Undesired sound is frequently attenuated by having it pass through ducts with absorbing side walls. If the absorptive properties of the walls are described by a normal impedance, and if there is no air flow in the duct, the resulting attenuation is easily predicted on the basis of existing theory. In many cases, however (e.g., in ventilating ducts or the exhaust ducts in certain wind tunnels or jet-engine test cells), there is also a flow of air through the duct which may affect the rate of sound attenuation. In order to investigate this phenomenon a study is made of the propagation of sound in both constant gradient shear flow and a turbulent shear flow above a flat surface. Curves are presented showing how, in the case of downstream propagation, the flow gradient tends to channel the sound energy into a narrow layer next to the wall. These results are used in estimating the effect of a flow on the attenuation of sound in a duct with absorbing side walls.

We investigate the balance of forces near an X line in a collisionless plasma having a reconnection electric field along the X line. Near an X line the generalized Ohm's law reduces to the electron momentum equation. We use a two‐dimensional magnetic field model and apply a simplified model of particle motion to evaluate terms in this momentum equation. Our results show that the gyroviscosity associated with off‐diagonal elements of the pressure tensor can balance the electric field at and near the X line, as has recently been suggested by Dungey. Particle velocity moments do not become large at the X line, and we conclude that neither collisions nor wave turbulence are necessary for an electric field along an X line to be maintained. Gyroviscosity results from single‐particle motion in the presence of the magnetic field gradients near the X line, but we find that large values for these gradients are not required. We find that conditions expected to occur in the distant geomagnetic tail yield reasonable values for the physical parameters associated with reconnection. However, the equations provide no actual constraints on parameters other than the requirement for force balance. Thus gyroviscosity does not impose restraints on the reconnection rate. The present application is to the geomagnetic tail, but our results should apply as well to collisionless reconnection in many situations.

A study is made of the propagation of sound in both a constant gradient shear flow and a turbulent shear flow above a flat surface. Curves are presented showing how, in the case of downstream propagation, the flow gradient tends to channel the sound energy into a narrow layer next to the wall. These results are used in estimating the effect of a flow on the attenuation of sound in a duct with absorbing side walls.

We propose that many features of the cusp and low‐latitude boundary layer (LLBL) observed near noon MLT can be explained by interpreting the LLBL as being on open lines with an inner boundary at the separatrix between open and closed magnetic field lines. This interpretation places the poleward boundary of the LLBL and equatorward boundary of the cusp along the field line that bifurcates at the cusp neutral point. The interpretation accounts for the abrupt boundary of magnetosheath particles at the inner edge of the LLBL, a feature that is inconsistent with LLBL formation by diffusion onto closed field lines, and for the distribution of magnetosheath particles appearing more as one continuous region than as two distinct regions across the noon cusp/LLBL boundary. Furthermore, we can explain the existence of energetic radiation belt electrons and protons with differing pitch angle distributions within the LLBL and their abrupt cutoff at the poleward boundary of the LLBL. By modeling the LLBL and cusp region quantitatively, we can account for a hemispherical difference in the location of the equatorial boundary of the cusp that is observed to be dependent on the dipole tilt angle but not on the interplanetary magnetic field (IMF) x component. We also find important variations and hemispherical differences in that the size of the LLBL that should depend strongly upon the x component of the IMF. This prediction is observationally testable. Finally, we find that when the IMF is strongly northward, the LLBL may include a narrow region adjacent to the magnetopause where field lines are detached (i.e., have both ends connected to the IMF).