Abstracts-Posters - Dynamics of the Sun & Stars: Honoring the Life & Work of Michael Thompson

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1. John Leibacher, NSO, LPL, IAS & Laurent Gizon, MPS, Univ. Göttingen, NYU Abu Dhabi

Title: Michael Thompson's support for science

Abstract: We recall some of Michael Thompson's contributions to groups beyond his own institution.

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2. Piyush Agrawal, University of Colorado

Title: Inverting Stokes data using Helioseismic inversion method

Abstract: As light travels through an atmosphere, it interacts with the medium through absorption, emission and scattering processes. Given a spectra, inferring the physical properties (for example T, Pg, velocity) of the medium it traversed, is called an inversion problem. To infer the unknown atmosphere, one usually starts with a depth-dependent guess atmospheric model and perturbs it until the synthesized spectra through this model matches the observed spectra. The desired perturbations are computed using response functions which is a measure of the sensitivity of spectra to changes in atmospheric variables. Due to the ill-posed nature of inverse problems, the solutions are non-unique and highly oscillatory. Thus, nodes are used to obtain a smooth solution. These nodes are a small number of evenly spaced depth locations where the perturbations are calculated. Perturbations at remaining depth points are interpolated using these nodal values. The final solution has a depth resolution set by the number of nodes, independent of the information content of the spectra. The solution thus obtained, most likely, is not the optimal solution.

The OLA inversion method used in helioseismology does not suffer from the limited resolution issues with nodes. In this method, the response functions are linearly combined in order to obtain a highly localized, average response kernel at a given target depth. The width of the kernel corresponds to the vertical resolution at that depth. The limit to how narrow an averaging kernel can be constructed depends on the amount of spectral information. The inverted result corresponds to kernel averaged quantity. The process is repeated for all depths and a smooth inverted solution is obtained.

In this work, we aim to apply the OLA method to spectroscopic data. To facilitate with the project, we used 1D smooth temperature profiles from MURaM, and SIR code to synthesize spectra. We added a small artificial perturbation to the temperature profile. The goal of the project is how well can we invert for this perturbed atmosphere using OLA method and how does it compare to the SIR inversion code.

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3. Sushanta Tripathy, National Solar Observatory

Title: Magnetoseismology of active regions

Abstract: The interpretation of acoustic waves surrounding the active regions has been a difficult task since the influence of magnetic field on the incident waves is not fully understood. As a result, structure and dynamics of active regions beneath the surface show significant uncertainties. Recent numerical simulations in active regions have demonstrated that the key to understand these complex processes requires a synergy between models and helioseismic inferences from observations. In this context, we characterize the spatio-temporal power distribution around active regions as a function of the height in the solar atmosphere and focus on the power enhancements seen around active regions as a function of wave frequencies, strength, inclination of magnetic field. We also analyze the relative phases of the observables and their cross-coherence functions. We expect that these effects will help in comprehending the interaction of acoustic waves with magnetic field and improve the structure and dynamics below the solar surface. Authors: S.C. Tripathy(1), K. Jain (1), S. Kholikov(1), F. Hill(1) and P. Cally(2): 1. National Solar Observatory, 3665 Discovery Drive, Boulder, CO 80303, USA, 2. Monash University, Clayton, Victoria 3800, Australia

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4. NA

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5. Alan Hoback, University of Detroit Mercy

Title: Direct travel time of X-ray class solar storms

Abstract: Plasma from solar events such as coronal mass ejections and solar flares can impact the Earth’s magnetosphere and cause disruptions of electrical systems on the ground. Warning systems exist to help prepare people for possible disruptions. Generally, it is thought that there would be a few days’ warning. However, larger flares on the scale of the Carrington Event have much faster travel times. For X-ray class flares above X of 17, all have travel times to one astronomical unit of 30 hours or less. Since those are more likely to have the greatest impact on the operation of electrical systems, their travel time is more important than the average event. However, it is shown that such occurrences have an average travel time of less than 24 hours.

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6. Junwei Zhao, Stanford University

Title: Helioseismic center-to-limb effect and measured travel-time asymmetries around sunspots

Abstract: Center-to-limb effect was found in time-distance helioseismic measurements, and exhibits as an asymmetry in the measured travel times for waves propagating away from and toward the solar disk center. The effect gets stronger when the measurements are made closer to the solar limb. The effect imposes a big challenge to a reliable inference of the Sun's interior meridional circulation. Meanwhile, another travel-time asymmetry between waves traveling away from and into sunspots is well known in the community, and how to interpret the measured travel-time asymmetry is controversial. It is possible that the both above-mentioned travel-time asymmetries are partially, albeit not entirely, due to different atmospheric heights, where oscillatory signals are taken for computing the acoustic travel times. For the center-to-limb case, observed atmosphere is higher near the limb due to the limb-darkening effect; for the sunspot case, sunspot's atmosphere is lower than the quiet Sun due to the Wilson depression. Through comparing these two different sets of travel-time asymmetry measurements, we assess what fraction of the asymmetry is due to the different atmospheric heights, and discuss why the atmospheric heights can be a cause of such a travel-time asymmetry.

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7. Laurent Gizon, Max Planck Institute for Solar System Research

Title: Center for Space Science at NYUAD

Abstract: The Center for Space Science at New York University Abu Dhabi is primarily focused on topics in solar and stellar physics through seismology and modelling, and exoplanet science. In this poster we present a few recent research highlights. Authors: The CSS Team.

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8. Laurent Gizon, Max Planck Institute for Solar System Research

Title: Improved helioseismic imaging of the farside

Abstract: We use the Porter-Bojarski formulation of helioseismic holography and accurate Green's functions to image active regions on the farside of the Sun. For the very large active region NOAA 12192 a clear improvement in the signal-to-noise ratio is obtained compared to the current pipeline. Authors: Dan Yang, Laurent Gizon, Damien Fournier, and Charles Lindsey.

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9. Laurent Gizon, Max Planck Institute for Solar System Research

Title: Global-scale Rossby waves in the solar interior

Abstract: We present multiple independent observations of global-scale solar Rossby waves. According to numerical and theoretical studies, these waves are potential probes of the deep convection zone. Authors: Laurent Gizon and MPS Team.

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10. Thomas Duvall, Max Planck Institute for Solar System Research

Title: Measurement of Rossby wave radial eigenfunctions using deep-focus time-distance helioseismology

Abstract: Recently sectoral solar Rossby waves have been detected by several techniques, including local correlation tracking, ring diagrams, time-distance helioseismology, and mode coupling. It would be most useful to constrain the radial eigenfunctions of these modes. That is the purpose of the present study, in which deep-focus time-distance helioseismology is used to measure the amplitude of the Rossby at different focus depths. Two models are found to be consistent with the measurements: 1) the amplitude goes as (r/R)**m, m being the azimuthal order, and 2) a model with a flat response from the surface to 35 Mm depth. Models that can be safely excluded are a completely flat model and shallow models, with significant amplitude from the surface to 2 Mm depth.

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11. Yuhong Fan, UCAR/HAO

Title: Probing the variation with depth of the solar meridional circulation using Legendre function decomposition

Abstract: The solar meridional circulation is a crucial component of magnetic flux transport and dynamo models. Despite decades of helioseismic study, no consensus exists regarding the variation of its properties with depth. It has become apparent that the main challenges consist of 1) overcoming realization noise with multi-year long datasets, and 2) the identification and robust removal for systematic center-to-limb effects. Here we apply the helioseismic methodology of Legendre Function Decomposition (LFD) to 7.5 years of Dopplergrams obtained by the Helioseismic and Magnetic Imager (HMI) as the basis of inferring the depth variation of the meridional flow between 20 and 60 degrees latitude in both hemispheres. The LFD method, first developed by Braun and Fan in 1998, probes subsurface flows through the Doppler-effect induced distortion of power spectra. The procedure is optimized for the detection of meridional flows and uses Legendre functions (of the first and second kind) to characterize poleward and equatorward wave propagation in spherical coordinates. For this study we have developed control procedures which assess and remove center-to-limb artifacts, using measurements obtained by applying the procedure to pseudo poles at the east and west limbs. Forward modeling is carried out to evaluate the consistency of the corrected LFD frequency shifts with models of the depth variation of the meridional circulation in the top half of the convection zone. Tentative results include, for both hemispheres, peak in the poleward flow (18 m/s) at a depth of 10 Mm. The flow decreases beneath this depth becoming undetectable within the noise (plus/minus 3 m/s) below a depth of 60 Mm. Authors: Braun, D.C. (NWRA), Birch, A. (MPS), and Fan, Y. (HAO/UCAR)

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12. Kaori Nagashima, Max Planck Institute for Solar System Research

Title: Monte-Carlo test of an improved multi-ridge fitting code for ring-diagram analysis

Abstract: The huge discrepancy between the amplitude of subsurface convection flows estimated by ring-diagram and time-distance helioseismology analyses (Hanasoge et al. 2016) is still an unsolved problem. As a step towards solving this, we investigated the ring-diagram analysis code used in the comparison, the multi-ridge fitting code ATLAS (Greer et al. 2014). Through our investigation of the code we identified potential issues in the code and corrected them. In the ATLAS code the power spectrum in two-dimensional Cartesian spatial coordinates is remapped into polar coordinates, smoothed in the angle of the horizontal wavevector, and rebinned to reduce the resolution in the angle of the wavevector. The probability distribution function of the rebinned and smoothed power is a chi-square distribution with a higher degree of freedom than that of the raw power. The logarithm of the rebinned and smoothed power is nearly Gaussian. In this case the maximum likelihood method reduces to a least-square estimate; we take this into account and changed the fitting algorithm to the least-squares fitting of the logarithm of the smoothed power spectrum. We also altered the model for the power spectrum to improve the stability of the fits. Here we report our Monte-Carlo tests of the improved code. The tests confirm that we have reduced the fitting bias as well as the error estimates of the parameters. Since the difference between the results by the original and improved codes are ~3% at most even when a huge flow (>200 m/s) is embedded in the simulation data, it is not large enough to solve the aforementioned discrepancy.

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13. Jason Jackiewicz, New Mexico State University

Title: Probabilistic inversions for time-distance helioseismology

Abstract: Inversions play a critical role in helioseismology, and M.J. Thompson was a pioneer in developing techniques for their efficient implementation. In global helioseismology, inversions of frequencies have been used to successfully determine the internal structure of the Sun and its differential rotation. In local helioseismology, such inversions are ubiquitous for measuring subsurface flows of all scales. However, optimization procedures that rely on matrix inversions often suffer from numerical instabilities caused by ill-conditioned matrices and typically a very irregular misfit function. This work presents the use of new MCMC techniques within a Bayesian framework for making inferences in local helioseismology for cases where a parametrization of the unknowns is possible. We will show a few examples for meridional circulation and supergranulation, and compare to more traditional inversion techniques to illustrate the advantages and disadvantages.

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14. Jishnu Bhattacharya, NYU Abu Dhabi

Title: Helioseismic sensitivity kernels to probe solar subsurface convection in spherical geometry

Abstract: Estimating subsurface convective flows remains one of the challenging fronts in helioseismology. Especially challenging in this regard is the profile of meridional flows, with a range of differing inferences that seem to indicate the importance of addressing systematics that seep into the inverse problem. Added to this is the challenge of dealing with noise, in which case improving the inferences necessitates averaging over data -- something that is challenging to achieve given the computational complexity of evaluating sensitivity kernels. In this work we present a new formalism to compute sensitivity kernels that addresses both these issues -- leading to significant speed-ups in evaluating sensitivity kernels alongside addressing systematic issues like line-of-sight projection and center-to-limb variation in formation heights of the observed spectral line. This approach of computing sensitivity kernels has the potential to improve the estimates of large-scale flows beneath the solar surface.

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15. NA

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16. Sylvain Korzennik, Center for Astrophysics, Harvard & Smithsonian

Title: On active region emergence precursors

Abstract: I present intriguing results from a systematic look at possible indicators of active region emergence precursors using both acoustic power mapping and time distance travel time anomalies. I analyzed a set of active regions that have emerged at low latitudes and close to the central meridian, and were observed with HMI. Such regions, since they emerged near disk center, could be tracked for days before and after their emergence. Data cubes of tracked surface observable were analyzed, using time-series of various length, using HMI's intensity, velocity and magnetic field data. While each active region displays its own particularities, this systematic approach allowed me to pick some interesting statistical properties that will be summarized. Using different time-series length allowed me to investigate the trade-off between precision and resolution, whether temporal or spatial. It also revealed to which extent one sees a tight correlation between various metric of surface activity and its effect of the measured acoustic signal, whether seen in raw of filtered power map or travel time anomalies.

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17. Doug, Braun, NorthWest Research Associates

Title: Active region flows and their contribution to varying global dynamics

Abstract: We explore the general properties of the near-surface dynamics of solar active regions (ARs) and show how AR flows may contribute to longitudinally averaged measurements of global properties such as meridional circulation and torsional oscillations. Helioseismic holography is applied to HMI Dopplergrams yielding about 5000 flow measurements of 336 ARs observed by SDO between 2010 and 2014. Ensemble averages of the AR flows are presented, binned into subsets based on total magnetic flux. These averages show converging flows, with speeds about 10 m/s and spanning 10 degrees from the active region centers, which are apparent for most ARs above the flux limit of our survey at 10^21 Mx. Retrograde flows are also detected, with amplitudes around 10 m/s, which predominantly, but not exclusively, flank the polar side of the ARs. The high signal-to-noise levels of these averages makes possible the assessment of individual AR contributions to time-varying global flows. We demonstrate this for several solar rotations, showing that individual active regions contribute substantially to these global flows. This work is supported by the Solar Terrestrial program of the National Science Foundation (award AGS-1623844) and by the NASA Heliophysics Division (awards 80NSSC18K0066 and 80NSSC18K0068).

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18. Aleczander Herczeg, New Mexico State University

Title: Convective blueshift and systematic errors in meridional flow measurements

Abstract: Time-distance helioseismology has been extensively used to study the meridional flow on the sun, from the near surface to the base of the convection zone. However, measurements of the solar meridional flow have been plagued by a systematic error that results in a spurious flow which, to first order, appears to be moving radially outward from disk center. This systematic error is present in the measurements whether the initial data was collected in space or on the ground, regardless of the instrument used (HMI, MDI, GONG, etc). This implies the source of the systematic error is the sun, and the center-to-limb variations that appear in the spurious flow point toward this being a surface effect. Currently, the cause of this effect is unknown, and the method for removing this spurious flow from the final model is unsatisfactory. Convective blueshift, the apparent average blueshift due to convection at the solar surface, qualitatively matches the center-to-limb systematic error: both effects vary with the spectral line used to measure it and with the resolution of the instrument, and both effects first increase upon moving from the disk center before decreasing to a minimum near the limb. Using HMI data, time-distance methods are used to compare inverted meridional flow results with and without correcting for convective blueshift before processing the raw data. If the systematic errors are caused - either in full or in part - by neglecting to account for this convective blueshift, this treatment of the data can lead to increased accuracy of all future meridional flow measurements made using the time-distance method.

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19. Bhishek Manek, University of California, Santa Cruz

Title: Solar hemispheric helicity selection rules: A new theory explaining recent solar observations

Abstract: The buoyant transport of magnetic fields from the solar interior toward the surface plays an important role in the emergence of active regions, the formation of sunspots, and the overall solar dynamo. Observations suggest that toroidal flux concentrations, often referred to as "flux tubes", rise from their region of initiation likely in the solar tachocline toward the solar surface due to magnetic buoyancy. Many studies have assumed the existence of such magnetic structures and studied the buoyant rise of an isolated flux tube in a quiescent, field-free environment. Here, motivated by the mechanisms of flux tube formation, we relax the latter assumption and study the rise of a toroidal flux tube embedded in a large-scale poloidal background magnetic field. We find that the presence of the large-scale background field severely affects the dynamics of the rising tube. A relatively weak background field, as low as 6% of the tube strength, can destroy the rise of a tube that would otherwise rise in the absence of the background field. Surprisingly, the rise of tubes with one sign of the twist is suppressed by a significantly weaker background field than the other. This demonstrates a potential mechanism for the selection of the preferred helicity of rising and emerging tubes for the solar case that is commensurate with many features of the hemispherical rule.

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20. Kiran Jain, National Solar Observatory

Title: Comparing last two solar activity minima using acoustic oscillation frequencies

Abstract: Various ground- and space-based instruments have been collecting uninterrupted data of the Sun's acoustic oscillations for more than two decades. These observations are crucial to understand the structure and dynamics of the solar interior. With improved spatial and temporal resolutions, we are able to reliably explore the connection between subsurface layers and the magnetic activity related changes above the surface. In addition, we also measure the solar differential rotation with reduced uncertainties. The acoustic oscillation frequencies do vary in phase with the surface magnetic activity with significantly high correlation, however , during the activity minimum between cycles 23 and 24, the low-degree modes that penetrate deeper layers sensed minimum about a year earlier than the surface activity. The helioseismic data also demonstrated the latitudinal dependence of the progression of the Cycle 23 . Here, we explore if these trends still continue in Cycle 24 and also compare similarities and/or discrepancies between Solar Cycles 23 and 24. Authors: Kiran Jain, Sushant Tripathy and Frank Hill (National Solar Observatory, Boulder, CO)

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21. Rudolf Komm, National Solar Observatory

Title: Long-lived activity complexes, their kinetic helicity, lifetime, and flare activity

Abstract: We study long-lived activity complexes using stackplots of magnetic activity derived from NSO/SOLIS synoptic magnetograms. We focus on the kinetic helicity below the surface determined with ring-diagram analysis applied to full-disk Dopplergrams from SDO/HMI during Solar Cycle 24. The kinetic helicity of activity complexes follows the hemispheric helicity rule with mainly positive values in the southern hemisphere and negative ones in the northern hemisphere. To distinguish between active and quiet regions, we divide the data into subsets with high and low levels of activity and create stackplots of surface magnetic activity and subsurface kinetic helicity for each subset. The distribution of flares in a stackplot resembles closely that of the high-activity subset. The flare-productive locations in long-lived complexes produce, on average, the same number of flares as those of short-lived ones. However, long-lived complexes have a larger number of these locations and thus a higher flare-production rate than short-lived ones. We will present the latest results. Authors: Rudolf Komm, Sanjay Gosain, National Solar Observatory, Boulder, CO

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22. Lekshmi B, Center of Excellence in Space Sciences India, IISER Kolkata

Title: Solar cycle variation of large-scale plasma flows

Abstract: Large scale plasma flows like differential rotation, and meridional flow play an important role in driving the solar dynamo process. These flows show spatial and temporal variations over the solar cycle and are not symmetric across the equator. We perform a long-term study (17yr) of the hemispherical asymmetry in torsional oscillation and meridional flow using the velocity data obtained from Global Oscillation Network Group (GONG) ring diagram analysis. Our results show that the hemispherical asymmetries in near-surface torsional oscillation and meridional flow are strongly related to the asymmetry in the sunspot cycle. We observe that there exists a time delayed correlation (anti-correlation) between the asymmetries in torsional oscillation (meridional flow) and the sunspot cycle, with the asymmetry in the flows preceding the cycle asymmetry. Also, we study the solar cycle variation of near-surface meridional flow and its role in the Surface Flux Transport (SFT) dynamo process. Our study puts new constraints on dynamo models of the solar cycle and may help in predicting the hemispherical cycle strength. While improved flow diagnostics and magnetohydrodynamic simulations are needed to understand the physical processes at play.

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23. Leif Svalgaard, Stanford University

Title: Three centuries of solar activity with monthly resolution

Abstract: I present a reconstruction of the sunspot Group Numbers since 1700 AD with a time resolution of one month. Comparison with proxies from geomagnetism show excellent agreement, thus resolving the current tension between recent other reconstructions.

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24. Loren Matilsky, JILA & CU Boulder

Title: Dynamo states with strikingly different symmetries coexisting in global solar simulations

Abstract: The origin of the Sun’s various cycles of magnetic activity remains one of the most pressing outstanding problems in solar physics. Global, 3D MHD dynamo simulations have recently made substantial contact with observations, yielding regular polarity-reversing cycles and equatorward propagation of interior magnetic field. Here we present a new suite of 3D, global simulations of a solar-like convection zone that remarkably reveal two distinct states of the dynamo coexisting simultaneously. One state consists of opposite-polarity reservoirs of magnetism—two in each hemisphere—that exhibit regular polarity reversals and equatorward propagation. Superimposed is another state, asymmetric about the equator, which consists of one reservoir of strong azimuthal field in a single hemisphere. The uni-hemispherical reservoir flips polarity and migrates poleward with the cycle, wandering between North and South on long timescales. We describe physically how the magnetic fields our simulated dynamos are maintained, and discuss our results in terms of the long-observed sunspot asymmetry, or solar “active hemispheres.”

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25. Jacob Noone Wade, University of California, Santa Cruz

Title: Influence of turbulence on an essentially nonlinear dynamo mechanism

Abstract: Through direct numerical simulations, we study the influence of turbulence on a novel essentially nonlinear dynamo (END) mechanism. This dynamo operates through the interaction of a velocity shear with magnetic buoyancy mitigated by secondary Kelvin-Helmholtz instabilities. As opposed to the traditional mean-field or kinematic theory treatment, this END paradigm only exhibits dynamo action if the initial magnetic configuration is finite and above a critical threshold. We first study the effects of the smallest scales of turbulence by injecting noise into the magnetic fields. We then seek to study the effect of a wider range of scales by analyzing the interactions of actual resolved convection. To achieve a setup that is perhaps more realistic, we ultimately embed the END mechanism in a two-layer penetrative simulation. These preliminary results indicate that turbulence in many forms does little to hinder the END mechanism and therefore this example of an END at least appears to be quite robust.

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26. Sylvain Breton, CEA Saclay

Title: A comparison of global helioseismic-instrument performances

Abstract: With more than 24 years of observations gathered for global helioseismology studies with either space instruments aboard the SoHO satellite (GOLF and VIRGO) or with fully deployed ground-based networks (GONG and BiSON), it is timely to carry out a comparative evaluation of their performances at different frequency ranges.

In this perspective, we first present the most up-to-date calibration of the GOLF data spanning from 1996 to 2019 including an improvement of the time stamp of the series as well as on the correction of pointing problems of the satellite. We compare these new time series with those of VIRGO, GONG (Sun-as-a-star mode), BiSON and the GOLF previous velocity calibrations.

Next, we evaluate the performance of the Solar-SONG data, an initiative that aims at performing helioseismic measurements simultaneously at different height of the solar atmosphere using the multi-wavelengths abilities of the SONG spectrograph placed at the Observatorio del Teide. We compare contemporaneous observations of GOLF, using the newly calibrated time series, and Solar-SONG done during the summer 2018. We show that for the analysis of the Doppler velocity extracted using all the available information, the 30-days long Solar-SONG time series present a better signal-to-noise ratio (SNR) than the contemporaneous GOLF (after 24 years in space). Solar-SONG spectrum has an amplitude SNR of 4 compared to 2.5 for GOLF in the p-mode region, opening the way to perform a deeper analysis of the data at different wavelengths. Authors: S. Breton, R. A. García, P. L. Pallé, S. Mathur, A. Jiménez, F. Grundahl, M. Fredslund-Andersen et al.

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27. Stuart Jefferies, Georgia State University

Title: The acoustic cut-off frequency in the solar atmosphere

Abstract: We present spatial maps of the acoustic cut-off frequency for two heights in the solar atmosphere. These maps have implications for the mechanical heating of the Sun's upper atmosphere by magneto-acoustic-gravity waves.

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28. Jesper Schou, Max Planck Institute for Solar System Research

Title: Waves, convection and systematic errors

Abstract: As the quantity and quality of helioseismic observations have increased, the random errors have decreased and systematic errors have become dominant in many cases. Some of these, such as the surface term and various center-to-limb effects, are due to our poor understanding of the interaction of waves and convection near the solar surface. Here we analyze large hydrocode simulations and show how the waves can be analyzed in a theoretical framework and that we are able to better understand and correct these systematic errors. Authors: J. Schou and A.C. Birch Max Planck Institute for Solar System Research

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29. Regner Trampedach, Space Science Institute

Title: Non-adiabatic helioseismology via 3D convection simulations

Abstract: I have extracted p-mode eigen-functions from a 3D simulation of a deep convective solar atmosphere. The part that is out of phase w.r.t. the density wave, is fitted to an analytical expression in depth and frequency and then applied to a non-adiabatic oscillation calculation. The resulting damping widths and frequency shifts are compared with helioseismic observations.

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30. Robert Loper, Air Force Institute of Technology

Title: Plasma structure of the deep solar interior

Abstract: Time-series analyses of helioseismic data and solar neutrino data have indicated the potential existence of a second tachocline at the core-radiative zone interface. While the helioseismic analysis conflicts with other results, both analyses call into question our knowledge of the deep solar interior (r/R_sun < 0.5). This work uses existing data regarding the deep solar interior to deduce its plasma structure through semi-classical magnetohydrodynamics (MHD), and considers the generation of a magnetic field in the deep solar interior and its diffusion outward. At low latitudes, the diffused field could serve as the seed field for the tachocline dynamo field and also interacts with it to provide a long-period solar cycle modulation. At high latitudes, the diffused field manifests as scattered regions of intense polar fields.

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31. Zhao Guo, Pennsylvania State University

Title: Granulation signature in radial velocities of solar-like stars

Abstract: The granulation amplitude and timescale have been calibrated by using the power spectrum of a sample of asteroseismic stars observed by Kepler in Kallinger et al. Much to the concern of the exoplanet community is the effect of granulation in the radial velocity (RV) time series. It is expected that the granulation is more prominent in RVs. We study the granulation signal by using the RV time series from SONG and HARPS and compare the result with the calibration from Kepler photometry. For each star, we perform a model comparison and determine whether the data can be explained by one, two, or three granulation components (granulation, super-granulation, meso-granulation, or activity). We find that the one-site quasi-continuous RV data of a few days are sufficient to reveal the granulation timescale and amplitude for the dwarfs and giants under study, and the timescales are generally in agreement with the calibration from Kepler photometry. We try to explain the observed granulation amplitude ratio in RVs and in photometry and put it on solid theoretical ground.

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32. Angela Santos, Space Science Institute

Title: On the limits of seismic inversions for radial differential rotation of solar-type stars

Abstract: Among other stellar properties, seismic data contains information on stellar internal rotation, which plays an important role on stellar dynamo models. Helioseismology has been successful in the characterization of internal rotation in the Sun. However, due to the uncertainties on the observations and on stellar models, determining internal rotation in stars other than the Sun, in particular solar-type stars, has been challenging. In this work, using artificial rotational splittings, we explore the limitations on the constraining of internal rotation profiles through the implementation of a regularized least squares inversion method. We further test the impact of using surface rotation estimates (for instance, inferred from spot modulation of stellar light curves) on the rotation profile determination. Authors: A. R. G. Santos, M. J. Thompson, S. Mathur, R. A. García

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33. Saskia Hekker, Max Planck Institute for Solar System Research

Title: The physical origin of the luminosity maximum of the bump

Abstract: The short phase of contraction and brief reversal in the trend of increasing luminosity during the red giant phase of evolution, the so-called bump, is a well known feature in models and in observations of both open and globular clusters. Nevertheless, the physics that drives the stellar structure changes associated with the bump are not fully understood. Here we investigate the bump in terms of the specific entropy and its time derivative. Our initial results reveal structural changes that might explain the bump.

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34. Margarida Cunha, Institute of Astrophysics and Space Sciences, Porto, Portugal

Title: A new utility to study strong chemical gradients in stellar interiors

Abstract: The asymptotic analysis of the pulsation equations predicts regularly-spaced periods for gravity modes. These periods get deflected when a sharp variation in the buoyancy frequency, often associated with strong chemical gradients, is present. These features are known as glitches and their study is one of the few methods allowing us to probe the physical conditions in a localized region inside a star. Theoretical works (e.g., Cunha et al. 2015, 2019) provide the analytical bases to interpret the signatures of glitches of arbitrary amplitude in seismic observables. However, testing the analytical predictions is not straightforward, because finding a model with a glitch with specific properties (e.g., position, amplitude, and width) implies, in the best-case scenario, to explore the space of stellar parameters until the desired glitch is attained. This approach has clear disadvantages like (1) no warranty to generate the pursued glitch, (2) time consuming, and (3) preclusion of a direct comparison of the effect of different glitches on the seismic data.

Here, we present a program that allows to artificially add/remove a Gaussian-like glitch into/from the buoyancy profile of an already existing stellar model, that way solving points (1-3). Comparison between the sharpness of the glitch with respect to the local wavelength of gravity modes is also provided. The modification to the buoyancy frequency is done in a consistent way by a correspondent modification of the first adiabatic exponent as described in Ball et al. 2018. The program counts with an interactive GUI as well as a command line mode and is available as a Github repository. Authors: Stefano Garcia, Margarida S. Cunha, Mathieu Vrard

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35. Margarida Cunha, Institute of Astrophysics and Space Sciences, Porto, Portugal

Title: 16 Cygni A: A testbed for stellar core physics

Abstract: Asteroseismology allows us to probe the innermost layers of stars and to improve the description of the physical processes that determine their structure and evolution. The work presented in this poster aims at developing a novel method that compares core-sensitive seismic diagnostics (namely, the frequency ratios) observed in a pulsating star to those derived for a set of models obtained through Forward Modelling of the same object. The main goal is to reduce the number of stellar models accepted by the Forward Modelling down to the ones that better represent the core of the specific star being studied. The method is illustrated on the bright, solar-like pulsator 16 Cygni A. By comparing the observed frequency ratios for this benchmark star to the ones derived from stellar models, we were able to downselect the set of models accepted by the Forward Modelling. More importantly, we were able to constrain further the fraction of hydrogen in the core, establishing the precise evolutionary state of this star. Authors: Rocha, C. I. S. A.; Pereira, C. J. G. N.; Cunha, M. S.; Monteiro, M. J. P. F. G.; Nsamba, B.; Campante, T. L.

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36. Connor Bice, CU Boulder

Title: Exploring the origins of intense magnetism in early M-dwarf stars

Abstract: M-type stars are quickly stepping into the forefront as some of the best candidates in searches for habitable, Earth-like exoplanets, and yet many M-dwarfs exhibit extraordinary flaring events which would bombard otherwise habitable planets with ionizing radiation. While the later (less massive) M-dwarfs tend to flare much more frequently than more massive stars, with typical quiescent intervals often lasting only hours, it is the more massive G- K- and M-dwarfs which are the usual hosts of "superflares," which can at their peaks outshine their host stars by factors as large as 10^4. These two classes of flaring stars are separated by what we have come to call the tachocline divide. Not only is the distribution of their magnetic characters strikingly different, main-sequence stars later than approximately M3.5 become fully convective and may no longer possess a tachocline, a layer of strong rotational shear revealed by helioseismology to separate the Solar RZ and CZ. We turn here to the more massive M-dwarfs to examine the effects such a layer may have on their internal dynamics. We present the results of simulations of rapidly rotating M2 stars, some with impenetrable boundaries at the base of the CZ, and others allowing overshoot into the underlying RZ and tachocline, computed using the spherical 3D MHD code Rayleigh. In doing so, we find that although a tachocline is not necessary for these stars to generate strong, axisymmetric magnetic fields, its presence can enable the existence of certain dynamo configurations that are impossible to maintain without it. Furthermore, we find that magnetic buoyancy induced transitions between certain dynamo states are capable of transmitting sufficient magnetic energy to the stellar surface to power even the largest of early M-dwarf superflares.

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37. Benjamin Fernando, University of Oxford

Title: Asteroseismic modelling of Lambda Bootis stars

Abstract: The Lambda Bootis stars are A-type Population I objects which are strangely metal deficient in their outer layers. An estimated 2% of the relevant spectral class are members of this group, with examples including canonical A-type stars such as Vega (a Lyrae). Potential variety in the interior stellar structures of these Lambda Bootis stars exists, with both young-main sequence stars and more evolved helium-core burning stars which have been stripped through binary interactions fitting the current temperature, luminosity and surface gravity observations. Asteroseismic modelling offers a powerful tool to differentiate between such internal structures, and I will present such results here. The data needed to make such a differentiation should be available through modern instruments (e.g. TESS), offering an opportunity for further investigation of this class of star. A discussion of these techniques as specifically applied to the Lambda Bootis star Tycho B (which has been suggested as a potential mass-donating companion to Tycho’s supernova, SN1572) will also be presented. Such work is of particular interest as if Tycho B can be confirmed as the mass-donor, a constraint can be placed on the proportion of SNIa which occur through the single-degenerate channel, offering insight into an important but as yet unanswered question in modern astrophysics.

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