The stability of stratified conducting shear flow in an aligned magnetic field

The stability of a horizontally stratified, electrically conducting fluid permeated by a uniform magnetic field aligned with the motion is investigated. The resulting linear stability problem for the special case of constant density gradient and linear shear in an unbounded fluid is reduced to the study of a third order differential equation in time. In the absence of dissipation, the linear shear eventually causes hybrid Alfvén-gravity waves to decay algebraically. The effect of the shear is to shorten the vertical length scale. So with the addition of even small diffusivity, dissipation is strongly stabilising and all modes eventually collapse exponentially, generally at a fast rate. The evolution from wave motion to exponential decay is examined for particular limiting cases. When the fluid is bounded by horizontal planes a nonlinear stability criterion is derived using the energy method

Screw dynamo in a time-dependent pipe flow

The kinematic dynamo problem is investigated for the flow of a conducting fluid in a cylindrical, periodic tube with conducting walls. The methods used are an eigenvalue analysis of the steady regime, and the three-dimensional solution of the time-dependent induction equation. The configuration and parameters considered here are close to those of a dynamo experiment planned in Perm, which will use a torus-shaped channel. We find growth of an initial magnetic field by more than 3 orders of magnitude. Marked field growth can be obtained if the braking time is less than 0.2 s and only one diverter is used in the channel. The structure of the seed field has a strong impact on the field amplification factor. The generation properties can be improved by adding ferromagnetic particles to the fluid in order to increase its relative permeability,but this will not be necessary for the success of the dynamo experiment. For higher magnetic Reynolds numbers, the nontrivial evolution of different magnetic modes limits the value of simple `optimistic' and `pessimistic' estimates.Comment: 10 pages, 12 figure

A Quantitative Model of Energy Release and Heating by Time-dependent, Localized Reconnection in a Flare with a Thermal Loop-top X-ray Source

We present a quantitative model of the magnetic energy stored and then released through magnetic reconnection for a flare on 26 Feb 2004. This flare, well observed by RHESSI and TRACE, shows evidence of non-thermal electrons only for a brief, early phase. Throughout the main period of energy release there is a super-hot (T>30 MK) plasma emitting thermal bremsstrahlung atop the flare loops. Our model describes the heating and compression of such a source by localized, transient magnetic reconnection. It is a three-dimensional generalization of the Petschek model whereby Alfven-speed retraction following reconnection drives supersonic inflows parallel to the field lines, which form shocks heating, compressing, and confining a loop-top plasma plug. The confining inflows provide longer life than a freely-expanding or conductively-cooling plasma of similar size and temperature. Superposition of successive transient episodes of localized reconnection across a current sheet produces an apparently persistent, localized source of high-temperature emission. The temperature of the source decreases smoothly on a time scale consistent with observations, far longer than the cooling time of a single plug. Built from a disordered collection of small plugs, the source need not have the coherent jet-like structure predicted by steady-state reconnection models. This new model predicts temperatures and emission measure consistent with the observations of 26 Feb 2004. Furthermore, the total energy released by the flare is found to be roughly consistent with that predicted by the model. Only a small fraction of the energy released appears in the super-hot source at any one time, but roughly a quarter of the flare energy is thermalized by the reconnection shocks over the course of the flare. All energy is presumed to ultimately appear in the lower-temperature T<20 MK, post-flare loops

Acute Severe Pain Is a Common Consequence of Sexual Assault

Sexual assault (SA) is common, but the epidemiology of acute pain after SA has not previously been reported. We evaluated the severity and distribution of pain symptoms in the early aftermath of SA among women receiving sexual assault nurse examiner (SANE) care, and the treatment of pain by SANE nurses. Severe pain (≥7 on a 0–10 numeric rating scale) was reported by 53/83 women sexual assault survivors (64% [95% CI, 53%–74%]) at the time of SANE evaluation and 43/83 women (52% [95% CI, 41%–63%]) one week later. Pain in four or more body regions was reported by 44/83 women (53% [95% CI, 42%–64%]) at the time of initial evaluation and 49/83 women (59% [95% CI, 48%–70%]) at one week follow-up. Among survivors with severe pain at the time of initial post-assault evaluation, only 7/53 (13% [95% CI, 6%–26%]) received any pain medication at the time of initial SANE treatment. These findings suggest that pain is common in SA survivors in the early post-assault period, but rarely treated

Inertial wave activity during spin-down in a rapidly rotating penny shaped cylinder

International audienc

Dynamics of rotating fluids

No abstract available

The onset of thermal convection in rotating sperical shells.

International audienc

The extinction problem for three-dimensional inward solidification

The one-phase Stefan problem for the inward solidification of a three-dimensional body of liquid that is initially at its fusion temperature is considered. In particular, the shape and speed of the solid-melt interface is described at times just before complete freezing takes place, as is the temperature field in the vicinity of the extinction point. This is accomplished for general Stefan numbers by employing the Baiocchi transform. Other previous results for this problem are confirmed, for example the asymptotic analysis reveals the interface ultimately approaches an ellipsoid in shape, and furthermore, the accuracy of these results is improved. The results are arbitrary up to constants of integration that depend physically on both the Stefan number and the shape of the fixed boundary of the liquid region. In general it is not possible to determine this dependence analytically; however, the limiting case of large Stefan number provides an exception. For this limit a rather complete asymptotic picture is presented, and a recipe for the time it takes for complete freezing to occur is derived. The results presented here for fully three-dimensional domains complement and extend those given by McCue et al. [Proc. R. Soc. London A 459 (2003) 977], which are for two dimensions only, and for which a significantly different time dependence occurs