Variability of Solar/Stellar
Magnetic Activity

Session organizers

• Damian Fabbian (Max Planck Institute for Solar System Research, Germany)
• Rosaria Simoniello (University of Geneva, Switzerland)
• Remo Collet (Aarhus University, Denmark)
• Serena Criscuoli (National Solar Observatory, USA)
• Heidi Korhonen (University of Copenhagen, Denmark)
• Natalie Krivova (Max Planck Institute for Solar System Research, Germany)
• Katalin Oláh (Konkoly Observatory, Hungary)
• Alexander Shapiro (Max Planck Institute for Solar System Research, Germany)
• Aline Vidotto (Trinity College Dublin, Ireland)
• Nikola Vitas (Instituto de Astrofisica de Canarias, Spain)

Scientific Motivation

The richness of solar observations, for which continuous monitoring is now available at extremely high spatial resolution and throughout the electromagnetic spectrum (e.g., HINODE, SDO), is complemented by a wealth of new spectro-polarimetric information at increasing temporal resolution in the new era of statistical studies of unprecedentedly huge samples of stars observed by space-borne telescopes like KEPLER and COROT

Together with the results of advanced theoretical and computational tools and the availability of massively-parallel supercomputers, these new data are rapidly changing our view of stellar magnetic activity and variability throughout stellar evolution.

Solar-like stars are known to show chromospheric activity similar to that on the Sun, e.g. in the Ca II H and K emission indicators. Magnetic activity shows erratic, multi or single periodic behavior, interrupted by long quiescent periods (grand minima). What is the origin of this broad range of variability in stellar activity and how does it relate to solar variability and activity?

In solar-like stars, the cyclic activity is ascribed to a dynamo mechanism maintained through differential rotation at the tachocline. However, the tachocline moves towards increasing depths with later spectral types, disappearing around spectral-type M4. Thus, a direct comparison between the stellar activity in solar-type with lower-mass stars is essential for the understanding of the effect of stellar mass on the resulting magnetic activity through a dynamo mechanism.

This Splinter is aimed at offering a synthetic view of the recent progresses in the domain of variability of stellar magnetism from different perspectives. We invite astrophysicists to present their latest results on this topic, particular in relation to the solar-stellar connection and the intimate interplay between magnetic activity and variability of our Sun and cool stars.

Programme

This splinter session will be split over two afternoons, Tuesday 7 June and Thursday 9 June. The full programme of the splinter session can be downloaded here.

In Session 1 (Tuesday 7 June), we will address the nature of solar/stellar magnetic variability based on observations of the Sun and other cool stars. We will focus on solar/stellar brightness variations on different time scales as well as at on the properties of solar/stellar magnetic fields

In session 2 (Thursday 9 June), we will address the origin and evolution of stellar magnetic activity in light of the latest results from survey and dynamo theory.

Click on the table below to expand the detailed programme for the various sessions:

• Tue 7 June | 14:30 - 17:30 | Session 1: Magnetic variability as a key to the solar/stellar connection
  • Tue 7 June | 14:30 - 15:45 | Session 1.1: Solar/stellar variability: observational properties and theory
    • 14:30    M. Giampapa (invited):  Photometric Variability in the Sun and Sun-like Stars
      The rich array of solar magnetic field-related phenomena we see occurs not only on stellar counterparts of our Sun but in stars that represent significant departures in their fundamental parameters from those of the Sun. Though these phenomena appear energetically negligible when compared to the total luminosity of stars, they nevertheless govern the angular momentum evolution and modulate the radiative and particle output of the Sun and late-type stars. The term “The Solar-Stellar Connection” has been coined to describe the solar-stellar synergisms in the investigation of the generation, emergence and coupling of magnetic fields with the outer solar-stellar atmosphere to produce what we broadly refer to as magnetic activity. With the discovery of literally thousands of planets beyond our solar system, the Solar-Stellar-Planet Connection is quickly emerging as a new area of investigation of the impacts of magnetic activity on exoplanet atmospheres. In parallel with this rapid evolution in our perspectives is the advent of transformative facilities for the study of the Sun and the dynamic Universe. The primary focus of this invited talk will be on photometric variations in solar-type stars and the Sun. These brightness variations are associated with thermal homogeneities typically defined by magnetic structures that are also spatially coincident with key radiative proxies. Photometric variability in solar-type stars and the Sun includes transient brightening, rotational modulation by cool spots and cycle-related variability, each with a characteristic signature in time and wavelength. The emphasis of this presentation will be on the relationship between broadband photometric variations and magnetic field-related activity in solar-type stars and the Sun. Facets of this topic will be discussed both retrospectively and prospectively as we enter a revolutionary, new era for astronomy.
    • 14:50    R. A. García:  Probing Stellar Magnetism with Space Photometry of Solar Analogs
      The surface magnetic field has substantial influence on various stellar properties that can be probed through various techniques. With the advent of new space-borne facilities such as CoRoT and Kepler, uninterrupted long high-precision photometry is available for hundred of thousand of stars. This number will substantially grow through the forthcoming TESS and PLATO missions. The unique Kepler observations –covering up to 4 years with a 30-min cadence– allows studying stellar variability with different origins such as pulsations, convection, surface rotation, or magnetism at several time scales from hours to years. We study the pho- tospheric magnetic activity of solar-like stars by means of the variability induced in the observed signal by starspots crossing the visible disk. We constructed a solar photometric magnetic activity proxy, Sph from SPM/VIRGO/SoHO, as if the Sun was a distant star and we compare it with several solar well-known magnetic proxies. The results validate this approach. Thus, we compute the Sph proxy for a set of CoRoT and Kepler solar-like stars for which pulsations were already detected. After characterizing the rotation and the magnetic properties of ∼300 solar-like stars, we use their seismic properties to characterize 18 solar analogs for which we study their magnetism. This allows us to put the Sun into context of its siblings.
    • 15:02    C. Marvin:  Measurements of Absolute Calcium II H & K in FGKM Stars
      M dwarfs are the most numerous stars in the universe, yet they still lack absolute chromospheric Ca II H and K (R'_HK ) calibrations to effectively compare their activity with FGK stars. We scale high-S/N, high-resolution template spectra, obtained by co-adding multiple HARPS spectra of the same star, to PHOENIX stellar atmosphere models, and obtain chromospheric line measurements of Ca II H & K in physical units of 106 M dwarfs. We also derive new Mt. Wilson S-index to R'_HK conversions appropriate for cooler stars, ranging from 0.82 ≤ (B − V ) ≤ 2.00. We establish a chromospheric activity database by combining archival data of FGK stars and using our technique to extend absolute chromospheric measurements to M dwarfs. Our results show that using model atmospheres provide a reliable way to scale uncalibrated spectra and also estimate photospheric flux for M dwarfs, but note that accurate stellar parameter determination is essential to compare chromospheric emission of different spectral types.
    • 15:14     R. Haywood:  Radial-Velocity Variability of the Sun as a Star with HARPS and HARPS-N
      Since we can resolve the surface of the Sun directly, we can explore the origin of radial-velocity variations induced by individual solar surface features such as faculae/plage, sunspots and granulation. I will present my recent investigation of the radial-velocity variations of the Sun as a star, based on high-resolution HARPS spectra of reflected sunlight and simultaneous images from the Solar Dynamics Observatory. We found that faculae are the dominant source of activity-induced radial-velocity variations, via suppression of convective blueshift. We investigated possible proxies for activity-induced radial-velocity variations and found that optical lightcurves can only provide a partial representation of these signals; the full-disc magnetic flux, however, is an excellent tracer. In addition to this dataset, the HARPS-N spectrograph has been operating with a new solar telescope feed since 2015 July. I will present results from the first year observations, which show radial-velocity variations of up to 7-8 m/s. Identifying proxies for solar radial-velocity variations is key to understanding the radial-velocity variability of other Sun-like stars, and is also essential for other investigations such as exoplanet detection surveys.
    • 15:26    J. R. Barnes:  Photospheric Acne at the Bottom of the Main Sequence
      Starspots are an important manifestation of stellar activity and yet their distribution patterns on the lowest mass stars is not well known. Time series spectra of fully convective M dwarfs taken in the red-optical with UVES reveal numerous line profile distortions which are interpreted as starspots. We derive Doppler images for four M4.5V - M9V stars and find that contrast ratios corresponding to photosphere-spot temperature differences of only 200-300 K are sufficient to model the timeseries spectra. Although more starspot structure is found at high latitudes, spots are reconstructed at a range of phases and latitudes with mean spot filling factors of only a few per cent. The occurrence of low-contrast spots at predominantly high latitudes is in general likely to be responsible for the low amplitude photometric variability seen in late-M dwarfs. The recovered starspot patterns are used to assess their effect on precision radial velocity surveys aimed at detecting planets around this population of stars.
    • 15:45 - 16:15    Coffee Break
      Coffee!
  • Tue 7 June | 16:15 - 17:30 | Session 1.2: Stellar magnetic fields and their impact on the surrounding environment
    • 16:15     S. Marsden (invited):  Magnetic Fields on Solar-Type Stars: The Solar-Stellar Connection
      Our understanding of solar magnetic fields is significantly more detailed than we can achieve for other stars. However, over the last few decades, techniques such as Zeeman Doppler Imaging (ZDI), have delivered major advances in the study of stellar magnetic fields, to the point where we can now map the global surface magnetic topology of other stars and use these to model the coronal magnetic fields and even the stellar wind produced by the star. These advances now allow us to compare the Sun with other solar-type stars and make increasingly detailed comparisons of the behaviour and generation of their dynamo magnetic fields. In this review I will give an overview of what ZDI has taught us about the magnetic fields of other stars and how our study of the Sun can inform our understanding of the dynamos operating across a range of solar-type stars.
    • 16:35    S. Aigrain (invited):  The Effects of Stellar Activity on Detecting and Characterizing Planets
      Intrinsic stellar variability associated with magnetic activity, rotation and convection, affects the detection of exoplanets via the transit and radial velocity methods, and the characterisation of their atmospheres. I will review the increasingly sophisticated methods developed in the last few years to mitigate this problem, and outline how stellar variability is likely to impact the field of exoplanets in the future. Planetary transits last a few hours, much shorter than the rotational modulation of star spots (day to weeks), but smaller-scale variability is nonetheless an important limiting factor in our ability to detect transits of Earth analogs in Kepler and Plato data. In radial velocity, the problem is even more severe, as the planet’s signal occurs on the orbital timescale, which can coincide with the range expected for stellar rotation periods or activity cycles - but the spectra used to extract radial velocities contain a wealth of information about stellar activity that can be used to disentangle the two types of signals. Finally, when using transits or phase curves to probe the composition and dynamics of planetary atmospheres, star spots must be accounted for very carefully, as they can mimic or mask planetary atmosphere signals. On the positive side, the sensitivity of planet search and characterisation experiments to stellar activity means that they are a treasure trove of information about stellar activity. The continued success of exoplanet surveys depends on our making the best possible use of this information.
    • 16:55    J. Llama:  The Impact of Stellar Activity on High Energy Exoplanet Transits
      High energy (X-ray / UV) observations of transiting exoplanets have revealed the presence of extended atmospheres around a number of systems. At these energies, stellar radiation is absorbed in the upper atmosphere of the planet, making X-ray / UV transits an exciting tool for investigating the composition of exoplanetary atmospheres. However, the effects of stellar activity on transits at these wavelengths is far from understood. In X-rays the stellar disk appears limb-brightened, and active regions appear as extended bright features that evolve on a much shorter timescale than in the optical. This makes measuring the true planet-to-star radius ratio challenging. The Sun offers a unique opportunity to study the impact of stellar activity on high energy transits. Using disk resolved soft X-ray and UV images from NASA’s Solar Dynamics Observatory taken over the last solar cycle I will show how both occulted and unocculted active regions can mimic an inflated planetary atmosphere by changing the depth and shape of a transit profile. I will also show how the disk integrated Lyman-α Solar irradiance varies on both short and long timescales and how this variability can also impact our ability to recover the true radius ratio of a transiting exoplanet. Finally, I will present techniques to overcome these challenges in high-energy transits.
    • 17:07    J. D. Alvarado-Gómez:  Simulating the Environment Around Planet-Hosting Stars
      Recent developments in instrumentation and observational techniques have opened a new window for stellar magnetic field studies. In particular Zeeman Doppler Imaging (ZDI) is now routinely used to recover the large-scale magnetic field topologies of stars different from the Sun, including several planet-hosting stars. These stellar magnetic fields intimately affect the environment around late-type stars by driving the coronal high-energy radiation (EUV/X-rays), transient events (e.g. flares and coronal mass ejections), and the development of stellar winds and astrospheres. These elements can have a strong impact in the evolution of planetary systems via star-planet interactions and erosion of exoplanetary atmospheres. In this context, the results from ZDI data-driven, detailed 3D MHD modeling of the coronal conditions and circumstellar environment around three planet hosting stars are presented. For one of the considered systems (HD 1237), we investigate the interactions of the magnetized stellar wind with the exoplanet, assuming a Jupiter-like magnetosphere around it.
    • 17:19     M. Jardine:  Predicting the Wind Speeds of Solar-Like Stars
      Some aspects of stellar magnetic activity such as X-ray emission are relatively easy to observe, while others, such as the geometry of the magnetic field are rather more difficult. Indeed, typically only the large-scale (or low-order) components of the field can be mapped. Most elusive of all is the hot, tenuous stellar wind, yet the wind speed is a crucial quantity governing the angular momentum loss of the star. We demonstrate, however, that wind speeds can be predicted reliably even from fairly low-resolution magnetograms. We use an empirically-derived model of the solar wind that predicts the distribution of wind speeds based on surface magnetograms. Using solar magnetic field measurements spanning several magnetic cycles, we demonstrate how changes in the surface magnetic field on various lengthscales affect the coronal structure and hence the wind speed. We find that only low-order field components are needed to characterise the wind speed. We compare the variations in wind speed with variations in the X-ray emission over several cycles and show that while the small-scale field has a significant effect Lx, it has little effect on the wind speed. This suggests that the large number of stellar magnetograms that are becoming available can be used to predict the stellar wind speeds.
    • 17:30     End of Session 1
• Thu 9 June | 14:30 - 17:30 | Session 2: Variability and activity throughout stellar evolution
  • Thu 9 June | 14:30 - 15:45 | Session 2.1: Age-Rotation-Activity relation from stellar surveys and theory
    • 14:30    S. K. Solanki (invited):  Variability of the Sun and Sun-like Stars on Different Time Scales
      Although many features can be seen moving across the sun at various wavelengths, the Sun is an almost constant star with regard to its radiative output and its radiative variability has generally only been discovered after the beginning of the space age. For Sun-like stars it is easier to measure their small levels of variability from the ground, leading to some unexpected discoveries that seemed to suggest a behavior rather different from that displayed by the Sun. Revisions of the measurements and advances in modelling have, however, in recent years shown that some of the main differences between the Sun and stars can be reconciled. A much richer data set of short and medium-term variability has been provided by the COROT and Kepler missions. Although a number of exciting results have already been obtained from these data, many more are expected in the coming years as the data are more thoroughly analyzed and models developed to describe solar variability are extended to describe stars similar to the Sun, but not exact solar analogs.
    • 14:50    G. Basri (invited):  Age-Rotation-Activity Relations: Observations and Theory
      The fact that stellar rotation and chromospheric emission are correlated with age was explicitly noted by Wilson (1963) and reinforced by Kraft (1967). Wilson knew that CaII emission was correlated with surface magnetic field in the Sun. Skumanich (1972) suggested a simple functional for the age-activity relation, and suggested that magnetic braking was the likely reason for the decline in activity. A theory for the rotation-activity connection was elucidated by Noyes et al. (1984), who invoked the Rossby number as important to the stellar dynamo. This calibrated the relation by convection zone depth and turnover time, although it was noted early and recently confirmed that it is not clear whether Rossby number is empirically superior to the rotation period itself in producing a clear rotation-activity relation. In fact, turnover times are hard to properly define, and the Rossby number is itself calibrated to tighten the relations. The number of stars in samples used to study this has increased dramatically, as have the diagnostics available to assess magnetic activity. It remains clear is that there is a strong relationship between magnetic activity and stellar rotation, and that magnetic braking forces both activity and rotation to decrease with age. These relations are also subject to modification as a function of stellar mass. There has recently been a great increase in the number of measured stellar rotation periods, and in the calibration of these relations using star clusters (whose ages can be independently assessed). I will summarize some of the ongoing progress on this topic.
    • 15:10     R. Booth:  An Improved Stellar Age-Activity Relationship for Ages Beyond a Gigayear
      When the first bio-marker in an exoplanetary atmosphere is found, it will most likely be found in a planet orbiting an inactive, old nearby star. In order to assess this exoplanet’s evolution, we will need to reliably estimate the age of the star, which is notoriously difficult for old field stars. Since Skumanich first presented evidence of a relationship between the rotation and the age of a star there has been much interest in developing a calibrated relationship which can be used to provide an estimate of a star’s age. Currently these age-activity relationships are only reliable for ages under a gigayear. My research uses the advancement in asteroseismology which has allowed large samples of solar and late-type stars’ ages to be determined and combines these with X-ray data from which I measure the stellar activity in order to derive a new and improved age-activity relationship for stars with ages greater than a gigayear.
    • 15:22    J. Lehtinen:  Rotation and Spot Activity of Young Solar-Type Stars
      We present results from a recent multidecade study of the spot activity of young solar-like stars, based on photometry gathered at the Fairborn observatory since 1987. Our period analysis of this photometry reveals systematic tendencies in the stellar rotation and activity behaviour with respect to stellar mass. Period variations seen in the light curves point to a flat rotation dependence of the absolute equator to pole differential rotation. Activity cycles are commonly seen in the light curve mean levels and amplitudes and their lengths fall on distinct branches when compared to the Rossby number and chromospheric activity levels. Our results indicate that there are systematic simultaneously excitable cycle modes present in the active stars in the form of a previously undescribed split into two parallel cycle branches. Several of the studied stars have super- imposed cycles belonging to both of the parallel branches. Active longitudes are also frequently seen on the more active stars and range from short lived structures lasting for a few years into highly stable longitudinal confinement of the observed activity over decades. We find, however, that there is a sharp transition between weakly and strongly active stars so that only stars with a sufficiently high activity level have clearly developed active longitudes. When active longitudes do appear, they commonly have a markedly shorter rotation period than the photospheric mean rotation period, recovered from direct light curve period analysis. This may be a signature of radial differential rotation or that the active longitudes follow an azimuthal dynamo wave.
    • 15:24    V. See:  What Can We Learn About Stellar Activity Cycles from ZDI?
      It is known that activity cycles, similar to the 11 year cycle of the Sun, can exist on other stars. Previous work suggests that stars may lie on two branches in a cycle period vs rotation period diagram though there is no definitive explanation for why this should be the case. Fundamentally, activity cycles occur as a result of the underlying dynamo. Indeed, a great deal has been learnt about the Sun’s activity cycle by studying how its magnetic field evolves over each activity cycle. In the same way, we should be able to learn about the activity cycles of others stars by studying their magnetic field properties. In this talk, I will present new insights into stellar activity cycles by analysing the magnetic maps of stars that are known to present activity cycles. I will show that stars along each of the branches appear to have different magnetic field topologies.
    • 15:45 - 16:15    Coffee Break
      Coffee!
  • Thu 9 June | 16:15 - 17:30 | Session 2.2: Constraining Solar/Stellar dynamo theory
    • 16:15     L. Jouve (invited):  Numerical Simulation of Solar/Stellar Dynamos
      In this talk, we will review some aspects of the stellar magnetism and in particular what numerical simulations tell us about the physical processes underlying the observations. In cool stars, a convective dynamo is thought to be responsible for the presence and evolution of magnetic fields. The question of the impact of the internal stellar structure on the magnetic field topology will be addressed. We will focus in particular on the role of differential rotation and of a tachocline. Another important aspect of stellar dynamos is the possible presence of magnetic cycles and how its period depends on the stellar parameters. Numerical simulations addressing this issue will be presented. Finally, one step of the dynamo process is the emergence of magnetic flux from the interior where it is created and organised to the exterior where it emerges as starspots. We will also show results of global 3D MHD numerical simulations of such a process.
    • 16:35    T. Metcalfe:  The Stellar Context for Solar Magnetism
      Precise photometry from the Kepler space telescope allows not only the measurement of rotation in solar-type field stars, but also the determination of reliable masses and ages from asteroseismology. These critical data have recently provided the first opportunity to calibrate rotation-age relations for stars older than the Sun. The evolutionary picture that emerges is surprising: beyond middle-age the efficiency of magnetic braking is dramatically reduced, implying a fundamental change in angular momentum loss beyond a critical Rossby number (Ro∼2). I will review detailed evolutionary modeling of the Kepler observations and discuss recent efforts to expand the sample and minimize systematic uncertainties. This will provide the context for multiple lines of evidence that the Sun is in a transitional evolutionary phase, and that its magnetic cycle may represent a special case of stellar dynamo theory.
    • 16:47    K. Augustson:  Magnetic Furnaces: Examining Fully Convective Dynamos and the Influence of Rotation
      The dynamo action likely present within fully convective regions are explored through global-scale 3-D simulations. These simulations provide a contextual analog for the convective dynamos that are likely operating deep within the interiors of fully convective low mass stars. A logarithmic range of rotation rates are considered, thereby capturing both convection barely sensing the effects of rotation to others in which the Coriolis forces are prominent. The vigorous dynamo action realized within all of these turbulent convective cores builds magnetic fields with peak strengths exceeding a megagauss, with the overall magnetic energy (ME) in the faster rotators reaching super-equipartition levels compared to the convective kinetic energy (KE). Such strong fields are able to coexist with the flows without quenching them through Lorentz forces. This state is achieved due to the velocity and magnetic fields being nearly co-aligned, and with peak magnetic islands being somewhat displaced from the fastest flows as the intricate evolution of these MHD structures proceeds. As the rotation rate is increased, the primary force balance shifts from nonlinear advection balancing Lorentz forces to a magnetostrophic balance between Coriolis and Lorentz forces.
    • 16:59    S. Engle (TBC):  Activity-Rotation-Age Relationships for M dwarfs and the Ages of Extrasolar Planets
      Red Dwarfs (M dwarfs or dM stars) make up ~75% of the local stellar inventory, and a recent statistical analysis data from the Kepler Mission indicates that ~15% of red dwarfs host Earth-size planets orbiting within their liquid water Habitable Zones (HZ). This is among the reasons they have been targeted by an increasing number of planet-hunting programs. As such, developing a method to accurately estimate the age of a field M dwarf is of critical importance to a number of fields. However, due to their long lifetimes and very slow nuclear evolution, the best method for determining ages is likely through “magnetic tracers” such as X-UV activity levels and stellar rotation rates. The Living with a Red Dwarf program’s database of M dwarfs with photometrically determined rotation periods (via starspot modulations) is becoming substantial, and a full range of “calibrators” is being realized. We report on our current results and continuing efforts to build reliable Activity-Rotation-Age relationships for M dwarfs, utilizing X-UV measures obtained with HST, IUE Chandra and XMM (both proposed by us, and archival). Such relationships permit the assessment of the habitability of planets hosted by red dwarfs, by delineating the X-UV radiation environments these planets are exposed to, and have been exposed to in the past. After proper calibration, the relationships can also permit the age of a field red dwarf (and any hosted planets) to be determined through measures of either the stellar rotation period or X-UV activity level.
    • 17:11 - 17:30    Final Discussion Session for Afternoons 1 & 2
      Peculiarities and Common Features of Magnetism and Activity in the Sun and in Solar-Like Stars
    • 17:30     End of Session 2

Invited speakers

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