Shukur Kholikov, Jason Jackiewicz, Markus Roth

In this study, we present first inversion results for deep meridional flow using spherical Born approximation kernels and time-distance helioseismology. The computation of Born approximation kernels for flows has only recently become available in spherical geometry. Compared to the ray approximation, the Born approximation is considered to provide a more realistic model of the advection and scattering processes in the solar interior, which are captured in travel time measurements. We first validate this method using artificial data from a linear 3D simulation of solar interior wave propagation. We find that the prediction of the Born approximation model coincides well with the simulated data. We then perform standard SOLA inversions of the solar meridional flow. First, inversion results of the simulated data are discussed and compared to the original flow profile included in the simulation. Finally, we apply the validated method to GONG data spanning periods of low, medium and high solar activity (2001-2003, 2004-2006, and 2007-2009). The results are discussed and compared to literature.

Conny Aerts, Peter Papics, Santiago Triana

Thanks to the unprecedented high quality space photometry provided by CoRoT and Kepler missions, our view on stellar oscillations in B-type stars and the physics of the upper HRD is progressively improving. O- and B-type stars harbor a fully mixed convective cores, and a radiative envelope. However, the interface between these two layers -- the so-called overshooting layer -- is not understood from first principles. Thus, the width and the mixing efficiency of the overshooting layer is always treated by simplistic schemes like step-function or exponentially diffusive mixing prescriptions. We modelled two rich main sequence pulsators observed by Kepler and CoRoT which also turn out to be very slowly rotating pulsating B stars. They are KIC 10526294 and HD 50230, respectively. Based on forward seismic modelling, we derive the width of the overshooting layer on top of their receding cores. Additionally, we show that extra diffusive mixing of 100 to 10 000 cm$^2$ sec$^{-1}$ in the radiative envelope of these two stars is essential to better fit their observed g-mode frequencies. We also compare the classical step-function versus exponential diffusive overshoot. The diffusive overshooting prescription outperforms the other to fit the observed frequencies by a factor 2 to 3 (in $\chi^2$ sense). The derived values for the overshooting and diffusive mixing coefficients are weakly dependent on the choice of opacities and chemical mixtures, and are considered robust constraints.

S. Brun

The aim of the present study is to characterize the effect of the rotation rate in building magnetic field via dynamo action in solar-like stars. We use the code ASH to model the convective dynamo for solar-like stars at various rotation rates and hence Rossby numbers. We find that stable magnetic configuration without cycling evolution; with steady low latitude magnetic field wreaths are found for slowly rotating cases with large Rossby number. For models rotating faster with a low Rossby number, the convective dynamo shows a cycling activity, leading to systematic pole inversion. We also note that a topology change of the stellar magnetic field occurs going from a dipolar-like to a quadrupolar-like structure when the system magnetic energy drops during the cyclic activity, in good agreement with our star the Sun.

M. J. P. F. G. Monteiro, T. L. Campante, D. R. Reese, T. R. White, A. Garc\'{i}a Hern\'{a}ndez, C. Jiang

The detection and analysis of oscillations in binary star systems is critical in understanding stellar structure and evolution. This is because such systems have the same initial chemical composition and age. Solar-like oscillations have been detected in both components of the asteroseismic binary HD 176465 by Kepler (White et al., 2016). This study presents an independent modelling of the two stars in this binary system. Stellar models generated using MESA (Modules for Experiment in Stellar Astrophysics) were fitted to both the observed individual frequencies and some spectroscopic parameters. The individual theoretical oscillation frequencies for the corresponding stellar models were obtained using GYRE as the pulsation code. A Bayesian approach was applied to find the Probability Distribution Functions of the stellar parameters using AIMS (Asteroseismic Inference on Massive Scale) as the optimization code. The age of the individual stars was found to agree with that obtained by White et al., (2016) of about 3.0 $\pm$ 0.5 Gyr old.

Laure Lef\`evre, Edward W. Cliver, Leif Svalgaard

In 2015, a new entirely recalibrated version of the sunspot number and group number series has been released. Important changes, by up to 40\%, have been applied to the original historical series, leading to a new picture of the secular evolution of solar activity, without significant upward trend between the $17^{th}$ and the $20^{th}$ century. We first describe the main changes and implications of this very first revision of the sunspot number series since its creation, more than 165 years ago. We also discuss the long-term non-linear relation between the sunspot number and the group number, as it is now free from artifacts. Both series are now largely reconciled, but do not fully overlap, clearly reflecting different properties of the solar cycle. Together with this major step, several important changes were simultaneously adopted regarding past conventions. We will explain the various modifications and their motivations, in order to help users making the proper adaptations. In particular, A.Wolfer was chosen as the new reference, in place of R. Wolf, thus dropping the fixed 0.6 Z\"urich factor. For the group numbers, given the time variability of the average number of spots per group, we don't apply anymore a constant scaling factor to match the average scale of the sunspot number, in contrast with the previous series by Hoyt and Schatten (1998). Finally, we present the new data sets and data formats adopted for this revised series and for the future production of the sunspot number. The new SILSO Web portal will provide access both to current and past versions of the series, allowing to keep track of future revisions and thus giving more flexibility to follow future progresses in sunspot science. Finally, we conclude on the redefinition of the base method used to routinely produce the sunspot number from all current and future observations of the SILSO worldwide network. New tools and statistical approaches derived directly from our global recalibration work will soon be ported to our operational software, improving the quality control and the long-term stability of the sunspot number series. This will complete the necessary modernization of our only direct long-term record of solar activity.

Saskia Hekker, Elisabeth Guggenberger, Nathalie Themessl

Over 35 years ago, it was proposed that there exists diagnostic potential in combining the small and large frequency separations for solar-like oscillators. Since then, we have been spoilt with a plethora long-timeseries photometric light curves from which individual stochastic oscillation modes can be extracted. These light curves allow the separations to be determined en masse and in power spectra with relatively low signal-to-noise ratios. The small-frequency separation in low-mass main-sequence stars and in subgiants is sensitive to the molecular weight gradient in the inner regions whilst the large-frequency separation is indicative of the mean density of the star. We investigate how accurately these parameters must be determined in order to place constraints on stellar parameters such as mass and age. We discuss some difficulties associated with calculating the necessary large grids of models (with different input physics) as well as the degeneracy that exists in later evolutionary phases.

It is not uncommon to see the references to recent (cycle 24) solar activity as “exceptionally low” or as an “extended long-term decline”. But how would we know that the current level of activity is unusual if we did not have historical data taken over many cycles? How would we know, for example, what are the strongest field strengths in sunspots and how they change with time or that the amplitude of next cycle could be defined by the strength of polar field in previous cycle if we did not have long-term records of solar activity? Truly, synoptic observations feed future research to solve issues that may not be identified at the time when data are acquired. In my talk, I will discuss the current state of long-term synoptic programs and present results of my recent projects on reconstructing the solar activity using historical data.

SOLAR-C WG

Solar-C is a Japan-led international solar mission designed to investigate the magnetic activities of the Sun, focusing on the study in heating and dynamical phenomena of the chromosphere and corona, and also to develop an algorithm for predicting short and long term solar evolution.It has long been known that the interplay between magnetic fields and plasmas is at the heart of most solar phenomena, but the details of this interplay are in many cases clouded in ambiguity and uncertainty. To dramatically improve the situation, SOLAR-C will carry three dedicated instruments; the Solar UV-Vis-IR Telescope (SUVIT), the EUV Spectroscopic Telescope (EUVST) and the High Resolution Coronal Imager (HCI), to jointly observe the entire visible solar atmosphere with essentially the same high spatial resolution (0.1-0.3 arcsec), performing high resolution spectroscopic measurements over all atmospheric regions and spectro-polarimetric measurements from the photosphere through the upper chromosphere. In addition, Solar-C will contribute to our understanding on the influence of the Sun-Earth environments with synergetic wide-field observations from ground-based and other space missions. I will present some leading science objectives and the mission concept, including the current status of SOLAR-C.

François Lignières (IRAP), Jérôme Ballot (IRAP)

Seismology of intermediate-mass and massive stars is limited by our lack of understanding of the effect of fast rotation on gravity modes. In particular, in this regime perturbative methods are unable to identify observed modes. We therefore develop an asymptotic theory for adiabatic gravito-inertial modes in uniformly rotating stars. We first derived a generalized dispersion equation taking the Coriolis force and the centrifugal deformation into account. The corresponding ray dynamics allowed us to explore the structure of the phase space thanks to a ray-tracing code. We observed three coexisting types of structures: (i) nearly integrable structures similar to non- rotating structures, (ii) island chains around stable periodic orbits, (iii) large chaotic zones. These three different types of structures are expected to give three different families of modes.

General description of a ring-like molecular structure discovered in the G332 sector, located at a V(lsr) = ~ -50 km/s with a minimum spectral width of ~7 km/s, at a distance of 3.7 kpc from Earth. The ring is observed in the 12CO, 13CO, C18O lines of the MopraCO Survey, and in the CI line measured from the HEAT telescope in Antarctica. Deeper investigation are ongoing to better characterise the physics of the ring and the related features interacting with the molecular structure

G. J. D. Petrie, & N. K.:Singh

The solar magnetic helicity has opposite signs not only in the two hemispheres, but also at large and small length scales. The latter can be detected by computing magnetic helicity spectra, but this must be done separately in each hemisphere. Here we utilize a two-scale method from mean-field dynamo theory that allows us to compute magnetic helicity spectra as a function of two different wavenumbers: one corresponding to rapidly varying scale and one corresponding to a slowly varying one. We generalize this method to spherical harmonics and compute in that way global magnetic helicity spectra for that part of the field that shows a global dipolar symmetry. We present results from simple one-dimensional model calculations, three-dimensional dynamo simulations, and the two-dimensional magnetic field from synaptic vector magnetograms.

Markus Roth, Jason Jackiewicz

We present the current status of an undergoing validation of a recently developed model for computing spherical Born approximation sensitivity functions for flows. In a first step, power spectra and reference cross-correlations from the model and a simulation of Hartlep et al. (2013) are matched. Some difficulties in obtaining such a match are discussed. In a second step, travel times from the forward model and from the simulation, which includes a standard meridional flow profile, are to be compared. The analysis procedure including the use of phase-speed filters is identical to the one employed in Jackiewicz et al. (2015). Furthermore, we present a novel approach for a fast computation of integrated sensitivity functions which can be used for interpreting rotationally symmetric flows such as differential rotation and meridional flow.