The principal one is that after you exclude geomagnetic storms, violent thunderstorms, and typhoons, we don't know why most of the remainder of the sudden declines in the electron density of the ionosphere occur.
最主要是，即使排除了太陽磁爆、劇烈雷雨或颱風等因素，仍有大半的電離層濃度驟減是不明原因的。