The equatorial electrojet (EEJ) is a strong ionospheric current along the magnetic equator driven by the day side eastward electric field. Current strength is affected by two‐stream and gradient‐drift instabilities which pose a serious obstacle to quantitative electrodynamic modeling of the equatorial ionosphere. Using highly accurate radar and magnetic field measurements taken over the past decade, we deduce empirical relations between the observed EEJ and the driving eastward electric field. These indicate that the current strength is largely unaffected by instabilities for eastward fields in the range of −0.4 mV/m to 0.07 mV/m. This is followed by a band of moderate current reduction from 0.07 mV/m to 1 mV/m, consistent with predictions for the gradient drift instability. At even stronger eastward fields, a further current reduction is observed, as expected for the two‐stream instability. These non‐linear empirical relations can be used to correct ionospheric electric field strengths inferred from space and ground‐based magnetic field measurements.
