F-region heating observed during the main phase of magnetic storms

The variation of the F-region electron temperature observed at Millstone Hill during geomagnetic storms that occurred in June, August, and September 1965 is discussed. These measurements, made by using the incoherent (Thomson) scatter technique, show the existence of nocturnal heating of the electrons. In the June storm there is also evidence of an anomalously large heat flux (∼1010 ev/cm²/sec) from the magnetosphere into the F region during the daytime. All periods of heating appear to occur during the main phase of the storm, as observed at Honolulu, and are usually accompanied by marked decreases in the intensity of the total field observed locally. On the basis of the present observations it is not possible to exclude precipitation of low-energy particles as the heating mechanism, but a more likely explanation seems to be that the heat is generated through the production or decay of the ring current and is conducted along field lines into the ionosphere. As such these observations provide support for the heat conduction theory developed by K. D. Cole (1955) to explain the stable red arc.