Earlier studies of ionospheric scintillations outlined the lower boundary of the high-latitude region where intense scintillations at 40 MHz were observed. The quiet-day scintillation boundary reached a lower position of 57° invariant latitude at 2200 LT. Dyson's (1969) recent observations with a Langmuir probe have verified the existence of a lower latitude boundary of small-scale irregularities. The boundary concept has been extended to include the effects of magnetic storms. Observations of satellite beacon signals at 40 and 54 MHz during 1961 to 1966 indicate that the mean change in the lower boundary latitude of the irregularity region is a decrease of approximately 1.6° per unit change in local K index. This is quite similar to the change of 1.8° per unit change in Kp noted for the trough position by Rycroft and Thomas (1970). In examining the data available from high-inclination and synchronous satellites, it was noted that the change in latitude with K index is a function of time. The maximum change of latitude as a function of K index, approximately 2° to 3° per unit K, occurred between 0300 and 0600 LT; the minimum change, about 1° per unit K, occurred over a broad interval from 1600 to 0200 LT. If the irregularity structure is produced by an interaction of the plasmapause with the ionosphere, the morphologic behavior of this region of the magnetosphere can be studied by reviewing the large inventory of scintillation and spread F data that has been amassed in the course of ionospheric research.
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