The equatorial electrojet (EEJ) represents an intense electric current flowing along the dip-equator in the ionospheric E region on the dayside. The primary reason for the high current density is the geomagnetic field geometry. Within a narrow band, about 500km wide, where the magnetic field lines are parallel to the Earth's surface, the conductivity is significantly enhanced. These strong currents flowing generally eastward produce a characteristic magnetic signature both on ground and in space.
CHAMP, passing the dayside equatorial region more than 15 times every day, is a very suitable platform for studying the features of the EEJ. Its high-resolution magnetometers allow recording the details of EEJ variations. We succeeded in inverting the magnetic signature such that the current distribution at about 108km height can be recovered (Lühr et al., 2004). From the obtained current profiles it is evident that the intense eastward current at the dip-equator is flanked by weaker westward current 5° to the south and to the north.
Recently, it was possible to investigate the correlation length of the EEJ variations. By comparing ground observations with CHAMP measurements Manoj et al. (2006) showed that the EEJ variations are coherent only over relative short scale length. From their ground-satellite correlation they found an excellent match between the results of ground-based observations and CHAMP when it passed overhead. The agreement drops off, however, rapidly when the longitudinal separation between the measurement points is more then 10°. In north-south direction a decoupling of the EEJ variations occurs already at a distance of 4° in latitude. All this implies that local current drivers play an important role for the intensity of the EEJ.