Observational evidence of the loading‐unloading substorm scheme

According to the loading‐unloading substorm scenario, the magnetic flux, stored in the magnetotail during the substorm growth phase,is dissipated in the course of the expansion phase. However so far only separate estimates of both accumulated and dissipated flux values were made due to the lack of methods to calculate those quantities, doing their direct comparison impossible. First, we analyzed the magnetotail magnetic flux at substorm onset as a function of solar wind parameters to show that the tail magnetic flux, stored during the growth phase (Δ F T ), depends mainly (CC = 0.95) on the merging electric field E m = V SW B t sin 3 θ/2. It implies the lack of threshold magnetic flux at substorm onset. Also, the magnetic flux through the auroral bulge at substorm maximum ( F B ) was calculated from Polar UVI images. Comparison of Δ F T and F B shows their correlation and confirms their balance. These results quantitatively support the substorm model, based on reconnection mechanism.