Prereversal Peak Research Articles

Temporal and altitudinal variability of the spread F observed by the VHF radar over Christmas Island

Abstract. The goal of this work is to study the time and altitude echo characteristics under different solar and seasonality conditions using the VHF radar range–time–intensity (RTI) images. The occurrence of equatorial spread F depends on the existence of conditions that can seed the Rayleigh–Taylor instability, and these conditions can change with solar flux, seasonality, longitude distributions, and day-to-day variability. So, the equatorial spread F is observed as its time and altitude occurrence. The VHF radar of Christmas Island (2.0∘ N, 157.4∘ W, 2.9∘ N dip latitude) has been operational in the equatorial region for some time, allowing long-term observations. The occurrence of echoes during solar minimum conditions is observed throughout the night since the post-reversal westward electric field is weaker than the solar maximum and the possibilities of the vertical plasma drift becoming positive are larger. On the other hand, echoes during solar maximum will be controlled by dynamics near the time of the pre-reversal peak (PRE). Our results indicate that the peak time occurrence of echoes along this period shows a well-defined pattern, with echoes distributed as closer to local sunset during solar maximum and around/closer to midnight during solar minimum conditions; meanwhile, the peak altitude occurrence of echoes shows a slightly regular pattern with higher-altitude occurrences during solar maxima and lower altitudes during solar minimum conditions.

Plasma transport process in the equatorial/low-latitude ionosphere

The behaviour of the equatorial/low-latitude ionosphere and the transport processes during magnetic disturbed and quiet periods of a high solar activity year, 2014, in the American sector are investigated. Parameters used include vertical drift (Vz), transport term (W), NmF2, hmF2 and scale-height (H). The F2 plasma variations followed the diurnal local solar pattern, being higher at daytime. The sunset maximum and sunrise minimum peaks of hmF2 were directly opposite to the scale height (H) pattern. The plasma distribution was basically controlled by combined actions of the electrodynamic convection/thermospheric composition, which is geomagnetic activity dependent. The annual, semi-annual and winter-anomalies of the F2 parameters were higher at the dip equator in comparison with the low-latitude. The Vz pre-reversal peak magnitude coincided with hmF2 peak and the effects are more pronounced during geomagnetic disturbed conditions. The transport term pattern was similar to that of the scale height and it is suggested as a proxy parameter for quantifying low-latitude plasma irregularities and distribution of thermospheric composition.

Numerical simulation of equatorial plasma bubbles over Cachimbo: COPEX campaign

The problem of day-to-day variability in onset of equatorial spread F (ESF) is addressed using data from the 2002 COPEX observational campaign in Brazil and numerical modeling. The observational results show that for values of virtual height of the F layer base less than 355km at around 18:35 LT, and for the prereversal peak enhancement of the vertical plasma drift (Vp) less than 30m/s, the spread-F (ESF) was absent on four nights over Cachimbo (9.5°S, 54.8°W, dip latitude=−2.1°). In this work we analyze the geophysical conditions for the generation of the irregularities by comparing the nights with and without the ESF. In the comparison a numerical code is used to simulate plasma irregularity development in an extended altitude range from the bottom of the equatorial F layer. The code uses the flux corrected transport method with Boris–Book’s flux limiter for the spatial integration and a predictor–corrector method for the direct time integration of the continuity equation for O+ and the SOR (Successive-Over-Relaxation) method for electric potential equation. The code is tested with different evening eastward electric fields (or vertical drifts Vp<30m/s and Vp>30m/s) in order to study the influence of the prereversal enhancement in the zonal electric field on plasma bubble formation and development. The code also takes into account the zonal wind, the vertical electric field and the collision frequency of ions with neutrals and the amplitude of initial perturbation. The simulation shows a good agreement with the observational results of the ESF. The results of the code suggest that the instability can grow at the F layer bottomside by the Rayleigh–Taylor mechanism only when the Vp>30m/s. In the analyzed cases we have considered the competition of other geophysical parameters in the generation of plasma structures.

Morphology of F-region vertical E×B drifts in the African sector using ionosonde measurements

F-region vertical velocities are derived from the ground-based ionosonde data for Ibadan (7.4°N, 3.9°E; dip 6°S: an equatorial station in the African zone), to study the general characteristics of electrodynamics of equatorial ionosphere, such as their variation with season, solar cycle, and magnetic activity at different local time sectors. The results show profound seasonal and geomagnetic effects. Except for equinoctial period, there is an excellent consistency in the magnitudes (nearly 20 m/s) and patterns of upward daytime F-region drifts at low and high solar activity periods. Evening F-region exhibits strong motion with absolute mean value for quiet-time (15 m/s) greater than on disturbed-time (10 m/s). The average downward quiet midnight-early morning hours sector value is well below than 10 m/s. The evening reversal time is earliest and latest during solstitial periods. Prereversal peak is season dependent and varies strongly with magnetic activity. We show that prereversal peak, daytime, and nighttime maximum drifts saturate at particular values of F10.7 cm solar radio flux index, effects not noticed with corresponding sunspot number. Our observations confirm several previous results from other equatorial sites utilizing different experimental techniques.

Equatorial F-region vertical ion drifts during quiet solar maximum

Median values of ionosonde h′ F data acquired at Ibadan (Geographic:7.4°N, 3.9°E, Magnetic: dip 6°S, and magnetic declination, 3°W), Nigeria, West Africa, have been used to determine vertical ion drift (electric field) characteristics in the postsunset ionosphere in the African region during a time of high solar activity (average F 10.7 −208). The database spans from January and December 1958 during the era of International Geophysical Year (IGY) for geomagnetic quiet conditions. Bimonthly averaged diurnal variations patterns are very similar, but differ significantly in magnitude and in the evening reversal times. Also, monthly variations of F-region vertical ion drift reversal times inferred from the time of h′ F maximum indicates early reversal during equinoxes and December solstice months except for the month of April. Late reversal is observed during the June solstice months. The equatorial evening prereversal enhancement in vertical ion drift ( V zp ) occurs largely near 1900 LT with typical values ∼20–45 m/s. Comparison of Ibadan ionosonde V zp with the values of prereversal peak velocity reported for Jicamarca (South America), Kodaikanal (India), and Scherliess and Fejer global model show considerable disparity. The changes of postsunset peak in virtual height of F-layer ( h′ F P ) with prereversal velocity peak V zp are anti-correlated. Investigation of solar effects on monthly values of V zp and h′ F P revealed that these parameters are independent of monthly averaged solar flux intensity during quiet-time sunspot maximum conditions.

Equatorial ionospheric vertical plasma drift model over the Brazilian region

Comparison between equatorial inospheric F region vertical plasma drift from satellite measurements [Fejer et al., 1995], for the Brazilian longitude sector, and the drifts derived from ionosonde measurements around sunset shows significant differences on the prereversal peak behavior during solstices of high solar activity periods. Using ionosonde measurements around sunset and satellite measurements at other local times, we constructed an ionospheric vertical plasma drift model that is representative of the equatorial region over the Brazilian longitudes, where the magnetic declination is around −20°. The so derived drift model, here called IDM (ionosonde drift model), is used as an input to the Sheffield University plasmasphere‐ionosphere model (SUPIM). It is shown that the F layer heights given by SUPIM with IDM are in good agreement with ionosonde measurements over the Brazilian longitudes and that IDM better simulates the F layer heights than the averaged drifts given by the satellite drift model [Fejer et al., 1995].

Electric fields in the low latitude F-region

The nighttime equatorial F-region zonal electric field E yF has been deduced from h′ F data at Trivandrum, a magnetic equatorial station in the Indian longitude sector. The nocturnal variation of E yF in different seasons in high and low solar activity periods is presented. It is found that the nocturnal variation of E yF at Trivandrum is quite different in solstices and in high solar activity periods from that in equinox and low solar activity periods. A linear relationship is established between the pre-reversal peak of E yF and 10.7 cm solar flux for the different seasons. The behaviour of E yF at Trivandrum is compared with that reported at Jicamarca, an equatorial station in American longitude sector.

Equatorial F region vertical plasma drifts: Seasonal and longitudinal asymmetries in the American sector

Longitudinal and seasonal asymmetries in the evening ionospheric F region plasma vertical drift (Vz) enhancements, between two longitudinally separated stations situated along the magnetic equator, in the American sector, are investigated under solar maximum conditions, based on results obtained from the analysis of ionosonde data for these stations. The two stations are Huancayo, Peru, and Fortaleza, Brazil, which have markedly different magnetic declination angles. The observed asymmetries are interpreted using a detailed numerical simulation of the E and F region electrodynamic coupling process that takes into account also its asymmetry about the magnetic equator arising from the finite magnetic declination angle. The results of the simulation show, in agreement with observations, that the occurrence time of the evening F region vertical drift prereversal peak and its seasonal variation at a station are controlled by the magnetic declination angle at that station, which determines the seasonal variation of the sunset times (and hence the integrated Pedersen conductivity longitudinal gradient) at its magnetic conjugate E layers. The amplitude of the prereversal peak, on the other hand, undergoes the influence of the magnetic declination angle as well as of the thermospheric zonal wind component.

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.

Spread F plasma bubble vertical rise velocities determined from spaced ionosonde observations

Systematic time differences in the onsets of spread F events in the ionograms are observed between the magnetic equatorial station Fortaleza (4°S, 38°W, dip latitude 1.8°S) and the low‐latitude station Cachoeira Paulista (23°S, 45°W, dip latitude 14°S), two stations in Brazil, located at close‐by magnetic meridional planes (actually some 12° of magnetic longitude apart). On the assumption, justified from different experimental observations, that the spread F irregularities occur in strongly field‐aligned plasma bubbles that extend several degrees on either side of the magnetic equator, and rise up in vertically elongated columns over the magnetic equator, we have related the observed time differences in the onsets of spread F events at the two stations to the plasma bubble vertical rise velocities over the magnetic equator. The vertical rise velocities of the plasma bubble so determined are found to be well within the values measured by VHF radar and satellite techniques, and further show, at times, good correlations with the amplitude of the prereversal peak in the vertical drift velocities and the heights of the evening equatorial F layer. Possible implications of these results are discussed.

Magnetic declination control of the equatorial F region dynamo electric field development and spread F

We have carried out a comparative study of the evening prereversal enhancements in the equatorial F region vertical ionization drift velocities (VZ) over Fortaleza (4°S, 38°W), Brazil, and Jicamarca (12°S, 77°W), Peru, two magnetic equatorial stations in the American zone. The results show profound dissimilarities in the seasonal trends in the times and widths of the VZ prereversal peak, which reflect in the spread F characteristics as well, at the two stations. The dissimilarities are shown to be arising mainly from the difference in the magnetic field declination angles that causes differences in the conjugate E region sunset durations and, hence, in the F region polarization electric field development rates at the two stations.

Some characteristics of spread F at the magnetic equatorial station Fortaleza

Spread F in ionograms of both the range and frequency spread types is studied for the magnetic equatorial station Fortaleza (geographic coordinates 38°W, 4°S; dip latitude −1.7°) for a 1‐year period during 1978–1979). Satellite traces suggesting ionization structures with scale sizes of 100–200 km are found to be necessary precursors to the formation of the irregularities that give rise to range type spread F, whereas frequency type spread F was observed under different circumstances. Striking differences are noted in the spread F characteristics and related parameters over Fortaleza as compared with Huancayo (geographic coordinates 75.3°W, 12.05°S; dip latitude 0.6°) and Jicamarca (geographic coordinates 76.8°W, 11.95°S; dip latitude 0.9°), situated within only ∼38° in longitude west of Fortaleza. The postsunset maximum in h′F, in winter, is delayed by about 1½ hr with respect to other seasons over Fortaleza, as compared with Huancayo, where it occurs earlier in winter than in other seasons. Similar differences also exist in the prereversal peak near sunset in the vertical drift velocity (Vz) between Fortaleza and Jicamarca. The differences are attributed possibly to the large difference in the magnetic field declination and to other factors such as the difference in the magnetic field intensity and the relative separation between geographic and magnetic equators that exist between Fortaleza and the other stations.