Responses of atmospheric electric field and air-earth current to variations of conductivity profiles

A global circuit model is constructed to study responses of air-earth current and electric field to a variation of atmospheric electrical conductivity profile. The model includes the orography and the global distribution of thunderstorm generators. The conductivity varies with latitude and exponentially with altitude. The thunderstorm cloud is assumed to be a current generator with a positive source at the top and a negative one at the bottom. The UT diurnal variations of the global current and the ionospheric potential are evaluated considering the local-time dependence of thunderstorm activity. The global distributions of the electric field and the air-earth current are affected by the orography and latitudinal effects. Assuming a variation of conductivity profile, responses of atmospheric electrical parameters are investigated. The non-uniform decrement of the conductivity with altitude increases both the electric field and the air-earth current. The result suggests a possibility that the increment of the electric field and the air-earth current after a solar flare may be caused by this scheme, due to Forbush decrease.     

Geomagnetic field variations at low latitudes and ionospheric electric fields

The solar cycle, seasonal and daily variations of the geomagnetic H field at an equatorial station, Kodaikanal, and at a tropical latitude station, Alibag, are compared with corresponding variations of the E-region ionization densities. The solar cycle variation of the daily range of H at either of the stations is shown to be primarily contributed to by the corresponding variation of the electron density in the E-region of the ionosphere. The seasonal variation of the ΔH at equatorial stations, with maxima during equinoxes, is attributed primarily to the corresponding variation of the index of horizontal electric field in the E-region. The solar daily variation of ΔH at the equatorial station is attributed to the combined effects of the electron density with the maximum very close to noon and the index of electric field with the maximum around 1030 LT, the resulting current being maximum at about 1110 LT. These results are consistent with the ionosphere E-region electron horizontal velocity measurements at the equatorial electrojet station, Thumba in India.     

Diurnal variations in the D-region during a storm after-effect

Measurements of electron concentration in the D- and lower E-regions of the ionosphere are reported for seven rocket flights from South Uist, Scotland, in April 1973. They took place during a 12-hour period starting 30 hours after the main phase of a severe geomagnetic storm. The principal feature of the results is that the electron concentrations below 85 km varied between 4 and 10 times the concentrations found on normal days. The variability was correlated with changes in radio wave absorption.The observed electron concentrations are compared with results of other workers for storm conditions and in particular with the model of Spjeldvik and Thorne (1975), and are found to be in reasonable agreement.The possibility of changes in positive ion composition occurring during the post-storm period is inferred.     

Diurnal variations in F2 peak density and ionization flux above Tahiti

The effects of composition and ionization fluxes on the diurnal variation of NmF2 at an equatorial anomaly zone station (Tahiti) are separated. The calculated diurnal variation of the fluxes agrees well with what would be expected from published equatorial E × B drift observations and global neutral wind models. A correlation analysis shows that lower hmF2 is often accompanied by larger NmF2, in spite of a much larger recombination rate. This illustrates the dominance of the fountain effect and neutral wind induced interhemispheric transport at this station.     

Diurnal variations in the flux of ionisation above the F2 peak in the northern and southern hemispheres

The flux of ionisation at 850 km height is calculated using the MSIS atmospheric model, a simplified form for the continuity equation at the peak of the F2-layer, and observed values of NmF2. Results are given for stations at latitudes of 32°N, 21°N, 21°S and 37°S during 1971 and for Tahiti (18°S) in 1980. Changes in the neutral atmosphere and in the hmF2 model have minor effects at low latitudes, where the fluxes are larger, but can appreciably alter the results at mid latitudes. Increased recombination due to N₂ vibrational excitation produces a large afternoon decrease in NmF2 in summer, near solar maximum, and an increased downward flux. At all stations the day-time flux has a much larger downward component in winter than in summer. Because of the eastward magnetic declination, zonal winds produce opposite effects on the diurnal variations of hmF2, NmF2 and flux in the northern and southern hemispheres. Downward fluxes are largest in the morning in the southern hemisphere and in the late afternoon and evening in the north. At ± 21° latitude, neutral winds have a major effect on the distribution of ionisation from the equatorial fountain. Thus, at the solstices the day-time flow is about 4 times larger in winter than in summer. Averaged over both hemispheres, the total flow at 21° latitude is approximately the same for solstice and equinox conditions. At mid latitudes there is a downwards flux of about 1–2 × 10¹² m² s⁻¹ into the night ionosphere.     

On the effect of electron precipitation on the fair-weather electric field

In this short paper we have estimated the influence of the diurnal modulation of the electron precipitation at low and middle latitudes of the South Atlantic Magnetic Anomaly (SAMA) on the fair-weather electric field. We have used simple exponential atmospheric conductivity models, together with the ion production rates determined from balloon and rocket measurements in the SAMA. An upper limit to this influence was also calculated and compared with the normal diurnal variation of the fair-weather electric field due to the diurnal variation of the global thunderstorm activity.     

Effect of the electric field on the equatorial ionospheric plasma distribution

It is known that on a counter electrojet day the noontime electron density at the equator shows enhanced values with no bite-out. The consequences of the absence of the normal equatorial electrojet on the electron density distribution at the equatorial station Kodaikanal (dip latitude 1.4°N, long. 77.5°E) and at an anomaly crest location Ahmedabad (dip latitude 18°N, long. 73°E) are discussed for a strong electrojet (SEJ) day and a counter electrojet (CEJ) day. The electron density distribution with height for a pair of SEJ and CEJ days at the two equatorial stations Kodaikanal and Huancayo (dip latitude 1°N, long. 75°W) are studied. The F-region peak height, hm and the semi-thickness parameter ym on the SEJ day followed a similar variation pattern. On the CEJ days ym exhibited a substantially low and mostly flattened daytime variation compared to the peaked values on the SEJ day. An attempt is made to interpret these differences in terms of the changes in the vertical drift pattern resulting from the E × B drift of plasma at the equator and the varying recombination rate β, which is also a height dependent and a local time dependent parameter.     

Balloon observations of stratospheric electricity above the South Pole: vertical electric field,conductivity, and conduction current

This paper summarizes the results of measurements of the electrical conductivity σ and vertical component of the vector electric field E_z acquired from eight stratospheric balloon flights launched from Amundsen-Scott Station, South Pole, in the austral summer of 1985–1986. The major findings of this research are as follows

• 1.(1) The data contribute to the set of global atmospheric electricity measurements and extend the work of COBB [(1977), Atmospheric electric measurements at the South Pole. In Electrical Processes in Atmospheres, Dolezalek H. and Reiter R. (eds), pp. 161–167. Steinkopf, Darmstadt, F.R.G.] to determine the electrical environment of the south polar region
• 2.(2) The average vertical profile of the conductivity at the South Pole, when compared with profiles obtained at other Antarctic locations, suggests that the conductivity scale height may increase with increasing geomagnetic latitude across the polar cap.
• 3.(3) The conductivity profiles measured at the South Pole and other Antarctic locations differ significantly from polar cap model profiles. On the basis of these measurements, the model profiles appear to require modification
• 4.(4) The magnitudes of the E_z profiles were observed to vary from day-to-day by a factor of > 2
• 5.(5) In all of the flights the air-Earth conduction current J_z, calculated as the product of E_z and σ, decreased with altitude in agreement with previous direct measurements of the air-Earth current by Cobb [( 1977), Atmospheric electric measurements at the South Pole. In Electrical Processes in Atmospheres, Dolezalek H. and Reiter R. (eds), pp. 161–167. Steinkopf, Darmstadt, F.R.G.]
• 6.(6) The magnitude of J_z was 2–3 times larger than the global average, which can be attributed to the lower columnar resistance of the atmosphere above the high-elevation Antarctic plateau. The magnitude of J_z agrees with that observed by Cobb, if the Cobb measurements are multiplied by the Few and Weinheimer [(1986), Factor of 2 error in balloon-borne atmospheric conduction current measurements. J. geophys. Res.91, 10937] correction factor of 2
• 7.(7) E_z from all of the flights during times of balloon float demonstrates characteristics of the classical ‘Carnegie’ diurnal variation, which is indicative of global influences on the ionospheric potential
• 8.(8) The influence of geomagnetic activity was observed as a decrease in the amplitude of the diurnal variation of E_z with increasing geomagnetic activity index K_p, which is the predicted effect at the South Pole of the magnetospheric polar-cap potential superimposed on the ‘Carnegie’ potential variation.

     

Fair weather electric field in Port Moresby

Measurement of the fair weather electric field at Port Moresby during the dry season in July–August 1988 shows no clear effect of the world-wide thunderstorm activity predicted by Mauchlyy S. J. (1923, J. geophys. Res. 28, 61). The observed field is generally low—approximately, 60V/m—during daytime but remains relatively higher—about 150 V/m—at night. The field strength increases slowly after sunset but decreases sharply after sunrise. The levels observed are typical of isolated sites and the frequent temperature inversion observed during the period is suggested as the likely explanation of the observed behaviour of the field.     

Structure in the seasonal variations of ionospheric Es occurrence in the South Pacific

A survey is presented of the occurrence of strong ionospheric sporadic-E for South Pacific ionosonde stations covering a period of many years. The seasonal characteristics indicate the presence of strong non-solar effects with, for example, subtropical data showing strong afternoon enhancements of E_s activity in the autumn.     

Analytical modelling of the electric field distribution in the high-latitude ionosphere

An analytical approach is implemented for self-consistent modelling of the high-latitude convection electric field. Input parameters are determined as distributions of field-aligned currents and height-integrated conductivity. The high-latitude ionosphere is approximated with an arbitrary number (N) of concentric rings. The height-integrated conductivity (∑) is independent of co-latitude within any ring, but depends on the longitude ~ sin λ. The field-aligned currents flow only along the boundaries of each ring and are presented by Fourier series in longitude. The analytical solution for the potential φ as a function of longitude is also presented as a Fourier series. An analytical solution is obtained for the potential dependencies on co-latitude. For the extreme case, when the integrated conductivity does not depend on longitude, this solution coincides with the analytical results, obtained by other authors. Based on this solution, the potential distribution in the high-latitude ionosphere, an example with N = 5 is shown, the values of conductivity and field-aligned currents being similar to those values used by other authors.     

Non-neutral field-aligned current sheet and the auroral electric field

Equilibrium configurations of the non-neutral field-aligned electron current sheet in a Maxwellian plasma are obtained from solutions of the time-independent Vlasov-Maxwell equations. This is the first field-aligned non-neutral current sheet model in a hot plasma in which the electrons are allowed to be nonadiabatic. The current in this model has a perpendicular (i.e. diamagnetic) as well as a parallel component to the external magnetic field. The electric field of the current sheet is pointing normally toward the midplane of the sheet. This zeroth-order perpendicular electric field is identified as the primary electric field which gives rise to a few keV potential drop along auroral field lines.     

Horizontal transmission of the polar electric field to the equator

A mode which makes possible the instantaneous transmission of the polar ionospheric electric field to the equator is obtained. The problem is solved analytically as an initial-boundary value problem by assuming a plane Earth-ionosphere waveguide system composed of the metallic ionosphere with vertical static magnetic field and the perfectly conducting Earth, The TM₀ (zeroth-order transverse magnetic) waveguide mode excited by the polar electric field propagates instantaneously to the low-latitude accompanying the electric field with the same direction as the polar field. The simultaneous occurrence of the PRI (preliminary reverse impulse) of the SC1 and the DP-2 geomagnetic variations in the high-latitude and equatorial regions are interpreted in terms of the polar originated electric field associated with the TM₀ mode. The geometrical attenuation of the transmitted electric field due to the finite scale of the polar field causes the disappearance of the PRI at low-latitudes, but the PRI appears again in the dayside equatorial region because of the enhanced ionospheric conductivity. The twin current vortices for the high-latitude PRI are interpreted as the Hall current caused by the polar electric field with finite scale. It is suggested that the direct current flows between the magnetosphere and the equatorial ionosphere via the polar ionosphere, when a large scale electric field is generated in the magnetosphere. From the mid-latitude ionospheric part of this current, the electric field may be mapped upward along the magnetic lines of force into the low L-value hydromagnetic region and as a result the penetration of the magnetospheric electric field into the plasmasphere is achieved by way of the Earth-ionosphere waveguide.     

Midlatitude measurements of the ionospheric electric field during the ALADDIN programme

Three measurements of ionospheric electric field were made during the 24 h ALADDIN rocket programme at Wallops Island (37°50′N, 75°29′W) on June 29–30, 1974. The first of these used a double probe instrument, flown at 1500 Local Solar Time, and the second and third measurements were made by barium cloud releases at evening and morning twilight. These three electric field vectors have been compared with the predictions of a number of models of electric field due to the dynamo effects of various atmospheric tides, and also of a possible magnetospheric origin. On the assumption that the measurements were made at a location equatorward of the afternoon convergence and poleward of the morning divergence in the electric field patterns related to the Sq current cystem, Stening's model of the diurnal variation of the electric field induced by the (1, −2) tidal model at the time of the Summer solstice correctly predicts the directions of the observed electric field. Forbes and Lindzen's model, incorporating the three major propagating tidal modes as well as the evanescent (1, −2) mode, also bears an acceptable relationship to the ALADDIN electric field directions. The ALADDIN E-field magnitudes are comparable with those obtained by ground-based observations (incoherent scatter) from Millstone Hill and from Saint Santin but are about half of Stening's model values, and three times those of Forbes and Lindzen.While the Millstone Hill E-field directions are compatible with the ALADDIN observations, Saint Santin E-field directions, at the same latitude but 75° difference in longitude, are distinctly different from ALADDIN, implying that longitudinal differences are significant.     

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.     

Middle atmospheric electrodynamic modification by particle precipitation at the South Atlantic Magnetic Anomaly

Evidence for a localized middle atmospheric electrodynamic modification at low latitudes (southern Brazilian coast) of the South Atlantic Magnetic Anomaly (SAMA), in association with enhanced geomagnetic activity, are presented in a unified way combining recent observational efforts and related numerical studies. They involve a distortion effect in the fair weather electric field at balloon altitudes. This effect is attributed to a local intensification of energetic electron precipitation through a related middle atmospheric ionization enhancement and is elucidated by numeric simulation. From the electric field measurements and the numeric simulation, the intensification of precipitation is considered to occur in fairly narrow regions at the observed low L values (around L = 1.13) of the SAMA, with horizontal extensions of the order of a few hundred kilometers. A physical mechanism that could be responsible for this sort of intensification is suggested. Furthermore, a comparison of the phenomenon of middle atmospheric electrodynamic modification at the SAMA with a similar one at auroral latitudes, in response to enhanced solar and geomagnetic activity, is also given.     

Retrieval of east-west wind in the equatorial electrojet from the local wind-generated electric field

The paper presents a method to retrieve the height-varying east-west wind U(z) in the equatorial electrojet from the local wind generated electric field E_W(z), or from the radar-measured phase velocity V_p^II(z) of the type II plasma waves. The method is found to be satisfactory when E_w ⪢ E_p, where E_p is the vertical polarization electric field generated by the global scale east-west electric field, EY, and E_y < 0.2 mV m⁻¹. Measurements of V_p^II by a VHF backscatter radar can be inverted to obtain the causative wind profile by this method. The method is tested using a simulation study in which E_w(z) and V_p^II(z) as generated by two different wind models are used. The retrieved winds are compared with the original wind profiles and it is found that the error in the retrieved winds is mostly under 5%, for the case of no errors in the model Pedersen conductivity (σ₁) profile and the E_w(z) or V_p^II(z)(z) profiles used in the inversion. Even with a ±20% error in the above profiles, the errors in the retrieved winds are found to be less than 20% over 75% of the altitude range and 20–30% for the remaining 25% of the altitude range, on the average.     

Lunar and solar daily variations of the geomagnetic field at Italian stations

Mean hourly values of magnetic declination D, horizontal intensity H and vertical intensity Z observed at Italian stations have been analyzed to determine solar and luni-solar diurnal components, together with the corresponding terms O₁ and N₂ of the lunar tidal potential.The results, showing the variations of the first four harmonic components with season, degree of magnetic activity and annual sunspot number, are tabulated and discussed. Differences between the dependence of S and L on season and sunspot number are considered and tentative explanations offered. The oceanic tidal effect has been determined and it is apparent that this is more likely to show the influence of the Atlantic Ocean rather than the Mediterranean Sea.