Climatology and Characteristics of Medium-Scale F-region Ionospheric Plasma Irregularities Observed by COSMIC Radio Occultation Receivers

Medium-scale ionospheric ionization structures are a persistent global feature of the earth's ionosphere. COSMIC radio occultation measurements are well suited to address the incomplete global observational picture of plasma density irregularities, including the global climatology in both bottomside and topside F-region layers, and the irregularity structure in the vertical dimension. A climatological database of F-region ionospheric irregularities and their characteristics (e.g. amplitude, scale size) has been developed through detection of total electron content (TEC) perturbations by Global Positioning System (GPS) receivers onboard COSMIC satellites. COSMIC observations provide unique insight into irregularity characteristics, which complements existing ground-based and in-situ irregularity observations.

This study presents climatological results from four years of data, two during solar minimum (2008-2009) and two during the ascending phase of solar cycle 24 (2012-2013). Irregularities were found to occur frequently at high latitudes, and during nighttime in equatorial to mid-latitude regions in both bottom and topside F-region layers. High latitude irregularity occurrence and intensity is largest in the summer hemisphere in active solar years, and has a significant dependence on solar wind and interplanetary magnetic field conditions. At equatorial and mid-latitudes, longitudinal-seasonal occurrence trends indicate geographically localized gravity wave occurrence or enhancements in plasma instability growth rates. Seasonal occurrence patterns indicate a high occurrence of irregularities in regions corresponding to the solar terminator, confined primarily to altitudes below 300 km. The local time-altitude distributions of equatorial and mid-latitude irregularity occurrence, amplitude, and scale size provide further insight into irregularity generation mechanisms, and include features consistent with "spread-F" irregularities and travelling ionospheric disturbances (TIDs). In addition, the irregularity occurrence and characteristic features in all regions varied with occultation geometry (e.g. orientation of satellite-to-receiver ray path with respect to geomagnetic field orientation), providing further insight into the irregularity structure.