C/NOFS observations of plasma density and electric field irregularities at post-midnight local times



[1] We report on plasma densities and electric fields measured by the C/NOFS satellite between 10 and 20 June 2008. Midway through the interval, geomagnetic conditions changed from quiescent to disturbed as a high speed stream (HSS) in the solar wind passed Earth. During the HSS passage C/NOFS encountered post-midnight irregularities that ranged from strong equatorial plasma bubbles to longitudinally broad depletions. At the leading edge of the HSS the interplanetary magnetic field rapidly intensified and rotated causing auroral electrojet currents to rise and fall within a few hours. As the electrojet relaxed, C/NOFS witnessed a rapid transition from a weakly to a strongly disturbed equatorial ionosphere that lasted ∼10 hours. Eastward polarization electric fields intensified within locally depleted flux tubes. We discuss relative contributions of gravity-driven currents, overshielding electric fields and disturbance dynamos as drivers of post-midnight depletions.