Oral Presentation

Reversal of density perturbations near the dawn terminator in association with geomagnetic activity

Author(s): Sunny W. Y. Tam (Institute of Space and Plasma Sciences, National Cheng Kung University, Taiwan), Chien-Han Chen (National Property Administration, Ministry of Finance, Taiwan), Kaiti Wang (Department of Aerospace Engineering, Tamkang University, Taiwan)

Presenter: Sunny W. Y. Tam (Institute of Space and Plasma Sciences, National Cheng Kung University)

Although observations of solar activity and the associated effects in the interplanetary space may enable one to predict periods of geomagnetic disturbances, it is difficult to forecast the resulting effects on our ionosphere. In particular, according to observations reported in the literature, geomagnetic activity may have positive or negative effect on ionospheric electron density at subauroral and middle magnetic latitudes. The observed positive and negative perturbations were responses to multiple driving mechanisms associated with geomagnetic disturbances.

Based on the vertical electron density profiles observed by the GPS Occultation Experiment aboard the FORMOSAT-3/COSMIC satellites from August 2006 to July 2009, a statistical study is performed to examine the influence of geomagnetic disturbances on the ionosphere at subauroral latitudes near the dawn terminator. Three ranges of solar zenith angles are adopted to characterize transitions between the pre- and post-daybreak ionosphere. Results indicate a reversal of plasma density perturbations from darkened to sunlit locations between 50° and 65° magnetic latitudes. The darkened topside ionosphere features density increases associated with geomagnetic activity, while density reductions mark its sunlit counterpart. In the 55°–65° magnetic latitude range, the dominant contributors to the density perturbation are auroral electron precipitation for the darkened region and enhanced penetration electric fields for the sunlit region, with the transition occurring across the terminator local times. At lower magnetic latitudes, dominance shifts first to electric fields at 50°–55°, then to aurora-induced neutral wind at 45°–50°, suggesting that during disturbed times electric fields seldom penetrate below 50°. Findings in this statistical study should contribute to the study of space weather and the understanding of non-local influences of geomagnetic activity on topside dynamics.