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.     

Field aligned airglow observations of transequatorial bubbles in the tropical F-region

It is possible to form images of the tropical F-region ionization structures, variously labelled as ‘bubbles’, ‘plumes’, or ‘depletions’, in a plane perpendicular to the magnetic field by observing the airglow emissions associated with them in a field aligned direction. Structures which are present at altitudes from 250 km to more than 700 km above the dip equator map down to the 250–350 km region, where recombination and associated airglow emissions occur, ranging from the equator to dip latitudes of 15° or more. The structures can be viewed in a field aligned direction from sites in the range 17°–23° dip latitude. Measurements with high angular resolution (as small as 0.1° in the meridian) could show structures as small as 2 km. It is possible to make simultaneous measurements in both 6300 and 7774 Å recombination emissions, from which the height h_max of the peak plasma concentration n(e)_max on the field line can be estimated from a ratio of the emission rates. It is possible to make maps of n(e)_max and h_max either by raster scanning the sky in the two emissions or by imaging them onto an imaging detector. Useful data can be obtained from one site over a range of 20° in dip latitude and 10° in dip longitude. Observations in the same magnetic meridian as a backscatter radar system are desirable, as also are observations from near magnetic conjugate points. Imaging characteristics for the observation sites in the range of dip latitude 17°–23° have been calculated.     

Low latitude electrodynamic plasma drifts : a review

Radar and radio measurements have provided detailed information on the dependence of F-region electrodynamic drifts on height, season, solar cycle and magnetic activity. Recently, satellite ion drift and electric field probes have determined the variation of low latitude ionospheric drifts over a large range of altitudes and latitudes. The general characteristics of the quiet time plasma can be explained as resulting from E- and F-region dynamo and interhemispheric coupling processes. The low latitude and equatorial zonal and upward/poleward components of the plasma drift respond differently to geomagnetic activity. Disturbance dynamo effects are responsible for the drift perturbations following periods of enhanced magnetic activity. The prompt penetration of high latitude electric fields to lower latitudes produces large perturbations on the upward/poleward drifts, but has no significant effect on the low latitude and equatorial zonal drifts. A number of processes such as ‘overshielding’, ‘fossil wind’ and magnetic reconfiguration were suggested as being responsible for the direct penetration of high latitude electric fields to lower latitudes. Detailed low latitude and global numerical models were used to study the characteristics of low latitude and equatorial plasma drifts and their response to changes in the polar cap potential drop or in the high latitude field-aligned currents. These models can reproduce the latitudinal variation of the perturbation electric fields and their diurnal variations, but are still unable to account for several aspects of the experimental data as a result of the complexity of the high latitude and magnetospheric processes involved.     

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.     

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.     

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.     

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.     

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.     

Spread-F occurrence in mid- and low-latitude regions related to various levels of geomagnetic activity

The occurrence of spread-F from seven latitude regions for ionosonde stations (78 in all) located from L-shell = 3.3 to 1.05 has been investigated (using the superposed-epoch technique) relative to four different levels of geomagnetic activity. Data for 14.5 years were used. For moderate, high and very-high geomagnetic activity a significant peak in spread-F occurrence is found for the four latitude regions closest to the auroral zone. These peaks are delayed (after the geomagnetic activity) by a matter of days, the delays being greater for the lower levels of activity and also greater for regions further from the auroral zone. Similarly, delayed dips in spread-F occurrence are found for very-low geomagnetic activity. Analyses for the remaining three regions (those closest to the equator) failed to show corresponding delayed peaks or dips in the occurrence of spread-F relative to the appropriate levels of geomagnetic activity. It is suggested that (for the three highest levels of geomagnetic activity) the mechanism which is responsible for the suppression of spread-F in equatorial regions may operate at these low latitudes and thus counterbalance the other mechanism which is responsible for the positive correlation found for the higher-latitude regions.     

Modeling the low latitude ionospheric total electron content

The undisturbed ambient total electron content of the ionosphere in the equatorial region exhibits two characteristic features:

• 1.(i) a longitudinal behavior of the post-sunset variation of the ionization near the crests of the equatorial anomaly
• 2.(ii) an enhancement at lower latitudes following the post-sunset decay. During high solar activity periods the southern crest of the equatorial anomaly in the African longitude sector is characterized by a post-sunset maximum often exceeding the afternoon maximum. In the Indian and other longitude zones, the post-sunset peak is not so prominent. Instead, a ledge is obtained in the corresponding local time period. At lower magnetic latitudes, the ionization decays very rapidly around sunset, but an enhancement lasting 2–4 h is observed afterwards.

Numerical solution of the plasma continuity equation, including the effects of ionization production by solar ultraviolet radiation, loss through charge exchange and transport by diffusion, electrodynamic drift and neutral wind, has been used to investigate the above two features. It is found that the pre-reversal peak of the E × B drift at the magnetic equator around sunset is the dominant mechanism responsible for the post-sunset behavior near the crests of the equatorial anomaly. The zonal wind causes an asymmetry of the total content in the northern and southern hemispheres. In African longitudes, where the magnetic declination is about 20°W, the southern crest is more developed at the expense of the northern counterpart. The north-south asymmetry is practically absent in the Asian sector, with its low (< 5°) declination angle. In the Pacific area, an easterly declination (about 9°E) results in a higher post-sunset ionization at the northern crest, although the asymmetry is less pronounced than that in the African zone. The night-time enhancement at lower latitudes has been found to be controlled by the post-sunset increase in the vertical drift, possibly also modulated by the neutral wind.     

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.     

The effect of vertical drift on the equatorial F-region stability

Owing to the high conductivity along magnetic field lines, the stability of the night-time equatorial F-region is determined by magnetic field line integrated quantities. However, slow vertical diffusion near the magnetic equator plus the rapid increase in ion chemistry rates at lower altitude combine to give a very small positive scale height for the electron concentration on the bottomside of the region. As a result, the field line averaged quantities are reasonably approximated by their equatorial values, provided that the E-region does not contribute significantly. The time-dependent behavior of the growth rate for the Rayleigh-Taylor gravitational instability on the F-region bottomside is examined here as a function of the vertical E × B drift velocity using reasonable chemistry to obtain approximate equatorial vertical profiles of ionospheric parameters. It is found that the growth rate exceeds the chemical recombination rate over most of the bottomside F-layer even without vertical drift, but that a realistic E × B drift can result, after about 1 h, in an increase of this growth rate by an order of magnitude. The absolute growth rate is so small (< 10⁻³ s⁻¹) with zero vertical drift that a seeding mechanism would probably be required for the formation of bubbles. The rapid appearance of bubbles shortly after sunset appears likely only after a period of upward drift, as is observed.     

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.     

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.     

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.     

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.     

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.     

The response of geomagnetic spectral composition to solar wind during storm time

In this paper 16 geomagnetic storms in 1968–1978 recorded at 8 magnetic observatories located from polar to equatorial regions in the λ= 120°E longitudinal zone and its vicinity have been analysed. The horizontal component H traces of 27 h intervals have been sampled once every 1.5 min. The time sequences of the data thus obtained have been processed by the method of digital filtering and maximum entropy spectral analysis (MESA).The results of the analysis are compared with the associated solar wind parameters. It confirms that the geomagnetic disturbances are controlled by the solar wind in several ways, i.e. geomagnetic disturbances respond differently to various solar wind parameters or to the different ranges of them. The north-south component of the interplanetary magnetic field (IMF) B_z., the IMF latitude θ and the solar wind velocity V play the most important part in inducing geomagnetic storms.     

Neutral atom precipitation—a review

The production of energetic neutral atoms by charge exchange of ring current ions with neutral hydrogen in the geocorona was predicted many years ago, and there are now a number of measurements of the effect of the impact of these energetic atoms on the thermosphere. Theoretical models of the process have been developed. The latitude variation of the precipitating flux depends very much on the pitch angle distribution of the ions in the ring current, and on the L shell on which they are located.The production of a belt of trapped particles at low altitude near the magnetic equator may occur when neutral atoms re-ionize and become trapped on impacting the thermosphere, and this belt has been found in particle measurements near the equator and is enhanced during periods of magnetic activity.A region of enhanced optical emission due to precipitating neutrals is found in the thermosphere near the magnetic equator in both disturbed and quiet times, implying a low L value and/or pancake pitch angle distribution for the ring current particles that give rise to these neutrals. An isotropic pitch angle distribution is present in parts of the ring current at time during magnetic storms. This gives rise to neutral atom precipitation at all latitudes, and particularly of particles near 90° pitch angle in the region of SAR arc occurrence, about 10° in dip latitude equatorward of the isotropic region.The rate of energy deposition and the rate of production of ionization in the thermosphere depend on the ion species present in the ring current; their energy spectra, and on the distributions of the ions with L value and pitch angle. The rate of energy deposition may at times reach 10⁻² to 10⁻¹ mWm⁻², sufficient for significant heating and wind generation. The rate of production of ionization in the thermosphere at night may be much greater than that of other low latitude night-time ionization sources.     

Ionospheric changes in response to IMF variations

This paper attempts to summarize the results of investigations of IMF effects on the ionosphere, published mostly in Russian, and to place them in context in up-to-date knowledge of IMF/magnetosphere/ionosphere relationships. Effects of the IMF sector structure and of the IMF B_z component turnings on the ionospheric F-layer are considered, including variations of position of the main ionospheric trough (MIT). The paper includes results of both theoretical calculations and observational data obtained mostly by the Cosmos-900, Intercosmos-19 and Cosmos-1809 satellites at subauroral, middle and low latitudes. The MIT position dependence on longitude has been derived as a background for further study. It has been shown that the nightside winter trough position at the storm growth phase correlates best with K_P index taken with a time delay τ, which is proportional to a disturbance growth rate ΔK_p/Δt. The MIT position dependence on D_st, B_z and B_y is also shown. Two troughs have been found to be formed usually in the storm recovery phase at postmidnight hours: these are the MIT (main ionospheric trough) and RIT (ring ionospheric trough) associated with the DR-current. In general the MIT position's response to B_z southward turnings corresponds well to changes of the amoral diffuse precipitation equatorial edge. For B_z southward turnings the height of the equatorial night-time F-layer lowers, and at equatorial latitudes f_oF2 decreases sharply, the latter effect being most pronounced at 03 LT. Large-scale internal gravity waves arriving at equatorial latitudes from the auroral oval cause intensification of the equatorial anomaly, both in daytime and night-time. A schematic pattern of a global ionospheric response to a magnetic disturbance is constructed using as an example the strong storm on 3–4 April 1979.