Origin Of Low Frequency Oscillations Research Articles

Study of dusty plasma as origin of low-frequency oscillations in space

By using the fundamental fluid equations of continuity and momentum transfer, we derive an eigenvalue equation for Rayleigh Taylor (RT) instability and solve the equation numerically. The result shows that the growth rate of the RT mode is considerably higher in a dusty plasma than in a pure plasma. This is a first step, but it shows quite unambiguously that the dust might have a crucial role in explaining the origin of the low-frequency fluctuations in the space, the origin of the low-frequency oscillations is not understood at present.

On Stabilizing Role of Parallel Inhomogeneous Flow on Low-frequency Space Fluctuations

A fundamental reality throughout the space plasma is the existence of magnetic field-aligned flows. It is usually believed that the spatial transverse shear in the parallel flow destabilizes many low frequency oscillations and this may be the origin of low frequency oscillations in the ionosphere (V. V. Gavrishchaka et al., 1998, Phys. Rev. Lett., 80, 728 and Phys. Rev. Lett.: 2000, 85, 4285). Here we show that this notion of destabilizing influence of the shear in the parallel flow can be changed altogether if one takes the effect of the flow curvature (second spatial derivative) into account. The transverse curvature in the parallel flow can overcome the destabilizing influence of the shear and can render the low frequency modes stable. It is shown that unlike flow shear the effect of flow curvature is sign- and mode-dependent.

Mitigation of low-frequency oscillations by parallel inhomogeneous flow

A fundamental reality throughout the space and laboratory plasmas is the existence of magnetic field-aligned flows. It is usually believed that the spatial transverse shear in the parallel flow destabilizes many low-frequency instabilities and this may be the origin of low-frequency oscillations in the ionosphere. Here we show that this notion of the destabilizing influence of the shear in the parallel flow canbe changed altogether if one takes the effect of the flow curvature(second spatial derivative) into account. The transverse curvature in the parallel flow can overcome the destabilizing influence of the shear and can render the low-frequency ion temperature gradient-type instabilities stable.

Origin of Low-Frequency Oscillations in the Ionosphere

The threshold current for the ion-acoustic branch is shown to be significantly lower than the ion-cyclotron branch and insensitive to the ion/electron temperature ratio if there is a transverse gradient in the relative magnetic field aligned drift, ${V}_{d}$, and $|({k}_{y}{/k}_{z})(1/{\ensuremath{\Omega}}_{i})({\mathrm{dV}}_{d}/dx)|$ is sufficiently large. The effect persists even when $|{\mathrm{dV}}_{d}/dx|\ensuremath{\rightarrow}0$ provided $({k}_{z}{/k}_{y})\ensuremath{\rightarrow}0$, where ${k}_{z}$ and ${k}_{y}$ are wave vectors along and across the magnetic field and ${\ensuremath{\Omega}}_{i}$ is the ion gyrofrequency. Therefore, the ion-acoustic branch is more central to the plasma processes in the ionosphere than is currently believed.

The Origin of Low-Frequency Oscillations in the Southern Mid-Atlantic Bight

Abstract Distinct offshore and upward phase propagation at periods of 12 and 24 days was previously observed in the horizontal flow field on the outer shelf in the southern Mid-Atlantic Bight during the Shelf Edge Exchange Processes Experiment in spring 1988. A linear, forced-wave model is invoked to explain the observed phase propagation. It is found that the observed phase propagation can exist when currents are in resonance with a vorticity source on the continental slope. It is essential that the thermohaline field be characterized by an upper-ocean stratification an the slope for resonance to occur. The phase difference near resonance is weakly dependent on the bottom frictional parameter and the alongshore length scale of forcing. Resonance is due to onshore propagation of topographic waves. It is suggested that low-frequency oscillations on the outer shelf in the Mid-Atlantic Bight in winter an associated with sources on the upper slope.