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

Abstract

We address the need for an easily observable indicator of thermospheric state, and particularly one sensitive to the neutral concentration ratio, [O]/[N 2 ]. Accepting foF2 as the observable of choice, several options for a derived thermospheric daily index are considered; these include midday and nighttime mean values of foF2 2 , and others based on the rate of change of foF2 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 solar cycles, derived from data in the National Geophysical Data Center Ionospheric Digital Database (NGDC/IDD) and similar databases. There are well‐defined dependencies on solar activity, season, geography, and storm‐disturbance level. We compare the indices to ESRO4 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 and season, satellite/ground‐site proximity, satellite altitude, etc. Using median foF2 2 to compare our sunrise index to the Mass Spectrometer‐Incoherent Scatter (MSIS‐90) model atmosphere at a constant pressure level, we find similar latitude variations in three longitude sectors and two solar cycle extremes. A successful index of [O]/[N 2 ], available daily from monitoring ionosondes (and perhaps from Global Positioning System. Total Electron Content (GPS/TEC) monitors, derived similarly), would have important applications in aeronomy, space weather, and telecommunications forecasting.