P. Muralikrishna

Long term trends in sporadic E layers and electric fields over Fortaleza, Brazil

Long term trends in the sporadic E-layer occurrences and associated electric fields over the equatorial station, Fortaleza are analysed using 16 years (1975–1990) of ionosonde data. The secular drift of the magnetic equator and hence that of the equatorial electrojet current (EEJ) to northward of Fortaleza has resulted in marked long term changes in the occurrence characteristics of the different types of Es layers: the q-type (arising from electrojet instability processes) and the 1, f, c and h types (resulting from wind/wind shear mechanisms). Systematic decrease in the occurrence rates of the q-type Es, accompanied with increases in the remaining types of Es, was registered from 1975 to 1990, during which period the magnetic equator drifted to north of Fortaleza by ∼400km. The long term trend manifests competing roles of the equatorial electric field and wind systems, in the generation of the different types of Es layers as a function of the distance from the electrojet center. The F-layer dynamo electric field is seen to exercise strong control of Es layer formation near sunset. Solar cycle variations, represented by the Fl0.7cm flux, seems to be present in elctrojet intensity and the lower thermospheric wind system.

On the deviations of measured electron density profiles from IRI predictions for low latitudes

In situ electron density profiles obtained from the equatorial stations in Brazil using conventional Langmuir probes and High Frequency Capacitance probes are compared with the IRI predictions in the light of the spectral distribution of the plasma density irregularities observed. Plasma instability mechanisms, especially the Rayleigh-Taylor and the Cross-Field instability mechanisms are considered responsible for the generation of the plasma irregularities observed at different height regions along the electron density profile, in the post sunset ionosphere. A comparative study of the observed characteristics of the plasma irregularities with those expected from the theories can give us information on the reliability of the observed profiles. This reliability estimate is all the more important because of the fact that the techniques used for the measurement of electron density are known to be associated with some inherent problems. The presence or absence of plasma irregularities in a given height region can, therefore, give us information as to whether the deviations of the observed electron density profiles from the IRI predictions are genuine or not. From this comparative study one can also obtain information as to what are the physical parameters responsible for the observed deviations in the profiles and thereby suggest some possible improvements in the methods used for IRI predictions for low latitudes.

Equatorial F-region irregularities generated by the Rayleigh-Taylor instability mechanism: rocket observations from Brazil

Several sounding rockets carrying plasma diagnostic experiments were launched from the Brazilian rocket launching stations in Natal (5.9 ◦ S, 35.2 ◦ W Geog. Lat.) and Alcˆ (2.31 ◦ S, 44.4 ◦ W Geog. Lat.). Langmuir Probes (LP) were used to measure the height profiles of electron density and electron temperature and High Frequency Capacitance (HFC) probes were used to measure electron density. The LPs were also used to measure the amplitude of electron density fluctuations. A detailed study of the characteristic features of electron density irregularities in the scale size range of less than a meter to a few kilometres, observed by rocket-borne electron density probes under different ionospheric conditions is presented here. The main objective of these studies is the identification of these irregularities from the point of view of the dynamic and electrodynamic processes responsible for their generation. The existing theories of the Rayleigh Taylor Instability (RTI) mechanism applicable for conditions of the geomagnetic equatorial ionosphere over Brazil are critically examined in the light of the observations. The observed characteristics of the irregularities during four rocket launches are compared with the theoretically expected characteristics. These results confirm the operation of a cascade process that is responsible for the generation of a wide spectrum of irregularities. The large-scale irregularities are first generated probably by the RTI mechanism that creates conditions favourable for the operation of the Cross-Field (CFI) or the Gradient Drift Instability (GDI) mechanism. From the k-spectra of irregularities it is possible to obtain the spectral index n corresponding to the height chosen. In most of the cases the estimated spectral indices agree well with those published in the literature for the cases of irregularities generated by the GDI mechanism.

Spectral features of E- and F-region plasma irregularities as observed by rocket-borne electron density probes from Brazil

The height variation of the ionospheric electron density was measured with rocket-borne electron density probes from Alcˆ (2.31 ◦ S; 35.2 ◦ W) in Brazil. A Black Brant X sounding rocket was launched on 14-th October 1994 at 19h55min (LT) to investigate the phenomenon of high-altitude equatorial spread-F events. Ground equipments were operated during the campaign to ensure that the rocket was launched under conditions favorable for the generation of plasma bubbles in the F-region. The electron density was measured by three different types of probes. A High Frequency Capacitance probe (HFC) gave density data with low height resolution, while a conventional Langmuir Probe (LP) and a Plasma Frequency Probe (PFP) measured the electron density and the spatial fluctuations in it. The k -spectra of the plasma irregularities were obtained by the spectral analysis of the electron density fluctuation data. An important feature observed was the continuous presence of plasma irregularities of a large range of vertical scale sizes in the altitude range of 340 km to 817 km. The electron number density varied considerably in these spatial structures, for example a decrease by a factor of 2.6 in a vertical extension of 1 km near the altitude of 497 km. Near 535 km altitude the electron density increased by a factor of 1.8 within a height range of 2.7 km. Density structures of vertical scale sizes in the range of hundreds of meters also were observed superposed on the large-scale structures. During the rocket upleg two height regions of intense irregularities were observed, one between 366 and 480 km and the other between 684 and 812 km. The Langmuir Probe (LP) could make measurements of irregularities of vertical scale sizes more than 8 m in these height ranges, while the Plasma Frequency Probe, could make measurements of irregularities of vertical scale sizes as small as 0.5 m. Spectral features of these irregularities as observed by the two plasma probes at different height regions are presented and discussed here.

Permanent changes in sporadic E layers over Fortaleza, Brazil

Abstract Data available on sporadic E layers in ionograms, from 1975 to 1995 over Fortaleza (38°W, 4°S) in Brazil are analysed as an extension of a previous study to examine permanent changes in the occurrence characteristics of the different types of E s layers. The so called q type E s produced by plasma instability (type II) process of the equatorial electrojet and the other types of E s produced by wind/wind shear mechanism are studied separately. The results show that while the occurrence of the former variety steadily decreased during the period of analysis, the latter type showed marked increases. These characteristics are shown to be associated mainly with the steady drift of the magnetic equator to northward of Fortaleza at a rate of ∼ 0.2 degree per year. The long term trends are, however, modulated by solar activity cycle in F 10.7 flux. Also there is a suggestion of a possible effect from the June 1991 Pinatubo volcanic eruption. The competing roles of the neutral winds and electric fields in the formation of the low latitude sporadic E layer is demonstrated.

Global scale Earth's upper atmosphere monitoring by SACI-1 micro-satellite

Abstract The first Brazilian scientific micro-satellite, SACI-1, is scheduled to be launched in the 2nd semester of 1999. It will be placed in a sun-synchronous polar orbit at a height of 750 km. Four scientific payloads, airglow photometer, high energy particle telescopes, plasma probes and high sensitive magnetometer, are on board to monitor the Earths upper atmosphere environment. The main scientific goal is focused on upper atmospheric dynamics, ionospheric plasma irregularities, anomalous cosmic ray and field-aligned electric current system in the magnetosphere. International collaboration will be necessary, and this will be a key point in order to maximize the utilization of the data.