A new aspect of daily variations of the geomagnetic field at low latitude

Data from the unique network of low latitude geomagnetic observatories in India extending from the dip equator to the northern focus of the Sq current system have shown a new type of Sq current distribution different from those associated with the normal or the counter electrojet currents. On 3 December 1985 both the horizontal as well as the vertical components of the geomagnetic field at Annamalainagar showed maximum values around the midday hours. The abnormal feature described seems to be rather a rare phenomenon. The solar daily range of H field is found to be fairly constant from the dip equator up to about 12° dip latitude, suggesting the complete absence of the equatorial enhancement of ΔH, typical of the equatorial electrojet. The cancellation of the equatorial electrojet is suggested to be caused by a westward flowing current system much wider than the conventional equatorial electrojet. This additional current system could be due to the excitation of certain tidal modes at low latitudes on such abnormal days.     

New results on the dip equator and the equatorial electrojet in India

The geophysical implications are examined of the continuing southward migration of the magnetic dip equator in India since 1965, its precise ground location in 1971, and thereafter its drift at 1–6 km/yr accelerating to 7 km/yr in the mid-1980s near its mean central position in the 80-yr secular oscillation, estimated to be about 10 km south of Trivandrum. Simultaneously its drift northwards near the antipodal point at Huancayo Observatory, in Peru (South America), is also observed.The ground projection of the mean axis of the equatorial electrojet for 1980 is clearly delineated about 55 km to the north of the dip equator in India, with positive Sq(Z) values of 25 nT recorded right on the dip equator, based on the ground geomagnetic survey 1971 and the magnetometer array experiment of 1980. The half-width and midday peak total current intensity of the Indian electrojet are determined from the H data recorded at Trivandrum, Annamalainagar and Hyderabad for the solar minimum year 1976 (146 ± 46 km, 137 ± 25 Amp/km) and the maximum year 1980 (169 ± 39 km, 203 ±49 Amp/km), assuming a uniform west-east current band model at a height of 107 km centred on its newly discovered axis. These new results are quite different from those of earlier determinations. Severe induction anomalies observed in the region due to subsurface geological bodies are also appropriately incorporated.     

Geomagnetically quiet day ionospheric currents over the Indian sector—III. Counter equatorial electrojet currents

Counter equatorial electrojet (CEJ) occurring at all hours from 0700 to 1700 h LT in the Indian sector have been studied. The percentage occurrence of morning and afternoon CEJ in each hour and each season are given and discussed. The first quantitative determinations of eight landmark parameters that depict the structures of hourly latitudinal profiles of CEJ current have been presented and discussed.These are the peak height integrated forward current density (or intensity) at the centre of the current, the peak return current intensity, the ratio of the peak return to the peak forward current intensity, the total forward current, the dip latitude of the current centre, the distance of the focus from the current centre, the distance of the peak return current intensity from the centre, and the latitudinal extent of the current. They reveal new and interesting features of diurnal and seasonal variations, and marked contrasts between morning and afternoon CEJ. Other evidences support our focal distance w of CEJ. Comparison with equatorial electrojet (EEJ) shows that five of the parameters are similar for EEJ and morning CEJ but substantially different for afternoon CEJ. The counter worldwide part of Sq currents have also been compared with CEJ.     

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.     

Latitudinal and vertical parameters of the equatorial electrojet from an autonomous data set

The magnetic field expressions from the current ribbon and thick current versions of the continuous distribution of current density model and their merits have been presented. For the first time both the latitudinal and vertical parameters of the equatorial electrojet (EEJ) have been derived from the same set of data. The local noon and daytime means of certain key parameters of the EEJ are shown to be in good agreement with those from other sources. Selected local noon means include: peak current density j_o, 10.58 ± 0.34 A/km²; peak current intensity j_o, 224 ± 9 A/km; total eastward current I₊, 74 ± 5 kA ; EEJ current focal distance w, 300 ± 5 km ; half thickness at half of peak current density p, 7.0 ± 0.1 km; peak westward current location x_m, 5.13 ± 0.08° dip latitude; and EEJ latitudinal extent L₁, 12 ± 1° dip latitude. The problem of model calculated landmark distances of EEJ being consistently shorter than observations, encountered by Onwumechiliet al. [J. geomagn. Geoelecl. 41, 443 (1989)] has been solved.     

Ionospheric electrodynamic model with an eccentric dipole geomagnetic field

A computer model of ionospheric electrodynamic processes using an eccentric dipole (ED) for the geomagnetic field has been developed. This is a development from existing models which are based on the centred dipole (CD) coaxial with the geographic axis. The ED dynamo model introduces or modifies the effects of hemispherical asymmetry and longitudinal variation in the dynamo processes through two explicit parameters—the geomagnetic field intensity and the length of the field lines. These parameters of the ED field have been quantified and displayed. An additional contribution to the above effects comes implicitly from the ionospheric parameters—plasma density and atmospheric tidal winds—which become asymmetric relative to the ED dip equator. The integrated effect of the geomagnetic and ionospheric parameters produces significant variation in the field line integrated ionospheric conductivity. The ED dynamo model shows that the peak height of the equatorial electrojet (EEJ) moves by over 2 km and height profiles of the EEJ display strong hemispherical asymmetry.     

Effect of severe auroral disturbances on the equatorial ionosphere

It is now an established fact that during extremely strong magnetic storms a sudden anomalous decrease in the F-layer critical frequency foF2 is sometimes noticed at the equator around noon-time and the duration of this effect is known to be anywhere between some tens of minutes to several hours. As an extension of earlier work by Turunen and Rao, 1980, seven severe auroral storm events based on AE index have been selected during the period July 1958–June 1960 and their effects on the equatorial ionosphere have been investigated utilizing the published ionospheric data for the chain of Indian stations starting from equatorial latitudes and extending up to the mid-latitudes. From this study, it is noted that at the equator around noontime the foF2 values decrease and the noon bite-out phenomena are enhanced. However, as one goes towards mid-latitudes this trend is reversed. Because of this, the Appleton anomaly is also enhanced during disturbed days. Besides, the fF_s values at the magnetic equator show an increase during disturbed days indicating thereby that the eastward equatorial electrojet current is enhanced on disturbed days. This suggests that the auroral electrojet current is coupled to the equatorial electrojet current possibly via the magnetosphere.     

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.     

Equatorial F-layer irregularity patches at anomaly latitudes

Recent studies of the physics of F-layer irregularities in the equatorial ionosphere have been concerned with the development of plumes or patches. A series of observations in the equatorial anomaly region in a year of high solar flux has been analyzed for the radio propagation effect of scintillations. The observations were made on patches in the developing, mature and decay phases. Although irregularities develop on the west wall of the patches, the intensity of scintillation does not appear to diminish within the patch; the patches contain bursts of high level activity.Patch characteristics at microwave wavelengths match airglow depletion images when two considerations are introduced, i.e. the westward tilt of the patch as shown by optical and radar observations and the effective path length of the irregularities affecting the radio propagation path. Using optical images of depletions the effective thickness of the layer of irregularities above the peak of the F2-layer can be estimated; it is relatively short, i.e. of the order of 70 km for the gigaHertz frequencies and 150 km for the 257 MHz transmissions. The total path length is 110 km for the microwave frequencies and 220 km for the lower levels of scintillation at 257 MHz. The decrease in microwave scintillations compared to meter wavelength observations in the midnight and post-midnight time period in these anomaly observations is due to the combination of decay of electron density as well as the relatively rapid decay of smaller scale irregularities, as has previously been noted in observations at the magnetic equator.     

Spatial dependence of day-time vertical polarisation of Pc 3–5 magnetic pulsations in Sri Lanka

The rate of change of the horizontal and vertical components of magnetic pulsations in the period range 20–600 s have been recorded at four stations in Sri Lanka, namely, Vavuniya, Maradankadawela, Maho and Colombo. An analysis of the records shows that the horizontal polarisation of the pulsations is predominantly along the magnetic meridian at all four stations. The vertical polarisation as measured by the ratio ΔZ/ΔH increases with increase in period and for signals in a given periodic band, except for those recorded at Maho ; there is also an increase of vertical polarisation with distance of the recording station from the magnetic equator. At Maho, there is a local decrease of the vertical polarisation at all periods probably due to anomalous electrical conductivity in this region. Although the spatial and period dependence of the vertical polarisation of the pulsations recorded at the other three stations can be explained in terms of an oscillating ionospheric current band of half-width about 105 km flowing east-west in the neighbourhood of the magnetic equator, and its image current in a uniform conducting earth, such a model may not be realistic in that it neglects possible effects due to induced currents in the ocean deflected round the coasts of Sri Lanka. It is suggested that the observed day-time polarisation may represent the effect of a wider equatorial ionospheric current system, such as the equatorial electrojet and additional effects due to induced currents in the ocean.     

On the equatorial electrojet influence on geomagnetic pulsation amplitudes

It is well known that several types of geomagnetic pulsations show a significant amplitude enhancement near the dip equator due to the daytime equatorial electrojet. In the present study, the dependence of this enhancement on the period and type of geomagnetic variations is examined. The results show that, in general, the amplitude enhancement appears to be more or less uniform, amounting to a factor of 2.0–2.5, over a wide range of periods. However, for pulsations, there is a fairly sharp cut-off of the equatorial enhancement around a 20 s period, the shorter period end of Pc3 pulsations. Further, shorter period pulsations (<20 s) sometimes suffer an attenuation at the dip equator near noon. These results are discussed in the light of the transmission characteristics of the ionosphere, including the possible relation to the equatorial anomaly in the ionospheric F-region.     

Evidence for a large scale electric field gradient at the onset of equatorial spread-F

Barium-strontium release experiments were conducted at Sriharikota Rocket Range (SHAR. 5.5 N dip latitude) at the onset of equatorial spread-F. We report here an unusual phenomenon of the development of two barium ion clouds from a single release around 200 km altitude, moving with different speeds indifferent directions. This is the first experimental evidence for the presence of large scale electric field gradients with a scale size of 15km. By incorporating neutral wind measured during spread-F into a numerical model for equatorial electrojet we interpret these gradients to be the manifestation of effects due to the meridional winds and wind shears. It is possible that the electric field gradients observed may lead to the generation of plasma holes during the onset of equatorial spread-F.     

Study of equatorial plasma bubble dynamics using GHz scintillation observations in the Indian sector

To study equatorial plasma bubble dynamics, telemetry signals (4 GHz) were recorded simultaneously from two geostationary satellites. INSAT-1B (74°E) and INSAT-1C (94°E) at Sikandarabad satellite Earth station (dip 42.0°) from January to December 1989 and at the Chenglepet satellite Earth station (dip 10.5°) during September–October 1989 along the same geomagnetic meridian. The characteristics and occurrence pattern of the scintillations suggest that these are equatorial plasma bubble induced events. Observations from the two satellites recorded simultaneously at each of these locations were utilized to estimate the east-west plasma bubble irregularity motion. Plasma bubble rise velocities over the magnetic equator were calculated from the systematic onset time differences observed between an equatorial and a low latitude station. The east-west plasma bubble velocity estimated at Sikandarabad, corresponding to 1200 km altitude in the equatorial plane, shows a night time variation pattern with a peak at around 2100 LT. The mean values over Chenglepet, which correspond to 400 km altitude, start decreasing right from 1900 LT and seem to be influenced by the plasma bubble rise velocities. The differences in magnitude observed between the present results and those reported elsewhere by other techniques are interpreted in terms of vertical shears in the plasma zonal flow over the equator. The near alignment of the two observing stations along a common geomagnetic meridian and the simultaneous use of two satellites located twenty degrees apart in longitude provided an excellent data base to study plasma bubble dynamics.     

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.     

Formation of ionisation layers responsible for blanketing Es during daytime counter-electrojet over magnetic equator

A model using photochemistry and transport due to electric fields and gravity wave winds has been used to explain the formation of ionisation layers observed over an equatorial station Thumba (dip 0°47′S) with rocket-borne Langmuir probes during two daytime counter-electrojet periods. These layers were seen as blanketing E_s-layers with an ionosonde at Thumba. Convergence of the metallic ions due to three-dimensional gravity wave winds and a westward electric field appears to be mainly responsible for the observed ionisation layer over the equator.     

Geomagnetic daily variations in H at Alcântara and Eusébio in the Brazilian equatorial region

Daily variations in H at Alcântara (dip +3.8°) and Eusébio (dip − 6.5°), two stations in the equatorial electrojet region in Brazil, are compared. The electrojet in NE Brazil exhibits N-S asymmetry. The discrepancy between the daily variation range in H indicates the presence of complicated ionospheric currents in this region and warrants a detailed investigation.     

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.     

Day-time Pi pulsations at equatorial latitudes

Low latitude Pi2 pulsations are considered to be the best indicators of the onset of magnetospheric substornis (Rostoker and Olson, 1978; Saito, 1979) and are hitherto believed to be mainly night-time phenomena. It is seen from this study utilising the pulsation records from Choutuppal (geomagnetic: 7°.5, 149°.3 E)and Etaiyapuram (geomagnetic: –0°.6.147°.5 E)and the “Common Scale Magnetograms” from the Auroral Electrojet (AE) stations during January–April 1976, that Pi2s do appear even during day-time on many occasions at equatorial latitudes in simultaneity with the onset of magnetospheric substorms at AE stations located in the night hemisphere. It is also found that the day-time Pis, unlike the night-time Pi2s, show enhancement in their amplitudes of Hx component at Etaiyapuram, situated at the dipequalor as compared to those at Choutuppal, well away from it. The results thus not only show the appearance of Pi pulsations during daytime in the equatorial zone, but also bring out the possible influence of the equatorial electrojet on their amplitudes at the dip equator.     

Equatorial penetration of magnetic disturbance effects in the thermosphere and ionosphere

The neutral dynamic and electrodynamic coupling between high and low latitudes, and the mutual interactions between these two processes, are investigated. For 22 March 1979, when a sudden increase in magnetic activity occurred, we have analyzed the following experimental data: (a) neutral densities and cross-track neutral winds as a function of latitude (0°–80°) near 200 km from a satellite-borne accelerometer; (b) hourly mean H-component magnetic data from the Huancayo Observatory (0.72°S, 4.78°E; dipole geomagnetic coordinates) magnetometer; and (c) hourly mean foF2 measurements from the ionosonde at Huancayo. Comparisons are also made with a self-consistent thermosphere-ionosphere general circulation model and with observationally-based empirical models of winds and density.In concert with the increase in magnetic activity to K_p levels of 5–7, a nighttime (2230 LT) westward intensification of the neutral wind approaching 400 ± 100 ms⁻¹ occurred near the magnetic equator on 22 March 1979, accompanied by a 35% increase in neutral mass density. About 2 h after each of two substorm commencements associated with periods of southward IMF, ∼100γ and ∼200γ reductions in the daytime Huancayo H-component (corrected for ring current effects) are interpreted in terms of ∼0.5 and ∼1.0 mVm⁻¹ westward perturbation electric fields, respectively. An intervening 2-hour period of northward IMF preceded a positive equatorial magnetic perturbation of about 200γ. Time scales for field variations are a few hours, suggesting that processes other than Alfven shielding are involved. Variations in f₀F2 (∼ ± 1.0 MHz) over Huancayo are consistent with the inferred electric fields and magnetic variations. Similar equatorial perturbations are found through examination of other magnetic disturbances during 1979.     

Absolute electron density measurements in the equatorial ionosphere

Accurate measurement of the electron density profile and its variations is crucial to further progress in understanding the physics of the disturbed equatorial ionosphere. To accomplish this, a plasma frequency probe was included in the payload complement of two rockets flown during the CONDOR rocket campaign conducted from Peru in March 1983. In this paper we present density profiles of the disturbed equatorial ionosphere from a night-time flight in which spread-F conditions were present and from a day-time flight during strong electrojet conditions. Results from both flights are in excellent agreement with simultaneous radar data in that the regions of highly disturbed plasma coincide with the radar signatures. The spread-F rocket penetrated a topside depletion during both the upleg and downleg. The electrojet measurements showed a profile peaking at 1.3 × 10⁵cm⁻³ at 106 km, with large scale fluctuations having amplitudes of roughly 10 % seen only on the upward gradient in electron density. This is in agreement with plasma instability theory. We further show that simultaneous measurements by fixed-bias Langmuir probes, when normalized at a single point to the altitude profile of electron density, are inadequate to correctly parameterize the observed enhancements and depletions.