Author name code: dsilva ADS astronomy entries on 2022-09-14 author:"D'Silva, Sydney" ------------------------------------------------------------------------ Title: Equivalence between Duvall's Law and the Time-Distance Curve Authors: D'Silva, S. Bibcode: 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. Title: Time-Distance Helioseismology at High Frequencies Authors: D'Silva, S. Bibcode: 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. Title: A Note on Helioseismic Tomography Authors: D'Silva, S. Bibcode: 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. Title: The Meaning of Travel Time in Time-Distance Helioseismology Authors: D'Silva, S. Bibcode: 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. Title: ``Dispersion'' in Time-Distance Helioseismology Authors: D'Silva, S. Bibcode: 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. Title: Computing Travel Time in Time-Distance Helioseismology Authors: D'Silva, S. Bibcode: 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:

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.

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. Title: Time-Distance Helioseismology: Challenges in Computing Travel Time Authors: D'Silva, Sydney Bibcode: 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. Title: Sounding the Sun's Chromosphere Authors: Jefferies, S. M.; Osaki, Y.; Shibahashi, H.; Harvey, J. W.; D'Silva, S.; Duvall, T. L., Jr. Bibcode: 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. Title: Helioseismic Tomography Authors: D'Silva, Sydney; Duvall, Thomas L., Jr.; Jefferies, Stuart M.; Harvey, John W. Bibcode: 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. Title: Theoretical Foundations of Time-Distance Helioseismology Authors: D'Silva, Sydney Bibcode: 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.

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.

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. Title: Measuring the Solar Internal Rotation Using Time-Distance Helioseismology. I. The Forward Approach Authors: D'Silva, Sydney Bibcode: 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. Title: Downflows under sunspots detected by helioseismic tomography Authors: Duvall, T. L.; D'Silva, S.; Jefferies, S. M.; Harvey, J. W.; Schou, J. Bibcode: 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 proposed1 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 helioseismology2 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-1, 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. Title: Flux Retraction and Recycling: Negative Buoyancy--induced Oscillations as an Alternative to Parker's Thermal Relaxation Oscillations Authors: D'Silva, Sydney Bibcode: 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. 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. Bibcode: 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. Title: Brunt-Vaeisaelae Growth Rate and the Radial Emergence of Equipartition Fields Authors: D'Silva, S. Bibcode: 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 N2; 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 N2 is shown to play a negligible role. However, in an adiabatic rise the role of absolute value of N2 is dominant; if absolute value of N2 is larger than roughly 10-12/sq sec in the lower CZ, magnetic buoyancy is shown to rise exponentially as the flux tube emerges. Further if absolute value of N2 greater than 4 x 10-11/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 N2 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 N2 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. Title: Measuring Local Sub-Surface Flows Using Time-Distance Helioseismology Authors: D'Silva, Sydney Bibcode: 1995SPD....26..404D Altcode: 1995BAAS...27..955D No abstract at ADS Title: Time-Distance Helioseismology in the Vicinity of Sunspots Authors: D'Silva, Sydney; Duvall, Thomas L., Jr. Bibcode: 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. Title: Acoustic Mode-Mixing in Sunspots Authors: D'Silva, S. Bibcode: 1995ASPC...76..276D Altcode: 1995gong.conf..276D No abstract at ADS Title: Flux Retraction and Recycling Authors: D'Silva, Sydney Bibcode: 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. Title: Acoustic Mode Mixing in Sunspots Authors: D'Silva, Sydney Bibcode: 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 kh, gets dispersed into a wide range of wavenumbers. We also predict the observational signatures of mode mixing in sunspots. Title: Sunspot Rotation and the Field Strengths of Subsurface Flux Tubes Authors: D'Silva, Sydney; Howard, Robert F. Bibcode: 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. Title: Magnetic Activity in Thick Accretion Disks and Associated Observable Phenomena. II. Flux Storage Authors: D'Silva, Sydney; Chakrabarti, Sandip K. Bibcode: 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. Title: Magnetic Activity in Thick Accretion Disks and Associated Observable Phenomena. I. Flux Expulsion Authors: Chakrabarti, Sandip K.; D'Silva, Sydney Bibcode: 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. Title: Constraints on magnetic fields that produce sunspots Authors: D'Silva, S. Bibcode: 1994smf..conf..136D Altcode: No abstract at ADS Title: Limits on the Magnetic Field Strength at the Base of the Solar Convection Zone Authors: D'Silva, Sydney; Howard, Robert F. Bibcode: 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. Title: Limits on magnetic fields that produce sunspots. Authors: D'Silva, Sydney Bibcode: 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. Title: A theoretical model for tilts of bipolar magnetic regions Authors: D'Silva, S.; Choudhuri, A. R. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 - 104 G flux tubes with radii of a few hundred km being dragged radially by upflows of 70 m s-1. Title: Influence of turbulence on rising flux tubes in the solar convection zone Authors: Choudhuri, A. R.; D'Silva, S. Bibcode: 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. Bibcode: 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. Bibcode: 1989IAUS..134..531K Altcode: No abstract at ADS