An Absence of Equatorial Scintillation Activity Prior to Large Geomagnetic Storms

A recent study has established a technique for forecasting the occurrence or non-occurrence of equatorial scintillation activity after sunset by observing the virtual height of the bottom-side F layer (h’F) from a ground-based digital sounder on the magnetic equator. As measured by the UHF S4 Index at Ancon, Peru, there exists a “threshold” in h’F at 1930 local time (h’Fthr) above which scintillation activity is likely to occur and below which scintillation activity is unlikely to occur. This h’Fthr value increases linearly with increasing F10.7 cm flux values. For an F10.7 cm value of 70, h’Fthr = 260 km and for F10.7 cm flux = 180; h’Fthr= 400 km. Several days prior to large geomagnetic storms, the F10.7 cm flux values begin increasing from their nominally quiet values to much greater values. For example, prior to the October, 2003 Halloween storm (Oct. 5 - 17), F10.7 cm flux was about 100 and beginning on Oct. 19, the F10.7 values began increasing to a value of 298 on the 26th, just prior to the Halloween storm period. The consequence of this increase is to increase the h’Fthr values such that the day-to-day h’F (1930 LT) values begin falling below the h’Fthr values and scintillation activity decreases as measured by the S4 Index at Ancon, Peru SCIntillation Network Decision Aid (SCINDA) site. We present results prior to the Halloween, 2003 geomagnetic storm and the October-November, 2001 geomagnetic storm that support this argument. We compare the average F10.7 cm fluxes, h’Fthr, h’F (1930 LT), and scintillation S4 Index values for the “quiet” period where the F10.7 values are relatively constant with the same averages over the period where F10.7 cm flux is increasing prior to the geomagnetic storms. We find that the average Total Hourly Mean S4 (THMS4) value just prior to the Halloween geomagnetic storm is 50% lower than the average THMS4 value for the “quiet” period. The possibilities for global verification of these results are discussed.