A study of rocket measurements of ionospheric currents-IV. Ionospheric currents in the transition zone and the overview of the study

Abstract

The study of rocket measurements suggests the following conclusions. (i) Ionospheric currents are basically in two layers. (ii) The upper current layer with a steady altitude extent of 18 f 3 km is global and should be regarded as the worldwide part of the Sq current system. (iii) The intense eastward lower current layer at the magnetic dip equator that terminates in a focus within 2" to 4" dip latitude, together with the reverse westward lower current layer that peaks around 5.3 f 0.7" dip latitude, should be regarded as the equatorial electrojet (EEJ). (iv) The EEJ and the worldwide part of the Sq current system are coupled, sometimes overlapping in varying degrees, especially within 0" to 0.5" dip latitude where the overlap often leads to a hybrid current system. (v) The lunar geomagnetic variations result from the modulations of the above current layers and not from a separate current layer. An interesting latitudinal profile of the vertical cross-section of ionospheric currents from the dip equator to about 60" latitude has been presented. Seven of the nine altitude structure parameters of the current layers equatorward of the EEJ current focus differ by 10 to 30 km from their values beyond the EEJ current focus. Six of these altitude parameters tend to recover after their minima at the EEJ current focus and the Sq current focus. The lower current layer is thicker, the upper current layer is higher, and the entire current system is thicker equatorward of the EEJ current focus than beyond it. The half thickness at half of the peak current density of the eastward EEJ is about 2.9 times that of the reverse westward lower current layer, and about 34.3 f 0.7 per cent greater than that of the worldwide part of the Sq current layer equatorward of the Sq current focus. The continuous distribution of current density model has elucidated the latitudinal variations of AF, A H and their ratio in the vicinity of the dip equator. It thus explains some observations of Davis, Burrows & Stolarik (1967) and some discrepancies that accompanied the detection of the meridional currents of the equatorial electrojet.

Relationship between the equatorial electrojet and global Sq currents at the dip equator region

“…Furthermore, since our analysis used daily values, the weak correlation also implies that day-to-day variability in Sq and EEJ is largely uncorrelated. In addition, some studies have suggested that the EEJ has its own circuit whose return paths and intensity variations are different from the global Sq current (Ogbuehi et al 1967;Onwumechili, 1992b). This could further contribute to the weak correlation observed at the dip equator.…”

“…Rocket measurements by Onwumechili (1992b) showed that the EEJ and Sq at the dip equator flow at different altitudes where the effective conductivities, electric fields, and winds are different. This suggests that Sq and EEJ are driven by different factors, which could naturally result in a weak correlation between the two.…”

“…Detailed studies of both currents can be found in Onwumechili (1992b);Stening (1995), and Onwumechili (1997).…”

“…A rocket study by Onwumechili (1992b) has revealed the existence of an intense lower current layer and a weak upper current layer that peak at altitudes of 107 ± 2 km and 136 ± 8 km, respectively. The eastward lower current layer, which consists mainly of a Hall current, is defined as the equatorial electrojet (EEJ), and it practically corresponds to j H .…”