Evolution of open magnetic flux during substorms: the effects of dipole tilt angle

There is a long-standing debate on the causes of the semiannual variation of geomagnetic activity. One of the prevailing hypotheses is that the Earth’s dipole tilt angle Ψ modulates the dayside reconnection rate, causing the so-called equinoctial effect. Here we perform the first large-scale statistical study to test this hypothesis. We identified isolated substorms in 2010-2019 and used the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) measurements to determine the open magnetic flux variations and estimates of the dayside reconnection rate during these substorm events. We find no significant Ψ dependence of the dayside reconnection rate, opposing earlier studies. However, we find that during low Ψ (equinoxes) a greater amount of open flux is stored in the tail prior to the expansion phase. This suggests that a different mechanism, operating in the magnetotail, contributes to the equinoctial effect and the semiannual variation of geomagnetic activity.