The response of the ionospheric F region to the large solar flare that occurred near 1500 UT on August 7, 1972, has been monitored by means of Faraday rotation measurements made at 17 stations in North America, Europe, and Africa. With observations spanning more than 10 hours in local time and more than 70 deg in latitude, the first truly global morphology of a flare‐induced F region event was obtained. The sizes of the individual sudden increases in the total electron content (Sitec) ranged from 1.8 to 8.6 × 1016 el/m²; on a percentage basis, all the Sitec fell within the 15 to 30% range. No obvious relationship was found between the sizes of the increases and the solar zenith angles at the various subionospheric points, nor between the observed Sitec and the sudden flare effects (SFE) seen on nearby magnetometer recordings. The latitudinal behavior provided the only simple ordering parameter found in the data, the lower latitudes having larger observed increases than the higher latitudes. Millstone Hill incoherent scatter data showed that nearly 40% of the total content enhancement observed at that site came from heights above 300 km. All the Sitec had a rise time of about 10 min, during which the Tec rate of change showed an excellent correlation with the time development of the solar radio burst monitored at 35,000 MHz.

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The values of electron content obtained by observations of Faraday effect for the period July 1961 to October 1964 are presented. These values, when combined with values obtained by other workers at a time when the sun was more active, enable us to study the diurnal, seasonal, and sunspot dependence of the electron content and the slab thickness. These experimentally measured quantities are then related to the physical quantities such as the diffusion coefficient and the electron and ion temperatures by using a diffusion transport model. The seasonal dependence of the electron content and the slab thickness are not in agreement. Possible causes of seasonal anomaly are examined in terms of a diffusion transport layer. The most favored mechanism seems to be the change of composition of the minor constituent O2 or N2. The slab thickness is found to increase with increasing magnetic activities.

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