arXiv subjects

S. P. Rajaguru

Publications and source records attributed to S. P. Rajaguru.

43 records · Page 3Linked to original sources

arXiv2008

Impact of Locally Suppressed Wave sources on helioseismic travel times

Wave travel-time shifts in the vicinity of sunspots are typically interpreted as arising predominantly from magnetic fields, flows, and local changes in sound speed. We show here that the suppression of granulation related wave sources in a sunspot can also contribute significantly to these travel-time shifts, and in some cases, an asymmetry between in and outgoing wave travel times. The tight connection between the physical interpretation of travel times and source-distribution homogeneity is confirmed. Statistically significant travel-time shifts are recovered upon numerically simulating wave propagation in the presence of a localized decrease in source strength. We also demonstrate that these time shifts are relatively sensitive to the modal damping rates; thus we are only able to place bounds on the magnitude of this effect. We see a systematic reduction of 10-15 seconds in $p$-mode mean travel times at short distances ($\sim 6.2$ Mm) that could be misinterpreted as arising from a shallow (thickness of 1.5 Mm) increase ($\sim$ 4%) in the sound speed. At larger travel distances ($\sim 24$ Mm) a 6-13 s difference between the ingoing and outgoing wave travel times is observed; this could mistakenly be interpreted as being caused by flows.

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arXiv2008

Separating surface magnetic effects in sunspot seismology: new time-distance helioseismic diagnostics

Time-distance helioseismic measurements in surface- and deep-focus geometries for wave-paths that distinguish surface magnetic contributions from those due to deeper perturbations beneath a large sunspot are presented and analysed. Travel times showing an increased wave speed region extending down to about 18 Mm beneath the spot are detected in deep-focus geometry that largely avoids use of wave field within the spot. Direction (in- or out-going wave) and surface magnetic field (or focus depth) dependent changes in frequency dependence of travel times are shown and identified to be signatures of wave absorption and conversion in near surface layers rather than that of shallowness of sunspot induced perturbations.

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arXiv2007

Contamination by Surface Effects of Time-distance Helioseismic Inversions for Sound Speed Beneath Sunspots

Using Doppler velocity data from the SOI/MDI instrument onboard the SoHO spacecraft, we do time-distance helioseismic inversions for sound-speed perturbations beneath 16 sunspots observed in high-resolution mode. We clearly detect ring-like regions of enhanced sound speed beneath most sunspot penumbrae, extending from near the surface to depths of about 3.5 Mm. Due to their location and dependence on frequency bands of p-modes used, we believe these rings to be artifacts produced by a surface signal probably associated with the sunspot magnetic field.

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arXiv2006

Radiative transfer effects on Doppler measurements as sources of surface effects in sunspot seismology

We show that the use of Doppler shifts of Zeeman sensitive spectral lines to observe wavesn in sunspots is subject to measurement specific phase shifts arising from, (i) altered height range of spectral line formation and the propagating character of p mode waves in penumbrae, and (ii) Zeeman broadening and splitting. We also show that these phase shifts depend on wave frequencies, strengths and line of sight inclination of magnetic field, and the polarization state used for Doppler measurements. We discuss how these phase shifts could contribute to local helioseismic measurements of 'surface effects' in sunspot seismology.

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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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arXiv2002

Convective intensification of magnetic flux tubes in stellar photospheres

The convective collapse of thin magnetic flux tubes in the photospheres of sun-like stars is investigated using realistic models of the superadiabatic upper convection zone layers of these stars. The strengths of convectively stable flux tubes are computed as a function of surface gravity and effective temperature. We find that while stars with T$_{eff}\ge$ 5500 K and log $g$$\ge$ 4.0 show flux tubes highly evacuated of gas, and hence strong field strengths, due to convective collapse, cooler stars exhibit flux tubes with lower field strengths. Observations reveal the existence of field strengths close to thermal equipartition limits even in cooler stars, implying highly evacuated tubes, for which we suggest possible reasons.

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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.

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