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, layers. The so called q type E, produced by plasma instability (type II) process of the equatorial electrojet and the other types of E, 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 drifi 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,, 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. INTRODUC'ITON'Ihere is an increasing interest in the scientific community to understand the permanent or long term changes that take place in the upper layers of the earth's atmosphere. It has been found that the ionosphere-thermosphere-mesosphere system presents better sensitivity to certain aspects of the long term changes than does the lower atmosphere (Rishbeth, 1996, this issue). Such changes could occur from natural such as solar-geophysical processes, secular changes in the earth's interior source of the geomagnetic field, volcanic eruptions, etc., or by anthropogenic sources. Reliable long term observational data base is necessary for investigating such changes. Further the task of identifying an observed change as arising from, say, anthropogenic activities requires detailed assessment of all other possible contributions from natural sources of such changes, and vice versa. In this paper we have focussed attention on aspects of long term changes arising from natural sources. We have examined the long term trend in the sporadic E-layer phenomenon over an equatorial station, Fortaleza, in Brazil using a 21-year (1975-1995) data base. This work which is an extension of a recent study published by Abdu et al. (1996), #at covered only the period [1975][1976][1977][1978][1979][1980][1981][1982][1983][1984][1985][1986][1987][1988][1989][1990], shows that significant changes of permanent nature have taken place for the sporadic E layer development conditions over Fortaleza. Such changes arise mainly from natural processes as will be explained in this paper. SPORADIC E-LAYER DATA SET Different types of E, layers occur in the equatorial electrojet and its low latitude vicinity regions. The q-type layer observed at 105-110 km in the day time equatorial ionograms is a manifestation of the type II irregularities of the equatorial electrojet (EEJ) produced by the well-known gradient E x B instability mechanism (Reid, 1960) that is primarily driven by...

Occurrence and Modeling Examination of Sporadic‐E Layers in the Region of the South America (Atlantic) Magnetic Anomaly

“…The sequence of ionograms analyzed for JAT and SJC showed that the Es h type tends to present a descending movement in the ionosphere until to become an Es c type around~105-120 km. This is a well-known behavior already reported in previous studies (e.g., Abdu et al, 1997;MacDougall, 1974). The occurrence of both Es h and Es c types is larger over JAT than over SJC for all months.…”

“…The sequence of ionograms analyzed for JAT and SJC showed that the Es h type tends to present a descending movement in the ionosphere until to become an Es c type around~105-120 km. This is a well-known behavior already reported in previous studies (e.g., Abdu et al, 1997;MacDougall, 1974 Journal of Geophysical Research: Space Physics and SJC, the Es h and Es c occurrences are almost absent during nighttime, but it is clearly noticed that there is an enhancement after sunrise. A maximum Es h occurrence of~10.3-13.5% is observed between around 12:00 and 15:00 LT during December over JAT, while over SJC, a maximum is not well defined, but it seems to occur during morning hours.…”

“…As mentioned before, the Es f/l type is the most frequent throughout the months analyzed for both stations. The Es f/l occurrence is also higher during nighttime than during the daytime; a typical feature also reported by Abdu et al (, ) over the station of Fortaleza (38°W, 4°S), Brazil. The Es f/l type showed significant increase in the occurrence during postmidnight hours in December (maximum percentage of ~47.2%) and during premidnight hours in June (maximum percentage of ~55.3%) over both JAT and SJC.…”

“…In contrast, other types of Es (also known as blanketing sporadic‐ E ) induced by horizontal neutral winds and wind shear mechanisms (e.g., the l‐ , f‐ , c‐ , and h‐ types) started to be observed more frequently as the dip latitude of Fortaleza changed to the edge of the latitude range covered by the EEJ. Abdu et al () also showed that the solar flux variations could modulate the occurrence rate of different types of Es with a maximum during the peak in the solar activity cycle. Moreover, Abdu et al () presented evidences of frequent nighttime occurrence of blanketing Es around the Brazilian South America (also known as South Atlantic) Magnetic Anomaly (SAMA) region, associated with a regular ionization source caused by energetic particle precipitation.…”

New Findings of the Sporadic E (Es) Layer Development Around the Magnetic Equator During a High‐Speed Solar (HSS) Wind Stream Event

“…Therefore, there is a strong condition (as the density gradient and polarization electric field) to develop a physical mechanism to form only the Es q layers during daytime over BLM. This statement is supported by the observations conducted by Abdu et al (1997) and Resende et al (2013). Some ionograms illustrating the Es layers (red arrows) are shown in Figure 5 on May 05, 2018.…”