Author(s): , , ,
Institution(s): 1. Korea Astronomy & Space Science Institute, 2. National Observatory of Athens
Solar cycle 24 is very modest compared to previous solar cycles. The solar maximum phase may have been reached in the middle of 2014 and the sunspot number has decreased since the beginning of 2015. During this period, it has been reported that only few events produced strong X-class flares, solar proton events, and geomagnetic storms. In this study we have investigated causes of the multiple geomagnetic storms occurred on September 12-13, 2014. The geomagnetic storm forecast model based on the CME observations was used for identification of the causes of the geomagnetic storms. Details of the solar source region were investigated to give an answer why the geomagnetic storms were not so strong even though they were related to fast coronal mass ejections with large earth-ward direction. As a result, we found that the first weak storm was driven by the CME related to M4.6 flare and the second minor storm was driven by one of the fast CMEs related to strong X1.6 flare. Our result shows that the reason why the second storm was not strong is that it was caused by the CME with northward magnetic field. Therefore we suggest that one of the essential ingredients for geomagnetic storm forecasting is to find out the magnetic field direction of earth-ward CMEs, which can be accomplished by investigating magnetic fields of their solar source regions a few days before their arrival to the earth.