Why Focus on Sun-Earth Connection Events?

Sun-Earth connection events are those which have been observed starting from their origin at the Sun, their interplanetary propagation, and impact on Earth. Compiling all relevant data from various instruments (ground and space), analyzing them, and interpreting them using models help us understand the event in full detail. Such an analysis helps us compare and contrast understand but also compare it with other events so that we can test current ideas on what governs a typical cradle-to-grave event. Identical solar events can result in very different geospace impacts depending on the conditions in the solar wind and geospace conditions. The third largest geomagnetic storm occurred on 2018 August 26, towards the end of solar cycle 24. The storm had a Dst index of -175 nT the third largest in solar cycle. However, the solar source of the storm was an inconspicuous filament channel eruption with no X-ray signature and a faint slow (~126 km/s) coronal mass ejection. It turns out that a combination of three things made this storm so intense: (i) the CME continued to accelerate beyond the coronagraph field of view, (ii) the CME flux rope started out as a low-inclination structure but ended up at the magnetosphere as a high-inclination magnetic cloud indicating large rotation, and (iii) there was a large density enhancement at the back of the cloud that suddenly boosted the ring-current injection and hence the high storm intensity. In addition to illustrating the 2018 August 26 storm, I will introduce other events considered for this session.