Altitude and latitude dependence of the equatorial electrojet

The effects of day-to-day or seasonal variation of altitude and latitude profiles of the Elayer plasma density in the equatorial ionosphere on equatorial electrojet (EEJ) structure are examined numerically using a self-consistent and high resolution dynamo model. It is found that variations in the E-layer peak altitude and amplitude and its gradient below significantly affect EEJ structure. For any realistic shape, the EEJ peak appears at or below the E-layer peak altitude. Distinct double peaks appear in the EEJ structure, such as revealed by rocket measurements, if the E-layer peak is above 105 km or the gradient is large, as when sporadic-E is present. The influence of the latitudinal variation of ionospheric field line integrated conductivities upon the amplitude and altitude of the EEJ peak is demonstrated.     

Electrodynamic effects of metal ions in the noon equatorial E-region

The observations of metal ions in significant concentrations and in layer formation in the lower E-region are briefly reviewed. It is expected that a metal ion layer may alter the electro-conductivity of the ionosphere and modify the distribution of ionospheric dynamo current. The variation in the electroconductivity is theoretically calculated, and it is shown that the Pedersen conductivity is reduced by less than 14%. In the equatorial ionosphere, field line integration of metal ion effects is likely to be large. A metal ion layer, similar to those observed, is numerically modelled in the equatorial region and its electrodynamic effects on the equatorial electrojet (EEJ) are numerically examined using our self-consistent model of the ionospheric dynamo. The effects are found to be significant on the amplitude of the EEJ, but not effectively large on its peak altitude.     

Geomagnetic field variation and the equivalent current system generated by an ionospheric dynamo at the solstice

The geomagnetic field variation and equivalent current system produced by an asymmetrical ionospheric dynamo action under a solstitial condition are simulated and compared with the observational results. Results of our simulation reproduce well most of the observational features of the solstitial Sq system. For example, the latitude of the current vortex center is higher in summer than in winter and the local time of the center in the summer hemisphere is located earlier than that in the winter hemisphere. In the morning and afternoon sector the current vortex in the summer hemisphere invades the winter hemisphere. The first feature is attributed to the ionospheric currents, but the second and third features are due to the field-aligned currents generated by the asymmetry of the ionospheric dynamo.     

Three dimensional ionospheric currents and field aligned currents generated by asymmetrical dynamo action in the ionosphere

Three dimensional ionospheric currents and field aligned currents generated by asymmetrical ionospheric dynamo are calculated self-consistently, using the assumption of infinite parallel conductivity. Tidal winds of (1, −2) mode, which are generally accepted as a main cause of Sq fields, are adopted as a wind model. Variation in universal time (UT) is examined by considering the discordance between conductivity and wind distribution, which are assumed to follow the geographic coordinate system, and geomagnetic dipole field. Observed UT variation of Sq current system is partly reproduced by our calculation. Calculation for solstice condition is performed by shifting conductivity distribution by 23.5° in latitude. Height integrated westward currents are much smaller in the winter hemisphere than in the summer hemisphere, though eastward currents are not so different in both hemispheres. This unbalance is compensated by the field aligned currents mainly from summer to winter hemisphere in the morning and vice versa in the afternoon. In both above asymmetric cases, structure of the equatorial electrojet is almost symmetric with respect to the equator. Total field aligned currents are rather large and comparable to currents in the ionosphere.     

Electric fields and currents in the equatorial electrojet deduced from VHF radar observations—III. Comparison of observed ΔH values with those estimated from measured electric fields

From VHF backscatter radar measurements at Thumba (dip: 56′S) of the phase velocities of type II irregularities in the equatorial electrojet (EEJ), electric field (E_y) values are estimated for different times of the day. Using the electric field values thus deduced and the Pedersen and Hall conductivities calculated using model values of electron densities and the collision frequencies of ions and electrons, the height integrated current intensity in the EEJ is estimated. The surface level geomagnetic field perturbation ΔH produced by this ionospheric current is then calculated. The calculated values of ΔH are compared with observed values of ΔH (after subtracting the magnetospheric contribution of D_st) for a number of days. The comparisons show good agreement between observed ΔH values and those calculated from measured electric fields. The agreement is found to be good even when type I irregularities are present at higher altitudes in the EEJ. This comparative study demonstrates the validity of estimating electric field values from VHF radar measurements and it indicates the possibility of deducing electric field values from ground level ΔH values, at least for statistical studies.     

Model calculation and analysis of magnetosphere-ionosphere-thermosphere coupling

Model calculations are used to analyse the function of the magnetosphere-ionosphere electrodynamic coupling and that of the ionospheric tide-dynamo. The phenomena concerned are discussed.It is shown that for geomagnetic and ionospheric research in low-latitude and equatorial regions, the penetration and shielding of magnetospheric disturbances should be taken into account. On the other hand, the effect of the tidal dynamo on the ionosphere of the auroral region should not be ignored, especially on geomagnetically disturbed days when the conductivity is very high. The features and the relative importance of these two effects are different at different stages of a magnetic storm, and depend on latitude. There is an evident dawn-dusk asymmetry in these effects. The influence of weak geomagnetic disturbances on the low-latitude ionosphere should also be taken into account.     

Time variation of instantaneous equivalent Sq current system

We have examined the time variation of the instantaneous equivalent S_q current system during 1–18 March 1980, by removing the averaged UT variation caused by the difference between geographic and geomagnetic coordinate systems. The additional current system thus obtained shows several typical patterns which explain the variation field. Some of them may be explained by the IMF polarity effect or by the geomagnetic disturbance field of polar origin, but others appear to be caused by the variation of the ionospheric dynamo. The latter lasts for several hours or sometimes for over ten hours and may be explained by the variation of diurnal or semi-diurnal tidal winds.     

The F-region dynamo

This paper reviews the theory of the F-region dynamo which drives about 10–15% of the total mid-latitude ionospheric current by day, and the major part at night (Section 2). Polarization fields associated with the dynamo cause marked effects in the night-time F-region, notably the mean eastward wind (Section 3). The paper also discusses the equipotentiality of geomagnetic field lines (Section 4 and Appendix) and the question of location of Sq and L current systems (Section 5).     

Low latitude electrodynamic plasma drifts : a review

Radar and radio measurements have provided detailed information on the dependence of F-region electrodynamic drifts on height, season, solar cycle and magnetic activity. Recently, satellite ion drift and electric field probes have determined the variation of low latitude ionospheric drifts over a large range of altitudes and latitudes. The general characteristics of the quiet time plasma can be explained as resulting from E- and F-region dynamo and interhemispheric coupling processes. The low latitude and equatorial zonal and upward/poleward components of the plasma drift respond differently to geomagnetic activity. Disturbance dynamo effects are responsible for the drift perturbations following periods of enhanced magnetic activity. The prompt penetration of high latitude electric fields to lower latitudes produces large perturbations on the upward/poleward drifts, but has no significant effect on the low latitude and equatorial zonal drifts. A number of processes such as ‘overshielding’, ‘fossil wind’ and magnetic reconfiguration were suggested as being responsible for the direct penetration of high latitude electric fields to lower latitudes. Detailed low latitude and global numerical models were used to study the characteristics of low latitude and equatorial plasma drifts and their response to changes in the polar cap potential drop or in the high latitude field-aligned currents. These models can reproduce the latitudinal variation of the perturbation electric fields and their diurnal variations, but are still unable to account for several aspects of the experimental data as a result of the complexity of the high latitude and magnetospheric processes involved.     

On the global atmospheric electrical circuit

A model of global atmospheric electric circuit has been developed, which incorporates the pollution due to aerosol particles of anthropogenic and volcanic origins and the ionization caused by the coronal discharges due to intense electric field beneath thunderclouds in the atmospheric layers close to the earth's surface. Also the effects of solar activity and of Stratospheric Aerosol Particles (SAP) on the parameters of the circuit have been studied. The global distribution of Aerosol Particles (AP) of man-made origin has been assumed on the basis of world population density and that of volcanic origin on the basis of global distribution of volcanic activities. The thunderstorms are assumed to be the current generators of the global circuit. The latitudinal, longitudinal and height variations of atmospheric conductivity have been calculated from the known variations of ionization caused by cosmic rays, the radio-active emanations and intense electric fields beneath thunderclouds (over continents), along with the assumed variation of AP concentration.On increasing the AP concentration over the northern hemisphere by about 5 times that of the southern hemisphere, the ionospheric potential increases to about 6% and electric field increases in non-mountainous regions more than that in the high mountain regions. A large increase in SAP serves to increase the global resistance while both global current and ionospheric potential decrease. However, for low SAP concentration, the ionospheric potential increases. The SAP affect the electrical structure of the stratosphere without much influencing the troposphere except in the volcanically active regions, where conductivity is low due to high AP concentration. The major influence on the global atmospheric electric circuit occurs in response to solar activity. In the absence of local effects, the calculated variation of ionospheric potential is 14%. However, in real atmosphere (where both local and global processes act simultaneously), the ionospheric potential is found to vary by only about 3%. This has little effect on the ground electrical properties where more than 30% of the variations have been found to be caused by the local effect.     

Characteristics of ionospheric ELF radiation generated by HF heating

This paper describes new observations of the characteristics of ELF generation produced by modulation of the dynamo current system from HF heating of the ionospheric D-region. A model of the ELF antenna structure embedded in the D-region is described and stepped ELF frequency observations are shown to support the model assumptions. Presented are data on the phase height of the ELF ionospheric antenna versus ELF frequency, polarization of the downgoing wave and relationship to the dynamo current direction, correlation of ELF field strength with per cent cross-modulation, power linearity tests and duty cycle results. All observations used the high power heater facility of the Arecibo Observatory.     

The vernal-autumnal asymmetry in the seasonal variation of geomagnetic activity

In intervals in which the polarity of the main solar dipole field is stabilized, a 12 month wave occurs in geomagnetic activity (indices aa, Ap, Dst) with its maximum in one of the equinoctial periods. Whether the vernal or the autumnal maximum is greater depends on the polarity of the main solar dipole; the existence of the wave may be explained by the north-south asymmetry in the main solar dipole field. The results favour the southward component of the interplanetary magnetic field as the decisive factor for geomagnetic activity.     

Annual and semiannual variations of the geomagnetic field at equatorial locations

For a year of quiet solar-activity level, geomagnetic records from American hemisphere observatories located between about 0° and 30° north geomagnetic latitude were used to compare the annual and semiannual variations of the geomagnetic field associated with three separate contributions: (a) the quiet-day midnight level, MDT; (b) the solar-quiet daily variation, Sq; (c) the quiet-time lunar semidiurnal tidal variation, L(12). Four Fourier spectral constituents (24, 12, 8, 6 h periods) of Sq were individually treated. All three orthogonal elements (H, D and Z) were included in the study.The MDT changes show a dominant semiannual variation having a range of about 7 gammas in H and a dominant annual variation in Z having a range of over 8 gammas. These changes seem to be a seasonal response to the nightside distortions by magnetospheric currents. There is a slow decrease in MDT amplitudes with increasing latitude.The Sq changes follow the patterns expected from an equatorial ionospheric dynamo electrojet current system. The dominant seasonal variations occur in H having a range of over 21 gammas for the 24 h period and over 12 gammas for the 12 h period spectral components. The higher-order components are relatively smaller in size. The Sq(H) amplitudes decrease rapidly with increasing latitude. Magnetospheric contributions to the equatorial Sq must be less than a few per cent of the observed magnitude.The L(12) variation shows the ionospheric electrojet features by the dominance of H and the rapid decrease in amplitude with latitude away from the equator. However, the seasonal variation range of over 7 gammas has a maximum in early February and minimum in late June that is not presently explainable by the known ionospheric conductivity and tidal behavior.     

Production of electromagnetic field disturbances due to the interaction between acoustic gravity waves and the ionospheric plasma

The problem of electromagnetic field disturbances produced by the interaction between winds of acoustic gravity waves (AGW) origin and the ionospheric plasma has been considered. It is shown that, when not allowing the electrostatic approach, electromagnetic field disturbances represent shear Alfvén and compressional modes modified by ionospheric Pedersen and Hall conductivities. It is further shown that the quasielectrostatic Alfvén type disturbances give the main contribution to electric field perturbations. Magnetic field perturbations due to Alfvén and compressional modes have the same order of magnitude. Two numerical models for simulation of the problem under consideration have been developed. The first model is intended for the simulation of Alfvén type disturbance production and transmission into the magnetosphere, taking into account the dipole geometry of the geomagnetic field, but a mutual transformation of Alfvén and compressional modes is ignored. The second model is constructed for the simulation of both electromagnetic field disturbance production and their mutual transformation in the ionosphere. The results of numerical simulations with these models show that there is an opportunity for AGW activity monitoring in the lower thermosphere by ground-and satellite-based recordings of magnetic and electric field variations.     

Modeling equatorial ionospheric electric fields

This paper gives a brief overview of the processes responsible for the equatorial electric field, and reviews relevant modeling work of these processes, with emphases on basic aspects and recent progress. Modeling studies have been able to explain most of the observed features of equatorial electric fields, although some uncertainties remain. The strong anisotropy of the conductivity and the presence of an east-west electric field lead to a strong vertical polarization electric field in the lower ionosphere at the magnetic equator, whose magnitude can be limited by plasma irregularities. Local winds influence the structure of the equatorial polarization field in both the E and F regions. The evening pre-reversal enhancement of the eastward electric field has been modeled by considering a combination of effects due to the presence of a strong eastward wind in the F region and to east-west gradients of the conductivity, current, and wind. Models of coupled thermosphere-ionosphere dynamics and electrodynamics have demonstrated the importance of mutual-coupling effects. The low-latitude east-west electric field arises mainly from the global ionospheric wind dynamo and from the magnetospheric dynamo, but models of these dynamos and of their coupling have not yet attained accurate predictive capability.     

Plasmaspheric zonal electric fields and coupling fluxes from the Dunedin VLF Doppler experiment,for 180°E,L ≈ 2.3, at solstice and equinox

Group delays and Doppler shifts from ducted whistler-mode signals are measured using the VLF Doppler experiment at Dunedin, New Zealand (45.8°S, 170.5°E). Equatorial zonal electric field and plasmasphere-ionosphere coupling fluxes are determined for L ≈ 2.3 at June solstice and equinox during magnetically quiet periods. The general features of the electric field measured at Dunedin agree with those predicted from ionospheric dynamo theory with a (1,−2) tidal component. Some seasonal variations are observed, with the electric field measured during equinox being smaller and predominantly westward during the night. The electric field at June solstice is also westward during the evening and for part of the night, but turns sharply eastward during the pre-dawn and dawn period at the duct entry site. The June electric field appears to follow a diurnal variation whereas the equinox electric field shows a possible 4-hourly periodic variation. Seasonal variations in the neutral wind pattern, altering the configuration of the ionospheric dynamo field, are the probable cause of the seasonal differences in the electric field. The seasonal variation of the coupling fluxes can be explained by the alteration of the E x B drift pattern, caused by the changes in the electric field.     

Electric fields and currents in the equatorial electrojet deduced from VHF radar observations—I. A method of estimating electric fields

Using the measured Doppler spectra of the VHF backscatter radar signals from type II ionization irregularities in the equatorial electrojet (EEJ) at Thumba (dip. 56′S), the height profiles of the phase velocity V_p of the plasma waves in the EEJ are determined. It is shown that the east-west electrostatic field E_y in the EEJ can be deduced from the experimental height profiles of V_p using an appropriate model of ion and electron collision frequencies. The theoretical basis and the practical application of the method for deducing E_y are described. The usefulness of the method even when type I irregularities are present at the higher altitudes of the EEJ is demonstrated.It is shown that the collision frequencies of ions and electrons are likely to have a significant diurnal variation, which may be caused by diurnal variations of the neutral densities and temperatures in the E-region.     

A new role for the plasmaphere in “quiet” ionospheric electrodynamics

Global scale longitudinal gradients of pressure in the plasmasphere may be formed naturally by ionospheric processes, or caused by electrostatic fields of ionospheric dynamo origin. It is shown that plasmaspheric gradients of pressure, orthogonal both to the magnetic field (B) and to grad B, generate geophysically significant field-aligned currents. Considering the ionosphere and plasmasphere as a coupled electrodynamic system, these currents alter non-negligibly the self-consistent ionospheric electric field and current. Criteria are established for this coupling mechanism (a kind of plasmaspheric impedance) to be significant. This has implications for the relationships of ionospheric electric fields and currents, F-region drifts, and magnetic variations, due to upper atmosphere tides and winds.     

Electric field sources in the quiet plasmasphere from whistler observations

Existing evidence for the ionospheric dynamo being the source of quiet time electric fields in the plasmasphere is reviewed. Part of a 24 h set of whistler data recorded continuously at Sanae, Antarctica (L = 4), during quiet magnetic (average K_p = 1) is analysed to obtain westward electric fields in the equatorial plane. These electric fields are examined as a function of L-value in order to infer their source. It is found that for periods of outward flow of plasma during the noon-midnight local time period, the electric fields are consistent with the dominant source being the ionospheric dynamo. There is some evidence that during the evening period of inward flow the electric fields are magnetospheric in origin, although this could also be consistent with a refined dynamo model. The observed whistler duct convection patterns do not fit either of two theoretical models, which invoke a magnetospheric field but not a dynamo field.     

Geomagnetically quiet day ionospheric currents over the Indian sector—II. Equatorial electrojet currents

With 1986 quiet days data of Indian observatories, the equatorial electrojet (EEJ) has been studied in terms of 8 landmark parameters that reveal the structures of hourly latitudinal profiles of EEJ current from 0700 to 1700 h local time. The landmark distances suggest that near dawn, the EEJ is widest and its centre and focus are most northerly before it contracts towards local noon with its centre and focus moving southwards. The seasonal means of peak current intensity and the total forward current seem to peak earlier when the intensity of EEJ is higher than when it is lower. The seasonal order of EEJ intensity is found not to be the same at all hours of the day. This implies that seasonal variation of EEJ is not semiannual at certain daytime hours. The landmark distances of EEJ current have semi-annual variations with minima in the vernal and autumnal equinoxes and maxima at June and December solstices, but annual variations of the measures of EEJ current are just the reverse. Certain properties of the worldwide part of Sq are found to be markedly different from those of EEJ including some key features of diurnal and seasonal variations.