Session Summary Session 11. Long-Term Solar/Stellar Variability: Closing the Rift Between Models and Observations Organizers: Lisa Upton, Irina Kitiashvilli, Travis Metcalfe, Andres Munjoz-Jaramillo Scene Setters: Ricky Egeland and Axel Brandenburg

Session Description: Understanding the dynamics of solar and stellar interiors and how they produce long-term cyclic behavior continues to challenge both the astronomy and solar communities. The stars offer a broader perspective, showing a wide variety of rotational behaviors, magnetic topologies, as well as cycle amplitudes and periods. Ground and space- based observations of the Sun offer enormous amounts of data which allow techniques, such as helioseismology, to explore magnetism and convection zone dynamics in far greater detail. Advances in computational science, such as data assimilation and machine learning, are inspiring studies which allow us to begin the unification of theory, models, and observations. Despite these advances, there are still challenges to creating physics-based models which are consistent with observations. These include gaps in observations (e.g., lack of a global observation of any star, including our own Sun’s polar regions), comparatively short duration of observational data (e.g., only a few cycles), the many time and spatial scale lengths involved in creating complete models, as well as the difficulty bridging historical separation between observers, theoreticians and modelers. This session will address these and other challenges for understanding solar and stellar dynamics and long-term activity. (Brown et al. 2011)

The Rift Between Models and Observations 1.Dynamo models omit the surface and as a consequence produce no directly observable results. 2.Observations are unable to detect magnetism in the interior, and therefore provide no discerning value. 3.Dynamo models are parameterized by quantities that cannot be measured nor calculated for real stars. What can the variety in solar/stellar variability tell us about the physics of stellar interiors and the dynamo?

´ Cycles can be regular or chaotic. ´ Many stars have persistent secondary cycles. ´ Why don't all stars cycle? ´ Non-cycling stars also provide important constraints. ´ Cycle period increases with rotational period: ´ Appear to be two branches. ´ Sun doesn't fit with either (transitioning). ´ Fully convective stars have cycles: ´ Tachocline is not required. ´ Probably still need shear layers. What can the variety in solar/stellar variability tell us about the physics of stellar interiors and the dynamo?

´ Cycles can be regular or chaotic. ´ Many stars have persistent secondary cycles. ´ Why don't all stars cycle? ´ Non-cycling stars also provide important constraints. ´ Cycle period increases with rotational period: ´ Appear to be two branches. ´ Sun doesn't fit with either (transitioning). ´ Fully convective stars have cycles: ´ Tachocline is not required. ´ Probably still need shear layers. What can the variety in solar/stellar variability tell us about the physics of stellar interiors and the dynamo?

´ Cycles can be regular or chaotic. ´ Many stars have persistent secondary cycles. ´ Why don't all stars cycle? ´ Non-cycling stars also provide important constraints. ´ Cycle period increases with rotational period: ´ Appear to be two branches. ´ Sun doesn't fit with either (transitioning). ´ Fully convective stars have cycles: ´ Tachocline is not required. ´ Probably still need shear layers. Are cycle to cycle fluctuations in variability, including distinct phases such as the Maunder Minimum, generated by fundamental changes in the dynamo or naturally by random processes?

´ Evidence that this could be caused by the random stochastic fluctuations in active region emergence (e.g., Joy’s Tilt).

(Karak & Miesch 2018) What observational data is needed from future missions to provide the most significant advances in the study of long-term solar/stellar variability?

´ Better meridional flow measurements in the Sun - at high latitudes and in the throughout the convection zone. ´ Measurements of the magnetic field in the Sun’s convection zone. ´ More accurate measurements of stellar properties – mass, composition, ages (e.g. Methuselah older than universe) with known rotation and cycle periods. ´ Convective properties of stars (e.g., turbulent pumping, turnover, Rossby #, etc.) ´ Differential Rotation of stars. Are there anti-solar stars? ´ Long term synoptic observations of many stars: ´ Only ~ 300 stars have 10+ years. ´ Only ~ 150 stars have 30+ years. ´ Zeeman-Doppler imaging of stars: ´ Latitudinal distribution of magnetic field. Alvarado-Gómez et al. (in prep) What are the challenges to creating physics-based models, which are consistent with observations, to better understand and forecast solar activity?

´ Currently model the Interior or Surface, but not both – How do we merge the two in models? ´ Highly stratified, with many scale heights. ´ Unable to produce near-surface shear layer at all latitudes. ´ Initial conditions - models require a long time to get to a stable solution. ´ Need to be able to reproduce rotation/cycle period/activity level regimes that are consistent with the observations. ´ Not clear how to discriminate between models. ´ Convection simulations produce too much power at low wavenumber. ´ Solar dynamo models don’t produce the precise form of the differential rotation (non-cylindrical, closer to radial). Outstanding Questions

´ To cycle or not to cycle? What factors determine where a star will cycle or not (e.g., rotation, stellar type, stellar twins, etc.)?

´ Is the slope for the ratio of Prot/Pcyc positive or negative? ´ Why doesn’t the Sun have an anti-solar differential rotation? ´ Is there a critical Rossby # that serves as a dynamo threshold? ´ How do stars enter/come out of a Maunder Minimum? ´ Why do most stars have two cycles? Is this a signature of 2 dynamos operating simultaneously? ´ Can spots be made near the surface or only in the deep interior? ´ How important is the near-surface shear layer? ´ What would be a satisfactory dynamo solution? Key Take Away Messages

´ The solar dynamo is the longest unsolved problem in astrophysics! ´ At present, we cannot believe dynamo models because they cannot reproduce observations. ´ If we make the models more realistic, we WILL get there. ´ Bringing together the solar and stellar communities is essential to advancing our understanding of how the dynamo works. ´ Stellar observations are breaking new ground (e.g., Zeeman Doppler Imaging => magnetic cycle maps, star spot emergence and decay). ´ We’ve barely scratched the surface – we need a lot more observations. ´ We have a multi-faceted problem that requires large-number statistics. ´ In particular, a long-term synoptic program. Suprathermal Ion abundance variations in corotating interaction regions over two solar cycles Robert C Allen Johns Hopkins University Applied Physics Lab Far Beyond the Sun: Mapping the Magnetic Cycle of the Young Solar-Analog iota Horologii Julián David Alvarado-Gómez Center for Astrophysics | Harvard & Smithsonian GPS, decrypting brightness variations of the Sun and Sun-like Eliana Maritza Amazo-Gómez Max-Planck-Institut fur Sonnensystemforschung Long-term reconstruction of Solar UV indices Serena Criscuoli National Solar Observatory Magnetic Properties of Asterospheres of Exoplanet Systems Alison Farrish Rice University Do Sun-like stars experience a magnetic mid-life crisis? Investigation from a new Ca HK activity survey using LCO NRES Adam Goga Coastal Carolina University Work Done by Lorentz Force Drives Solar-Stellar Magnetic Cycles Sushant Sushil Mahajan Georgia State University Lifetimes and emergence/decay rates of star spots on solar-type stars estimated by Kepler data in comparison with those of sunspots Kosuke Namekata Kyoto University Do superflares really occur on slowly-rotating Sun-like stars in the long-term activity changes? -– Latest statistical results using Kepler and Gaia-DR2 data Yuta Notsu University of Colorado Boulder Numerical Modeling of a Laboratory Star Fredy Ramirez University of Colorado, Boulder