A study of rocket measurements of ionospheric currents-III. Ionospheric currents at the magnetic dip equator

Abstract

The 31 daytime rocket flight results within 0" or 2" dip latitude provide a good study of several issues leading to the following findings. In this zone, 60 per cent or more of ionospheric current systems have two layers. The lower layer has a peak density of 7 f 4 A k m -' at an altitude of 1 0 6 f 2 k m , a half thickness at half peak of 5.7 f 0.7 km and an altitude extent of 34 f 9 km. The upper layer has a peak density of about 1 A km-' at an altitude of 136 f 8 km and an altitude extent of 19 f 5 km. In some altitude structures, the single layer system is like a hybrid of the lower and the upper layers. The thickness of the equatorial electrojet decreases as its peak current density increases. From this and earlier findings, the electrojet contracts vertically and latitudinally as its peak current density increases. From the sample examined, there is not much difference between the intensities of the lower layers over India and Peru. However, the intensity of the upper layer over India is only about half of its intensity over Peru. The decrease of the electrojet current density in the afternoon is related more to the decrease in the east-west electrostatic field than to electron density or conductivity. A C electric field fluctuations caused by the cross-field instability have been observed in situ in the east-west direction only, in the frequency range of 5 to 120 Hz at altitudes of 85 to 105 km. It is shown that the geometrical and current density structures of the electrojet substantially influence the ratios of its internal to external magnetic field components, all of which initially increase in magnitude with dip distance. Within 0" to 2" dip latitude the ratios are 0.28 f 0.08 for the northward component X, -0.17 f 0.2 for the vertical component 2, and 0.23 f 0.02 for the total field F. A proposed simple measure of day-to-day variability shows promise.

Day-to-day variability of geomagnetic hourly amplitudes at low latitudes

“…The altitude of the peak current density of the upper current layer decreases with latitude from about 136 ± 8 km at the dip equator to about 120 km outside the EEJ zone. From their characteristics, the lower current layer is associated with the EEJ and the upper current layer is associated with the W Sq (Onwumechili 1992a,b). The W Sq is mainly a Pedersen current driven by the zonal component E y of the global Sq dynamo polarization electric field E .…”

“…where SV stands for the sequential variability (Onwumechili 1992a), C stands for D, H and Z and c stands for d, h and z . Only consecutive days are used in obtaining the differences in eq.…”

“…The random variation of the phase of variability of the amplitude of Sq makes it very difficult to study its magnitude. Onwumechili (1992a) therefore defined the sequential variability ( SV ), with which we can conveniently study the magnitudes of the day‐to‐day variability of Sq amplitudes without regard to sign. The sequence of values of SV t in eq.…”

“…(1996). An in situ direct measurement of the day‐to‐day variability of the EEJ current parameters by means of rockets was described by Onwumechili (1992a).…”

Wind dynamo effects on ground magnetic perturbations and vertical drifts

“…Also, the ground magnetic perturbations which are induced by the EEJ in the equatorial region have been observed regularly at several magnetic observatories since the 1920s [e.g., Bartels and Johnston, 1940;Egedal, 1947;Doumouya et al, 1998;Rastogi et al, 2004;Manoj et al, 2006]. In situ rocket measurements and ground and satellite-borne magnetometers have provided extensive information about the structure and characteristics of the EEJ [e.g., Onwumechili, 1992;Jadhav et al, 2002;Lühr et al, 2004;Lühr and Maus, 2006]. In addition, theoretical and empirical models have been built not only to examine physical features and to understand large-scale spatial variation in the EEJ, but to reveal its relations with other ionospheric phenomena [e.g., Richmond, 1973;Doumouya et al, 2003;Klimenko et al, 2007;Alken and Maus, 2007].…”