Electrodynamic Drift Research Articles

Modeling the low latitude ionospheric total electron content

The undisturbed ambient total electron content of the ionosphere in the equatorial region exhibits two characteristic features: 1. (i) a longitudinal behavior of the post-sunset variation of the ionization near the crests of the equatorial anomaly 2. (ii) an enhancement at lower latitudes following the post-sunset decay. During high solar activity periods the southern crest of the equatorial anomaly in the African longitude sector is characterized by a post-sunset maximum often exceeding the afternoon maximum. In the Indian and other longitude zones, the post-sunset peak is not so prominent. Instead, a ledge is obtained in the corresponding local time period. At lower magnetic latitudes, the ionization decays very rapidly around sunset, but an enhancement lasting 2–4 h is observed afterwards. Numerical solution of the plasma continuity equation, including the effects of ionization production by solar ultraviolet radiation, loss through charge exchange and transport by diffusion, electrodynamic drift and neutral wind, has been used to investigate the above two features. It is found that the pre-reversal peak of the E × B drift at the magnetic equator around sunset is the dominant mechanism responsible for the post-sunset behavior near the crests of the equatorial anomaly. The zonal wind causes an asymmetry of the total content in the northern and southern hemispheres. In African longitudes, where the magnetic declination is about 20°W, the southern crest is more developed at the expense of the northern counterpart. The north-south asymmetry is practically absent in the Asian sector, with its low (< 5°) declination angle. In the Pacific area, an easterly declination (about 9°E) results in a higher post-sunset ionization at the northern crest, although the asymmetry is less pronounced than that in the African zone. The night-time enhancement at lower latitudes has been found to be controlled by the post-sunset increase in the vertical drift, possibly also modulated by the neutral wind.

Equatorial F-region ionization differences between March and September, 1979

Ionospheric total electron content and the F-region maximum electron density at a number of stations in the equatorial region, during the recent solar activity maximum period 1979 to 1980, show significant differences between the two equinoctial periods. Ionization during the month of March is higher than in September, irrespective of the station location both in northern and southern hemispheres, and in different longitude sectors. The observed pattern is compared with those predicted by different models, in particular with one of the authors which includes processes such as ionization production, loss, electrodynamic drifts, winds and global composition changes involved in the equatorial ionosphere. It is found that a change in the neutral composition is primarily responsible for the observed F-region density differences between March and September.

Ion‐neutral momentum coupling near discrete high‐latitude ionospheric features

We have developed a two‐dimensional numerical model to study the momentum coupling between the ionosphere and neutral atmosphere in the vicinity of discrete high‐latitude features, such as convection channels and plasma density troughs. The model, which is based on generalized magnetohydrodynamic equations, takes account of global pressure gradients, viscous dissipation, ion drag, the Coriolis force, and electrodynamic drifts. From our initial steady state investigation, we have found the following: (1) in convection channels, significant shears and rotations of the thermospheric flow can occur below 200 km if a minimum in the electron density profile is present between the E and F regions; (2) in convection channels, the thermospheric wind decreases with height in the F region owing to the effects of horizontal viscosity; (3) at low altitudes, the boundaries of convection channels may produce Ekman spirals; (4) a convection channel acts to induce a thermospheric motion over a region that extends up to 1000 km on both sides of the convection channel owing to the effects of horizontal viscosity; (5) in general, thermospheric winds driven by large‐scale pressure gradients have an appreciable effect on the thermospheric flow in and near convection channels; (6) narrow channels of rapidly convecting, low density plasma produce a small thermospheric response; and (7) Joule heating is generally the dominant heat source inside convection channels, while viscous heating dominates outside. Frequently, however, viscous heating can be the dominant heat source both inside and outside of convection channels.

Ionospheric storm of 4–5 August 1972 in the Asia-Australia-Pacific sector

The ionospheric storm of 4–5 August 1972 is analyzed using data from 35 middle and low latitude ionospheric stations and 7 magnetic stations in the Asia-Australia-Pacific longitude sector. The largest ionospheric and geomagnetic disturbances were both observed in the mid-Pacific. Strong upward electrodynamic drifts occurred in the western Pacific during the initial phase of the storm. True-height profiles of electron density in the Pacific area show that the ionosphere was raised during the first several hours after the geomagnetic storm sudden commencement, that large variations later occurred in the mid-Pacific, possibly caused by wave-like thermospheric winds, and that the F-layer was reduced in density and raised in height during the main phase of the storm on the following day. Synoptic pictures of the variations in foF2 are presented, showing that electron density enhancements occurred during the storm initial phase in the western Pacific at middle latitudes, peaking in the 20°–45° geomagnetic latitude range, and becoming density deficits during the storm main phase. The observed variations are interpreted in terms of possible physical mechanisms.

Latitude dependence of ionosphere total electron content: Observations during sudden commencement storms

The latitude dependence of the changes in the ionosphere total electron content (TEC) has been studied during 12 magnetic storms. TEC observations were obtained at Hamilton, Massachusetts, and Arecibo, Puerto Rico. Definite latitude differences are observed in the TEC responses during the magnetic storms: both TEC enhancements and TEC depletions are observed at Hamilton, while only enhancements are measured at Arecibo. A pre-local midnight TEC ‘ledge’ is frequently observed in the Arecibo storm data but is seldom observed in the higher-latitude Hamilton data. These enhanced ledges in the Arecibo TEC, together with the nighttime TEC depletions measured at Hamilton, produce large south-to-north differences in the ionosphere TEC. The ledges at Arecibo may arise from electrodynamic drift effects associated with westward electric fields; the role of the motion term in the continuity equation for the TEC is assessed under these conditions. The local evening enhancements in the TEC during certain large storms are discussed in the context of the latitude dependence of the storm time magnetic field variations measured from Great Whale River to San Juan. It is shown that the large TEC enhancements appear to be associated with large positive geomagnetic bays at middle to high latitudes.

Ionospheric electron content and equivalent slab thickness in the equatorial region

The ionospheric total electron content and equivalent slab thickness in the equatorial region around 75°E have been studied. It is found that the slab thickness exhibits a strong latitudinal dependence with a maximum at the magnetic equator and a minimum around the crest of the F2 anomaly, indicating that the F region is thick near the equator and most skewed near the crest. The results are consistent with the electrodynamic drift and diffusion in the equatorial ionosphere.

The ionospheric dynamo and equatorial magnetic variations

World-wide magnetic variations observed at four universal times on a quiet day are simulated with coupled numerical models of the global ionospheric dynamo and of the equatorial electrojet. The global dynamo model uses a tilted dipole geomagnetic field with realistic conductivity distributions and height-varying tidal winds, and allows for current flow along field lines through the magnetosphere between northern and southern hemispheres. The simulations furnish information about upper atmospheric winds, electric fields and currents. Our results indicate that the winds driving the dynamo are functions not only of geographic latitude and local time, but also have a universal time dependence. In addition to the diurnal tidal wind component, the semidiurnal component appears to play a prominent role in generating currents and electric fields. The daily variation of the electric field and the consequent E × B electrodynamic drift at the magnetic equator show considerable longitudinal variability. Large vertical drifts around sunrise and sunset sometimes appear in the simulations, which are interpreted in terms of polarization charge accumulations in regions of sharp conductivity gradients. Geomagnetic-field-aligned current flow through the magnetosphere is significant, and is associated with part of the observed east-west ground magnetic variations at the magnetic equator. This field-aligned current is driven primarily by winds of the (2, 3) antisymmetric semidiurnal tidal mode.

Source and identification of heavy ions in the equatorialFlayer

Further evidence is presented to show that the interpretation of some Ogo 6 retarding potential analyzer (RPA) results in terms of ambient Fe+ ions is correct. The Fe+ ions are observed only within dip latitudes of ±30°, and the reason for this latitudinal specificity is discussed in terms of a low-altitude source region and F region diffusion and electrodynamic drift. It is shown that the polarization field associated with the equatorial electrojet will raise ions to 160 km out of a chemical source region below 100 km but it will do so only in a narrow region centered on the dip equator. Subsequent vertical E × B drift, coupled with motions along the magnetic fields, can move the ions to greater heights and greater latitudes. There should be a resultant fountain of metallic ions rising near the equator that subsequently descends back to the E and D layers at tropical latitudes. Even if the metallic ions do have access to the lower F region at higher latitudes, the downward plasma flow at the F peak will restrict their access to the upper F region.

Auroral infrasonic wave-generation mechanism

The morphology of auroral infrasonic wave (AIW) substorms, as determined from infrasonic observations at Inuvik (68.4°N), College (64.9°N), and Palmer (60.8°N) along a magnetic meridian through Alaska, shows that AIW are never observed propagating in a poleward direction, even though auroral activity frequently occurred south of the stations. AIW have been shown to be infrasonic bow waves generated by supersonic westward, equatorward, or eastward motions of auroral electrojet arcs. All AIW are observed from azimuths that vary from parallel to the auroral oval to transverse to the oval in an equatorward direction depending on the local magnetic time. Specific examples of auroral poleward expansions that cross the Inuvik zenith have been studied. It was found that even highly supersonic poleward motions of arcs with strong electrojets do not produce infrasonic bow wages. When a reversal in direction of the poleward expansion occurs and the arcs subsequently move equatorward, strong AIW are observed from the arcs. This asymmetry in the occurrence of AIW with respect to direction of motion of an arc is interpreted as an intrinsic asymmetry in the generation mechanism within the auroral arcs and not as a propagation effect. It is postulated that the basic acoustic pulse within the electrojet arcs is caused by collisions with the neutral gas of positive ions that are driven by electrodynamic drift in the E region of the auroral arc. Thus Lorentz force is the coupling mechanism between the electrojet current carriers and the neutral gas. An ionization-collision process occurs within the auroral arcs for those arcs that are moving supersonically in a direction parallel to the direction of the neutral ionization drift. The resulting increase in ion density for such arcs reduces the time constant for ion drag, and thus the Lorentz-force coupling is effective in producing an infrasonic shock wave. If the supersonic translation of the primary auroral electron sheet has a component of motion parallel to the electrodynamic drift of the positive ions, an auroral infrasonic shock wave will be produced in the E-region ionosphere and propagate to the ground as a modified shock or bow wave. If, on the other hand, the auroral-arc motion is antiparallel to the drift of the positive ions, no AIW will be produced.

Electrodynamic drift in the nocturnal F-region ionosphere caused by conjugate point sunrise

A rapid change in the electrical conductivity of the ionosphere occurs at the time of sunrise. When the conjugate point is in darkness, this change causes a change in the component of the dynamo electrostatic field normal to the sunrise line. This change in electrostatic field should cause a corresponding change in the vertical component of ion drift velocities in the F-region. The theory we develop predicts changes in ion drift velocities that are of the same order of magnitude, tens of meters per second, as the dynamo region winds. The effect may be detectable as an upward or downward movement of the nocturnal ionosphere at the time of conjugate point sunrise.

Lunar-solar tides in electron density at fixed heights of the ionosphere

Lunar tidal variations in the electron density N at fixed heights of the ionosphere for different solar hours have been computed at the equatorial station Huancayo, and the mid-latitude station Puerto Rico. It is found that the tidal amplitudes increase rapidly with altitude between 180 and 250km. At Huancayo the phase i.e. time of maximum positive deviation of lunar semi-diurnal variations in N is 07–08 lunar hr for the lower regions (below 180 km); it changes sharply at heights around 200 km, above which it is 03–04 lunar hr. The amplitude of the tide at heights around 200 km is independent of solar hr. At Puerto Rico, the phase is continuously retarded with increasing height; it is about 01 lunar hr in E-region heights and changes to about 09–10 lunar hr at heights near h max . It is seen that lunar oscillations in the E- and F 1-regions, at both the places, are in phase with corresponding magnetic field variations. Various observed results are explained as due to lunar variations in the magnetic field through the variations in the vertical electrodynamic drift velocities.

Electrodynamic drift effects in mid-latitude F-region storm phenomena

Calculations have been performed of changes in N m F 2 due to vertical drifts associated with electric fields in the storm time mid-latitude F-region. Vertical drifts depend on changes of electric field strength and vanish after a short time for steady fields in the daytime F-region because of motion set up in the neutral air. Various forms of electric field variation have been used. It is shown that electrodynamic drifts may be responsible for one type of reported phenomenon but not for the very large increases in N m F 2 which are sometimes observed. One result of interest is that that a sinusoidal electric field produces drifts which probably cause a nett increase in N m F 2. A ‘circuit analogue’ method has been developed to estimate the vertical drifts caused by electric fields in the F-region.

Distributions of Nighttime F‐Region Molecular Ion Concentrations and 6300‐Å Nightglow Morphology

A numerical technique is employed in solving the coupled, nonlinear system of equations for the O+, NO+, and O2+ number densities in the nighttime F region, including the effects of diffusion, E×B drift, and neutral air motions. The best fits to the ‘sunset’ [Johnson, 1967] and ‘midnight’ [Holmes et al., 1965] rocket observations of the ion profiles obtained at White Sands, New Mexico, are achieved by iteratively varying the neutral wind, the electrodynamic drift, the neutral atomic nitrogen and nitric oxide concentrations, and the ion‐atom interchange and dissociative recombination rate coefficients. The best fit results for both sets of profiles with a Jacchia model atmosphere are achieved with the rate coefficients (cm3 sec−1) at 300°K given by γN2 = 1.1×10−12, γO2 = 2.0×10−11, αNO+ = 3.5×10−7, and αO2+ = 3.1×10−7, each with an assumed inverse temperature dependence. In addition, the ‘sunset’ best fit solutions require that the chemical time constant τ for reaction of O2+ with N and NO be 500 seconds at 200 km and that the net vertical plasma transport velocity wz due to E×B drift and a neutral wind be downward with a speed of 14 m sec−1. The midnight best fit solutions are achieved with τ = 2.5×103 at 200 km and wz = 10 m sec−1 upward. The 6300‐Å nightglow morphology in the inter‐tropical region is computed directly from the ion concentrations calculated for assumed drift and neutral wind models representative of equinoctial, sunspot‐minimum conditions. Isophote contours of zenith intensity are presented to illustrate the nightglow morphology associated with the decay of the Appleton anomaly. Calculated results are compared with the observations of Barbier [1964] to demonstrate the influence of electrodynamic drift on the morphology of nightglow enhancements. The results show that the electrodynamic drifts required to produce the observed enhancements are consistent with the drifts observed at Jicamarca.

Anomalous nighttime increases in total electron content

A study of anomalous nighttime increases in total electron content at Hawaii from September 1964 to September 1968 shows that the typical increase lasts from 2 to 4 hr, has a mean size of 6 × 10 16electrons/m 2, and peaks during 2100–2200 HST and 0100–0200 HST. Its amplitude depends upon the preceding peak daytime content and the concurrent background content. The increases exhibit both seasonal and solar cycle dependences, but no correlation was found with magnetic activity. A source of ionization which is external to the column of content observed and which may be different from that which maintains the nighttime ionosphere is required to explain the data. Two possible sources are conjugate point exchange of ionization and ionization movements due to electrodynamic drifts. The ionization source has a westward velocity component. Its effects occasionally are highly localized since data at four locations spaced a few hundred kilometers apart exhibit different patterns. A pre-sunrise dip in content can be interpreted as the content returning to its former nighttime decay rate after the occurrence of an anomalous increase near sunrise.

Calculations for an Equatorial F2‐region Solar Eclipse

The effects of a total solar eclipse on the equatorial F2 layer are studied by obtaining transient solutions of the time‐dependent continuity equation. Electron concentrations for solar minimum conditions are calculated for two meridians that intersect the eclipse path, each at a different latitude. Inter‐hemispherical coupling of ionization during an eclipse does not appreciably affect the electron concentrations. After maximum solar obscuration, an upward moving ionospheric layer develops below the F2 peak (on ionograms it appears as a cusp between f0F1 and f0F2 and is known as the eclipse F1.5 layer), and it is explained in this paper in terms of electrodynamic drift. The assumed vertical component of drift is upward and approximately 14 m/sec at noon. The latitudinal extent of the eclipse F1.5 layer is approximately ±10° magnetic latitude. Qualitatively there is good agreement between the calculations and equatorial ionospheric observations. The technique used by ionospheric observers for deducing the daytime electron production rate q and the linear loss coefficient β from eclipse data is applied to the calculated profiles to judge its efficiency in recovering these parameters. At 300 km, q is approximately 100 cm−3 sec−1 and β is approximately 1×10−4 sec−1. If a drift of 14 m/sec is ignored, the calculated value of β at 300 km altitude is too large by approximately a factor of 3.

Combined world-wide neutral air wind and electrodynamic drift effects on the F2-layer

Three time-varying solutions of the world-wide continuity equation for electrons in the F2-region are obtained under equinoctial conditions of symmetry. The effects of production, loss, diffusion, electrodynamic ‘ E × B ’ drift and neutral air winds are taken into account. The three solutions correspond to various combinations of an ‘ E × B ’ drift of amplitude 20 m/sec −1 and a meridional neutral air wind of maximum magnitude 100 m/sec −1. The ‘ E × B’ drift chosen is simple harmonic, being upwards during the day and downwards at night. The meridional wind chosen is simple harmonic, being polewards during the period 4.5–16.5 hr LT and equatorwards for the remaining period; it varies with latitude, has a latitudinal maximum at ± 45° and vanishes both at the equator and at the poles. The results show that, at midlatitudes, wind effects alone can explain the observed diurnal variations in N m F2 and that at equatorial stations winds combined with ‘ E × B ’ drifts can explain the observed noon bite-out and the post-sunset anomalous enhancement of N m F2. The ‘ E × B ’ drift produces a realistic equatorial anomaly and is responsible for the noon bite-out observed at the dip equator. Neutral air winds may be responsible for the maintenance of the night-time equatorial F2-layer, for the considerable day-to-night change in h m F2 and for a night-time anomalous behaviour in h m F2 which is similar both to that observed in the winter hemisphere and to h' F2 observed during equinoctial months. Thus it is shown that neutral air winds combined with ‘ E × B ’ drifts produce realistic ionospheric models.

Auroral infrasonic waves

The morphology of auroral infrasonic waves at College, Alaska, is related to the temporal and spatial distributions of supersonic auroral motions that develop within the auroral oval during polar magnetic substorms. The break-up phase of the auroral substorm results in the rapid auroral motions that generate the observed infrasonic waves. Direct observations of infrasonic wave packets from overhead supersonic auroral forms, as well as the comparison of the morphologies of infrasonic waves and auroral motions are used to verify a shock wave model for the generation of auroral infrasonic waves. Electrodynamic drift and joule heating associated with the auroral electrojets in auroras that produce infrasonic shock waves are accepted as the basic processes that generate the initial acoustic pulse within a moving auroral form.

A theoretical technique for evaluating the time-dependent effects of general electrodynamic drifts in theF2 layer of the ionosphere

A technique for solving the full global diffusion equation for theF2 layer, including electro­dynamic ‘E x B’ drift, is presented. This involves considering the form of the diffusion equation in axes moving with the electrodynamic ‘E x B’ drift. In addition, it is shown how the region of integration, which involves complicated dipole geometry, may be mapped on to a semi-infinite rectangle. The method is illustrated by calculating world curves of electron density for a simple model atmosphere in which the electrodynamic ‘E x B’ drift and the electron production are time dependent and the electron loss coefficient is time indepen­dent. The calculations are compared with previous experimental results and confirm electrodynamic lifting as one of the major factors controlling the dip equatorialF2 layer and the associated trough of ionization as suggested by D. F. Martyn.

NighttimeF2layer at the equator

The behavior of the nighttime equatorial F2 layer under the influence of ambipolar diffusion and an electrodynamic drift has been studied theoretically. It is shown that there exists a maximum in the nighttime variation of the electron number density at constant height and that this maximum depends mainly on the nature of an electrodynamic drift. The effect due to ambipolar diffusion is comparatively small, and the rate of decay of ionization is affected to an appreciable degree by an increase in the magnitude of the drift. The results lead us to believe that, apart from the thermal contraction of the atmosphere, an electromagnetic drift may be an important factor in the formation of the maxima observed in electron profiles at a constant height.

Power series solution of the F2-layer diffusion equation

If the equatorial electron density profile in the region is known, the equilibrium electron density everywhere else can in theory be computed. A method is given for carrying out this process for regions within 30° of latitude of the dip equator for an exponential atmosphere of constant scale height and for symmetry about the dip equator. This is achieved by simplifying the diffusion equation for an isothermal atmosphere, then expanding in a power series, convergent at great height and whose coefficients are functions of latitude. The method is applied to a study of the distributions of electron density expected from an equatorial profile with a single maximum, similar to a Chapman profile. Attention is confined to the case of no production and loss, with an upward electrodynamic drift at right angles to the magnetic field lines and uniform at the dip equator. The results are of interest in connexion with the electron density at great height in the F2-layer, and illuminate the nature of the diffusion equation. The electrodynamic drift causes the latitudinal peaks in electron density at constant height to move in towards the dip equator, and a concentration of ionization at low heights closer to the dip equator. It is suggested by analogy with a straight field line model that in fact the electron density becomes infinite at infinitely low heights in this region of concentration, corresponding to a distribution of sources there. Thus an equilibrium layer, in absence of production and loss must be supported from below.