Magnetospheric Behaviour Associated with the Geosynchronous Magnetopause Crossings

High solar wind dynamic pressure accompanied by southward interplanetary magnetic field (IMF) results in a strong magnetospheric compression. The dayside magnetopause may occasionally pass through geosynchronous orbit for one to several hours. Geosynchronous spacecraft may or may not observe corresponding geosynchronous magnetopause crossings (GMCs) depending on their position. However, GMCs can be reasonably well predicted using empirical magnetopause models. In this work, we make a list of GMCs using solar wind OMNI data and Lin et al.'s (2010) magnetopause model. Then we study the magnetospheric response associated with the GMCs using the SME (AE) and Dst indices. We show that both strong magnetic substorms and storms follow GMCs in more than half of the cases. Within the next 48 hours, the minimal hourly Dst drops below -100 nT in 54% of events, and the maximal hourly SME exceeds 1500 nT in 55% of events. The magnetosphere stays relatively quiet with Dst>-50 nT and SME<1000 nT only in 13.5% of events. We find that the average time lag between GMC and maximum of SME/AE is only one hour, while the time lag between GMC and minimum of Dst is 8 hours. We consider GMCs as an initial magnetospheric response to solar wind pressure pulses which often precede large substorms and storms. We speculate that contrary to storms and substorms, GMCs can be better predicted using upstream solar wind data at L1.