How the magnetosphere-ionosphere/thermosphere-ground system is coupled by waves

<p>We study the polar magnetospheric coupling to the ionosphere/thermosphere and ground system along the field by examining a spectrum of perturbations propagating from the magnetosphere downward. What distinguishes this study from conventional treatments of the magnetosphere-ionosphere/thermosphere system is that our treatment self-consistently includes the responses of the neutrals to the magnetic and plasma velocity perturbations. The thermosphere is coupled to the magnetosphere-ionosphere (M-I) system in a degree that depends on the time scale of the perturbations. There are three major processes that affect the perturbation propagation: damping that reduces the energy flux while producing heating, the neutral-inertia loading that reduces the propagation speed, and reflection which, associated with structures of the ionosphere and thermosphere, reduces the downward energy flux. The damping is stronger in higher frequencies, 10<sup>-2~0</sup> Hz for M-I coupling. As a result of reflection, significant energy fluxes of the magnetospheric perturbations cannot reach the lower ionosphere and hence the ground although some heating and energization may occur in the lower ionosphere resulting from the strong damping of high frequency fluctuations. However, the amplitude of the magnetic fluctuations of the transmitted flux into the lower ionosphere can be enhanced in lower frequencies because of the decrease in the propagation speed due to strong neutral-inertia loading. Combining the attenuation and amplitude enhancement effects, the net enhanced amplitudes occur in frequencies less than few Hertz, which may explain the ready observations of PC waves that are enhanced magnetic oscillations in periods from 0.5 sec to 30 min on the ground while little enhancement is observed below this period range. On the other hand, the smallness of the propagation velocity results in very small electric perturbations, forming a magneto-static condition for coupling from the lower ionosphere to the ground in low-frequencies, casting doubts on any ionosphere-ground coupling mechanisms based on static electric field in the lower frequencies.</p>

Effect of Imitation Phenomenon on Two-lane Traffic Safety in Fog Weather

A neighboring lane’s vehicles are potentially important influence factors of traffic safety. In fog weather, drivers will automatically imitate the behaviors demonstrated by other vehicles in the neighboring lane. To illustrate the effect of the imitation phenomenon on traffic safety, this paper develops an extended two-lane car-following model in fog weather. Numerical simulations are carried out to study the effect of imitation on multiple-vehicle collision induced by a sudden stop, as well as perturbation propagation when a small perturbation is added to the uniform traffic flow. The results indicate that the number of collisions depends on the influence coefficient of neighboring lane’s vehicles, sensitivity, headway and initial velocity. Furthermore, the number of crumpled vehicles decreases when the imitation phenomenon is taken into account. In addition, lower vehicular velocity in the neighboring lane can reduce the magnitude of acceleration and fluctuation of headway. The perturbation can be absorbed under certain given conditions regarding the imitation phenomenon. Therefore, traffic safety can be improved by considering the effect of the imitation phenomenon on two-lane traffic flow in fog weather. The findings in this study can provide a theoretical reference for the development of multi-lane intermittent release measures in fog weather.

Shock Wave in a Pile Immersed into Viscoplastic Medium

The wave problem of perturbation propagation along an elastic pile interacting with the medium is investigated using the model of viscoplastic friction. An exact solution of the problem is obtained using the Laplace transforms for an arbitrary time of the loading period. The diagrams for velocity and stresses have been constructed.

Effect of Localization on the Stability of Mutualistic Ecological Networks

The relationships between the core-periphery architecture of the species interaction network and the mechanisms ensuring the stability in mutualistic ecological communities are still unclear. In particular, most studies have focused their attention on asymptotic resilience or persistence, neglecting how perturbations propagate through the system. Here we develop a theoretical framework to evaluate the relationship between architecture of the interaction networks and the impact of perturbations by studying localization, a measure describing the ability of the perturbation to propagate through the network. We show that mutualistic ecological communities are localized, and localization reduces perturbation propagation and attenuates its impact on species abundance. Localization depends on the topology of the interaction networks, and it positively correlates with the variance of the weighted degree distribution, a signature of the network topological hetereogenity. Our results provide a different perspective on the interplay between the architecture of interaction networks in mutualistic communities and their stability.

Well posed problem in sense of Hadamard, for a source produced acoustic perturbation propagation in a lined duct, carrying a gas flow

The purpose of this paper is to replace Brambley’s concept of ”well posed differential equation” (J. Sound Vibr., 322 (2009) 1026-1037), related to the source produced acoustic perturbation propagation in a rectangular lined duct carrying a gas flow, with that of ”well posed problem in Hadamard sense”. The reason is that the concept introduced by Brambley is confusing, i.e. the same equation can be well posed or ill posed depending on the functional framework. For the source produced acoustic perturbation (SPAP) propagation problem the concept of ”well posed problem in Hadamard sense” is defined as follows: for any SPAP, from a given set of SPAP-s , there exists a unique solution to the problem of propagation and the solution depends continuously on the SPAP. Necessary or sufficient conditions are derived for that the propagation problem be well posed in sense of Hadamard.