The horizontal magnetic field lines at the equator produce a unique current system like that described below. In the dayside equatorial ionosphere, currents driven by tidal wind through the dynamo mechanism cause an accumulation of charges, which are positive at dawn and negative at dusk terminators, and this results in an eastward electric field, , along the magnetic equator. The cross fields of this electric field and northward magnetic field results in an eastward Pedersen current and downward Hall current. The Pedersen current, , flows dominantly at about 130-km altitude in response to the peak Pedersen conductivity there. The downward Hall current leads to an accumulation of charges at the upper and lower edges of the dynamo layer, which results in the formation of an upward polarized electric field, , with a magnitude about 20 times larger than . This vertical polarization electric field induces a strong eastward Hall current, . This Hall current flows and peaks around 110-km altitude in response to the peak Hall conductivity there (Forbes, [1981]; Onwumechili, [1992a]; Prölss, [2004]).
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 . The upper current layer, which consists mainly of a Pedersen current, is thought to be part of the global Sq current and essentially corresponds to . The global Sq current system is characterized by dayside vortices that are counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. Both currents overlap to give the total current at the dip equator: , where σ3 is the Cowling conductivity (Hirono, [1950], [1952]) that perturbs the geomagnetic northward (H) component of equatorial ground magnetometer observations. Detailed studies of both currents can be found in Onwumechili ([1992b]); Stening ([1995]), and Onwumechili ([1997]).
The relationship between the EEJ and Sq current has been studied for many years, but until now, agreement still appears to be lacking on this topic. Some previous studies found a good correlation between them, while others have showed nonexistent or only weak correlations. This conflict likely results from the lack of good continuous data and the difficulty of isolating global Sq and EEJ at dip equator stations. Most studies have used the two-station method to calculate the EEJ as the difference between measurements taken at a dip equator station and at an off-dip equator station, and data from the off-dip equator station are typically used directly to represent the global Sq contribution at the dip equator. In many cases, no significant correlation is obtained as shown in Ogbuehi et al. ([1967]), Okeke et al. ([1998]), and Okeke and Hamano ([2000]). In contrast, studies by Kane ([1971]) and Yamazaki et al. ([2010]), which used the total H component at the dip equator to represent the EEJ (which we hereafter refer to as the total current), revealed a good correlation with Sq at off-dip equator stations. This discrepancy might be understood by the insight that the correlation coefficient between two time series x
1
and x1 + x2 will usually be different from that between x1 and x2 (Mann and Schlapp, [1988]).
One also needs to keep in mind that the EEJ current varies drastically with latitude, especially within ±6° across the dip equator. This fact introduces some uncertainty in the EEJ estimation from ground-based data as it is often impossible to locate the station exactly at the dip latitude. So far, this problem was encountered by all previous researchers on this topic. Furthermore, the Sq current at the dip equator also differs from the one outside this region as the Sq current is also known to vary with latitude. Most previous studies directly used the Sq measured at off dip-equator stations, and this will certainly affect the study of the EEJ-Sq relationship at the dip equator.
In the present study, we reexamined the EEJ-Sq relationship by using long-term ground-based magnetometer data simultaneously from station pairs in three longitude sectors. Furthermore, to overcome the above-mentioned uncertainties due to the latitudinal variation of EEJ and Sq, we normalized the observation data to the dip equator using the CM4 model by estimating peak EEJ and Sq values at the dip equator to yield more accurate results. Additionally, we compared the EEJ-Sq relationship with the total current-Sq relationship. Possible mechanisms are then discussed to explain the results obtained.