Quantitative analysis of the relationship between polar ionospheric currents and auroral electrojet indices

The ionospheric currents in the polar region are caused mainly by field-aligned currents. The quiet polar current system consists of a pair of current vertices at dawn and dusk. When substorm occurs, however, intense magnetic disturbances are generated by enhanced polar currents, especially the westward electrojet of a millions Amperes in the auroral oval. The intensity of the auroral electrojet is commonly described by the auroral electrojet indices AL, AU, AE and AO. In this paper the relationship between the polar current system and the auroral electrojet indices is quantitatively studied by means of polar current functions obtained from the data recorded at 71 magnetic stations in the northern polar region during the International Magnetospheric Study (IMS). Fairly well linear correlation of AL (or AU) with AE index has been found, indicating that AE index multiplied by a proper factor can approximately substitute AL and AU indices. The total polar current, JT, and the strengths of the dawn and dusk current vortices, JT1 and JT2, can be characterized by AE index, an increment of 1nT in AE index corresponding to 1000A in JT. A fairly well direct proportion is found between AE and the maximal westward current density, an increment of 1 nT in AE corresponding to 1 A/km of the maximal westward current density at magnetic midnight. The latitudinal profiles of the current density are similar for different local time in night sector. The maximal density of the westward electrojet usually occurs at geomagnetic latitude 65°-70°around midnight, as for the eastward electrojet, it is around 80°. The analysis shows that for 5-mimute mean values, the saturation values of AE and AL are 700 nT and -500 nT, respectively. Accordingly, a caution should be taken when the indices greater than the saturation values would be used for studying magnetospheric or ionospheric processes.