Abstract:
The ionospheric annual anomaly or nonseasonal anomaly of the ionosphere is characterized by
globally increased ionization in December solstice than in June solstice. Though this phenomenon was
reported several decades ago, the causal mechanisms have not been fully understood till today. In this paper,
the F2 layer peak electron density (NmF2) data from Formosa satellite 3/Constellation Observing System for
Meteorology, Ionosphere, and Climate-radio occultation observations during the low solar activity year 2009
were systematically analyzed to investigate the physical mechanisms responsible for annual anomaly and
its local time, latitudinal, and longitudinal variability. It is found that the annual anomaly is primarily dominant
at Southern Hemisphere at all local times, with significant enhancements at equatorial ionization anomaly
crest latitudes during noon to afternoon hours and at high latitudes during nighttimes. The annual anomaly
in Northern Hemisphere occurs with relatively smaller magnitudes and confined only to morning to early
afternoon hours (08–14 LT). This study brings out the important roles of effective neutral winds due to the
geomagnetic field configuration and the offset between geomagnetic equator and subsolar point for
the enhanced plasma density in the Southern Hemisphere during December that majorly contributes to the
ionospheric annual anomaly. These results provide new insights to the responsible mechanisms behind
the ionospheric annual anomaly and its local time latitudinal, and longitudinal variation.
