Occurrence climatology of F region field-aligned irregularities in middle latitudes as observed by a 40.8MHz coherent scatter radar in Daejeon, South Korea

摘要

A new 40.8 MHz coherent scatter radar was built in Daejeon, South Korea (36.18°N, 127.14°E, dip latitude: 26.7°N) on 29 December 2009 and has since been monitoring the occurrence of field-aligned irregularities (FAIs) in the northern middle latitudes. We report on the occurrence climatology of the F region FAIs as observed by the Daejeon radar between 2010 and 2014. The F region FAIs preferentially occur around 250–350km at 18:00–21:00 local time (postsunset FAI), around 350–450 km near midnight (nighttime FAI), around 250–350 km before sunrise (presunrise FAI), and around 160–300 km after 05:00 local time (postsunrise FAI). The occurrence rates of nighttime and presunrise FAIs are maximal during summer, though the occurrence rates of postsunset and postsunrise FAIs are maximal during the equinoxes. FAIs rarely occur during local winter. The occurrence rate of F region FAIs increases in concert with increases in solar activity. Medium-scale traveling ionospheric disturbances (MSTIDs) are known as an important source of the F region FAIs in middle latitudes. The high occurrence rate of the nighttime FAIs in local summer is consistent with the high occurrence rate ofMSTIDs in that season. However, the dependence of the FAI activity on the solar cycle is inconsistent with theMSTID activity. The source of the F region FAIs inmiddle latitudes is an open question. Our report of different types of FAIs and their occurrence climatology may provide a useful reference for the identification of the source of the middle latitude FAIs.