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Author name code: dsilva
ADS astronomy entries on 2022-09-14
author:"D'Silva, Sydney" 

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Title: Equivalence between Duvall's Law and the Time-Distance Curve
Authors: D'Silva, S.
2001ApJ...549L.135D    Altcode:
  Duvall's law is shown to be equivalent to the phase time-distance
  curve. The two are connected through a simple transformation. Thus,
  like Duvall's law, the phase time-distance curve can be analytically
  inverted to obtain the radial sound speed. The use of the phase
  information of the time-distance technique makes the inversion of the
  time-distance Duvall law particularly advantageous over the traditional
  inversion of Duvall's law. The radial sound speed can also be obtained
  numerically through the radial limit of the global or local tomography
  of the Sun. With global tomography, one can take advantage of the higher
  order skips of wave packets to obtain local tomograms of any section of
  the Sun, including the interior of the farside, that currently cannot
  be directly observed.

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Title: Time-Distance Helioseismology at High Frequencies
Authors: D'Silva, S.
2001ApJ...548L.107D    Altcode:
  In time-distance helioseismology, computed travel time is believed
  to be the shortest time taken by a wave packet to travel between two
  spatial locations on the surface of the Sun separated by the shortest
  distance. Typically, it is computed by cross-correlating oscillation
  signals at the two locations and identifying the position of the
  envelope peak of the cross-correlation function. When the oscillation
  signals are measured in the region where the waves are propagating,
  correlation techniques do not necessarily provide the shortest travel
  time. Instead, they are shown to give the total time for the wave
  packet to take one round-trip between the two boundaries of the cavity
  in which the waves are enclosed. High-frequency oscillations (above
  the chromospheric acoustic cutoff frequency of approximately 5 mHz)
  are believed to be reflected by the corona-chromosphere boundary, and
  their signals are measured in the region where they propagate. Travel
  time computed by correlation techniques indicates the time the wave
  packet takes to return to the observing plane the second time after
  it encounters both the upper and lower turning points. Correlation
  techniques do not directly provide the shortest travel time, which would
  be the time to return to the observing plane after an encounter with
  either the upper or lower turning points. Inversions of travel time at
  high frequencies should include the path of the wave packet through the
  chromosphere between the observing plane and the corona-chromosphere
  boundary where travel time can be significantly affected by the local
  thermal, magnetic, and flow properties of the chromosphere.

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Title: A Note on Helioseismic Tomography
Authors: D'Silva, S.
2001ApJ...547L..81D    Altcode:
  Helioseismic tomography is a form of the tomographic techniques
  adapted to image the interior of the Sun from observations of the
  acoustic oscillations at the surface. The important adaptation is the
  computation of travel time. Phase travel time, a measure of the time
  a wave packet takes to travel between two spatially separated surface
  locations, is computed by cross-correlating the oscillation signals and
  identifying a zero-crossing of the correlation function. To improve
  the signal-to-noise ratio, the oscillation signals, or individual
  correlation functions, are spatially averaged. It is tacitly assumed
  that the travel time of the averaged signal, or correlation function, is
  the average of the individual travel times. In general, this assumption
  is false; the phase travel time of the average signal is a solution to
  a nonlinear equation and depends on the amplitudes of the individual
  correlation functions. This demands suitable modifications of the
  computation of travel times and the tomographic equations.

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Title: The Meaning of Travel Time in Time-Distance Helioseismology
Authors: D'Silva, S.
1999AAS...194.4206D    Altcode: 1999BAAS...31..883D
  Oscillation signals measured at two spatial locations on the Sun are
  cross-correlated and the position of the envelope peak or a phase peak
  of the correlation function is called the travel time. The relation
  of this observationally derived quantity and the theoretical travel
  time of a wavepacket between the two locations is not unique. The
  conditions that establish uniqueness are given and the effects of
  breaking the uniqueness criteria are demonstrated. The oscillation
  signals are measured over a finite area at each spatial location. The
  effect of spatial averaging and the meaning of the observationally
  derived quantity called travel time is discussed.

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Title: “Dispersion” in Time-Distance Helioseismology
Authors: D'Silva, S.
1998ApJ...499L.211D    Altcode:
  In time-distance helioseismology, travel time is the time taken by a
  wave packet to travel between two spatially separated locations on the
  surface of the Sun. It is computed by cross-correlating oscillation
  signals at the two locations and identifying the position of the
  envelope peak of the cross-correlation function, or the position of
  one of its phase peaks, as the travel time. The wave packet spectrum
  is a subset of the signal spectra. Adding more frequencies to the
  wave packet spectrum is shown to not necessarily narrow the width of
  the envelope of the cross-correlation function. “Dispersion” in the
  travel time across the spectrum restricts the minimum width of the
  cross-correlation function and shifts the position of the envelope and
  phase peaks as a function of the central frequency and width of the
  wave packet spectrum. Wave packets at the surface of polytropes show
  no dispersion in travel time; hence, Gaussian spectra yield Gaussian
  envelopes, and envelope widths at constant central frequency go to zero
  with increasing spectral width, showing no shift in the envelope peak
  or phase peaks. In the Sun, however, dispersion is inherent: Envelope
  and phase peaks are functions of the central frequency and width of
  the spectrum, and Gaussian spectra do not yield Gaussian envelopes
  and can even conspire to resemble a sum of two or more Gaussians.

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Title: Computing Travel Time in Time-Distance Helioseismology
Authors: D'Silva, S.
1998ApJ...498L..79D    Altcode:
  In time-distance helioseismology, the position of the peak of the
  envelope of a cross-correlation function is identified as the travel
  time of a wave packet between two spatially separated locations
  on the Sun. When a cross-correlation function is a sum of two or
  more cross-correlation functions, the following theorem forbids this
  identification: <P />THEOREM. The envelope of the analytic signal of a
  sum of two real functions is not equal to the sum of the envelopes of
  the analytic signals of the individual functions, unless (1) the ratio
  of the two functions is equal to the ratio of their Hilbert transforms
  and the sum of the product of the two functions and the product of
  their Hilbert transforms is nonnegative or (2) the envelopes of the
  two functions are disjoint. <P />The individual functions should be
  isolated before the travel times are identified with their envelope
  peaks. Typically, in time-distance helioseismology, the envelopes
  are Gaussians and are never disjoint, nor do the cross-correlation
  functions and their Hilbert transforms automatically satisfy condition
  1. This Letter derives the conditions under which condition 1 is
  satisfied and the conditions under which the envelope of a sum can
  be approximated to the sum of the individual envelopes so that the
  errors committed are minimal. These precautionary measures need to be
  taken while computing travel times from solar oscillations, so that
  the subsequent theoretical interpretations that derive the internal
  structure of the Sun can be reliable.

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Title: Time-Distance Helioseismology: Challenges in Computing
    Travel Time
Authors: D'Silva, Sydney
1998ESASP.418..627D    Altcode: 1998soho....6..627D
  Time-distance helioseismology is a methodology that computes the
  travel time of acoustic wavepackets that travel between two spatially
  separated locations on the surface of the Sun and derives information on
  structures beneath its surface through the theoretical interpretations
  that follow. Travel time is computed by cross-correlating oscillation
  signals at the two locations and identifying the position of the
  peak of the envelope, or the position of one of the phase peaks,
  of the cross-correlation function. There are numerous conceptual
  difficulties surrounding this first crucial step of computing travel
  time : (1) When a cross-correlation function is a sum of two or more
  cross-correlation functions (which is the case at supergranulation
  distances), a theorem demands a cautious use of this identification. (2)
  Travel time is not a unique quantity, it is not only a function of the
  central frequency of the wavepacket spectrum, but also a function of
  the spectral width. Hence, a time-distance diagram, of travel time as
  a function of travel distance, is not unique. (3) Relative values of
  travel times, like travel time differences between wavepackets traveling
  in opposite directions between two locations, are usually associated
  with Doppler shifts of the intervening medium. Unless the oppositely
  traveling wavepackets are identical the travel time difference could
  reflect the difference in travel times due to adifference in the
  central frequencies, or widths of the two wavepackets. Identical
  filtering will not necessarily ensure that the oppositely traveling
  wavepackets are identical, particularly in the vicinity of sunspots. (4)
  Oscillation spectra filtered with Gaussian filters do not necessarily
  yield cross-correlation functions with Gaussian envelopes and the
  envelope might appear to be a sum of Gaussian envelopes. These and
  several other conceptual difficulties in computing travel time leave
  the theoretical interpretations that follow unreliable. A detailed
  study of these challenges is needed to improve the methodology of
  time-distance helioseismology. This work was supported by NASA grants
  NAG 5-4031 and NSF AST 9521785.

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Title: Sounding the Sun's Chromosphere
Authors: Jefferies, S. M.; Osaki, Y.; Shibahashi, H.; Harvey, J. W.;
   D'Silva, S.; Duvall, T. L., Jr.
1997ApJ...485L..49J    Altcode:
  Time-distance analysis of solar acoustic waves with frequencies above
  the nominal atmospheric acoustic cutoff frequency (~5.3 mHz) shows
  partial reflection of the waves at both the Sun's photosphere and a
  layer located higher in the atmosphere. This result supports recent
  reports of chromospheric modes.

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Title: Helioseismic Tomography
Authors: D'Silva, Sydney; Duvall, Thomas L., Jr.; Jefferies, Stuart
   M.; Harvey, John W.
1996ApJ...471.1030D    Altcode:
  "Helioseismic tomography" is a method using observations to construct
  slices of the Sun's internal structure. It is based on a reduction of
  observations to time-distance surfaces and hypersurfaces. We present
  a procedure for measuring time-distance surfaces and hypersurfaces,
  and thereby a method of studying localized inhomogeneities in the
  interior of the Sun, such as abnormalities in the sound speed (e.g., a
  thermal shadow, Parker 1987a), or local subsurface flows, or magnetic
  fields. We also present a simulation of measuring time-distance
  surfaces and illustrate how to measure the size of an inhomogeneity,
  its location in depth, and the deviation of its sound speed compared
  to its local surroundings.

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Title: Theoretical Foundations of Time-Distance Helioseismology
Authors: D'Silva, Sydney
1996ApJ...469..964D    Altcode:
  Time-distance helioseismology (Duvall et al.) measures the signal due
  to solar oscillations at any two points on the surface of the Sun and
  cross-correlates them to obtain the time taken by the signal to travel
  between the two points. The travel time provides information on the
  solar interior through which the oscillations propagate. Traditional
  helioseismology, on the other hand, studies the mode structure of the
  power spectrum of the oscillations, which also provides information on
  the internal structure of the Sun. <P />In this paper, a theoretical
  basis for time-distance helioseismology is presented. Its departure
  from traditional helioseismology is described. The theory can be
  applied to any dispersive or nondispersive medium. In time-distance
  helioseismology, it provides a method of computing theoretical
  cross-correlation functions from solar models for the signals
  of acoustic-gravity waves measured at any two points on the solar
  surface. <P />One of the applications of time-distance helioseismology
  will be to measure subsurface flows and rotation. The theory provides
  a method of computing theoretical cross-correlation functions from
  solar models in the presence of any subsurface flow, or rotation.

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Title: Measuring the Solar Internal Rotation Using Time-Distance
    Helioseismology. I. The Forward Approach
Authors: D'Silva, Sydney
1996ApJ...462..519D    Altcode:
  We set up the ray equations for acoustic-gravity waves in spherical
  geometry, in the presence of rotation or any horizontal subsurface
  flows. Rotation lifts the degeneracy and splits the single time-distance
  curve for the nonrotating Sun (D'Silva &amp; Duvall 1995) into a
  family of closely spaced curves, of which the prograde and retrograde
  curves are at the extreme ends. We calculate time-distance curves
  (for travel times as measured by Duvall and coworkers for a variety
  of synthetic radial rotation profiles, including solid-body rotation
  and the known equatorial differential rotation. The travel time and
  distance differences between the pro grade and retrograde rays show
  specific signatures of the rotation profile. Hence, the forward problem
  to measure rotation is studied in detail.

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Title: Downflows under sunspots detected by helioseismic tomography
Authors: Duvall, T. L.; D'Silva, S.; Jefferies, S. M.; Harvey, J. W.;
   Schou, J.
1996Natur.379..235D    Altcode:
  SUNSPOTS are areas of cooler gas and stronger magnetic fields in the
  Sun's photosphere (its 'surface'), but just how they form and are
  maintained has long been a puzzle. It has been proposed<SUP>1</SUP>
  that small vertical magnetic flux tubes, generated deep within the Sun,
  develop downflows around them when they emerge at the surface. The
  downflows bring together a large number of flux tubes in a cluster
  to form a sunspot, which behaves as a single flux bundle as long as
  the downflows bind the flux tubes together. Until now, however,
  it has not been possible to test this model with subsurface
  observations. Here we use the recently developed technique of
  travel-time helioseismology<SUP>2</SUP> to detect the presence of
  strong downflows beneath both sunspots and the bright features known
  as plages. The flows have a velocity of ~2 kms<SUP>-1</SUP>, and they
  persist to a depth of about 2,000 km. The data suggest, however, that
  the vertical magnetic field can be a coherent flux bundle only to a
  depth of ~600 km; below this depth it is possible that the downflows
  hold together a loose collection of flux tubes to maintain the sunspots
  that we see.

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Title: Flux Retraction and Recycling: Negative Buoyancy--induced
    Oscillations as an Alternative to Parker's Thermal Relaxation
    Oscillations
Authors: D'Silva, Sydney
1995ApJ...448..459D    Altcode:
  Observations show that magnetically active regions on the Sun cluster
  together as activity complexes. They are maintained at an almost
  constant flux level by recurring flux emergences accompanied by local
  disappearance of flux, and the in situ flux disappearance shows evidence
  of flux retraction. Parker (1987a, b) suggested thermal shadows to
  act as dynamical barriers, suppressing magnetic buoyancy in the lower
  convection zone (CZ), and intermittently letting flux emerge through
  a thermal relaxation oscillation, which could explain the recurrent
  flux injection into these activity complexes. We suggest negative
  buoyancy (Vainshtein &amp; Levy 1991) to account for flux retraction
  and its subsequent recycling. Flux tubes can cool and become heavier
  than their surroundings. The negatively buoyant flux tube can fall
  through the CZ, hit the top of the stable radiative zone where it has
  to encounter a rapidly rising Brunt-Väisälä frequency N, and bounce
  back to the surface in an oscillatory fashion. The oscillation can be
  damped by dissipative processes such as viscosity, the tube can be
  fragmented by instabilities, or the rising flux tube can grab flux
  from the overshoot region when it reemerges; overall, the negative
  buoyancy-induced oscillation could retract flux, recycle it, and
  explain the recurrent flux injection and in situ flux disappearance
  observed in activity complexes.

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Title: Sunspot Velocity Correlations: Are They Due to Reynolds
    Stresses or to the Coriolis Force on Rising Flux Tubes?
Authors: D'Silva, Sydney; Howard, Robert F.
1995SoPh..159...63D    Altcode:
  Observations have consistently pointed out that the longitudinal
  and latitudinal motions of sunspots are correlated. The magnitude
  of the covariance was found to increase with latitude, and its sign
  was found to be positive in the N-hemisphere and negative in the
  S-hemisphere. This correlation was believed to be due to the underlying
  turbulence where the sunspot flux tubes are anchored, and the covariance
  had the right sign and magnitude needed to explain the transfer of
  angular momentum toward the equator through Reynolds stresses.

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Title: Brunt-Vaeisaelae Growth Rate and the Radial Emergence of
    Equipartition Fields
Authors: D'Silva, S.
1995ApJ...443..444D    Altcode:
  It is believed that the dynamo operates in the overshoot region at the
  base of the solar convection zone (CZ), and the magnetic features we see
  at the surface are formed when flux tubes rise through the CZ and appear
  at the photosphere. Studies of dynamics of flux tubes have pointed
  out that 10 kG tubes, which are nearly in energy equipartition with
  the velocity field at the base of the CZ, are weakly buoyant and hence
  overwhelmed by the Coriolis force. They move parallel to the rotation
  axis and emerge at very high latitudes, well above the sunspot zone,
  which makes it difficult to explain the formation of sunspots. Influence
  of the Coriolis force was found to be overcome only if flux tubes
  were stronger than roughly a 100 kG. The Brunt-Vaisala growth rate
  (we define as the square root of the absolute value of N<SUP>2</SUP>;
  where N is the Brunt-Vaisala frequency) of the CZ plays an imporatnt
  role in the dynamics of rising flux tubes. In an isothermal rise, when
  the flux tube is in thermal equilibrium with its surroundings, absolute
  value of N<SUB>2</SUB> is shown to play a negligible role. However,
  in an adiabatic rise the role of absolute value of N<SUB>2</SUB> is
  dominant; if absolute value of N<SUB>2</SUB> is larger than roughly
  10<SUP>-12</SUP>/sq sec in the lower CZ, magnetic buoyancy is shown
  to rise exponentially as the flux tube emerges. Further if absolute
  value of N<SUB>2</SUB> greater than 4 x 10<SUP>-11</SUP>/sq sec, the
  exponential rise is sufficiently rapid to enable equipartition fields
  to overcome the influence of the Coriolis force and emerge rapidly. In
  the CZ of the solar model of Christensen-Dalsgaard, Proffitt, &amp;
  Thompson (1993; model CPT) equipartition fields are found to emerge at
  high latitudes. However, an increase of absolute value of N<SUP>2</SUP>
  in the lower CZ, on average, roughly by a factor of 8 would make them
  emerge radially to sunspot latitudes. If this is possible, there would
  be no need for the dynamo to produce extraordinarily strong fields to
  explain the formation of sunspots. Conversely, if such a large absolute
  value of N<SUB>2</SUB> is not possible for the lower layers of the CZ,
  then our results actually reinforce the conclusion in previous work
  that field strengths at the CZ base of order 100 kG are necessary for
  sunspot strength magnetic fields to emerge at sunspot latitudes.

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Title: Measuring Local Sub-Surface Flows Using Time-Distance
    Helioseismology
Authors: D'Silva, Sydney
1995SPD....26..404D    Altcode: 1995BAAS...27..955D
  No abstract at ADS

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Title: Time-Distance Helioseismology in the Vicinity of Sunspots
Authors: D'Silva, Sydney; Duvall, Thomas L., Jr.
1995ApJ...438..454D    Altcode:
  We use the ray description of acoustic-gravity modes to calculate
  time-distance diagrams for the quiet Sun and for regions in the vicinity
  of a sunspot with a monolithic flux-tube structure. Time-distance
  curves for the quiet Sun match the observations of Duvall et al. In
  the vicinity of a sunspot these quiet Sun curves split into a family
  of closely spaced curves. The structure of this bandlike feature is
  found to be sensitive to the sunspot model and can be a diagnostic of
  the subsurface geometry of the sunspot flux tube.

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Title: Acoustic Mode-Mixing in Sunspots
Authors: D'Silva, S.
1995ASPC...76..276D    Altcode: 1995gong.conf..276D
  No abstract at ADS

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Title: Flux Retraction and Recycling
Authors: D'Silva, Sydney
1994AAS...185.8608D    Altcode: 1994BAAS...26.1465D
  Observations show that magnetically active regions on the sun
  cluster together as activity complexes. They are maintained at an
  almost constant flux level by recurring flux emergences accompanied
  by local disappearance of flux, and the in situ flux disappearance
  shows evidence of flux retraction. Parker (1987) suggested thermal
  shadows to act as dynamical barriers, suppressing magnetic buoyancy
  in the lower convection zone (CZ), and intermittently letting flux
  emerge through a thermal relaxation oscillation, which could explain
  the recurrent flux injection into these activity complexes. We suggest
  negative buoyancy to account for flux retraction. Flux tubes can cool
  and become heavier than their surroundings. The negatively buoyant flux
  tube can fall through the CZ, hit the top of the stable radiative zone
  where it has to encounter a rapidly rising Brunt-Vaisala frequency N and
  bounce back to the surface in an oscillatory fashion. The oscillation
  can be damped by dissipative processes like viscosity, the tube can be
  fragmented by instabilities, or the rising flux tube can grab flux from
  the overshoot region when it re-emerges; overall, the negative buoyancy
  induced oscillation could retract flux, recycle it, and explain the
  recurrent flux injection, and in situ flux disappearance observed in
  activity complexes.

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Title: Acoustic Mode Mixing in Sunspots
Authors: D'Silva, Sydney
1994ApJ...435..881D    Altcode:
  Sunspots are known to behave as efficient sinks of acoustic wave
  energy, absorbing almost 50% of the energy from the acoustic waves
  impinging on them. The physical properties of the flux tube that forms
  the sunspot are believed to be responsible for the absorption. We
  show that the absorption coefficient is closely connected not only
  to the physical properties of the sunspot flux tube, but also to its
  geometry beneath the surface. Using geometric acoustics, we show that
  a sunspot with a monolithic fluxtube structure mixes acoustic modes;
  energy in an incoming mode, at any horizontal wavenumber k<SUB>h</SUB>,
  gets dispersed into a wide range of wavenumbers. We also predict the
  observational signatures of mode mixing in sunspots.

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Title: Sunspot Rotation and the Field Strengths of Subsurface
    Flux Tubes
Authors: D'Silva, Sydney; Howard, Robert F.
1994SoPh..151..213D    Altcode:
  Observations show that bipolar magnetic regions (BMRs) have differential
  rotation profiles that are faster than the local Doppler velocity
  profiles by about 5%, and thep-spots in the growing sunspot groups
  rotate faster than thef-spots. Also, the smaller spots rotate
  faster than the larger ones. We present detailed observations of
  the functional dependence of the residual rotation of sunspots on
  the spot size of thep- andf-spots of growing sunspot groups. Through
  numerical calculations of the dynamics of thin flux tubes we show that
  flux loops emerging from the bottom of the convection zone acquire a
  rotation velocity faster than the local plasma velocities, in complete
  contradiction to what angular momentum conservation would demand. The
  sunspot flux tubes need not be anchored to regions rotating faster than
  the surface plasma velocities to exhibit the observed faster rotation;
  we show that this occurs through a subtle interplay between the forces
  of magnetic buoyancy and drag, coupled with the important role of the
  Coriolis force acting on rising flux tubes. The dynamics of rising
  flux tubes also explains the faster rotation of smaller sunspots; we
  show that there is no need to evoke a radial differential rotation and
  anchoring of smaller spots to faster rotating regions. The simulated
  differential rotation profiles of thep- andf-legs of flux loops emerging
  in the convection zone, with a latitudinal differential rotation and
  velocity contours constant along cones, mimic the observed profiles
  for growing sunspot groups only when the flux loops emerge radially
  and obey Joy's law. (The `legs' are defined to be the vertical part
  of the loops.) Also the rotation-size relation of growing sunspots
  is obeyed only by radially emerging loops which obey Joy's law. This
  constrains the fields at the bottom of the convection zone that are
  possible for producing the BMRs we see, to lie between 60 and 160 kG,
  which is in agreement with previous claims.

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Title: Magnetic Activity in Thick Accretion Disks and Associated
    Observable Phenomena. II. Flux Storage
Authors: D'Silva, Sydney; Chakrabarti, Sandip K.
1994ApJ...424..149D    Altcode: 1993astro.ph.11009D
  In Paper I (Chakrabarti &amp; D'Silva 1994), we have studied the
  conditions under which flux tubes are expelled from adiabatic thick
  accretion disks. In the present paper, we explore a few other models
  of thick disks, where flux tubes could be stored. We show that flux
  tubes with sufficiently weak fields are not expelled out if they move
  adiabatically inside an isothermal disk; they continue to oscillate
  around mean equipotential surfaces inside the disk. If the field in the
  flux tube is amplified due to the shear, they are eventually expelled
  away. We explore a "toy" model also, where the entropy increases outward
  from the center of the thick disk and find a similar behavior. Flux
  storage in the disk, as in the case of the Sun, in general, enhances
  the possibility of sustained magnetic activity and formation of coronae
  in the chimney region. The existence of coronae on the disk surface
  may explain the short-time variability in the spectra of blazars
  and the emission of energetic particles from active galactic nuclei
  and quasars. It may also supply matter to the cosmic jets through
  magnetized winds.

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Title: Magnetic Activity in Thick Accretion Disks and Associated
    Observable Phenomena. I. Flux Expulsion
Authors: Chakrabarti, Sandip K.; D'Silva, Sydney
1994ApJ...424..138C    Altcode: 1993astro.ph.11010C
  We study the dynamics of toroidal magnetic flux tubes, symmetric
  about the rotation axis, inside nonmagnetic thick accretion disks
  around black holes. We present model equations which include effects
  of gravity, centrifugal force, pressure gradient force, Coriolis
  force, drag, magnetic tension, and magnetic buoyancy. We solve them
  assuming the disk to be adiabatic. We show that under a wide range
  of parameters describing the size and the field strength, as well as
  angular momentum distribution inside the disk, buoyant flux tubes,
  either released on the equatorial plane or at the outer edge of the
  disk, can gather in the chimney-like openings near the axis, This
  behavior makes the chimneys magnetically most active and could shed
  light on the origin and acceleration of cosmic jets, as well as the
  variabilities observed in blazars.

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Title: Constraints on magnetic fields that produce sunspots
Authors: D'Silva, S.
1994smf..conf..136D    Altcode:
  No abstract at ADS

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Title: Limits on the Magnetic Field Strength at the Base of the
    Solar Convection Zone
Authors: D'Silva, Sydney; Howard, Robert F.
1993SoPh..148....1D    Altcode:
  Howard (1993) finds a relationship between the tilt angles of
  BMRs (Bipolar Magnetic Regions) and the separation between their
  leading and following polarities; the tilt angle increases with
  polarity separation. Here we present a more detailed analysis of
  this relationship and show that this effect constrains the strength
  of the magnetic field at the bottom of the convection zone to a value
  between 40 and 150 kG, which confirms the constraints put by D'Silva and
  Choudhuri (1993) based on Joy's law (the tilt-latitude relationship),
  through an entirely different approach.

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Title: Limits on magnetic fields that produce sunspots.
Authors: D'Silva, Sydney
1993BASI...21..371D    Altcode:
  The solar dynamo is believed to operate in a thin stable region at
  the bottom of the convection zone and the bipolar magnetic regions
  (BMRs) that we see on the surface are produced by magnetic flux
  tubes generated there. These flux tubes emerge as Omega shaped loops
  (Parker 1955, 1979) due to magnetic buoyancy, and the regions where
  they intersect the surface are called BMRs. These BMRs obey Joy's law
  (Hale et al. 1919; Wang &amp; Sheeley 1989, 1991; Howard 1992), which
  states that the line joining the two poles of BMR makes an angle with
  the latitudinal line, called the tilt, which increases with increase in
  latitude and the p-spot (preceding region of the BMR which is Westward)
  is closer to the equator. We give a theoretical model for these tilts
  (D'Silva &amp; Choudhuri 1993). We also show that if the BMRs produced
  by flux tubes emerging from the bottom of the convection zone have to
  exhibit the tilts measured by observations (Wang &amp; Sheeley 1989,
  1991; Howard 1992), then the field strength at the bottom of the
  convection zone has to lie between 60 and 160 kG. For fields stronger
  than 160 kG, magnetic buoyancy dominates over Coriolis force and the
  tilts produced are very small compared to the observed values. Whereas,
  for fields weaker than 60 kG, Coriolis force dominates over buoyancy
  and makes them emerge at very high latitudes, well above the typical
  sunspot latitudes.

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Title: A theoretical model for tilts of bipolar magnetic regions
Authors: D'Silva, S.; Choudhuri, A. R.
1993A&A...272..621D    Altcode:
  Joy's law (Hale et al. 1919) states that bipolar magnetic regions (BMRs)
  are inclined to the latitudinal line, with the p-spot (preceding spot)
  of the BMR closer to the equator and the tilt angle increasing with
  latitude. It is believed that the solar dynamo operates in the overshoot
  region just below the convection zone and the BMRs are produced by
  the flux loops rising from there due to magnetic buoyancy. These
  rising loops are expected to be twisted by the Coriolis force so that
  they eventually emerge on the solar surface with a tilt. We extend
  the numerical calculations of Choudhuri (1989) to study the tilts
  produced on the rising flux loops by the Coriolis force. We find
  that the theoretically calculated tilts match the observations only
  if the magnetic field of the flux loops lies in the range between 60
  and 160 kG. For such flux loops, the tilt has the correct magnitude
  and also varies correctly with the latitude. If the magnetic fields
  were stronger than 160 kG, then Coriolis force is much weaker than
  magnetic buoyancy and is only able to produce tilts which are very
  small in overall magnitude, though they still vary correctly with
  latitude. On the other hand, if the fields were smaller than 60 kG, then
  the Coriolis force would have been so overpowering that the flux loops
  would move parallel to the rotation axis as found earlier (Choudhuri
  1989). Such flux loops appear only in high latitudes and do not obey
  Joy's law. On changing the drag on the flux tube, these conclusions
  are not changed. If we change the footpoint separation of the flux
  loop, then we find that magnetic tension may halt the rise of the flux
  loop if the footpoint separation is below a critical value. However,
  for flux tubes which are able to reach the surface, the range from 60
  to 160 kG for the magnetic field still approximately holds. Thus our
  calculations seem to rule out either equipartition fields (about 10 kG)
  or very strong megagauss fields.

---------------------------------------------------------
Title: Can Equipartition Fields Produce the Tilts of Bipolar Magnetic
    Regions?
Authors: D'Silva, Sydney
1993ApJ...407..385D    Altcode:
  The effect of turbulence on the nonaxisymmetric flux rings of
  equipartition field strength in bipolar magnetic regions is studied
  on the basis of the small-scale momentum exchange mechanism and the
  giant cell drag combined with the Kelvin-Helmholtz drag mechanism. It
  is shown that the giant cell drag and small-scale momentum exchange
  mechanism can make equipartition flux loops emerge at low latitudes,
  in addition to making them exhibit the observed tilts. However, the
  sizes of the flux tubes have to be restricted to a couple of hundred
  kilometers. An ad hoc constraint on the footpoints of the flux loops
  is introduced by not letting them move in the phi direction, and it
  is found that equipartition fields of any size can be made to emerge
  at sunspot latitudes with the observed tilts by suitably adjusting
  the footpoint separations.

---------------------------------------------------------
Title: Dynamics of Flux Tubes in Thick Accretion Disks
Authors: D'Silva, Sydney; Chakrabarti, Sandip K.
1993NYASA.688..726D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Dynamics of Magnetic Flux Tubes in Thick Accretion Disks in
    AGNs and Associated Observational Effects
Authors: Chakrabarti, S. K.; D'Silva, S.
1992AAS...181.2905C    Altcode: 1992BAAS...24.1166C
  We study the dynamics of the magnetic flux tubes in the thick accretion
  disks around black holes. It is shown that due to strong pressure
  gradient force inside the disk, buoyant flux tubes predominantly gather
  in the openings near to the axis, known as funnels, from where the
  cosmic radio jets are believed to be originated. We discuss various
  observational signatures of such behaviour of the flux tubes, such as
  microvariabilities in Blazars, particle acceleration, and jet formation
  close to the funnel.

---------------------------------------------------------
Title: Can sunspots be produced by equipartition magnetic fields
    residing at the bottom of the convection zone;?
Authors: D'Silva, Sydney
1992AAS...181.9401D    Altcode: 1992BAAS...24.1269D
  Joy's law states that the line joining the two poles of a bipolar
  magnetic region (BMR) makes an angle with the latitudinal line, called
  the tilt, which increases with increase in latitude. If the solar
  dynamo operates at the bottom of the convection zone and the BMRs
  on the surface are produced by the fields generated there, then they
  should obey Joy's law. We give a theoretical model for these tilts,
  and show that the observations severely constrain the field strength
  at the bottom of the convection zone between 60 and 160 kG. For fields
  stronger than 160 kG, magnetic buoyancy dominates over Coriolis force
  and the tilts produced are very small compared to the observed. Whereas,
  for fields weaker than 60 kG, Coriolis force dominates over buoyancy and
  makes them emerge at very high latitudes, well above the typical sunspot
  latitudes. Fields above 60 kG are an order of magnitude stronger than
  the fields that can be in energy equipartition with the velocity fields
  at the bottom of the convection zone. Such strong fields will severely
  inhibit dynamo action. In addition, we do not know how a dynamo could
  produce such a strong field. We propose a couple of mechanisms by which
  equipartition fields could possibly produce BMRs with the observed
  tilts: (a) Giant cells, if they exist, can dominate over Coriolis
  force and drag these equipartition fields in their updraughts, (b)
  Small scale turbulence can interact with the flux tube and exchange
  momentum with it, thus suppressing Coriolis force and making them
  emerge at the sunspot latitudes. We show that these two mechanisms
  can make equipartition fields emerge at the sunspot latitudes with
  the proper tilts, provided their sizes are smaller than a couple of
  hundred kilometers. We also show that special anchoring mechanisms
  have to be invoked in order to make equipartition fields of any size
  produce BMRs with the observed tilts.

---------------------------------------------------------
Title: Joy's Law and Limits on the Magnetic Field Strength at the
    Bottom of the Convection Zone
Authors: D'Silva, Sydney
1992ASPC...27..168D    Altcode: 1992socy.work..168D
  No abstract at ADS

---------------------------------------------------------
Title: The Effect of Kelvin-Helmholtz Instability on Rising Flux
    Tubes in the Convection Zone
Authors: D'Silva, Sydney Z.; Choudhuri, Arnab R.
1991SoPh..136..201D    Altcode:
  If the solar dynamo operates at the bottom of the convection zone,
  then the magnetic flux created there has to rise to the surface. When
  the convection zone is regarded as passive, the rising flux is
  deflected by the Coriolis force to emerge at rather high latitudes,
  poleward of typical sunspot zones (Choudhuri and Gilman, 1987;
  Choudhuri, 1989). Choudhuri and D'Silva (1990) included the effects
  of convective turbulence on the rising flux through (a) giant cell
  drag and (b) momentum exchange by small-scale turbulence. The momentum
  exchange mechanism could enable flux tubes of radii not more than a few
  hundred km to emerge radially at low latitudes, but the giant cell drag
  mechanism required unrealistically small flux tube radii (a few meters
  for a reasonable giant cell upflow) to counteract the Coriolis force. We
  now include the additional effect of Kelvin-Helmholtz instability in a
  symmetrical flux ring caused by the azimuthal flow induced during its
  rise. The azimuthal flow crosses the threshold for the instability only
  if there is a giant cell upflow to drag the flux tubes appreciably. In
  the absence of such a drag, as in the case of a passive convection
  zone or in the case of momentum exchange by small-scale turbulence, the
  azimuthal velocity never becomes large enough to cause the instability,
  leaving the results of the previous calculations unaltered. The giant
  cell drag, aided by Kelvin-Helmholtz instability, however, becomes
  now a viable mechanism for curbing the Coriolis force - 10<SUP>4</SUP>
  G flux tubes with radii of a few hundred km being dragged radially by
  upflows of 70 m s<SUP>-1</SUP>.

---------------------------------------------------------
Title: Influence of turbulence on rising flux tubes in the solar
    convection zone
Authors: Choudhuri, A. R.; D'Silva, S.
1990A&A...239..326C    Altcode:
  The role of turbulence in facilitating the flux tubes generated at low
  solar latitudes at the bottom of the convection zone to emerge at the
  typical sunspot latitudes is investigated. It is found that large scale
  turbulence on the scale of the giant cells cannot dominate the Coriolis
  force, since such domination would require either an unreasonably large
  updraft velocity in the giant cells or an unreasonably small flux tube
  radii. On the other hand, small-scale turbulence can suppress the
  Coriolis force by exchanging angular momentum between the flux tube
  and the surroundings, provided the flux tubes have radii smaller than
  a few hundred km.

---------------------------------------------------------
Title: Effect of Turbulence on Emerging Magnetic Flux Tubes in the
    Convection Zone
Authors: D'Silva, S.; Choudhuri, A. R.
1990IAUS..142...60D    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: VLA Observations of an Optically Deep Sample of Molonglo
Quasars: Aspect Dependence of the Optical Continuum
Authors: Kapahi, V. K.; Subrahmanya, C. R.; D'Silva, S.
1989IAUS..134..531K    Altcode:
  No abstract at ADS