On the mechanism of seasonal and solar cycle NmF2 variations: A quantitative estimate of the main parameters contribution using incoherent scatter radar observations

Seasonal (winter/summer) and solar cycle NmF2 variations as well as summer saturation
effect in NmF2 have been analyzed using Millstone Hill incoherent scatter radar (ISR)
daytime observations. A self‐consistent approach to the Ne(h) modeling has been applied to
extract from ISR observations a consistent set of main aeronomic parameters and to estimate
their quantitative contribution to the observed NmF2 variations. The retrieved aeronomic
parameters are independent of uncertainties in thermosphere and solar EUV empirical
models, and this is a distinguishing feature of the present consideration. Different
temperatures in winter and in summer in the course of solar cycle overlapped on the O++N2
reaction rate coefficient temperature dependence result in different NmF2 dependences
on solar activity: a steep practically linear increase with a tendency to turn up in January
(contrary to international reference ionosphere prediction) and a slow increase with a
tendency to saturate at high solar activity in July despite increasing solar EUV irradiation.
In winter the EUV flux and thermospheric parameters provide approximately equal
contributions to the NmF2 increase, while in summer the contribution of thermospheric
parameters is small. Both in winter and in summer the variations of atomic oxygen [O] are
small at the F2 layer peak, and its contribution is small compared to linear loss coefficient, b.
It is shown that the summer saturation effect in NmF2 under high solar activity is not just
reduced to O/N2 or EUV flux solar cycle variations but is determined by b via the g1
temperature dependence. A new mechanism (qualitative) to explain the December anomaly
in NmF2 is proposed. It is based on the idea that the areas of atomic oxygen production and its
loss are spatially separated and that time is required to transfer [O] from one area to the other
where [O] associates in a three‐body collision. Therefore, under a 7% increase in the O2
dissociation rate due to the Sun‐Earth distance decrease in December–January compared to
June–July, an accumulation of atomic oxygen should take place in the thermosphere in the
vicinity of the December solstice resulting in a 21% NmF2 increase, which is close to the
observed global December effect.

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