Climatology of Pi2 Pulsations Deduced From the Wp Index

Abstract We examine the statistical properties of the Pi2 occurrence rate, especially the seasonal and universal‐time (UT) variations by analyzing the Wp index. The Wp index represents the wave power of Pi2 pulsations, and Pi2 events can be identified from the Wp index. We then examine the seasonal and UT variations of the Pi2 occurrence frequency. The results show that the Pi2 occurrence frequency is strongly dependent on the dipole tilt angle, which is equal to the geomagnetic latitude of the subsolar point. This means that the diurnal and annual variations of the Pi2 activity are mostly controlled by the so‐called equinoctial effect. We also evaluate the contribution of the Russell–McPherron effect and axial effect, both of which are attributed to the seasonal and UT variations of the solar wind observed around the Earth. We employ a machine‐learning‐based model trained to predict the Pi2 occurrence rate, and various synthetic data are fed into this model to discriminate the contribution of the solar wind variations from the equinoctial effect. It is found that the Russell–McPherron and axial effects hardly affect the Pi2 occurrence rate.