Speaker
Description
The solar wind, a stream of charged particles emanating from the Sun, interacts with Earth’s magnetosphere. This interaction creates space weather phenomena such as geomagnetic storms, substorms, and auroras. Space weather is driven by solar wind and the interplanetary magnetic field (IMF) which drive ionospheric electric currents in the Earth’s magnetosphere and ionosphere. It is well-known that geomagnetic activity is stronger during equinoxes compared to solstices, an effect known as the semiannual variation of geomagnetic activity. There is a long-standing debate on the causes of semiannual variation. One of the suggested hypotheses is that the dipole tilt angle ψ modulates the dayside reconnection rate. Here we perform the first large-scale statistical study to test this hypothesis. We identified about 1300 isolated substorms in 2010-2023 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 a greater amount of open flux is stored in the tail prior to the substorm expansion phase during low ψ than during large ψ. This is due to a dipole tilt dependence of the dayside reconnection rate, and possibly another mechanism operating in the magnetotail. These two effects contribute to the semiannual variation of geomagnetic activity.