Longitudinal Structure of the Mid- and Low-Latitude Ionosphere Observed by Space-borne GPS Receivers

Abstract

This study presents longitudinal structures of the mid- and low-latitude ionosphere using the GPS radio occultation observation on board the COSMIC satellite mission. The longitudinal structure seen in the equatorial and low-latitude ionospheric regions results from modification of the daily dynamo electric field by upward propagating atmospheric tides that are generated by latent heat release of the tropical rainstorms. Changes of the dynamo electric field modify the equatorial plasma fountain and thereby enhance the equatorial ionization anomaly (EIA). With capa- bility of three-dimensional global ionospheric observation, altitudinal, local time, and monthly variations of this recent discovered fascinating feature are obtained for further understanding of the underlying physical mechanism. Through comparison between electron densities at various altitudes, the longitudinal structure is prominently seen at upper part of the ionosphere. Additionally, COSMIC observations provide three-dimensional structure of the Weddell Sea anomaly which is featured by the greater nighttime electron density than daytime. Not only occurring at the southern hemisphere near the Weddell Sea region of the Antarctica, a similar nighttime density enhancement feature is also found in the northern hemisphere during local summer by COSMIC observations. The anomalous signatures in both hemispheres share very similar characteristics in electron density structure, latitudinal distribution, and appearance time. They are, therefore, categorized as the mid-latitude summer nighttime anomaly (MSNA).