The variability of ionospheric dynamo currents

The method of Hibberd is used to study the variability of ionospheric dynamo currents from day to day, with solar rotation and with the solar cycle. The method eliminates many sources of disturbance by using the difference of H at two magnetic observatories having the same longitude but different latitudes. In this way, a measure of the strength of the ionospheric currents can be obtained almost every day, even during magnetically disturbed periods. It is concluded that the currents are produced partly by tidal modes driven by in situ heating in the thermosphere, and that variations in the amplitude of these modes are mainly responsible for the solar rotation and the solar cycle effects which are observed. There are also random day-to-day changes, uncorrelated from one day to the next, and these suggest that upward propagating tidal modes are also important in driving the currents.     

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.     

Effects of the ionospheric conductivity on the longitudinal structure of the F-region equatorial anomaly

A longitudinal comparison has been undertaken of the vertical ion velocities in terms of ion mobilities and the eastward electrojet currents in terms of the electric conductivities overhead the magnetic equator at African and West Asian regions to search for longitudinal differences in the developments of the equatorial anamoly. It is shown that the vertical ion velocities and electrojet currents in both regions are different. It is proposed that longitudinal differences in (k²_1i+k²_2i)/k_2i and K_1i/K_2i ratios give rise to a different vertical ion velocities in these regions. This is likely to cause different equatorial F-layer plasma fountain magnitudes resulting in different developments of the equatorial anomaly.     

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.     

Modelling studies of the conjugate-hemisphere differences in ionospheric ionization at equatorial anomaly latitudes

The relative importance of the equatorial plasma fountain (caused by vertical E x B drift at the equator) and neutral winds in leading to the ionospheric variations at equatorial-anomaly latitudes, with particular emphasis on conjugate-hemisphere differences, is investigated using a plasmasphere model. Values of ionospherec electron content (IEC) and peak electron density (Nmax) computed at conjugate points in the magnetic latitude range 10–30° at longitude 158°W reproduce the observed seasonal, solar activity, and latitudinal variations of IEC and Nmax, including the conjugate-hemisphere differences. The model results show that the plasma fountain, in the absence of neutral winds, produces almost identical effects at conjugate points in all seasons; neutral winds cause conjugate-hemisphere differences by modulating the fountain and moving the ionospheres at the conjugate hemispheres to different altitudes.At equinox., the neutral winds, mainly the zonal wind, modulate the fountain to supply more ionization to the northern hemisphere during evening and night-time hours and, at the same time, cause smaller chemical loss in the southern hemisphere by raising the ionosphere. The gain of ionization through the reduction in chemical loss is greater than that supplied by the fountain and causes stronger premidnight enhancements. in IEC and Nmax (with delayed peaks) in the southern hemisphere at all latitudes (10–30°). The same mechanism, but with the hemispheres of more flux and less chemical loss interchanged, causes stronger daytime IEC in the northern hemisphere at all latitudes. At solstice, the neutral winds, mainly the meridional wind, modulate the fountain differently at different altitudes and latitudes with a general interhemispheric flow from the summer to the winter hemisphere at altitudes above the F-region peaks. The interhemispheric flow causes stronger premidnight enhancements in IEC and Nmax and stronger daytime Nmax in the winter hemisphere, especially at latitudes equatorward of the anomaly crest. The altitude and latitude distributions of the daytime plasma flows combined with the longer daytime period can cause stronger daytime IEC in the summer hemisphere at all latitudes.     

Periodic amplitude variations as precursors of plumes of equatorial ionospheric irregularities

Periodic amplitude fluctuations of VHF signals from a geostationary satellite monitored from near the magnetic equator have been observed in the evening hours as precursors of strong Rayleigh fading associated with plumes of irregularities. These periodic fluctuations called “amplitude waves” exhibit amplitude changes of only 1 to 2 dB and have been observed for up to 30 minutes before the onset of strong scintillations. Individual fades are correlated over distances of at least 120 km in the magnetic eastwest direction. The velocity of these wavelike disturbances has been found to be approximately 140 ms⁻¹ eastward with a corresponding wavelength of 25 km. No wavelike behavior of Faraday rotation, a measure of the background changes in TEC, was observed during these times. Several mechanisms are examined as the cause of these amplitude waves; however, none was found to be completely satisfactory in explaining the observations.     

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.     

Computer simulations of the seasonal variations of the ionospheric equatorial anomaly in East Asia under solar minimum conditions

The previous dynamical, computer simulation model of the ionosphere at low latitudes of Chan H. F. and Walker G. O. (1984a, J. atmos. terr. Phys. 46, 1103; 1984b, J. atmos. terr. Phys. 46, 1113) has been modified to (1) include photoionization of molecular species NO⁺, N₂⁺ and O₂⁺ below 300km, (2) decouple the ionization and wind calculations below 180 km and (3) expand the geographical coverage to 46°N-30°S latitude. The first two modifications improved the model stability and the latter reduced the effect of the lateral boundaries on the equatorial anomaly. Results are presented for the representative seasonal months of January, April and July for East Asia, during solar minimum, comprising latitudinal-local standard time (120°E) contour plots of (1) the atmospheric pressure, (2) the computed meridional wind at 300 km, (3) the foF2 and (4) hmF2, together with latitudinal profiles of foF2 and N_T (electron content) showing the daytime development and nighttime decay of the equatorial anomaly.Comparisons have been made between the computer simulations and various experimental measurements of foF2, M(3000) F2 and N_T obtained in East Asia during periods of low solar activity. Most of the gross features of the development and decay of the equatorial anomaly at the various seasons were reproducible by the model simulations, the best agreement occurring for the equinoctial month of April.     

Weak scattering theory applied to equatorial ionospheric scintillation for a wide range of parametric values

The model of ionospheric fluctuations used by Booker and Ferguson (1978) to describe spread-F is applied to ionospheric scintillation in the band from 100 MHz to 10 GHz in equatorial regions. Calculations are based on long Isotropie field-aligned irregularities possessing an inverse power-law spectrum extending from an outer scale [wavelength/(2π)]linked to the properties of the neutral atmosphere down to an inner scale of the order of the ionic gyroradius. Spectral indices from 0 to 6 are considered, with special attention to the range from 1 to 4. The r.m.s. fluctuation of ionization density is assumed to be proportional to the ambient ionization density throughout the plasmasphere, but the effect is shown of removing the fluctuations at heights above 500, 750 and 1000 km. Using a height-distribution of phase-changing screens, calculations are made, for evening and presunrise conditions, of the mean square fluctuations both of phase and of fractional amplitude for situations in which an Earth terminal and a stationary satellite are both in the magnetic equatorial plane. Heights of equivalent single phase-changing screens are deduced for both phase and amplitude fluctuations; they are different from each other and from the height of maximum ionization density. It is concluded that the weak scattering theory can satisfactorily explain weak scintillation, but that amplitude scintillation at strengths of practical importance for radio communications requires the inclusion of refractive scattering in addition to diffractive scattering.     

Some effects of geomagnetic activity on spread-F occurrence and ionospheric height variations in equatorial regions

AE indices have been used to investigate, at times of increased geomagnetic activity, the possibility of significant changes to both spread-F occurerence and h′F values for 3 stations in equatorial latitudes. The investigation covered a sunspot minimum period. Furthermore, data for each of these parameters have been considered for both a pre-midnight period (interval A) and a post-midnight period (interval B). The use of the AE indices at 12 different times at 2 h intervals allows the measurement of the delay times, after increased geomagnetic activity, of any significant changes in the parameters being investigated.The results show that for interval A significant suppressions of spread-F occurrence are recorded at delay times of approximately 3 h and 9 h. These delays correspond to enhanced geomagnetic activity at local times of 1800 and 1200, respectively. Also, for interval A the h′F variations suggest that h′F is suppressed at times of spread-F suppression. For interval B spread-F occurrence seems to be controlled by two opposing effects. For several hours after enhanced geomagnetic activity spread-F occurrence increases significantly, followed by a sharp decline culminating in suppressed occurrence, again related to increased geomagnetic activity at 1800 local time for the maximum effect. Also, for interval B h′F values lift abruptly a few hours after enhanced geomagnetic activity, followed by a gradual decline when delays of up to 20 h are considered. Further work on these delays may allow reliable short-term forecasting of some ionospheric behaviour in equatorial regions.     

A diurnal study of the electrical structure of the equatorial middle atmosphere

Electrical parameters measured from 115 km down to below 20 km during the Project Condor campaign at the Punta Lobos Rocket Range near Lima, Peru, are presented Ten rocket-launched payloads measured electrical conductivity. A strong diurnal influence due to solar ultraviolet radiation is shown. Nine of the payloads also measured electric fields. No large mesospheric vertical electric fields are found in the data. A calculation of the d.c. global conduction current density at 18 km is smaller than previously measured at low latitudes and does not show the conventional diumal variation.     

North-south asymmetry in the ionospheric equatorial anomaly in the African and the West Asian regions produced by asymmetrical thermospheric winds

The effect of asymmetrical thermospheric winds on NmF2 at the dip I = 30° and its magnetic conjugate point have been computed for equinox conditions to study asymmetry in the ionospheric equatorial anomaly in the African and West Asian regions. The wind models of I11 et al. and Chan and Walker have been used in our computations. During the daytime, due to the winds NmF2 in the northern crest becomes greater than NmF2 in the southern crest; at night the reverse is true in both regions. It is shown that the observed asymmetry in NmF2 at the equatorial crest in the African sector can be well explained by considering the effects of asymmetrical winds with respect to those in the West Asian sector.     

Reconstruction of horizontally-inhomogeneous ionospheric structure from oblique-incidence backscatter experiments

Fridman and Fridman [(1994) J. atmos. terr. Phys. 56, 115] suggested a method of reconstructing the horizontally-inhomogeneous ionospheric structure using vertical- and oblique-incidence backscatter sounding (OBS) ionograms measured at a single location. In the present paper this technique has been used to analyze experimental data and tested against independent vertical sounding (VS) measurements. By using the OBS and VS ionograms measured at Irkutsk as source data for the method we reconstructed ionization profiles over Tomsk (1050 km to the west of Irkutsk). We found that the reconstructed profiles are in reasonable agreement with the profiles obtained from VS measurements at Tomsk.     

Fine scale structure and turbulence parameters in the equatorial middle atmosphere

Two rockets carrying identical spherical probe payloads were launched from Thumba to measure positive ion density of the mesosphere and lower thermosphere over an equatorial location. Data obtained show the presence of strong irregularities in the ion density. From the measured positive ion current, the spectra of the spatial density fluctuation, turbulent velocity, energy dissipation rate and eddy diffusion coefficients have been derived in the altitude range of 70–100 km. The results are found to be different from those at middle and high latitudes.     

Distortions in the height structure of the equatorial electrojet during counter electrojet events

This paper presents simultaneous observations made near the magnetic equator during counter electrojet events using a coherent VHF backscattcr radar, magnetometer and digital ionosonde to understand the physical processes that generate the counter electrojet conditions. The VHF backscatter radar gives the height structure of the drift velocity or the ionization irregularities, the equatorial electrojet current variations are obtained from the magnetometer and the digital ionosonde provides the presence of blanketing E-layers at the F-region heights which give rise to the backscatter signals. These observations have provided direct experimental evidence for the theoretically predicted distortions in the height structure of the polarization electric field in the equatorial electrojet due to the local effects of shearing zonal neutral winds.     

Double structure of ionospheric conductivity in the midnight auroral oval during a substorm

Measurements of precipitating particles on board DMSP F7 spacecraft are used to analyze the distribution of ionospheric conductance in the midnight auroral zone during substorms. The distribution is compared with the meridional profile of ionospheric currents calculated from magnetic data from the Kara meridional chain. Two regions of high Hall conductance are found; one of them is the traditional auroral zone, at latitudes 64–68°, and the other is a narrow band at latitudes 70–73°. The position of high conductance zones is in agreement with the location of the intense westward currents. The accelerated particle population is typical of electrons E_e > 5 keV in the high conductance region.     

Internal structure of Middle Sabaean bayt

The Middle Sabean bayt , clan community, appears to have been a very complex and highly stratified social entity. It seems possible to identify its following main components: [1] the clan nucleus, consisting of [1a] the "leading group" ("patriarch/s" and his/their brothers and sons), [1b] their partrilineal close senior (' hy -) and junior ( bny -) relatives, [1c] their distant patrilineal relatives ( d -'^c dr ), as well as their [1d] unmarried daughters ( bnt ) and [1e] children ( wld;'wld ); it also seems possible to identify within the "clan nucleus" sub-groups of "senior" and "junior" adults; [2] the wives of the members of the "clan nucleus" who would normally come from other clan communities and who would form a distinct sub-group within the system of the Middle Sabean bayt (in general, it seems possible to suppose the existence of the "male" and "female" halves of the bayt ); together the above-mentioned sub-groups constituted the "free" part of the bayt (' hrr ); its other important part was formed by [3] its "unfree" members (' dm ; though most Middle Sabean' dm appear to have lived by their own autonomous bayts ); within large noble bayts an important role seems to have been played by maqtawīs , "personal assistants", of the bayts 'leaders.