arXiv subjects

H. M. Antia

Publications and source records attributed to H. M. Antia.

At least 91 records · Page 5Linked to original sources

arXiv2001

Global oscillation analysis of solar neutrino data with helioseismically constrained fluxes

A seismic model for the Sun calculated using the accurate helioseismic data predicts a lower $^{8}{B}$ neutrino flux as compared to the standard solar model (SSM). However, there persists a discrepancy between the predicted and measured neutrino fluxes and it seems necessary to invoke neutrino oscillations to explain the measurements. In this work, we have performed a global, unified oscillation analysis of the latest solar neutrino data (including the results of SNO charged current rate) using the seismic model fluxes as theoretical predictions. We determine the best-fit values of the neutrino oscillation parameters and the $χ^2_{\mathrm min}$ for both $ν_e-ν_{\mathrm active}$ and $ν_e -ν_{\mathrm sterile}$ cases and present the allowed parameter regions in the $Δm^2 - \tan^2 θ$ plane for $ν_e-ν_{\mathrm active}$ transition. The results are compared with those obtained using the latest SSM by Bahcall and his collaborators.

hep-ph↗

arXiv2001

Temporal variations of solar rotation rate at high latitudes

Frequency splitting coefficients from Global Oscillation Network Group (GONG) and Michelson Doppler Imager (MDI) observations covering the period 1995--2001 are used to study temporal variations in the solar rotation rate at high latitudes. The torsional oscillation pattern in the Sun is known to penetrate to a depth of about $0.1R_\odot$ with alternate bands of faster and slower rotating plasma. At lower latitudes the bands move towards equator with time. At higher latitudes, however, the bands appear to move towards the poles. This is similar to the observed pole-ward movement of large scale magnetic fields at high latitudes. This also supports theoretical results of pole-ward moving bands at high latitudes in some mean field dynamo models. The polar rotation rate is found to decrease between 1995 and 1999 after which it has started increasing.

astro-ph↗

arXiv2001

Ring diagram analysis of the characteristics of solar oscillation modes in active regions

The presence of intense magnetic fields in and around sunspots is expected to modify the solar structure and oscillation frequencies. Applying the ring diagram technique to data from the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO), we analyze the characteristics of high-degree f and p modes near active regions and compare them with the characteristics of the modes in quiet regions. As expected from earlier results, the f- and p-mode frequencies of high degree modes are found to be significantly larger in magnetically active regions. In addition, we find that the power in both f and p modes is lower in active regions, while the widths of the peaks are larger, indicating smaller lifetimes. We also find that the oscillation modes are more asymmetric in active regions than those in quiet regions, indicating that modes in active regions are excited closer to the surface. While the increase in mode frequency is monotonic in frequency, all other characteristics show more complex frequency dependences.

astro-ph↗

arXiv2001

Seismic study of stellar convective cores

It has been shown that a discontinuity in the derivatives of the sound speed at the edge of the convective regions inside a star gives rise to a characteristic oscillatory signal in the frequencies of stellar oscillations. This oscillatory signal has been suggested as a means to study the base of the outer convection zone in low mass stars and possibly the outer edge of the convective core in high mass stars. Using stellar models we show that because of a phenomenon similar to aliasing in Fourier transform, it may not be possible to use this signal to detect the convective core. Nevertheless, it may be possible to determine the size of convective cores using the frequency separation ν_{n+1,l}-ν_{n,l}.

astro-ph↗

arXiv2001

Seismic detection of stellar tachoclines

Helioseismic inversions for the rotation rate have established the presence of a tachocline near the base of the solar convection zone. We show that the tachocline produces a characteristic oscillatory signature in the splitting coefficients of low degree modes, which could be observed on distant stars. Using this signature it may be possible to determine the characteristics of the tachocline using only low degree modes. The limitations of this technique in terms of observational uncertainties are discussed, to assess the possibility of detecting tachoclines on distant stars.

astro-ph↗

arXiv2001

A study of possible temporal and latitudinal variations in the properties of the solar tachocline

Temporal variations of the structure and the rotation rate of the solar tachocline region are studied using helioseismic data from the Global Oscillation Network Group (GONG) and the Michelson Doppler Imager (MDI) obtained during the period 1995--2000. We do not find any significant temporal variation in the depth of the convection zone, the position of the tachocline or the extent of overshoot below the convection zone. No systematic variation in any other properties of the tachocline, like width, etc., is found either. Possibility of periodic variations in these properties is also investigated. Time-averaged results show that the tachocline is prolate with a variation by about 0.02R_sun in its position. The depth of the convection zone or the extent of overshoot does not show any significant variation with latitude.

astro-ph↗

arXiv2000

The Sun's acoustic asphericity and magnetic fields in the solar convection zone

The observed splittings of solar oscillation frequencies can be employed to separate the effects of internal solar rotation and to estimate the contribution from a large-scale magnetic field or any latitude-dependent thermal perturbation inside the Sun. The surface distortion estimated from the rotation rate in the solar interior is found to be in good agreement with the observed oblateness at solar surface. After subtracting out the estimated contribution from rotation, there is some residual signal in the even splitting coefficients, which may be explained by a magnetic field of approximately 20 kG strength located at a depth of 30000 km below the surface or an equivalent aspherical thermal perturbation. An upper limit of 300 kG is derived for a toroidal field near the base of the convection zone.

astro-ph↗

arXiv2000

Temporal variations of the rotation rate in the solar interior

The temporal variations of the rotation rate in the solar interior are studied using frequency splittings from Global Oscillations Network Group (GONG) data obtained during the period 1995-99. We find alternating latitudinal bands of faster and slower rotation which appear to move towards the equator with time - similar to the torsional oscillations seen at the solar surface. This flow pattern appears to persist to a depth of about 0.1R_sun and in this region its magnitude is well correlated with solar activity indices. We do not find any periodic or systematic changes in the rotation rate near the base of the convection zone.

astro-ph↗

arXiv2000

Possible solar cycle variations in the convection zone

Using data from the Global Oscillations Network Group (GONG) that covers the period from 1995 to 1998 we study the change in frequencies of solar oscillations with solar activity. From these frequencies we attempt to determine any possible variation in solar structure with solar activity. We do not find any evidence of a change in the convection zone depth or extent of overshoot below the convection zone during the solar cycle.

astro-ph↗

arXiv2000

Solar cycle variation in solar f-mode frequencies and radius

Using data from the Global Oscillation Network Group (GONG) covering the period from 1995 to 1998, we study the change with solar activity in solar f-mode frequencies. The results are compared with similar changes detected from the Michelson Doppler Imager (MDI) data. We find variations in f-mode frequencies which are correlated with solar activity indices. If these changes are due to variation in solar radius then the implications are that the solar radius decreases by about 5 km from minimum to maximum activity.

astro-ph↗

arXiv2000

Solar cycle variations of large scale flows in the Sun

Using data from the Michelson Doppler Imager (MDI) instrument on board the Solar and Heliospheric Observatory (SOHO), we study the large-scale velocity fields in the outer part of the solar convection zone using the ring diagram technique. We use observations from four different times to study possible temporal variations in flow velocity. We find definite changes in both the zonal and meridional components of the flows. The amplitude of the zonal flow appears to increase with solar activity and the flow pattern also shifts towards lower latitude with time.

astro-ph↗

arXiv1999

Effect of asymmetry in peak profiles on solar oscillation frequencies

Most helioseismic analyses are based on solar oscillations frequencies obtained by fitting symmetric peak profiles to the power spectra. However, it has now been demonstrated that the peaks are not symmetric. In this work we study the effects of asymmetry of the peak profiles on the solar oscillations frequencies of p-modes for low and intermediate degrees. We also investigate how the resulting shift in frequencies affects helioseismic inferences.

astro-ph↗

arXiv1999

Large scale flows in the solar interior: Effect of asymmetry in peak profiles

Ring diagram analysis can be used to study large scale velocity fields in the outer part of the solar convection zone. All previous works assume that the peak profiles in the solar oscillation power spectrum are symmetric. However, it has now been demonstrated that the peaks are not symmetric. In this work we study how the explicit use of asymmetric peak profiles in ring-diagram analysis influences the estimated velocity fields. We find that the use of asymmetric profiles leads to significant improvement in the fits, but the estimated velocity fields are not substantially different from those obtained using a symmetric profile to fit the peaks. The resulting velocity fields are compared with those obtained by other investigators.

astro-ph↗

arXiv1999

Limits on the proton-proton reaction cross-section from helioseismology

Primary inversions of solar oscillation frequencies coupled with the equations of thermal equilibrium and other input physics, enable us to infer the temperature and hydrogen abundance profiles inside the Sun. These profiles also help in setting constraints on the input physics that is consistent with the accurately measured oscillation frequencies data. Helioseismic limits on the cross-section of proton-proton nuclear reaction as a function of heavy element abundance in the solar core are derived. We demonstrate that it is not possible to infer the heavy element abundance profile, in addition to temperature and hydrogen abundance profiles, with the helioseismic constraints.

astro-ph↗

arXiv1999

High frequency and high wavenumber solar oscillations

We determine the frequencies of solar oscillations covering a wide range of degree (100< l <4000) and frequency (1.5 <ν<10 mHz) using the ring diagram technique applied to power spectra obtained from MDI (Michelson Doppler Imager) data. The f-mode ridge extends up to degree of approximately 3000, where the line width becomes very large, implying a damping time which is comparable to the time period. The frequencies of high degree f-modes are significantly different from those given by the simple dispersion relation ω^2=gk. The f-mode peaks in power spectra are distinctly asymmetric and use of asymmetric profile increases the fitted frequency bringing them closer to the frequencies computed for a solar model.

astro-ph↗

arXiv1998

Ring diagram analysis of near-surface flows in the Sun

Ring diagram analysis of solar oscillation power spectra obtained from MDI data is carried out to study the velocity fields in the outer part of the solar convection zone. The three dimensional power spectra are fitted to a model which has a Lorentzian profile in frequency and which includes the advection of the wave front by horizontal flows, to obtain the two components of the sub-surface flows as a function of the horizontal wave number and radial order of the oscillation modes. This information is then inverted using OLA and RLS methods to infer the variation in horizontal flow velocity with depth. The average rotation velocity at different latitudes obtained by this technique agrees reasonably with helioseismic estimates made using frequency splitting data. The shear layer just below the solar surface appears to consist of two parts with the outer part up to a depth of 4 Mm, where the velocity gradient does not show any reversal up to a latitude of 60 degrees. In the deeper part the velocity gradient shows reversal in sign around a latitude of 55 degrees. The zonal flow velocities inferred in the outermost layers appears to be similar to those obtained by other measurements. A meridional flow from equator polewards is found. It has a maximum amplitude of about 30 m/s near the surface and the amplitude is nearly constant in the outer shear layer.

astro-ph↗

arXiv1998

Can Naked Singularities Yield Gamma Ray Bursts?

Gamma-ray bursts are believed to be the most luminous objects in the Universe. There has been some suggestion that these arise from quantum processes around naked singularities. The main problem with this suggestion is that all known examples of naked singularities are massless and hence there is effectively no source of energy. It is argued that a globally naked singularity coupled with quantum processes operating within a distance of the order of Planck length of the singularity will probably yield energy burst of the order of M_pc^2\approx2\times 10^{16} ergs, where M_p is the Planck mass.

gr-qc↗

arXiv1998

Ring diagram analysis of velocity fields within the solar convection zone

Ring diagram analysis of solar oscillation power spectra obtained from MDI data is performed to study the velocity fields within the solar convection zone. The three dimensional power spectra are fitted to a model with a Lorentzian profile in frequency and includes the advection of the wave front by horizontal flows to obtain the two horizontal components of flows as a function of the horizontal wave number and radial order of the oscillation modes. This information is then inverted using the OLA and RLS techniques to infer the variation in flow velocity with depth. The resulting velocity fields yield the mean rotation velocity at different latitudes which agrees reasonably with helioseismic estimates. The zonal flow inferred in the outermost layers also appears to be in agreement with other measurements. A meridional flow from equator polewards is found to have an amplitude of about 25 m/s near the surface and the amplitude appears to increase with depth.

astro-ph↗