Prospects for an ionospheric index of neutral thermospheric composition, with space-weather applications

We address the need for an easily observable indicator of thermospheric sta te, and particularly one sensitive to the neutral concentration ratio, [O]/ [N-2]. Accepting foF2 as the observable of choice, several options for a de rived thermospheric daily index are considered; these include midday and ni ghttime mean values of foF2(2), and others based on the rate of change offo F2(2) during sunrise, which might serve as indicators of [O]/[N-2]. We show the global morphology of a promising sunrise index over more than three so lar cycles, derived from data in the National Geophysical Data Center Ionos pheric Digital Database (NGDC/IDD) and similar databases. There are well-de fined dependencies on solar activity, season, geography, and storm-disturba nce level. We compare the indices to ESR04 satellite global observations of [O]/[N-2] and find correlation coefficients as low as 0.3 (southern winter ) and as high as 0.8 (close passes, all data), depending upon hemisphere an d season, satellite/ground-site proximity, satellite altitude, etc. Using m edian foF2(2) to compare our sunrise index to the Mass Spectrometer-Incoher ent Scatter (MSIS-90) model atmosphere at a constant pressure level, we fin d similar latitude variations in three longitude sectors and two solar cycl e extremes. A successful index of [O]/[N-2], available daily from monitorin g ionosondes (and perhaps from Global Positioning System/Total Electron Con tent (GPS/TEC) monitors, derived similarly), would have important applicati ons in aeronomy, space weather, and telecommunications forecasting.