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) …