Geosynchronous Magnetopause Crossings and Their Relationships With Magnetic Storms and Substorms. Issue 6 (10th June 2021) - Samsonov, A. A.
Abstract: The paper investigates the strengthening of magnetospheric activity related to geosynchronous magnetopause crossings (GMCs). We make a list of GMC events using the empirical magnetopause model (Lin et al., 2010, https://doi.org/10.1029/2009ja014235 ) and hourly averaged OMNI data and find which solar wind and magnetospheric conditions accompany and follow the GMCs. The GMCs are mostly caused by the impact of interplanetary coronal mass ejections (ICMEs) and/or interplanetary shocks often with a strong increase in the density and a moderate increase in velocity. The average solar wind density during the first GMC hour is higher than 20 cm −3 in 70% cases, while the velocity is higher than 500 km/s in 56% cases. The hourly interplanetary magnetic field (IMF) B Z is negative in 87% cases. The average over all events SMU (SML), Kp, and PC indices reach maxima (minima) in 1 h after the GMC beginning, while the delay of the minimum of the Dst index is usually 3–8 h. These average time delays do not depend on the strength of the storms and substorms. The SML (Dst) minimum is less than −500 nT (−30 nT) in the next 24 h in 95% (99%) cases, that is, the GMC events are mostly followed by magnetic storms and substorms. We compare solar wind and magnetospheric conditions for GMCs connected with ICMEs and stream interaction regions (SIRs). Our study confirms that the ICME‐related events are characterized by stronger ring current and auroral activity than the SIR‐related events. The difference might be explained by the different behavior of the solar wind velocity. Plain Language Summary: With an ever‐increasing number of satellites in geostationary orbit, it is crucial to predict when they will be located outside of the Earth's magnetosphere which shields satellites from the interplanetary medium, including the low‐energy part of the solar energetic particle spectra which can potentially damage exposed satellite systems. Here, using a well‐established empirical model, we show that such satellites are outside of the protective magnetic shield in rare cases of extreme magnetospheric compression, as only 100 cases were identified over 24 years. We demonstrate that most of the cases occur during the times when the Sun is most active and are associated with Interplanetary Coronal Mass Ejections, giant bubbles of plasma occasionally emitted from the Sun and propagating through space. The disturbances of the near‐Earth space associated with the times of extreme magnetospheric compression are analyzed. It is shown that such compressions are followed by local geomagnetic activity at the high‐latitudes in the nightside ("magnetospheric substorm") with an average time delay of 1 h and by global magnetic disturbances ("magnetic storm") with the maximum effect observed by 3–8 h later. Key Points: Geosynchronous magnetopause crossings are studied in connection with solar wind drivers and accompanying geomagnetic activity Dst index often reaches a minimum in 3–8 h after geosynchronous magnetopause crossings Average SML (SMU) index reach minimum (maximum) in one hour after geosynchronous magnetopause crossings