Solar activity effects on equatorial plasma bubble zonal velocity and its latitude gradient as measured by airglow scanning photometers

Using a new mode of scanning 630-nm photometer operation the zonal velocities of ionospheric plasma depletions were measured over Cachoeira Paulista in Brasil in two east-west planes tilted 30°N and 30° S with respect to zenith. The measurements cover a time period of approximately 2 years, from January 1988 to January 1990, a period marked by significant increase in solar activity of the ongoing cycle. The results have permitted a rather detailed evaluation of the local time and latitude variations in the zonal plasma bubble velocity as a function of solar activity. Although the mean trend in the velocity local time variation is a decrease from early evening to post-midnight hours, a strong tendency for velocity peaks is observed near 21 LT and midnight. The velocities as well as their height (latitude) gradients show perceivable increases with solar activity represented as sunspot numbers. The present results are compared with the ambient plasma velocities measured using the Jicamarca radar by Fejer el al. (1985), J. Geophys. Res. 90, 12249, with that measured on board the DE 2 satellite on the equatorial latitudes by Coley and Heelis (1989), J. geophys. Res. 94, 6751, and with various theoretical calculations, in an attempt to bring out the salient features of the plasma dynamics of the equatorial ionosphere.     

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.     

A model for the electron temperature variation along geomagnetic field lines and its effect on electron density profiles and VLF paths

A realistic model for the temperature variation along geomagnetic field lines is described. For high altitudes (>1500 km) the temperature is taken to increase as the nth power of radial distance (n−2), giving temperatures consistent with those measured in situ by high altitude satellites. For realistic temperatures at low altitude an extra term is included. The temperature gradient along the field line is then 0.9–1.6° km⁻¹ during the day and 0.5–0.7° km⁻¹ during the night at 1000 km, reducing to about half these values at 2000 km, for the latitude range 35–50°. This is consistent with calculations made from nearly simultaneous satellite measurements at 1000 and 2500 km. It is shown that assuming diffusive equilibrium, including the new temperature model, more realistic equatorial electron density profiles result than for isothermal field lines.The temperature gradient model is also purposely formulated to be of a form that enables the temperature modified geopotential height to be obtained without numerical integration. This renders the model particularly suitable for ray-tracing calculations. A ray-tracing model is developed and it is shown that unducted ray paths are significantly altered from the corresponding paths in an equivalent isothermal model; there is greater refraction and magnetospheric reflection takes place at lower altitudes. For summer day conditions, an inter-hemispheric unducted ray path becomes possible from 26° latitude that can reach the ground at the conjugate.     

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.     

A revised corrected geomagnetic coordinate system for Epochs 1985 and 1990

A new set of corrected geomagnetic coordinates (CGM) has been calculated from the magnetic field model DGRF for Epoch 1985 and the IGRF model for Epoch 1990. A new approach to determine the ‘dip’ magnetic equator has been developed, which is based on the vertical (along Re) projection on the Earth's surface of the B-minimum value point (apex) on each geomagnetic field line. A strip along the ‘dip’ magnetic equator line has been defined where the corrected geomagnetic coordinates could not be found by the definition of CGM. Linear interpolation between the locations of the two last definable CGM latitudes in both hemispheres has been used to calculate the CGM longitudes in the equatorial region. Interpolation between locations of the last definable CGM latitude and ‘dip’ equator in both hemispheres has been used to calculate the CGM latitudes in this region. The constant B-min geomagnetic coordinate system (CBM) is proposed and analysed to replace CGM in the equatorial region.     

In-situ observations of magnetospheric electron scattering by a VLF transmitter

High resolution pitch angle measurements of outer zone electrons in the energy range 12 keV−1.6 MeV were obtained at high altitude in the region of the high power VLF transmitter UMS [300 kW radiated at 17.1 kHz (Watt A. D., 1967, VLF Radio Engineering, Pergamon Press, Oxford)] while a resonant wave-particle interaction was in progress. Additional complementary electron measurements in the range of 36–316 keV were obtained in the drift loss cone by another satellite at low altitude along the drift path 75° east of the interaction region. The data from the low-altitude satellite confirm that UMS was precipitating particles in the inner zone, in the slot, and in the outer zone at the time that the high-altitude satellite was obtaining its data. The high-altitude pitch angle distributions indicate that, for this event, two types of scattering interactions were in progress. Particles with small pitch angles, up to 17.2° at the Equator, were being removed, resulting in an enhanced loss cone. Particles which were mirroring between 6500 km and the altitude of the spacecraft (7200) km were also being strongly scattered, resulting in a relative minimum in the pitch angle distribution around 90°. The data are interpreted as indicating that a cyclotron mode interaction with UMS waves was precipitating electrons with equatorial pitch angles up to 17.2° and that another process, perhaps electrostatic (ES) waves arising from the UMS radiations through a mode-conversion process, was present in the region above 6500 km and was efficiently scattering those particles which mirrored in that region     

Contours of equatorial electrojet current density

A set of 17 rocket measured altitude profiles of the equatorial electrojet current density have been used to determine the parameters of a two-dimensional model of the equatorial electrojet with which the contours of equal current density of the electrojet have been constructed. The contours are in full agreement with contours by other workers constructed from wind models of the electrojet. They show the existence of return (westward) currents of the equatorial electrojet, on both flanks of the dip equator, extending from about 250 km to about 1000 km or more, with a peak at about 500 km–600 km from the dip equator, whose peak intensity is about 30% of the peak intensity of the eastward current at the dip equator. Other evidences of the westward current, the location and intensity of its peak have been mentioned.     

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.     

Low latitude thermospheric meridional winds between 250 and 450 km altitude: AE-E satellite data

The daily variations of the meridional wind at ±18° latitude have been obtained for summer and winter between 1977 and 1979 using the in situ measurements from the Atmosphere Explorer-E (AE-E) satellite. The AE-E altitude increased from about 250 to about 450 km during this period, with solar activity increasing simultaneously. Data are presented at three altitudes, around 270, 350 and 440 km. It was possible to average the data to obtain the 24 h variations of the meridional wind simultaneously at northern and southern latitudes and thereby study the seasonal variation of the meridional wind in the altitude range covered. Two features are found showing significant seasonal variation: (a) a late afternoon maximum of the poleward wind occurring only in winter at 1800 LT at all three altitudes; (b) a night-time maximum in the equatorward wind—the summer equatorward wind abating earlier (near 2130 LT) and more rapidly than the winter wind (after 2300 LT). Furthermore, in summer the night-time wind reaches higher amplitudes than in winter. The night-time feature is consistent with the observed seasonal variation of the equatorial midnight temperature maximum, which occurs at or before midnight in summer and after midnight in winter, showing a stronger maximum in summer. The observed night-time abatement and seasonal variations in the night-time winds are in harmony with ground based observations at 18° latitude (Arecibo). The time difference found between summer and winter abatements of the night-time equatorward wind are in large part due to a difference between the phases of the summer and winter diurnal (fundamental) components, and diurnal amplitudes are larger in summer than in winter at all threee altitudes. However, the higher harmonics play an important role, their amplitudes being roughly 50% of the diurnal and in some instances larger. The 24 h variation is mainly diurnal at all altitudes in both summer and winter, except in winter around 2700 km altitude where the semi- and ter-diurnal components are approximately equal to or larger than the diurnal.     

Parachute measurements of positive ion density of the middle atmosphere over the dip equator by spherical probe

A parachute-borne gridded spherical probe has been used to measure the total positive ion density. Two launches were made, using Soviet M-100 rockets, on 22 and 29 April 1987, at 1200 UT, from an equatorial station, Thumba (8°N, 76°E) India. Data were obtained for the altitude region 10 to 80 km. A broad maximum around 15 km and a broad minimum around 60 km have been noticed in the ion current profiles obtained in both flights. The theory of the operation of the probe has been given. A detailed discussion of the results obtained has also been included.     

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.     

Optical interferometer measurements of thermospheric temperature at Kavalur (12.5°N, 78.5°E), India

First results on the behaviour of thermospheric temperature over Kavalur (12.5°N, 78.5°E geographic; 2.8°N geomagnetic latitude) located close to the geomagnetic equator in the Indian zone are presented. The results are based on measurements of the Doppler width of O(¹D) night airglow emission at 630 nm made with a pressure-scanned Fabry-Perot interferometer (FPI) on 16 nights during March April 1992. The average nighttime (2130-0430 IST) thermospheric temperature is found to be consistently higher than the MSIS-86 predictions on all but one of the nights. The mean difference between the observed nightly temperatures and model values is 269 K with a standard error of 91 K. On one of the nights (9/10 April 1992, Ap = 6) the temperature is found to increase by ~250 K around 2330 IST and is accompanied by a ‘midnight collapse’ of the F-region over Ahmedabad (23°N, 72°E, dip 26.3°N). This relationship between the temperature increase at Kavalur and F-region height decrease at Ahmedabad is also seen in the average behaviour of the two parameters. The temperature enhancement at Kavalur is interpreted as the signature of the equatorial midnight temperature maximum (MTM) and the descent of the F-region over Ahmedabad as the effect of the poleward neutral winds associated with the MTM.     

A study of equatorial waves in the Indian zone

An equatorial wave campaign was conducted at Trivandrum (8.5°N, 77°E), Minicoy (8.3°N, 73°E) and Port Blair (11.7°N, 92.7°E) during June-July 1988. The campaign provided balloon-measured daily wind profiles at all the three stations for 48 days in the 0–30 km altitude range and rocket-measured daily wind profiles at Trivandrum for 42 days in the 31–60 km altitude range. Using these daily wind data a study was made on different equatorial wave modes present in this region. The study revealed evidence of Kelvin waves with period 12–16 days and vertical wavelength ∼ 10 km in the lower stratosphere, with period 6–9.6 days and vertical wavelength of ∼ 10–15 km in the stratospheric-lower mesospheric region and MRG waves with periods 4–4.4 days and vertical wavelength of 10 km in the upper troposphere and lower stratosphere.     

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.     

Tropical behavior of mesospheric ozone as observed by SMM

The seasonal behavior of low latitude mesospheric ozone, as observed by the SMM satellite solar occultation experiment, is detailed for the 1985–1989 period. Annual as well as semi-annual waves are observed in the 50–70 km altitude region. In the latitude range of ±30 the ozone phase and amplitude are functions of temperature and seasonal changes in solar flux. Temperature is the controlling factor for the equatorial region and seasonal changes in solar flux become more dominant at latitudes outside the equatorial zone (greater than ±15). There is a hemispheric asymmetry in the ozone annual wave in the 20 30 region, with northern hemispheric ozone having a larger amplitude than southern hemispheric ozone. In this region temperature is nearly in phase with ozone in both hemispheres and is reduced in amplitude in the northern hemisphere. The equatorial region is characterized by a strong semi-annual wave in addition to the annual variation, and temperature is nearly out of phase with ozone. At all latitudes there is a larger ozone concentration at sunrise than at sunset. The sunrise sunset difference increases with increasing altitude     

Small-scale ionospheric structures associated with mid-latitude spread-F

Slant-F traces on ionograms recorded by a modern ionosonde in a sunspot-minimum period have revealed the existence of field-aligned irregularities at times of spread-F occurrence. This appears to be the first investigation in a mid-latitude region around 36° (geomagnetic) to detect these irregularities at F₂-region heights using an ionosonde. Although such traces were observed frequently near sunspot minimum they were seldom recorded for periods close to sunspot maximum. Also, for a specific spread-F event in August 1989, both the ionograms from the modern ionosonde and scintillations of 150 MHz transmissions from a Transit satellite indicate the existence in the ionosphere of periodic structures (period around 11 min). The scintillation recording also included rapidly fading signals indicative of small-scale structures. The satellite had a path close to the magnetic meridian which passed through the recording station (Brisbane, Australia). Because of the enhanced signal fluctuations in the scintillation recording on this occasion it seems likely (with the support of other evidence on the ionograms) that the small-scale structures present were field-aligned.     

Broad-band auroral VLF hiss and inverted-V electron precipitation in the polar magnetosphere

A polar map of the occurrence rate of broad-band auroral VLF hiss in the topside ionosphere was made by a criterion of simultaneous intensity increases more than 5 dB above the quiet level at 5, 8, 16 and 20 kHz bands, using narrow-band intensity data processed from VLF electric field (50 Hz–30 kHz) tapes of 347 ISIS passes received at Syowa Station, Antarctica, between June 1976 and January 1983.The low-latitude contour of occurrence rate of 0.3 is approximately symmetric with respect to the 10–22 MLT (geomagnetic local time) meridian. It lies at 74° around 10 MLT, and extends down to 67° around 22 MLT. The high-latitude contour of 0.3 lies at invariant latitude of about 82° for all geomagnetic local times. The polar occurrence map of broad-band auroral VLF hiss is qualitatively similar to that of inverted-V electron precipitation observed by Atmospheric Explorer.(AE-D) (Huffman and Lin, 1981, American Geophys. Union, Geophysics Monograph, No. 25, p. 80), especially concerning the low-latitude boundary and axial symmetry of the 10–22 h MLT meridian.The frequency range of the broad-band auroral VLF hiss is discussed in terms of whistler Aode Cerenkov radiation by inverted-V electrons (1–30 keV) precipitated from the boundary plasma sheet. High-frequency components, above 12 kHz of whistler mode Cerenkov radiation from inverted-V electrons with energy below 40 keV, may be generated at altitudes below 3200 km along geomagnetic field lines at invariant latitudes between 70 and 77°. Low-frequency components below 2 kHz may be generated over a wide region at altitudes below 6400 km along the same field lines. Thus, the frequency range of the downgoing broad-band auroral hiss seems to be explained by the whistler mode Cerenkov radiation generated from inverted-V electrons at geocentric distances below about 2 R_E (Earth's radius) along polar geomagnetic field lines of invariant latitude from 70 to 77°, since the whistler mode condition for all frequencies above 1 kHz of the downgoing hiss is not satisfied at geocentric distance of 3 r_e on the same field lines.     

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.     

Spread-F and ionization anomaly belt

The present investigation attempts to bring out the dynamics of the F-region at magnetic equatorial and low latitudes in the American zone. Data are examined for two sets of nights, one with strong range-type spread at Huancayo another with complete absence of spread-F. A prominent bulge of the F-region was observed within and below a latitude 10°N in the evening hours of the spread-F nights. Contours of electron distribution during post-sunset hours at the equatorial latitude, Huancayo (Dip 2°N); low latitude, Talara (dip 13°N); and a location near the anomaly crest location, Panama (dip 38°N), indicated a much steeper gradient in electron density at fixed heights on spread-F nights compared to a rather low gradient on the nonspread-F nights. Enhanced concentration of electrons at the anomaly crest location Panama, and a lower density at the equatorial location Huancayo, were observed on spread-F present nights. This is attributed to the phenomena of an evening plasma fountain in operation at equatorial latitudes on spread-F nights.