The ionosphere as a measure of solar activity

Critical frequencies of all regular ionospheric layers vary diurnally, seasonally, with geographic latitude and longitude, and with solar activity so that, for any location and any season urn:x-wiley:00968013:media:tma33214:tma33214-math-0001 where f 0 is the critical frequency, G and H are functions of ( t ), the time of day, and S is the sunspot‐number. For locations where such ionospheric trends are well established, observations of critical frequency may be used to determine an ionospheric “sunspot‐number”. If values of F 2‐layer critical frequency for hours near local noon are used, since these generally have the most pronounced variation with solar activity, the ionospheric “sunspot‐number” obtained is considerably closer to its running‐average value than are ordinary sunspot‐numbers to their running averages. In addition, this measure is practically independent of varying atmospheric conditions and personal variation among observers, and therefore probably presents a more precise index of solar activity.