The Magnitude of IMF By Influences the Magnetotail Response to Solar Wind Forcing

The dynamics of substorms are known to be dominated by the North-South (Bz) component of the Interplanetary Magnetic Field (IMF), which is the most important driver of the dayside reconnection. Even though the dawn-dusk (By) component is also known to play a role in substorm dynamics, its effects are not yet fully understood. In this paper we study how IMF By modulates the onset latitude, strength and occurrence frequency of substorms as well as the isotropic boundary (IB) latitude of energetic protons. We show that the substorm onset latitude and the IB latitude are about one degree lower for large magnitude By (|By|>3 nT) than for small By. In contrast, the substorm occurrence frequency is larger for small |By|. We suggest that the magnetotail is more stable during large |By|, requiring the magnetotail lobes (and hence the polar cap) to contain more flux to initiate a substorm compared to the situation when By is small.