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.     

Night-time ionospheric scintillations at the magnetic equator

The nocturnal and seasonal variations of equatorial ionospheric scintillations are presented. Scintillations are classified into two classes, namely, class I and class II depending on their fading rates and association with bottomside spread-F. Power spectra and frequency indices for class I and class II scintillations are presented and their theoretical implications are discussed.     

Solar-induced variation of proton precipitation near the equator

This paper reports the solar condition dependences of the quasi-trapped component (low energy) of the proton population of energy 0.65–35 MeV which peaks in the equatorial zone centered on the minimum magnetic field equator in the altitude range 170–850 km. The proton populations compared pertain to AZUR observation in 1969–1970, S81-1 mission observation in 1982 and EXOS-C observation during 1984–1986. In the equatorial zone, the dependence of the flux normalization constant, which represents the absolute proton population, upon factors like L (1.1–1.3), B (0.29–0.32 gauss), latitude ( ± 20°), longitude (0–360°) and anisotropy index q (~6–12) of the pitch angle distribution function is not so significant in the given range of these factors as it is upon the solar epoch. It is found that the absolute proton flux in 1982 was, at least, forty times that in 1984–1986 and, almost, three times that in 1969–1970, possibly, due to, varying solar conditions in those epochs.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Ionospheric absorption at the magnetic equator using the A1 technique

A₁ absorption data on four operating frequencies, 1.8, 2.0, 2.2 and 2.5 MHz over a period of two years (1972–74) at Trivandrum (dip 0.6°S, geograph. lat. 8.6°N, long. 76.9°E) has been analysed to study its diurnal, day to day, long term and frequency variations. An empirical relation has been established between noon absorption and solar X-ray and 10.7 cm fluxes. Using this relation, the seasonal cosχ index has been evaluated. The deviation of the estimated absorption using this empirical relation from the observed absorption is found to be less than 10%.     

Vertical plasma drifts in the post-sunset F-region at the magnetic equator

HF doppler observations of vertical plasma drifts in the post-sunset equatorial F-region at Trivandrum (dip 0.9°S), conducted over a range of solar and geomagnetic conditions, are presented. The observations show that under magnetically quiet conditions, the characteristic post-sunset enhancement in the vertical plasma drift is quite sensitive to solar activity; the peak velocity drops by about a factor of 3 as the solar flux index (S_10.7) changes from about 125 to 70. It is found that the drift velocity enhancement has strong magnetic activity dependence only during high solar activity; the drift velocity drops by more than a factor of 2 from quiet to moderate activity, but builds back to the quiet day level for high magnetic activity. The occurrence of equatorial spread-F (ESF) is seen to be closely linked to the post-sunset enhancement in the vertical drift velocity, both showing essentially the same dependence on solar and magnetic activities. A comparison with Jicamarca observations shows that while the gross characteristics of the drift velocity pattern are about the same for the two stations, there are significant differences in the detailed variations, particularly for magnetically disturbed conditions.     

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.     

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.     

A model passive probe for free atmosphere electric field measurements

A model of a passive probe for determination of electric field strength in the free atmosphere is developed. This model shows that the same probe can be used to determine mean monosigned ion conductivity and also conduction current density. Simulation of typical experimental conditions is used to derive the response to small altitude-dependent conductivity fluctuations. Results from a passive probe sounding indicate that conduction current density was nondivergent to (at least) 26 km and that small-scale conductivity fluctuations were only around ± 3% of the mean value.     

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.     

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.     

The earth's conductivity at the Peruvian dip equator: indication of anomalous conductivity

A method involving the use of integral transforms has been applied to separate the daily geomagnetic variations into its internal and external parts without a previous separation into planetary and incremental parts. As a result of its application, a remarkable anomaly in the conductivity has been found at the Peruvian coastal stations. This anomaly influences specially the horizontal component of the magnetic field.