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The occurrence of sudden ionospheric disturbances is studied in relation to changes in the conductivity parameter and the conductivity gradient in the lower ionosphere. The analysis is based on phase a•d amplitude data from a great number of such events measured at several very low frequencies. The change of the conductivity parameter was found to be more important as the SID affects deeper regions. The results also suggest that during these anoma/ies the ionospheric conductivity gradient is reduced, generally by small amounts. Such disturbed gradients may be determined if initial conditions for daytime norma/ gradient, •nd for collision frequencies above the D region, are known. A first estimate shows that, for the majority of the events selected, the new gradient changes from the normal daytime value of 0.30 to 0.28 km-L The existence of special kinds of SID is discussed; they produce an important modification in the gradient and a relatively small height variation for the earth-ionosphere waveguide upper boundary.

Solar protons in the events of August 28 and September 2, 1966, produced phase disturbances on the 21.4-kHz VLF transmissions from station NSS, Annapolis, Maryland, to S5o Paulo, Brazil, and also on the 26.1-kHz VLF transmissions from station NPM, Hawaii, to S5o Paulo, Brazil. Both these paths lie well below the polar zone, but partially traverse the geomagnetic anomaly. A quantitative connection between the observed phase advances and satellite observations of the proton flux is used to calculate the length of path affected and the height changes in the disturbed portion of the waveguide. The VLF variations are accounted for by exponential ionospheric models with a reference height equal to the effective reflecting height of the sharply bound model.

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