Hemispheric asymmetries in the longitudinal structure of the low-latitude nighttime ionosphere

Abstract

Observations performed with the Low Resolution Airglow and Aurora Spectrograph (LORAAS), which flew aboard the Advanced Research and Global Observation Satellite (ARGOS), show evidence of wave number four longitudinal variations in the nighttime equatorial anomaly during March 2001 and March 2002. LORAAS limb scans of O I 135.6 nm emission features are used to reconstruct electron density profiles from which maps of the monthly average peak F region electron densities and heights at 0230 LT are generated. The longitudinal structure is nearly identical in both years. The locations of the longitudinal maxima in the Northern Hemisphere match that of previous studies, but the LORAAS data show a pronounced hemispheric asymmetry in the locations of the maxima in the Southern Hemisphere. The LORAAS results are compared with the International Reference Ionosphere (IRI-90) to show the derived electron densities and peak altitudes are consistent with the climatology, but that the observed longitudinal structure is not. A physics-based model of the ionosphere (Sami is Another Model of the Ionosphere (SAMI2)) is used to study the role of thermospheric neutral winds in generating the observed longitudinal pattern. It is demonstrated that longitudinally varying F region winds along with effects due to the offset of the geographic and magnetic equator are the plausible cause of the hemispheric asymmetry. Together, the effects enhance the observed wave number four pattern in the nighttime ionosphere. Copyright 2008 by the American Geophysical Union.