Density Gradient Field Research Articles

Solar radio emission very near the plasma frequency

view Abstract Citations (3) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Solar radio emission very near the plasma frequency Wentzel, D. G. Abstract The propagation of signals from the solar corona emitted very near the plasma frequency is investigated. The group velocity near the plasma frequency is analyzed; near the edge of the cone of emission, the bandwidth of the escaping radiation is much smaller than the inherent bandwidth of emission. Finite burst duration is observed because of the differential delays in the various parts of the signal, even when there is no subsequent coronal scattering. A formula for the burst duration when the density gradient and magnetic field are parallel is presented. For a homogeneous corona, the burst duration is 0.1 s longer than observed. The signal can be shortened by invoking a smaller density scale height near the plasma level or letting the direction of the density gradient lie outside the cone of emission. A coronal model to account for the observations is suggested. Publication: The Astrophysical Journal Pub Date: July 1983 DOI: 10.1086/161117 Bibcode: 1983ApJ...270..250W Keywords: Plasma Frequencies; Solar Corona; Solar Radio Emission; Intensity; Solar Magnetic Field; Solar Radio Bursts; Solar Physics full text sources ADS |

A new method for density gradient measurements in compressible flows

We suggest the use of a new method, moire deflectometry, for analysis of density gradients in compressible flows. This method, which is simpler to apply than schlieren and does not need precalibration, is capable of providing a fully quantitative diagnosis and map of the density gradient field. We demonstrate the method by a quantitative example of a two-dimensional diamond airfoil in a supersonic wind tunnel and compare it with other diagnostic methods widely used today. HE analysis of density distribution in compressible flows is of major importance in aerodynamics. Among the known methods for density measurements three are com- monly used: interferometry, schlieren, and shadowgraphy. Each method is sensitive to a different density derivative. The readings in interferometr y are proportional to the density, those in schlieren to the density gradients, and those in shadowgraphy to the Laplacian of the density. Although the most quantitative method is interferometry, it is only seldom applied. This is due to the severe mechanical stability requirements, about A/10, which dictate the use of either very heavy equipment to carry the optical components or short laser pulses. The most popular method is schlieren photography, which is a rather simple real-time method, and is more sensitive than shadowgraphy. The purpose of this paper is to demonstrate a new method, moire deflectometry, which was suggested recently,1 for the analysis and mapping of density gradient fields in supersonic wind tunnels. The method, which is based on moire effect, is capable of measuring the deflection of rays of a collimated light beam. The optical system consists of a collimated light source, of the type used in schlieren photography, and two gratings, Gj and G2, as shown schematically in Fig. 1. The moire pattern may be observed on a mat screen, S, attached to the grating G2. The two gratings, G, and G2, are oriented at an angle 6 to each other, so that the resultant moire pattern consists of straight fringes separated by a distance Pr given by2

Stability properties of cold mantle systems for current-driven modes

The stability problem of the boundary regions of cold blanket systems with induced currents parallel to the lines of force is formulated. Particular interest is focused on two types of mode: first electrostatic modes driven by the combined effects of a transverse resistivity gradient due to a spatially non-uniform electron temperature and a longitudinal current, second electromagnetic kink-like modes driven by the torque arising from a transverse current density gradient and magnetic field perturbations. It is found that the combination of various dissipative and neutral gas effects introduces strong stabilizing effects within specific parameter ranges. For particular steady-state models investigated it is shown that these effects become of importance in laboratory plasmas at relatively high densities, low temperatures and moderate magnetic field strengths. Stability diagrams based on specific steady-state cold plasma blanket models will be presented.

Nonlinear Theory of the E x B Instability

A weakly ionized plasma will go unstable in the presence of a density gradient and parallel electric field, both at right angles to a magnetic field, if the electric field is of the right sign and sufficiently large. The nonlinear limit of the first mode to go unstable when E is increased is determined as a function of Δ [Δ ≡ (E - Ec)/Ec]. A general theory of this small amplitude behavior is used. The mode amplitude, frequency shift, density flattening, electric field change, and anomalous plasma transport are obtained as functions of Δ. It is shown that mode coupling corrections are small, and hence that the quasilinear result is sufficient.