543 research outputs found
Pattern formation during diffusion limited transformations in solids
We develop a description of diffusion limited growth in solid-solid
transformations, which are strongly influenced by elastic effects. Density
differences and structural transformations provoke stresses at interfaces,
which affect the phase equilibrium conditions. We formulate equations for the
interface kinetics similar to dendritic growth and study the growth of a stable
phase from a metastable solid in both a channel geometry and in free space. We
perform sharp interface calculations based on Green's function methods and
phase field simulations, supplemented by analytical investigations. For pure
dilatational transformations we find a single growing finger with symmetry
breaking at higher driving forces, whereas for shear transformations the
emergence of twin structures can be favorable. We predict the steady state
shapes and propagation velocities, which can be higher than in conventional
dendritic growth.Comment: submitted to Philosophical Magazin
Theory of dendritic growth in the presence of lattice strain
Elastic effects due to lattice strain modify the local equilibrium condition
at the solid-solid interface compared to the classical dendritic growth. Both,
the thermal and the elastic fields are eliminated by the Green's function
techniques and a closed nonlinear integro-differential equation for the
evolution of the interface is derived. In the case of pure dilatation, the
elastic effects lead only to a trivial shift of the transition temperature
while in the case of shear transitions, dendritic patterns are found even for
isotropic surface energy
Nonlinear Two-Dimensional Green's Function in Smectics
The problem of the strain of smectics subjected to a force distributed over a
line in the basal plane has been solved
Empirical predictive models of daily relativistic electron flux at geostationary orbit: Multiple regression analysis
The daily maximum relativistic electron flux at geostationary orbit can be predicted well with a set of daily averaged predictor variables including previous day's flux, seed electron flux, solar wind velocity and number density, AE index, IMF Bz, Dst, and ULF and VLF wave power. As predictor variables are intercorrelated, we used multiple regression analyses to determine which are the most predictive of flux when other variables are controlled. Empirical models produced from regressions of flux on measured predictors from 1 day previous were reasonably effective at predicting novel observations. Adding previous flux to the parameter set improves the prediction of the peak of the increases but delays its anticipation of an event. Previous day's solar wind number density and velocity, AE index, and ULF wave activity are the most significant explanatory variables; however, the AE index, measuring substorm processes, shows a negative correlation with flux when other parameters are controlled. This may be due to the triggering of electromagnetic ion cyclotron waves by substorms that cause electron precipitation. VLF waves show lower, but significant, influence. The combined effect of ULF and VLF waves shows a synergistic interaction, where each increases the influence of the other on flux enhancement. Correlations between observations and predictions for this 1 day lag model ranged from 0.71 to 0.89 (average: 0.78). A path analysis of correlations between predictors suggests that solar wind and IMF parameters affect flux through intermediate processes such as ring current (Dst), AE, and wave activity
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