Magnetohydrodynamical torsional oscillations from thermo-resistive instability in hot jupiters

Hot jupiter atmospheres may be subject to a thermo-resistive instability where an increase in the electrical conductivity due to ohmic heating results in runaway of the atmospheric temperature. We introduce a simplified one-dimensional model of the equatorial sub-stellar region of a hot jupiter which includes the temperature-dependence and time-dependence of the electrical conductivity, as well as the dynamical back-reaction of the magnetic field on the flow. This model extends our previous one-zone model to include the radial structure of the atmosphere. Spatial gradients of electrical conductivity strongly modify the radial profile of Alfv\'en oscillations, leading to steepening and downwards transport of magnetic field, enhancing dissipation at depth. We find unstable solutions that lead to self-sustained oscillations for equilibrium temperatures in the range $T_\mathrm{eq}\approx 1000$--$1200$~K, and magnetic field in the range $\approx 10$--$100$~G. For a given set of parameters, self-sustained oscillations occur in a narrow range of equilibrium temperatures which allow the magnetic Reynolds number to alternate between large and small values during an oscillation cycle. Outside of this temperature window, the system reaches a steady state in which the effect of the magnetic field can be approximated as a magnetic drag term. Our results show that thermo-resistive instability is a possible source of variability in magnetized hot jupiters at colder temperatures, and emphasize the importance of including the temperature-dependence of electrical conductivity in models of atmospheric dynamics.Comment: Submitted to The Astrophysical Journa

A coupled 2 x 2D Babcock-Leighton solar dynamo model. II Reference dynamo solutions

In this paper we complete the presentation of a new hybrid 2 × 2D flux transport dynamo (FTD) model of the solar cycle based on the Babcock–Leighton mechanism of poloidal magnetic field regeneration via the surface decay of bipolar magnetic regions (BMRs). This hybrid model is constructed by allowing the surface flux transport (SFT) simulation described in Lemerle et al. to provide the poloidal source term to an axisymmetric FTD simulation defined in a meridional plane, which in turn generates the BMRs required by the SFT. A key aspect of this coupling is the definition of an emergence function describing the probability of BMR emergence as a function of the spatial distribution of the internal axisymmetric magnetic field. We use a genetic algorithm to calibrate this function, together with other model parameters, against observed cycle 21 emergence data. We present a reference dynamo solution reproducing many solar cycle characteristics, including good hemispheric coupling, phase relationship between the surface dipole and the BMR-generating internal field, and correlation between dipole strength at cycle maximum and peak amplitude of the next cycle. The saturation of the cycle amplitude takes place through the quenching of the BMR tilt as a function of the internal field. The observed statistical scatter about the mean BMR tilt, built into the model, acts as a source of stochasticity which dominates amplitude fluctuations. The model thus can produce Dalton-like epochs of strongly suppressed cycle amplitude lasting a few cycles and can even shut off entirely following an unfavorable sequence of emergence events

A Coupled 2 × 2D Babcock–Leighton Solar Dynamo Model. I. Surface Magnetic Flux Evolution

The need for reliable predictions of the solar activity cycle motivates the development of dynamo models incorporating a representation of surface processes sufficiently detailed to allow assimilation of magnetographic data. In this series of papers we present one such dynamo model, and document its behavior and properties. This first paper focuses on one of the model's key components, namely surface magnetic flux evolution. Using a genetic algorithm, we obtain best-fit parameters of the transport model by least-squares minimization of the differences between the associated synthetic synoptic magnetogram and real magnetographic data for activity cycle 21. Our fitting procedure also returns Monte Carlo-like error estimates. We show that the range of acceptable surface meridional flow profiles is in good agreement with Doppler measurements, even though the latter are not used in the fitting process. Using a synthetic database of bipolar magnetic region (BMR) emergences reproducing the statistical properties of observed emergences, we also ascertain the sensitivity of global cycle properties, such as the strength of the dipole moment and timing of polarity reversal, to distinct realizations of BMR emergence, and on this basis argue that this stochasticity represents a primary source of uncertainty for predicting solar cycle characteristics

Geologic Framework for the Assessment of Offshore CO2 Storage Resources: West Florida Platform

The West Florida Platform is a broad continental shelf west of peninsular Florida. The platform contains abundant sinks and seals that appear favorable for future commercial storage of CO2. Proven offshore storage technologies, like those that have been deployed in the North Sea, are likely transferrable to the West Florida Platform. This thesis is part of a larger study to evaluate the CO2 storage potential of the Eastern Gulf of Mexico continental shelf adjacent to Mississippi, Alabama, and Florida. This project involves a detailed analysis of data from eight exploratory wells in the area of the Sarasota Arch, and includes analysis of geophysical logs, interpretation of 2D reflection seismic profiles, and volumetric analysis of the CO2 storage resource.The Sarasota Arch is the primary controlling structure associated with reservoir development. Porous dolomite is concentrated in this structure, and the dolomite passes into nonporous limestone in the flanks. The porous strata identified in the Sarasota Arch have a potential storage resource of more than 878 Gt of CO2. Limestone and dolomite in Cretaceous and Paleogene strata are the primary targets for CO2 storage. The Lower Cretaceous Punta Gorda, Gordon Pass, and Panther Camp assessment units appear suitable for injection and storage of supercritical CO2 and contain abundant stacked dolomitic reservoirs, which are separated by regionally continuous anhydrite confining units. Reservoirs assessed in the Upper Cretaceous and Lower Paleogene Cedar Keys assessment unit contain a potential storage resource of approximately 600 Gt of CO2, however the lateral extent of the confining anhydrite beds is not as great as in the older units. Storage potential and sealing potential are greatest in Lower Cretaceous strata, which can hold a potential storage resource of about 278 Gt. Multi-gigatonne storage potential on the West Florida Platform could provide a viable storage option in the Eastern Gulf of Mexico, and consequently reduce the emissions footprint in the south-eastern United States.Geolog

The Lick-Carnegie Survey: Four New Exoplanet Candidates

We present new precise HIRES radial velocity (RV) data sets of five nearby stars obtained at Keck Observatory. HD 31253, HD 218566, HD 177830, HD 99492 and HD 74156 are host stars of spectral classes F through K and show radial velocity variations consistent with new or additional planetary companions in Keplerian motion. The orbital parameters of the candidate planets in the five planetary systems span minimum masses of M sin i = 27.43 M_{earth} to M sin i = 8.28 M_{jup}, periods of 17.05 to 4696.95 days and eccentricities ranging from circular to extremely eccentric (e ~ 0.63). The 5th star, HD 74156, was known to have both a 52-day and a 2500-day planet, and was claimed to also harbor a 3rd planet at 336d, in apparent support of the "Packed Planetary System" hypothesis. Our greatly expanded data set for HD 74156 provides strong confirmation of both the 52-day and 2500-d planets, but strongly contradicts the existence of a 336-day planet, and offers no significant evidence for any other planets in the system.Comment: 13 pages, 15 figures. Accepted for publication in ApJ. Fixed typos in Table 2. Additional material at http://www.ucolick.org/~smeschia/4planet.ph