Properties of outflow‐driven sawtooth substorms

Abstract

The mechanisms by which ionospheric O+ stretches the plasma sheet causing variations in polar cap flux during magnetospheric sawtooth substorms are investigated using the multi‐fluid Lyon‐Fedder‐Mobarry simulation code. O+ outflow is induced in the simulation by Alfvénic Poynting flux flowing to low altitude during the sawtooth expansion and growth phases. The O+ fluid populates the plasma sheet and causes it to stretch tailward in response to the increased mass density. The ionospheric outflow thus functions as a feedback loop in the magnetosphere‐ionosphere system wherein O+ ions released during the expansion phase alter the magnetospheric configuration and enable the development of the next substorm. We also find that the nightside reconnection rate is strongly dependent on the position of the tail merging line. As the x‐line moves tailward, the inflow speed and magnetic field strength are reduced, creating an imbalance between dayside and nightside merging. This imbalance is responsible for the buildup and release of open magnetic flux in the magnetosphere during the sawtooth cycle.