explanation      blue bibcodes open ADS page with paths to full text
Author name code: thomas-jack
ADS astronomy entries on 2022-09-14
=author:"Thomas, J.H." OR =author:"Thomas, John H." -author:"Aggarwal" -author:"Akiba" -author:"Mangini" -author:"Marshall"

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Title: New Horizons: Scalar and Vector Ultralight Dark Matter
Authors: Antypas, D.; Banerjee, A.; Bartram, C.; Baryakhtar, M.; Betz,
   J.; Bollinger, J. J.; Boutan, C.; Bowring, D.; Budker, D.; Carney, D.;
   Carosi, G.; Chaudhuri, S.; Cheong, S.; Chou, A.; Chowdhury, M. D.;
   Co, R. T.; Crespo López-Urrutia, J. R.; Demarteau, M.; DePorzio,
   N.; Derbin, A. V.; Deshpande, T.; Chowdhury, M. D.; Di Luzio, L.;
   Diaz-Morcillo, A.; Doyle, J. M.; Drlica-Wagner, A.; Droster, A.;
   Du, N.; Döbrich, B.; Eby, J.; Essig, R.; Farren, G. S.; Figueroa,
   N. L.; Fry, J. T.; Gardner, S.; Geraci, A. A.; Ghalsasi, A.; Ghosh,
   S.; Giannotti, M.; Gimeno, B.; Griffin, S. M.; Grin, D.; Grin, D.;
   Grote, H.; Gundlach, J. H.; Guzzetti, M.; Hanneke, D.; Harnik, R.;
   Henning, R.; Irsic, V.; Jackson, H.; Kimball, D. F. Jackson; Jaeckel,
   J.; Kagan, M.; Kedar, D.; Khatiwada, R.; Knirck, S.; Kolkowitz, S.;
   Kovachy, T.; Kuenstner, S. E.; Lasner, Z.; Leder, A. F.; Lehnert,
   R.; Leibrandt, D. R.; Lentz, E.; Lewis, S. M.; Liu, Z.; Manley, J.;
   Maruyama, R. H.; Millar, A. J.; Muratova, V. N.; Musoke, N.; Nagaitsev,
   S.; Noroozian, O.; O'Hare, C. A. J.; Ouellet, J. L.; Pappas, K. M. W.;
   Peik, E.; Perez, G.; Phipps, A.; Rapidis, N. M.; Robinson, J. M.;
   Robles, V. H.; Rogers, K. K.; Rudolph, J.; Rybka, G.; Safdari, M.;
   Safdari, M.; Safronova, M. S.; Salemi, C. P.; Schmidt, P. O.; Schumm,
   T.; Schwartzman, A.; Shu, J.; Simanovskaia, M.; Singh, J.; Singh, S.;
   Smith, M. S.; Snow, W. M.; Stadnik, Y. V.; Sun, C.; Sushkov, A. O.;
   Tait, T. M. P.; Takhistov, V.; Tanner, D. B.; Temples, D. J.; Thirolf,
   P. G.; Thomas, J. H.; Tobar, M. E.; Tretiak, O.; Tsai, Y. -D.; Tyson,
   J. A.; Vandegar, M.; Vermeulen, S.; Visinelli, L.; Vitagliano, E.;
   Wang, Z.; Wilson, D. J.; Winslow, L.; Withington, S.; Wooten, M.;
   Yang, J.; Ye, J.; Young, B. A.; Yu, F.; Zaheer, M. H.; Zelevinsky,
   T.; Zhao, Y.; Zhou, K.
2022arXiv220314915A    Altcode:
  The last decade has seen unprecedented effort in dark matter model
  building at all mass scales coupled with the design of numerous new
  detection strategies. Transformative advances in quantum technologies
  have led to a plethora of new high-precision quantum sensors and dark
  matter detection strategies for ultralight ($<10\,$eV) bosonic dark
  matter that can be described by an oscillating classical, largely
  coherent field. This white paper focuses on searches for wavelike
  scalar and vector dark matter candidates.

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Title: Explaining the observed relation between stellar activity
    and rotation.
Authors: Blackman, E. G.; Thomas, J. H.
2015MNRAS.446L..51B    Altcode: 2014arXiv1407.8500B
  Observations of late-type main-sequence stars have revealed empirical
  scalings of coronal activity versus rotation period or Rossby number
  Ro (a ratio of rotation period to convective turnover time) which has
  hitherto lacked explanation. For Ro ≫ 1, the activity observed as
  X-ray to bolometric flux varies as Ro<SUP>-q</SUP> with 2 ≤ q ≤
  3, whilst |q| &lt; 0.13 for Ro ≪ 1. Here, we explain the transition
  between these two regimes and the power law in the Ro ≫ 1 regime by
  constructing an expression for the coronal luminosity based on dynamo
  magnetic field generation and magnetic buoyancy. We explain the Ro ≪
  1 behaviour from the inference that observed rotation is correlated
  with internal differential rotation and argue that once the shear
  time-scale is shorter than the convective turnover time, eddies will
  be shredded on the shear time-scale and so the eddy correlation time
  actually becomes the shear time and the convection time drops out
  of the equations. We explain the Ro ≫ 1 behaviour using a dynamo
  saturation theory based on magnetic helicity buildup and buoyant loss.

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Title: Sunspots and Starspots
Authors: Thomas, John H.; Weiss, Nigel O.
2012sust.book.....T    Altcode:
  Preface; 1. The sun among the stars; 2. Sunspots and starspots:
  a historical introduction; 3. Overall structure of a sunspot;
  4. Fine structure of the umbra; 5. Fine structure of the penumbra;
  6. Oscillations in sunspots; 7. Sunspots and active regions; 8. Magnetic
  activity in stars; 9. Starspots; 10. Solar and stellar activity cycles;
  11. Solar and stellar dynamos; 12. Solar activity, space weather,
  and climate change; 13. The way ahead; Appendices; References; Index.

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Title: Theoretical Models of Sunspot Structure and Dynamics
Authors: Thomas, J. H.
2010ASSP...19..229T    Altcode: 2010mcia.conf..229T; 2009arXiv0903.4106T
  Recent progress in theoretical modeling of a sunspot is reviewed. The
  observed properties of umbral dots are well reproduced by realistic
  simulations of magnetoconvection in a vertical, monolithic magnetic
  field. To understand the penumbra, it is useful to distinguish between
  the inner penumbra, dominated by bright filaments containing slender
  dark cores, and the outer penumbra, made up of dark and bright filaments
  of comparable width with corresponding magnetic fields differing in
  inclination by some 30° and strong Evershed flows in the dark filaments
  along nearly horizontal or downward-plunging magnetic fields. The role
  of magnetic flux pumping in submerging magnetic flux in the outer
  penumbra is examined through numerical experiments, and different
  geometric models of the penumbral magnetic field are discussed in the
  light of high-resolution observations. Recent, realistic numerical
  MHD simulations of an entire sunspot have succeeded in reproducing
  the salient features of the convective pattern in the umbra and the
  inner penumbra. The siphon-flow mechanism still provides the best
  explanation of the Evershed flow, particularly in the outer penumbra
  where it often consists of cool, supersonic downflows.

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Title: Sunspots and Starspots
Authors: Thomas, John H.; Weiss, Nigel O.
2008sust.book.....T    Altcode:
  Preface; 1. The sun among the stars; 2. Sunspots and starspots:
  a historical introduction; 3. Overall structure of a sunspot;
  4. Fine structure of the umbra; 5. Fine structure of the penumbra;
  6. Oscillations in sunspots; 7. Sunspots and active regions; 8. Magnetic
  activity in stars; 9. Starspots; 10. Solar and stellar activity cycles;
  11. Solar and stellar dynamos; 12. Solar activity, space weather,
  and climate change; 13. The way ahead; Appendices; References; Index.

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Title: Flux Pumping and Magnetic Fields in the Outer Penumbra of
    a Sunspot
Authors: Brummell, Nicholas H.; Tobias, Steven M.; Thomas, John H.;
   Weiss, Nigel O.
2008ApJ...686.1454B    Altcode:
  The filamentary structure of a sunspot penumbra is believed to be
  magnetoconvective in origin. In the outer penumbra there is a difference
  in inclination of up to 30°-40° between the magnetic fields associated
  with bright and dark filaments, and the latter fields plunge downward
  below the surface toward the edge of the spot. We have proposed that
  these fields are dragged downward by magnetic pumping caused by the
  external granular convection. In this paper we model this process in
  a more elaborate idealized configuration that includes the curvature
  force exerted by an arched magnetic field in addition to magnetic
  buoyancy, and demonstrate that magnetic pumping remains an efficient
  mechanism for holding flux submerged. We discuss the implications of
  these results for the magnetic structure of the outer penumbra.

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Title: Evershed Flows along Penumbral Flux Tubes in Sunspots
Authors: Thomas, J. H.
2006ASPC..354..224T    Altcode:
  Theoretical models of the Evershed flow in a sunspot based on the
  thin flux tube approximation are compared. The super-Alfvénic,
  “sea-serpent” flow configurations found by Schlichenmaier (2002, 2003)
  are shown to be gravitationally unstable. If indeed super-Alfvénic
  flow speeds can be achieved along penumbral flux tubes, any undulations
  that occur will form preferentially in a horizontal plane and hence will
  not explain the observed Evershed downflows or outward-moving penumbral
  grains. On the other hand, sub-Alfvénic, arched flow configurations,
  such as the siphon-flow models of Montesinos and Thomas (1997), are
  gravitationally stable. The outer part of a siphon-flow flux tube is
  submerged, in opposition to its magnetic buoyancy, by downward magnetic
  flux pumping in the granular convective layer outside the sunspot.

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Title: On the fine structure of magnetic fields in sunspot penumbrae
Authors: Thomas, J. H.; Weiss, N. O.; Tobias, S. M.; Brummell, N. H.
2006A&A...452.1089T    Altcode:
  Recent observations have revealed the interlocking-comb structure of
  the magnetic field in the outer penumbra of a sunspot. We have argued
  that this structure owes its origin in part to downward pumping of
  magnetic flux by vigorous granular convection in the region surrounding
  the spot. Here we stress the difference between the inner and outer
  penumbra, and correct some misleading assertions in a recent paper by
  Spruit &amp; Scharmer.

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Title: Extracting rotational energy in supernova progenitors:
    Transient Poynting flux growth vs. turbulent dissipation
Authors: Blackman, Eric G.; Nordhaus, Jason T.; Thomas, John H.
2006NewA...11..452B    Altcode: 2004astro.ph.10716B
  Observational evidence for anisotropy in supernovae (SN) may signal
  the importance of angular momentum and differential rotation in the
  progenitors. Free energy in differential rotation and rotation can be
  extracted magnetically or via turbulent dissipation. The importance
  that magnetohydrodynamic jets and coronae may play in driving SN
  motivates understanding large scale dynamos in SN progenitors. We
  develop a dynamical large scale interface dynamo model in which the
  differential rotation and rotation deplete both through Poynting flux
  and turbulent diffusion. We apply the model to a differentially rotating
  core surrounded by a convection zone of a SN progenitor from a initial
  15 M<SUB>⊙</SUB> star. Unlike the Sun, the dynamo is transient because
  the differential rotation is primarily due to the initial collapse. Up
  to ∼10 <SUP>51</SUP> erg can be drained into time-integrated Poynting
  flux and heat, the relative fraction of which depends on the relative
  amount of turbulence in the shear layer vs. the convection zone
  and the fraction of the shear layer into which the magnetic field
  penetrates. Both sinks can help facilitate explosions and could lead
  to different levels of anisotropy and pulsar kicks. In all cases,
  the poloidal magnetic field is much weaker than the toroidal field,
  and the Poynting flux is lower than previous estimates which invoke
  the magnitude of the total magnetic energy. A signature of a large
  scale dynamo is the oscillation of the associated Poynting flux on
  ∼1 s time scales, implying the same for the energy delivery to a SN.

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Title: Flows along penumbral flux tubes in sunspots. Instability of
    super-Alfvénic, serpentine solutions
Authors: Thomas, J. H.
2005A&A...440L..29T    Altcode:
  The super-Alfvénic, undulating "sea-serpent" flow configurations
  along thin magnetic flux tubes, proposed by Schlichenmaier (2002,
  Astron. Nachr., 323, 303; 2003, ASP Conf. Ser., 286, 24) to explain
  both the Evershed flow and moving penumbral grains in a sunspot, are
  shown to be gravitationally unstable. Any undulations that occur for
  super-Alfvénic flow speeds will form preferentially in a horizontal
  plane and hence will not explain Evershed downflows or outward-moving
  penumbral grains. Sub-Alfvénic, arched flow configurations, on the
  other hand, are gravitationally stable.

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Title: Fine Structure in Sunspots
Authors: Thomas, John H.; Weiss, Nigel O.
2004ARA&A..42..517T    Altcode:
  Important physical processes on the Sun, and especially in sunspots,
  occur on spatial scales at or below the limiting resolution of
  current solar telescopes. Over the past decade, using a number of
  new techniques, high-resolution observations have begun to reveal
  the complex thermal and magnetic structure of a sunspot, along with
  associated flows and oscillations. During this time remarkable advances
  in computing power have allowed significant progress in our theoretical
  understanding of the dynamical processes, such as magnetoconvection,
  taking place within a sunspot. In this review we summarize the latest
  observational results and theoretical interpretations of the fine
  structure in sunspots. A number of projects underway to build new solar
  telescopes or upgrade existing ones, along with several promising new
  theoretical ideas, ensure that there will be significant advances in
  sunspot research over the coming decade.

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Title: The Origin of Penumbral Structure in Sunspots: Downward
    Pumping of Magnetic Flux
Authors: Weiss, Nigel O.; Thomas, John H.; Brummell, Nicholas H.;
   Tobias, Steven M.
2004ApJ...600.1073W    Altcode:
  This paper offers the first coherent picture of the interactions
  between convection and magnetic fields that lead to the formation of
  the complicated filamentary structure of a sunspot penumbra. Recent
  observations have revealed the intricate interlocking-comb structure
  of the penumbral magnetic field. Some field lines, with associated
  Evershed outflows, plunge below the solar surface near the edge of
  the spot. We claim that these field lines are pumped downward by
  small-scale granular convection outside the sunspot. This mechanism
  is demonstrated in numerical experiments. Magnetic pumping is a key
  new ingredient that links several theoretical ideas about penumbral
  structure and dynamics; it explains not only the abrupt appearance of
  a penumbra as a pore increases in size but also the behavior of moving
  magnetic features outside a spot.

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Title: Theory of sunspot structure
Authors: Thomas, John H.
2004IAUS..223..161T    Altcode: 2005IAUS..223..161T
  Recent high-resolution observations provide us with key information
  that allows us to begin to assemble a coherent theoretical picture of
  the formation and maintenance of a sunspot and its complex thermal
  and magnetic structure. A new picture of penumbral structure has
  emerged from observations, involving two components having different
  magnetic field inclinations and remaining essentially distinct over
  the lifetime of the spot, with little interchange of magnetic flux. The
  darker component, with more nearly horizontal magnetic field, includes
  "returning" magnetic flux tubes that dive down below the surface
  near the outer edge of the penumbra and carry much of the Evershed
  flow. The configuration of these flux tubes can be understood to be a
  consequence of downward pumping of magnetic flux by turbulent granular
  convection in the moat surrounding a sunspot. This process has been
  demonstrated in recent three-dimensional numerical simulations of fully
  compressible convection. The process of flux pumping is an important
  key to understanding the formation and maintenance of the penumbra,
  the hysteresis associated with the transition from a pore to a sunspot,
  and the behavior of moving magnetic features in the moat.

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Title: Fuel-Supply-limited Stellar Relaxation Oscillations:
    Application to Multiple Rings around Asymptotic Giant Branch Stars
    and Planetary Nebulae
Authors: Van Horn, Hugh M.; Thomas, John H.; Frank, Adam; Blackman,
   Eric G.
2003ApJ...585..983V    Altcode: 2002astro.ph..4029V
  We describe a new mechanism for pulsations in evolved stars: relaxation
  oscillations driven by a coupling between the luminosity-dependent
  mass-loss rate and the H fuel abundance in a nuclear-burning shell. When
  mass loss is included, the outward flow of matter can modulate the
  flow of fuel into the shell when the stellar luminosity is close to
  the Eddington luminosity L<SUB>Edd</SUB>. When the luminosity drops
  below L<SUB>Edd</SUB>, the mass outflow declines and the shell is
  resupplied with fuel. This process can be repetitive. We demonstrate
  the existence of such oscillations and discuss the dependence of the
  results on the stellar parameters. In particular, we show that the
  oscillation period scales specifically with the mass of the H-burning
  relaxation shell (HBRS), defined as the part of the H-burning shell
  above the minimum radius at which the luminosity from below first
  exceeds the Eddington threshold at the onset of the mass-loss
  phase. For a stellar mass M<SUB>*</SUB>~0.7 M<SUB>solar</SUB>,
  luminosity L<SUB>*</SUB>~10<SUP>4</SUP> L<SUB>solar</SUB>, and
  mass-loss rate |M|~10<SUP>-5</SUP> M<SUB>solar</SUB> yr<SUP>-1</SUP>,
  the oscillations have a recurrence time of ~1400 yr~57τ<SUB>fsm</SUB>,
  where τ<SUB>fsm</SUB> is the timescale for modulation of the
  fuel-supply in the HBRS by the varying mass-loss rate. This period
  agrees very well with the ~1400 yr period inferred for the spacings
  between the shells surrounding some planetary nebulae. We also
  find the half-width of the luminosity peak to be ~0.39 times the
  oscillation period; for comparison, the observational shell thickness
  of ~1000 AU corresponds to ~0.36 of the spacing between pulses. We find
  oscillations only for models in which the luminosity of the relaxation
  shell is ~10%-15% of the total stellar luminosity and for which energy
  generation occurs through the p-p chain. We suggest this mechanism as
  a natural explanation for the circumnebular shells surrounding some
  planetary nebulae, which appear only at the end of the AGB phase.

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Title: Solar physics: The Sun under a microscope
Authors: Thomas, John H.
2002Natur.420..134T    Altcode:
  Fine details of the filamentary structure of sunspots are revealed in
  new observations. These high-resolution measurements herald the quality
  of data to be expected from a new generation of solar telescopes.

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Title: Downward pumping of magnetic flux as the cause of filamentary
    structures in sunspot penumbrae
Authors: Thomas, John H.; Weiss, Nigel O.; Tobias, Steven M.; Brummell,
   Nicholas H.
2002Natur.420..390T    Altcode:
  The structure of a sunspot is determined by the local interaction
  between magnetic fields and convection near the Sun's surface. The dark
  central umbra is surrounded by a filamentary penumbra, whose complicated
  fine structure has only recently been revealed by high-resolution
  observations. The penumbral magnetic field has an intricate and
  unexpected interlocking-comb structure and some field lines, with
  associated outflows of gas, dive back down below the solar surface
  at the outer edge of the spot. These field lines might be expected
  to float quickly back to the surface because of magnetic buoyancy,
  but they remain submerged. Here we show that the field lines are kept
  submerged outside the spot by turbulent, compressible convection,
  which is dominated by strong, coherent, descending plumes. Moreover,
  this downward pumping of magnetic flux explains the origin of the
  interlocking-comb structure of the penumbral magnetic field, and the
  behaviour of other magnetic features near the sunspot.

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Title: Magnetic flux pumping and the structure of a sunspot penumbra
Authors: Thomas, J. H.; Weiss, N. O.; Tobias, S. M.; Brummell, N. H.
2002AN....323..383T    Altcode:
  We propose an overall scenario for the development and maintenance of
  a sunspot penumbra, in which turbulent magnetic flux pumping plays a
  key role. Recent high-resolution observations have revealed arched,
  "returning" magnetic flux tubes that emerge in the inner or middle
  penumbra, dive back down below the solar surface near the outer edge
  of the penumbra, and carry much of the Evershed flow. Some mechanism
  is required to keep the outer parts of the returning flux tubes
  submerged in spite of their magnetic buoyancy. We have proposed that
  the relevant mechanism is downward turbulent pumping of magnetic flux
  by granular convection in the moat outside the sunspot. This mechanism
  is demonstrated by means of an appropriate three-dimensional numerical
  simulation of turbulent compressible convection in the strongly
  superadiabatic granulation layer. We suggest that a penumbra first
  forms through a convectively driven instability at the outer edge
  of a growing pore. The nonlinear development of this instability
  pUSA)roduces the filamentary penumbra with its interlocking-comb
  magnetic field geometry. Downward flux pumping of some of the nearly
  horizontal magnetic flux in the dark filaments produces the returning
  flux tubes, with their associated Evershed flow, and also establishes
  the subcritical nature of the bifurcation that produces the filamentary
  penumbra, which explains why there are pores larger than the smallest
  sunspots.

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Title: A new look at the relationship between activity, dynamo number
    and Rossby number in late-type stars
Authors: Montesinos, B.; Thomas, John H.; Ventura, P.; Mazzitelli, I.
2001MNRAS.326..877M    Altcode:
  The correlation between stellar activity, as measured by the
  indicator ΔR<SUB>HK</SUB>, and the Rossby number Ro in late-type
  stars is revisited in light of recent developments in solar dynamo
  theory. Different stellar interior models, based on both mixing-length
  theory and the full spectrum of turbulence, are used in order to
  see to what extent the correlation of activity with Rossby number is
  model dependent, or otherwise can be considered universal. Although
  we find some modest model dependence, we find that the correlation of
  activity with Rossby number is significantly better than with rotation
  period alone for all the models we consider. Dynamo theory suggests
  that activity should scale with the dynamo number. A current model of
  the solar dynamo, the so-called interface dynamo, proposes that the
  amplification of the toroidal magnetic field by differential rotation
  (the ω-effect) and the production of the poloidal magnetic field
  from toroidal by helical turbulence (the α-effect) take place in
  different, adjacent layers near the base of the convection zone. A
  new scale analysis based on the interface dynamo shows that the
  appropriate dynamo number does not depend on the Rossby number alone,
  but also depends on an additional dimensionless factor related to
  the differential rotation. This leads to a new interpretation of the
  correlation between activity and Rossby number, which in turn leads
  to some conclusions about the magnitude of differential rotation in
  the dynamo layers of late-type main-sequence stars.

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Title: Dynamos in asymptotic-giant-branch stars as the origin of
    magnetic fields shaping planetary nebulae
Authors: Blackman, Eric G.; Frank, Adam; Markiel, J. Andrew; Thomas,
   John H.; Van Horn, Hugh M.
2001Natur.409..485B    Altcode: 2001astro.ph..1492B
  Planetary nebulae are thought to be formed when a slow wind from
  the progenitor giant star is overtaken by a subsequent fast wind
  generated as the star enters its white dwarf stage. A shock forms near
  the boundary between the winds, creating the relatively dense shell
  characteristic of a planetary nebula. A spherically symmetric wind
  will produce a spherically symmetric shell, yet over half of known
  planetary nebulae are not spherical; rather, they are elliptical or
  bipolar in shape. A magnetic field could launch and collimate a bipolar
  outflow, but the origin of such a field has hitherto been unclear,
  and some previous work has even suggested that a field could not be
  generated. Here we show that an asymptotic-giant-branch (AGB) star
  can indeed generate a strong magnetic field, having as its origin a
  dynamo at the interface between the rapidly rotating core and the more
  slowly rotating envelope of the star. The fields are strong enough to
  shape the bipolar outflows that produce the observed bipolar planetary
  nebulae. Magnetic braking of the stellar core during this process may
  also explain the puzzlingly slow rotation of most white dwarf stars.

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Title: A new analysis of the correlation between activity and Rossby
    number in late-type stars
Authors: Montesinos, B.; Thomas, J. H.
2001hsa..conf..393M    Altcode:
  No abstract at ADS

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Title: Dynamos in AGB Stars and Magnetic Shaping of Planetary Nebulae
Authors: Thomas, J. H.; Blackman, E. G.; Frank, A.; van Horn, H. M.;
   Markiel, J. A.
2001ASPC..248..439T    Altcode: 2001mfah.conf..439T
  No abstract at ADS

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Title: Fine-Scale Magnetic Effects on P-Modes and Higher Frequency
    Acoustic Waves in a Solar Active Region
Authors: Thomas, John H.; Stanchfield, Donald C. H., II
2000ApJ...537.1086T    Altcode:
  We present results from a 5 hr time series of simultaneous
  high-resolution measurements of oscillations in the photosphere
  and the chromosphere on fine spatial scales for a rapidly evolving
  solar active region and examine their relation to the vector
  magnetic field. The photospheric oscillations are determined from
  Doppler shifts in the Zeeman-insensitive spectral line Fe I 557.6
  nm, whereas the chromospheric oscillations are determined from the
  intensity fluctuations seen in the Ca II K line. The vector magnetic
  field configurations just prior to and just after the time-series
  measurements are obtained from the full Stokes inversion of the line
  profiles of Fe I 630.15 and 630.25 nm. In both the photosphere and
  the chromosphere, p-mode power is suppressed by the magnetic field,
  decreasing with increasing field strength. At higher frequencies (above
  the acoustic cutoff) power is also suppressed at the highest magnetic
  field strengths in the photosphere and the chromosphere but is enhanced
  in the photosphere in localized patches (“halos”) of intermediate
  field strength surrounding the patches of highest field strength
  (as first reported by Brown et al.). The evidence for similar halos
  in the chromosphere (as first seen by Braun et al. and Toner &amp;
  LaBonte) and their association with the photospheric halos is less
  clear. We find seismic evidence, in the forms of suppression of p-mode
  and higher frequency power and a halo of enhanced higher frequency
  power, for a developing pore prior to its appearance as a dark patch
  in Fe I 557.6 nm core intensity. This result shows that p-mode power
  suppression cannot be due primarily to the greater transparency of
  the cooler pore atmosphere.

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Title: Solar Interface Dynamo Models with a Realistic Rotation Profile
Authors: Markiel, J. Andrew; Thomas, John H.
1999ApJ...523..827M    Altcode:
  Extensions of the interface dynamo model of Parker are considered
  through two-dimensional numerical simulations in spherical geometry. In
  the interface model, the production of the poloidal and toroidal
  components of the magnetic field occur in two separate regions coupled
  by diffusion. A large discontinuous jump in the diffusivity at the
  interface allows the production of a sufficiently strong toroidal
  magnetic field in the lower region while avoiding the difficulty
  of alpha quenching. When the rotation rate is assumed to vary only
  radially, dynamo waves that closely resemble the analytical solutions
  in Cartesian geometry found by Parker are found propagating along
  the interface. However, when a fit to the solar rotation profile--as
  determined from helioseismology, with both latitudinal and radial
  dependence--is included, no fully satisfactory solar-like oscillatory
  solutions are found. For an appropriately large diffusivity contrast,
  only steady modes are found for negative dynamo number, and only purely
  decaying solutions are found for positive dynamo number. Here the effect
  of the latitudinal variation of rotation is to suppress the oscillatory
  interface modes driven by the radial shear. Oscillatory solutions can
  be found for a small diffusivity contrast, but these solutions have
  field strengths that are too low for the solar case. The hybrid mode
  of Charbonneau &amp; MacGregor found from similar calculations is
  shown to result from an incorrect boundary condition imposed at the
  interface and thus is not a valid solution.

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Title: The Solar Physics Division
Authors: Thomas, John H.
1999aasf.book..238T    Altcode:
  No abstract at ADS

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Title: Solar physics:  The Sun at small scales
Authors: Thomas, John H.
1998Natur.396..114T    Altcode:
  No abstract at ADS

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Title: The Evershed Effect in Sunspots: A Theoretical Explanation
Authors: Montesinos, Benjamin; Thomas, John H.
1998Ap&SS.263..323M    Altcode: 1999Ap&SS.263..323M
  The siphon flow model, consisting in the simulation of a flow of gas
  moving along a thin magnetic flux tube and driven by the pressure
  drop between its footpoints, is proposed to explain the observational
  features of the Evershed effect, one of the longstanding problems in
  solar physics.

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Title: Velocity and Magnetic Field Fluctuations in the Photosphere
    of a Sunspot
Authors: Lites, Bruce W.; Thomas, John H.; Bogdan, Thomas J.; Cally,
   Paul S.
1998ApJ...497..464L    Altcode:
  We use a data set of exceptionally high quality to measure oscillations
  of Doppler velocity, intensity, and the vector magnetic field at
  photospheric heights in a sunspot. Based on the full Stokes inversion
  of the line profiles of Fe I 630.15 and 630.25 nm, in the sunspot umbra
  we find upper limits of 4 G (root mean square [rms]) for the amplitude
  of 5 minute oscillations in magnetic field strength and 0.09d (rms)
  for the corresponding oscillations of the inclination of the magnetic
  field to the line of sight. Our measured magnitude of the oscillation
  in magnetic field strength is considerably lower than that found in
  1997 by Horn, Staude, &amp; Landgraf. Moreover, we find it likely
  that our measured magnetic field oscillation is at least partly due to
  instrumental and inversion cross talk between the velocity and magnetic
  signals, so that the actual magnetic field strength fluctuations are
  even weaker than 4 G. In support of this we show, on the basis of the
  eigenmodes of oscillation in a theoretical model of the sunspot umbra,
  that magnetic field variations of at most 0.5 G are all that is to
  be expected. The theoretical model also provides an explanation of
  the shift of power peaks in Doppler velocity to the 3 minute band in
  chromospheric umbral oscillations, as a natural consequence of the
  drastic change in character of the eigenmodes of oscillation between
  frequencies of about 4.5 and 5.0 mHz due to increased tunneling through
  the acoustic cutoff-frequency barrier. Using measurements of the
  phase of velocity oscillations above the acoustic cutoff frequency,
  we determine the relative velocity response height in the umbra of
  four different photospheric spectral lines from the phase differences
  between velocities in these lines, assuming that the oscillations
  propagate vertically at the local sound speed. In spacetime maps of
  fluctuations in continuum intensity, Doppler velocity, magnetic field
  strength, and field inclination, we see distinct features that migrate
  radially inward from the inner penumbra all the way to the center of
  the umbra, at speeds of a few tenths of a kilometer per second. These
  moving features are probably a signature of the convective interchange
  of magnetic flux tubes in the sunspot, although we failed to find any
  strong correlation among the features in the different quantities,
  indicating that these features have not been fully resolved.

---------------------------------------------------------
Title: The Seismology of Sunspots: A Comparison of Time-Distance
    and Frequency-Wavenumber Methods
Authors: Bogdan, T. J.; Braun, D. C.; Lites, B. W.; Thomas, John H.
1998ApJ...492..379B    Altcode:
  A pair of formulae are developed that relate the absorption coefficient
  and partial-wave phase shift concepts of frequency-wavenumber local
  helioseismology to the center-annulus cross-correlation function
  of time-distance helioseismology, under the general circumstances
  that both induced and spontaneous sunspot oscillations may be
  present. These formulae show that spontaneous emission of p-modes
  by magnetic and Reynolds stresses within the spot and the mode
  mixing between incoming and outgoing p-modes affect only the
  outgoing center-annulus cross-correlation time τ<SUP>+</SUP>,
  and they caution that real or spurious phase lags of the umbral
  oscillation signal lead to differences in the incoming and outgoing
  correlation times, resulting in τ<SUP>-</SUP> ≠ τ<SUP>+</SUP>. The
  application of these methods to actual helioseismic data obtained
  by the Global Oscillation Network Group (GONG) project is carried
  out in order to provide a tangible illustration of how time-distance
  and frequency-wavenumber ideas can profitably be combined to yield
  deeper insight into the seismic probing of sunspots. <P />By using the
  helioseismic GONG data in conjunction with concurrent observations of
  Doppler velocities and vector magnetic fields obtained by the High
  Altitude Observatory/National Solar Observatory (HAO/NSO) Advanced
  Stokes Polarimeter (ASP) for the 1995 October disk passage of active
  region NOAA 7912, we demonstrate that the inferred GONG umbral
  signal actually originates from the umbra-penumbra boundary about 6
  Mm distant from the center of the spot. Further, the ASP observations
  show that the 5 minute oscillations at the umbra-penumbra boundary lag
  behind those in the center of the umbra by approximately 1 minute,
  which is precisely the difference between the incoming and outgoing
  correlation times for NOAA 7912 recently determined by Braun. This
  remarkable result underscores the perils of using umbral oscillations
  in time-distance helioseismology, and it calls into question previous
  claims that correlation time differences constitute direct evidence
  for the existence of a steady downflow in and around sunspots. Taken
  together, the observational and theoretical evidence suggest that
  the p-mode forcing of the spot leads to the generation of upwardly
  propagating slow magnetoatmospheric waves. These waves are in turn
  responsible for the decreased amplitudes of the outwardly propagating
  p-modes in the surrounding quiet Sun, and the dispersion in their travel
  times between the hidden subsurface layer where they are forced and
  the overlying level where the Doppler signals originate leads to the
  observed phase lag between the umbral and penumbral oscillations and
  the corresponding correlation time differences. <P />This work utilizes
  data obtained by the Global Oscillations Network Group (GONG) project,
  managed by the National Solar Observatory, a Division of the National
  Optical Astronomy Observatories, which is operated by AURA, Inc.,
  under a cooperative agreement with the National Science Foundation.

---------------------------------------------------------
Title: Handbook of vacuum science and technology
Authors: Hoffman, Dorothy M.; Singh, Bawa; Thomas, John H.
1998hvst.book.....H    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Evershed effect in sunspots as a siphon flow along a
    magnetic flux tube
Authors: Montesinos, Benjamin; Thomas, John H.
1997Natur.390..485M    Altcode:
  The Evershed effect-a wavelength shift and profile asymmetry in
  the spectral lines observed from the outer regions of sunspots
  (the penumbra)-has been interpreted as a radial outflow of gas
  from the sunspot, but the dynamics of the flow have not been fully
  understood. Although the Evershed effect seems to stop abruptly at the
  outer edge of the penumbra, the outflow itself must continue, though
  tracing its path has proved difficult. Theoretical, and observational
  studies have suggested that much of the continuing flow may follow
  magnetic field lines that go below the visible surface of the Sun at or
  just beyond the edge of the penumbra, and recent observations have now
  confirmed this picture. Here we show, using theoretical calculations
  based on a more realistic model, that the flow acts like a siphon which
  is driven along a magnetic flux tube by the pressure drop between the
  endpoints of the tube.

---------------------------------------------------------
Title: The Evershed Effect in Sunspots as a Siphon Flow
Authors: Thomas, J. H.; Montesinos, B.
1997AAS...191.7407T    Altcode: 1997BAAS...29.1325T
  A classic problem in solar magnetohydrodynamics has been to explain
  the Evershed effect, a wavelength shift and asymmetry of spectral
  line profiles which indicate a nearly horizontal, radial outflow of
  gas across a sunspot penumbra. Although the Evershed effect ceases
  rather abruptly at the outer edge of the penumbra, the flow itself must
  continue in some way, and tracing the course of this flow has proved
  difficult. The siphon-flow model, in which the Evershed flow is driven
  along an arched magnetic flux tube by a pressure drop between the two
  footpoints of the tube, predicts that much of this flow is directed
  along magnetic field lines that dive below the solar surface near
  the outer edge of the penumbra. Recent observations have suggested
  that this is indeed true, and now the very recent observations of
  Westendorp Plaza et al. (WP; 1997, Nature, 389, 47) have confirmed this
  picture. We present the results of our latest siphon-flow model of the
  Evershed flow, which allows us to follow flux tubes that cross the outer
  penumbral boundary into the surrounding field-free photosphere, as well
  as flux tubes that return to the surface within the penumbra. Both of
  these cases are observed by WP, and the results of our model are in
  good quantitative agreement with these observations.

---------------------------------------------------------
Title: The Vector Magnetic Field, Evershed Flow and Intensity in
    a Sunspot
Authors: Stanchfield, D. C. H.; Thomas, J. H.; Lites, B. W.
1997ASNYN...5b..14S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Vector Magnetic Field and Evershed Flow in a Sunspot
Authors: Stanchfield, D. C. H.; Thomas, J. H.; Lites, B. W.
1997ASNYN...5b..12S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Simple solar dynamo models with variable alpha and omega
    effects
Authors: Roald, Colin B.; Thomas, John H.
1997MNRAS.288..551R    Altcode:
  We examine a pair of radially averaged, pseudo-Cartesian dynamo models,
  one with a dynamically variable alpha effect, and the other with a
  dynamically variable omega effect. Our models are kept deliberately
  minimal in order to permit extensive numerical analysis, including
  in particular direct solution of periodic orbits and continuation
  of unstable branches. Despite some differences in the formulations
  used, the bifurcation structure of the variable-omega model is found
  to resemble closely that computed by Jennings &amp; Weiss, whereas
  that of the variable-alpha model matches the structure computed by
  Schmalz &amp; Stix somewhat less well. Our two systems, however,
  resemble each other not at all, despite having the same linearized
  form. Quenching of the alpha and omega effects is confirmed to have
  dramatically different effects, with omega-quenching perhaps producing
  better resemblance to the Sun. Also, we find the variable-alpha model
  to contain periodic solutions for positive dynamo number that do not
  show poleward propagation, contrary to conventional wisdom.

---------------------------------------------------------
Title: Velocity and Magnetic Field Fluctuations in the Photosphere
    of a Sunspot
Authors: Thomas, J. H.; Lites, B. W.; Bogdan, T. J.
1997SPD....28.0236T    Altcode: 1997BAAS...29..899T
  We use a data set of exceptionally high quality, obtained with
  the HAO/NSO Advanced Stokes Polarimeter, to measure oscillations
  of Doppler velocity, intensity, and the vector magnetic field at
  photospheric heights in a sunspot. We find an upper limit of 4 G
  (rms) for 5-min oscillations in magnetic field strength in the
  umbra, based on the full Stokes inversion of the line profiles of
  Fe I 6301.5 and 6302.5. This magnitude of the oscillation in field
  strength is considerably lower than that found recently by Horn,
  Staude, and Landgraf (1997). We show, on the basis of the eigenmodes
  of oscillation in a simple theoretical model of the sunspot umbra,
  that magnetic field variations of order 1 G are all that is to be
  expected. Using measurements of velocity oscillations above the acoustic
  cutoff frequency, we determine the relative heights of formation in
  the umbra of four different photospheric spectral lines from the phase
  differences between velocities in these lines, assuming the oscillations
  propagate vertically at the local sound speed. In space-time maps of
  fluctuations in continuum intensity, Doppler velocity, magnetic field
  strength, and field inclination we see distinct features that migrate
  radially inward from the inner penumbra all the way to the center of the
  umbra. These moving features are probably a signature of the convective
  interchange of magnetic flux tubes in the sunspot, although we failed
  to find any strong correlation among the features in the different
  quantities, indicating that these features have not been fully resolved.

---------------------------------------------------------
Title: The Seismology of Sunspots: A Comparison of Time-Distance
    and Frequency-Wavenumber Methods
Authors: Bogdan, T. J.; Braun, D. C.; Lites, B. W.; Thomas, J. H.
1997SPD....28.0210B    Altcode: 1997BAAS...29..894B
  A pair of formulae are developed which relate the absorption coefficient
  and partial wave phase-shift concepts of frequency-wavenumber local
  helioseismology to the center- annulus cross correlation function
  of time-distance helioseismology, under the general circumstances
  that both induced and spontaneous sunspot oscillations may be
  present. These formulae caution that real or spurious phase lags of
  the umbral oscillation signal lead to differences in the incoming and
  outgoing correlation times for sunspots, as first observed by Duvall
  et al. (1996, Nature, 379, 430) and recently confirmed by Braun (1997,
  ApJ, submitted). By using helioseismic data obtained by the GONG project
  in conjunction with concurrent observations of Doppler velocities and
  magnetic fields obtained by the HAO/NSO Advanced Stokes Polarimeter
  for the October 1995 disk passage of active region NOAA 7912, we
  demonstrate that the inferred GONG umbral oscillation signal actually
  originates from the umbra-penumbra boundary some 6 Mm distant from the
  center of the spot. Further, the ASP observations show that the 5-min
  oscillations at the umbra-penumbra boundary lag those in the center of
  the umbra by approximately 1 min, which is precisely the difference
  between the incoming and outgoing cross correlation times for NOAA
  7912 recently determined by Braun. The evidence suggests that p-mode
  forcing of the spot results in the generation of upward propagating
  slow MAG waves. These waves are responsible for the absorption of
  the p-modes, and the dispersion in their travel times between the
  subsurface layer where they are forced and the overlying level where
  the Doppler signals originate leads to the observed phase lag between
  the umbral and penumbral oscillations, and the corresponding correlation
  time differences.

---------------------------------------------------------
Title: The Vector Magnetic Field, Evershed Flow, and Intensity in
    a Sunspot
Authors: Stanchfield, Donald C. H., II; Thomas, John H.; Lites,
   Bruce W.
1997ApJ...477..485S    Altcode:
  We present results of simultaneous observations of the vector magnetic
  field, Evershed flow, and intensity pattern in a nearly axisymmetric
  sunspot, made with the Advanced Stokes Polarimeter at the Vacuum
  Tower Telescope at NSO (Sacramento Peak). The vector magnetic field is
  determined from the Stokes profiles of the magnetically sensitive lines
  Fe I 630.15 and 630.25 nm, and Doppler velocities and intensities are
  measured in several lines including the weak C I 538.03 nm line, formed
  in the deepest layers of the atmosphere. The strength of the magnetic
  field decreases with increasing zenith angle (angle of inclination to
  the local vertical), and this decrease is nearly linear over most of the
  range of values in the sunspot. Magnetic field strength and continuum
  intensity are inversely related in the sunspot in a manner similar to
  the characteristic nonlinear relationship found by Kopp &amp; Rabin
  in the infrared line Fe I 1564.9 nm. A different relationship is found
  between magnetic field strength and core intensity (in Fe I 630.25 nm),
  however, with the curve doubling back to give two distinct values of
  field strength at the same core intensity in the penumbra--the higher
  and lower field strengths corresponding to the inner and outer penumbra,
  respectively. In the penumbra the magnetic field pattern consists
  of spokelike extensions of stronger, more vertical magnetic field
  separated by regions of weaker, nearly horizontal magnetic field,
  as found by Degenhardt &amp; Wiehr and Lites et al. The penumbral
  magnetic field extends outward beyond the outer continuum boundary
  of the sunspot, forming a canopy at the height of formation of Fe I
  630.25 nm. Our results for the Evershed flow confirm the discovery
  by Rimmele that this flow is generally confined to narrow, elevated
  channels in the penumbra. In the Fe I 630.25 nm line and other strong
  photospheric lines we see isolated, radially elongated channels of
  Evershed flow crossing the outer penumbra. These flow channels lie
  in regions of the penumbra where the magnetic field is very nearly
  horizontal. In the weak C I 538.03 nm line (formed at a height h =
  40 km) the flow pattern shows small, isolated patches of upflow, lying
  at the inner end of the Fe I flow channels where the magnetic field is
  more inclined to the horizontal. These patches presumably correspond to
  the upstream footpoints of the arched magnetic flux tubes carrying the
  Evershed flow. For some of the flow channels we find isolated patches
  of strong downflow in the C I line just outside the penumbra that might
  correspond to the downstream footpoints of these flux tubes. There is
  a weak association between the Evershed flow channels and the dark
  filaments seen in continuum intensity in the penumbra, but a much
  stronger association between the flow and the dark filaments seen in
  core intensity measured in the same spectral line.

---------------------------------------------------------
Title: 1-D and 2-D calculations of dynamo-generated magnetic fields
    in white dwarfs
Authors: Markiel, J. A.; Thomas, J. H.; van Horn, H. M.
1997ASSL..214..405M    Altcode: 1997whdw.work..405M
  No abstract at ADS

---------------------------------------------------------
Title: a Nonlinear Solar Dynamo Model with Variable α and Ω Effect
Authors: Roald, Colin B.; Thomas, John H.
1996ASNYN...4j...7R    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: A Nonlinear Solar Dynamo Model With Variable alpha and
    omega Effects
Authors: Roald, Colin B.; Thomas, John H.
1996AAS...188.6901R    Altcode: 1996BAAS...28..935R
  In the usual alpha omega dynamo, runaway growth of the magnetic
  field is prevented by introducing arbitrary quenching factors. These
  roughly represent the back-reaction of the magnetic field on the flow
  by multiplying alpha and omega by some decreasing function of the mean
  toroidal magnetic field strength. This approach, while straightforward,
  has only limited physical justification. Here we take an alternate
  approach by allowing alpha or omega to vary with latitude and time
  and derive dynamical equations for them using principles of energy
  conservation. The resulting system of nonlinear partial differential
  equations is approximated by a truncated Fourier-Galerkin expansion,
  leading to a system of nonlinear ordinary differential equations
  whose behavior is studied using the methods of nonlinear dynamical
  systems. The results show that low-order truncations of the system
  give misleading results, but at higher orders the system converges to
  give consistent behavior, independent of truncation order. Somewhat
  surprisingly, the results depend strongly on which effect we choose
  to make dynamically variable. The back-reaction on omega appears to
  be far more important in reproducing Sun-like dynamo oscillations.

---------------------------------------------------------
Title: Vector Magnetic Fields and the Evershed Flow in Sunspots
Authors: Stanchfield, D. C. H., II; Thomas, J. H.; Lites, B. W.
1996AAS...188.3507S    Altcode: 1996BAAS...28..872S
  We present results of simultaneous observations of the vector magnetic
  field and the Evershed effect in two sunspots, made with the Advanced
  Stokes Polarimeter at the Vacuum Tower Telescope at NSO (Sacramento
  Peak). The vector magnetic field is determined from the Stokes profiles
  of the magnetically sensitive line Fe I 630.25, and Doppler velocities
  are measured in several lines including the weak C I 538.03 line,
  formed in the deepest layers of the atmosphere. In addition to maps
  of the vector magnetic field and the line-of-sight Doppler velocity,
  we present maps of the true flow speed assuming that the flow is
  everywhere aligned with the magnetic field. The results confirm
  the recent discovery by Rimmele (1995), that the Evershed flow is
  generally confined to narrow, elevated channels in the penumbra. In
  the Fe I 537.96 line (formed at a height of about 230 km) we see
  isolated, radially elongated channels of Evershed flow in the outer
  penumbra. These flow channels lie in regions of the penumbra where the
  magnetic field is most nearly horizontal. In the C I line (formed at a
  height of about 40 km) the flow pattern shows small, isolated patches
  of upflow lying at the inner end of the Fe I flow channels, where the
  magnetic field is more highly inclined to the horizontal. These patches
  presumably correspond to the upstream footpoints of the arched magnetic
  flux tubes carrying the Evershed flow. In some cases, along a radial
  line extending outward from a flow channel, we find isolated patches
  of downflow in the C I line just outside the penumbra, with magnetic
  field inclination slightly beyond the horizontal (i.e., magnetic field
  diving beneath the surface). These patches might well correspond to
  the downstream footpoints of these flux tubes.

---------------------------------------------------------
Title: Siphon flows in solar magnetic flux tubes and sunspots.
Authors: Thomas, J. H.
1996ASIC..481...39T    Altcode:
  The behavior of steady siphon flows along thin, arched magnetic flux
  tubes in the solar atmosphere is discussed, with particular attention
  to the case of "flexible" flux tubes in the solar photosphere, where
  the plasma beta is of order unity. Qualitative features of subcritical
  and supercritical siphon flows are illustrated by the simple case of
  isothermal flow in an isothermal external atmosphere. More realistic
  flows, including representations of the temperature-stratified external
  solar atmosphere, the radiative transfer of energy between the flux tube
  and its surroundings, and variable ionization fraction along the flow
  are also discussed. The jump conditions for an adiabatic tube shock in
  a supercritical siphon flow are analyzed in some detail. Siphon flows
  offer the most plausible explanation of the Evershed effect in sunspots.

---------------------------------------------------------
Title: Dynamo Generation of Magnetic Fields in White Dwarfs
Authors: Thomas, John H.; Markiel, J. Andrew; van Horn, H. M.
1995ApJ...453..403T    Altcode:
  Our earlier analysis of αω dynamos in white dwarfs, aimed specifically
  at explaining the time-varying magnetic field in the DBV white dwarf GD
  358 discovered by Winget et al., is improved and extended to a broader
  range of white dwarf types. Our nonlinear, local dynamo equations,
  based on those of Robinson and Durney, are modified to account for new
  evidence concerning the Sun's αω dynamo. We calculate dynamo magnetic
  fields for both He-envelope (DB) and H-envelope (DA) white dwarfs,
  for a range of values of mass, luminosity, rotation rate, and amount
  of differential rotation, and for convective envelopes computed with
  different formulations of convection theory. The results strengthen
  the case for dynamo generation of the magnetic field observed in GD
  358. The results also show that dynamo generation of magnetic fields
  could occur in many DB and DA white dwarfs, including other pulsating
  DBV stars (besides GD 358) and DAV stars for which the magnetic fields
  may be detectable by asteroseismological techniques.

---------------------------------------------------------
Title: Analytical Properties of Shock Waves in Isolated Magnetic
    Flux Tubes
Authors: Stanchfield, Donald C. H., II; Thomas, John H.
1995ASNYN...4h..13S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Analytical Properties of Shock Waves in Isolated Magnetic
    Flux Tubes
Authors: Stanchfield, D. C. H., II; Thomas, J. H.
1995SPD....26.1002S    Altcode: 1995BAAS...27..977S
  No abstract at ADS

---------------------------------------------------------
Title: Dynamo Generation of a Magnetic Field in the White Dwarf GD 358
Authors: Markiel, J. Andrew; Thomas, John H.; van Horn, H. M.
1994ApJ...430..834M    Altcode:
  On the basis of Whole Earth Telescope observations of the g-mode
  oscillation spectrum of the white dwarf GD 358, Winget et al. find
  evidence for significant differential rotation and for a time-varying
  magnetic field concentrated in the surface layers of this star. Here
  we argue on theoretical grounds that this magnetic field is produced
  by an alpha omega dynamo operating in the lower part of a surface
  convection zone in GD 358. Our argument is based on numerical
  solutions of the nonlinear, local dynamo equations of Robinson &amp;
  Durney, with specific parameters based on our detailed models of
  white-dwarf convective envelopes, and universal constants determined
  by a calibration with the the Sun's dynamo. The calculations suggest
  a dynamo cycle period of about 6 years for the fundamental mode,
  and periods as short as 1 year for the higher-order modes that are
  expected to dominate in view of the large dynamo number we estimate for
  GD 358. These dynamo periods are consistent with the changes in the
  magnetic field of GD 358 over the span of 1 month inferred by Winget
  et. al. from their observations. Our calculations also suggest a peak
  dynamo magnetic field strength at the base of the surface convection
  zone of about 1800 G, which is consistent with the field strength
  inferred from the observations.

---------------------------------------------------------
Title: Chromospheric oscillations
Authors: Lites, B. W.; Rutten, R. J.; Thomas, J. H.
1994ASIC..433..159L    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The cause of the Evershed effect in sunspots: flows or waves?
Authors: Thomas, J. H.
1994ASIC..433..219T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Evidence for siphon flows with shocks in solar magnetic
    flux tubes
Authors: Degenhardt, D.; Solanki, S. K.; Montesinos, B.; Thomas, J. H.
1993A&A...279L..29D    Altcode:
  We synthesize profiles of the infrared line Fe I 15648.5 A (g = 3) for a
  recently developed theoretical model of siphon flows along photospheric
  magnetic loops. The synthesized line profiles are compared with the
  observations from which Rueedi et al. (1992) deduced the presence of
  such flows across the neutral line of an active region plage. This
  comparison supports the interpretation of Rueedi et al. (1992). It
  also suggests that the average footpoint separation of the observed
  loops carrying the siphon flow is 8-15 sec and that the siphon flow
  experiences a standing tube shock in the downstream leg near the top
  of the arch.

---------------------------------------------------------
Title: Book-Review - Sunspots - Theory and Observations
Authors: Thomas, J. H.; Weiss, N. O.; Parkinson, J. H.
1993Obs...113..145T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Absorption of p-Modes by Sunspots: Variations with Degree
    and Order
Authors: Bogdan, Thomas J.; Brown, Timothy M.; Lites, Bruce W.;
   Thomas, John H.
1993ApJ...406..723B    Altcode:
  A spherical harmonic decomposition of the p-modes into inward and
  outward propagating waves is employed to investigate the absorption
  of solar p-modes by an isolated sunspot. The absorption coefficient
  (averaged over frequency and azimuthal order) is found to increase
  with increasing horizontal wavenumber k over the range 0-0.8/Mm. For
  larger horizontal wavenumbers, in the range 0.8-1.5/Mm, the absorption
  coefficient decreases with increasing k. The absorption along each
  individual p-mode ridge tends to peak at an intermediate value of the
  spherical harmonic degree in the range 200-400. The highest absorption
  is found along the p(1) ridge, and the absorption decreases with
  increasing radial order.

---------------------------------------------------------
Title: A Siphon-Flow Model of the Photospheric Evershed Flow in
    a Sunspot
Authors: Thomas, John H.; Montesinos, Benjamin
1993ApJ...407..398T    Altcode:
  The Evershed flow at photospheric heights in a sunspot penumbra
  is modeled theoretically as a siphon flow along individual, arched
  magnetic flux tubes embedded in an atmosphere permeated by a uniform
  magnetic field. This approach is suggested by the recent evidence that
  the penumbra is a deep structure with a significant amount of emerging
  flux, rather than a shallow structure overlying field-free gas. The
  model resolves two problems associated with siphon-flow models based
  on a shallow penumbra: it produces arched flux tubes of sufficient
  horizontal extent, and it explains how the optically thin flux tubes
  carrying the Evershed flow can appear dark.

---------------------------------------------------------
Title: Sunspot dynamics
Authors: Thomas, John H.
1993runy.rept.....T    Altcode:
  This report describes recent results of our theoretical and
  observational work on dynamical phenomena in sunspots. The overall
  goal of this research has been a better understanding of the various
  oscillatory, transient, and steady motions in a sunspot and their
  relation to the basic structure of the sunspot. The principal topics of
  the research reported here are the following: (1) sunspot seismology,
  i.e., the study of the interaction of solar p-modes with a sunspot as
  a probe of the subsurface structure of a sunspot; (2) local sources
  of acoustic waves in the solar photosphere; and (3) siphon flows in
  isolated magnetic flux tubes and their relation to the photospheric
  Evershed flow and to intense magnetic elements outside of sunspots.

---------------------------------------------------------
Title: Siphon Flows in Isolated Magnetic Flux Tubes. V. Radiative
    Flows with Variable Ionization
Authors: Montesinos, Benjamin; Thomas, John H.
1993ApJ...402..314M    Altcode:
  Steady siphon flows in arched isolated magnetic flux tubes in the
  solar atmosphere are calculated here including radiative transfer
  between the flux tube and its surrounding and variable ionization
  of the flowing gas. It is shown that the behavior of a siphon flow
  is strongly determined by the degree of radiative coupling between
  the flux tube and its surroundings in the superadiabatic layer just
  below the solar surface. Critical siphon flows with adiabatic tube
  shocks in the downstream leg are calculated, illustrating the radiative
  relaxation of the temperature jump downstream of the shock. For flows
  in arched flux tubes reaching up to the temperature minimum, where the
  opacity is low, the gas inside the flux tube is much cooler than the
  surrounding atmosphere at the top of the arch. It is suggested that
  gas cooled by siphon flows contribute to the cool component of the
  solar atmosphere at the height of the temperature minimum implied by
  observations of the infrared CO bands at 4.6 and 2.3 microns.

---------------------------------------------------------
Title: Localized Sources of Propagating Acoustic Waves in the Solar
    Photosphere
Authors: Brown, Timothy M.; Bogdan, Thomas J.; Lites, Bruce W.;
   Thomas, John H.
1992ApJ...394L..65B    Altcode:
  A time series of Doppler measurements of the solar photosphere with
  moderate spatial resolution is described which covers a portion of the
  solar disk surrounding a small sunspot group. At temporal frequencies
  above 5.5 mHz, the Doppler field probes the spatial and temporal
  distribution of regions that emit acoustic energy. In the frequency
  range between 5.5 and 7.5 mHz, inclusive, a small fraction of the
  surface area emits a disproportionate amount of acoustic energy. The
  regions with excess emission are characterized by a patchy structure
  at spatial scales of a few arcseconds and by association (but not
  exact co-location) with regions having substantial magnetic field
  strength. These observations bear on the conjecture that most of the
  acoustic energy driving solar p-modes is created in localized regions
  occupying a small fraction of the solar surface area.

---------------------------------------------------------
Title: A Siphon-Flow Model of Photospheric Evershed Flow in a Sunspot
Authors: Thomas, J. H.; Montesinos, B.
1992AAS...180.0701T    Altcode: 1992BAAS...24..738T
  No abstract at ADS

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Title: The Theory of Sunspots
Authors: Thomas, John H.; Weiss, Nigel O.
1992ASIC..375....3T    Altcode: 1992sto..work....3T
  This review covers the present state of our theoretical understanding
  of the physics of sunspots, along with the principal observational
  results that need to be explained. The topics covered range from the
  detailed structure of an individual sunspot to the broad connection
  between sunspots and the global solar magnetic field and the solar
  cycle. Our aim is to give a critical discussion of the theoretical ideas
  and models without presenting mathematical details. After outlining
  the historical development of the basic concepts associated with the
  magnetohydrodynamic theory of sunspots, we discuss recent treatments of
  their properties and structure, placing special emphasis on developments
  that have occurred within the last ten years. There have been remarkable
  improvements in the theoretical modelling of sunspots, led by new
  ideas and by more elaborate and realistic numerical simulations. At
  the same time, new observations have raised new theoretical questions
  or caused old ones to be reconsidered. In particular, measurements
  of oscillations in and around sunspots have opened up the new field
  of sunspot seismology, while recent high-resolution observations have
  forced us to rethink the structure of a sunspot penumbra.

---------------------------------------------------------
Title: Sunspots - Theory &amp; Observations: NATO Cambridge, 1992
Authors: Thomas, John H.; Weiss, Nigel O.
1992ASIC..375.....T    Altcode: 1992sto..work.....T
  The papers contained in this volume focus on theoretical problems
  associated with sunspots and present results of recent high-resolution
  observations of sunspots. In particular, attention is given to the
  evolution of sunspots, overall structure and fine structure of sunspots,
  waves and oscillations in sunspots, and the relation of sunspots to the
  global solar magnetic field. Specific topics discussed include continuum
  observations and empirical models of the thermal structure of sunspots,
  fine structure of umbrae and penumbrae, magnetohydrodynamic waves in
  structural magnetic fields, and the motion of magnetic flux tubes in
  the convection zone and the surface origin of active regions.

---------------------------------------------------------
Title: Sunspot Seismology
Authors: Thomas, J. H.
1992nysa.conf...35T    Altcode: 1992LDP....17...35T; 1992ASNYN...4b...4T; 1992nysa.proc...35T
  No abstract at ADS

---------------------------------------------------------
Title: Siphon Flows in Isolated Magnetic Flux Tubes. IV. Critical
    Flows with Standing Tube Shocks
Authors: Thomas, John H.; Montesinos, Benjamin
1991ApJ...375..404T    Altcode:
  Critical siphon flows in arched, isolated magnetic flux tubes are
  studied within the thin flux tube approximation, with a view toward
  applications to intense magnetic flux concentrations in the solar
  photosphere. The results of calculations of the strength and position
  of the standing tube shock in the supercritical downstream branch of
  a critical siphon flow are presented, as are calculations of the flow
  variables all along the flux tube and the equilibrium path of the flux
  tube in the surrounding atmosphere. It is suggested that arched magnetic
  flux tubes, with magnetic field strength increased by a siphon flow,
  may be associated with some of the intense, discrete magnetic elements
  observed in the solar photosphere.

---------------------------------------------------------
Title: Sunspot dynamics
Authors: Thomas, John H.
1990runy.reptQ....T    Altcode:
  The goal of this research was the understanding of the various
  oscillatory, transient, and quasi-steady motions in sunspots and the
  basic structure of a sunspot. The research involved both theoretical
  modeling (based on thermohydrodynamic theory) and observations of
  dynamical phenomena in sunspots. The principal topics of the research
  were sunspot seismology (the interaction of solar p-modes with a
  sunspot as a probe of the subsurface structure of a sunspot); three
  minute umbral oscillations and their relation to the structure of
  the umbral atmosphere; siphon flows in isolated magnetic flux tubes
  and their relation to the photospheric Evershed flow and to intense
  magnetic elements outside of sunspots; and more general theoretical
  work on magneto-atmospheric waves. Here, a summary of results is given.

---------------------------------------------------------
Title: Siphon Flows in Isolated Magnetic Flux Tubes. III. The
    Equilibrium Path of the Flux-Tube Arch
Authors: Thomas, John H.; Montesinos, Benjamin
1990ApJ...359..550T    Altcode:
  It is shown how to calculate the equilibrium path of a thin magnetic
  flux tube in a stratified, nonmagnetic atmosphere when the flux tube
  contains a steady siphon flow. The equilbrium path of a static thin
  flux tube in an infinite stratified atmosphere generally takes the
  form of a symmetric arch of finite width, with the flux tube becoming
  vertical at either end of the arch. A siphon flow within the flux
  tube increases the curvature of the arched equilibrium path in order
  that the net magnetic tension force can balance the inertial force
  of the flow, which tries to straighten the flux tube. Thus, a siphon
  flow reduces the width of the arched equilibrium path, with faster
  flows producing narrower arches. The effect of the siphon flow on
  the equilibrium path is generally greater for flux tubes of weaker
  magnetic field strength. Examples of the equilibrium are shown for
  both isothemal and adiabatic siphon flows in thin flux tubes in an
  isothermal external atmosphere.

---------------------------------------------------------
Title: Siphon Flows in Isolated Magnetic Flux Tubes: Equilibrium
    Paths and Standing Tube Shocks
Authors: Thomas, J. H.; Montesinos, B.
1990BAAS...22..880T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The structure of photospheric flux tubes
Authors: Thomas, John H.
1990GMS....58..133T    Altcode:
  Basic physical mechanisms for producing the observed intense magnetic
  flux tubes in the solar photosphere are reviewed. The mechanism of
  flux expulsion by convective cells can concentrate magnetic flux up
  to the equipartition field strength, which is only about 200 G at the
  solar surface for the observed granular convection. Other mechanisms
  that partially evacuate the flux tube are needed to produce further
  concentration of magnetic flux to the observed values of 1000-1500
  G. Two such mechanisms are discussed: concentration by convective
  collapse of a vertical flux tube in the superadiabatic layer just below
  the solar surface, and concentration by a siphon flow in an arched,
  isolated flux tube.

---------------------------------------------------------
Title: Magnetic Flux Concentration by Siphon Flows in Isolated
    Magnetic Flux Tubes
Authors: Thomas, J. H.; Montesinos, B.
1990IAUS..138..263T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Transmission and Reflection of Compressive Waves at a
    Nonmagnetic-Magnetic Interface
Authors: Abdelatif, Toufik E.; Thomas, John H.
1989ApJS...70..239A    Altcode:
  The transmission and reflection properties of compressive waves at
  a plane interface between uniform nonmagnetic and magnetic regions
  in the absence of gravity are examined. Using stereographic polar
  projection, these properties are presented as functions of the two
  angles determining the direction of incidence. It is shown that
  the reflection coefficient and the direction of propagation of the
  transmitted wave are dependent on the direction of propagation of the
  incident wave for several representative parametric values. It is found
  that the incident, reflected, and transmitted wavenumber vectors always
  lie in the same plane, although the group velocity of the transmitted
  wave does not always lie in this plane. When the transmitted wave is
  a fast mode, there is generally weak reflection.

---------------------------------------------------------
Title: Siphon Flows in Isolated Magnetic Flux Tubes. II. Adiabatic
    Flows
Authors: Montesinos, Benjamin; Thomas, John H.
1989ApJ...337..977M    Altcode:
  This paper extends the study of steady siphon flows in isolated
  magnetic flux tubes surrounded by field-free gas to the case of
  adiabatic flows. The basic equations governing steady adiabatic
  siphon flows in a thin, isolated magnetic flux tube are summarized,
  and qualitative features of adiabatic flows in elevated, arched flux
  tubes are discussed. The equations are then cast in nondimensional form
  and the results of numerical computations of adiabatic siphon flows
  in arched flux tubes are presented along with comparisons between
  isothermal and adiabatic flows. The effects of making the interior
  of the flux tube hotter or colder than the surrounding atmosphere
  at the upstream footpoint of the arch is considered. In this case,
  is it found that the adiabatic flows are qualitatively similar to
  the isothermal flows, with adiabatic cooling producing quantitative
  differences. Critical flows can produce a bulge point in the rising part
  of the arch and a concentration of magnetic flux above the bulge point.

---------------------------------------------------------
Title: Siphon Flows in Isolated Magnetic Flux Tubes
Authors: Thomas, John H.
1988ApJ...333..407T    Altcode:
  Previous studies of siphon flows in magnetic flux tubes have been
  limited to the case of a low-β plasma, appropriate for an embedded
  flux tube in the solar corona or chromosphere. Here the author studies
  steady siphon flows in isolated, thin magnetic flux tubes surrounded
  by field-free gas, with plasma β ≥ 1, appropriate for conditions
  in the solar photosphere. The author presents the basic equations
  governing steady siphon flows in a thin, isolated magnetic flux tube,
  and discusses the case of a purely horizontal flux tube, for which
  gravitational forces do not come into play. Qualitative features of
  isothermal flows in elevated, arched flux tubes are then studied. The
  author presents the results of some numerical computations of isothermal
  siphon flows in arched flux tubes and discusses the nature of critical
  flows and the need for standing "tube shocks" in these flows. Finally,
  he discusses applications to intense magnetic flux tubes in the solar
  photosphere and considers the possible interpretation of the Evershed
  flow in the penumbral photosphere of a sunspot as a siphon flow along
  isolated magnetic flux tubes.

---------------------------------------------------------
Title: Sunspot Seismology
Authors: Thomas, J. H.; Lites, B. W.; Abdelatif, T. E.
1988IAUS..123..181T    Altcode:
  The 5 minute oscillations in a sunspot umbra are the response of the
  sunspot to forcing by the 5 minute p-modes in the surrounding convection
  zone (Thomas 1981). This interaction of solar p-modes with a sunspot
  can be used to probe the structure of a sunspot beneath the visible
  surface of the Sun (Thomas, Cram, and Nye 1982). Here the authors
  report briefly the results of both an observational study and a simple
  theoretical analysis of this interaction.

---------------------------------------------------------
Title: The Interaction of Solar p-Modes with a Sunspot. II. Simple
    Theoretical Models
Authors: Abdelatif, Toufik E.; Thomas, John H.
1987ApJ...320..884A    Altcode:
  The interaction of solar p-modes with a sunspot magnetic flux tube is
  investigated theoretically by means of two simple models. An increase
  in horizontal wavelength between the nonmagnetic and magnetic regions,
  due to the different characteristic wave speeds in the two regions,
  explains the corresponding observed wavelength shift of powe in the
  umbral k-omega power spectrum. The variation of the transmission
  coefficient with wavenumber along the p-mode diagnostic curves, due
  to resonant transmission, is responsible for the observed selective
  filtering of the p-modes by the sunspot.

---------------------------------------------------------
Title: Theoretical Models of the Interaction of Solar p-Modes With
    a Sunspot
Authors: Thomas, J. H.; Abdelatif, T. E.
1987BAAS...19R.936T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Simultaneous Measurements of Sunspot Umbral Oscillations in
    the Photosphere, Chromosphere, and Transition Region
Authors: Thomas, John H.; Lites, Bruce W.; Gurman, Joseph B.; Ladd,
   Edwin F.
1987ApJ...312..457T    Altcode:
  Measurements of umbral oscillations in a sunspot were made
  simultaneously from space (with the SMM/UVSP instrument) in the
  C IV transition-region line and from the ground (with the tower
  telescope at NSO/sunspot) in spectral lines formed in the photosphere
  and chromosphere. The power spectra of velocity and intensity
  variations show multiple peaks in the 3 min band (4.5-10 mHz). A
  strong oscillation at 5.5 mHz is coherent between the chromosphere
  and transition region. Another strong oscillation mode at 7.5 mHz is
  coherent between the photosphere and transition region and appears to
  have a node in the chromosphere. The rms velocity in the 3 min band is
  a little over 12 km/sec in both the chromosphere and transition region,
  but the kinetic energy density is lower in the transition region (by
  a factor of 10 or more) due to the lower mass density there. These
  measurements of amplitude and phase of the waves at different heights
  provided a new, independent method of testing or fitting models of
  the vertical temperature distribution in the umbral chromosphere and
  transition regions.

---------------------------------------------------------
Title: The Interaction of Solar p-Modes with a
    Sunspot. I. Observations
Authors: Abdelatif, Toufik E.; Lites, Bruce W.; Thomas, John H.
1986ApJ...311.1015A    Altcode:
  Time series of velocity maps of two isolated sunspots and their
  surroundings were recorded in the Fe I line and the umbral line Ti
  I. Both 3 and 5 min umbral oscillations were detected at photospheric
  heights. The 5 min oscillations have reduced amplitude in the umbra,
  which appears to act as a filter in transmitting selected frequencies
  in the power spectrum of 5 min p-mode oscillations of the surrounding
  convection zone. The k-omicron power spectrum of the umbral oscillations
  shows this selective transmission and also shows a shift of power to
  longer horizontal wavelengths. This behavior is exhibited by a simple
  theoretical model of the interaction of p-modes with a sunspot. The
  3 min umbral oscillations are concentrated in the dark central part
  of the umbra. In both sunspots, the kinetic energy density of the 3
  min umbral oscillation in the photosphere is much greater than the
  corresponding kinetic energy density at chromospheric heights measured
  in other sunspots.

---------------------------------------------------------
Title: Oscillation Spectra of Neutron Stars with Strong Magnetic
    Fields
Authors: Carroll, B. W.; Zweibel, E. G.; Hansen, C. J.; McDermott,
   P. N.; Savedoff, M. P.; Thomas, J. H.; van Horn, H. M.
1986ApJ...305..767C    Altcode:
  The authors have investigated the effects of a strong vertical
  magnetic field on the oscillation spectrum of a cylindrical slab
  model for the surface layers of a neutron star. In particular, they
  have considered the effects on those modes known to be concentrated
  in the surface layers: the pseudo-toroidal t- and a-modes and the
  pseudo-spheroidal s-, i-, and m/g-modes. The authors have derived
  and solved the Newtonian pulsation equations for the cylindrical
  model. The electromagnetic boundary conditions at the neutron star
  surface match the magnetohydrodynamic motions within the star to
  outgoing electromagnetic radiation in the evacuated waveguide above the
  surface; pseudo-toroidal modes couple to TM electromagnetic modes in
  the waveguide, while pseudo-spheroidal modes match to TE radiation. The
  periods of the t- and s-modes are similar to the quasi-periodicities
  observed in some pulsars.

---------------------------------------------------------
Title: Sunspot Umbral Oscillations in the Photosphere, Chromosphere,
    and Transition Region
Authors: Thomas, J. H.; Lites, B. W.; Gurman, J. B.; Ladd, E. F.
1986BAAS...18..678T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: A Model of Electromagnetic Damping Mechanisms for Neutron
    Star Oscillations
Authors: Carroll, B. W.; Zweibel, E. G.; Hansen, C. J.; McDermott,
   P. N.; Savedoff, M. P.; Thomas, J. H.; van Horn, H. M.
1985BAAS...17..855C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Hydromagnetic Waves in the Photosphere and Chromosphere
Authors: Thomas, J. H.
1985tphr.conf..126T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Sunspot umbral oscillations in the photosphere and low
    chromosphere
Authors: Lites, B. W.; Thomas, J. H.
1985ApJ...294..682L    Altcode:
  In the present simultaneous measurement of sunspot umbrae velocity
  oscillations in one spectral line formed in the low photosphere, and
  in another formed in the low chromosphere, just above the temperature
  minimum, the velocity power spectrum in each is found to exhibit both
  5-min and 3-min oscillations, with the kinetic energy of the latter
  being at least 5 times greater in the low photosphere than in the
  low chromosphere. The 3-min umbral oscillation has the character of a
  coherent, vertically standing wave in the photosphere. These results
  imply a photospheric, rather than chromospheric, resonant origin for
  the fundamental 3-min umbral oscillation. A negative phase difference
  at frequencies around 2 mHz suggests the presence of gravity waves in
  the umbra.

---------------------------------------------------------
Title: Sunspot Umbral Oscillations in the Photosphere and Low
    Chromosphere
Authors: Thomas, J. H.; Lites, B. W.
1985BAAS...17..631T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The effect of a vertical magnetic field on neutron star
    oscillations.
Authors: Carroll, B. W.; McDermott, P. N.; Savedoff, M. P.; Thomas,
   J. H.; van Horn, H. M.; Zweibel, E. G.; Morrow, C. A.; Hansen, C. J.
1985ASNYN...2...27C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Oscillations in sunspots
Authors: Thomas, J. H.
1985AuJPh..38..811T    Altcode:
  Recent observational and theoretical work on oscillations in sunspots
  is reviewed. The characteristic 3-minute umbral oscillations and
  flashes are resonant modes of the sunspot itself, whereas the 5-minute
  oscillations in the umbra are a passive response to forcing by p
  modes in the surrounding convection zone. The observational evidence
  suggests that the fundamental cause of the 3-minute oscillations is
  the photospheric fast-mode resonance, with chromospheric slow-mode
  resonances perhaps producing additional oscillation frequencies in the
  chromosphere. Observations and theoretical models of the interaction of
  5-minute p-mode oscillations with a sunspot offer a means of probing the
  structure of a sunspot magnetic flux tube beneath the solar surface. The
  observed differences between running penumbral waves in the chromosphere
  and in the photosphere may be explained by the effect of the Evershed
  flow on trapped magneto-atmospheric waves in the penumbra.

---------------------------------------------------------
Title: Hydromagnetic waves in the photosphere and chromosphere.
Authors: Thomas, J. H.
1985MPARp.212..126T    Altcode:
  The theory of waves in a plane stratified magnetic atmosphere and
  in an isolated magnetic flux tube is reviewed in some detail. For an
  expanding magnetic flux tube in a stratified atmosphere, the theory
  has been thus far limited to the thin flux tube approximation. The
  theory of surface waves along a magnetic interface and the dissipation
  of waves by resonant absorption and phase mixing are also discussed.

---------------------------------------------------------
Title: Magneto-atmospheric waves from a localized source
Authors: Adam, J. A.; Thomas, J. H.
1984Ap&SS.106..125A    Altcode:
  A technique developed by Lighthill (1960, 1965, and 1967) for finding
  the asymptotic solution of an inhomogeneous wave equation with
  constant coefficients is applied to the study of wave propagation in
  magneto-atmospheres. The geometry of the wavenumber surface plays
  an important role in determining the generation and propagation of
  various types of magneto-atmospheric waves from a localized forcing
  region. Examples of these wavenumber surfaces are exhibited for various
  magnetic-field strengths and wave frequencies. The asymptotic far
  field is tabulated for a time-harmonic spatially Gaussian localized
  forcing term.

---------------------------------------------------------
Title: Dynamical phenomena in sunspots. I - Observing procedures
    and oscillatory phenomena.
Authors: Thomas, J. H.; Cram, L. E.; Nye, A. H.
1984ApJ...285..368T    Altcode:
  High resolution spectra consisting of at least 1 hr periods were
  obtained of the sunpost atmosphere. The Ca II H and K lines were
  scanned to characterize umbral oscillations and flashes. The former
  displayed peaks lasting 150-197 sec, while penumbral oscillations
  peaked in the 197-300 sec range. Quiet sun oscillations exhibited no
  peaks under 300 sec. The Ca II K line umbral flashes were ubiquitous
  for all observational periods and were associated with light bridges
  in the umbra. Magnetic field, vertical velocity, and chromospheric
  intensity measurements taken during the 1 hr scans covered moving
  magnetic features (MMF), which traversed the moats around sunspots. MMF
  areas increased while the magnetic field intensity decreased with MMF
  movement away from a sunspot. Bright Ca II K line wings were apparent
  in the MMFs, but cores of the lines were not observed, suggesting that
  flux loops generating the line are low in the photosphere.

---------------------------------------------------------
Title: Dynamical Phenomena in Sunspots - Part Two - a Moving Magnetic
    Feature
Authors: Nye, A. H.; Thomas, J. H.; Cram, L. E.
1984ApJ...285..381N    Altcode:
  High-resolution observations of the magnetic field, vertical velocity,
  and chromospheric intensity in a typical moving magnetic feature (MMF)
  are obtained over a period of 1 hr as it moves outward across the
  moat surrounding a large sunspot. The area of the MMF increases and
  the magnetic field strength decreases as it moves outward. The MMF
  contains a strong downdraft with a velocity of 400 m s<SUP>-1</SUP>
  in the photosphere. The lifetime assigned to the MMF depends upon the
  signature by which it is observed.

---------------------------------------------------------
Title: Neutron Star Oscillations in the Presence of a Vertical
    Magnetic Field
Authors: Carroll, B. W.; McDermott, P. N.; Savedoff, M. P.; Thomas,
   J. H.; van Horn, H. M.; Zweibel, E. G.; Morrom, C. A.; Hansen, C. J.
1984BAAS...16..943C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Half-life of <SUP>26</SUP>Al
Authors: Thomas, J. H.; Rau, R. L.; Skelton, R. T.; Kavanagh, R. W.
1984PhRvC..30..385T    Altcode:
  We have measured the half-life of <SUP>26</SUP>Al because data on
  <SUP>21</SUP>Ne production rates in meteorites has indicated that the
  half-life may have been too low by 30-40%. We produced <SUP>26</SUP>Al
  using the <SUP>26</SUP>Mg(p,n)<SUP>26</SUP>Al reaction on thick natural
  Mg, the yield being calculated from cross section data. The activity
  of two such samples was measured with a Ge(Li) detector and the
  calculated half-life is t<SUB>12</SUB>=(7.8+/-0.5)×10<SUP>5</SUP>
  years, in agreement with the accepted half-life of
  <SUP>26</SUP>Al:t<SUB>12</SUB>=(7.16+/-0.32)×10<SUP>5</SUP>
  years. Therefore, another explanation must be found for the anomalous
  <SUP>21</SUP>Ne production rate based on <SUP>26</SUP>Al ages in
  meteorites.

---------------------------------------------------------
Title: Umbral oscillations in sunspots
Authors: Thomas, J. H.
1984A&A...135..188T    Altcode:
  Contrary to the claim by von Uexküll et al. (1983), their observations
  of upward phase propagation of umbral oscillations in the chromosphere
  are in agreement with the photospheric resonance theory of Thomas and
  Scheuer (1982) and in contradiction to the chromospheric resonance
  theory of Zhugzhda et al. (1983). Other observational evidence also
  indicates that the fundamental 3-min umbral oscillation is due to a
  photospheric resonance, although the closely-spaced multiple peaks
  sometimes seen in the power spectrum of chromospheric oscillations
  may well be due to chromospheric resonances.

---------------------------------------------------------
Title: Oscillatory Phenomena in a Sunspot
Authors: Thomas, J. H.; Cram, L. E.; Nye, A. H.
1984BAAS...16..531T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Flow in an Isolated Magnetic Flux Tube
Authors: Thomas, J. H.
1984ssdp.conf..276T    Altcode:
  The author considers the case of one-dimensional, steady, ideal flow
  along a thin isolated magnetic flux tube and assumes that the internal
  and external temperatures are equal at each point along the tube
  (the case of rapid thermal exchange).

---------------------------------------------------------
Title: Oscillations in a Sunspot and the Surrounding Photosphere
Authors: Abdelatif, T. E.; Lites, B. W.; Thomas, J. H.
1984ssdp.conf..141A    Altcode:
  Velocity oscillations at photospheric heights in a sunspot and its
  surroundings have been measured. The average temporal power spectrum of
  oscillations in the sunspot umbra shows the five-minute oscillations
  splitting into several distinct modes and also shows the existence
  of the three-minute umbral oscillation at photospheric heights, with
  greater kinetic energy density than in the chromosphere.

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Title: <SUP>10</SUP>Be concentrations and the long-term fate of
    particle-reactive nuclides in five soil profiles from California
Authors: Monaghan, M. C.; Krishnaswami, S.; Thomas, J. H.
1983E&PSL..65...51M    Altcode:
  Concentration-depth profiles of cosmic-ray-produced <SUP>10</SUP>Be
  ( t<SUB>1/2</SUB> = 1.5m.y.) have been measured by accelerator-mass
  spectrometry in five soil profiles. These measurements were made in
  an effort (1) to understand the retentivity of soil surfaces for
  particle-reactive tracers depositing from the atmosphere on time
  scales of 10 <SUP>4</SUP>-10 <SUP>6</SUP> years, and (2) to explore
  the application of <SUP>10</SUP>Be as a chronometer of geomorphic
  surface age. The profiles sampled are from two wave-cut terraces
  located near Mendocino, California, a table mountain top and an
  alluvial fan, both located near Friant, California. The ages of
  the Mendocino terraces are inferred to be (1-5) × 10 <SUP>5</SUP>
  years based on amino-stratigraphic correlations and models of terrace
  evolution; those of the table mountain top and alluvial fan are 9.5 ×
  10 <SUP>6</SUP> years and 6.0 × 10 <SUP>5</SUP> years, respectively,
  based on K-Ar analyses. All the surfaces sampled are nearly flat and
  exhibit few erosional features. In addition to <SUP>10</SUP>Be we
  measured <SUP>210</SUP>Pb, <SUP>239,240</SUP>Pu and <SUP>7</SUP>Be to
  ascertain the retentivity of the soils for particle-reactive nuclides
  and to assess the present-day delivery rate of nuclides from the
  atmosphere. The <SUP>7</SUP>Be inventory is 4.0 dpm/cm <SUP>2</SUP>
  similar to those observed at nearby locations. The inventories of
  <SUP>210</SUP>Pb and Pu isotopes conform to those predicted from
  model calculations and suggest that the soil surfaces sampled retain
  the entire burden of particle-reactive nuclides delivered to them over
  short time scales, ∼ 100 years. The <SUP>10</SUP>Be concentrations in
  the sample range between (0.2 and 7) × 10 <SUP>8</SUP> atoms/g soil and
  show strong correlations with leachable Fe and/or Al. The inventory of
  <SUP>10</SUP>Be in the soil domain sampled is 1-2 orders of magnitude
  lower than that expected from the geological age of the surface and
  an average delivery rate of <SUP>10</SUP>Be from the atmosphere,
  5.2 × 10 <SUP>5</SUP> atoms/cm <SUP>2</SUP> yr. The low inventory of
  <SUP>10</SUP>Be is attributed to its loss from the soil domain sampled
  by solution transport. Based on a simple ☐-model type calculation
  with a first-order removal process for <SUP>10</SUP>Be, the residence
  time of <SUP>10</SUP>Be in the soil domains sampled is determined to
  be of the order of 10 <SUP>4</SUP> years. The low residence time of
  <SUP>10</SUP>Be in the soil domains sampled requires that it be found
  either deeper in the regolith or in ground waters. In either case, the
  application of <SUP>10</SUP>Be as a chronometer of geomorphic surface
  age is severely constrained. However, the study of <SUP>10</SUP>Be in
  soils provides the only entry into the long-term (10 <SUP>4</SUP>-10
  <SUP>6</SUP> years) behavior of particle-reactive nuclides in soils and,
  hence, could be important for understanding the behavior of analogous
  nuclides introduced into soils by natural and anthropogenic processes.

---------------------------------------------------------
Title: On the Mode of Three-Minute Umbral Oscillations in Sunspots
Authors: Thomas, J. H.
1983BAAS...15..952T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Oscillations in Sunspots and the Surrounding Photosphere
Authors: Abdelatif, T.; Lites, B. W.; Thomas, J. H.
1983BAAS...15R.719A    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Magneto-atmospheric waves
Authors: Thomas, J. H.
1983AnRFM..15..321T    Altcode:
  A theoretical treatment of magneto-atmospheric waves is presented
  and applied to the modelling of waves in the solar atmosphere. The
  waves arise in compressible, stratified, electrically conductive
  atmospheres within gravitational fields when permeated by a magnetic
  field. Compression, buoyancy, and distortion of the magnetic field all
  contribute to the existence of the waves. Basic linearized equations
  are introduced to describe the waves and attention is given to
  plane-stratified atmospheres and their stability. A dispersion relation
  is defined for wave propagation in a plane-stratified atmosphere when
  there are no plane-wave solutions. Solutions are found for the full
  wave equation in the presence of either a vertical or a horizontal
  magnetic field. The theory is applied to describing waves in sunspots,
  in penumbrae, and flare-induced coronal disturbances.

---------------------------------------------------------
Title: The local dispersion relation for magneto-atmospheric waves
Authors: Thomas, J. H.
1982ApJ...262..760T    Altcode:
  The local dispersion relation for magneto-atmospheric waves is discussed
  in terms of the linearized theory of waves in a plane-stratified,
  inviscid, perfectly conducting atmosphere under uniform gravity. The
  normally used local dispersion relation is demonstrated to not
  be unique, depending instead on the order of derivation from the
  fundamental first-order perturbation equations of continuity,
  momentum, energy, and induction. Furthermore, it is shown that the
  local dispersion relation predicts that the cutoff frequency decreases
  with increasing magnetic field strength, while the WKB approximation
  method projects an increase in the cutoff frequency with increasing
  magnetic field strength. A new form of the local dispersion relation is
  developed, and consideration is given to the special case of a global
  dispersion relation in conditions of an isothermal atmosphere with a
  horizontal magnetic field.

---------------------------------------------------------
Title: Umbral Oscillations in a Detailed Model Umbra
Authors: Thomas, J. H.; Scheuer, M. A.
1982SoPh...79...19T    Altcode:
  Our theory of umbral oscillations as resonant modes of
  magneto-atmospheric waves (Scheuer and Thomas, 1981) is extended and
  confirmed by calculating the resonant modes in a much more detailed
  model of the umbral atmosphere. The depths of forcing required to
  produce observed oscillation periods (roughly 140 to 185 s) are in
  good agreement with the depths of overstable convection found in other
  studies (Moore, 1973; Mullan and Yun, 1973).

---------------------------------------------------------
Title: Five-minute oscillations as a subsurface probe of sunspot
    structure
Authors: Thomas, J. H.; Cram, L. E.; Nye, A. H.
1982Natur.297..485T    Altcode:
  Observations are reported here which show that the 5-min oscillations
  in a sunspot umbra actually split into several individual modes
  of different period. We interpret these modes of oscillation as
  the response of the sunspot to forcing by the 5-min p-modes in the
  surrounding quiet atmosphere. Also, we show how detailed observations
  of the multiple 5-min modes in a sunspot may be used as a probe of
  the structure of a sunspot beneath the solar surface.

---------------------------------------------------------
Title: High Resolution Observations of a Moving Magnetic Feature
Authors: Nye, A. H.; Thomas, J. H.; Cram, L. E.
1982BAAS...14..624N    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Five-minute oscillations as a subsurface probe of sunspot
    structure.
Authors: Thomas, J. H.; Cram, L. E.; Nye, A. H.
1982ASNYN...2b..25T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The physics of sunspots
Authors: Cram, Lawrence E.; Thomas, John H.
1981Natur.293..101C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Umbral Oscillations as Resonant Modes of Magneto-Atmospheric
    Waves
Authors: Scheuer, M. A.; Thomas, J. H.
1981SoPh...71...21S    Altcode:
  Umbral oscillations in sunspots are identified as a resonant response
  of the umbral atmosphere to forcing by oscillatory convection in the
  subphotosphere. The full, linearized equations for magneto-atmospheric
  waves are solved numerically for a detailed model of the umbral
  atmosphere, for both forced and free oscillations. Resonant `fast' modes
  are found, the lowest mode having a period of 153 s, typical of umbral
  oscillations. A comparison is made with a similar analysis by Uchida
  and Sakurai (1975), who calculated resonant modes using an approximate
  (`quasi-Alfvén') form of the wave equations. Whereas both analyses
  give an appropriate value for the period of oscillation, several new
  features of the motion follow from the full equations. The resonant
  modes are due to upward reflection in the subphotosphere (due to
  increasing sound speed) and downward reflection in the photosphere and
  low chromosphere (due to increasing Alfvén speed); downward reflection
  at the chromosphere-corona transition is unimportant for these modes.

---------------------------------------------------------
Title: Five-Minute Oscillations in Sunspots
Authors: Thomas, J. H.; Cram, L. E.; Nye, A. H.
1981BAAS...13..858T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: An EPR spectroscopic examination of heavy metals in humic
    and fulvic acid soil fractions
Authors: Abdul-Halim, A. L.; Evans, J. C.; Rowlands, C. C.; Thomas,
   J. H.
1981GeCoA..45..481A    Altcode:
  Electron paramagnetic spectra of humic acid and various fractions of
  fulvic acid from a deep peat soil were studied and related to some
  of the metals present. In fulvic acid, VO <SUP>2+</SUP> occurred in
  complexed form. The Mn <SUP>2+</SUP> components all had a high degree
  of ionicity. In the humic acid fraction Cu <SUP>2+</SUP> was present
  as a copper porphyrin-type complex.

---------------------------------------------------------
Title: The physics of sunspots. Sacramento Peak Observatory
    conference, held at Sunspot, New Mexico, 14 - 17 July 1981.
Authors: Cram, L. E.; Thomas, J. H.
1981phss.conf.....C    Altcode:
  An overview of current observational and theoretical research
  into the physics of sunspots is presented. Observations of the
  appearance and evolution of sunspots are reviewed, noting possible
  connections with solar granulation and a potential larger flow
  pattern. Theoretical aspects of a connection between magnetic fields
  and convective forces are considered, with particular attention given
  to the sunspot energy deficit as reflected in variations in the solar
  luminosity. The discussion is extended to similarities between sunspots
  and starspots. Details of sunspot characteristics are explored,
  including fine-scale, time-dependent changes in morphology and the
  origin of waves and oscillation in sunspots. Possible relationships
  between the Evershed flow and penumbral filaments, umbral dots and
  the umbral magnetic fine structure, and waves in the umbra and the
  penumbra are explored. The Sunspot sunspot model is introduced as a
  synthesis of various sunspot models and examined for applications to
  phenomena common to different sunspots.

---------------------------------------------------------
Title: Theories of dynamical phenomena in sunspots
Authors: Thomas, J. H.
1981phss.conf..345T    Altcode:
  Attempts that have been made to understand and explain
  observed dynamical phenomena in sunspots within the framework of
  magnetohydrodynamic theory are surveyed. The qualitative aspects of the
  theory and physical arguments are emphasized, with mathematical details
  generally avoided. The dynamical phenomena in sunspots are divided
  into two categories: aperiodic (quasi-steady) and oscillatory. For
  each phenomenon discussed, the salient observational features that any
  theory should explain are summarized. The two contending theoretical
  models that can account for the fine structure of the Evershed motion,
  namely the convective roll model and the siphon flow model, are
  described. With regard to oscillatory phenomena, attention is given
  to overstability and oscillatory convection, umbral oscillations and
  flashes. penumbral waves, five-minute oscillations in sunspots, and
  the wave cooling of sunspots.

---------------------------------------------------------
Title: Observations of dynamical phenomena in sunspots
Authors: Nye, A. H.; Cram, L. E.; Beckers, J. M.; Thomas, J. H.
1981phss.conf..313N    Altcode:
  A preliminary report of the results of one observing run based on data
  from one spectral line, the photospheric magnetic line Fe 6303, is
  presented as part of a series of observations of dynamical phenomena
  in sunspots using photographic spectra with the SPO vacuum tower
  telescope and echelle spectrograph. The ejection of a magnetic feature
  from the outer edge of the penumbra was observed. The initial total
  field strength of the feature was about 1000 gauss, which appeared to
  decrease as the feature moved away from the sunspot. The proper motion
  was about 2 km/s, and the velocity field measured in the V profile
  showed a downflow of 400 m/s on the spot-ward side of the moving
  magnetic feature. Umbral oscillations at the photospheric level with a
  herringbone structure characteristic of horizontally propagating waves,
  suggesting some overtone mode of membrane oscillation in the umbra,
  were seen. The peak amplitude of the oscillation was about 200 m/s,
  and the mean power spectrum had several clear peaks.

---------------------------------------------------------
Title: Conjectures regarding the structure of a sunspot penumbra
Authors: Cram, L. E.; Nye, A. H.; Thomas, J. H.
1981phss.conf..384C    Altcode:
  A study of simple radiative transfer models for some of the dark
  filaments on the outer edge of penumbras, which observations suggest
  may lie several hundred km above the base of the quiet photosphere,
  has determined that elevated dark filaments probably have higher
  temperatures and densities than the surrounding atmosphere. The
  possibility of a connection between the dark filaments, the photospheric
  Evershed flow, and umbral dots, is discussed. An important observational
  test of the present model would involve an attempt to separate
  temperature and optical depth effects in the dark filaments. This
  could be accomplished by either continuum photometry at a few, widely
  separated wavelengths, or by spectroscopic studies.

---------------------------------------------------------
Title: Resonant Modes of Umbral Oscillation in Sunspots
Authors: Thomas, J. H.; Scheuer, M. A.
1980BAAS...12..897T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Resonant Modes of Umbral Oscillation in Sunspots
Authors: Thomas, J. H.; Scheuer, M. A.
1980BAAS...12..477T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Dynamical phenomena in sunspots
Authors: Thomas, J. H.; Clark, A., Jr.; Scheuer, M. A.
1980runy.rept.....T    Altcode:
  The reflection of upward and downward propagating Alfven waves in
  sunspots is studied, in order to assess the possibility of cooling
  by Alfven waves. Wave reflection is studied by means of a three layer
  model of the umbral atmosphere. The results show very strong downward
  reflection of Alfven waves in the photosphere and low temperature,
  but only weak upward reflection in the convection zone. Further
  study of more realistic magneto-atmospheric waves (including the
  effects of compression and buoyancy) shows strong upward reflection
  in the convection zone as well as strong downward reflection in the
  photosphere and low chromosphere. These results tend to rule out
  significant sunspot cooling by waves. A study of a simple thermal
  model of a sunspot, based on the concept of partial inhibition of
  convection, shows that the inhibition mechanism can yield acceptable
  distributions of surface temperature. The results of this model also
  show that: (1) the edge of the umbra is sharp, even for deep spots;
  (2) deep spots produce weak bright rings, but shallow spots produce
  intense bright rings in conflict with observations; and (3) only a
  shallow surface layer of the sunspot is cool, the rest being warmer
  than the surroundings. Umbral oscillations in sunspots are studied and
  identified as a resonant response of the umbral atmosphere to forcing
  by oscillatory convection in the subphotosphere. The full linearized
  equations for magneto-atmospheric waves are solved numerically for
  a detailed model of the umbral atmosphere. It is suggested that the
  Sun's radius and surface temperature vary with the solar cycle due to
  the variation of total magnetic buoyancy in the convection zone over
  the cycle of the solar dynamo.

---------------------------------------------------------
Title: Variations of the Sun's radius and temperature due to magnetic
    buoyancy
Authors: Thomas, J. H.
1979Natur.280..662T    Altcode:
  LIVINGSTON<SUP>1</SUP> has recently measured a decrease in the surface
  temperature of the Sun coincident with increased solar activity. He
  interpreted the temperature drop as implying a corresponding reduction
  in luminosity. I point out here that surface cooling could also be
  due to a radial expansion of the Sun, with no attendant reduction
  in luminosity. There is a plausible physical mechanism for such an
  expansion; namely, variations in magnetic buoyancy due to variations
  in the magnetic flux in the convection zone over the solar cycle.

---------------------------------------------------------
Title: Moving lithospheric plates and mantle convection
Authors: Lux, Richard A.; Davies, Geoffrey F.; Thomas, John H.
1979GeoJ...58..209L    Altcode: 1979GeoJI..58..209L
  No abstract at ADS

---------------------------------------------------------
Title: Variations of the Sun's Radius and Temperature Due to Magnetic
    Buoyancy.
Authors: Thomas, J. H.
1979BAAS...11..437T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Refraction of Rossby waves on a multiple β-plane
Authors: Thomas, John H.; Lux, Richard A.
1978DyAtO...2..411T    Altcode:
  A multiple β-plane is introduced to explore the relation between plane
  and spherical Rossby waves. The fundamental problem, the refraction
  of a plane Rossby wave across a discontinuity in β, is solved. It is
  shown that refraction on the multiple β-plane agrees in the limit with
  refraction on the full sphere only if a suitable correction is made for
  the geometric distortion of the β-plane. The full spherical modes of
  Rossby waves trapped in a band about the equator (Longuet-Higgins, 1964)
  have their counterpart in a simple model consisting of an "equatorial"
  β-plane bounded above and below by "polar" β-planes.

---------------------------------------------------------
Title: The reflection of Alfvén waves and the cooling of sunspot.
Authors: Thomas, J. H.
1978ApJ...225..275T    Altcode:
  As one means of evaluating the possibility that sunspots are cooled by
  a flux of Alfven waves, the reflection of vertically propagating Alfven
  waves in a three-layer model of a sunspot umbra is studied. The results
  show strong downward reflection of Alfven waves in the photosphere and
  low chromosphere, with very little wave energy penetrating as high
  as the corona. This is in agreement with recent observations. The
  model umbra also shows very weak upward reflection of Alfven waves
  propagating downward into the convection zone. The results suggest that,
  if sunspots are indeed cooled by Alfven waves, these waves must escape
  downward into the solar interior.

---------------------------------------------------------
Title: Reflection of Alfven Waves and the Cooling of Sunspots.
Authors: Thomas, J. H.
1978BAAS...10..415T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Solar magneto-atmospheric waves and penumbral waves
Authors: Thomas, J. H.; Nye, A. H.; Clark, A., Jr.
1976runy.rept.....T    Altcode:
  The linearized theory of magneto-atmospheric waves (involving the
  combined restoring forces due to buoyancy, compressibility, and magnetic
  field) is developed for the case of a horizontal magnetic field which
  may vary with height. The convective stability of the basic atmosphere
  is considered, and it is shown that a nonuniform horizontal magnetic
  field may be destabilizing as well as stabilizing. It is found that
  penumbral waves may be identified with magneto-atmospheric waves of the
  plus type that are vertically trapped at photospheric levels. Although
  most of the wave energy is contained in the penumbral photosphere and
  subphotosphere, the maximum vertical velocity occurs in the chromosphere
  where the waves are evanescent (and where they are observed in H
  alpha). An exact analytical solution for magneto-atmospheric wave modes
  is found in the case of an isothermal atmosphere permeated by a uniform
  horizontal magnetic field, without making the usual short-wavelength
  approximation. The exact solution mentioned above is also used in
  conjunction with a simple two-layer model of a sunspot penumbra to
  further study the mode of running penumbral waves. The lowest plus
  eigenmode of the model is in good agreement with observations of
  penumbral waves.

---------------------------------------------------------
Title: Observations of penumbral wave in the photosphere.
Authors: Musman, S.; Nye, A. H.; Thomas, J. H.
1976ApJ...206L.175M    Altcode:
  Simultaneous observations have been made of velocities in the
  chromosphere (in H-alpha) and in the photosphere (in the nonmagnetic
  Fe I line at 5526-A) of three sunspots. The results reveal waves
  propagating horizontally outward across the penumbra in the photosphere
  with about the same period as the running penumbral waves in H-alpha
  (250-290 s). The photospheric waves are more intermittent and have
  higher horizontal phase velocity (by a factor of 2 or more) than the
  chromospheric penumbral waves.

---------------------------------------------------------
Title: Solar magneto-atmospheric waves. I. An exact solution for a
    horizontal magnetic field.
Authors: Nye, A. H.; Thomas, J. H.
1976ApJ...204..573N    Altcode:
  The linearized theory of magneto-atmospheric waves (involving the
  combined restoring forces due to buoyancy, compressibility, and magnetic
  field) is developed for the case of a horizontal magnetic field. A
  general propagation equation is derived for adiabatic perturbations with
  arbitrary vertical distributions of the sound speed c, Alfve'n velocity
  VA, and local density scale height H. An exact analytical solution
  to the propagation equation is obtained for the case of an isothermal
  atmosphere permeated by a uniform horizontal magnetic field, without
  making the usual short- wavelength assumption. This solution is applied
  to an idealized model of the low-corona- chromosphere transition region
  for comparison with observations of flare-induced coronal waves. The
  results show that disturbances may propagate horizontally in the low
  corona in a wave guide formed by the sudden density increase into the
  chromosphere below and by the rapidly increasing AlfVe'n velocity with
  height in the corona. The group velocity of the guided wave modes is
  nearly independent of wavelength, so that a disturbance propagates as a
  compact wave packet. Subject headings: hydromagnetics Sun: atmospheric
  motions - Sun: corona - Sun: magnetic fields

---------------------------------------------------------
Title: Solar Magneto-Atmospheric Waves. 11. a Model for Running
    Penumbral Waves
Authors: Nye, Alan H.; Thomas, John H.
1976ApJ...204..582N    Altcode:
  A simple two4ayer model of a sunspot penumbra is used to study the
  mode of running penumbral waves. Exact solutions of the linearized
  wave equation, not limited to the small-wavelength approximation,
  are employed in each layer. The lowest "plus" eigenmode of
  magneto-atmospheric waves in the model penumbra is in good agreement
  with observations of running penumbral waves. The results indicate that
  running penumbral waves should be observable in a photo spheric pectral
  sline. Subject headings: hydromagnetics - Sun: atmospheric motions -
  Sun: magnetic fields - Sun: sunspots

---------------------------------------------------------
Title: Observations of Penumbral Waves in the Photosphere
Authors: Musman, S.; Nye, A. H.; Thomas, J. H.
1976BAAS....8..344M    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Convective instability in the presence of a nonuniform
    horizontal magnetic field
Authors: Thomas, J. H.; Nye, A. H.
1975PhFl...18..490T    Altcode:
  Newcomb's criterion for convective stability in the presence
  of a horizontal magnetic field is derived as an equation which
  explicitly shows the effect of vertical variations in the magnetic
  field strength. For a uniform magnetic field, the equation is reduced
  to a form which is identical to the Schwarzschild criterion in the
  absence of a magnetic field, demonstrating that such a field has no
  effect on the condition for the onset of convective instability. For
  a nonuniform magnetic field, it is shown that convective instability
  will result if the magnetic pressure decreases more rapidly than the
  critical temperature rate.

---------------------------------------------------------
Title: Magneto-Atmospheric Waves and Moreton's Wave Phenomenon
Authors: Nye, Alan H.; Thomas, John H.
1975BAAS....7..361N    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: The Nature of Running Penumbral Waves
Authors: Nye, A. H.; Thomas, J. H.
1974SoPh...38..399N    Altcode:
  A model of a sunspot penumbra, including the effects of magnetic field,
  compressibility, and buoyancy, is studied in order to identify the
  mode of running penumbral waves. It is found that the penumbral waves
  may be identified with gravity-modified magneto-acoustic waves of the
  `plus' type that are vertically trapped at photospheric levels. Although
  most of the wave energy is contained in the penumbral photosphere and
  subphotosphere, the maximum vertical velocity occurs in the chromosphere
  where (i) the waves are evanescent and (ii) the vertical velocity is
  in fact observed (in Hα).

---------------------------------------------------------
Title: The Nature of Running Penubral Waves in Sunspots
Authors: Nye, A. H.; Thomas, J. H.
1974MitAG..35..266N    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: On the Evolution of Magnetic White Dwarfs
Authors: Fontaine, Gilles; Thomas, John H.; van Horn, H. M.
1973ApJ...184..911F    Altcode:
  We show that the magnetic DC white dwarfs cannot have evolved from
  stars of spectral type DA. A separate origin is necessary, and a
  direct evolutionary connection between the magnetic DC stars and the
  nuclei of planetary nebulae seems plausible. Such a connection implies
  photospheric field strengths 10 -10 gauss in planetary nuclei, which may
  be large enough to be detectable. In addition, we find that magnetic
  white dwarfs must be hotter than some (weakly massdependent) limiting
  effective temperature. This prediction is potentially susceptible
  to observational verification. Subject headings: magnetic stars -
  stellar evolution - white dwarf stars

---------------------------------------------------------
Title: Horizontal Propagation of Solar Atmospheric Oscillations
Authors: Thomas, J. H.; Clark, P. A.; Clark, A., Jr.
1972ApL....12...31T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Solar Seeing and the Spatial Properties of the Five-Minute
    Oscillations
Authors: Thomas, John H.
1972SoPh...24..262T    Altcode:
  A numerical simulation of observations of the spatial properties of the
  five-minute oscillations is carried out, assuming the oscillations are
  internal gravity waves excited by granular convection according to the
  theory of Thomas et al. (1971). The simulation includes the effects of
  seeing and finite aperture. The details of the simulation are chosen
  to model the observational method of Frazier (1968a, b). The results
  show that the peak in the observed power spectrum of the oscillations
  can occur at a wavelength considerably longer than the true wavelength
  of the oscillations. In particular, the peak in Frazier's observed
  power spectra at wavelength λ ≈ 5000 km is consistent with the
  considerably shorter true wavelength λ ≈ 1500 km predicted by the
  gravity wave theory.

---------------------------------------------------------
Title: Trapped Gravity Waves and the Five-Minute Oscillations of
    the Solar Atmosphere
Authors: Thomas, John H.; Clark, Patricia André; Clark, Alfred, Jr.
1971SoPh...16...51T    Altcode:
  The various modes of hydrodynamic waves are considered for a model
  of the solar atmosphere which is based on the Bilderberg model and
  includes the effects of ionization. The atmosphere forms a `potential
  well' for internal gravity waves, since the stability is low at the base
  (near the convection) and low again in the region of partial ionization
  in the chromosphere. Calculations show that there are two resonant
  (trapped) modes of internal gravity waves for horizontal wavelengths
  based on the scale of the granulation. The properties of these modes
  are in close agreement with the two modes of oscillation observed by
  Frazier (1968). Trapped acoustic modes are found to have periods too
  short to account for the observations.

---------------------------------------------------------
Title: An Observational Test to Distinguish the Mode of Chromospheric
    Oscillation
Authors: Clark, Alfred, Jr.; André Clark, Patricia; Thomas, John H.
1970BAAS....2T.303C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Trapped Gravity Waves and the Chromospheric Oscillations.
Authors: Thomas, John H.; Clark, Patricia André; Clark, Alfred, Jr.
1970BAAS....2Q.349T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Solar Differential Rotation and Oblateness
Authors: Clark, Alfred, Jr.; Thomas, John H.; André Clark, Patricia
1969BAAS....1..275C    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Solar Differential Rotation and Oblateness
Authors: Clark, Alfred, Jr.; Thomas, John H.; Clark, Patricia Andre
1969Sci...164..290C    Altcode:
  An investigation of the time development of differential rotation
  produced by the solar wind torque indicates that the sun has a rapidly
  rotating core.