Two‐step evolution of auroral acceleration at substorm onset

The sudden formation of parallel electric fields in the magnetosphere‐ionosphere (M‐I) coupling system is essential to complete substorm onset. From this standpoint, we focus substorm ignition on field‐aligned acceleration by studying the dynamical behavior of auroral kilometric radiation. Field‐aligned auroral acceleration shows a distinct two‐step evolution at substorm onset: the activation of low‐altitude acceleration (h ∼ 4000–5000 km) which corresponds to auroral initial brightening and the subsequent abrupt breakout of high‐altitude acceleration (h ∼ 6000–12,000 km) which corresponds to auroral breakup. Cases when only low‐altitude acceleration (first‐step evolution) is activated are pseudosubstorms. This indicates that the second evolution of field‐aligned acceleration divides full substorm from pseudosubstorm. The statistical relationship between the plasma flow burst in the plasma sheet and its response to the M‐I coupling region shows that about 65% of flow bursts cause pseudobreakup/initial brightening (low‐altitude acceleration) and one third of them develops into full substorm (low‐altitude and high‐altitude accelerations), while the magnitude of flow velocity does not necessarily distinguish between pseudobreakup and full substorm. This suggests that some plasma flow bursts originate field‐aligned current which first enhance low‐altitude acceleration, and the increasing field‐aligned current induces second acceleration above the preexisting low‐altitude acceleration as a consequence of current/current‐driven instabilities. In this sense, the substorm is finally ignited in the auroral M‐I coupling region.