Average Drift Velocity Research Articles

On the variability of equatorial F-region vertical plasma drifts

We use incoherent scatter radar measurements from the Jicamarca Observatory to study the variability of equatorial F-region vertical plasma drifts. The daytime average upward drifts do not vary much with solar activity, but the evening upward and the nighttime downward drifts increase from solar minimum to solar maximum. Our data indicate that the quiet-time variability of the Jicamarca vertical drifts is local time, seasonal, and solar cycle dependent. This variability is largest in the dawn–noon sector and during March equinox solar minimum periods, when the midday average upward drift velocity from consecutive magnetically quiet days can often change by more than 10 m/ s . The day-to-day variability of the vertical drifts decreases in the afternoon sector and with the increase of solar activity for all seasons. There are several possible processes responsible for the quiet-time plasma drift variability.

Discrete velocity random motion in an external field

We consider the effects of an external constant force on the one-dimensional transport of a particle whose velocity stochastically fluctuates between two fixed values, $\ifmmode\pm\else\textpm\fi{}v.$ Transport in the presence of a single trap is analyzed in detail. It is found that in the long time limit the trapping probability is decreased compared to that for the overdamped diffusion by the factor ${1/(1+v}_{d}/v)$ where ${v}_{d}$ is the average drift velocity.

Rhythmical eye-head-torso rotation alters fore-aft head stabilization during treadmill locomotion in humans*

A repetitive manoeuvre called torso rotation (TR) is known to temporarily reduce the gain of the horizontal vestibulo-ocular reflex by 10-15% in healthy humans. TR consists of a series of rhythmical rotations of the eyes, head and upper body executed continuously for 30 minutes while standing. Our aim was to investigate whether TR affects the ability to hold the head in a fixed fore-aft position relative to space while walking on a treadmill with eyes closed. Ten healthy subjects stood in a carefully standardized position on a stationary treadmill. The treadmill started unexpectedly and ran for 4 s at 29 cm/s. The test stimulus was a linear acceleration in the fore-aft direction at the moment of treadmill start-up. Linear head position (i.e., ability to stabilize the head) was measured during and following the stimulus. A mechanical system prevented head rotation. Two series of 60 trials were performed before TR (control 1 and control 2 series) and one after TR. Before TR, subjects drifted rearward at an average drift velocity +/- S.D. = 3.1 +/- 0.9 cm/s. This drift was reasonably stable over time within and between the two control series. After TR, head holding ability was further impaired, with subjects having more difficulty to stabilize their head after treadmill start-up. In the first 10 trials after the arrest of TR, the average drift velocity was significantly larger than before TR(6.1 +/- 1.5 cm/sec, p < 0.01). Recovery to control values followed a roughly exponential time course, with 67% recovery occurring in the first 3.4 minutes after TR. Our results indicate that TR impairs the ability to sense and/or respond to fore-aft linear accelerations of the head following treadmill start-up in the absence of vision.

Observations of precipitable water vapor fluctuations in convective boundary layer via microwave interferometry

At microwave frequencies, each centimeter of precipitable water vapor (PWV) causes about 6.45 cm of extra electrical path length relative to the “dry” air. The fluctuations of the water vapor dominate the changes of the effective path length through the atmosphere in a relatively short time period of a few hours. In this paper we describe a microwave interferometer developed for water vapor investigations and present the observation results. The interferometer consists of 10 antennas along two orthogonal 400‐m arms that form many baselines (antenna pairs) ranging from 100 to 400 m. All the antennas receive a common CW signal (11.7 GHz) from a geostationary television satellite, and phase differences between pairs of antennas are measured. The phase differences reflect the column‐integrated water vapor differences from the top of the atmosphere to the spatially separated antennas at the ground. The interferometric, baseline‐differential measurements allow us to study the statistical properties of the PWV fluctuations, as well as the turbulent activity of the convective boundary layer (CBL). Structure function analysis of the interferometer measurements shows good agreement with results obtained from the Very Large Array (VLA) and with a theoretical model developed for radio astronomical very long baseline interferometry (VLBI), reported previously by other investigators. The diurnally varying structure constant correlates remarkably well with the combination of the latent and sensible heat fluxes measured simultaneously from a 10‐m meteorological tower. The average drift velocity of the PWV over the interferometer was also derived from the measurements. The derived velocity agrees well during the morning hours with the wind measured by an anemometer at the center of the interferometer.

Transient transport of electrons in thin film electroluminescent devices

Transient transport of electrons in ZnS-type thin film electroluminescent devices is studied with Monte Carlo method. The variation rule of electrons' average kinetic energy, average drift velocity and intervalley transfer process are obtained. The transient time and drift length before the balance of acceleration and scattering are about 200 fs and 30–40 nm, respectively. Field emission process of electrons trapped at interface states only affects the transient process of electron transport and makes no influence on the steady state. The new explanations about the “dead layer” phenomenon and the overshoot in the average drift velocity are proposed based on the calculation results.

Stationary magnetospheric convection on November 24, 1981. 2. Small-scale structures in the dayside cusp/cleft

Abstract. A case study of the dayside cusp/cleft region during an interval of stationary magnetospheric convection (SMC) on November, 24, 1981 is presented, based on detailed measurements made by the AUREOL-3 satellite. Layered small-scale field-aligned current sheets, or loops, superimposed to a narrow V-shaped ion dispersion structure, were observed just equatorward from the region of the "cusp proper". The equatorward sheet was accompanied by a very intense and short (less than 1 s) ion intensity spike at 100 eV. No major differences were noted of the characteristics of the LLBL, or "boundary cusp", and plasma mantle precipitation during this SMC period from those typical of the cusp/cleft region for similar IMF conditions. Simultaneous NOAA-6 and NOAA-7 measurements described in Despirak et al. were used to estimate the average extent of the "cusp proper" (defined by dispersed precipitating ions with the energy flux exceeding 10-3 erg cm-2 s-1) during the SMC period, as ~0.73° ILAT width, 2.6-3.4 h in MLT, and thus the recently merged magnetic flux, 0.54-0.70 × 107 Wb. This, together with the average drift velocity across the cusp at the convection throat, ~0.5 km s-1, allowed to evaluate the cusp merging contribution to the total cross-polar cap potential difference, ~33.8-43.8 kV. It amounts to a quite significant part of the total cross-polar cap potential difference evaluated from other data. A "shutter" scenario is suggested for the ion beam injection/penetration through the stagnant plasma region in the outer cusp to explain the pulsating nature of the particle injections in the low- and medium-altitude cusp region.Key words. Magnetospheric physics (current systems; magnetopause · cusp · and boundary layers; solar wind-magnetosphere interactions).

Electron Acceleration Process in ZnS-Type Thin Film Electroluminescence Devices

Based on the calculations about scattering rates, the electron acceleration process of ZnS-type thin film electroluminescent devices is studied in detail through Monte Carlo method. The acceleration time and drift length before the balance of acceleration and scattering are about 200 fs and 30-40 nm, respectively. The steady average kinetic energy of electrons increases with electric field. Field emission process of electrons trapped at interface states only affects the acceleration process of electron transport and makes no influence on steady state. New explanations about the "dead layer" phenomenon and the overshoot in the average drift velocity are proposed based on the calculations.

Electromagnetic proton/proton instabilities in the solar wind: Simulations

Proton velocity distributions in the high‐speed solar wind are sometimes observed as two components of approximately equal temperature with an average relative drift velocity parallel to the background magnetic field. This relative drift gives rise to several proton/proton instabilities; for representative parameters, linear Vlasov theory demonstrates that the electromagnetic modes most likely to grow are a magnetosonic instability and an Alfvén instability. Two‐dimensional initial value hybrid simulations of both instabilities are carried out in a homogeneous plasma. Enhanced fluctuating fields from both instabilities yield a reduction in the relative drift speed and a characteristic heating of the more tenuous component temperature perpendicular to the background magnetic field. Ensembles of simulations yield constraints on the fluctuating field energy density, the relative drift speed, and component anisotropies. Some of these constraints are cast as concise analytic expressions which may be indicators of proton/proton instability activity in the solar wind.

Transient acceleration process of electrons in ZnS-type thin film electroluminescence devices

The transient acceleration process of electrons in ZnS-type thin film electroluminescent devices is studied in detail for the first time through the Monte Carlo method. The transient time and drift length before the balance of acceleration and scattering are of the order of 200 fs and 30 nm respectively. The steady average kinetic energy of electrons increases with electric field. Field emission processes of electrons trapped at interface states only affect the transient process of electron transport and have no influence on the steady state. New explanations of the `dead layer' phenomenon and the overshoot in the average drift velocity are proposed based on our calculation results.

Kinetic ballooning instability for substorm onset and current disruption observed by AMPTE/CCE

A new interpretation of AMPTE/CCE observation of substorm onset and current disruption and the corresponding physical processes are presented. Toward the end of late growth phase the plasma β increases to ≥50 and a low frequency instability with a wave period of 50–75 sec is excited and grows to a large amplitude at the current disruption onset. At the onset, higher frequency instabilities are excited so that the plasma and electromagnetic field form a turbulent state. Plasma transport takes place to modify the ambient pressure profile so that the ambient magnetic field recovers from a tail‐like geometry to a more dipole‐like geometry. A new theory of kinetic ballooning instability (KBI) is proposed to explain the low frequency instability and the high β threshold (βc≥50) observed by AMPTE/CCE. The stabilizing effect is mainly due to kinetic effects of trapped electrons and finite ion Larmor radii which give rise to a large parallel electric field and hence a parallel current that greatly enhances the stabilizing effect of field line tension. As a result βc is greatly increased over the ideal MHD ballooning instability threshold by ≥ O(10² ‐ 10³). The wave‐ion magnetic drift resonance effect produces a perturbed resonant ion velocity distribution centered at a duskward velocity roughly equal to the average ion magnetic drift velocity. This perturbed ion distribution explains the enhanced duskward ion flux during the explosive growth phase and can excite higher frequency instabilities (such as the cross‐field current instability).

Evaluation of energetic particle parameters in the near-Earth magnetotail derived from flux asymmetry observations

Abstract. The flux asymmetries measured by spectrometers on board spacecraft contain information on particle parameters. The net flux intensity (NFI) method provides a tool to evaluate these parameters. The NFI method is valid when both the spin period of the spacecraft and the time resolution of the particle spectrometers are much shorter than the characteristic time-scale of the particle flux variations. We apply the NFI analysis to the flux asymmetry measurements made by GEOS 2 at the nightside geosynchronous orbit in the late substorm growth phase. The cross-tail current of energetic ions, their pressure gradient and average drift velocity, as well as a field-aligned flows are investigated. Current disruption at substorm onset and the "convection surge" mechanism during dipolarization of the magnetic field are directly observed.Key words. Flux asymmetry · Net flux intensity · GEOS 2 · Energetic particles

Evaluation of energetic particle parameters in the near-Earth magnetotail derived from flux asymmetry observations

The flux asymmetries measured by spectrometers on board spacecraft contain information on particle parameters. The net flux intensity (NFI) method provides a tool to evaluate these parameters. The NFI method is valid when both the spin period of the spacecraft and the time resolution of the particle spectrometers are much shorter than the characteristic time-scale of the particle flux variations. We apply the NFI analysis to the flux asymmetry measurements made by GEOS 2 at the nightside geosynchronous orbit in the late substorm growth phase. The cross-tail current of energetic ions, their pressure gradient and average drift velocity, as well as a field-aligned flows are investigated. Current disruption at substorm onset and the "convection surge" mechanism during dipolarization of the magnetic field are directly observed.Key words. Flux asymmetry · Net flux intensity · GEOS 2 · Energetic particles

Relationship Between Structure and Electromigration Characteristics of Pure Aluminum Films

ABSTRACTAluminum films were deposited from a high purity aluminum source by the self-ion assisted technique onto oxidized silicon wafers with TiN sub-layers. The ions were accelerated toward the substrate by potentials of 0, 3 and 6 kV. The films were patterned into strips 670 μm long and 8 μm wide using photo-lithographic procedures and wet etching. Average drift velocities were measured in the films tested under electromigration conditions. Electromigration activation energy was obtained for the films. It was found that electromigration activation energy increased with the acceleration potential. The strength of the (111) fiber texture, however, decreased with the acceleration potential. Therefore, the weaker textures resulted in higher electromigration activation energies. These results can be explained in terms of grain boundary structure, which controls electromigration behavior. By using orientation imaging microscopy to characterize the structures, it was shown that the weaker textured specimens contained a high fraction of low angle and low diffusivity grain boundaries.

A theory for generation of the paired region 1 and region 2 field‐aligned currents

We present a new theoretical model for generation of a pair of region 1 and region 2 field‐aligned currents (FACs) under the condition of a southward interplanetary magnetic field. On the basis of the satellite observations it is assumed that the hot (≳1 keV) plasma particles are distributed in a magnetic shell connected to two ovals of diffuse auroras on the northern and southern polar ionospheres. The hot plasma population contained in this magnetic shell having several degrees of latitude in width is called the hot plasma torus (HPT). It is proposed that the region 1/region 2 FACs can be generated as a result of natural distortion of the HPT due to the solar wind convection. When the interplanetary magnetic field has a southward component, i.e., the IMF Bz is negative, the solar wind flow across open geomagnetic field lines gives rise to electric field convection patterns over the polar caps, which are modeled as twin vortex cells with antisunward flows in the center of the polar caps. The convection thus driven by the solar wind is referred to as the solar wind convection. If it were not for an E × B convection flow, the HPT would be shaped such that the HPT particles are contained in the “magnetic drift shells,” which are tangent to the averaged total magnetic drift velocity. In the presence of the solar wind convection, the configuration of the HPT will be deformed from the magnetic drift shells. Because of the distortion of the HPT, the pressure gradient in the HPT gains a component parallel to the magnetic drift. Therefore the HPT can be polarized because of oppositely directed magnetic drifts of the HPT electrons and protons: the high‐latitude and low‐latitude sides of the HPT on the eveningside are negative and positive, respectively, and the polarity is reversed on the morningside. The resulting pattern of large‐scale field‐aligned currents due to the polarization of the HPT is consistent with the observations of region 1 and region 2 FACs. Moreover, provided that the solar wind acts as a voltage generator in the interaction with the open field lines, as a long‐term characteristic of the paired region 1 and region 2 FACs we can obtain the relationship between the FAC intensity and the ionospheric conductivity: both the region 1 and region 2 intensities increase linearly with the Pedersen conductivity, while the proportionality constant for the region 2 FAC is smaller than that for the region 1 FAC. Our predicted relation for geomagnetic quiet conditions quantitatively agrees with the regression lines between the current intensities and the Pedersen conductivities obtained on the basis of Magsat satellite observations by Fujii and Iijima [1987].

Nonlinear theory of a high efficiency E×B drifting electron laser

In a drifting electron laser (DEL), powered by a relativistic beam of E×B drifting electrons in crossed, spatially modulated electric and magnetic fields, the emitted radiation energy and momentum come, respectively, from the change in the electrostatic energy eE0δX and vector potential eB0δX of the electron, δX being the recoil of the guiding center location perpendicular to the drift direction. Since the average drift velocity and the resonance condition are energy-independent one avoids the low efficiency limit η≤1/2Nw placed on free electron lasers (FELs) from energy detuning; in particular there is no inverse relation between per-pass gain and efficiency. In the large radiation amplitude and spot size limit it is shown that the trapped particles do not ‘‘turn around the bucket’’ but keep converting potential energy to radiation until they exit the interaction space, yielding typical DEL efficiency 30%–40% independent of the wiggler length. In addition DELs exhibit much higher tolerance to thermal beam spreads than FELs. In the case of small radiation spot size, gain saturation via the off-axis walk of the emitting electrons is avoided and overlap between the beam and the radiation is maintained by a small tilt of the resonator axis relative to the E×B drift direction.

Monte Carlo simulation of real and reciprocal space transfer of electrons in a pseudomorphically strained GaAs/In 0.15Ga 0.85As/GaAs quantum well

We present results of transient and steady-state Monte Carlo simulations of parallel electron transport in a strained GaAs/In 0.15Ga 0.85As/GaAs quantum well structure. Both real and reciprocal space transfer is studied for electrons in the three lowest conduction bands. Confined states in the Γ, L and X valleys are described via the effective mass model, taking into account the differences in the effective masses of the well and barrier materials. Polar optical, acoustic and inter-valley phonon scattering are included for all electrons, as well as alloy scattering for those confined in the well. Emission of electrons from the well is described in terms of quantum mechanical transitions between confined and unconfined states. The average electron energy and drift velocity, as well as the real and k-space distributions, are examined for a variety of field strengths, including fields large enough that reciprocal space as well as real space transfer is probable. It was found that for low fields (≈1 kV cm −1), the steady-state is reached only after many tens of picoseconds. At higher fields (>5 kV cm −1), the structure exhibits velocity overshoot followed by a significant transient build-up of the population of the confined satellite valley states in a time of the order of a picosecond. Then, after a few picoseconds, the structure reaches the steady-state in which most of the electrons reside in unconfined states. The steady-state velocity-field characteristic for the structure exhibits a higher low field mobility than GaAs, but is very similar in general form.

Bragg scattering and effective force-balance equations in superlattice miniband transport

The two existent balance-equation methods for electron transport, which incorporate Bragg scattering and give rise to bulk negative differential mobility, are further investigated using the Boltzmann transport equation for Bloch electrons. We point out that the ansatz employed in the earlier Büttiker–Thomas (BT) scheme to relate the average drift velocity and the average momentum, should be modified for a general energy band. Numerical calculation of superlattice miniband conduction shows that the modified BT force-balance method yields a velocity–field behavior almost equivalent to that predicted by the acceleration-balance method of Lei, if the appropriate momentum function is used in the calculation of the BT frictional force, such that Bragg scattering is properly taken into account in the process of electron scattering.

Iceberg – structure interaction probabilities for design

When designing systems to operate in regions through which icebergs travel, one of the first inputs of interest is the expected numbers of encounters with icebergs. In this paper, geometric solutions are presented for a cylindrical gravity-based structure, a floating production vessel, and a cargo vessel. The required inputs are the average areal density of icebergs, the average iceberg water line length, and the average iceberg drift velocity. Example calculations are presented for the Grand Banks region off Canada's east coast. Key words: iceberg, encounter, probability, risk, design, oil, production, Grand Banks, structure, tanker.

Alfvén modes in dusty cometary and planetary plasmas

Dusty plasmas contain charged dust grains which are much bigger than protons and usually carry high negative charges owing to preferential capture of electrons. These dust grains do not have a fixed charge, and perturbations in the plasma induce fluctuations in the grain charges owing to liberation or capture of additional electrons and protons. The dispersion relation for low-frequency, long-wavelength and nonresonant electromagnetic fluctuations in a multibeam dusty plasma is derived by taking into account the momentum loss of electrons and protons owing to dust charge fluctuations. When the charging and discharging effects conserve momentum between any two species, the modes become unstable provided that a normalized averaged drift velocity exceeds the Alfvén velocity. However, when the charging and discharging effects are more complicated, drift dissipative type modes are excited when the normalized averaged drift velocity is below the Alfvén velocity. These modes could generate spatial structures over a wide range from hundreds to tens of millions of kilometres in dusty cometary tails. The analysis could be applied to certain planetary rings and to a dusty interstellar medium.

The Observations of Sporadic E Irregularities in Equatorial Anomaly Crest Zone by Using Chung-Li VHF Radar.

With employing 52 MHz Chung-Li VHF radar, the ionospheric irregularities embedded in sporadic E layer in equatorial anomaly crest zone are observed. It shows that in general the echoes of sporadic E irregularities in association with plume-like structures are occurred in the height range from 99 km to 122 km and centered at around 108 km, quite consistent with the observations made by local HF ionosonde. Spectrally analyzing the radar returns indicates that the low-velocity type 1 (or type 3) and type 2 radar spectra are both observed and may appear simultaneously in the same Doppler spectrum. Their characteristics are estimated and discussed in the text. Differing from those obtained by using VHF radars situated in equatorial and auroral zone, the Doppler velocities of type 1 (or type 3) radar spectra observed here are generally within 95∼115 ms-1, fairly smaller than the former ones. The typical Doppler velocity and spectral width for type 2 radar spectra are within -150-160 ms-1 and 16-30 Hz, respectively. The average drift velocity of 3-meter irregularities deduced from type 2 spectra is also presented. It displays that the drift is toward the radar at the lower height, while away above approximately 104 km. Moreover, below the altitude of around 108 km the drift velocity varies substantially with height at the gradient of about 10 ms-1/km. Above that height, the drift velocity changes slightly with height. The plausible process corresponding to these two distinct behaviors of height variation of drift velocity are also discussed in this article.