The relationship between small interplanetary magnetic flux rope and the substorm expansion phase

Small magnetic flux ropes are commonly observed by spacecraft in interplanetary space near 1 AU. Like magnetic clouds, the configuration of small flux ropes often presents a sustaining southward interplanetary magnetic field. It is known that the substorm expansion phase can be triggered by northward turnings after a sustaining southward interplanetary magnetic field. Hence, potentially the small magnetic flux ropes may be more effective for triggering substorms. In this study, 26 small magnetic flux ropes, which were observed by Wind during the period from 2000 to 2002, were identified by fitting the constant alpha force-free model to the observed magnetic fields. Then we statistically analyzed the relationship between the small magnetic flux ropes and the magnetospheric substorms based on the 26 small magnetic flux ropes. The substorms were identified by the local 10-s resolution IL index. The statistical results show that 18 (69%) of the 26 small flux ropes triggered magnetospheric substorms. Among the 18 triggered substorms, 14 substorm expansion phases were triggered by the northward turning of the interplanetary magnetic field in the small magnetic flux ropes. For the other 4 events, their expansion phases were triggered by sudden changes in solar wind dynamic pressure. All the 18 triggers were also related to mangentic directional discontinuities in the small flux ropes. These observational facts suggest that small magnetic flux ropes are an important source of interplanetary disturbances of substorms.