Drift velocity

About: Drift velocity is a research topic. Over the lifetime, 6897 publications have been published within this topic receiving 129602 citations. The topic is also known as: drift speed.

A power law phase screen model for ionospheric scintillation: 2. Strong scatter

Abstract: In this paper the weak scatter scintillation theory is reformulated to show explicitly the ramifications of an arbitrarily large ionospheric outer scale. The measured temporal phase spectrum, for example, is effectively truncated at a fixed frequency corresponding to the detrend time or the length of the data interval over which it is measured (whichever is smaller). As a consequence, the rms phase exhibits a complicated dependence on the relative irregularity drift velocity and the propagation geometry. This effect has not been included in previous analyses. By comparison, intensity scintillation data are intrinsically high-pass filtered by the diffraction process. By taking advantage of this fact a simple closed form expression for the S4 intensity scintillation index has been derived. The theory is applied to representative data sets from the Wideband satellite. The interpretation of the ionospheric parameters deduced from the analysis is also discussed.

Velocity Modification of H I Power Spectrum

Abstract: The distribution of atomic hydrogen in the Galactic plane is usually mapped using the Doppler shift of 21 cm emission line, and this causes the modification of the observed emission spectrum. We calculate the emission spectrum in velocity slices of data (channel maps) and derive its dependence on the statistics of velocity and density fields. We find that, (1) if the density spectrum is steep, i.e., n < -3, the large k asymptotics of the emissivity spectrum are dominated by the velocity fluctuations; and (2) the velocity fluctuations make the emission spectra shallower, provided that the data slices are sufficiently thin. In other words, turbulent velocity creates small-scale structure that can erroneously be identified as clouds. The effect of thermal velocity is very similar to the change of the effective slice thickness, but the difference is that, while an increase of the slice thickness increases the amplitude of the signal, the increase of the turbulent velocity leaves the measured intensities intact while washing out fluctuations. The contribution of fluctuations in warm H I is suppressed relative to those in the cold component when the velocity channels used are narrower than the warm H I thermal velocity and small angular scale fluctuations are measured. We calculate how the spectra vary with the change of velocity slice thickness and show that the observational 21 cm data is consistent with the explanation that the intensity fluctuations within individual channel maps are generated by turbulent velocity fields. As the thickness of velocity slices increases, density fluctuations begin to dominate emissivity. This allows us to disentangle velocity and density statistics. The application of our technique to Galactic and SMC data reveals spectra of density and velocity with power law indexes close to -11/3. This is a Kolmogorov index, but the explanation of the spectrum as due to the Kolmogorov-type cascade faces substantial difficulties. We generalize our treatment for the case of a statistical study of turbulence inside individual clouds. The mathematical machinery developed is applicable to other emission lines.

Velocity Distributions in Molecular Beams from Nozzle Sources

Abstract: The velocity distributions in molecular beams of argon produced by the Kantrowitz‐Grey supersonic nozzle technique have been investigated using a time‐of‐flight method It is shown that the transition from continuum to free molecular flow in free jet expansion limits the minimum width of the velocity distribution which may be obtained The Mach number corresponding to the axial velocity distribution of the expanding gas approaches a limit which varies inversely with the two‐fifths power of the Knudsen number at the nozzle With argon at room temperature the narrowest velocity distribution obtained corresponds to a Mach number of 23 (90% of the molecules within 54% of the most probable velocity)

Hot electrons in germanium and Ohm's law

Abstract: The data of E. J. Ryder on the mobility of electrons in electric fields up to 40,000 volts per cm are analyzed. The mobility decreases many fold due to the influence of scattering by optical modes and due to increases of electron energy. It is estimated that electron “temperatures” as high as 4000°K have been produced in specimens having temperatures of atomic vibration of 300° K. The critical drift velocity above which there are deviations from Ohm's law is about 2.6 × 106 cm/sec. This is three times higher than the elementary theory and on explanation in terms of complex energy surfaces is proposed.

Anomalous Diffusion Arising from Microinstabilities in a Plasma

Abstract: A plasma is considered in which a Maxwellian distribution of electrons with thermal velocity ve and drift velocity vD is drifting relative to a Maxwellian distribution of ions with thermal velocity vi. For vD ≲ ve the usual ion acoustic waves are stable, however, electrostatic ion cyclotron waves with ω ≅ Ωi are unstable for vD ≳ 5vi. In the case when 5vi ≲ vD ≲ ve, and Te/Ti < 2 the electrostatic ion cyclotron waves grow to a nonlinear equilibrium spectrum. This spectrum of waves leads to a diffusion of electrons across the field lines with a diffusion coefficient D = αρ2eΩe, where ρe is the electron Larmor radius and Ωe is the electron Larmor frequency. α, the ratio of the resulting diffusion coefficient to the Bohm diffusion coefficient, is given by a constant × (vD/ve)5(Te/Ti)2.