arXiv subjects

M. J. Thompson

Publications and source records attributed to M. J. Thompson.

50 records · Page 3Linked to original sources

arXiv2007

The Effect of Abnormal Granulation on Acoustic Wave Travel Times and Mode Frequencies

Observations indicate that in plage areas (i.e. in active regions outside sunspots) acoustic waves travel faster than in quiet sun, leading to shortened travel times and higher p-mode frequencies. While it is clear that the ultimate cause of any difference between quiet sun and plage is the presence of magnetic fields of order 100 G in the latter, the mechanism by which the magnetic field exerts its influence has not yet been conclusively identified. One possible such mechanism is suggested by the observation that granular motions in plage areas tend to be slightly ``abnormal'', dampened compared to quiet sun. In this paper we consider the effect that abnormal granulation observed in active regions should have on the propagation of acoustic waves. Any such effect is found to be limited to a shallow surface layer where sound waves propagate nearly vertically. The magnetically suppressed turbulence implies higher sound speeds, leading to shorter travel times. This time shift Dt is independent of the travel distance, while it shows a characteristic dependence on the assumed plage field strength. As a consequence of the variation of the acoustic cutoff with height, Dt is expected to be significantly higher for higher frequency waves within the observed regime of 3-5 mHz. The lower group velocity near the upper reflection point further leads to an increased envelope time shift, as compared to the phase shift. $p$-mode frequencies in plage areas are increased by a corresponding amount, Dnu/nu = nu*Dt. These characteristics of the time and frequency shifts are in accordance with observations. The calculated overall amplitude of the time and frequency shifts are comparable to, but still significantly (factor of 2 to 5) less than suggested by measurements.

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arXiv2006

Sensitivity of time-distance helioseismic measurements to spatial variation of oscillation amplitudes I. Observations and a numerical model

It is well known that the observed amplitude of solar oscillations is lower in sunspots than in quiet regions of the Sun. We show that this local reduction in oscillation amplitudes combined with the phase-speed filtering procedure in time-distance helioseismic analyses could be a source of systematic errors in the range of 5 - 40% in the measured travel-time anomalies of acoustic waves around sunspots. Removing these travel time artifacts is important for correctly inferring the subsurface structure of sunspots. We suggest an empirical correction procedure and illustrate its usage for a small sunspot. This work utilizes data from MDI/SOHO.

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arXiv2005

Seismic analysis of the second ionization region of helium in the Sun: I. Sensitivity study and methodology

The region of the second ionization of helium in the Sun is a narrow layer near the surface. Ionization induces a local change of the adiabatic exponent $Γ_1$, which produces a characteristic signature in the frequencies of p-modes. By adapting the method developed by (Monteiro et al. 1994), we propose a methodology for determining the properties of this region by studying such a signature in the frequencies of oscillation. Using solar data we illustrate how the signal from the helium ionization zone can be isolated. Using solar models with different physics -- theory of convection, equation of state and low temperature opacities -- we establish how the characteristics of the signal depend on the different aspects contributing to the structure in the ionization layer. We further discuss how the method can be used to measure the solar helium abundance in the envelope and to constrain the physics affecting this region of the Sun. The potential usefulness of the method we propose is shown. It may complement other inversion methods developed to study the solar structure and to determine the envelope helium abundance.

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arXiv2002

On Variation of the Latitudinal Structure of the Solar Convection Zone

The latitudinal sound-speed structure of the Sun's convection zone gives insight into the physical processes occurring there, specifically the cellular convection and possibly the presence of magnetic fields. Using helioseismic data from the GONG network and MDI instrument on SOHO, we map the latitudinal acoustic structure of the convection zone from 1995 to 2002. The temporally averaged structure confirms previous findings of an excess in sound speed at the $10^{-4}$ level at 60 degrees latitude. There also appear to be some variations with time, with the peak in sound-speed asphericity at 60 degrees growing towards the maximum of solar activity according to the MDI data. However, we present some evidence that such variation may be associated with instrumental variation between the epochs before and after SOHO was temporarily lost in 1998. Nonetheless, some genuine temporal variation may be present, and we discuss the possible physical causes of that.

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arXiv2001

Rotation of the solar interior

Helioseismology has allowed us to infer the rotation in the greater part of the solar interior with high precision and resolution. The results show interesting conflicts with earlier theoretical expectations, indicating that the the Sun is host to complex dynamical phenomena, so far hardly understood. This has important consequences for our ideas about the evolution of stellar rotation, as well as for models for the generation of the solar magnetic field. Here we provide an overview of our current knowledge about solar rotation, much of it obtained from observations from the SOHO spacecraft, and discuss the broader implications.

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arXiv2001

Asteroseismic Inference for Solar-Type Stars

The oscillation spectra of solar-type stars may in the not-too- distant future be used to constrain certain properties of the stars. The CD diagram of large versus small frequency separations is one of the powerful tools available to infer the properties - including perhaps masses and ages - of stars which display a detectable spectrum of oscillation. Also, the border of a convective region in a solar-type star gives rise to a characteristic periodic signal in the star's low-degree p-mode frequencies. Such a signature contains information about the location and nature of the transition between convective and non-convective regions in the star. In this work we address some of the uncertainties associated with the direct use of the CD diagram to evaluate the mass and age of the star due to the unknown contributions that make the stars different from the evolutionary models used to construct our reference grid. We also explore the possibility of combining an amplitude versus period diagram with the CD diagram to evaluate the properties of convective borders within solar-type stars.

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arXiv2001

SOLA inversions for the core structure of solar-type stars

The Subtractive Optimally Localized Averages (SOLA) method, developed and extensively used in helioseismology, is applied to artificial data to obtain measures of the sound speed inside a solar-type star. In contrast to inversion methods which fit models to some aspect of the data, methods such as SOLA provide an honest assessment of what can truly be resolved using seismic data, without introducing additional assumptions such as that the space of admissible stellar models is small. The resulting measures obtained from SOLA inversion can subsequently be used to eliminate putative stellar models. Here we present results of experiments to test the reliability of SOLA inferences using solar models and models of solar-type stars.

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arXiv2001

On Inverting Asteroseismic Data

Some issues of inverting asteroseismic frequency data are discussed, including the use of model calibration and linearized inversion. An illustrative inversion of artificial data for solar-type stars, using least-squares fitting of a small set of basis functions, is presented. A few details of kernel construction are also given.

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arXiv2001

The subsurface radial gradient of solar angular velocity from MDI f-mode observations

We report quantitative analysis of the radial gradient of solar angular velocity at depths down to about 15 Mm below the solar surface for latitudes up to 75 degree using the Michelson Doppler Imager (MDI) observations of surface gravity waves (f modes) from the Solar and Heliospheric Observatory (SoHO). A negative outward gradient of around -400 nHz/R, equivalent to logarithmic gradient of the rotation frequency with respect to radius which is very close to -1, is found to be remarkably constant between the equator and 30 degree of latitude. Above 30 degree it decreases in absolute magnitude to a very small value at around 50 degree. At higher latitudes the gradient may reverse its sign: if so this reversal takes place in a thin layer extending only 5 Mm beneath the visible surface, as evidenced by the most superficial modes (with degrees l>250). The signature of the torsional oscillations is seen in this layer, but no other significant temporal variations of the gradient and value of the rotation rate there are found.

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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.

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arXiv2000

Deeply penetrating banded zonal flows in the solar convection zone

Helioseismic observations have detected small temporal variations of the rotation rate below the solar surface corresponding to the so-called `torsional oscillations' known from Doppler measurements of the surface. These appear as bands of slower and faster than average rotation moving equatorward. Here we establish, using complementary helioseismic observations over four years from the GONG network and from the MDI instrument on board SOHO, that the banded flows are not merely a near-surface phenomenon: rather they extend downward at least 60 Mm (some 8% of the total solar radius) and thus are evident over a significant fraction of the nearly 200 Mm depth of the solar convection zone.

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arXiv1999

Effects of line asymmetries on the determination of solar internal structure

Despite the strong evidence that the peaks in the spectrum of solar oscillations are asymmetric, most determinations of mode frequencies have been based on fits of symmetric Lorentzian profiles to the Fourier or power spectra of oscillation time strings. The systematic errors resulting from neglecting the line asymmetry could have serious effects on inversions for the solar internal structure and rotation. Here we analyse artificial data based on simple models of the intrinsic line asymmetry, using GONG mode parameters with asymmetries found by one of us (Rosenthal \cite{rosenthal2}). By fitting Lorentzians to the resulting spectra, we estimate the likely properties of the errors introduced in the frequencies. We discuss whether these frequency shifts have a form similar to the near-surface layers uncertainties and are therefore suppressed in the solar structure inversion. We also estimate directly their contribution, if any, in the solar sound-speed and density determinations using the SOLA technique.

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arXiv1997

Solar internal sound speed as inferred from combined BiSON and LOWL oscillation frequencies

Observations of the Sun with the LOWL instrument provide a homogeneous set of solar p-mode frequencies from low to intermediate degree which allows one to determine the structure of much of the solar interior avoiding systematic errors that are introduced when different data sets are combined, i.e., principally the effects of solar cycle changes on the frequencies. Unfortunately, the LOWL data set contains very few of the lowest-degree modes, which are essential for determining reliably the structure of the solar core - in addition, these lowest-degree data have fairly large associated uncertainties. However, observations made by the Birmingham Solar-Oscillations Network (BiSON) in integrated sunlight provide high-accuracy measurements of a large number of low-degree modes. In this paper we demonstrate that the low-degree mode set of the LOWL data can be successfully combined with the more accurate BiSON data, provided the observations are contemporaneous for those frequencies where the solar-cycle-induced effects are important. We show that this leads to a factor-of-two decrease in the error on the inferred sound speed in the solar core. We find that the solar sound speed is higher than in solar models for r < 0.2Rsun. The density of the solar core is, however, lower than that in solar models.

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arXiv1995

A modified R1 X R1 method for helioseismic rotation inversions

We present an efficient method for two dimensional inversions for the solar rotation rate using the Subtractive Optimally Localized Averages (SOLA) method and a modification of the R1 X R1 technique proposed by Sekii (1993). The SOLA method is based on explicit construction of averaging kernels similar to the Backus-Gilbert method. The versatility and reliability of the SOLA method in reproducing a target form for the averaging kernel, in combination with the idea of the R1 X R1 decomposition, results in a computationally very efficient inversion algorithm. This is particularly important for full 2-D inversions of helioseismic data in which the number of modes runs into at least tens of thousands.

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