Magnetospheric substorms are considered to be one of the major energy release processes as a consequence of the interaction between the solar wind and the Earth’s magnetosphere. The interaction is known to sensitively depend on the direction and magnitude of the north-south component of the interplanetary magnetic field that the solar wind brings together. Substorms are manifested by sudden auroral brightening due to particle precipitation along magnetic field, energetic charged particle generation, ULF magnetic pulsation excitation, abrupt changes of the magnetospheric magnetic field, and magnetic reconnection. One of the intriguing features of substorms is that they sometimes occur quasi-periodically with a period of ~1-3 hrs. This is commonly observed when corotating high-speed solar streams lasting several days impinge upon the magnetosphere. The high-speed streams accompany large amplitude Alfvenic fluctuations of the interplanetary magnetic field, leading to alternating polarities between north and south, which may be responsible for the repetitive substorms. Interestingly, <br /><br />
we have recently found that quasi-periodic substorms do also occur even when the interplanetary magnetic field is purely northward for which the magnetic reconnection at the dayside magnetopause is hardly expected to occur. Also, substorm features are seen for sawtooth-type quasi-periodic oscillations of energetic particle injections under prolonged southward conditions of the accompanied interplanetary magnetic field. The fact that such diverse interplanetary conditions lead to quasi-periodic substorms poses a real mystery to substorm scientists. This talk will introduce our recent effort to follow up a clue to the mystery.