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 Earth and Space Sciences Colloquia

Abstract: The dynamo effect is a magnetohydrodynamics instability, allowing the existence of a self-sustaining magnetic field in a flow of an electrically conductive fluid. This effect is considered to be at the origin of the magnetic field of the Earth and the stars. Until now, it had only been evidenced experimentally in cases where internal boundaries guide the flow. I will present the first observation of a dynamo in a homogeneous flow of liquid sodium (VKS2 experiment), where the turbulent fluctuations are of the same order of magnitude as the mean flow. The behaviour of this instability close to the threshold will be described, as well as scaling laws for its saturation. One striking feature of such a dynamo, that makes it very different from the previous constrained dynamos, is that it displays a large variety of dynamical regimes, including chaotic reversals, strongly reminiscent of the observed reversals of the Earth's magnetic field.

Abstract: Here I discuss the atmospheric ciculations on the giant planets in our solar system -- Jupiter, Saturn, Uranus, and Neptune -- as well as on some of the 200-odd giant planets discovered around other stars. The atmospheric circulations of our local giant planets are characterized by banded cloud patterns, numerous east-west jet streams, and stable vortices at the cloud level. The question of what causes these features has remained a puzzle since high-resolution spacecraft images of these planets were returned in the 1970s and 1980s. A probable hypothesis is that turbulence injected into the cloud layer reorganizes into zonal jets and large vortices. However, existing models have been insufficient to test the hypothesis. Here, I describe basic dynamical ideas and numerical simulations to investigate whether this process can produce Jupiter-like jets, and, if so, whether such jets penetrate deeply into the interior or remain confined to the forcing layer. The simulations show development of vortices and jets that resemble the giant planets in broad outline. But the details -- such as whether jets dominate over vortices, whether the jets penetrate deeply into the interior or remain confined to the forcing level, and whether the jet pattern (if any) resembles Jupiter and Saturn -- depends on the type of forcing, the vertical temperature structure, and other parameters. I then extend these ideas to extrasolar planets, focusing on those planets orbiting very close to their stars -- the so-called "hot Jupiters." These planets are intensely heated on their daysides and are expected to have a vigorous circulation that shapes the day-night temperature difference, infrared lightcurve, spectra, albedo, and atmospheric composition. Recent observations place constraints on the wind speeds, day-night temperature difference, and albedos of several hot Jupiters. I will describe theoretical ideas and numerical simulations of the atmospheric circulation of these planets with the goal of interpreting these and future observations of these planets.

The eastern Himalayas are a unique range to quantify the contribution of tectonics and climate to long-term erosion rates because uniform and steady tectonics have persisted for several Myr, while monsoonal precipitations have varied in space and time. The change in precipitation distribution was caused by the uplift of Shillong Plateau 4-5 Myr ago and by formation of an orographic barrier to the prevailing winds transporting moisture from the Bay of Bengal toward the Himalaya. The effects are condensation and precipitation on plateau’s windward side, and a consequent drop in precipitation in the lee. The decrease in rainfall in the Bhutan has led to modification of landscape and decrease in rate of surface erosion. These in turn have caused modifications of tectonic processes in the Himalaya.

Using apatite fission-track data we document the change in erosion rates between Late Miocene and Pliocene registered in two adjacent areas (one in the lee of the Shillong Plateau and one not). We recognise low-relief relict landscape that can be used to gauge the surface uplift rate as well as the incision rate of the modern landscape and thus separate tectonic- and climate-induced exhumation and surface uplift rates. What is more important, we can compare two adjoining areas with and without climate change and confidently deduce the causes of modifications in tectonics, erosion rate and landscape.

We suggest that the presented area is a natural system that is appropriate to answer questions on the nature and efficiency of the coupling between erosion, climate and tectonics (e.g. strength of coupling, time- and length-scale of coupling, how to demonstrate feedbacks). In addition, this work may provide a field study that can compellingly test the erosionally-controlled channel extrusion model.