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.
Bibcode: 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.
Title: Explaining the observed relation between stellar activity
and rotation.
Authors: Blackman, E. G.; Thomas, J. H.
Bibcode: 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-q with 2 ≤ q ≤
3, whilst |q| < 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.
Title: Sunspots and Starspots
Authors: Thomas, John H.; Weiss, Nigel O.
Bibcode: 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.
Title: Theoretical Models of Sunspot Structure and Dynamics
Authors: Thomas, J. H.
Bibcode: 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.
Title: Sunspots and Starspots
Authors: Thomas, John H.; Weiss, Nigel O.
Bibcode: 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.
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.
Bibcode: 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.
Title: Evershed Flows along Penumbral Flux Tubes in Sunspots
Authors: Thomas, J. H.
Bibcode: 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.
Title: On the fine structure of magnetic fields in sunspot penumbrae
Authors: Thomas, J. H.; Weiss, N. O.; Tobias, S. M.; Brummell, N. H.
Bibcode: 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 & Scharmer.
Title: Extracting rotational energy in supernova progenitors:
Transient Poynting flux growth vs. turbulent dissipation
Authors: Blackman, Eric G.; Nordhaus, Jason T.; Thomas, John H.
Bibcode: 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⊙ star. Unlike the Sun, the dynamo is transient because
the differential rotation is primarily due to the initial collapse. Up
to ∼10 51 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.
Title: Flows along penumbral flux tubes in sunspots. Instability of
super-Alfvénic, serpentine solutions
Authors: Thomas, J. H.
Bibcode: 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.
Title: Fine Structure in Sunspots
Authors: Thomas, John H.; Weiss, Nigel O.
Bibcode: 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.
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.
Bibcode: 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.
Title: Theory of sunspot structure
Authors: Thomas, John H.
Bibcode: 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.
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.
Bibcode: 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 LEdd. When the luminosity drops
below LEdd, 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*~0.7 Msolar,
luminosity L*~104 Lsolar, and
mass-loss rate |M|~10-5 Msolar yr-1,
the oscillations have a recurrence time of ~1400 yr~57τfsm,
where τfsm 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.
Title: Solar physics: The Sun under a microscope
Authors: Thomas, John H.
Bibcode: 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.
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.
Bibcode: 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.
Title: Magnetic flux pumping and the structure of a sunspot penumbra
Authors: Thomas, J. H.; Weiss, N. O.; Tobias, S. M.; Brummell, N. H.
Bibcode: 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.
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.
Bibcode: 2001MNRAS.326..877M
Altcode:
The correlation between stellar activity, as measured by the
indicator ΔRHK, 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.
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.
Bibcode: 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.
Title: A new analysis of the correlation between activity and Rossby
number in late-type stars
Authors: Montesinos, B.; Thomas, J. H.
Bibcode: 2001hsa..conf..393M
Altcode:
No abstract at ADS
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.
Bibcode: 2001ASPC..248..439T
Altcode: 2001mfah.conf..439T
No abstract at ADS
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
Bibcode: 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 &
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.
Title: Solar Interface Dynamo Models with a Realistic Rotation Profile
Authors: Markiel, J. Andrew; Thomas, John H.
Bibcode: 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 & 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.
Title: The Solar Physics Division
Authors: Thomas, John H.
Bibcode: 1999aasf.book..238T
Altcode:
No abstract at ADS
Title: Solar physics: The Sun at small scales
Authors: Thomas, John H.
Bibcode: 1998Natur.396..114T
Altcode:
No abstract at ADS
Title: The Evershed Effect in Sunspots: A Theoretical Explanation
Authors: Montesinos, Benjamin; Thomas, John H.
Bibcode: 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.
Title: Velocity and Magnetic Field Fluctuations in the Photosphere
of a Sunspot
Authors: Lites, Bruce W.; Thomas, John H.; Bogdan, Thomas J.; Cally,
Paul S.
Bibcode: 1998ApJ...497..464L
Altcode:
We use a data set of exceptionally high quality to measure oscillations
of Doppler velocity, intensity, and the vector magnetic field at
photospheric heights in a sunspot. Based on the full Stokes inversion
of the line profiles of Fe I 630.15 and 630.25 nm, in the sunspot umbra
we find upper limits of 4 G (root mean square [rms]) for the amplitude
of 5 minute oscillations in magnetic field strength and 0.09d (rms)
for the corresponding oscillations of the inclination of the magnetic
field to the line of sight. Our measured magnitude of the oscillation
in magnetic field strength is considerably lower than that found in
1997 by Horn, Staude, & Landgraf. Moreover, we find it likely
that our measured magnetic field oscillation is at least partly due to
instrumental and inversion cross talk between the velocity and magnetic
signals, so that the actual magnetic field strength fluctuations are
even weaker than 4 G. In support of this we show, on the basis of the
eigenmodes of oscillation in a theoretical model of the sunspot umbra,
that magnetic field variations of at most 0.5 G are all that is to
be expected. The theoretical model also provides an explanation of
the shift of power peaks in Doppler velocity to the 3 minute band in
chromospheric umbral oscillations, as a natural consequence of the
drastic change in character of the eigenmodes of oscillation between
frequencies of about 4.5 and 5.0 mHz due to increased tunneling through
the acoustic cutoff-frequency barrier. Using measurements of the
phase of velocity oscillations above the acoustic cutoff frequency,
we determine the relative velocity response height in the umbra of
four different photospheric spectral lines from the phase differences
between velocities in these lines, assuming that the oscillations
propagate vertically at the local sound speed. In spacetime maps of
fluctuations in continuum intensity, Doppler velocity, magnetic field
strength, and field inclination, we see distinct features that migrate
radially inward from the inner penumbra all the way to the center of
the umbra, at speeds of a few tenths of a kilometer per second. These
moving features are probably a signature of the convective interchange
of magnetic flux tubes in the sunspot, although we failed to find any
strong correlation among the features in the different quantities,
indicating that these features have not been fully resolved.
Title: 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.
Bibcode: 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+,
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- ≠ τ+. 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. 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. 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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 & Weiss, whereas
that of the variable-alpha model matches the structure computed by
Schmalz & 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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 & 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 & 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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 &
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.
Bibcode: 1994ASIC..433..159L
Altcode:
No abstract at ADS
Title: The cause of the Evershed effect in sunspots: flows or waves?
Authors: Thomas, J. H.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1992AAS...180.0701T
Altcode: 1992BAAS...24..738T
No abstract at ADS
Title: The Theory of Sunspots
Authors: Thomas, John H.; Weiss, Nigel O.
Bibcode: 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 & Observations: NATO Cambridge, 1992
Authors: Thomas, John H.; Weiss, Nigel O.
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 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
Bibcode: 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.
Bibcode: 1990BAAS...22..880T
Altcode:
No abstract at ADS
Title: The structure of photospheric flux tubes
Authors: Thomas, John H.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1985BAAS...17..855C
Altcode:
No abstract at ADS
Title: Hydromagnetic Waves in the Photosphere and Chromosphere
Authors: Thomas, J. H.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1985ASNYN...2...27C
Altcode:
No abstract at ADS
Title: Oscillations in sunspots
Authors: Thomas, J. H.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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-1
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.
Bibcode: 1984BAAS...16..943C
Altcode:
No abstract at ADS
Title: Half-life of 26Al
Authors: Thomas, J. H.; Rau, R. L.; Skelton, R. T.; Kavanagh, R. W.
Bibcode: 1984PhRvC..30..385T
Altcode:
We have measured the half-life of 26Al because data on
21Ne production rates in meteorites has indicated that the
half-life may have been too low by 30-40%. We produced 26Al
using the 26Mg(p,n)26Al 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 t12=(7.8+/-0.5)×105
years, in agreement with the accepted half-life of
26Al:t12=(7.16+/-0.32)×105
years. Therefore, another explanation must be found for the anomalous
21Ne production rate based on 26Al ages in
meteorites.
Title: Umbral oscillations in sunspots
Authors: Thomas, J. H.
Bibcode: 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.
Bibcode: 1984BAAS...16..531T
Altcode:
No abstract at ADS
Title: Flow in an Isolated Magnetic Flux Tube
Authors: Thomas, J. H.
Bibcode: 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.
Bibcode: 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.
Title: 10Be 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.
Bibcode: 1983E&PSL..65...51M
Altcode:
Concentration-depth profiles of cosmic-ray-produced 10Be
( t1/2 = 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 4-10 6 years, and (2) to explore
the application of 10Be 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 5
years based on amino-stratigraphic correlations and models of terrace
evolution; those of the table mountain top and alluvial fan are 9.5 ×
10 6 years and 6.0 × 10 5 years, respectively,
based on K-Ar analyses. All the surfaces sampled are nearly flat and
exhibit few erosional features. In addition to 10Be we
measured 210Pb, 239,240Pu and 7Be 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 7Be inventory is 4.0 dpm/cm 2
similar to those observed at nearby locations. The inventories of
210Pb 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 10Be concentrations in
the sample range between (0.2 and 7) × 10 8 atoms/g soil and
show strong correlations with leachable Fe and/or Al. The inventory of
10Be 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 10Be from the atmosphere,
5.2 × 10 5 atoms/cm 2 yr. The low inventory of
10Be 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 10Be, the residence
time of 10Be in the soil domains sampled is determined to
be of the order of 10 4 years. The low residence time of
10Be 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 10Be as a chronometer of geomorphic surface
age is severely constrained. However, the study of 10Be in
soils provides the only entry into the long-term (10 4-10
6 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.
Bibcode: 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.
Bibcode: 1983BAAS...15R.719A
Altcode:
No abstract at ADS
Title: Magneto-atmospheric waves
Authors: Thomas, J. H.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1982ASNYN...2b..25T
Altcode:
No abstract at ADS
Title: The physics of sunspots
Authors: Cram, Lawrence E.; Thomas, John H.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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 2+ occurred in
complexed form. The Mn 2+ components all had a high degree
of ionicity. In the humic acid fraction Cu 2+ 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1981phss.conf..384C
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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.
Bibcode: 1980BAAS...12..897T
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No abstract at ADS
Title: Resonant Modes of Umbral Oscillation in Sunspots
Authors: Thomas, J. H.; Scheuer, M. A.
Bibcode: 1980BAAS...12..477T
Altcode:
No abstract at ADS
Title: Dynamical phenomena in sunspots
Authors: Thomas, J. H.; Clark, A., Jr.; Scheuer, M. A.
Bibcode: 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.
Bibcode: 1979Natur.280..662T
Altcode:
LIVINGSTON1 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1978BAAS...10..415T
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No abstract at ADS
Title: Solar magneto-atmospheric waves and penumbral waves
Authors: Thomas, J. H.; Nye, A. H.; Clark, A., Jr.
Bibcode: 1976runy.rept.....T
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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.
Bibcode: 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.
Bibcode: 1976ApJ...204..573N
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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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1975PhFl...18..490T
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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.
Bibcode: 1975BAAS....7..361N
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No abstract at ADS
Title: The Nature of Running Penumbral Waves
Authors: Nye, A. H.; Thomas, J. H.
Bibcode: 1974SoPh...38..399N
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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.
Bibcode: 1974MitAG..35..266N
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No abstract at ADS
Title: On the Evolution of Magnetic White Dwarfs
Authors: Fontaine, Gilles; Thomas, John H.; van Horn, H. M.
Bibcode: 1973ApJ...184..911F
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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.
Bibcode: 1972ApL....12...31T
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No abstract at ADS
Title: Solar Seeing and the Spatial Properties of the Five-Minute
Oscillations
Authors: Thomas, John H.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 1970BAAS....2Q.349T
Altcode:
No abstract at ADS
Title: Solar Differential Rotation and Oblateness
Authors: Clark, Alfred, Jr.; Thomas, John H.; André Clark, Patricia
Bibcode: 1969BAAS....1..275C
Altcode:
No abstract at ADS
Title: Solar Differential Rotation and Oblateness
Authors: Clark, Alfred, Jr.; Thomas, John H.; Clark, Patricia Andre
Bibcode: 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.