explanation      blue bibcodes open ADS page with paths to full text
Author name code: roberts
ADS astronomy entries on 2022-09-14
=author:"Roberts, B." 

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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.
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.

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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.
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.

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Title: Simulation Studies for a Surface Veto Array to Identify
    Astrophysical Neutrinos at the South Pole
Authors: Euler, S.; Gonzalez, J.; Roberts, B.
2015ICRC...34.1070E    Altcode: 2015PoS...236.1070E
  No abstract at ADS

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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.
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.

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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.
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.

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Title: The period ratio for kink and sausage modes in a magnetic slab
Authors: Macnamara, C. K.; Roberts, B.
2011A&A...526A..75M    Altcode:
  <BR /> 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,
  P<SUB>1</SUB>/2P<SUB>2</SUB>, between the period P<SUB>1</SUB> of
  the fundamental mode and twice the period P<SUB>2</SUB> of its first
  overtone, is one such tool of coronal seismology and its departure from
  unity provides information about the structure of the corona. <BR />
  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. <BR /> 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.

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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.
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.

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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.
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.

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Title: Effects of thermal conduction and compressive viscosity on
    the period ratio of the slow mode
Authors: Macnamara, C. K.; Roberts, B.
2010A&A...515A..41M    Altcode:
  <BR /> 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, P<SUB>1</SUB>/2P<SUB>2</SUB>,
  between the period P<SUB>1</SUB> of the fundamental mode and the
  period P<SUB>2</SUB> of its first overtone, is one such tool of coronal
  seismology and its departure from unity provides information about the
  structure of the corona. <BR /> Methods: We consider analytically the
  effects of thermal conduction and compressive viscosity on the period
  ratio for a longitudinally propagating sound wave. <BR /> 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. <BR /> Conclusions:
  Damping typically has a small effect on the period ratio, suggesting
  that longitudinal structuring remains the most significant effect.

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Title: Coronal Seismology by Means of Kink Oscillation Overtones
Authors: Andries, J.; van Doorsselaere, T.; Roberts, B.; Verth, G.;
   Verwichte, E.; Erdélyi, R.
2009SSRv..149....3A    Altcode:
  The detection of overtones of coronal loop kink oscillations has been
  an important advance in the development of coronal seismology. It has
  significantly increased the potential of coronal seismology and has
  thus initiated important theoretical and observational improvements. New
  detections of overtones have been made and a reduction of the error bars
  has been obtained. The efforts of theoreticians to extend eigenmode
  studies to more general coronal loop models is no longer a matter
  of checking the robustness of the model but now also allows for the
  estimation of certain equilibrium parameters. The frequencies of
  the detected (longitudinal) overtones are in particular sensitive to
  changes in the equilibrium properties along the loop, especially the
  density and the magnetic field expansion. Also, attempts have been
  made to use the limited longitudinal resolution in combination with
  the theoretical eigenmodes as an additional seismological tool.

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Title: THEMIS Operations
Authors: Bester, M.; Lewis, M.; Roberts, B.; McDonald, J.; Pease,
   D.; Thorsness, J.; Frey, S.; Cosgrove, D.; Rummel, D.
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.

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Title: Progress in coronal seismology
Authors: Roberts, B.
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.

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Title: Analytical determination of coronal parameters using the
    period ratio P<SUB>1</SUB>/2P<SUB>2</SUB>
Authors: McEwan, M. P.; Díaz, A. J.; Roberts, B.
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 P<SUB>1</SUB>/2P<SUB>2</SUB>
  from unity provide information about the extent of longitudinal
  structuring within the loop. <BR />Aims: Here we develop an analytical
  approximation that describes the shift in P<SUB>1</SUB>/2P<SUB>2</SUB>
  in terms of the ratio L/Λ<SUB>c</SUB> of the length 2L of a
  coronal loop and the density scale height Λ_c. <BR />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
  P<SUB>1</SUB> and P<SUB>2</SUB>. We also present a variational approach
  to determining the period ratio and show how the WKB method may be
  used. <BR />Results: Analytical approximations to the period ratio
  P<SUB>1</SUB>/2P<SUB>2</SUB> 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. <BR />Conclusions: Our simple formula
  and approximate approaches highlight a useful analytical tool for
  coronal seismology. We demonstrate that P<SUB>1</SUB>/2P<SUB>2</SUB>
  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.

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Title: Effect of coronal structure on loop oscillations: exponential
    profiles
Authors: Díaz, A. J.; Donnelly, G. R.; Roberts, B.
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. <BR />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. <BR />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. <BR />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.

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Title: Effect of longitudinal density structure on a straight magnetic
    field modelling coronal arcade oscillations
Authors: Donnelly, G. R.; Díaz, A. J.; Roberts, B.
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. <BR />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. <BR />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_1/2P<SUB>2</SUB> and P_1/3P<SUB>3</SUB>
  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. <BR
  />Conclusions: We show analytically for simple arcade modes that
  the shift in the fundamental period proves to be small, but the ratio
  P_1/2P<SUB>2</SUB> depends strongly on the density structure. Moreover,
  transversal propagation also shifts the ratio P_1/2P<SUB>2</SUB> 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. <P />Appendix A is
  only available in electronic form at http://www.aanda.org

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Title: On the period ratio P<SUB>1</SUB>/2P<SUB>2</SUB> in the
    oscillations of coronal loops
Authors: McEwan, M. P.; Donnelly, G. R.; Díaz, A. J.; Roberts, B.
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 P<SUB>1</SUB>/2P<SUB>2</SUB> between the period P<SUB>1</SUB>
  of the fundamental mode and the period P<SUB>2</SUB> of its first
  harmonic; in an homogeneous medium this ratio is one, but in a more
  complex configuration it is shifted to lower values.<BR /> 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 P<SUB>1</SUB>/2P<SUB>2</SUB>.<BR /> 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.<BR
  /> Conclusions: .The departure of P<SUB>1</SUB>/2P<SUB>2</SUB> 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.

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Title: Slow MHD oscillations in density structured coronal loops
Authors: Díaz, A. J.; Roberts, B.
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. <BR /> 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.<BR /> 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).<BR
  /> 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.

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Title: Effect of coronal structure on loop oscillations
Authors: Donnelly, G. R.; Díaz, A. J.; Roberts, B.
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.<BR /> 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.<BR /> 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.<BR /> 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.

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Title: Fast magnetohydrodynamic oscillations in a force-free line-tied
    coronal arcade
Authors: Díaz, A. J.; Zaqarashvili, T.; Roberts, B.
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.<BR />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.<BR />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.<BR />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.

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Title: Comment on “Note on the Initial Value Problem for Coronal
    Loop Kink Waves” By P. S. Cally
Authors: Ruderman, M. S.; Roberts, B.
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.

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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.
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.

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Title: Asteroseismology of sdB stars with FUSE.
Authors: Kuassivi; Bonanno, A.; Ferlet, R.; Roberts, B.; Caplinger, J.
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.

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Title: Two-wave interaction in ideal magnetohydrodynamics
Authors: Zaqarashvili, T. V.; Roberts, B.
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.

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Title: Fast Magnetohydrodynamic Oscillations in a Fibril Prominence
    Model
Authors: Díaz, A. J.; Roberts, B.
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.
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.
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.
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.
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.
2005A&A...433...15S    Altcode:
  A&amp;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.
2005A&A...429..767S    Altcode: 2004astro.ph..8114S
  The recently suggested swing interaction between fast magnetosonic
  and Alfvén waves (Zaqarashvili &amp; 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.
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.
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.
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.
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. <P />Appendix A is only available in
  electronic form at http://www.edpsciences.org

---------------------------------------------------------
Title: Coronal Loop Oscillations
Authors: Donnelly, G. R.; Roberts, B.
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.
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.
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. <P />Link: <A
  href="http://www.springer.com/east/home?SGWID=5-102-22-3362=5696-0&amp;changeHeader=true">http://www.springer.com/east/home?SGWID=5-102-22-3362=5696-0&amp;changeHeader=true</A>

---------------------------------------------------------
Title: ION-Neutral Collisions Effect on MHD Surface Waves
Authors: Kumar, Nagendra; Roberts, B.
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.
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.
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.
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.
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 ω<SUB>k</SUB> 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&lt;&lt;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/ω<SUB>k</SUB> 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.
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.
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.
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.
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<SUP>-1</SUP>) 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<SUP>-1</SUP>
  and ω<SUP>-1</SUP><SUB>k</SUB>R<SUP>1/3</SUP>, where R is the
  Reynolds number and ω<SUB>k</SUB> is the frequency of the kink
  mode. Under coronal conditions (when γ<SUP>-1</SUP>&lt;&lt;ω<SUP>-
  1</SUP><SUB>k</SUB>R<SUP>1/3</SUP>), the characteristic damping
  time of global oscillations is γ<SUP>-1</SUP>. During this time,
  most of the energy in the initial perturbation is transferred into
  a resonant absorption layer of thickness of order l<SUP>2</SUP>/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/ω<SUB>k</SUB> 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 V<SUB>p</SUB>/V<SUB>s</SUB> 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.
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 V<SUB>p</SUB>/V<SUB>s</SUB> were constructed. The
  average V<SUB>p</SUB>/V<SUB>s</SUB> 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). V<SUB>p</SUB>/V<SUB>s</SUB> 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.
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) × 10<SUP>6</SUP> meters], thin
  [diameter (2.0 ± 0.36) × 10<SUP>6</SUP> 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.
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.
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.
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.
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.
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.
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.
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.
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 (&gt;300 km s<SUP>-1</SUP>) 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.
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.
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.
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.
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.
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.
1997joso.proc...64N    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Dynamics of Flux Tubes in the Solar Atmosphere: Theory
Authors: Roberts, B.; Ulmschneider, P.
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.
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 × 10<SUP>3</SUP> km, field
  strength 50 G and number density 5 × 10<SUP>15</SUP> m<SUP>−3</SUP>,
  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.
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.
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.
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.
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 &amp; 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.
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.
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&lt;-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&gt;=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.
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.
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.
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 velocityC<SUB>g</SUB>of 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 inC<SUB>g</SUB>moves 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.
1995Obs...115...97S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: High Frequency Waves and Chromospheric Magnetism
Authors: Johnston, A.; Roberts, B.; Wright, A. N.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 &amp; 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.
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.
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.
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 ω<SUP>2</SUP> = 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.
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 ω<SUP>2</SUP>
  = gk(ρ<SUB>1</SUB>-ρ<SUB>2</SUB>)/(P<SUB>1</SUB>+P<SUB>2</SUB>)
  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.
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.
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.
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.
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.
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.
1993ASPC...42..181J    Altcode: 1993gong.conf..181J
  No abstract at ADS

---------------------------------------------------------
Title: MHD waves: a variety of phenomena.
Authors: Roberts, B.
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.
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 - k<SUB>x</SUB>
  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.
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.
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.
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.
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.
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.
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.
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.
1992Natur.355..230E    Altcode:
  RECENTLY Libbrecht and Woodard<SUP>1</SUP> and Elsworth et
  al.<SUP>2</SUP> 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 investigated<SUP>3-5</SUP>simple 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.
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.
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.
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.
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.
1991assm.conf..105R    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Magnetoacoustic-Gravity Surface Waves (With 1 Figure)
Authors: Miles, A. J.; Roberts, B.
1991mcch.conf..508M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Magnetohydrodynamic Surface Waves (With 2 Figures)
Authors: Roberts, B.
1991mcch.conf..494R    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Properties of Non-Parallel Magnetoacoustic Surface Waves
    (With 1 Figure)
Authors: Jain, R.; Roberts, B.
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.
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.
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.
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.
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.
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.
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.
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.
1990ppsa.conf...77M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Properties and models of photospheric flux tubes
Authors: Roberts, B.
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.
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.
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.
1989Obs...109...33T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Resonance absorption of energy by a cold inhomogeneous plasma
Authors: Sedlacek, Z.; Roberts, B.
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.
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.
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.
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.
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.
1988cspp.book....1R    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: MHD waves in the solar atmosphere.
Authors: Edwin, P. M.; Roberts, B.
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.
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.
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.
1987rfsm.conf..292M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Guided MHD waves as a coronal diagnostic tool.
Authors: Roberts, B.
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.
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. Observations<SUP>1,2</SUP> of p-modes, in particular,
  have permitted a theoretical determination<SUP>3</SUP> 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
  measurements<SUP>4</SUP>. Recently, Woodard and Noyes<SUP>5</SUP> 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 5×10<SUP>5</SUP>
  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.
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.
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.
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.
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.
1985Obs...105..240K    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Solitary waves in a magnetic flux tube
Authors: Roberts, B.
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.
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.
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.
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.
1984ESASP.220..257S    Altcode: 1984ESPM....4..257S
  Following the work of Nye &amp; 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.
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.
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.
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.
1984ost1.conf..183R    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Non-equilibrium and instability in coronal structures.
Authors: Roberts, B.
1984ost1.conf...75R    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Wave Propagation in a Magnetic Cylinder
Authors: Edwin, P. M.; Roberts, B.
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.
1983Natur.305..688R    Altcode:
  Pulsations in radio emission from the solar coronal plasma have
  been detected for over a decade<SUP>1-9</SUP>. The oscillations are
  quasiperiodic, with periods of typically a second or so. Recently,
  sub-second time structures have been found in hard X rays<SUP>10</SUP>,
  and simultaneously in hard X rays and microwaves<SUP>11</SUP>. 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 (ω<SUB>ν</SUB>)
  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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 R<SUB>m</SUB>, reconnexion can occur at a
  rate no faster than a fraction π/(4 log R<SUB>m</SUB>) of the Alfvén
  speed. Alternative models of the region outside that of diffusion have
  been put forward by Yeh &amp; 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 v<SUB>e</SUB> 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 R<SUB>m</SUB> is calculated, and it is shown, for log<SUB>10</SUB>
  R<SUB>m</SUB> 1, to be approximately 20% of Petschek's value. Typical
  values are 0·10v<SUB>Ae</SUB> for R<SUB>m</SUB> = 10·<SUP>2</SUP> and
  0·02v<SUB>Ae</SUB> for R<SUB>m</SUB> = 10<SUP>6</SUP>. (v<SUB>Ae</SUB>
  is the Alfvén speed at large distances from the diffusion region.)

---------------------------------------------------------
Title: Adiabatic transverse waves in a conducting gas
Authors: Roberts, B.; Sozou, C.
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.