Multipoint observations of substorm pre‐onset flows and time sequence in the ionosphere and magnetosphere

In this paper, we use a rare and close longitudinal conjunction of the THEMIS spacecraft with the Sondrestrom radar to present multipoint observations of important features of the time sequence of substorm pre‐onset plasma flows in the ionosphere and magnetotail on March 5, 2008. We found that the onset was preceded sequentially by enhanced polar cap flows heading equatorward near the polar cap boundary, and then by tail fast flows from the mid‐tail to the near‐Earth region. We also observed in situ fluctuations in short‐period Pi2 band (∼30–50 s) in both the magnetic field and plasma pressure during the initial couple of min of the fast flows, occurring nearly simultaneously with the dipolarization and the fast flows and propagating earthward. Our results suggest that these fluctuations may be triggered by the fast flows, and may play an important role in the substorm onset process. Our event suggests that localized tail reconnection may be triggered by the enhanced polar cap flows, though the reconnection location cannot be unambiguously determined. Earthward fast flows are generated as a result of the tail reconnection and reach the inner magnetosphere to initiate the substorm onset. The presented case is consistent with that predicted by the Nishimura et al. (2010a) scenario. Key Points Multipoint observations of the sequence of preonset flows are presented The flows starting from polar cap and moving through midtail to near‐Earth space The flows are responsible for triggering a substorm