Phase reversal of the diurnal cycle in the midlatitude ionosphere

The typical diurnal cycle of the midlatitude F region electron density consists of a midday maximum and a midnight minimum. However, a phase reversal of this diurnal cycle has been found to occur in three distinct regions on the globe. They are the East Asian (EA) region centered around (53 degrees N, 150 degrees E), the Northern Atlantic (NA) region centered around (45 degrees N, 50 degrees W) and the South Pacific (SP) region centered around (60 S, 110 degrees W). The intensively reported Weddell Sea Anomaly falls inside the SP region. The phase reversal occurs during March-August in EA and NA regions, and between August and March in SP region, being most prominent in local summer. Furthermore, this diurnal anomaly is more pronounced at solar minimum than at solar maximum, and more pronounced in SP region than in NA and EA regions, in terms of larger diurnal magnitude and more months it lasts in a year. It is emphasized that the diurnal anomaly consists of not only a nighttime enhancement, but also a concurrent noontime depletion. Hence, midlatitude summer nighttime enhancements reported in previous studies are just part of this reversed diurnal cycle. The cause for the phase reversal involves several interplaying physical processes. Among these, the neutral wind combined with the geomagnetic field configuration plays a pivotal role. It generates a one-wave longitudinal pattern at southern middle latitudes and a two-wave pattern at northern middle latitudes, whose wave peaks correspond to the center of the SP, EA, and NA regions, respectively. The seasonal variation of neutral winds and downward motion of the ionization induced by thermal contraction of the ionosphere at sunset may largely control the occurring local time of the nighttime density enhancement and how long it persists in different months. The phase reversal occurs as a result of close ion-neutral coupling. It is further noted that winter anomaly in the EA, NA, and SP regions is very weak or missing.