Component Of Drift Velocity Research Articles

Longitudinal and interhemispheric variations of auroral ionospheric electrodynamics in a realistic geomagnetic field

We investigate the influences of nondipolar features of the geomagnetic field (field strength, orientation, and magnetic coordinate distortion) on auroral ionospheric electrodynamics. We present a conceptual model in which both the electric potential and the distribution functions of precipitating auroral particles are invariant in the magnetic latitude/magnetic local time reference frame, and we explore the predictions of this model concerning longitudinal (or universal time) and interhemispheric (north/south) variations of auroral electrodynamic parameters at ±68° magnetic latitude for a fixed magnetic local time and solar zenith angle. The conductances, electric fields and currents, ion drift velocity components, and Joule heating tend to have two minima and maxima with respect to longitude in the northern hemisphere but have a single minimum and maximum in the southern hemisphere. Particularly large variations are found for the field‐aligned current density (maximum/minimum = 1.76) and the regional Joule heating (maximum/minimum = 1.88), with maxima near northwest Iceland and northwest Alaska, and minima over north‐central Siberia and Hudson Bay. The variations of field‐aligned current intensity may imply a tendency for more frequent field‐aligned electron acceleration and thus brighter aurora near northwest Iceland and northwest Alaska than elsewhere. The longitudinal variations of Joule heating may contribute significantly to universal time variations of total hemispheric Joule heating. Whether or not the conceptual model is correct, at least some of the various electrodynamic parameters must have longitudinal and hemispheric variations of the general magnitudes the model predicts.

The adverse effect of perpendicular ion drift flow on cylindrical triple probe electron temperature measurements

The cylindrical triple probe method is an attractive technique for measuring electron temperatures (Te) and electron number densities (ne) in a variety of plasmas sources. In practice, however, the cylindrical triple probe can be sensitive to sources of error that affect all Langmuir probe techniques. In particular, the presence of an ion drift velocity component that is perpendicular to the probe axis has been known to result in erroneous measurements of ne. Less obvious, however, is that ion flow perpendicular to the probe has a significant effect on the indicated Te. The purpose of this note is to make researchers aware of such an effect and to demonstrate a technique which can mitigate it. The approach taken to investigate this phenomenon was to make Te measurements in the plume of a 20 kW magnetoplasmadynamic thruster with the probe oriented at several angles with respect to the local ion flow.

Magnetic aspect angle effects in radar aurora at 48.5 MHz, corrected for refraction

It is now widely recognized that at lower VHF frequencies, refraction by electron density structures in the auroral E region can greatly affect the character of radio auroral backscatter, making the quantitative study of magnetic aspect angle effects difficult or impossible. In the present study of data taken with the 48.5‐MHz Bistatic Auroral Radar System (BARS) in central Canada, these difficulties have been minimized by using only data obtained from spatially and temporally uniform events. After correcting for effects of refraction, it has been found that true large aspect angle backscatter occurs for magnetic aspect angles exceeding 6°. Doppler velocities from the smaller aspect angle Red Lake radar behaved as if proportional to the line‐of‐sight component of the convection drift velocity; the resulting flow directions were found to be in good agreement with estimates obtained from magnetometers located in the BARS field of view. The constant of proportionality between the radar velocity and the drift velocity component appears to decrease by a factor of approximately 2 as the aspect angle increases from 3° to 6°. At 3° aspect angle, the backscatter power decreased with increasing aspect angle by about 13 dB/deg (the aspect sensitivity), similar to what has been found in other studies at UHF. In common with those other studies this aspect sensitivity is found to decrease with increasing aspect angle, but more rapidly, falling to less than 4 dB/deg at aspect angles of 5°. In contrast to previous studies at small aspect angle, there is no evidence of a dependence of backscattered power on the flow angle, the angle between the radar line of sight and the direction of E × B drift.

Simulating the movement of bed and suspended loads in the coastal environment

A dynamic computer simulation model is implemented to simulate wave-induced transport of bed and suspended material, and the associated development of nearshore profiles. The model uses the Airy, Stokes', cnoidal and solitary wave theories to compute the height, length, celerity, steepness, breaker indices, and horizontal, veritical, mass drift and orbital velocities of random waves propagating along a flat slope. With shoreward wave propagation both bed and suspended loads are computed at discrete grid points. The FORTRAN '77 simulation program is executed for 3000 iterations, with each iteration corresponding to one twelfth of a tidal cycle. The simulated results demonstrate that sediment movement is controlled by the magnitude of the drift, orbital and horizontal wave velocity components, with wave breaking having a direct influence on the deposition of transported bed load. The shallow water regions of cnoidal and solitary waves are characterized by the presence of high concentrations of suspended material. Over the runlength of the simulation, transport of bed and suspended material cause the initialized profile state to change through a sequence of ill-defined transient states to a final morphological state. Examination of the sequence of profile states indicate weak resemblances of barred and nonbarred profile configurations.

Erratum: Velocity-dependent fluctuations: Breaking the randomness of Brownian motion

Any object in thermodynamic equilibrium (TEQ) with a heat bath executes thermal (Brownian) motion. This motion is completely random; i.e., the object is equally likely to move in any direction. This motion is completely random because TEQ is the state of maximum randomness (maximum entropy). In this paper, we prove that if the value of an appropriate fluctuating property of our object functionally depends upon the object's velocity of thermal motion (i.e., if velocity-dependent fluctuations exist), then the randomness of our object's thermal motion at TEQ will be broken---so that, at TEQ, our object will have a net systematic nonrandom drift-velocity component in a preferred direction. This decrease in randomness (in entropy) is shown to occur without any compensating increase either within the object or elsewhere; i.e., it is shown to occur in violation of the second law of thermodynamics. It is also shown how this systematic drift-velocity component can, in turn, be utilized in a process whose only systematic net effect is the conversion, at TEQ, of heat from our heat bath into work, again in violation of the second law of thermodynamics. Both the systematic drift velocity and the time rate of (i.e., the power output associated with) the conversion of heat into work are calculated quantitatively for a simple example.

Velocity-dependent fluctuations: Breaking the randomness of Brownian motion.

Any object in thermodynamic equilibrium (TEQ) with a heat bath executes thermal (Brownian) motion. This motion is completely random; i.e., the object is equally likely to move in any direction. This motion is completely random because TEQ is the state of maximum randomness (maximum entropy). In this paper, we prove that if the value of an appropriate fluctuating property of our object functionally depends upon the object's velocity of thermal motion (i.e., if velocity-dependent fluctuations exist), then the randomness of our object's thermal motion at TEQ will be broken---so that, at TEQ, our object will have a net systematic nonrandom drift-velocity component in a preferred direction. This decrease in randomness (in entropy) is shown to occur without any compensating increase either within the object or elsewhere; i.e., it is shown to occur in violation of the second law of thermodynamics. It is also shown how this systematic drift-velocity component can, in turn, be utilized in a process whose only systematic net effect is the conversion, at TEQ, of heat from our heat bath into work, again in violation of the second law of thermodynamics. Both the systematic drift velocity and the time rate of (i.e., the power output associated with) the conversion of heat into work are calculated quantitatively for a simple example.

The structure of the plasma sheet‐lobe boundary in the Earth's magnetotail

The structure of the magnetotail plasma sheet‐plasma lobe boundary is studied by observing the properties of tailward flowing O+ ion streams. These have been detected by the ISEE 2 plasma experiment and the ISEE 1 ion composition experiment inside the boundary during three time periods in April 1978; data from the ISEE 1 electron spectrometer are used to define the location of the boundary. The density of the O+ ion streams has a relative maximum, and their velocity has a relative minimum at the boundary. The xy drift velocity component across the local magnetic field reverses at the boundary, indicating a reversal of the Ez electric field component. These changes are consistent with the topology of the electric field lines in the tail as mapped from the ionosphere.

Excitation of LHR electrostatic waves in a warm magnetoplasma by current pulses: Application to the VLF echoes excited by the Isis 2 HF transmitter

The potential created by a current pulse is derived using a warm‐plasma description in the vicinity of the lower hybrid resonance (LHR). The results are somewhat different from the previous ones obtained with the cold‐plasma theory; the amplitude of the excited wave has an asymptotic decay with time as t−1 rather than t−½. The spatial distribution of the potential shows a propagation effect that gives a reasonable explanation of the duration of the VLF echoes excited by the Isis 2 HF transmitter; it might also provide a technique for the measurement of the ionic thermal velocity and the drift velocity component perpendicular to the magnetic field.

A preliminary comparison ofFregion plasma drifts andEregion irregularity drifts in the auroral zone

During several days in April–May 1976 the Chatanika, Alaska, incoherent scatter radar and a temporary Doppler auroral radar located at Aniak, Alaska, were directed toward ionospheric volumes along a common magnetic field line in order to compare E region and F region drifts and associated electric fields. The Chatanika radar measured F region plasma drifts via the incoherent scatter technique, while the Aniak radar measured the drifts of E region irregularities (i.e., the radar aurora). The radar geometry was arranged so that both radars measured approximately the same velocity component of a magnetically westward or eastward motion. Preliminary data show good agreement between the drift velocity components measured by the two techniques during most of the experimental period. This result indicates that relatively modest auroral radar systems may be used, with some qualifications, to determine auroral electric fields.

A few results of three-dimensional statistical analysis of ionospheric drift records for an equatorial station

Fedor [1967] proposed a general statistical method of analysis based on the concepts of Briggs et al. [1950] and Banerji [1960]. According to this technique, the best-fitting values of the average steady drift (horizontal and vertical), of the random drift, and of the anisotropy parameters and the orientation of the characteristic ellipsoid of the ground diffraction pattern are obtained from an adequate combination of cross correlograms among spaced receiver and spaced frequency fading records. The method offers three advantages over the previous treatments of this problem in that (1) it provides a best fit to the available data in accordance with the principle of least squares, (2) it places no upper limit on the quantity of data that may be fitted, and (3) the method permits a complete description of the average three-dimensional structure of the measured quantity. It has been shown by Fedor that the horizontal drift velocity components are not given correctly by the two-dimensional methods of Briggs et al. [1950] if the vertical movements of anisometric irregularities occur.

Dynamics of a charged particle in a spiral field

The adiabatic guiding center motion of an energetic charged particle in an idealized representation of the interplanetary magnetic field is discussed. Expressions are presented for all drift velocity components. A comparison of these drifts establishes that, for the cases of physical interest, the electric drift velocity is by far the dominant drift velocity. Considering only the electric drift velocity, an expression is derived for the time of flight of a particle leaving the sun. Similarly, it is shown analytically that the particles remain attached to the same field line on which they left the sun, thus indicating their point of origin. High-energy unscattered particles are shown to be rapidly channeled down the field lines, which results in a highly anisotropic particle distribution in the vicinity of the earth. These analytic results are in excellent agreement with numerical integration of the complete drift equation and compare favorably with the experimental data.

North–South Ionospheric Drifts near the Magnetic Equator

WHEN the well known Mitra method, with three aerials spaced at distances of 1–2 wavelengths apart, is used to measure ionospheric drifts close to the magnetic equator, it is difficult to measure the northward component of drift velocity, VN. This is because the irregularities of the ground diffraction pattern, and also presumably the ionospheric irregularities producing this pattern, are field-aligned and highly elongated in the north–south direction. In consequence the time delays between aerials oriented in this direction are very small; and in the measurements made hitherto at Ibadan, Nigeria (dip 6° S)—for example, by Skinner, Lyon and Wright1—it has not been possible to obtain significant results for VN. Some evidence has, however, recently been published2 of significant daytime values of VN at Thumba, India (dip 0.6° S).

Quantum Effects in the Amplification of Sound in the Presence of a Magnetic Field

A quantum-mechanical treatment is used to calculate the magnetic-field dependence of sound, which is amplified by interaction with conduction electrons in the presence of crossed dc electric and magnetic fields. It is shown that oscillatory behavior as a function of the strength of the applied magnetic field occurs under conditions of amplification. The oscillations which occur in the amplification have an amplitude equal to the nonoscillatory part of the amplification for nontransverse orientations of sound-wave vector q and magnetic field H, as long as there are electrons that have a component of drift velocity in the direction of q which is equal to the sound velocity ${V}_{s}$. The amplification occurs when the drift velocity in the crossed fields, ${\mathrm{V}}_{H}$, has a component in the direction of q which exceeds ${V}_{s}$.

Quantum resonances in the amplification of sound

Giant quantum oscillations in the atienuation of sound by conduction electrons in the presence of a magnetic field at low temperatures were recently predicted. This is brought about when there are electrons that have a component of drift velocity in the direction of propagation of the sound wave which is equal to the sound velocity. It is shown that the giant quantum oscillations can be observed under conditions of amplification.