Multiyear Trends of the Ion Composition and Temperature of the Mid-Latitude Lower Thermosphere

Experimental data on measurements of the nitric oxide NO content in the mid-latitude lower thermosphere demonstrate its significant dependence on solar (and correspondingly) geomagnetic activity, which varies from undisturbed to strongly disturbed conditions up to three or four times or more. Such a dependence of [NO] on heliogeophysical factors cannot but affect the content of NO+ ions, which predominate (together with O_2^ + ions) at heights of 105–120 km. The analysis in the present work has confirmed this assumption: the φ+ = [NO+]/[ O_2^ + ] ratio in the mid-latitude E layer of the ionosphere is indeed intricately correlated with solar activity. At the same time, data from vertical sounding of the ionosphere for 86 years of observations indicates a systematic multiyear increase of the critical frequency of the E layer (foE) at a fixed level of solar activity. Taking into account these circumstances made it possible to estimate the long-term variations of the neutral atmosphere temperature near the peak height of the E layer (hmE = 110–115 km). The calculations showed that between1931 and 2017, the annual mean temperature of the mid-latitude lower thermosphere increased at a linear rate exceeding 0.3–0.5 K/yr.