Laboratory simulations of coastally trapped waves with rotation, topography and stratification

We describe observations of the generation and propagation of coastally trapped waves in the laboratory and their comparison with theory, over a range of values of several experimental parameters. The topography and stratification used consisted of a sloping continental shelf and vertical continental slope with three-layer stratification that could be approximated by an extended version of the Gill and Clarke model [Gill, A.E., Clarke, A., 1974. Wind-induced upwelling, coastal currents and sea level changes. Deep Sea Res. 21, 325–345]. The latter was modified to accommodate a central mixed layer, curved geometry, and friction on the shelf. This configuration represents coastal geometry with large Burger number. The experiments were successful in realizing coastally trapped waves that were consistent with the theoretical expectations. However, the waves propagated more slowly, and for narrow shelves were damped more rapidly than predicted by the theory. The first was attributed to: (i) the effect of stratification on fluid on the shelf, reducing the topographic Rossby wave effect; (ii) the parameterization of the viscosity. The second difference was attributed to the mechanism of generation: the paddle used did not always generate sinusoidal waves, and the subsequent dispersion resulted in a net loss of amplitude.     

Time variation of martian regional contrasts

Observations of contrast changes in blue light on Mars in 1969 and 1971 are presented. Seasonal effects evidently can be evaluated from these data, although nonseasonal effects are also present. A diurnal variation is noted for certain features during both apparitions. This variation cannot be fully accounted for without considering meteorological effects. An example of a contrast reversal in ultraviolet light is presented. The suggestion is made that during the Martian season covered herein, variability of the bright areas controlled the apparent contrasts observed.     

The disappearance of OH from comet P/Encke

Photoelectric observations of Comet P/Encke during its 1980 apparition are combined with other published data to relate molecular production rates to the visual lightcurve. In addition to a substantial asymmetry about perihelion which is already well known, there are shorter-term variations in specific molecules which have not been duplicated by models. The most dramatic of these fluctuations is a rapid decrease by more than a factor of 3 in the production of OH at 0.75 AU preperihelion.     

Simulating Gravity Changes in Topologically Realistic Driven Earthquake Fault Systems: First Results

Currently, GPS and InSAR measurements are used to monitor deformation produced by slip on earthquake faults. It has been suggested that another method to accomplish many of the same objectives would be through satellite-based gravity measurements. The Gravity Recovery and Climate Experiment (GRACE) mission has shown that it is possible to make detailed gravity measurements from space for climate dynamics and other purposes. To build the groundwork for a more advanced satellite-based gravity survey, we must estimate the level of accuracy needed for precise estimation of fault slip in earthquakes. We turn to numerical simulations of earthquake fault systems and use these to estimate gravity changes. The current generation of Virtual California (VC) simulates faults of any orientation, dip, and rake. In this work, we discuss these computations and the implications they have for accuracies needed for a dedicated gravity monitoring mission. Preliminary results are in agreement with previous results calculated from an older and simpler version of VC. Computed gravity changes are in the range of tens of μGal over distances up to a few hundred kilometers, near the detection threshold for GRACE.     

Oscillating pulsar polar gaps

An analytical model for oscillating pair creation above the pulsar polar cap is presented in which the parallel electric field is treated as a large amplitude, superluminal, electrostatic wave. An exact formalism for such wave is derived in one dimension and applied to both the low-density regime in which the pair plasma density is much lower than the corotating charge density and the high-density regime in which the pair plasma density is much higher than the corotating charge density. In the low-density regime, which is relevant during the phase leading to a pair cascade, a parallel electric field develops resulting in a rapid acceleration of particles. The rapid acceleration leads to bursts of pair production and the system switches to the oscillatory phase, corresponding to the high-density regime, in which pairs oscillate with net drift motion in the direction of wave propagation. Oscillating pairs lead to a current that oscillates with large amplitude about the Goldreich–Julian current. The drift motion can be highly relativistic if the phase speed of large amplitude waves is moderately higher than the speed of light. Thus, the model predicts a relativistic outflow of pairs, a feature that is required for avoiding overheating of the pulsar polar cap and is also needed for the pulsar wind.     

Saturated magnetic field amplification at supernova shocks

Cosmic ray streaming instabilities at supernova shocks are discussed in the quasi-linear diffusion formalism which takes into account the feedback effect of wave growth on the cosmic ray streaming motion. In particular, the non-resonant instability that leads to magnetic field amplification in the short wavelength regime is considered. The linear growth rate is calculated using kinetic theory for a streaming distribution. We show that the non-resonant instability is actually driven by a compensating current in the background plasma. The non-resonant instability can develop into a non-linear regime generating turbulence. The saturation of the amplified magnetic fields due to particle diffusion in the turbulence is derived analytically. It is shown that the evolution of parallel and perpendicular cosmic ray pressures is predominantly determined by non-resonant diffusion. However, the saturation is determined by resonant diffusion which tends to reduce the streaming motion through pitch angle scattering. The saturated level can exceed the mean background magnetic field.