Abstract
Abstract. We present a method for augmenting spacecraft measurements of thermospheric composition with quantitative estimates of daytime thermospheric composition below 200 km, inferred from ionospheric data, for which there is a global network of ground-based stations. Measurements of thermospheric composition via ground-based instrumentation are challenging to make, and so details about this important region of the upper atmosphere are currently sparse. The visibility of the F1 peak in ionospheric soundings from ground-based instrumentation is a sensitive function of thermospheric composition. The ionospheric profile in the transition region between F1 and F2 peaks can be expressed by the “G” factor, a function of ion production rate and loss rates via ion–atom interchange reactions and dissociative recombination of molecular ions. This in turn can be expressed as the square of the ratio of ions lost via these processes. We compare estimates of the G factor obtained from ionograms recorded at Kwajalein (9∘ N, 167.2∘ E) for 25 times during which the Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics (TIMED) spacecraft recorded approximately co-located measurements of the neutral thermosphere. We find a linear relationship between G and the molecular-to-atomic composition ratio, with a gradient of 2.55±0.40. Alternatively, using hmF1 values obtained by ionogram inversion, this gradient was found to be 4.75±0.4. Further, accounting for equal ionisation in molecular and atomic species yielded a gradient of 4.20±0.8. This relationship has potential for using ground-based ionospheric measurements to infer quantitative variations in the composition of the neutral thermosphere via a relatively simple model. This has applications in understanding long-term change and the efficacy of the upper atmosphere on satellite drag.
Highlights
A small fraction of the Earth’s upper atmosphere, the thermosphere, is ionised, principally by solar extreme ultraviolet and X-ray radiation, to form the ionosphere
Data from the Global Ultraviolet Imager (GUVI; Yee et al, 2003) on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) spacecraft (Kusnierkiewicz, 2003) were used for comparison with the ground-based ionospheric parameters derived in this study
The assumption that the ionospheric profile within the transition region is dominated by atomic ions will only be applicable to mid- to low-latitude stations under conditions of low geomagnetic activity
Summary
A small fraction of the Earth’s upper atmosphere, the thermosphere, is ionised, principally by solar extreme ultraviolet and X-ray radiation, to form the ionosphere. Gardiner et al, 1982), exploiting the fact that ionisation reflects high-frequency (HF) radio waves In this way, detailed records have been obtained of the long-term variation of the ionosphere in response to changes in season, solar activity, space weather events and phenomena such as solar eclipses. In 2018 the Global-scale Observations of the Limb and Disk (GOLD) instrument (Eastes et al, 2017), hosted by the STS-14 commercial spacecraft, was launched into a geostationary orbit from where it makes column-integrated measurements of the thermosphere over an entire hemisphere and height profiles at the limb Despite these advances, information about thermospheric composition is limited to dayside above around 200 km. A comparison with co-located measurements of the thermospheric composition from the TIMED spacecraft provides an opportunity to determine if the shape of the ionogram profile in the F1 to F2 layer transition can be used to infer a quantitative estimate of the thermospheric composition at the same altitudes
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