear periodic waves in solar coronal loops which are approximated by smoothed slabs of enhanced gas density embedded within a uniform magnetic field. This method uses a fast Fourier transform technique to calculate spatial derivatives and a modified Euler algorithm for the time scheme for solving cold magnetohydrodynamic equations that govern nonlinear perturbations. The numerical results show that nonlinearity can play a significant role, leading to wave breaking of the kink wave and slab demolition for the sausage one. The kink periodic wave adjusts better to the smoothed slab than the sausage wave. Title: Numerical Simulations of Fast Magnetohydrodynamic Waves in a Coronal Plasma - Part Two Authors: Murawski, K.; Roberts, B. Bibcode: 1993SoPh..144..101M Altcode: The temporal evolution of impulsively generated waves in solar coronal loops is studied in the framework of linearized lowβ MHD by means of numerical simulations. Loops are approximated by smoothed slabs of gas density in an otherwise uniform magnetic field. The simulations show that there is an energy leakage from the slab, associated with the propagation of wave packets which exhibit both periodic, quasi-periodic and decay phases. The quasi-periodic phase possesses the strongest amplitudes and is thus most likely to correspond with observed time scales. Title: P-Mode Frequency Shifts and Chromospheric Magnetism Authors: Jain, R.; Roberts, B. Bibcode: 1993ASPC...42...53J Altcode: 1993gong.conf...53J No abstract at ADS Title: Numerical Simulations of Fast Magnetohydrodynamic Waves in a Coronal Plasma - Part One Authors: Murawski, K.; Roberts, B. Bibcode: 1993SoPh..143...89M Altcode: The temporal evolution of ducted waves under coronal conditions is studied in the framework of linearized lowβ MHD by means of numerical simulations. Coronal loops are represented by smoothed slabs of enhanced gas density embedded within a uniform magnetic field. The simulations show that for a smoothed density profile there is an energy leakage from the slab, associated with the propagation of sausage and kink waves. Wave energy leakage in the kink wave is generally small, whereas the wave energy in sausage waves leaks more strongly for long wavelengths and smoother slabs. Title: Solar p-modes and magnetic activity. Authors: Roberts, B. Bibcode: 1993wpst.conf..115R Altcode: The remarkable fact that the Sun supports global acoustic oscillations, and that those oscillations vary over the solar activity cycle, is viewed from the standpoint of an elementary model. The model consists of a field-free polytrope (representing the solar convection zone and below) over which lies a horizontal magnetic field embedded in an isothermal atmosphere (representing the magnetized canopy structure of the chromosphere). It is argued that chromospheric magnetism has an important influence on the variability of p-modes. Title: A Modified Bohr-Sommerfeld Condition for P-Modes Authors: Johnston, A.; Wright, A. N.; Roberts, B. Bibcode: 1993ASPC...42..181J Altcode: 1993gong.conf..181J No abstract at ADS Title: MHD waves: a variety of phenomena. Authors: Roberts, B. Bibcode: 1993wpst.conf....3R Altcode: Theoretical aspects of magnetohydrodynamic waves are reviewed, stressing the variety of phenomena that such waves embrace. Title: Magnetoacoustic Gravity Surface Waves - Part One Authors: Miles, Alan J.; Roberts, B. Bibcode: 1992SoPh..141..205M Altcode: The linearized theory for the parallel propagation of magnetoacoustic-gravity surface waves is developed for an interface of a horizontal magnetic field above a field-free medium. The media either side of the interface are taken to be isothermal. The dispersion relation is obtained for the case of a constant Alfvén speed. In the absence of gravity the interface may support one or two surface modes, determined by the relative temperatures and magnetism of the two media. The effect of gravity on the modes is examined and dispersion diagrams and eigenfunctions are given. In the usual ω - kx diagnostic diagram, the domain of evanescence is shown to be divided into two distinct regions determining whether a given mode will have a decaying or growing vertical velocity component. In the absence of a magnetic field the transcendental dispersion relation may be rewritten as a polynomial. This polynomial possesses two acceptable solutions only one of which may exist in any given circumstances (depending on the ratio of the densities). If the gas density within the field exceeds that in the field-free medium, then the f-mode may propagate. The f-mode exists in a restricted band of horizontal wavenumber and only when the field-free medium is warmer than the magnetic atmosphere. An analytical form for the wave speed of the f-mode is obtained for small values of the Alfvén speed. It is shown that the f-mode is related to the fast magnetoacoustic surface wave, merging into that mode at short wavelengths. Title: Magnetoacoustic Gravity Surface Waves - Part Two Authors: Miles, Alan J.; Allen, H. R.; Roberts, B. Bibcode: 1992SoPh..141..235M Altcode: The linearized theory for the parallel propagation of magnetoacoustic-gravity surface waves is developed and a dispersion relation obtained for the case of an isothermal interface of a uniform horizontal magnetic field residing above a field-free medium. The transcendental dispersion relation is solved numerically for a range of parameters and the resulting dispersion curves and corresponding eigenfunctions plotted. As in the case of a uniform Alfvén speed (Paper I), the existence of the fast and slow magnetoacoustic-gravity surface modes and the f-mode (modified by the presence of the uniform magnetic field) is determined by the relative temperatures of the two media either side of the interface. If the lower field-free region is cooler than the upper magnetic atmosphere only the slow magnetoacoustic-gravity surface mode may propagate. In addition to these three surface modes we find higher harmonic-type trapped modes. The existence of these modes also depends on the temperatures either side of the interface. They propagate only when both the field-free region is warmer than the magnetic field region and the Alfvén speed is greater than the corresponding sound speed in the magnetic atmosphere. Title: The modes of oscillation of a prominence. II - The slab with transverse magnetic field Authors: Joarder, P. S.; Roberts, B. Bibcode: 1992A&A...261..625J Altcode: We investigate theoretically the possible magnetoacoustic modes of oscillation in a prominence that may arise solely due to a weak transverse magnetic field component. The prominence is modelled as an infinitely long plasma slab embedded in a uniform magnetic field that threads the prominence transversely and is anchored to rigid walls on either side of the prominence. The effect of gravity is ignored. Particularly interesting in this study is the appearance of magnetosonic modes that are analogous to the fundamental mode of vibration of a stretched string loaded with a heavy point mass at its center. These modes may correspond to some of the long-period (1 h) oscillations in a quiescent prominence. Intermediate periodicities, in the range 8-10 min, may be due to an acoustic-type mode in the prominence slab. Title: Waves in solar magnetic flux tubes - The observational signature of undamped longitudinal tube waves Authors: Solanki, S. K.; Roberts, B. Bibcode: 1992MNRAS.256...13S Altcode: Linear calculations of undamped magnetoacoustic waves utilizing the thin-tube approximation are presented and their influence on the Stokes I and V line parameters of a set of photospheric spectral lines is analyzed. To improve the observational diagnostics of flux tube waves, the dependence of various line parameters, such as the Stokes V zero-crossing wavelength, Stokes V amplitude, width area and asymmetry, on wave parameters is studied. It is determined that it is possible to set a lower limit on the energy flux transported by longitudinal tube waves by measuring time series of the Stokes V zero-crossing wavelength of a selected group of lines. Title: The modes of oscillation of a prominence. I. The slab with longitudinal magnetic field. Authors: Joarder, P. S.; Roberts, B. Bibcode: 1992A&A...256..264J Altcode: The authors consider theoretically the modes of oscillation of a solar quiescent prominence, treating the prominence as a three-dimensional, magnetized plasma slab embedded in a uniform magnetic field. They examine the normal modes of oscillation of such a system. The existence of fast MHD surface modes with periods of the order of an hour is of particular interest. These modes, which are closely analogous to the classical modes of vibration of an elastic membrane, may correspond to the observed long-period oscillations of quiescent prominences. Shorter periodicities, in the range 3-5 min, may be associated with the fundamental of the magnetic Pekeris mode and the first harmonic of the magnetic Love mode. Title: Surface and body waves in solar wind flow tubes Authors: Mann, G.; Marsch, E.; Roberts, B. Bibcode: 1992sws..coll..495M Altcode: Parker (1963) already assumed that the solar wind might be fine-structured in form of flow tubes. Such spatial structures can give rise to surface and body waves with characteristic frequencies. These waves are studied here by means of the ideal magnetohydrodynamic equations. The resulting dispersion relations are discussed for typical parameters of solar wind flow tubes observed by the two HELIOS probes. These waves might be able to transport photospheric oscillations into the interplanetary space. Title: Magnetohydrodynamic Waves in Structured Magnetic Fields Authors: Roberts, B. Bibcode: 1992ASIC..375..303R Altcode: 1992sto..work..303R The nature of magnetohydrodynamic waves in structured magnetic fields is discussed, with particular reference to sunspots. The modes of oscillation of an isolated flux tube are reviewed. We consider the nature of surface waves on an interface between two isothermal gases, one of which is embedded within a horizontal magnetic field. Title: Magnetic flux tubes as communication channels Authors: Roberts, B. Bibcode: 1992AIPC..267...24R Altcode: 1992ecsa.work...24R The role of magnetic flux tubes as communication channels connecting the photosphere with the chromosphere and above is reviewed. Title: Interpretation of solar-cycle variability in high-degree p-mode frequencies Authors: Evans, David J.; Roberts, B. Bibcode: 1992Natur.355..230E Altcode: RECENTLY Libbrecht and Woodard1 and Elsworth et al.2 have demonstrated that the frequencies of solar acoustic p-mode oscillations vary significantly over the solar cycle. We have previously suggested that cyclic variations in the magnetic activity of the Sun could modulate the p-mode frequencies in a similar way. In particular, we investigated3-5simple models of the 'magnetic canopy', which permeates the solar atmosphere and overlies all of the Sun's surface, to determine its influence on p-mode frequencies. Here we make a comparison of our model predictions with the observations of Libbrecht and Woodard. We find that, despite the simplicity of our model, we are able to obtain good agreement with the observed frequency shifts for modes of frequency less than 4 mHz, through a mechanism in which an increasing magnetic field induces 'stiffening' of the Sun's chromosphere. Above this frequency there is clearly something missing from our model. We speculate that the behaviour above 4 mHz is related to a cutoff frequency in the solar atmosphere, above which waves are trapped only partially. Title: Magnetoacoustic surface waves at a single interface Authors: Jain, Rekha; Roberts, B. Bibcode: 1991SoPh..133..263J Altcode: The occurrence of magnetoacoustic surface waves at a single magnetic interface one side of which is field-free is explored for the case of non-parallel propagation. Phase-speeds and penetration depths of the waves are investigated for various Alfvén speeds, sound speeds and angles of propagation to the applied field. Both slow and fast magnetoacoustic surface waves can exist depending on the values of sound speeds and propagation angle. The fast waves penetrate more than the slow waves. Title: The Sensitivity of Chromospherically Induced p- and f-Mode Frequency Shifts to the Height of the Magnetic Canopy Authors: Evans, David J.; Roberts, B. Bibcode: 1991ApJ...371..387E Altcode: Previous investigations of the effect of a magnetic canopy on the solar p- and f-modes are extended to allow for variations in height of the magnetic canopy. Analytical solutions in the limit of long horizontal wavelength are obtained; the solutions exhibit explicitly the dependence of frequency shifts on magnetic field strength, wavenumber, and canopy height. Frequency shifts are principally due to changes in canopy height. Full numerical solutions are also presented. It is argued that changes in chromospheric magnetism will be manifest in p- and f-mode data sets gathered at different phases of the solar cycle. A comparison of theoretical results obtained here with the recent data obtained by Libbrecht and Woodard (1990) is made. Title: Magnetohydrodynamic waves in the Sun. Authors: Roberts, B. Bibcode: 1991gamp.conf..105R Altcode: Taken overall, this brief outline of oscillatory phenomena observed in the Sun serves perhaps to show that the Sun is indeed a complex medium capable of supporting a large variety of wave phenomena. Contents: 1. Introduction. 2. Magnetohydrodynamic waves. 3. Surface waves. 4. Waves in magnetic flux tubes. 5. Stratification effects. Title: Waves in the solar atmosphere Authors: Roberts, B. Bibcode: 1991GApFD..62...83R Altcode: It is pointed out that there are a great variety of wave phenomena in the solar atmosphere, an understanding of which rests upon a full appreciation of the dual roles of magnetic structuring and gravitational stratification in determining the modes. The basic waves of magnetic flux tubes are described in some detail and applied to photospheric magnetic flux tubes, sunspots, and coronal loops. Oscillations in quiescent prominences and the influence of the chromospheric magnetic canopy on p-modes are also discussed. Title: Magnetohydrodynamic Waves in the Sun Authors: Roberts, B. Bibcode: 1991assm.conf..105R Altcode: No abstract at ADS Title: Magnetoacoustic-Gravity Surface Waves (With 1 Figure) Authors: Miles, A. J.; Roberts, B. Bibcode: 1991mcch.conf..508M Altcode: No abstract at ADS Title: Magnetohydrodynamic Surface Waves (With 2 Figures) Authors: Roberts, B. Bibcode: 1991mcch.conf..494R Altcode: No abstract at ADS Title: Properties of Non-Parallel Magnetoacoustic Surface Waves (With 1 Figure) Authors: Jain, R.; Roberts, B. Bibcode: 1991mcch.conf..511J Altcode: No abstract at ADS Title: The Influence of a Chromospheric Magnetic Field on the Solar p- and f-Modes. II. Uniform Chromospheric Field Authors: Evans, David J.; Roberts, B. Bibcode: 1990ApJ...356..704E Altcode: A dispersion relation for the p- and f-modes is derived in a model atmosphere containing a uniform chromospheric field. Asymptotic solutions to this equation have been derived for the case of small horizontal wavenumber. Numerical solutions for larger horizontal wavenumbers indicate that, in all cases, the effect of the chromospheric field is to increase the frequency of the modes. The magnitude of the frequency shift increases monotonically with radial order n, degree l and field strength B(0). Comparison with observed frequencies is difficult; however, results suggest that the f-mode in particular may provide a useful indicator of whether or not chromospheric fields are important in determining mode frequencies. Solar cycle variations of high-degree modes and comparisons between active and quiet regions are also likely to reveal the effects of chromospheric magnetism. Only detailed high-resolution data sets of high-degree modes are likely to reveal the importance of chromospheric magnetism for helioseismology. Title: The Observational Signature of Flux Tube Waves and an Upper Limit on the Energy Flux Transported by Them Authors: Solanki, S. K.; Roberts, B. Bibcode: 1990IAUS..138..259S Altcode: No abstract at ADS Title: Waves in solar photospheric flux tubes and their influence on the observable spectrum Authors: Solanki, S. K.; Roberts, B. Bibcode: 1990GMS....58..181S Altcode: Linear calculations of undamped magnetoacoustic waves in thin solar magnetic flux tubes are presented and their influence on the Stokes V profiles of various iron lines is studied. This is a necessary first step for the diagnostics of the properties of flux tube waves, in particular the amount of energy transported by them into the upper atmosphere. It is shown that, with sufficiently high spatial resolution, observations can distinguish between standing and propagating waves on the basis of line parameters of photospheric spectral lines alone. Particular attention is given to exploring quantitative diagnostics for the wave amplitude, since it is currently the most important unknown parameter determining the energy flux carried by the waves. It is found that although this parameter can be derived relatively simply if the thermal fluctuations produced by the wave are ignored (i.e., for an isothermal wave), the task becomes much more complex for the more realistic case of a coupled variation of temperature and velocity. Title: Waves in Magnetic Flux Tubes Authors: Roberts, B. Bibcode: 1990IAUS..142..159R Altcode: The basic aspects of wave propagation in a magnetic flux tube are reviewed, with particular emphasis on the types of flux tube that occur in the solar atmosphere. Two fundamental speeds arise naturally in a description of wave propagation in a flux tube: the slow magnetoacoustic (cusp) speed cT, which is both subsonic and sub-Alfvenic, and a mean Alfven speed ck. Both surface and body modes are supported by a tube. It is stressed that a flux tube may act as a wave guide, similar to the guidance of light by a fiber optic, or sound in an ocean layer, or seismic waves in the earth's crust. Title: The Oscillations of a Magnetic Flux Tube and Its Application to Sunspots Authors: Evans, David J.; Roberts, B. Bibcode: 1990ApJ...348..346E Altcode: The modes of oscillation of an isolated magnetic flux tube in the absence of gravity is examined, with parameters chosen to mimic a sunspot. Gravitational stratification of the umbral atmosphere leads to consider two cases, distinguished primarily by the ordering of the Alfven speed and the external sound speed. The transition between these two regimes occurs at about the level where the optical depth, tau(c), is equal to 1 in the umbra. The modes given by the model, taken together with the observations, suggest that 3 minute oscillations are slow-body modes (driven by overstable convection) and that a sunspot consists of a bundle of pore-sized flux tubes rather than a single 'monolithic' one. Fast-body modes are identified in the tube with the observed 5 minute oscillations of the umbral photosphere and below. The excitation of these modes propagating up or down the tube may explain the recent observation that sunspots act as sinks for p-modes propagating in their environment. Running penumbral waves are associated with fast- and slow-surface modes. The fast-surface wave could arise from fast-body modes driven below the level where tau(c) = 1; the slow-surface waves may arise from granular buffeting or overstable convection. Title: Evolution of fast sausage mode wave packets in coronal flux tubes. Authors: Edwin, P. M.; Mann, G.; Roberts, B.; Zhelyazkov, I. Bibcode: 1990PDHO....7..138E Altcode: 1990dysu.conf..138E Flux tube waves can be generated in coronal loops due to the flare process. Wave packets of the fast sausage mode can appear for several seconds in the solar corona. The radio signature of these wave packets manifests in slowly drifting stripes of enhanced emission in dynamic frequency spectra during solar type-IV radio bursts. Title: Oscillations in the Sun. Authors: Roberts, B. Bibcode: 1990sppp.conf....1R Altcode: The author describes some of the current thinking about the Sun, in particular its oscillatory aspects. Examples of solar oscillations include: the well-known p- and g-modes (pressure and gravity oscillations) of the global Sun, for which magnetic effects are of some interest but are not central to the modes; oscillations in sunspots and isolated flux tubes, for which magnetism, pressure and gravity effects are all important; and coronal oscillations, which are governed principally by magnetic forces. Title: Magnetoacoustic Surface Waves in the Solar Atmosphere. Authors: Miles, A. J.; Roberts, B. Bibcode: 1990ppsa.conf...77M Altcode: No abstract at ADS Title: Properties and models of photospheric flux tubes Authors: Roberts, B. Bibcode: 1990GMS....58..113R Altcode: The gross properties of isolated solar magnetic flux tubes, from the intense tube to the sunspot, are summarized, paying particular attention to how these properties determine the basic wave propagation speeds in the structures. An overview of the fundamental models of oscillations is attempted, considering both thin and wide tubes, and a comparison with observations of oscillations is made. The potential seismological value of such information is stressed. Title: On the Properties of Magnetoacoustic Surface Waves Authors: Miles, Alan J.; Roberts, B. Bibcode: 1989SoPh..119..257M Altcode: The nature of magnetoacoustic surface waves at a single magnetic interface, one side of which is field-free, is explored for the case of parallel propagation. The interface may support a slow surface wave or both slow and fast surface waves, depending upon the ordering of the sound speeds in the two media. Phase-speeds and penetration depths of the waves and the associated pressure perturbations and motions are investigated for a variety of field strengths and sound speeds. The fast wave disturbs the interface more than the slow wave. In the magnetic field region the slow wave is mainly longitudinal in nature whilst the fast surface wave is transverse for strong fields, longitudinal for weaker fields. In the field-free region both waves are longitudinal in character. The running penumbral wave phenomenon may provide an example of a magnetoacoustic surface mode, though any direct comparison requires the inclusion of gravitational effects. Title: The Influence of a Chromospheric Magnetic Field on the Solar p- and f-Modes Authors: Campbell, W. R.; Roberts, B. Bibcode: 1989ApJ...338..538C Altcode: The influence of a chromospheric magnetic field on p- and f-mode frequencies is evaluated theoretically for a simple model of the solar plasma, consisting of a polytrope in the solar interior, above which is an isothermal atmosphere. The atmosphere is permeated by a horizontal magnetic field. Frequency changes and shifts in phase factors due to the presence of a magnetic atmosphere are calculated analytically in the long-wavelength limit, and numerically for arbitrary wavelength. The results reveal that, at low to moderate degree l, an increase in chromospheric magnetic field leads to a frequency increase for the n = 1 p-mode, whereas the overtones (n = 2, 3, etc.) suffer a frequency decrease. At high l, all the p-modes suffer a frequency decrease. Title: Book-Review - Particle Acceleration and Trapping in Solar Flares Authors: Trottet, G.; Pick, M.; Roberts, B. Bibcode: 1989Obs...109...33T Altcode: No abstract at ADS Title: Resonance absorption of energy by a cold inhomogeneous plasma Authors: Sedlacek, Z.; Roberts, B. Bibcode: 1989JPlPh..41...97S Altcode: The active and reactive power resonantly absorbed by a linear passive system is calculated in terms of its continuum eigenfunctions and the spatial-temporal spectral density of the external force. An application to the resonance absorption of electromagnetic radiation by a cold inhomogeneous plasma is given. Title: Magnetohydrodynamic waves in stratified atmospheres. Authors: Roberts, B. Bibcode: 1989pig..conf..687R Altcode: The behaviour of magnetohydrodynamic waves in a stratified medium is examined, paying particular attention to applications in the solar atmosphere. As a special case, the author discusses in detail the p- and g-modes occurring in the Sun and brings out some of the recent advances in helioseismology. Then possible magnetic effects on these modes are examined. The effects of stratification are also important in describing surface waves and instabilities in the solar atmosphere. The author considers briefly the so-called 'running penumbral wave' that is observed to emanate from sunspots. Finally, he turns to the behaviour of magnetoacoustic waves in magnetic flux tubes, which are believed to be the principal form of magnetic field in the surface layers of the Sun. The role of gravity in introducing propagation cut-off is examined. Title: The influence of a chromospheric magnetic field on p- and f-modes. Authors: Roberts, B.; Campbell, W. R. Bibcode: 1988ESASP.286..311R Altcode: 1988ssls.rept..311R The influence of a magnetic chromosphere on p- and f-modes of oscillation is explored theoretically, modelling the chromosphere by an isothermal atmosphere permeated by a horizontal magnetic field and the field-free convection zone by a polytrope. It is pointed out that active regions, characterized by magnetic field strengths of the order of 100 gauss, have a significant effect on the frequencies of high degree modes. The frequency of the f-mode is increased by the presence of a magnetic field; the frequencies of p-modes are decreased. In an evolving active region the frequencies of high degree p- and f-modes are systematically split apart by the changing magnetic field. These effects should be apparent in a comparison of high resolution data sets for active regions with either low resolution sets or high resolution sets for quiet regions. Title: Employing analogies for ducted MHD waves in dense coronal structures Authors: Edwin, P. M.; Roberts, B. Bibcode: 1988A&A...192..343E Altcode: Analogies of fast MHD waves propagating along a dense coronal structure are exploited to examine how the size and shape of the inhomogeneity affect the properties of the Love- and Pekeris-type waves. The profile's shape determines the dispersive nature of the waves. Excited impulsively, magnetic Love and Pekeris waves give rise to quasi-periodic oscillations with a duration and time scale that depend on the cross-sectional area and strength of the inhomogeneity. More diffuse coronal inhomogeneities support impulsively generated periodic oscillations, with the quasi-periodic signature being absent. Title: Magnetic Effects on Solar P-Modes Authors: Campbell, W. R.; Roberts, B. Bibcode: 1988IAUS..123..161C Altcode: Both analytical and numerical studies of the effect of a magnetic field at the base of the convection zone on p-modes are presented. It is argued that the recently reported changes in the low degree p-mode frequencies, from 1980 to 1984, may result from corresponding changes in the magnetic field strength. Title: Solar Magnetohydrodynamics Authors: Roberts, B. Bibcode: 1988cspp.book....1R Altcode: No abstract at ADS Title: MHD waves in the solar atmosphere. Authors: Edwin, P. M.; Roberts, B. Bibcode: 1987ESASP.275..169E Altcode: 1987sspp.symp..169E In this review the authors outline the theory of MHD waves in a magnetically structured atmosphere. Applications of the theory are made to intense magnetic flux tubes and their solitons, to rapid oscillations in the corona, to spicules, and to running penumbral waves. A brief application to helioseismology is also made. Title: On Magnetohydrodynamic Solitons in Jets Authors: Roberts, B. Bibcode: 1987ApJ...318..590R Altcode: Nonlinear solitary wave propagation in a compressible magnetic beam model of an extragalactic radio jet is examined and shown to lead to solitons of the Benjamin-Ono type. A number of similarities between such magnetic beam models of jets and models of solar photospheric flux tubes are pointed out and exploited. A single soliton has the appearance of a symmetric bulge on the jet which propagates faster than the jet's flow. Title: Thermal Dissipation in Slender Flux Tubes and Structured Media Authors: Edwin, P.; Roberts, B. Bibcode: 1987rfsm.conf..250E Altcode: The authors examine the behaviour of waves in flux tubes when dissipative effects are important. Such a theory has applications to isolated tubes and to sunspots (including umbral dots). Title: Joint Discussion on Topics of Sessions 9 and 10 Authors: Mein, P.; Roberts, B. Bibcode: 1987rfsm.conf..292M Altcode: No abstract at ADS Title: Guided MHD waves as a coronal diagnostic tool. Authors: Roberts, B. Bibcode: 1986NASCP2442..325R Altcode: 1986copp.nasa..325R A description is provided of how fast magnetoacoustic waves are ducted along regions of low Alfven velocity (high density) in the corona, exhibiting a distinctive wave signature which may be used as a diagnostic probe of in situ coronal conditions (magnetic field strength, density inhomogeneity, etc.). Some observational knowledge of the start time of the impulsive wave source, possibly a flare, the start and end times of the generated wave event, and the frequency of the pulsations in that event permits a seismological deduction of the physical properties of the coronal medium in which the wave propagated. With good observations the theory offers a new means of probing the coronal atmosphere. Title: Magnetic field corrections to solar oscillation frequencies Authors: Roberts, B.; Campbell, W. R. Bibcode: 1986Natur.323..603R Altcode: The presence of a magnetic field both deep within the Sun and in its atmosphere raises the question of the field's influence on the p- and g-modes of oscillation and the implications for helioseismology. Observations1,2 of p-modes, in particular, have permitted a theoretical determination3 of the sound speed within the solar interior, thus providing a seismological probe of the Sun's depths. Magnetic fields within the Sun are likely to be too weak to significantly affect this determination of the sound speed. Nonetheless, magnetic fields may modify the oscillation frequencies in a distinctive fashion, thereby raising the possibility of placing limits on interior field strengths through frequency measurements4. Recently, Woodard and Noyes5 have reported a slight but systematic decrease in frequencies of low-degree p-modes from 1980 to 1984. Here we argue that the frequencies of both p- and g-modes are modified by a magnetic field. In particular, we attribute the decrease in p-mode frequencies to a magnetic field within the solar interior evolving over the solar cycle. Field strengths at the base of the convection zone of at least 5×105 G are required. Title: Dispersive ducting of MHD waves in the plasma sheet: A source of Pi2 wave bursts Authors: Edwin, P. M.; Roberts, B.; Hughes, W. J. Bibcode: 1986GeoRL..13..373E Altcode: Fast magnetoacoustic waves can be ducted by plasma inhomogeneities such as the plasma sheet. As this ducting is dispersive an impulsive source will give rise to a well-defined, quasi-periodic wave packet with timescales determined by the width of the inhomogeneity and characteristic speeds in the wave duct and surrounding medium. The duration of the wave packet depends upon the distance from the source. We argue that an impulsive source in the plasma sheet at substorm onset will produce a wave packet near earth with characteristics similar to pi2 wave bursts and put this idea forward as a mechanism for the generation of pi2 pulsations. Title: On the Behavior of Hydromagnetic Surface Waves Authors: Lee, M. A.; Roberts, B. Bibcode: 1986ApJ...301..430L Altcode: The behavior of velocity, magnetic field, and pressure perturbations about a continuously varying interface in pressure equilibrium is investigated in detail within ideal incompressible magnetohydrodynamics. A specific initial value problem is solved in quadrature for a thin interface and compared with the solution for a discontinuous interface. The unattenuated surface wave about a discontinuous interface is replaced at a thin interface by a collective surface disturbance which decays, with the associated energy density flowing into local oscillations within the interface. At long times the envelope of the local oscillations is concentrated within a small fraction of the thin interface (gradients within the envelope increase linearly with time, eventually resulting in a breakdown of the linearized ideal theory). Thus, the derived decay rate of the surface disturbance gives a mode-conversion rate rather than a heating rate. In applications to the propagation and dissipation of surface waves in the solar corona, this rate cannot in general be interpreted as a coronal heating rate. Title: Impulsively generated fast coronal pulsations. Authors: Edwin, P. M.; Roberts, B. Bibcode: 1986NASCP2449..347E Altcode: 1986rfsf.nasa..347E A mechanism has been offered to try to explain some of the quasi-periodic, rapid, oscillatory behaviour observed in the solar corona. It has been shown that the explanation does not depend on the coronal loop having uniform density. Variations in density across the loops would modify the duration of the quasi-periodic phase and the time scale of the periodic phase, but observations and results from radio wave, hard X-ray and other data could still be accounted for. Title: Dynamical Processes in Magnetic Flux Tubes Authors: Roberts, B. Bibcode: 1986ssmf.conf..169R Altcode: In this review of dynamical processes in isolated flux tubes, the author concentrates on adiabatic effects, thereby allowing a relatively straightforward exposition of the basic modes of a tube. Title: Book-Review - Unstable Current Systems and Plasma Instabilities in Astrophysics - I.A.U. SYMP.107 Authors: Kundu, M. R.; Holman, G. D.; Roberts, B. Bibcode: 1985Obs...105..240K Altcode: No abstract at ADS Title: Solitary waves in a magnetic flux tube Authors: Roberts, B. Bibcode: 1985PhFl...28.3280R Altcode: The propagation of weakly nonlinear long-wavelength (weakly dispersive) sound waves in both a magnetic slab and a magnetic cylinder (flux tubes in the solar atmosphere) is analyzed. The slab geometry is shown to lead to the Benjamin-Ono equation, and the cylindrical geometry yields an allied form of this equation. It is noted that the magnetic flux tube provides a simple illustration of these two equations, hitherto considered only in separate physical systems. Title: Magnetohydrodynamic waves. Authors: Roberts, B. Bibcode: 1985ssmf.conf...37R Altcode: Contents: Structuring and stratification. Waves in a magnetically structured atmosphere. Waves in a uniform medium. Waves in discretely structured media. Oscillations in a low-β gas. Damped Alfvén waves. Waves in stratified atmospheres. Slender flux tubes. Title: Surface solitary waves and solitons Authors: Hollweg, J. V.; Roberts, B. Bibcode: 1984JGR....89.9703H Altcode: The solar atmosphere and solar wind are magnetically structured. The structuring can include tangential discontinuities, which can support surface waves. Such waves can be dispersive. This means that dispersion and nonlinearity can balance in such a way that solitary waves (or solitons) can result. This general point is illustrated by a two-dimensional nonlinear analysis which explicitly demonstrates the presence of long-wavelength solitary waves propagating on tangential discontinuities. If the waves are only weakly nonlinear, then they obey the Korteweg-de Vries equation and are true solitons. Title: Waves in inhomogeneous media. Authors: Roberts, B. Bibcode: 1984ESASP.220..137R Altcode: 1984ESPM....4..137R Recent developments in the theory of magnetohydrodynamic waves in a magnetically structured atmosphere are reviewed. Both continuously and discretely structured media are considered, with applications to coronal loops, open field regions, sunspots, and Hα fibrils. Analogies with other subjects (e.g. oceanography, fibre optics) are drawn. The propagation of an impulsively generated fast magnetoacoustic wave in a density duct leads to periodicities of about a second in the corona and tens of seconds in the chromosphere. Title: On running penumbral waves. Authors: Small, L. M.; Roberts, B. Bibcode: 1984ESASP.220..257S Altcode: 1984ESPM....4..257S Following the work of Nye & Thomas the authors model running penumbral waves as magnetoacoustic-gravity modes propagating along the lower boundary of the penumbral magnetic field. They examine the theoretical properties of surface modes on the boundary and show that it is not necessary to invoke trapping of magnetoacoustic-gravity modes to produce the confinement of the modes to the photospheric chromospheric region. Running penumbral waves are interpreted as fast magnetoacoustic surface modes. Title: On coronal oscillations Authors: Roberts, B.; Edwin, P. M.; Benz, A. O. Bibcode: 1984ApJ...279..857R Altcode: Magnetoacoustic oscillations in a solar coronal inhomogeneity (e.g., coronal loop) are shown to take place with two distinct periodicities, one on an acoustic (long) time scale and the other on an Alfvenic (short) time scale. The short period modes - fast magnetoacoustic waves - are trapped in regions of low Alfven speed: typically, this corresponds to high density loops or dense open field regions. Their periods may be on the order of seconds. The form of the fast oscillations is discussed for both standing modes in a closed loop and impulsively generated disturbances in a loop or open field structure. Impulsively generated waves in a density enhancement exhibit both periodic and quasi-periodic phases. Symmetric oscillations (sausage modes) are analogous to Pekeris waves in oceanography; asymmetrical (kink) disturbances are akin to Love waves in seismology. It is suggested that fast magnetoacoustic waves may explain the observed pulsations in Type IV radio events, the sausage waves providing the desired 1 s periodicities. Magnetoacoustic oscillations provide a potentially useful diagnostic tool for determining physical conditions in the inhomogeneous corona. Title: The creation of fine structure by magnetic fields Authors: Roberts, B. Bibcode: 1984AdSpR...4h..17R Altcode: 1984AdSpR...4...17R The solar plasma is strongly structured by the presence of magnetic field. This structuring is manifest in the photosphere in the form of flux tubes, from the readily visible sunspots to the sub-telescopic intense tubes, so that the atmosphere is divided into strong-field media or field-free media. In the corona, by contrast, the magnetic field permeates the whole of the atmosphere and structuring consists principally of density and temperature inhomogeneities. We discuss some of the causes of magnetic structuring, including kinematic concentration, convective collapse and magnetoconvection for photospheric tubes, spicules in the chromosphere, and thermal instability for coronal loops. Title: Discussions in the MHD theory group Authors: Roberts, B. Bibcode: 1984ost1.conf..179R Altcode: No abstract at ADS Title: Suggestions for future investigations Authors: Roberts, B.; Einaudi, G.; Toricelli, G.; Cargill, P.; Durrant, C. J. Bibcode: 1984ost1.conf..183R Altcode: No abstract at ADS Title: Non-equilibrium and instability in coronal structures. Authors: Roberts, B. Bibcode: 1984ost1.conf...75R Altcode: No abstract at ADS Title: Wave Propagation in a Magnetic Cylinder Authors: Edwin, P. M.; Roberts, B. Bibcode: 1983SoPh...88..179E Altcode: The nature of oscillations in a magnetic cylinder embedded in a magnetic environment is investigated. It is shown that the standard slender flux tube analysis of a kink mode in a cylinder excludes the possibility of a second mode, which arises under photospheric conditions. Under coronal conditions, two widely separated classes of oscillation can be freely sustained, one on an acoustic time-scale and the other on an Alfvénic time-scale. The acoustic-type oscillations are always present, but the much shorter period, Alfvénic-type, oscillations arise only in high density (strictly, low Alfvén velocity) loops. An application to waves in fibrils is also given, and suggests (following Wentzel, 1979) that they are fast kink waves propagating in a density enhancement. Title: Fast pulsations in the solar corona Authors: Roberts, B.; Edwin, P. M.; Benz, A. O. Bibcode: 1983Natur.305..688R Altcode: Pulsations in radio emission from the solar coronal plasma have been detected for over a decade1-9. The oscillations are quasiperiodic, with periods of typically a second or so. Recently, sub-second time structures have been found in hard X rays10, and simultaneously in hard X rays and microwaves11. Here we examine whether magnetohydrodynamical oscillations in a density enhancement, treated for simplicity as a straight magnetic slab, can explain the observed short periods. A dense region in the corona (for example, a loop) can act as a wave trap, and symmetrical oscillations within that trap must be of short wavelength with correspondingly short period. An impulsive source (such as a flare) naturally gives rise to a quasiperiodic disturbance. Such oscillations are closely akin to the Pekeris modes of oceanography, the Love waves of seismology and the dielectric waves of fibre optics. Title: Wave propagation in intense flux tubes. Authors: Roberts, B. Bibcode: 1983SoPh...87...77R Altcode: The nature of non-adiabatic wave propagation in a slender magnetic flux tube is explored. The results of the theory are compared with the observations of Giovanelli et al. (1978), and found to be in general agreement. Those observations, of tubes in the photosphere-chromosphere, show outwardly propagating waves, with periods of 300 s, which take some 19 s to propagate from one level of line formation to another level higher in the atmosphere. In sharp contrast to this, is the time of 7 s for a similar disturbance outside the tube to propagate between the same two levels of line formation, estimated to be some 600 km apart in the field-free atmosphere. Title: Magnetic flux tubes on the Sun Authors: Spruit, H. C.; Roberts, B. Bibcode: 1983Natur.304..401S Altcode: Magnetic fields are the cause of almost all forms of solar activity. Near the solar surface, and possibly in the entire convection zone, these fields occur in the form of isolated flux tubes. In recent years, new views have been developed (and older ones revived) in which this property plays a central role. Here we review these ideas, dealing with the nature of the solar cycle, sunspot structure, the origin of spicules and the source of mechanical heating in the solar atmosphere. The ideas are illustrated with the aid of a simple mathematical model for the behaviour of thin magnetic flux tubes. The properties of inhomogeneities in the corona (coronal loops) are also discussed. Title: Wave Diagrams for Magnetohydrodynamic Modes in a Magnetically Structured Atmosphere Authors: Rae, I. C.; Roberts, B. Bibcode: 1983SoPh...84...99R Altcode: Phase-speed diagrams, showing the allowable spectrum of surface and body waves in a magnetically structured atmosphere, are constructed for the interface and the slab. The diagrams (illustrated for photospheric flux tubes, photosphere-chromosphere magnetic canopy, and coronal conditions) classify disturbances for both the normal modes of a structure and incident wave propagation on a structure, allowing a simple application once sufficiently detailed observations of waves become available. Title: MHD wave motion in magnetically structured atmospheres Authors: Rae, I. C.; Roberts, B. Bibcode: 1983A&A...119...28R Altcode: An examination of the MHD wave motions expected in magnetically structured regions of the solar atmosphere is completed using an idealised plane interface model. The steady-state solutions are characterised by wave energy fluxes possessing infinities when the equilibrium is at resonance. Under certain conditions wave energy is totally transmitted through magnetoacoustic boundaries. The transient solutions are characterised by a wave front trailing an oscillatory wake. A discussion of the predicted form of narrow-band solar MHD wave motions in magnetically structured regions is given. Title: Solitons in magnetic flux tubes Authors: Roberts, B. Bibcode: 1983IAUS..102...61R Altcode: The highly inhomogeneous nature of the solar atmosphere leads the author to suggest that solitons may occur in magnetic structures such as coronal loops or photospheric flux tubes. The theory is outlined for the simple case of a magnetic slab in a field-free atmosphere and shown to lead to the Benjamin-Ono equation. Title: On MHD wave propagation in inhomogeneous plasmas and the mechanism of resonant absorption Authors: Rae, I. C.; Roberts, B. Bibcode: 1982MNRAS.201.1171R Altcode: The propagation of MHD waves in inhomogeneous plasmas is discussed using both the eikonal and differential equation approaches. The cusp resonance, which arises in a compressible plasma, and has been largely ignored in the literature, is discussed in some detail. Resonant heating may take place at those locations where the phase-speed of an incident wave matches the local Alfven or cusp speeds. The Alfven resonance can only be excited by wave tunnelling, whereas the cusp resonance is excited by propagating slow waves. Cusp resonance by slow waves may therefore be important as a means of heating the solar corona. Title: Pulse propagation in a magnetic flux tube Authors: Rae, I. C.; Roberts, B. Bibcode: 1982ApJ...256..761R Altcode: The linear development of a pulse as it propagates adiabatically along an isothermal magnetic flux tube embedded in a gravitationally stratified atmosphere is studied. It is shown that, for a quiescent environment, longitudinal disturbances in the tube are governed by an equation of the Klein-Gordon type. An impulsively generated disturbance results in a wave front propagating at the subsonic and subAlfvenic tube speed; the wave front trails a wake oscillating at the tube frequency. The results are illustrated for solar photospheric conditions. Title: Wave Propagation in a Magnetically Structured Atmosphere - Part Three - the Slab in a Magnetic Environment Authors: Edwin, P. M.; Roberts, B. Bibcode: 1982SoPh...76..239E Altcode: The propagation of waves in a magnetic slab embedded in a magnetic environment is investigated. The possible modes of propagation are examined from the general dispersion relation, both analytically and numerically, for disturbances which are evanescent in the environment. Approximate dispersion relations governing propagation in a slender slab of field are derived both from the general dispersion relation and from an application of the slender flux tube approximation. Title: Solitons in solar magnetic flux tubes Authors: Roberts, B.; Mangeney, A. Bibcode: 1982MNRAS.198P...7R Altcode: It is theoretically demonstrated that a solar photosphere magnetic flux tube can support the propagation of two types of soliton, or solitary wave: (1) the tube soliton, governed by the Benjamin-Ono equation, and (2) the external soliton, governed by the Korteweg-de Vries equation. Under photospheric conditions, the former may propagate at speeds of about 7 km/sec, and the latter at approximately 11 km/sec. It is speculated that the tube soliton may manifest itself in the atmosphere as a spicule. Title: Bound oscillations on thin magnetic flux tubes - Convective instability and umbral oscillations Authors: Hollweg, J. V.; Roberts, B. Bibcode: 1981ApJ...250..398H Altcode: The possibility that 'tube waves' can be trapped on slender solar magnetic flux tubes is investigated. For rigid isothermal flux tubes, it is found that the flux tube geometry can by itself lead to waves which are trapped on the part of the tube that expands with height. Some geometries lead to trapped modes with eigenperiods near 180 s, if parameters appropriate to sunspot umbrae are chosen. It is possible that the umbral oscillations are a manifestation of such trapped waves, if sunspot umbrae consist of an assembly of slender flux tubes, as in the spaghetti model of Parker (1979). For flux tubes which have a constant ratio of Alfven speed to sound speed, it is found that it is primarily the variation of temperature with height which determines whether trapped waves can exist. Certain temperature profiles lead to disturbances for which omega squared is less than zero, corresponding to convective instability or Rayleigh-Taylor instability. Title: Wave Propagation in a Magnetically Structured Atmosphere - Part Two - Waves in a Magnetic Slab Authors: Roberts, B. Bibcode: 1981SoPh...69...39R Altcode: Magnetic fields may introduce structure (inhomogeneity) into an otherwise uniform medium and thus change the nature of wave propagation in that medium. As an example of such structuring, wave propagation in an isolated magnetic slab is considered. It is supposed that disturbances outside the slab are laterally non-propagating. The effect of gravity is ignored. Title: Wave Propagation in a Magnetically Structured Atmosphere - Part One - Surface Waves at a Magnetic Interface Authors: Roberts, B. Bibcode: 1981SoPh...69...27R Altcode: The solar atmosphere, from the photosphere to the corona, is structured by the presence of magnetic fields. We consider the nature of such inhomogeneity and emphasis that the usual picture of hydromagnetic wave propagation in a uniform medium may be misleading if applied to a structured field. We investigate the occurrence of magnetoacoustic surface waves at a single magnetic interface and consider in detail the case where one side of the interface is field-free. For such an interface, a slow surface wave can always propagate. In addition, a fast surface wave may propagate if the field-free medium is warmer than the magnetic atmosphere. Title: Surface waves and the heating of the corona Authors: Rae, I. C.; Roberts, B. Bibcode: 1981GApFD..18..197R Altcode: The Sun's atmosphere is highly structured by magnetic fields, and this modifies the nature of the usual Alfvén waves which propagate in an everywhere uniform medium. Two simple magnetic field structures are considered: the interface profile (for which a discontinuity separates two media with different, but uniform, unidirectional magnetic fields), and the monotonic profile (for which the magnetic field is unidirectional and continuous but not everywhere constant). An initial-value problem is formulated in ideal MHD, and the perturbations from equilibrium are written in terms of a Bromwich integral. It is well known that the poles of the Bromwich kernel lead to the usual discrete normal mode solutions, and the branch-cuts of the kernel lead to a continuum. An analysis reveals that the interface profile supports Alfvén surface and body waves, whilst the monotonic profile allows only a continuous spectrum of waves. However, by suitably deforming the contour, spectral poles (not related to normal modes) can be located off the physical (principal) Riemann sheet. The residues from these poles yield solutions resembling the interfacial surface waves, but they decay on a timescale T. This property of temporal decay in a non-dissipative system has led lonson (1978) and Wentzel (1979b) to suggest that the spectral residues show the behaviour of a surface wave when the interface is made continuous, and they have interpreted T as the time-scale for dissipation of such a wave in a non-ideal medium. Here we argue that, on the contrary, the time T is in fact the scale for the build-up of the continuous spectrum of oscillations; the spectral residues indicate restructuring of wave motions. It is shown that the process of phase-mixing causes motions along the magnetic field to build-up, whilst those transverse to the field decay. However, the status of wave propagation in a highly structured medium as a means of heating the corona, by dissipating through irreversible processes in non-ideal MHD, remains open until a fully dissipative investigation of their character becomes available. Title: Waves in magnetic structures Authors: Roberts, B. Bibcode: 1981phss.conf..369R Altcode: An examination is presented of the theoretical nature of wave propagation in a magnetically structured and gravitationally stratified atmosphere. It is shown that a magnetic structure such as a sunspot or photospheric intense flux tube supports the propagation of both surface and body waves. The phase speeds of such modes in the absence of gravity are subsonic with respect to the sound speed in the field-free gas. The body waves, which propagate at just below the sound speed in the magnetic field and thread completely across the field (sunspot), are shown to be related to observed umbral oscillations, while the surface waves are largely confined to the edges of the field. In addition, it is shown that the Klein-Gordon equation can describe the motion of vertically propagating waves, including longitudinal waves; acoustic-gravity waves in a slender, rigid, or magnetic tube; kink waves in a slender flux tube; and Alfven waves in a uniform field. Title: The Thermal Statics of Coronal Loops Authors: Roberts, B.; Frankenthal, S. Bibcode: 1980SoPh...68..103R Altcode: The thermal statics of constant pressure coronal loops is discussed, with particular emphasis on non-equilibrium and scaling relations. An analytical solution showing explicitly the occurrence of non-equilibrium in radiation dominated loops is presented. In addition, the general scaling law for hot loops is given. However, in view of the uncertainties in the coronal heating function and the observational determined loop parameters, it is suggested that scaling laws are currently of limited value. Title: Vertical Motions in an Intense Magnetic Flux Tube - Part Five - Radiative Relaxation in a Stratified Medium Authors: Webb, A. R.; Roberts, B. Bibcode: 1980SoPh...68...87W Altcode: It is of interest to examine the effect of radiative relaxation on the propagation of waves in an intense magnetic flux tube embedded in a stratified atmosphere. The radiative energy loss (assuming Newton's law of cooling) leads to a decrease in the vertical phase-velocity of the waves, and to a damping of the amplitude for those waves with frequencies greater than the adiabatic value (ων) of the tube cut-off frequency. The cut-off frequency is generalized to include the effects of radiative relaxation, and allows the waves to be classified as `mainly progressive' or `mainly damped'. The phase-shift between velocity oscillations at two different levels and the phase-difference between temperature and velocity perturbations are compared with the available observations. Title: Vertical Motions in an Intense Magnetic Flux Tube - Part Four - Radiative Relaxation in a Uniform Medium Authors: Webb, A. R.; Roberts, B. Bibcode: 1980SoPh...68...71W Altcode: Radiative damping of waves is important in the upper photosphere. It is thus of interest to examine the effect of radiative relaxation on the propagation of waves in an intense magnetic flux tube embedded in a uniform atmosphere. Assuming Newton's law of cooling, it is shown that the radiative energy loss leads to wave damping. Both the `damping per wavelength' and the `damping per period' reach maximum value when the sound and radiative timescales are comparable. The stronger the magnetic field, the greater is the damping. Title: Recent development in the theory of perfect MHD waves in magnetically structured media. Authors: Roberts, B. Bibcode: 1980AnPh....5..453R Altcode: 1980mhda.conf..453R No abstract at ADS Title: Vertical Motions in an Intense Magnetic Flux Tube - Part Three - on the Slender Flux Tube Approximation Authors: Roberts, B.; Webb, A. R. Bibcode: 1979SoPh...64...77R Altcode: In a recent discussion of intense photospheric magnetic fields we gave an expansion procedure that lead to a tractible system of differential equations governing vertical motions in a slender flux tube embedded in a quiescent environment. Transverse variations were taken into account in our discussion. In support of this expansion scheme we considered the special case of a straight flux tube in a uniform atmosphere. Wilson (1978, 1979b) has now criticised our treatment of this special case. We discuss his objections here, and show them to be without foundation. Title: A model for quiescent solar prominences. Authors: Milne, A. M.; Priest, E. R.; Roberts, B. Bibcode: 1979ApJ...232..304M Altcode: A one-dimensional model is computed for a quiescent prominence in both magnetohydrostatic equilibrium and thermal equilibrium (under a balance between the thermal condition, radiation, and wave heating). The effects of changing the coronal plasma pressure, the horizontal magnetic field strength, and the inclination of the horizontal magnetic field to the prominence normal are investigated. It is found that an equilibrium state is impossible when either the plasma beta or the magnetic field shear is too high. One feature of this model is that the magnetohydrostatics is coupled to the energetics, giving a fourth-order two-point boundary value problem, with two symmetric conditions applied at the center of the structure and the coronal temperature and density specified at a fixed outer edge. Title: Spicules: The resonant response to granular buffeting? Authors: Roberts, B. Bibcode: 1979SoPh...61...23R Altcode: We propose that spicules are the direct result of a resonant buffeting of the sides of an intense (kilogauss) slender flux tube by turbulent granular motions. For a critical wavespeed, the granular motion drives a high-speed flow along the axis of the tube. We illustrate this effect by considering in detail a flux tube embedded in a uniform compressible atmosphere, thereby modelling the development of the `seedflow' in the photosphere that eventually manifests itself in the chromosphere as the spicule. Title: A Modified KIPPENHAHN-SCHLÜTER Model for Quiescent Prominences. Authors: Priest, E. R.; Milne, A. M.; Roberts, B. Bibcode: 1979phsp.coll..184P Altcode: 1979IAUCo..44..184P The effect of varying magnetic field strengths and shears on a model quiescent solar prominence is assessed. Following Kippenhahn and Schlueter (1957) the prominence is modeled by a one-dimensional slab configuration. Their work is extended to include thermal effects, using an energy equation which expresses a balance between thermal conduction, radiative loss and wave heating. At some specified distance from the center of the prominence, the coronal temperature and pressure are prescribed while at the center of the prominence there are two symmetry conditions, which results in a two-point boundary value problem. Title: Vertical motions in an intense magnetic flux tube. II: convective instability. Authors: Webb, A. R.; Roberts, B. Bibcode: 1978SoPh...59..249W Altcode: The nature of convective instability in a slender magnetic flux tube is explored. A sufficient condition for stability is derived for the case of anarbitrary temperature profile in the external medium. The discussion allows for the possibility of a temperature difference between the interior and exterior of the tube. Special cases of our sufficiency condition reduce to Schwarzschild's criterion and its generalisation by Gough and Tayler (1966). Title: Vertical motions in an intense magnetic flux tube. Authors: Roberts, B.; Webb, A. R. Bibcode: 1978SoPh...56....5R Altcode: The recent discovery of localised intense magnetic fields in the solar photosphere is one of the major surprises of the past few years. Here we consider the theoretical nature of small amplitude motions in such an intense magnetic flux tube, within which the field strength may reach 2 kG. We give a systematic derivation of the governing `expansion' equations for a vertical, slender tube, taking into account the dependence upon height of the buoyancy, compressibility and magnetic forces. Several special cases (e.g., the isothermal atmosphere) are considered as well as a more realistic, non-isothermal, solar atmosphere. The expansion procedure is shown to give good results in the special case of a uniform basic-state (in which gravity is negligible) and for which a more exact treatment is possible. Title: Intense magnetic fields and umbral dots. Authors: Roberts, B. Bibcode: 1976SoPh...50..329R Altcode: Intense magnetic fields (of one or two kilogauss) situated in a background of weak field, and umbral dots (regions of weak field) situated in a background of intense field (the sunspot) have both received considerable, though largely unrelated, discussion in the recent literature. Here we suggest that the two phenomena may in fact be less disparate than hitherto presumed. Applying a simple (hydrostatic) model calculation to both phenomena, and assuming temperature differences to be constant with height, we are able to estimate the depth of each of these structures. Title: Overstability and cooling in sunspots. Authors: Roberts, B. Bibcode: 1976ApJ...204..268R Altcode: The role played by overstable Alfven modes in magnetic structures such as sunspots is considered in detail for a column of magnetic field. It is demonstrated explicitly that overstable Alfven waves cool the interior of the magnetic column. It is suggested that these waves account for the cooling in sunspot umbrae, and therefore, in concurrence with Parker, we conclude that a sunspot is a region of enhanced heat transport. The calculations indicate that sunspots have small regions at normal photospheric brightness, and we tentatively suggest that these regions are umbral dots. We also suggest that cooling by overstable Alfven waves may explain the existence of the intense small magnetic flux tubes that constitute the general solar magnetic field. Title: On the maximum rate of magnetic-field reconnexion for Petschek's mechanism Authors: Roberts, B.; Priest, E. R. Bibcode: 1975JPlPh..14..417R Altcode: The standard model for fast magnetic-field reconnexion (Petschek 1964) is qualitatively valid, despite numerous criticisms of its quantitative details. It contains four slow magnetohydrodynamic shock waves, which radiate from a central diffusion region. On the basis of Petschek's rough analysis, it is generally stated that, for large values of the magnetic Reynolds number Rm, reconnexion can occur at a rate no faster than a fraction π/(4 log Rm) of the Alfvén speed. Alternative models of the region outside that of diffusion have been put forward by Yeh & Axford (1970), whose general solutions Vasyliusnas (1975) proved invalid, and by Sonnerup (1970), whose model is mathematically useful, but of limited practical applicability. But their results suggest that reconnexion can occur at any rate whatsoever, with the diffusion-region dimensions responding accordingly. The present paper analyses the external region for Petschek's mechanism in greater detail than hitherto, with the object of deciding whether or not there is a maximum rate. The inclinations of the shock waves are calculated as a function of the fluid speed ve at large distances, which is taken as a measure of the reconnexion rate. It is found that, in agreement with Petschek's rough analysis, there is indeed an upper limit on the allowable rate of magnetic-field reconnexion. Its variation with Rm is calculated, and it is shown, for log10 Rm 1, to be approximately 20% of Petschek's value. Typical values are 0·10vAe for Rm = 10·2 and 0·02vAe for Rm = 106. (vAe is the Alfvén speed at large distances from the diffusion region.) Title: Adiabatic transverse waves in a conducting gas Authors: Roberts, B.; Sozou, C. Bibcode: 1971JPlPh...6..249R Altcode: This paper is an investigation of the effect of a magnetic field on transverse waves in a perfectly conducting gas which is rotating like a Rankine vortex about the axis of a cylinder. The magnetic field is assumed to be in the axial direction. There are three waves: two waves are rotating in the same direction as the gas, one faster and the other slower than the core speed of the gas, and one wave rotates in the opposite direction.