A Combined Effect of the Earth's Magnetic Dipole Tilt and IMF By in Controlling Auroral Electron Precipitation

Auroral particle precipitation (<30 keV) is usually assumed to be equally strong for both signs of the By component of the interplanetary magnetic field (IMF). However, recent statistical studies have showed that geomagnetic activity is significantly modulated by the signs and amplitudes of IMF By and the Earth's dipole tilt angle Ψ. Here we quantify this By dependence for auroral electron precipitation for the first time. Furthermore, we make a case study on a sequence of high-speed stream (HSS) driven events of auroral and medium energy (>30 keV) particle precipitation. We show that when HSSs are comparable in terms of IMF and solar wind parameters, HSSs with opposite signs of By and Ψ can lead to systematically stronger particle precipitation in individual events. We perform a superposed epoch analysis of 485 HSSs giving further evidence that the By-effect is especially significant during HSSs. This is likely due to the persistent IMF By polarity during HSSs.