Flux transport dynamos have been successful in reproducing a number of behaviors seen in the Sun. This includes in particular the equatorward migration of the toroidal magnetic field and the rush to the poles of poloidal magnetic field at high solar latitudes. Full numerical simulations have failed to produce these aspects in a satisfactory manner, so something must be wrong with the simulations. The problem starts already when comparing the large-scale flows, which display markedly cylindrical patterns in both angular velocity and meridional circulation, which appear incompatible with helioseismology. Some simulations display equatorward migration, but this is because of a localized dip in angular velocity at mid-latitude, which again is not found in the Sun. In principle, of course, the simulations ought to reproduce the solar behavior eventually, as better resolution can be achieved. The biggest resolution deficit is near the surface, where global simulations are unable to capture the effects of what is known as entropy rain. In my talk, I will review aspects of the solar dynamo and will discuss how the effects of entropy rain may change the picture. I will finish with speculations of how one might shortcut some of these hurdles.

1. History of Exoplanets at NCAR
1. 1995–1996: Prehistory: Planets of sunlike stars are first detected
2. 1997–1998: HAO/Lowell stirrings: An alliance is formed, and a campaign to find transiting planets is begun
3. 1999: Breakout: The right toli is forged, and the first transiting planet discovered
4. 2000: Exploitation: HD209458b begins to yield its secrets
2. Present Knowledge
1. Twenty years of bullet points (a selection)
2. Probing the composition & structure of exo-atmospheres
1. Phase curves, mostly IR
2. Imaging plus spectroscopy
3. Transmission spectroscopy
3. Future NCAR-Planet Opportunities
1. Upcoming large observing facilities, ground-based and in space
2. Transit spectroscopy—hot giant planets
3. Transit spectroscopy—Earth-like planets
4. Visiting planets with atmospheres in our own sliar system

The Mauna Loa Solar Observatory (MLSO) in Hawaii has been operated by the National Center for Atmospheric Research (NCAR) High Altitude Observatory (HAO) since 1965. MLSO provides observations needed to understand the Sun’s continuous release of magnetized plasma and energy. This talk covers past, present and future MLSO instrumentation and observations with an emphasis on the 15 year period when Michael Knӧlker was HAO Director.

I will discuss the prospects of habitability in binary and higher-order systems. Aside from single stars, planets have also been identified in binary systems, both regarding S-type and P-type orbits, as well as in a notable number of triple and quadruple systems. Previous results on the prospects of habitability in stellar binaries include the calculation of habitable regions based on both gravitational and radiative constraints. Additionally, other aspects relevant to the facilitation of habitability as, e.g., the impact of stellar UV/X-ray radiation and (super-)flares also deserve detailed consideration. Moreover, new perspectives on habitability have emerged through the tentative discovery of exomoons, which may also offer homestead of habitability if, e.g., hosted by a Jupiter-type planet in a stable orbit while embedded into the respective stellar habitable zone.

Solar eruptions such as flares and coronal mass ejections (CMEs) are all manifestations of the explosive release of magnetic energy stored in the current carrying, sheared or twisted coronal magnetic fields. In this talk I will present MHD simulations of CME initiation as a result of loss of equilibrium of twisted coronal magnetic flux ropes. It is found that even the ideal MHD instability is the mechanism that initiates the eruption, magnetic reconnection is playing an important role in the evolution leading up to the eruption.  With the explicit inclusion of the non-adiabatic effects, I will also show MHD simulations of coronal flux ropes with the formation of prominence condensations, and the development of CMEs with associated prominence eruptions. It is found that the magnetic field supporting the prominence can be significantly non-force-free despite the low plasma-β. The prominence weight can significantly affect the stability of the flux rope and increase the loss of equilibrium height. Inducing prominence draining can initiate an eruption of the flux rope. Finally, I will show our recent progress in carrying out observationally guided simulations of realistic CME events.

The 1.6 m clear aperture solar telescope has been in regular operation for more than a decade. It is outfitted with visible light (400-1100 nm), near-IR (900-1500 nm) and mid-IR (1000-5000 nm) spectroscopes, as well as a broad-band filter imager (BFI) and the fast imaging solar spectrograph (FISS). Single deformable mirror (DM) adaptive optics (AO) and ground-layer AO (GLAO) systems correct visible and NIR light and multi-conjugate AO (MCAO) operates in the visible with MCAO now being extending  into the NIR. 

After an introduction to the 1.6 m telescope and its focal plane instrumentation, the talk will concentrate on the enabling technology, adaptive optics, efforts made in Big Bear –present, past and planned.

The circumstellar habitable zone is defined as the region around a star in which a planet can support liquid water at its surface. This definition, which is sometimes criticized for being too narrow, serves a practical purpose: For the foreseeable future, any observations of exoplanets will need to be made remotely. For life to be detected on such a planet, it must exist at the surface, so that it can modify the atmosphere in a way that can be detected from Earth, or from Earth orbit. And if that life does not depend on liquid water, it will be difficult to convincingly identify.

The boundaries of the habitable zone can be estimated by using climate models. The inner edge is determined by one of two related phenomena: i) a runaway greenhouse, in which surface water evaporates entirely, or ii) a ‘moist greenhouse’, in which the surface remains wet but water vapor becomes abundant in the upper atmosphere, leading to rapid photodissociation followed by escape of hydrogen to space. In older models (1), the runaway greenhouse limit was well inside the moist greenhouse limit. But in more recent models (2), which include updated H2O absorption coefficients from the HITEMP database, the two limits lie almost on top of each other, near 1.0 AU for a Sun-like star. Both models referenced above are cloud-free, 1-D climate models with fully saturated tropospheres, which likely overestimate the greenhouse effect of H2O. A more sophisticated 3-D calculation (3), also with HITEMP absorption coefficients, moves the inner edge back to 0.95 AU, which is where it had been thought to lie for many years (1,4). 

The outer edge of the habitable zone in these models is defined by a phenomenon termed the ‘maximum CO2 greenhouse’. Several assumptions are embedded in this definition: i) The only two greenhouse gases considered are CO2 and H2O. ii) The planet is assumed to be volcanically active and to emit CO2 at a roughly Earth-like rate. And, iii) CO2 is assumed to build up in a planet’s atmosphere as its surface cools because removal by silicate weathering and carbonate deposition slows down (5). But greenhouse warming is still limited because the albedo of a dense CO2-rich atmosphere is high and because condensation of CO2 to form clouds lowers the tropospheric lapse rate, thereby reducing the greenhouse effect. 1-D models put the ‘maximum greenhouse’ limit at ~1.67 AU for a Sun-like star (2).

While making estimates of habitable zone boundaries is fun, and hopefully useful, the real excitement will come when we are able to observe rocky exoplanets within the habitable zones of nearby stars and take spectra of their atmospheres. This may be possible for transiting M-star planets using NASA James Webb Space Telescope, currently scheduled to launch in 2021. Non-transiting M-star planets may be observable from the ground using 30 m-class telescopes such as ESO’s ELT (Extemely Large Telescope), which could be completed by 2024. Earth-like planets around sun-like stars are more difficult to observe but could potentially be studied in the mid-to-late 2030’s by large space telescopes such as NASA’s HabEx and LUVOIR concepts. The search for such planets and for biosignature gases in their atmospheres promises to be one of the most exciting areas of astronomy for the next several decades.

The High Altitude Observatory (HAO) was founded in 1940 by Harvard astronomer Walter Orr Roberts, who later merged the solar physics research laboratory into the National Center for Atmospheric Research (NCAR) when it was established 20 years later. Through most of its history, HAO has recognized the importance of maintaining a connection to the stellar astrophysics community. I use bibliometric data to review the legacy of the highest-impact contributions of HAO scientists to stellar astrophysics research, including: (1) the definition of an age-rotation-activity relation for solar-type stars, (2) the development of a widely-used equation of state for stellar envelopes, and (3) the discovery of the first transiting exoplanet. I conclude with a brief overview of how these investigations of other stars have also advanced our understanding of solar physics over the past 50 years.

In this talk I will review developments of the MURaM radiative MHD code over the past 15 years that were in part spearheaded by HAO while Michael Knoelker was director. After the initial developments of the MURaM code at the Max Planck Institute for Solar System Research, HAO research expanded the code capabilities to tackle solar active regions, including sunspot fine structure and active region evolution, and extended the code into the lower solar corona. With these developments, the MURaM code has evolved into a multi-purpose solar radiative MHD code that has been applied to the full spectrum of quiet to active sun, including solar flares and the initiation of coronal mass ejections. I will review a few recent applications of the code and discuss current developments.

The 4m Daniel K. Inouye Solar Telescope (DKIST) on Haleakala, Maui has achieved first engineering solar light in December of 2019. First solar images were recorded with adaptive optics and the Visible Broadband Imager. In early 2020 HAO’s Visible Spectro-Polarimeter was integrated into the DKIST instrument lab and has obtained first spectra. The infrared polarimeters are undergoing final acceptance testing and VTF is making excellent progress towards delivery to Maui at the end of 2020.  The DKIST instruments will produce complex data sets, which will be distributed through the NSO/DKIST Data Center. The start of the operations commissioning phase is scheduled for mid-2020. We summarize the status of DKIST and present first images, and touch on Michael Knoelker’s many and important contributions to DKIST.

The Visible Tunable Filter is a spectro-polarimeter based on Fabry-Perot interferometers. It takes monochromatic images of the Sun at several wavelengths across a spectral line in rapid sequence. Each image covers an area of 40000 x 40000 km on the Sun with a spectral sampling of 3 Picometers. The operating wavelength can be chosen between 520 nm and 870 nm. The VTF allows to observe the evolution of the structure of the solar photosphere and chromosphere, together with the line-of-sight component of the material flow. When used as a polarimeter, the magnetic field configuration is measured simultaneously with the intensity and velocity. The spatial resolution of the VTF is only limited by the telescope and the atmospheric conditions. As an imaging instrument, post-facto image-reconstruction will be regularly applied to achieve diffraction-limited resolution down to 20 km on the Sun. The VTF plays an important role in the Critical Science Plan of DKIST, as it is used in about 50% of all science use cases. Two tunable Fabry-Perot interferometers (‘Etalons’) for wavelength isolation are the heart of the instrument. With a clear aperture of 25 cm, these devices, developed at KIS, together with industrial partners, are by far the largest tunable Etalons ever built. The VTF will initially be operated with one Etalon and a reduced spectral range. The first Etalon is being assembled at KIS, while the plates for the 2nd Etalon are being processed in industry. The instrument is presently being set up in its laboratory at KIS in Freiburg, where end-to-end tests of the full instrument, including 3 4kx4k science cameras, and the VTF software developed at KIS, embedded in the DKIST software environment, will be performed in spring and summer of 2020. The VTF will see First Light at the DKI Solar Telescope in early 2021.

Obviously, no other star provides us with so much information about the workings of its dynamo process, extending over wide ranges of spatial and temporal scales. Surface observations reach from the global dipole down to structures at the resolution limit of the biggest telescopes, covering time scales from fractions of a second to centuries. Natural terrestrial archives allow us to reconstruct solar activity levels over millenia into the past. Helioseismology reveals the structure of the solar interior, as well as meridional and rotational flows in the convection zone. Although the Sun tells us so much, we still do not have a definitive model of the solar dynamo - is it still not sufficient or do we perhaps not listen closely enough? Can stellar observations help out?  This talk will give a brief overview of the observational basis for our understanding of the dynamo, what it implies for dynamo models, and what we are still missing.

Sunrise is a balloon-borne solar observatory dedicated to the investigation of the physics of the magnetic field and its interaction with convective plasma flows and waves. The Sunrise observatory is designed for operation in the stratosphere (at heights up to 40 km) in order to avoid the image degradation due to turbulence in the Earth’s lower atmosphere and to gain access to the UV spectral range. The first science flights of Sunrise, in June 2009 and June 2013, for which HAO provided the gondola under the leadership of Michael Knoelker, led to many new results described in over 100 papers in refereed journals. This includes the first time that magnetic features were fully resolved in the quiet Sun, the first time that the life history of individual magnetic features could be followed, the discovery of a canopy of ubiquitous fibrils in the lower chromosphere that carries a variety of waves, discovery of a new method by which coronal loops are heated, etc. This success has shown the huge potential of the Sunrise approach. The recovery of the largely intact payload offers an opportunity for a third flight. Sunrise III will have greatly extended capabilities, in particular to measure weaker magnetic field over a greater range of heights (covering both photosphere and chromosphere). To this end, Sunrise III will carry two new instruments as well as upgrades of its present instruments.

Measurements of coronal and chromospheric magnetic fields are arguably the most important observables required for advances in our understanding of the processes responsible for coronal heating, coronal dynamics and the generation of space weather that affects communications, GPS systems, space flight, and power transmission. The Coronal Solar Magnetism Observatory (COSMO) is a proposed ground-based suite of instruments designed for routine study of coronal and chromospheric magnetic fields and their environment, and to understand the formation of coronal mass ejections (CME) and their relation to other forms of solar activity. This new facility will replace the current NCAR Mauna Loa Solar Observatory. COSMO will enhance the value of existing and new observatories on the ground and in space by providing unique and crucial observations of the global coronal and chromospheric magnetic field and its evolution. The design and current status of the COSMO will be reviewed.

The GREGOR solar telescope at the Teide Observatory on Tenerife is Europe's largest solar telescope, and still, the second largest world-wide. It observes the Sun in the visible and near-infrared spectral domains with a diffraction-limited resolution of better than 0.1 arcsec. This presentation will review the capabilities of GREGOR and its recent scientific successes.

In this talk I will review recent advances in 3D climate modeling of planetary atmospheres, applied both to exoplanets and solar system bodies. Over the past decade, 3D general circulation and climate system models have emerged as powerful tools for studying the atmospheres of objects beyond our Earth. 3D models have been used to explore possible early atmospheres of Earth, Mars, and Venus, each of which took unique evolutionary tracks to reach their current states. While observations of terrestrial exoplanets remain sparse, 3D models have already proven useful for refining predictions of the habitable zone, elucidating the physics of novel climate states, and constraining the efficacy of observational techniques for remotely characterizing such worlds. Where better data is available, 3D models have already yielded verified predictions of atmospheric processes occuring on hot Jupiters. The use of 3D models in (exo)planetary research has broadened our understanding of planetary atmospheres, and will continue to be useful as the quality of our observational data improves over time.

Discovery and characterization of the coolest regions in the solar photosphere, chromosphere and corona are new frontiers in the modern solar physics. DKIST and its instruments will allow us to have unprecedented records of the chemistry and dynamics of cool magnetic and non-magnetic regions in the solar atmosphere. In this talk I will show examples of simultaneous multiwavelength spectropolarimetric measurements that may allow us to unveil magnetic fields and their evolution on the smallest scale ever achieved in four dimensions (space and time). Interestingly, modeling atmospheres of the coolest Sun and those of hot Jupiters have a few challenges in common, such as strong irradiation and chemical processes. I will demonstrate how solar models can be further developed by including irradiation of cool solar plasma.

We describe a Heliophysics flagship mission to explore and monitor the solar polar regions: the Solar Polar Observing Constellation (SPOC) mission. SPOC will follow on the pioneering polar glimpses provided by Hinode/SOT and Solar Orbiter/PHI to enable long-duration (solar cycle) helioseismology, surface flows, magnetic field dynamics, and CME observations from above or below the ecliptic that will unlock the mysteries of the solar dynamo while also greatly improving our ability to model and predict the solar wind and CME transport throughout the heliosphere.

I will review the history of long-term synoptic observations of Sun-like stars and what their results teach us about the Sun and its enigmatic dynamo. Multi-decadal programs from the Mount Wilson Observatory HK Project, the Lowell Observatory Solar-Stellar Spectrograph, and the Fairborn Observatory Automated Photometric Telescope program serve as a basis of comparison for similar decadal- or century-scale observational records for the Sun. In particular, they reveal the importance of rotation in the history of solar variability, and allow us to estimate limits to the amplitude of solar variability over time scales impossible to approach with direct observation or proxy methods. Finally, I will discuss the importance of dedicated synoptic observation programs for the Sun and the stars, and how increased observational efforts might help to answer fundamental questions about the dynamo.

We present a comprehensive investigation of the Mg II k&h lines profiles under quiet solar conditions, as seen by IRIS. An in depth analysis of the various profile characteristics is provided, considering datasets covering diverse surrounding large scale magnetic configurations and different local activity levels, from pure quiet sun to regions underlying coronal holes or located under the active region canopy. The line profile characteristics are identified, the differences between Network and Inter-network regimes are characterised and the evolution of similar activity conditions is discussed. The statistical properties of the different activity levels are explored and the correlations between the different computed parameters are evaluated. Overall, the Inter-network regime is dominated by signatures of convective motions, which become suppressed in the Network, where the magnetic field leads to enhanced emission.

The detailed processes occurring in proto-planetary disks set the stage for the formation of planetesimals. It has become increasingly apparent that forming stars do not accrete their mass through the steady accretion of material from their surrounding disks, but rather primarily through isolated, stochastic accretion events. During these rare events, the central protostars are in hydrostatic but not in thermal equilibrium; they undergo large excursions in the HRD. The Stratospheric Observatory For Infrared Astronomy (SOFIA) has observed a few such events. I will present theoretical calculations of stellar evolution during stochastic accretion and review relevant SOFIA observations.