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.     

Night-time equatorial F-region zonal plasma drifts from VHF backscatter radar observations

Night-time equatorial F-region plasma drifts are deduced from VHF backscatter radar observations of F-region irregularities. The zonal drifts reveal large vertical shears. It is found that the irregularity polarization electric field (though small compared to the ambient field) is significant in affecting the observed zonal drifts.     

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.     

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.     

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.     

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.     

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.     

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.     

Simulation of ionospheric currents and geomagnetic field variations of Sq for different solar activity

Variations of ionospheric Sq electric currents and fields caused by changes in electric conductivity due to changes in solar activity are studied using the International Reference Ionosphere (IRI) model. Calculations are made for R (sunspot number) = 35 and 200 on the assumption of constant (1, −2)mode tidal winds. It is shown that electric fields grow when solar activity is high, because the ratio of the conductivity in the F-region to that in the E-region increases. Currents in the F-region become stronger than those in the E-region, and nocturnal currents are not negligible when solar activity becomes high. F-region currents also play an important role in the westward currents on the high latitude side of the current vortex. The calculated geomagneticH component at the equator has a depression around 1600 LT for R = 35, while it decreases smoothly from 1100 LT to 1900 LT for R = 200. This difference is consistent with the observed geomagnetic field variation. The ratio of total Sq currents obtained by our simulation is about 3.5, which is a little larger than is found in the observed results.     

Nocturnal high-latitude E-region in winter during extremely quiet conditions

The quiet night-time E-region at high latitudes has been studied using the EISCAT UHF radar. Data from three subsequent nights during a long period of low magnetic activity are shown and typical features of electron density are described. The background electron density is observed to be 5·10⁹ m⁻³ or smaller. Two types of enhancements above this level are observed ; one is due to charged particle precipitation associated with the F-region trough and the other is composed of sporadic-E layers due to waves in the neutral atmosphere. The sporadic-E is observed to exist almost continuously and to exhibit a regular diurnal behaviour. In addition to the typical afternoon and morning sequential layers, a third major descending layer is formed at night after the passage of the F-region trough The afternoon layer disappears simultaneously with the enhancement of the northward trough-associated electric field and the night-time layer appears at high altitudes after the field has again been reduced to a small value. It is suggested that metal ions from low altitudes are swept by the electric field to the upper E-region where they are again compressed to the night-time layer. A set of steeply descending weaker layers, merging to the main night-time layer are also observed. These layers are most probably caused by atmospheric gravity waves. Theoretical profiles for molecular ions indicate that the strongest layers are necessarily composed of metal ions but, during times when the layers are at their weakest, they may be mainly composed of molecular ions.     

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.     

A numerical model of the ionospheric dynamo—I. Formulation and numerical technique

A new method of numerically solving a suitably formulated ionospheric wind dynamo equation for electrostatic potential and field is developed. Unlike in many other dynamo models, the upper boundary does not exist and the formulation asymptotically approaches the equatorial boundary condition. Therefore, it naturally incorporates the symmetric, asymmetric E- and F-region dynamo actions in any given ionosphere and any given global or local wind field. It also enables the equation to be posed as an initial value problem and solved numerically using an efficient, accurate, stable and fast integration method of ordinary differential equations. The numerical technique can be extended to compute three dimensional dynamo-generated electric currents in the ionosphere.     

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.     

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.     

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.     

Incoherent scatter observations of thin ionization layers at Sondrestrom

High resolution incoherent-scatter observations of E-region thin (1–3 km) metallic ion layers are presented. Data were collected during three different periods from August 1990 to August 1991, in three different experimental modes. First, the antenna was directed vertically and the entire duty cycle was devoted to Barker coded multi-pulse [Zamlutti (1980) J. atmos. terr. Phys.42, 975–982] measurements to determine the densities and temperatures in the E-region with 300 m resolution. The second experiment measured the F-region electric field as well as the high resolution E-region densities. For the third experiment the antenna was scanned magnetic north-south while only the E-region densities were measured. The experiments were carried out on 16 different nights for a period of 4 h each night at a time near magnetic midnight. Thin ionization layers were observed on 12 of the 16 nights. The first experiment demonstrated that the thin layers are composed of a significant fraction of heavy metallic ions; assuming the layers are composed of a mixture of Fe⁺ and Mg⁺ a composition estimate of 63% Fe⁺ was obtained in one example. The second experiment investigated the relationship between the direction of the electric field and the presence of the thin layers. In these observations thin layers were only present when the electric field was pointed in the magnetic north-west or south-west quadrants, most frequently when the field was near magnetic west. Correlation between layer altitude and field direction was also observed, layers occurring at higher altitudes for fields directed in the north-west, and lower altitudes for fields directed to the south-west. The observations are compatible with the electric field mechanism for thin ionization layer formation. The scanning experiment showed that the layers were of a limited latitudinal extent, typically about 100 km up to a maximum of about 200 km.     

EISCAT observations of tidal modes in the lower thermosphere

EISCAT has made regular measurements of plasma velocity at heights between 101 and 133 km in the E-region and at 279 km in the F-region as part of the Common Programme CP1. Correcting for the effect of the electric field as determined in the E-region, it is possible to estimate the neutral wind velocity in the E-region for a number of days in the period 1985–1987 when magnetic conditions were relatively quiet. These velocities display diurnal and semi-diurnal tidal oscillations. The diurnal tide varies considerably from day to day in both amplitude and phase. The semi-diurnal tide also varies in amplitude but displays a fairly consistent phase at each height and the variation of phase with height below 110 km indicates a dominant (2,4) mode. Above 120 km the variation of phase with height is slower which suggests that at these heights the (2, 4) mode is attenuated and the (2, 2) mode is more important. The results agree well with previous measurements at high latitude.     

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.     

Studies of ionospheric F-region irregularities from geomagnetic mid-latitude conjugate regions

Scintillation data from near Boston, U.S.A., and spread-F data from Argentine Islands, Antarctica are used to investigate the diurnal and seasonal variations of the simultaneous occurrence of medium-scale (~ 1–10 km) irregularities in the electron concentration in the F-region of the ionosphere at conjugate magnetic mid-latitude regions. It is found that these two stations near 52° CGL observe similar irregularity occurrence on ~75% of occasions at night when the data are considered on an hour by hour basis. During solstices, the relationship is dominated by occasions when irregularities are absent from both ends of the geomagnetic field lines; however, at equinoxes, periods of the simultaneous occurrence and non-occurrence of irregularities are approximately equally frequent. During periods of high geomagnetic activity, processes associated with the convection electric field and particle precipitation are likely to be important for the formation and transport of irregularities over these higher mid-latitude observatories. These processes are likely to occur simultaneously in conjugate regions. On days following geomagnetic activity, two processes may be operating that enhance the probability of the temperature-gradient instability, and hence lead to the formation of irregularities. These are the presence of stable auroral red arcs which occur simultaneously in conjugate locations, and the negative F-region storm effects whereby latitudinal plasma concentration gradients are increased; these effects are only similar in conjugate regions. During very quiet geomagnetic periods, F-region irregularities are occasionally observed, but seldom simultaneously at the two ends of the field lines. There is also an anomalous peak in the occurrence of irregularities over Argentine Islands associated with local sunrise in winter. No explanation is offered for these observations. Photo-electrons from the conjugate hemisphere appear to have no effect on irregularity occurrence.