The highest (see note) frequency which the ionosphere will reflect vertically is called foF2. These foF2 measurements from various sites can be used to create a map of foF2. The data used to produce the map of the USA region are from USAF observing sites and obtained from Space Environment Centre, Boulder Colorado. The above map can be used as a guide to NVIS ionospheric frequency support. Map is updated at approximately 40 minutes past the hour. Note: The geomagnetic field splits a radio wave in the ionosphere into two separate components, termed the ordinary(o) and extraordinary(x) waves. It is the o-wave which is routinely scaled from ionograms.

This HAP map comes from the Australian government.