What is a compound storm?

Coronal mass ejection is the main driving force of space weather effects, including geomagnetic storms. How its solar-terrestrial spatial propagation characteristics affect the generation and intensity of geomagnetic storms is an important topic in space weather research and prediction, and it is also one of the main obstacles to accurately predict space weather. On September 30th, 20 12, the space weather forecast center of the National Oceanic and Atmospheric Administration (NOAA) predicted a geomagnetic storm of G 1 (small), but it was later revised to G3 (strong) level, which was considered to be caused by a single solar burst event. However, Liu Ying, a researcher at the State Key Laboratory of Space Meteorology, National Space Science Center, Chinese Academy of Sciences, and others, combined with the wide-field imaging observation of stereoscopic satellites and the in-situ measurement of solar wind, proved that this magnetic storm was actually a two-stage magnetic storm, which was produced by two solar bursts separated by more than 60 hours (see figure 1). The second coronal mass ejection (CME) broke out at 23:57 UT on September 27th, 20 12, which was a rapid event. It is further confirmed that there are pulse acceleration stage, rapid deceleration stage and nearly constant speed propagation stage in space propagation between the sun and the earth.