Inferring thermospheric composition from ionogram profiles: A calibration with the TIMED spacecraft

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. We present a technique thatdeducesquantitative estimates of thermospheric composition from ionospheric data, for which there is a global network of stations. 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 bythe \textquoteleftG\textquoteright factor, a function of ion production rate and loss rates via ion-atom interchange reactionsand 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 estimatesof the G factor obtained from ionograms recorded at Kwajalein (9oN, 167.2oE) for 25 times during which theTIMED spacecraftrecordedapproximately co-located measurements of the neutral thermosphere.We find alinear relationship between √G and the molecular: atomic composition ratio,with agradient of 2.23 \textpm0.17 and an offset of 1.66 \textpm 0.19. This relationship reveals the potential for using ground-based ionospheric measurements to infer quantitative variations in the composition of the neutral thermosphere. Such information can be used to investigate spatial and temporal variations in thermospheric compositionwhich in turn has applications such as understanding the response of thermospheric composition to climate change and the efficacy of the upper atmosphere on satellite drag.