Seasonal and solar cycle variations of the auroral electrojet indices

Using hourly mean auroral electrojet indices for the past 20 years, we exam ine the seasonal and solar cycle variations of the AU and AL indices as wel l as the smaller time-scale fluctuations in these indices. The AU and AL in dices maximize during summer and equinoctial months, respectively. By remov ing the effects of the solar conductance from the AU index, it is found tha t the electric field contribution to the AU index exhibits the same semiann ual variation pattern as the AL index, indicating that the semiannual magne tic variations are controlled by the electric field. Since the auroral elec trojets are mostly Hall currents flowing in the east-west direction, the fl uctuations of the auroral electrojet indices can be interpreted in terms of fluctuations in the north-south component of the electric field and the Ha ll conductance. The AU fluctuation is largely due to that of the electric f ield, while the AL fluctuation is attributed to both the electric field and Hall conductance with their contributions being comparable. The high fluct uation of AL compared to that of AU is attributed to particle precipitation associated with substorm activity. However, the fluctuations of the electr ic field and conductance do not show any noticeable seasonal dependence. Th e variation pattern of the yearly mean AL index follows the mirror image of the AU index during the past 20 years, indicating that the absolute values of the two indices are proportional to each other. This suggests again tha t the electric field is the main modulator of magnetic disturbance. On the other hand, they show a tendency to become higher during the declining phas e of the solar cycle. This is the same variation pattern confirmed from the aa index. However, the fluctuations of the electric field and the Hall con ductance do not show any apparent dependence on the solar cycle. (C) 2000 E lsevier Science Ltd. All rights reserved.