Post-sunset F-region vertical velocity variations at magnetic equator

The vertical drift velocity of the F-region in the post-sunset period at the magnetic equatorial station Trivandrum has been studied using a HF phase path sounder. The study revealed the presence of quasi-periodic fluctuations with periods in the range 4 30 min superposed on a steady vertical motion as a regular feature of the equatorial F-region in the post-sunset period. The fluctuations in the vertical velocity arc attributed to the east west electric field fluctuations generated by internal atmospheric gravity waves. The vertical velocity fluctuations can provide the necessary seed perturbations for the growth of equatorial spread-F irregularities.     

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.     

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.     

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.     

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.     

On the IRI model predictions for the low-latitude ionosphere

Measurements of ionospheric electron density vertical profiles, carried out at a magnetic equatorial station located at Fortaleza (4°S, 38°W; dip latitude 2°S) in Brazil, are analyzed and compared with low-latitude electron density profiles predicted by the International Reference Ionosphere (IRI) model. The analysis performed here covers periods of high (1979/1980) and low (1986) solar activities, considering data obtained under magnetically quiet conditions representative of the summer, winter and equinox seasons. Some discrepancies are found to exist between the observed and the IRI model-predicted ionospheric electron density profiles. For high solar activity conditions the most remarkable one is the observed fast upward motion of the F-layer just after sunset, not considered in the IRI model and which precedes the occurrence of nighttime ionospheric plasma irregularities. These discrepancies are attributed mainly to dynamical effects associated with the low latitude E × B electromagnetic plasma drifts and the thermospheric neutral winds, which are not satisfactorily reproduced either in the CCIR numerical maps or in the IRI profile shapes. In particular, the pre-reversal enhancement in the vertical E × B plasma drifts around sunset hours has a great influence on the nighttime spatial distribution of the low-latitude ionospheric plasma. Also, the dynamical control exerted by the electromagnetic plasma drifts and by the thermospheric neutral winds on the low-latitude ionospheric plasma is strongly dependent on the magnetic declination angle at a given longitude. These important longitudinal and latitudinal dependences must be considered for improvement of IRI model predictions at low latitudes.     

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

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

On the occurrence of equatorial spread-F in the evening hours

By comparing electron drift velocities at Jicamarca with corresponding ionograms and VHF radio scintillation records at Huancayo it has been shown that the day-to-day variability in the occurrence of equatorial spread-F irregularities in the post-sunset period depends critically on the time of reversal of the Sq electric field. The field reversal before sunset does not produce any spread-F in the evening hours, while the continuation of the day-time electric field for a couple of hours after sunset at normal strength is a favourable condition for generating spread-F.     

Satellite observations of zonal electric fields near sunrise in the equatorial ionosphere

We report here on a number of examples of anomalous enhancements of eastward electric fields near sunrise in the equatorial ionospheric F-region. These examples were selected from the data base of the equatorial satellite, San Marco D (1988), which measured ionospheric electric fields during a period of solar minimum. The eastward electric fields reported correspond to vertical plasma drifts. The examples studied here are similar in signature and polarity to the pre-reversal electric field enhancements seen near sunset from ground-based radar systems. The morphology of these sunrise events, which are observed on about 14% of the morning-side satellite passes, are studied as a function of local zonal velocity, magnetic activity, geographic longitude and altitude. The nine events studied occur at locations where the zonal plasma flow is generally measured to be eastward, but reducing as a function of local time and at satellite longitudes where the magnetic declination has the opposite polarity as the declination of the sunrise terminator.     

Meridional electric currents in the equatorial F-region

A study of the boundary conditions for the equatorial thermospheric transport equations by the authors has led to the theoretical prediction of the vertical electric field at the base of the F-region. Earlier, this result was applied to the calculation of the zonal wind field in the equatorial F-region. In this work, the aforementioned model is applied to the calculation of the F-region electric current field in the meridional plane as a function of time and the east-west magnetic field generated by these currents. In particular, the field at sunset is compared with the observations made by Magsat.     

Equatorial plasma bubble eastward velocity characteristics from scanning airglow photometer measurements over Cachoeira Paulista

Regular observations were carried out of airglow depletion patch motion, associated with equatorial plasma bubble events, using east-west and meridional scan OI 6300 Å airglow photometers over Cachoeira Paulista (22°42′S, 45°W, dip −26°), Brazil. Results from the east-west scan photometer for 31 nights obtained during the period January 1980–February 1981 that are analysed in the present work show that eastward motion of airglow depletion patches is a regular phenomenon in the post-sunset period, extending often to early morning hours (~02 LT), during the summer-equinoctial months. Such moving airglow structures were absent during the winter months (June, July and August). The main results from this first extensive study of the airglow depletion patch zonal movements over Cachoeira Paulista are the following:

• 1.(1) the night-time mean eastward drift velocities for the months January–May 1980 are higher than those for the period September 1980–February 1981;
• 2.(2) plots of eastward velocity versus local time for the second period mentioned in (a) show a decrease of the drift velocity with local time, while for the first group the velocity oscillates around an apparently local time-independent mean value;
• 3.(3)the drift velocities vary in varying degrees with geomagnetic activity index K_p. These results are discussed in the light of other reported measurements of the zonal F-region plasma motions.

     

Plasma convection and auroral precipitation processes associated with the main ionospheric trough at high latitudes

Intervals of F-region electron density depletions associated with the main (mid-latitude) ionospheric trough have been studied using latitude scanning experiments with the EISCAT UHF radar. From 450 h of measurements over a one year period at solar minimum (April 1986–April 1987) the local time of appearance of the trough at a given latitude is observed to vary by up to about 8 h. No seasonal dependence of location is apparent, but troughs are absent in the data from summertime experiments. A weak dependence of trough location on K_p is found, and an empirical model predicting the latitude of the trough is proposed. The model is shown to be more appropriate than other available quantitative models for the latitudes covered by EISCAT. Detailed studies of four individual days show no relationship between local magnetic activity and time of observation of the trough. On all four of these days, however, the edge of the auroral oval, evidenced by enhanced electron densities in the E-region, is found to be approximately co-located with, or up to 1° poleward of, the F-region density minimum. Simultaneous ion drift velocity measurements show that the main trough is a region of strong (> several hundred metres per second) westward flow, with its boundary located approximately 1°–2° equatorward of the density minimum. Within the accuracy of the observations this relationship between the convection boundary, the trough minimum and the precipitation boundary is independent of local time and latitude. The relevance of these results is discussed in relation to theoretical models of the F-reregion at high latitudes.     

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.     

Zonal irregularity drifts and neutral winds measured near the magnetic equator in Peru

Measurements of zonal irregularity drifts were made by the spaced receiver scintillation and radar interferometer techniques from Huancayo and Jicamarca, respectively. The Fabry-Perot Interferometer operated at Arequipa provided the zonal neutral winds. These simultaneous measurements were performed during evening hours in the presence of equatorial spread-F on three nights in October 1988. The zonal drift of 3-m irregularities obtained with the 50-MHz radar showed considerable variation as a function of altitude. The drift of hundreds of m-scale irregularities obtained by the scintillation technique agreed with the drift of 3-m irregularities when the latter were measured near the F-peak. The neutral winds, on the other hand, sometimes exceeded the irregularity drifts by a factor of two. This is a possible result of the partial reduction of the vertical polarization electric field in the F-region caused by the effects of integrated Pedersen conductivity of the off-equatorial night-time E-region coupled to the F-region at high altitudes above the magnetic equator.     

Role of vertical winds on the Rayleigh-Taylor mode instabilities of the night-time equatorial ionosphere

In view of the recent observations on the presence of vertical winds in the equatorial ionosphere in the evening and night-time, the role of vertical winds in the Rayleigh-Taylor (R-T) mode instability has been re-examined. The mathematical treatment of Chiu and Straus, earlier developd for a case of horizontal winds, is extended to evaluate the role of vertical winds in causing the R-T mode instability. It is shown that the vertical (downward) winds of small magnitude have a very significant effect on the instability growth rate in the. F-region. A downward wind of l m s⁻¹ can cause the same growth rate as a 200 m s⁻¹ eastward wind at 260 km altitude. Furthermore, a downward wind of 16m s⁻¹ at 300 km can be as effective as that due to the gravitational drift itself. Similarly, an upward wind can inhibit the instability on the bottomside of the F-region. It appears that the polarity of the vertical winds (upward or downward) at the base of the F-layer plays an important role in the growth of the R-T mode plasma instability in the equatorial ionosphere.     

A servo-model of the night-time F2-layer above St. Santin

A modified form of the ionospheric servo-model is used to describe the night-time F2-layer above St. Santin. Data taken by the incoherent scatter radar on nine nights in 1974–1977 were used to determine the height profiles of electron density, electron and ion temperature and electric field. The servo-model was then used to compute the theoretical variation through the night of the height of the F2 peak and the field-aligned plasma velocity, using gas concentrations and horizontal pressure gradients derived from the MSIS79 atmospheric model. On magnetically quiet nights these calculated values agreed closely with the observations. On disturbed nights, however, the calculations and observations began to diverge an hour or so after the onset of a substorm. The divergence could be explained by an enhanced southward wind.     

The equatorial ring-current energy during magnetic storms as related to Dst activity indices

On the basis of geomagnetic activity indices Dst and solar wind parameters (velocity and density), we calculate the kinetic energy of the equatorial ring-current during the development of a magnetic storm, following the hypothesis of Sckopke's theorem.Supposing that the relaxation mechanisms of the plasmaspheric drift current which lead to the pre-storm state are always present and depend solely on the kinetic energy, it is found that the major part of the energy gain is lost, except during the main phase, when the interplanetary magnetic field permits it to be stored.     

Response of night-time equatorial F-region to magnetic disturbances

The response of the equatorial night-time F-region to magnetic stormtime disturbances has been examined using mainly ionograms recorded at Trivandrum and magnetograms recorded at high, middle and low latitudes during the magnetic storm of 23–26 November 1986. The analysis revealed a close coupling between the equatorial F-region and high latitude magnetic field disturbances originating in solar wind-magnetosphere interactions. The presence of spread-F on ionograms during this period is found to be consistent with the Rayleigh-Taylor instability mechanism for the growth of the irregularities.     

The seasonal variations in the nighttime 6300 Å emission at midlatitudes from ISIS-II spacecraft

Optical limb observations at F-region heights from the ISIS-II satellite have been used to study the seasonal variations in the 6300 Å limb emission for nighttime conditions and the aeronomic implications. The observations were carried out over the American zone at northern midlatitudes, and refer mainly to the period 1973–1975 of low solar activity.The observed seasonal variations in the emission seem to be mainly controlled by the electron density at F-region heights for nighttime and quiet geomagnetic conditions. The winter minimum is found to be deeper than the summer minimum. The obervations give clear evidence of semiannual variation in the emission. The phase variations agree closely with that of the semiannual variations in electron density and neutral atmospheric density at F-region heights. However, the amplitude variations of the semiannual variations are found to be larger than suggested by the observed F-region electron density. The observations during highly disturbed conditions possibly show the presence of gravity waves with wavelengths around 500 km, which could transport auroral energy to lower latitudes. The midlatitude enhancements observed during disturbed conditions seem to be related to the inward movement of the plasmapause.     

Spatial and temporal distributions of midlatitude ionospheric scintillations observed by low-altitude satellites

Spatial and temporal distributions of ionospheric scintillations have been observed at Kashima (36.0°N, 140.7°E) using VHF and UHF signals from low-altitude satellites. From these observations, three different types of prevailing ionospheric scintillations seen from Japan are identified. Scintillations of type I are rather weak scintillations, occur most frequently during the daytime in summer and are primarily associated with the sporadic E-layer. However, considerable occurrences of type I scintillations are also observed during the night in summer and autumn, not necessarily due to the sporadic E-layer but occasionally due to F-layer irregularities which originate from localized midlatitude processes. Type II scintillations are much stronger than type I and occur near the equatorward horizon during spring, summer and autumn. Their occurrences start after sunset, reach a maximum before midnight and decrease subsequently, with a tendency for negative and positive correlations with the magnetic and solar activities, respectively. It is concluded that type II scintillations are the midlatitude aftermath of equatorial plume-associated irregularities and cause trans-equatorial propagation of VHF waves. From observations of type I and II scintillations, the boundary between midlatitude and equatorial scintillations is clearly identified. Type III scintillations are as strong as type II and appear only during magnetically active periods. They can be regarded as another aspect of the severe scintillation events observed on gigahertz waves from geostationary satellites as reported by Tanaka (1981).