Two-Step Acceleration of Auroral Particles at Substorm Onset as Derived From Auroral Kilometric Radiation Spectra

The evolution of field-aligned auroral acceleration in the magnetosphere-ionosphere (M-I) coupling region at substorm onset is derived by means of a unique analysis of auroral kilometric radiation (AKR) frequency spectra. The derived AKR source dynamics showed two-step field-aligned acceleration at auroral substorm onset. The first step is intensification of low-altitude acceleration at an altitude of 3000-5000 km, which induces initial brightening. The second step is breakout of high-altitude acceleration (8000-12,000 km) above the preexisting low-altitude acceleration, which results in violent auroral breakup and poleward expansion. Low-altitude acceleration that is not followed by the breakout of high-altitude acceleration (one-step evolution) is a pseudosubstorm. The close relationship between development of low-altitude acceleration and gradually increasing field-aligned current (FAC) during initial brightening suggests that FAC plays an important role in linking sub storm initiation in the plasma sheet with sudden auroral acceleration in the M-I coupling region.