Aurora, Amazing Northern Lights

The peaking of Bz during this time is a result of geometry. The IMF comes from the Sun and is carried outward with the solar wind. The rotation of the Sun causes the IMF to have a spiral shape called the Parker spiral. The southward (and northward) excursions of Bz are greatest during April and October, when Earth's magnetic dipole axis is most closely aligned with the Parker spiral.
However, Bz is not the only influence on geomagnetic activity. The Sun's rotation axis is tilted 8 degrees with respect to the plane of Earth's orbit. The solar wind blows more rapidly from the Sun's poles than from its equator, thus the average speed of particles buffeting Earth's magnetosphere waxes and wanes every six months. The solar wind speed is greatest – by about 50 km/s, on average – around 5 September and 5 March when Earth lies at its highest heliographic latitude.
Still, neither Bz nor the solar wind can fully explain the seasonal behavior of geomagnetic storms. Those factors together contribute only about one-third of the observed semiannual variations.