Magnetospheric substorms have long been the focus of magnetospheric physics.The main unsolved issue is the triggering process of the substorm expansion onset.Despite many studies in the last three decades,the chronological relationship between magnetotail reconnection,auroral break-up and near-Earth dipolarization remains a controversial question.The coordinated Cluster-Double Star measurements and the THEMIS mission have provided opportunities to resolve the controversy with observations of the substorm onset time history by employing multiple satellites and ground-based all-sky aurora imagers and magnetometers.This talk provides a review of recent case studies of Cluster-Double Star and THEMIS observations,with focus on the relationship between the initiation and evolution of substorm expansion phase and mid-tail magnetic reconnection.