The north and south symmetry of the ionospheric storms at magnetic conjugate points for low latitudes during the March 1976 severe geomagnetic storms and the relation between daily changes of the storms with geomagnetic activity indices

In this study, the behavior of the ionospheric storms at magnetic conjugate points for two low-latitude observatories during March 1976 severe geomagnetic storms and the relationship between the ionospheric storms and different geomagnetic indices was investigated. For this purpose, hourly ionospheric critical frequency (foF2) data measured from March 19 to April 17, 1976 at Akita and Brisbane stations were used. These stations are magnetic conjugate pairs. The K-p, D-st, a(p), and a(p), (tau) values are geomagnetic activity indicators for this research. The a(p), (tau) reflects an integration of the geomagnetic activity over a number of 3-h intervals, giving more weight to the recent past. The a(p) (tau) values could be better described the magnitude of main phase foF2 deviations using an integration of a(p) that takes into account the recent history of geomagnetic activity. To investigate the response of ionospheric storms at magnetic conjugate points to the geomagnetic activity changes during the severe geomagnetic storms, the foF2 values in the geomagnetic quiet days were subtracted from all the foF2 values and thus the delta foF2 values were obtained. The cross-correlation coefficient (R) between delta foF2 values of these stations was calculated and it was obtained as 0,93. Besides, the cross-correlation coefficients (R) between delta foF2 values at Brisbane and Akita stations with K-p, D-st, a(p), and a(p) (tau) values were calculated. The best the cross-correlation coefficient between the geomagnetic indices and the delta foF2 values at the Brisbane was obtained as 0,89 for a(p) (tau = 0,5) values. However the best cross-correlation coefficient between the geomagnetic indices and delta foF2 values at the Akita was obtained as 0,87 for a(p) (tau = 0,7) values. Next, the variations depending on local time of delta foF2 values at Brisbane and Akita were investigated for both the main phases and the recovery phases of two severe geomagnetic storms. During the main phases of the geomagnetic storms, the delta foF2 values at both Brisbane)and Akita gradually increase with decreasing D-st values. However during the recovery phases of the geomagnetic storms, the mean delta foF2 values (<(delta foF2))over bar> at both Brisbane and Akita have positive values during the local nighttime whereas they have negative values during the local daytime. It is presumed that prompt penetrating electric fields (PPEFs) that penetrate into the magnetosphere and the Earth's ionosphere during the geomagnetic storms are responsible for these results. (C) 2019 COSPAR. Published by Elsevier Ltd. All rights reserved.