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Title: Three-Dimensional Atmospheric Circulation Models of HD 189733b and HD 209458b with Consistent Magnetic Drag and Ohmic Dissipation
Authors: E. Rauscher, K. Menou

We present the first three-dimensional circulation models for extrasolar gas giant atmospheres with geometrically and energetically consistent treatments of magnetic drag and ohmic dissipation. Atmospheric resistivities are continuously updated and calculated directly from the flow structure, strongly coupling the magnetic effects with the circulation pattern. We model the hot Jupiters HD 189733b (Teq \approx 1200 K) and HD 209458b (Teq \approx 1500 K) and test planetary magnetic field strengths from 0 to 30 G. We find that even at B = 3 G the atmospheric structure and circulation of HD 209458b are strongly influenced by magnetic effects, while the cooler HD 189733b remains largely unaffected, even in the case of B = 30 G and super-solar metallicities. Our models of HD 209458b indicate that magnetic effects can substantially slow down atmospheric winds, change circulation and temperature patterns, and alter observable properties. These models establish that longitudinal and latitudinal hot spot offsets, day-night flux contrasts, and planetary radius inflation are interrelated diagnostics of the magnetic induction process occurring in the atmospheres of hot Jupiters and other similarly forced exoplanets. Most of the ohmic heating occurs high in the atmosphere and on the day side of the planet, while the heating at depth is strongly dependent on the internal heat flux assumed for the planet, with more heating when the deep atmosphere is hot. We compare the ohmic power at depth in our models, and estimates of the ohmic dissipation in the bulk interior (from general scaling laws), to evolutionary models that constrain the amount of heating necessary to explain the inflated radius of HD 209458b. Our results suggest that deep ohmic heating can successfully inflate the radius of HD 209458b for planetary magnetic field strengths of B \geq 3 - 10 G.

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Title: Dust cloud lightning in extraterrestrial atmospheres
Authors: Christiane Helling (Univ St Andrews), Moira Jardine (Univ St Andrews), Declan Diver (Univ Glasgow), Soeren Witte (Univ Hamburg)

Lightning is present in all solar system planets which form clouds in their atmospheres. Cloud formation outside our solar system is possible in objects with much higher temperatures than on Earth or on Jupiter: Brown dwarfs and giant extrasolar gas planets form clouds made of mixed materials and a large spectrum of grain sizes. These clouds are globally neutral obeying dust-gas charge equilibrium which is, on short timescales, inconsistent with the observation of stochastic ionisation events of the solar system planets. We argue that a significant volume of the clouds in brown dwarfs and extrasolar planets is susceptible to local discharge events and that the upper cloud layers are most suitable for powerful lightning-like discharge events. We discuss various sources of atmospheric ionisation, including thermal ionisation and a first estimate of ionisation by cosmic rays, and argue that we should expect thunderstorms also in the atmospheres of brown dwarfs and giant gas planets which contain mineral clouds.

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Title: Thermo-Resistive Instability of Hot Planetary Atmospheres
Authors: Kristen Menou

The atmospheres of hot Jupiters and other strongly-forced exoplanets are susceptible to a thermal instability in the presence of ohmic dissipation, weak magnetic drag and strong winds. The instability occurs in radiatively-dominated atmospheric regions when the ohmic dissipation rate increases with temperature faster than the radiative (cooling) rate. The instability domain covers a specific range of atmospheric pressures and temperatures, typically P ~ 3-300 mbar and T ~ 1500-2500K for hot Jupiters, which makes it a candidate mechanism to explain the dayside thermal "inversions" inferred for a number of such exoplanets. The instability is suppressed by high levels of non-thermal photoionisation, in possible agreement with a recently established observational trend. We highlight several shortcomings of the instability treatment presented here. Understanding the emergence and outcome of the instability, which should result in locally hotter atmospheres with stronger levels of drag, will require global non-linear atmospheric models with adequate MHD prescriptions.

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Exoplanet weather
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Forecasting the weather on exoplanets


Credit AstroNow09

Dr. David Acreman explains how scientists are trying to predict the weather on planets around other stars.



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Exoplanet Atmospheres
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Title: A Detection Of H-alpha In An Exoplanetary Exosphere
Authors: Adam G. Jensen, Seth Redfield, Michael Endl, William D. Cochran, Lars Koesterke, Travis S. Barman

We report on a search for H-alpha absorption in four exoplanets. Strong features at H-alpha are observed in the transmission spectra of both HD 189733b and HD 209458b. We attempt to characterise and remove the effects of stellar variability in HD 189733b, and along with an empirical Monte Carlo test the results imply a statistically significant transit-dependent feature of (-8.721.48)x10^-4 integrated over a 16 Angstrom band relative to the adjacent continuum. We interpret this as the first detection of this line in an exoplanetary atmosphere. A previous detection of Ly-alpha in HD 189733b's atmosphere allows us to calculate an excitation temperature for hydrogen, T_exc=2.6x10^4 K. This calculation depends significantly on certain simplifying assumptions. We explore these assumptions and argue that T_exc is very likely much greater than the radiative equilibrium temperature (the temperature the planet is assumed to be at based on stellar radiation and the planetary distance) of HD 189733b. A large T_exc implies a very low density that is not in thermodynamic equilibrium the planet's lower atmosphere. We argue that the n=2 hydrogen required to cause H-alpha absorption in the atmosphere is created as a result of the greater UV flux at HD 189733b, which has the smallest orbit and most chromospherically active central star in our sample. Though the overall integration of HD 209458b's transmission spectrum over a wide band is consistent with zero, it contains a dramatic, statistically significant feature in the transmission spectrum with reflectional symmetry. We discuss possible physical processes that could cause this feature. Our remaining two targets (HD 147506b and HD 149026b) do not show any clear features, so we place upper limits on their H-alpha absorption levels.

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Scattered Light Could Reveal Alien Atmospheres

The light scattered off distant worlds could help reveal details about their atmospheres that no other method could uncover, scientists find.
Nearly all the information astronomers have of the atmospheres of alien planets or exoplanets comes from worlds whose orbits happen to be precisely aligned from our vantage point. Once per orbit, these exoplanets go in front of (transit) their host stars from our point of view, and the light from these stars passes through the atmospheres of these planets on its way to Earth. The molecules in these alien atmospheres absorb some of this starlight, resulting in patterns known as spectra that allow scientists to identify what they are.

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Title: Constraining High Speed Winds in Exoplanet Atmospheres Through Observations of Anomalous Doppler Shifts During Transit
Authors: Eliza Miller-Ricci Kempton, Emily Rauscher

Three-dimensional (3-D) dynamical models of hot Jupiter atmospheres predict very strong wind speeds. For tidally locked hot Jupiters, winds at high altitude in the planet's atmosphere advect heat from the day side to the cooler night side of the planet. Net wind speeds on the order of 1-10 km/s directed towards the night side of the planet are predicted at mbar pressures, which is the approximate pressure level probed by transmission spectroscopy. These winds should result in an observed blue shift of spectral lines in transmission on the order of the wind speed. Indeed, Snellen et al. (2010) recently observed a 2 1 km/s blue shift of CO transmission features for HD 209458b, which has been interpreted as a detection of the day-to-night winds that have been predicted by 3-D atmospheric dynamics modelling. Here we present the results of a coupled 3-D atmospheric dynamics and transmission spectrum model, which predicts the Doppler-shifted spectrum of a hot Jupiter during transit resulting from winds in the planet's atmosphere. We explore four different models for the hot Jupiter atmosphere using different prescriptions for atmospheric drag via interaction with planetary magnetic fields. We find that models with no magnetic drag produce net Doppler blue shifts in the transmission spectrum of ~2 km/s and that lower Doppler shifts of ~1 km/s are found for the higher drag cases, results consistent with -- but not yet strongly constrained by -- the Snellen et al. (2010) measurement. We additionally explore the possibility of recovering the average terminator wind speed as a function of altitude by measuring Doppler shifts of individual spectral lines and spatially resolving wind speeds across the leading and trailing terminators during ingress and egress.

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Exoplanet Aurora: An Out-of-this-World Sight

Cambridge, MA - Earth's aurorae, or Northern and Southern Lights, provide a dazzling light show to people living in the polar regions. Shimmering curtains of green and red undulate across the sky like a living thing. New research shows that aurorae on distant "hot Jupiters" could be 100-1000 times brighter than Earthly aurorae. They also would ripple from equator to poles (due to the planet's proximity to any stellar eruptions), treating the entire planet to an otherworldly spectacle.
Earth's aurorae are created when energetic particles from the Sun slam into our planet's magnetic field. The field guides solar particles toward the poles, where they smash into Earth's atmosphere, causing air molecules to glow like a neon sign. The same process can occur on planets orbiting distant stars, known as exoplanets.
Particularly strong aurorae result when Earth is hit by a coronal mass ejection or CME - a gigantic blast that sends billions of tons of solar plasma (electrically charged, hot gas) into the solar system. A CME can disrupt Earth's magnetosphere - the bubble of space protected by Earth's magnetic field - causing a geomagnetic storm. In 1989, a CME hit Earth with such force that the resulting geomagnetic storm blacked out huge regions of Quebec.
Cohen and his colleagues used computer models to study what would happen if a gas giant in a close orbit, just a few million miles from its star, were hit by a stellar eruption. He wanted to learn the effect on the exoplanet's atmosphere and surrounding magnetosphere.

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Title: Mapping Clouds and Terrain of Earth-like Planets from Photometric Variability
Authors: Hajime Kawahara (1), Yuka Fujii (2) ((1) Tokyo Metropolitan University, (2) The University of Tokyo)

We develop an inversion technique of annual scattered light curves to sketch a two-dimensional albedo map of exoplanets. As a test-bed for future observations of extrasolar terrestrial planets, we apply this mapping technique to simulated light curves of a mock Earth-twin. A primary feature in recovered albedo maps traces the annual mean distribution of clouds. To extract information of other surface types, we attempt to reduce the cloud signal by taking difference of two bands. We find that the inversion of reflectivity difference between 0.8-0.9 and 0.4-0.5 micron bands roughly recover the continental distribution, except for high latitude regions persistently covered with clouds and snow. The inversion of the reflectivity difference across the red edge (0.8-0.9 and 0.6-0.7 micron) emphasizes the vegetation features near the equator. The planetary obliquity and equinox can be estimated simultaneously with the mapping under the presence of clouds. We conclude that the photometric variability of the scattered light will be a powerful means for exploring the habitat of a second Earth

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Title: The Role of Drag in the Energetics of Strongly Forced Exoplanet Atmospheres
Authors: Emily Rauscher (1), Kristen Menou (2) ((1) University of Arizona, (2) Columbia University)

In contrast to the Earth, where frictional heating is typically neglected in atmospheric modelling, we show that drag mechanisms could act as an important heat source in the strongly-forced atmospheres of some exoplanets, with the potential to alter the circulation. We modify the standard formalism of the atmospheric energy cycle to explicitly track the loss of kinetic energy and the associated frictional (re)heating, for application to exoplanets such as the asymmetrically heated "hot Jupiters" and gas giants on highly eccentric orbits. We establish that an understanding of the dominant drag mechanisms and their dependence on local atmospheric conditions is critical for accurate modelling, not just in their ability to limit wind speeds, but also because they could possibly change the energetics of the circulation enough to alter the nature of the flow. We discuss possible sources of drag and estimate the strength necessary to significantly influence the atmospheric energetics. As we show, the frictional heating depends on the magnitude of kinetic energy dissipation as well as its spatial variation, so that the more localized a drag mechanism is, the weaker it can be and still affect the circulation. We also use the derived formalism to estimate the rate of numerical loss of kinetic energy in a few previously published hot Jupiter models with and without magnetic drag and find it to be surprisingly large, at 5-10% of the incident stellar irradiation.

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