Perpendicular Drift Velocity Research Articles

ARTEMIS Observations of Lunar Crustal Field‐Solar Wind Interaction and Impact on Reflected Plasma Under Weak Radial IMF

AbstractMagnetic anomalies or crustal fields are frequently observed close to the lunar surface, yet their interaction with the ambient plasma is not fully understood. Utilizing data from the Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) spacecraft, we perform case studies of such interactions under radial, weak interplanetary magnetic field. Our analysis shows that while the lunar crustal field direction did not directly affect the velocity and density of reflected ions, it locally increased their pitch angles, favoring growth of ultralow‐frequency waves. Additionally, we find that while the solar wind ions were demagnetized, retaining a nearly constant velocity, the magnetized electrons acquired significant perpendicular drift velocity due to the crustal field, resulting in a Hall current. This current altered the field strength near the lunar surface. The calculated loss cone angles of electrons reflected by the near‐surface field estimated from the Hall current are consistent with observations, suggesting that the crustal field was dominated by the Hall magnetic field. Our study advances our understanding of how lunar crustal fields interact with the solar wind and modify the nearby reflected plasma.

Transport coefficients for drifting Maxwellian plasmas: The effect of Coulomb collisions

We derive the collisional momentum and energy transport coefficients in Maxwellian plasmas with a general drift velocity with respect to the ambient magnetic field by using two approaches, the Fokker-Planck approximation and Boltzmann collision integral. We find the transport coefficients obtained from Fokker-Planck representation are similar to those obtained by using Boltzmann collision integral approach, and both results are presented in a closed form in terms of hypergeometric functions. This has been done for drifting Maxwellian plasmas with special emphasis on Coulomb collision, i.e. inverse-square force. Also, we calculate the transport coefficients for two special cases, firstly, when the drift velocity is parallel to the ambient magnetic field (i.e. u = u∥, and zero perpendicular drift velocity), and secondly, when the drift velocity is perpendicular to the ambient magnetic field (i.e. u = u⊥, and zero parallel drift velocity). It is worthy to mention that, up to our knowledge, none of the derived transport coefficients for the above mentioned case are presented in closed form and in terms of hypergeometric function.

Further research of the collisional instability. An extensive study of growth and a notable influence of the boundaries

AbstractIn a cylindrical geometry, a low-pressure magnetized argon plasma is lit, using the rf upper hybrid resonance. As a radial dc electric field is formed, a perpendicular drift velocity is obtained on the charged particles and a collisional instability is raised. The description, study and identification of that wave were given in the last published work from NCSR ‘Demokritos’, both experimentally and theoretically. Since then, the research has been continued and significant results have been obtained concerning the fields of plasma parameters, wave further understanding, growth research and boundaries' strong influence. The aim of the present paper is to inform that the rising instability has many of the characteristics of drift instabilities that are caused by the density gradient, as well as the ability of it's frequency to overcome the ion gyro-frequency. In addition, it gives an answer to the well-known problem of the need to consider a small component of the wave vector in the parallel direction, in order the wave to rise through the electron-neutral collisions' proceedings; a strong influence of cavity length on wave amplitude has been observed and measured.

16-Moment Approximation for Ion Velocity Distribution and its Application in Calculations of Incoherent Scatter Spectra

A sixteen moment approximation based on a bi-Maxwellian that contains the stress tensor and the heat flow vector is applied to describe the ion velocity distribution which influences the incoherent scatter spectra. A discussion is made about the effects on the incoherent scatter spectra caused by different values of the normalized perpendicular drift velocity D, aspect angle ϕ between the magnetic field and the line-of-sight direction, and the ratio α of the ion-neutral collision to ion cyclotron frequency. Numerical results show that the shifting and asymmetry of incoherent scatter spectra appear parallel to E × B and E as the normalized perpendicular drift velocity D increases due to the ion drift velocity, the stress tensor and the heat flow vector respectively. However, the spectrum is always typically double-humped Maxwellian parallel to B. The ion velocity distribution is more distorted from the Maxwellian as the aspect angle ϕ increases from 0° to 90°, and consequently the incoherent scatter spectra is no longer typically double-humped Maxwellian. As α increases, the ion velocity distribution becomes Maxwellian and the incoherent scatter spectra become typically double-humped Maxwellian even with a large value of the normalized perpendicular drift velocity D. It is reasonable to use the sixteen-moment approximation to describe the non-Maxwellian plasma characterized by the large temperature anisotropy.

The behavior of N/sub 2/ + SF/sub 6/ gas mixtures subjected to orthogonal electric and magnetic fields

The limiting equivalent electric fields in N/sub 2/+SF/sub 6/ binary gas mixtures due to Townsend discharges are evaluated directly from a Monte-Carlo simulation when the mixture is subjected to orthogonal electric and magnetic fields. Along with the limiting equivalent electric fields, transverse and perpendicular drift velocities, electron mean energies and collision frequencies are also determined within the scope of the Monte-Carlo simulation. The equivalent reduced electric field (EREF) concept for the corresponding limiting electric fields is also investigated for the calculated mean energy levels and collision frequencies. The EREF values are found to be in good agreement with the previously published limiting electric field data.

Transport coefficients for electrons in argon in crossed electric and magnetic rf fields

Monte Carlo simulations of electron transport have been performed in crossed electric and magnetic rf fields in argon. It was found that a magnetic field strongly affects electron transport, producing complex behaviour of the transport coefficients that cannot be predicted on the basis of dc field theory. In particular, it is important that a magnetic field, if it has sufficiently high amplitude, allows energy gain from the electric field only over a brief period of time, which leads to a pulse of directed motion and consequently to cyclotron oscillations being imprinted on the transport coefficients. Furthermore, this may lead to negative diffusion. The behaviour of drift velocities is also interesting, with a linear (sawtooth) dependence for the perpendicular drift velocity and bursts of drift for the longitudinal. Non-conservative effects are, on the other hand, reduced by the increasing magnetic field.

On the role of ion temperature anisotropy in the growth and propagation of shear‐modified ion‐acoustic waves

Oblique ion‐acoustic waves, excited by the combination of magnetic field‐aligned (parallel) electron drift and sheared parallel ion flow, are investigated in magnetized laboratory plasma that is characterized by ion temperature anisotropy. Direct measurements of the parallel and perpendicular ion temperatures, parallel and perpendicular ion drift velocities, electron temperature and parallel electron drift velocity, parallel and perpendicular wave vector components, and mode frequency and growth rate are used to elucidate the shear‐modified ion‐acoustic (SMIA) instability mechanism and document an observed correlation between ion temperature anisotropy and wave propagation angle. Experimental measurements show that anisotropy significantly influences the growth rate and propagation angle of oblique ion‐acoustic waves. These results support the ion‐acoustic wave interpretation of broadband waves in the auroral energization region where shear and anisotropy are known to exist and may have ramifications for many space plasmas in which anisotropy exists in the electron temperature or ion temperature.

Annual and semiannual variations of the midlatitude ionosphere under low solar activity

The annual and semiannual variations of the midlatitude ionosphere under low solar activity are studied using middle and upper (MU) radar (135°E, 35°N) incoherent scatter observations and Sheffield University plasmasphere‐ionosphere model (SUPIM). The variations of the daytime electron density (Ne) and electron and ion temperatures (Te and Ti) at 200–600 km altitudes measured by the radar under low solar activity (F10.7 ≤ 120) are satisfactorily reproduced for the first time by incorporating the radar measured values of the magnetic meridional neutral wind velocity (Uθ) and northward perpendicular plasma drift velocity (V⟂) into SUPIM that uses mass spectrometer incoherent scatter 1986 (MSIS‐86) for neutral densities and neutral temperatures. The study shows that the annual and semiannual variations of the midlatitude ionosphere during daytime at altitudes near and above the ionospheric peak under low solar activity depend more on the direct effect of the neutral wind arising through the changes in ionospheric height than on the indirect effect arising through the changes in thermospheric composition (or atomic to molecular concentration ratio); the indirect effect, however, predominates on the variations at altitudes below the ionospheric peak. The electron temperature (Te) undergoes similar but almost opposite seasonal and semiannual variations as the electron density. The ion temperature (Ti) is closer to neutral temperature than to electron temperature at altitudes up to ∼400 km and shows comparatively weak annual and semiannual variations.

Limiting equivalent electric field response in Ar+SF/sub 6/ subjected to orthogonal electric and magnetic fields

The limiting equivalent electric fields in Ar+SF/sub 6/ binary gas mixtures due to Townsend discharges are evaluated directly from a Monte-Carlo simulation when the mixture is subjected to orthogonal electric and magnetic fields. Along with the limiting equivalent electric fields, transverse and perpendicular drift velocities, electron mean energies and collision frequencies are also determined within the scope of the Monte-Carlo simulation. The equivalent reduced electric field (EREF) concept for the corresponding limiting electric fields is also investigated for the calculated mean energy levels and collision frequencies. The EREF values are found to be in good agreement with the previously published limiting electric field data.

Plasma flow in the outer region of the RFX reversed field pinch experiment

Langmuir probes and collector samples have been exposed to the edge plasma in the RFX reversed field pinch experiment to investigate the plasma flow. Parallel and perpendicular drifts have been observed for primary ions and carbon impurities. Two models valid for a magnetized and an unmagnetized plasma have been applied and compared in order to derive the parallel and perpendicular drift velocities. The perpendicular flow is accounted for by the radial electric field and shows a velocity shear, consistent with the change in sign of the radial electric field in the edge region. The parallel flow increases going into the plasma

Simulation of electron swarms in SF6 in uniform E*B fields

Reports electron swarm parameters in SF6 in uniform E*B fields employing a Monte Carlo simulation technique. The parameters studied are transverse and perpendicular drift velocities, magnetic deflection angle, collision frequency, mean energy and effective ionization coefficients for 30<or=E/p<or=200 V cm-1 Torr-1 and 0<or=B/p<or=6*10 T Torr-1. At a given E/p, application of a magnetic field causes decreases in the transverse drift velocity, the mean energy and the effective ionization coefficient. The validity of the effective reduced electric field concept is also investigated for the evaluated collision frequency, mean energy and effective ionization coefficient.

Measurement of the radial electric field in the ASDEX tokamak

Estimates of the radial electric field Er at the plasma periphery are obtained by measuring the drift velocities of low-Z impurity ions (He II, B IV, C III). The drift velocities are determined from the differential Doppler shift of visible line emission observed along opposite viewing directions. The principle of the measurement, including contributions from the diamagnetic drift, as well as radial gradients in the excitation rate and the effect of integrating along the line of sight are discussed in detail. The measured line of sight averaged drift velocities can be strongly influenced by the location and shape of the Er profile, especially if, as measured on other tokamaks, it is localized to a narrow region just within the separatrix. Values of Er estimated by assuming a constant radial profile underestimate maximum local values. During the H*-phase, however, high line of sight averaged perpendicular drift velocities of the B IV ions of a least 15 km/s in the electron diamagnetic drift direction are observed. From this, the presence of a strong negative radial electric field of at least 25 kV/m in the plasma edge region is inferred. Values of the B IV ion poloidal drift velocity calculated from an appropriate neoclassical theory are in the same direction as those measured. However, the calculated line of sight averaged values are much smaller than the measured ones. This reinforces the conclusion that a strong negative radial electric field is present just within the separatrix during the H-mode

Electron transport coefficients in mercury vapour in E*B fields

The influence of a magnetic field on electron transport coefficients in mercury vapour has been studied using a Monte-Carlo simulation technique. The ranges of parameters studied are 10<or=E/N<or=1000 Td and O<or=B/N<or=20*10-19T cm3 (where E is the electric field, B the magnetic field and N the gas number density). Data on ionization coefficients, transverse and perpendicular drift velocities and mean energy are evaluated at various magnetic field strengths. The results are examined in the light of the effective reduced electric field (EREF) concept. It is found that the EREF is a good approximation for the mean energy, collision frequency and ionization coefficient if the appropriate collision frequency which is dependent on E/N and B/N (instead of a constant collision frequency) is used in EREF calculations. As a result, the application of a magnetic field perpendicular to the electric field decreases the transverse drift velocity, mean energy and ionization coefficient. The influence of the magnetic field on the energy distribution function is also studied.

Theory of negative differential conductivity in a superlattice miniband.

Bloch equation conductivity perpendicular to the layers of a superlattice (period d) is evaluated using an extension of the balance-equation approach [X. L. Lei and C. S. Ting, Phys. Rev. B 32, 1112 (1985)] to narrow-band transport. The perpendicular peak drift velocity ${\mathit{v}}_{\mathit{p}}$ and the critical field ${\mathit{E}}_{\mathit{c}}$, at which the drift velocity peaks, are analyzed as functions of miniband width. Our theoretical prediction that ${\mathit{E}}_{\mathit{c}}$d increases with decreasing miniband width agrees well with the data of Sibille et al. [Phys. Rev. Lett. 64, 52 (1990)], even for the samples of narrowest miniband width in their experiment.

Equatorial fountain effect and dynamo drift signatures from AE‐E observations

Vertical and horizontal ion drift velocities for 1977–1979 from the Unified Abstract files of the satellite Atmosphere Explorer E were used in preliminary studies of the behavior of the low‐latitude (−20° to 20° dip latitude) F region. Sample diurnal variations obtained during 1978–1979 equinox time periods are very similar to those measured by the incoherent scatter radar at Jicamarca, Peru. Latitude profiles of the vector drift values clearly show the classic “fountain effect” responsible for the Appleton anomaly during the day. Investigation of longitude effects revealed that the average perpendicular (approximately vertical) drift velocity Vperp is independent of longitude, implying that the average east‐west electric field is proportional to B and thus is likely derived from classical dynamo winds.

Electron energy distributions and transport coefficients in N2inE × Bfields

Using the concept of energy-dependent effective field intensity, electron transport coefficients in nitrogen have been determined in E × B fields (E = electric field intensity, B = magnetic flux density) by the numerical solution of the Boltzmann transport equation for the energy distribution of electrons. It has been observed that as the value of B/p (p = gas pressure) is increased from zero, the perpendicular drift velocity increased linearly at first, reaches a maximum value, and then decreases with increasing B/p. In general, the electron mean energy is found to be a function of Eavet/p ( Eavet = averaged effective electric field intensity) only, but the other transport coefficients, such as transverse drift velocity, perpendicular drift velocity, and the Townsend ionization coefficient, are functions of both E/p and B/p.

Electric field induced drifts from the French Incoherent Scatter Facility

The Saint-Santin facility has been operating in its quadristatic configuration since October 1973. A set of 8 days and 7 nights (winter and equinox) of F region drift measurements perpendicular to the magnetic field is presented and provides information on the ionospheric electric fields. For quiet magnetic conditions the day-to-day variability is found to be weak, and the corresponding daily variations show the dominance of a 50 m/s diurnal oscillation in east-west drifts and of a 15–20 m/s terdiurnal oscillation in north-south drifts. None of the presently available models of quiet time electric fields have been found to reproduce this data over 24 hours. Comparison with other mid- and low-latitude electric field results shows a rather good agreement with incoherent scatter results south of Saint-Santin but a strong contrast with Millstone Hill results despite comparable geographic latitudes. For disturbed magnetic conditions, perpendicular drift velocities appear displaced from their quiet day values with a tendency to westward deviations. On some occasions those displacements reach large values of 100 m/s (4 mV/m electric field) and more over 1 or 2 hours. An interpretation in terms of penetration of the plasmasphere by electric fields associated with magnetospheric substorms is suggested.

Elerctron Transport Coefficients in N2 in Crossed Fields at Low E/p

The numerical solutions of Boltzmann transpott equation for the energy distribution of electrons moving in crossed fields in nitrogen have been obtained for 100 ? E/p ? 1000 V M-1 Torr-1 and for 0? B/p ? 0.02 Tesla Torr-1 using the concept of energy dependent effective field intensity. From the derived distribution functions the electron mean energy, the tranaverse and perpendicular drift velocities and the averaged effective field intensity (Eavef) which signifies the average field intensity experienced by electron swarms in E × B field have been derived. The maximum difference between the electron mean energy for a given E × B field and that corresponding to Eavef/p (p is the gas pressure) is found to be within ±3.5%.

Lower Hybrid Two Stream Instability in Quasi-Neutral Electron Beam

In a quasi-neutral electron beam of hollow cylindrical type the lower hybrid two stream instability is observed. By applying a radial electric field externally, the perpendicular drift velocity u ⊥ is controlled and the enhancement and suppression of the instability are made artificially. Further, the superposition of pulsed radial field shows that the unstable waves grow exponentially and the frequency and growth rate measured as functions of u ⊥ . These observed behaviors are interpreted by the linear theory of the lower hybrid two stream instability. Experimental results show that, after exponentially growing, the amplitude of the instability is saturated is saturated and the ions are heated.

Moderate and strong magnetic field electron drift velocities in nitrogen, oxygen and air†

Abstract By means of the equivalent reduced electric field concept, calculation of the electron transverse and perpendicular drift velocity and their ratio, tan θ, is extended to a moderate and a strong crossed magnetic field for nitrogen, oxygen and air, for average electron energies, from 0-1 to 10 ev. Data are presented on a constant electron energy rather than constant E/p basis. It is shown numerically that the prior assumed constant electron collision frequency values, derived from the various transport coefficients, inherently vary widely (by about a factor of 2(. Even for a given transport coefficient and constant electron energy, the collision frequency changes with applied magnetic field ( also by a factor of 2). The advantage of carrying out experiments at constant electron energy in the presence of a crossed magnetic field is discussed.