Black auroras are recognized as spatially welldefined regions within uniform diffuse aurora where the optical emission is significantly reduced. Although a well studied phenomenon, there is no generally accepted theory for black auroras. One theory suggests that black regions are formed when energetic magnetospheric electrons no longer have access to the loss cone. If this blocking mechanism dr...

Escherichia coli is a motile bacterium that moves up a chemoattractant gradient by performing a biased random walk composed of alternating runs and tumbles. Previous models of run and tumble chemotaxis neglect one or more features of the motion, namely (a) a cell cannot directly detect a chemoattractant gradient but rather makes temporal comparisons of chemoattractant concentration, (b) rather ...

We study the role of band structure anisotropy on the hole transport in 4H–SiC during the transient regime. For the same strength of the applied electric field, the drift velocity overshoot of the hole is stronger and reaches steady state later when the field is applied perpendicular to the c-axis, than when the field is in the c-axis direction. In both cases, the time for the hole drift veloci...

Polymer dynamics at large fields in Rubinstein-Duke repton model is investigated theoretically. Simple diagrammatic approach and analogy with asymmetric simple exclusion models are used to analyze the reptation dynamics of polymers. It is found that for polyelectrolytes the drift velocity decreases exponentially as a function of the external field with an exponent depending on polymer size and ...

A measurement of the drift velocity of free electrons in liquid argon has been performed. Free electrons have been produced by photoelectric e ect using laser light in a so called \laser chamber". The results on the drift velocity vd are given as a function of the electric eld strength in the range 0:5 kV=cm j ~ Ej 12:6 kV=cm and the temperature in the range 87 K T 94 K. A global parametrizatio...

Negative chemotaxis refers to the motion of microorganisms away from regions with high concentrations of chemorepellents. In this study, we set controlled gradients of NiCl2, a chemorepellent, in microchannels to quantify the motion of Escherichia coli over a broad range of concentrations. The experimental technique measured the motion of the bacteria in space and time and further related the m...

Here μ(E)E = v(E) is the (steady state) drift velocity for the case of homogeneous field E. The field is also space and time dependent, i.e. E = E(x, t). Mobility and diffusion coefficients are steady state quantities, but the carrier velocity may differ considerably from the steady state value v(E), due to abrupt space or time changes of the electric field. v(E) can be considered accurate only...

Ionospheric two-stream waves and gradient-drift waves nonlinearly drive a large-scale (D.C.) current in the E-region ionosphere. This current ows parallel to, and with a comparable magnitude to, the fundamental Pedersen current. The underlying mechanism is quite simple. E-region plasma waves generate oscillating electric elds causing electrons to ~ E ~ B drift perpendicular to the wave's propag...

Chemotaxis of the bacterium Escherichia coli is well understood in shallow chemical gradients, but its swimming behavior remains difficult to interpret in steep gradients. By focusing on single-cell trajectories from simulations, we investigated the dependence of the chemotactic drift velocity on attractant concentration in an exponential gradient. Whereas maxima of the average drift velocity c...

The Stokes drift argument put forth by Coy et al. (1986) to explain the monthly mean downward vertical velocities observed in the summer polar mesosphere by radars is critically re-exanfined. The size of the effect is quite sensitive to the choice of gravity wave wavenumber and phase speed. We reproduce the Coy et al. (1986) result for a monochromatic gravity wave and then generalize to a Garre...