10 research outputs found
Current collection in a magnetoplasma
The authors present a survey of a very incomplete subject, current collection in a magnetoplasma. The best-developed and simplest theories for current collection are steady-state collisionless theories, and these must be understood before departures from them can be analyzed usefully. Thus, the authors begin with a review of them. The authors include some recent numerical results which indicate that steady-state collisionless Laplace-limit currents remain substantially below the Parker-Murphy (1967) canonical upper bound out to very large electrode potentials, and approach it as a limit only very slowly if at all. Attempts to correct this theory for space-charge effects lead to potential disturbances which extend to infinite distance along the electrode's magnetic shadow, unless collisional effects are also taken into account. However, even a small amount of relative plasma drift motion, such as that involved in a typical rocket experiment, can change this conclusion fundamentally. It is widely believed that time-averaged current collection may be increased by effects of plasma turbulence, and the authors review the available evidence for and against this contention. Steady-state collisionless particle dynamics predicts the existence of a toroidal region of trapped orbits which surrounds the electrode. Light emissions from this region have been photographed, indicating that collisional ionization may also occur there, and this, and/or scattering by collisions or possibly turbulent fluctuations in this region, may also increase current collection by the electrode. The authors also discuss effects on particle motions near the electrode, associated with breakdown of magnetic insulation in the region of large electric fields near it
The vorticity dynamics of instability and turbulence in a breaking internal gravity wave
Gravity wave exclusion circles in background flows modulated by the semidiurnal tide
In this short paper the exclusion circles and
vertical phase locities for gravity waves launched from the ground into a
time-varying wind are studied using a ray-tracing technique. It is shown that
waves with initial observed phase speeds that should place them within the local
temporally varying exclusion circle, are often Doppler shifted outside of the
circle. This, and the finite lifetime of some critical levels, allow waves to
survive the critical layer and reach higher altitudes. Also, for slower
phase-speed waves, the temporally varying wind can shift the observed frequency
to negative values, so that the observed phase motions will be opposite (i.e.
horizontally reversed and vertically upward), even though the energy still
propagates upward. This effect can also cause the phase velocity to move inside
the local exclusion circle. Due to the directional filtering of wave sources by
the stratospheric wind, the percentage of such reverse-propagating waves will
change systematically with local time and height in our simplified but realistic
model. These results are related to ground-based systems, optical and radar,
which sample the wind field and gravity waves in the middle atmosphere
The influence of time-dependent wind on gravity-wave propagation in the middle atmosphere
Ray-tracing techniques are used to
computationally investigate the propagation of gravity waves through the middle
atmosphere, as characterized by the vertically varying CIRA-86 wind and
temperature models, plus a tidal wind model that varies temporally as well as
vertically. For the wave parameters studied here, the background wind variation
has a much stronger influence on the ray path and changes in wave
characteristics than does the temperature variation. The temporal variation of
the tidal component of the wind changes the observed frequency, sometimes
substantially, while leaving the intrinsic frequency unaltered. It also renders
temporary any critical levels that occur in the tidal region. Different starting
times for the rays relative to the tidal phase provide different propagation
environments, so that the temporary critical levels appear at different heights.
The lateral component of the tidal wind is shown to advect propagating wave
packets; the maximum lateral displacement of a packet varies inversely with its
vertical group velocity. Time-dependent effects are more pronounced in local
winter than in summer