Formation of substorm Pi2: A coherent response to auroral streamers and currents

Pi2 driving has been the subject of long standing debate involving magnetosphere‐ionosphere coupling. Using the THEMIS ground‐based all‐sky imager array, we have examined auroral disturbances evolving in the Pi2 frequency range to help identify where the Pi2 signal propagates from the magnetosphere toward the ionosphere. We demonstrate that expansion‐phase intensifications near the poleward edge of the auroral bulge occur quasiperiodically with one‐to‐one correspondence to Pi2 pulses, neither being strictly periodic. The expansion‐phase intensifications propagate toward the equatorward boundary of the auroral oval, and then often turn azimuthally. Based on the known relation between auroral streamers and plasma sheet flow bursts, the presence of the quasiperiodic streamers indicates that substorm Pi2s are driven by multiple plasma sheet flow bursts, each driving a Pi2 pulse. While it is difficult to tell precisely when auroral zone Pi2 starts because of the modest onset‐related negative bays, we found that midlatitude‐equatorial Pi2 tends to start a few minutes after initial substorm auroral onset, and that each Pi2 pulse starts to rise simultaneously over a wide latitude range from the auroral zone to the magnetic dip equator as well as over a wide longitudinal range. The observations furthermore show that auroral zone Pi2 near the streamer meridian is anti‐correlated with midlatitude‐equatorial Pi2. These features can be explained by an oscillating current wedge, whose upward field‐aligned current (FAC) corresponds to the auroral streamers. The midlatitude‐equatorial Pi2 is likely caused by the FACs of the current wedge, and the oscillating westward electrojet creates the auroral zone Pi2. Key Points Expansion phase streamers are quasiperiodic and correlated with Pi2 Streamers start to form minutes after substorm auroral onset Pi2 amplitude distribution is consistent with the current wedge model