Seasonal variations of the ionospheric electron densities retrieved from Constellation Observing System for Meteorology, Ionosphere, and Climate mission radio occultation measurements

Abstract

We collected the ionospheric electron density ( N e ) profiles from the FORMOSAT‐3/COSMIC (F3/C) radio occultation measurements to investigate the seasonal behaviors of daytime N e in the altitude range of 200–560 km. Harmonic analysis of the N e at different altitudes provides unprecedented detail of the seasonal behaviors of N e at low solar activity (LSA). Global maps of seasonal harmonic components indicate that there are strong annual and semiannual variations in daytime N e , which have distinct latitudinal and altitudinal dependency. The semiannual component predominates over the annual variation in the equatorial regions and at high latitudes in the East Asian and South Atlantic sectors at low altitudes, and at higher altitudes the semiannual component predominates in the equatorial region, but recedes in other regions. The semiannual variation peaks in equinoctial months in most regions, while it has maxima in solstice months, first in the South Pacific region (around 30°S, 120°W) at 250 km altitude and expanding over the South Pacific and South Atlantic oceans at higher altitudes. Moreover, there is a region around 45°S, 30°W with a dominant semiannual component, moving toward east‐north with increasing altitude in the range of 200–270 km. These two interesting features are novel but are not reported yet. The relative amplitude of the annual component of N e has hemispheric asymmetry, which is prominent at high altitudes in the Southern Hemisphere. The winter/seasonal anomaly widely exists in the Northern Hemisphere and southern low latitudes and in Indian Ocean region at low altitudes but gradually disappears at higher altitudes. Further, in equatorial regions, a new finding is the obvious wave‐like pattern in the longitudinal structure of the amplitudes of seasonal harmonic components in equatorial regions, which supports possible couplings of sources with lower atmospheric origins in the longitudinal variations of N e .