Finite Wave Velocity Research Articles

Clinical achievements of impedance analysis

Various models and derived measures of arterial function have been proposed to describe and quantify pulsatile hemodynamics in humans. A major distinction can be drawn between lumped models based on circuit theory that assume infinite pulse wave velocity versus distributed, propagative models based on transmission line theory that acknowledge finite wave velocity and account for delays, wave reflection, and spatial and temporal pressure gradients within the arterial system. Although both approaches have produced useful insights into human arterial pathophysiology, there are important limitations of the lumped approach. The arterial system is heterogeneous and various segments respond differently to cardiovascular disease risk factors including advancing age. Lumping divergent change into aggregate summary variables can obscure abnormalities in regional arterial function. Analysis of a limited number of summary variables obtained by measuring aortic input impedance may provide novel insights and inform development of new treatments aimed at preventing or reversing abnormal pulsatile hemodynamics.

On Jovian plasma sheet structure

We evaluate several models of Jovian plasma sheet structure by determining how well they organize several aspects of the observed Voyager 2 magnetic field characteristics as a function of Jovicentric radial distance. The present study focuses exclusively on the data from the Voyager 2 spacecraft because the magnetosphere was more stable during that flyby than it was during the Pioneer 10 and Voyager 1 flybys. It is shown that in the local time sector of the Voyager 2 outbound pass (near 0300 LT) the published hinged‐magnetodisc models with wave (i.e., models corrected for finite wave velocity effects) are more successful than the published magnetic anomaly model in predicting locations of current sheet crossings. We also consider the boundary between the plasma sheet and the magnetotail lobe which is expected to vary slowly with radial distance. We use this boundary location as a further test of the models of the magnetotail. The plasma‐sheet‐lobe boundary is often identified from the observed field gradients, the gradients in the lobes being much smaller than those in the plasma sheet. We show that the compressional MHD waves have much smaller amplitude in the lobes than in the plasma sheet and use this criterion to refine the identification of the plasma‐sheet‐lobe boundary. When the locations of crossings into and out of the lobes are examined, it becomes evident that the magnetic‐anomaly model yields a flaring plasma sheet with a halfwidth of ∼3 RJ at a radial distance of 20 RJ and ∼12 RJ at a radial distance of 100 RJ. The hinged‐magnetodisc models with wave, on the other hand, predict a halfwidth of ∼3.5 RJ independent of distance beyond 20 RJ. New optimized versions of the two models locate both the current sheet crossings and lobe encounters equally successfully. The optimized hinged‐magnetodisc model suggests that the wave velocity decreases with increasing radial distance. The optimized magnetic anomaly model yields lower velocity contrast than the model of Vasyliunas and Dessler (1981). The hinged‐magnetodisc models are shown to satisfy the constraints on the plasma sheet thickness imposed by MHD theory. The magnetic anomaly model can be made consistent with the expectations from MHD theory only by assuming plasma anisotropy so large that the plasma would be unstable to mirror mode instability. We recognize that the magnetic anomaly model is a comprehensive model designed to account for many features of the Jovian system as observed in multiple flyby missions. The plasma sheet structure for the Voyager 2 epoch is only one aspect of the model's predictions. Nonetheless, we believe that the model's failure to satisfy constraints on plasma sheet structure imposed by MHD equilibrium theory presents a challenge to the magnetic anomaly model as it is currently described.

Nonlinear shock acceleration. III - Finite wave velocity, wave pressure, and entropy generation via wave damping

The nonlinear theory of shock acceleration developed in earlier papers, which treated the waves as being completely frozen into the fluid, is generalized to include wave dynamics. In the limit where damping keeps the wave amplitude small, it is found that a finite phase velocity (V sub ph) of the scattering waves through the background fluid, tempers the acceleration generated by high Mach number shocks. Asymptotic spectra proportional to 1/E sq are possible only when the ratio of wave velocity to shock velocity is less than 0.13. For a given asymptotic spectrum, the efficiency of relativistic particle production is found to be practically independent of the value of V sub ph, so that earlier results concerning its value remain valid for finite V sub ph. In the limit where there is no wave damping, it is shown that for modest Alfven Mach numbers, approximately greater than 4 and less than 6, the magnetic field is amplified by the energetic particles to the point of being in rough equipartition with them, as models of synchrotron emission frequently take the field to be. In this case, the disordering and amplification of field energy may play a major role in the shock transition.