Radar Observations of Flows Leading to Substorm Onset Over Alaska

About 40 years after the discovery of the substorm, auroral observations were used to propose that substorm onset was triggered in the inner plasmas sheet (equatorward portion of the auroral oval) by an intrusion of the low entropy plasma by plasma sheet flow channels. Longitudinal localization makes such flow channels difficult to observe with sparse spacecraft, but they can be seen in the ionosphere via the broader, two‐dimensional coverage by ground‐based radars. We have analyzed all eight substorm auroral onset events with appropriate auroral and Poker Flat radar viewing during a radar campaign. These, together with five previously analyzed events provide evidence that intruding flow channels play an important role in substorm onsets. That the flux tube integrated entropy of this new plasma must be lower than that of the surrounding plasma implies that the change in the entropy distribution within the plasma sheet gives rises to the substorm onset instability that is seen via the growing auroral and electromagnetic waves seen with substorm auroral onset. Our analysis also adds to previous evidence that the flow channels leading to onset move into the plasma sheet/auroral oval from the open polar cap region of the magnetotail lobes/polar caps, that these flow channels move across the polar cap following their origin near the dayside boundary of the polar cap, and that substantial ground magnetic depressions associated with substorm do not occur with substorm onset but instead occur a few to several minutes later in association with expansion phase streamers.