A unified view on convection and field-aligned current patterns in the polar cap

During the last two decades measurements of polar cap ionospheric electric fields and currents, field-aligned currents, and global auroral forms have been made from ground-based and space-based platforms. An attempt is made to unify these observations into a large-scale view of polar phenomena. In this view, plasma convection patterns and the corresponding electrodynamics in the polar region can consistently be ordered by the orientation of the interplanetary magnetic field (IMF). The different patterns of the electric potential and of field-aligned currents depend on where the main interaction between the terrestrial and interplanetary fields occurs, on the morning or evening side of the central polar cap, or on the dayside portion of the ‘closed’ cusp region, or on the nightside portion of the ‘open’ cusp region. One of the essential elements of this unified view is that it is possible to account for various convection patterns ranging from the four-cell pattern (during periods of strong northward IMF and B_y ~ 0), to the three-cell pattern (B_z > 0 and |B_y| 2> 0), to the conventional two-cell pattern (B_z < 0) with its possible deformation into a convection throat near the dayside cusp (during southward IMF). We also discuss the way in which the complicated field-aligned current systems can consistently be accounted for in terms of these convection patterns.     

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.     

Regions of strongly enhanced perpendicular electric fields adjacent to auroral arcs

In a joint campaign involving EISCAT, the Cornell University Portable Radar Interferometer (CUPRI), and sounding rockets, we have observed short-lived elevations of E-region electron temperatures, indicating the presence of strong electric fields. The use of a new pulse-code technique has considerably improved our EISCAT data in regions of low ionospheric electron densities. It has been found that strong and apparently short-lived enhancements of electric fields and associated E-region electron temperatures occur more commonly than long-lived ones. However, earlier EISCAT data with simultaneous optical recordings (and also some CUPRI radar data from the ERRRIS campaign) indicate that many of these events are, in fact, not short-lived, but occur in localized regions and are associated with drifting auroral forms. We show that the observed elevations of electron temperatures are created by very intense electric fields which can be found within narrow regions adjacent to auroral arcs. We discuss our observations against the background of models for electric field suppression or enhancement in the vicinity of auroral precipitation.     

On the role of auroral electric fields in the formation of low altitude sporadic-E and sudden sodium layers

The characteristics of metallic and molecular ion sporadic-E (E_s) layers, formed by the action of strong electric fields at auroral latitudes, are examined using computer simulations. It is found that, for electric fields directed between northward and westward (northern hemisphere), thin metallic ion layers (<2 km thick) can be formed above about 105 km altitude. For electric fields directed from westward, through southward, to south-eastward, slightly thicker (4–6 km thick) metallic ion layers can form between 90 and 105 km altitudes. Thin layers of molecular ions can be formed by electric fields directed between north and west if the ion density is low. Examples of E_s layers observed by the EISCAT radar, together with simultaneous observations of electric fields and ion drifts are presented which show good agreement with the simulations. The relationship between the lower-altitude E_s layers and sudden sodium layers (SSLs) is discussed leading to an explanation of some of the characteristics of SSLs at high latitude. A possible involvement of smoke particles in the formation of both E_s layers and SSLs is proposed.     

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.

     

Dynamo-electric field-aligned currents in the plasmasphere

The dynamo-theory has been used to interpret antisymmetric S_q-variations. The following results have been found analytically: 1. The antisymmetric electric field is important only for driving field-aligned coupling currents between both hemispheres. 2. The antisymmetric part of the observed S_q-variations can be explained best by the dynamo-action of seasonally depending (1, 1)-,(1, −2)-, and (2, 2)-tidal modes. 3. At all seasons the field-aligned currents are strongest near noon where they have at north-summer in the northern hemisphere a source near 50° and a sink near 30° latitude.     

Ion composition changes during F-region density depletions in the presence of electric fields at auroral latitudes

F-region density depletions in the afternoon/evening sector of the auroral zone are studied with the EISCAT UHF radar. Four case studies are presented, in which data from three experiment modes are used. In each case the density depletion can be identified with the main ionospheric trough. For the two cases occurring in sunlit conditions the electron densities recovered significantly after the trough minimum. Tristatic ion velocity measurements show the development of poleward electric fields of typically 50–100 m Vm⁻¹, which maximize exactly in the trough minimum. A special analysis technique for incoherent scatter measurements is introduced, based on the ion energy equation. By assuming that the ion temperature should obey this equation it is possible to fix this parameter in a second analysis and to allow the ion composition to be a free parameter. The results from two experiments with accurate velocity measurements indicate that the proportion of O⁺ near the F-region peak decreased from 100% in the undisturbed ionosphere to only 10% and 30%, respectively, in the density minimum of the trough. The loss of O⁺ is explained by the temperature dependence of recombination with nitrogen molecules. Temperatures derived from radar measurements are very sensitive to the assumed ion composition. For the above case of 10% O⁺ the deduced electron temperature in the trough was transformed from a local minimum of < 2000 K to a local maximum of 4000 K.     

Auroral infrasonic waves and the auroral electrojet

Using models of a line current and a band current with a parabolic intensity distribution across the width, techniques to deduce the speed, the direction of motion and the zenith crossing time of the electrojet from observations of surface magnetic perturbations are studied.From the current motions deduced by these techniques and the observed traces of the Auroral Infrasonic Waves (AIW), the following four facts are established.

• 1.(1) The time between the zenith crossing of the current and the arrival of AIW at the ground is reasonable.
• 2.(2) The direction of travel of AIW is considered to be parallel to the current drift or parallel to the current flow.
• 3.(3) The trace velocity of AIW is equal to the calculated drift velocity of the current.
• 4.(4) AIW can be produced only by a line (or a narrow band) current.

Several minutes before the AIW arrive at the ground, the existence of certain motions of westward current which satisfy the AIW emission conditions proposed by Wilson (1972) have been confirmed. However there are several cases in which a succession of two equatorward supersonic zenith crossings of westward current have induced only one AIW.     

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.     

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.     

Simultaneous measurements of the ionospheric electric field by probes and radar methods

Ionospheric electric field values are presented, obtained simultaneously by the double probe technique on board a rocket and by two incoherent backscatter radar installations. The measurements were performed during auroral activity over northern Scandinavia. The spatial distribution of the field reveals pronounced local variations.     

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.     

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.     

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.     

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.     

The day to night absorption ratio in auroral and subauroral zone riometer measurements during auroral absorption

The day to night ratio of auroral absorption has been studied using data from auroral and subauroral latitudes and by application of different kinds of statistical analyses. Ratios between 0.5 and 3.0 are obtained, depending on the criteria applied to the selection of data. Previous studies obtained similar ratios, but reached different conclusions about the effective solar control of auroral absorption. It is concluded here that evidence of solar control of the day to night ratio of auroral absorption, or the lack thereof, cannot be extracted by these statistical analyses.     

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 atmospheric electric field on the South Polar ice-cap

Atmospheric electric field variations recorded under fair-weather conditions on the South Polar ice-shelf in summer show the site to be globally representative and therefore of possible use in monitoring variations in the electrosphere potential. Evidence is also produced which suggests that the contribution to global thunderstorm activity by oceanic thunderstorms should be regarded as itself having a diurnal variation of some 18% in amplitude.     

Auroral rays: filamentation in the auroral accelerator

In situ measurements of particles, fields and optical emissions from a rocket that encountered auroral rays are reported. The measurements give insight into the production of rays, as well as the production of large fluctuations in electric fields perpendicular to the magnetic field. The fine structure and rapid variations of the electron energy flux associated with the rays are apparently produced by modulation in the degree of electron acceleration. Rays are produced when the energy flux increases in localized regions to values even higher than those normally encountered in bright auroral forms. Close and consistent similarities in the variations of the electron energy flux, the light and the electric fields suggest that the field variations were produced as a direct result of the changes in the stream of accelerated electrons. In examining possible causes of the velocity changes that produce the rays, two acceleration processes are considered; acceleration as a consequence of a potential difference between the magnetosphere and the atmosphere and acceleration by waves.     

Simultaneous observations of E and F region electric field fluctuations at the magnetic equator

Simultaneous daytime observations of E region horizontal irregularity drift velocities in the equatorial electrojet and F region vertical plasma drifts were made on a few magnetically quiet days at the magnetic equatorial station of Trivandrum (dip 0.5°N). Measurements of the electrojet irregularity velocities by VHF backscatter radar and the F region vertical plasma drifts by HF Doppier radar are used to deduce the daytime East-West electric fields in the E and F regions, respectively. The fluctuating components of the electric fields are separated and subjected to power spectral analysis. The E and F region electric field fluctuations are found to be well correlated; the estimated correlation coefficient is in the range of 0.52–0.8. The fluctuation amplitudes are of the order of 15% over the background for the E region and 25% for the F region. The spectral analysis reveals dominant components in the range of 30–90 min with F region components stronger than those of the E region by a factor of about 1.5 on the average. The F region electric fields during daytime being coupled from the low latitude E region, the good correlation observed between the E and F region perturbations suggests that the electric fields in the E region at low and equatorial latitudes are coherent for the temporal scales of the order of few tens of minutes. The spectral characteristics are such that the commonly occurring medium scale gravity waves could possibly be the source for the observed fluctuations in the E and F region electric fields.