Evidence for short spatial correlation lengths of the noontime equatorial electrojet inferred from a comparison of satellite and ground magnetic data

The current density of the noontime equatorial electrojet (EEJ) as determined from CHAMP data is highly variable between successive passes of the satellite, which are separated by 23° in distance and 93 min in time. An open question is to which extent this variability is caused by temporal or spatial variations in the ionosphere. Another important question is the connection between EEJ and global solar‐quiet (S q ) current systems. We try to answer these questions by comparing the EEJ current density estimated from high‐quality scalar magnetic field measurements of the CHAMP satellite with the magnetic horizontal intensity variations at six equatorial observatory pairs distributed across the globe. Data taken during the period 2000–2002 were used for the present study. We apply corrections for the effect of local time (LT) and S q fields. By estimating the correlation coefficients between the ground and satellite data as a function of distances between measurements, new insights into the spatial structure of the EEJ have been obtained. The high correlation, when CHAMP passes directly over an observatory, decays quickly in eastern and western directions. Typically, within ±15° of longitudinal separation between satellite and observatory, the correlation falls well below the statistical significance level. This observation holds for all longitude sectors. Interestingly, the correlation between CHAMP‐inferred EEJ strength and observatory differences breaks down for the observatory pairs, outside of a ±4° latitudinal band. This implies that the EEJ and S q variations are uncorrelated for periods up to 1 hour. Additionally, it was found that monitoring of the EEJ can be performed best if the reference observatory is 4° to 5° apart from the dip equator.