Equinoctial asymmetry in mid-latitude NmF2 noontime peaks: A formation mechanism

Mid-latitude semiannual noontime NmF2 peaks were analyzed at four Northern Hemisphere stations (Boulder, Rome, Wakkanai, Juliusruh) and two Southern Hemisphere stations (Hobart, Port Stanley). The aeronomic parameters responsible for the observed NmF2 variations were determined by solving an inverse problem of aeronomy using the original THERION method.

On average, the NmF2 peak in autumn is larger than the vernal peak in both hemispheres under solar minimum conditions. This observed difference in NmF2 between the two peaks is attributed to variations in thermospheric parameters that are not directly related to solar and geomagnetic activity. While the vernal peak can occur over a span of three months in both hemispheres, the autumnal peak is confined to a shorter two-month period.

The primary factor influencing the difference between NmF2 in the two peaks is the abundance of atomic oxygen [O]. A distinct two-hump NmF2 variation, with a trough in December–January in the Northern Hemisphere, reflects a lower concentration of [O] during this period compared to October–November. This variation is driven by changes in [O] rather than by the solar zenith angle effect.

The empirical MSISE00 model, which is based on observational data, suggests a global increase in total atomic oxygen abundance during the equinoxes. However, this increase cannot be explained by a simple redistribution of [O] within the thermosphere, as it represents a global-scale enhancement of atomic oxygen levels. The most plausible mechanism for controlling the global abundance of [O] in the thermosphere is the downward transfer of atomic oxygen via eddy diffusion.

At present, no alternative explanation sufficiently accounts for the global increase in total atomic oxygen during the equinoxes. This phenomenon remains a key area of interest in understanding the aeronomic processes governing thermospheric composition and its impact on ionospheric variability.