THE SUNSPOT CYCLE AND AURORAL F-LAYER IRREGULARITIES

The use of the word ''aurora'' for many different observations at high latitudes has limited the concepts involved; this is particularly tru e for F region irregularities. Observations setting the position of th e auroral oval (Starkov and Fel'dshtein, 1970) were made using primari ly the 555.7-nm green line, which is emitted predominantly at E layer heights. These observations have shown that the change in position of the auroral oval for low values of Kp as a function of sunspot cycle i s of the order of 1 degrees to 2 degrees between sunspot maximum and s unspot minimum. However, irregularities in the F region show much larg er solar cycle variations in the locations of the equatorward boundary , typically 10 degrees. A review of scintillation data indicates that at a given auroral latitude, the scintillation activity increases with sunspot number. In addition, for a constant scintillation intensity, the equatorward boundary moves to lower latitudes as sunspot maximum i s approached. We review existing spread F studies and show that for qu iet geomagnetic conditions, there is lower occurrence during years of low sunspot numbers than during years of high sunspot numbers. However , the spread F index, related to Delta f/f(0)F(2), is higher during ye ars of low sunspot number than during years of high sunspot number. We demonstrate that this apparent dichotomy can be reconciled by using a new method of normalizing the spread F index by the maximum electron concentration of the F layer. We briefly discuss the possible explanat ions for the observed solar cycle variations of irregularity occurrenc e in terms of the absolute values and gradients of electron concentrat ion and the E region conductivity.