Geomagnetic Activity Following Interplanetary Shocks in Solar Cycles 23 and 24

Interplanetary shocks are important precursors of interplanetary coronal mass ejections (ICMEs) and corotating interaction regions (CIRs). The shock compression and draping effects on the interplanetary magnetic field (IMF) in sheaths can lead to significant geomagnetic activity. We identified 297 fast forward shocks observed by the Advanced Composition Explorer spacecraft upstream of the Earth, and analyzed their geomagnetic impacts in solar cycle (SC) 23 (1998-2008) and SC24 (2009-2018). The shock (normalized) occurrence rate is found to be significantly higher during SC23 compared to SC24, and it exhibits a stronger correlation with the sunspot number during SC23 (correlation coefficient r = 0.93) than during SC24 (r = 0.86). The average shock compressions of the IMF magnitude and plasma density are approximate to 2.0 and approximate to 2.4, respectively, with no significant correlation with geomagnetic activity. Variations of solar wind parameters and geomagnetic activity indices following the shock arrival are explored. An interval of 6 h (3 days) following the shock is characterized by the average peak values of solar wind speed V-sw = 525 (610) km s(-1), IMF B-z = -6.5 (-11.1) nT, and electric field E-y = 3.5 (6.1) mV m(-1), followed by the average peak geomagnetic indices of Dst = -36 (-83) nT, ap = 56 (92) nT, and AE = 733 (1061) nT. About 25% and 63% of the shocks are followed by geomagnetic storms with Dst <= -50 nT in the following 6-h and 3-day periods, respectively. The percentages of shocks followed by the auroral activity level AE > 500 nT are approximate to 65% and approximate to 96% for the short and long intervals, respectively. For the ap activity level (> 56 nT), the geoeffective shocks are approximate to 30 % and approximate to 60%, respectively. The overall increase in the geomagnetic activity after the shock arrival for the longer shock-preceded interval is possibly due to inclusion of contributions from shock driver (ICME or CIR) fields. It can be concluded that an interplanetary fast forward shock has a probability of 1/4 to be followed by geomagnetic storms, and of 2/3 to be followed by significant auroral activity. We derived probability distribution functions of geomagnetic indices for the 6-h and 3-day intervals following shocks. The results might be important for space weather modeling and applications.