Characteristics of Substorm‐Onset‐Related and Nonsubstorm Earthward Fast Flows and Associated Magnetic Flux Transport: THEMIS Observations

Abstract

AbstractSubstorms are closely associated with fast flows; however, many fast flows are not associated with substorm onsets and only lead to a localized, transient, and weak response in the magnetosphere and ionosphere. This study uses the midlatitude positive bay index to identify substorms. A case study investigates the magnetospheric and ionospheric responses to substorm‐onset‐related fast flows and nonsubstorm fast flows using observations from the THEMIS spacecraft and ionospheric currents. Statistics based on THEMIS observations made over 11 years show that substorm‐onset‐related fast flows are more likely to penetrate closer to the Earth and spread over a wider range of MLTs compared to nonsubstorm fast flows. The substorm‐onset‐related fast flow durations are slightly longer than nonsubstorm fast flow on average, and there is no significant difference between their peak velocity probability distributions. However, substorm‐onset‐related fast flows are statistically shown to be accompanied by substantially larger Bz increases that persist for longer periods of time, and hence result in 80% larger earthward‐directed magnetic flux transport rates.