A statistical survey of low-frequency electric field fluctuations around the dayside cusp/cleft region

Intense low-frequency (0.2 Hz-4.0 Hz) electric field fluctuations are frequently observed over the dayside auroral region. We studied the statistical properties of the spatial occurrence of the intense fluctuations and their dependence on interplanetary conditions based on electric field observations from the polar-orbiting Akebono (Exos-D) satellite. We found that the intense fluctuations are most frequently observed around the prenoon cusp/cleft region at low altitude (5000 km-8000 km) and that the intensity of the fluctuations tends to decrease with increasing altitude, which is less evident on the afternoon oval. The occurrence of the intense fluctuations depends on the spiral angle of the interplanetary magnetic field and on the solar wind velocity. The fluctuations are interpreted in terms of Alfven waves centered on the cusp/cleft region penetrating down to lower altitude without any significant dissipation and quasi-static structures associated with parallel potential drops decoupling the intense high-altitude electric fields from the ionosphere in the afternoon region of the auroral oval.