By generalizing the theory of convection to any type of thermal and compositional source terms (diabatic processes), we show that thermohaline convection in Earth's oceans, fingering convection in ...stellar atmospheres, and moist convection in Earth's atmosphere are derived from the same general diabatic convective instability. We also show that "radiative convection" triggered by the CO/CH4 transition with radiative transfer in the atmospheres of brown dwarfs is analogous to moist and thermohaline convection. We derive a generalization of the mixing-length theory to include the effect of source terms in 1D codes. We show that CO/CH4 "radiative" convection could significantly reduce the temperature gradient in the atmospheres of brown dwarfs similarly to moist convection in Earth's atmosphere, thus possibly explaining the reddening in brown dwarf spectra. By using idealized 2D hydrodynamic simulations in the Ledoux unstable regime, we show that compositional source terms can indeed provoke a reduction of the temperature gradient. The L/T transition could be explained by a bifurcation between the adiabatic and diabatic convective transports and seen as a giant cooling crisis: an analog of the boiling crisis in liquid/steam-water convective flows. This mechanism, with other chemical transitions, could be present in many giant and Earth-like exoplanets. The study of the impact of different parameters (effective temperature, compositional changes) on CO/CH4 radiative convection and the analogy with Earth moist and thermohaline convection is opening the possibility of using brown dwarfs to better understand some aspects of the physics at play in the climate of our own planet.
Aims.
Clouds are expected to form in a broad range of conditions in the atmosphere of exoplanets given the variety of possible condensible species. This diversity, however, might lead to very ...different small-scale dynamics depending on radiative transfer in various thermal conditions. Here, we aim to provide some insight into these dynamical regimes.
Methods.
We performed an analytical linear stability analysis of a compositional discontinuity with a heating source term that depends on a given composition. We also performed idealized two-dimensional simulations of an opacity discontinuity in a stratified medium, using the
ARK
code. We used a two-stream gray model for radiative transfer and explored the brown-dwarf and Earth-like regimes.
Results.
We revealed the existence of a radiative Rayleigh-Taylor instability (RRTI, hereafter, which is a particular case of diabatic Rayleigh-Taylor instability) when an opacity discontinuity is present in a stratified medium. This instability is similar in nature to diabatic convection and relies only on buoyancy with radiative transfer heating and cooling. When the temperature is decreasing with height in the atmosphere, a lower-opacity medium on top of a higher-opacity medium is shown to be dynamically unstable, whereas a higher-opacity medium on top of a lower-opacity medium is stable. This stability-instability behavior is reversed if the temperature is increasing with height.
Conclusions.
The existence of a RRTI could have important implications for the stability of the cloud cover with regard to a wide range of planetary atmospheres. In our Solar System, it could help explain the formation of mammatus cloud in Earth atmospheres and the existence of the Venus cloud deck. Likewise, it suggests that stable and large-scale cloud covers could be ubiquitous in strongly irradiated exoplanets, but might be more patchy in low-irradiated or isolated objects such as brown dwarfs and directly imaged exoplanets.
The 2D wavelet transform modulus maxima method is used to obtain quantitative information on the fractal/multifractal nature and anisotropic structure of Galactic atomic hydrogen (H I) from the ...Canadian Galactic Plane Survey. Five mosaics were analyzed in the second quadrant of the Milky Way, corresponding to the Local, Perseus, and Outer spiral arms, as well as two "interarm" regions located between these three spiral arms. A monofractal signature is found for all five mosaics. An anisotropic signature is detected: the rms roughness fluctuations of the mosaics in the latitude direction differ from those in the longitude direction. This anisotropy is scale-independent for the interarm regions while it is scale-dependent for the spiral arms. The longitudinal matter distribution of H I structure is similar for all five mosaics while the latitudinal distribution is smoother in the spiral arms. These results hold for all physical length scales studied, from 62 pc in the Local arm to 644 pc in the Outer arm. Several hypotheses are investigated to provide a physical explanation: the scale-height gradient, the density wave, star formation activity, photolevitation of dusty clouds, random motion of H I clouds, corrugation, and turbulence. The slopes of the power spectra for an increasing number of velocity channels were compared for 11 sections of the Local arm column density mosaic. All slopes are identical within the uncertainties (-3.0) and we do not detect for the Galactic plane the change in the power law index predicted by Lazarian and Pogosyan.
This contribution deals with the fluid modeling of multicomponent magnetized plasmas in thermo-chemical non-equilibrium from the partially- to fully-ionized collisional regimes, aiming at the ...predictive simulation of magnetic reconnection in Sun chromosphere conditions. Such fluid models are required for large-scale simulations by relying on high performance computing. The fluid model is derived from a kinetic theory approach, yielding a rigorous description of the dissipative and non-equilibrium effects and a well-identified mathematical structure. We start from a general system of equations that is obtained by means of a multiscale Chapman-Enskog method, based on a non-dimensional analysis accounting for the mass disparity between the electrons and heavy particles, including the influence of the electromagnetic field and transport properties. The latter are computed by using a spectral Galerkin method based on a converged Laguerre-Sonine polynomial approximation. Then, in the limit of small Debye length with respect to the characteristic scale in the Sun chromosphere, we derive a two-temperature single-momentum multicomponent diffusion model coupled to Maxwell's equations, which is able to describe fully- and partially-ionized plasmas, beyond the multi-fluid model of Braginskii, valid for the whole range of the Sun chromosphere conditions. The second contribution is the development and verification of an accurate and robust numerical strategy that is based on CanoP, a massively parallel code with adaptive mesh refinement capability, which is able to cope with the full spectrum of scales of the magnetic reconnection process, without additional constraint on the time steps compared to single-fluid Magnetohydrodynamics (MHD) models. The final contribution is a study of the physics of magnetic reconnection in collaboration with the heliophysics team of NASA Ames Research Center. We show that the model and methods allow us to retrieve the results of usual single-fluid MHD models in the highly collisional case at equilibrium, while achieving a more detailed physics description relevant to such applications in the weakly collisional case, where non-equilibrium effects become important.
The multifractal nature of solar photospheric magnetic structures is studied using the two-dimensional wavelet transform modulus maxima (WTMM) method. This relies on computing partition functions ...from the wavelet transform skeleton defined by the WTMM method. This skeleton provides an adaptive space-scale partition of the fractal distribution under study, from which one can extract the multifractal singularity spectrum. We describe the implementation of a multiscale image processing segmentation procedure based on the partitioning of the WT skeleton, which allows the disentangling of the information concerning the multifractal properties of active regions from the surrounding quiet-Sun field. The quiet Sun exhibits an average Hoelder exponent {approx}-0.75, with observed multifractal properties due to the supergranular structure. On the other hand, active region multifractal spectra exhibit an average Hoelder exponent {approx}0.38, similar to those found when studying experimental data from turbulent flows.
Magnetohydrodynamic turbulence is thought to be responsible for producing complex, multiscale magnetic field distributions in solar active regions. Here we explore the multiscale properties of a ...number of evolving active regions using magnetograms from the Michelson Doppler Imager (MDI) on the
Solar and Heliospheric Observatory
(SOHO). The multifractal spectrum was obtained by using a modified box-counting method to study the relationship between magnetic-field multifractality and region evolution and activity. The initial emergence of each active region was found to be accompanied by characteristic changes in the multifractal spectrum. Specifically, the range of multifractal structures (
D
div
) was found to increase during emergence, as was their significance or support (
C
div
). Following this, a decrease in the range in multifractal structures occurred as the regions evolved to become large-scale, coherent structures. From the small sample considered, evidence was found for a direct relationship between the multifractal properties of the flaring regions and their flaring rate.
The BAO Radio Acquisition System Charlet, D.; Abbon, P.; Ansari, R. ...
IEEE transactions on nuclear science,
08/2011, Volume:
58, Issue:
4
Journal Article
Peer reviewed
The BAO radio instrument is a demonstrator of a radio telescope dedicated to cosmological studies and dark energy observation. It will analyze the signal emitted by the hydrogen in the universe with ...the aim of the detection of BAO's (Baryon Acoustic Oscillations). The instrument will use cylindrical reflectors or a large number of small dishes associated with a digital beam-former correlator. The electronic chain covers a large frequency range (1.5 GHz) and has a high throughput rate (~300 MB/s).
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