Day to Day Longitudinal Variability of the Nighttime Low Latitude Terrestrial Ionosphere

Abstract

The Earth’s ionosphere is the region of upper atmosphere that is a partially ionized gas. It extends from the mesosphere and through the thermosphere to altitudes ~1000 km where it ultimately merges with the magnetosphere. The strong coupling of the ionosphere to the dense regions below and the solar-driven magnetosphere above make it the most variable component of the atmosphere. Sources of ionospheric variability or “weather” originate from solar and geomagnetic activity and meteorological influences. One motivation for studying the ionosphere is to improve techniques to predict ionospheric weather that affects space-borne and ground-based technological systems used for communication, navigation, surveillance and basic research. Geomagnetic storms can be particularly disruptive leading to significant satellite systems failures. Even quiet-time disturbances, such as scintillation and spread-F events, can impact high frequency radio communications, especially in the equatorial and high-latitude regions. To improve prediction capabilities, a better understanding of the drivers of the variability is needed. In this study we used recent ionospheric measurements, particularly remote ultraviolet (UV) sensing of the airglow, along with recently developed analysis techniques to better characterize the day to day and longitudinal variability of the nighttime low-latitude ionosphere and to advance the understanding of the origins of such variations. We performed a case study of the longitudinal variability in the occurrence of equatorial scintillation on 22-23 March 2002 and found evidence of longitudinal differences in the daytime and evening vertical plasma drifts that may affect the conditions for the occurrence of scintillation. This work prompted an investigation the day to day variability of the nighttime ionosphere using UV remote sensing data from the Low Resolution Airglow and Aurora Spectrograph (LORAAS). UV limb scans from March 2001 and March 2002 were used to determine the density and morphology of the post-midnight (~0230 LT) Equatorial Anomaly (EA), a prominent feature of the nighttime ionosphere. The most variable feature was the latitude and separation of the EA crests (46-67% variation about the mean). The least variable was the height of the peak densities in the EA crests (< 10% variation about the mean). The monthly mean values of the EA features are in agreement with the International Reference Ionosphere (IRI-90), a climatology model. We used the LORAAS data along with a physics-based model of the ionosphere (SAMI2) to investigate a wavelike pattern in the longitudinal variation of ionospheric densities in the EA region. We discovered a pronounced hemispheric asymmetry in the longitudinal variations of the EA crests and showed that this is due to both longitudinally varying thermospheric winds and effects associated with the offset of the geographic and geomagnetic equators.