Orientation of IMF Discontinuity Normals Across the Solar Cycles

Abstract

Transient events like hot flow anomalies and foreshock bubbles are common in the Earth's foreshock. These foreshock transients may play an important role in the solar wind‐magnetosphere interaction. They typically occur when backstreaming ions accumulate at the intersection of interplanetary magnetic field (IMF) discontinuities with the bow shock. Discontinuity orientations play a key role in determining when and where transients form in the foreshock and strike the magnetopause. They also play a role in determining the amplitude, and significance, of individual transients. We investigate properties of the IMF discontinuity normals across two solar cycles. We compare Advanced Composition Explorer (ACE) IMF discontinuity observations for 2001, 2002, 2012, and 2014 (solar maximum) and 2008, 2009, 2019, and 2020 (solar minimum). We employ both the Minimum variance analysis (MVA) and Cross‐product methods to determine phase front normals. Most discontinuity normals point earthward and dawnward at longitudes ranging from 180° to 250°. A greater range of normal can be seen during solar minimum than during solar maximum. Normals lie further from the Sun‐Earth line when solar wind densities are low and velocities are high. This provides a natural explanation for the observed tendency of transients to occur for low solar wind densities and high velocities in terms of longer discontinuity interaction times with the bow shock.