DIRECT PENETRATION OF THE POLAR ELECTRIC-FIELD TO THE EQUATOR DURING A DP-2 EVENT AS DETECTED BY THE AURORAL AND EQUATORIAL MAGNETOMETER CHAINS AND THE EISCAT RADAR

The quasi-periodic DP 2 magnetic fluctuations (period of 30-40 min) ap pearing coherently at the auroral and equatorial latitudes during the day are analyzed based on the high time resolution magnetometer data r ecorded at the International Monitor for Auroral Geomagnetic Effects ( IMAGE) stations in Scandinavia and at the Brazilian and African equato rial stations. It is shown that the correlation between the DP 2 magne tic fluctuations at both latitudes is excellent (correlation coefficie nt of 0.9). No discernible time shift has been found within the resolu tion of 25 s. The European incoherent scatter (EISCAT) radar observati ons in Scandinavia show that the DP 2 fluctuations at auroral latitude s are caused by an ionospheric Hall current which is controled by the convection electric field. The DP 2 fluctuations exhibit a strong decr ease in magnitude with decreasing latitude, however, it is enhanced co nsiderably at the dip equator with an amplitude comparable to that at the subauroral latitude. The considerable equatorial enhancement of th e magnitude of the DP 2 fluctuations with an enhancement ratio of 4 is due to the concentration of the electric current along the highly con ductive dayside equatorial ionosphere. These observational facts can b e explained in terms of an ionospheric current which is generated by t he magnetospheric electric field at the high latitude and extends to t he equatorial ionosphere almost instantaneously. From the viewpoint of the electric field penetration, we conclude that the magnetospheric e lectric field penetrates to the equatorial ionosphere through the pola r ionosphere almost instantaneously within the time resolution of 25 s . The nearly instantaneous propagation of the electric field to the eq uator can be explained primarily by a parallel plane transmission line model composed of the conductive Earth and ionosphere. In addition to our finding of the fast propagation of the DP 2 electric field, it is found that an impulsive magnetic change with a timescale of 100 s app ears at the dayside dip equator with a time delay of about 10 s, which requires to include the effect of the high conductivity of the daysid e equatorial ionosphere in future studies of the propagation model.