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