arXiv subjects

Sylvain G. Korzennik

Publications and source records attributed to Sylvain G. Korzennik.

9 recordsLinked to original sources

arXiv2026

Latitude-Dependent Time Variations of the Solar Tachocline

We have examined how the characteristics of the tachocline -- i.e., the change in rotation rate $δΩ$, or the "jump", the position of the midpoint of the tachocline, $r_d$, and the width of the tachocline, $w_d$, -- change as a function of time at different latitudes using 30 years of helioseismic data obtained by the GONG network. We find a statistically significant change in the jump, however, these changes do not have a simple correlation with solar activity. The dependence is different for solar Cycles 23 and 24, and for Cycle 25, it is more similar to that of Cycle 24. While our measured changes of the tachocline's width with time are marginally statistically significant, {the cross correlation is statistically significant and implies that the width is larger when the solar activity is smaller, suggesting that magnetic fields play a role in confining the tachocline. The position of the tachocline shows a significant secular change at low latitudes ($< \simeq 50^\circ$).} At these latitudes, the tachocline has been moving steadily closer to the base of the convection zone. This is consistent with other measurements that have shown that the overall complexity of solar activity has been decreasing over the last few decades. It leads us to speculate that strong magnetic fields tend to push the tachocline deeper into the radiative zone.

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arXiv2026

Resolving the Tachocline using Inversion of Rotational Splitting Derived from Fitting Very Long and Long Time Series

We use rotation splittings derived from very long and long time series, namely 25.2, 12.6 and 6.3 year long, computed by Korzennik (2023) independent methodology to characterize the solar tachocline and its variation with latitude and time. We use two different inversion methodologies and a model of the tachocline to derive its position, width and the amplitude of the radial shear. To validate our methodology we present results from simulated rotational splittings, whether including or not random noise commensurable with the current observational precision. We also describe how we leverage the fact that one of our methodologies uses an initial guess that can be chosen to include a priori information. In order to try to resolve the tachocline, we increased the radial density of the inversion grid and showed how it affect the inferences. We also show how the trade off between smoothing and noise magnification affects these, as well as the effectiveness of using an informed initial guess. Results derived from high-precision rotational splittings show clearly that the location of the tachocline at low latitudes is different for its position at high latitudes. The latitudinal variation of its width is not significantly constrained, but our results agree with estimates based on forward modeling. When using splittings derived from somewhat shorter time series, we find temporal variations that are neither definitive nor significant, since we see systematic differences when using different methodologies.

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arXiv2025

The Shape of the Solar Tachocline

Early helioseismic results have shown that the tachocline has a prolate shape. However, the models used in those studies constrained the tachocline to be either prolate or oblate. We use helioseismic data obtained from long time series (2304 and 4608 days) to determine the shape of the solar tachocline. Like previous work, we use forward modeling methods for this work; however, we allow more flexibility for the shape of the tachocline. We find that the tachocline does indeed deviate from a simple prolate structure and bulges out at mid latitudes. The center of the tachocline lies in the radiative zone at low latitudes, in the convection zone at intermediate latitudes, and back in the radiative zone at high latitudes. The high-latitude ($ > 60^\circ$) behavior is, however, uncertain and model dependent. Models that allow more variation of the shape indicate that the tachocline at high latitudes is almost coincident with the base of the convection zone.

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arXiv2025

Investigating Variations in Solar Differential Rotation by Helioseismology

Helioseismic signatures of dynamo waves have recently been discovered in variations of the solar differential rotation, offering valuable insights into the type of dynamo mechanism operating in the solar convection zone. To characterize these variations, we analyze p-mode frequency-splitting data estimated using time intervals of various lengths to enhance the signal-to-noise ratio in inversions of zonal flows. We introduce a novel time-dependent inversion method that inherently smooths the solution over time, eliminating the need for separate post-processing smoothing. By applying this approach to observational data from the SOHO Michelson Doppler Imager, SDO Helioseismic Magnetic Imager, and Global Oscillation Network Group, we identify similar dynamo wave patterns in both the zonal acceleration and the zonal flow throughout the entire convection zone. Our analysis shows that while using longer time series smooths out temporal variations, the fundamental features observed in the short time series (i.e. 72-day long) persist when inverting datasets covering different time periods. These findings reinforce earlier detections and offer further validation of solar dynamo models. We additionally investigate the dimensionless radial gradient of rotation. Its value is close to -1 and increases in the deeper layers, remaining nearly constant from the equator to mid-latitudes within the depth range of 13 to 35 Mm below the surface; the results at high latitudes remain somewhat inconclusive. The variation of this quantity displays a torsional oscillation-like pattern, albeit with certain differences.

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arXiv2024

Time Variation of the Solar Tachocline

We have used solar oscillation frequencies and frequency splittings obtained over solar cycles 23, 24 and the rising phase of solar cycle 25 to investigate whether the tachocline properties (jump i.e., the change in the rotation rate across the tachocline, width and position) show any time variation. We confirm that the change in rotation rate across the tachocline changes substantially, however, the change does not show a simple correlation with solar cycle unlike, for instance, changes in mode frequencies. The change during the ascending phase of solar cycle 25 is almost a mirror image of the change during the descending part of solar cycle 24, tempting us to speculate that the tachocline has a much longer period than either the sunspot or the magnetic cycle. We also find that the position of the tachocline, defined as the mid-point of the change in rotation rate, showed significant changes during solar cycle 24. The width of the tachocline, on the other hand, has showed significant changes during solar cycle 23, but not later. The change in the tachocline becomes more visible if we look at the upper and lower extents of the tachocline, defined as (position +/- width). We find that for epochs around solar maxima and minima, the extent decreases before increasing again - a few more years of data should clarify this trend. Our results reinforce the need to continue helioseismic monitoring of the Sun to understand solar activity and its evolution.

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arXiv2024

A SART-Based Iterative Inversion Methodology to Infer the Solar Rotation Rate from Global Helioseismic Data

We present a new iterative rotation inversion technique based on the Simultaneous Algebraic Reconstruction Technique developed for image reconstruction. We describe in detail our algorithmic implementation and compare it to the classical inversion techniques like the Regularized Least Squares (RLS) and the Optimally Localized Averages (OLA) methods. In our implementation, we are able to estimate the formal uncertainty on the inferred solution using standard error propagation, and derive the averaging kernels without recourse to any Monte-Carlo simulation. We present the potential of this new technique using simulated rotational frequency splittings. We use noiseless sets that cover the range of observed modes and associate to these artificial splittings observational uncertainties. We also add random noise to present the noise magnification immunity of the method. Since the technique is iterative we also show its potential when using an apriori solution. With the right regularization this new method can outperform our RLS implementation in precision, scope and resolution. Since it results in very different averaging kernels where the solution is poorly constrained, this technique infers different values. Adding such a technique to our compendium of inversion methods will allow us to improve the robustness of our inferences when inverting real observations and better understand where they might be biased and/or unreliable, as we push our techniques to maximize the diagnostic potential of our observations.

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arXiv2024

Asphericity of the base of the solar convection zone

We have used solar oscillation frequencies and frequency splittings obtained over solar cycles 23 and 24 to investigate whether the base of the solar convection zone shows any departure from spherical symmetry. We used the even-order splitting coefficients, $a_2$-$a_8$, and estimated the contributions from each one separately. The average asphericity over the two solar cycles was determined using frequencies and splittings obtained with a 9216-day time-series. We find that evidence of asphericity is, {\em at best}, marginal: the $a_2$ component is consistent with no asphericity, the $a_4$ and $a_6$ components yield results at a level a little greater than $1\,σ$, while the $a_8$ component shows a signature below $1\,σ$. The combined results indicate that the time average of the departure from the spherically symmetric position of the base of the convection zone is $\lesssim 0.0001R_\odot$. We have also used helioseismic data obtained from time-series of lengths 360 days, 576 days, 1152 days, and 2304 days in order to examine the consistency of the results and evaluate whether there is any time variation. We find that the evidence for time variation is statistically marginal in all cases, except for the $a_6$ component, for which tests consistently yield $p$ values of less than $0.05$.

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arXiv2017

Initial Results from Fitting Resolved Modes using HMI Intensity Observations

The HMI project recently started processing the continuum intensity images following global helioseismology procedures similar to those used to process the velocity images. The spatial decomposition of these images has produced time series of spherical harmonic coefficients for degrees up to $\ell=300$, using a different apodization than the one used for velocity observations. The first 360 days of observations were processed and made available. In this paper I present initial results from fitting these time series using my state of the art fitting methodology and compare the derived mode characteristics to those estimated using co-eval velocity observations.

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arXiv1999

An Upper Limit on the Reflected Light from the Planet Orbiting the Star tau Bootis

The planet orbiting tau Boo at a separation of 0.046 AU could produce a reflected light flux as bright as 1e-4 relative to that of the star. A spectrum of the system will contain a reflected light component which varies in amplitude and Doppler-shift as the planet orbits the star. Assuming the secondary spectrum is primarily the reflected stellar spectrum, we can limit the relative reflected light flux to be less than 5e-5. This implies an upper limit of 0.3 for the planetary geometric albedo near 480 nm, assuming a planetary radius of 1.2 R_Jup. This albedo is significantly less than that of any of the giant planets of the solar system, and is not consistent with certain published theoretical predictions.

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