Using five independent substorm onset lists, we show that substorms occur more frequently when the Interplanetary Magnetic Field (IMF) By component and the dipole tilt angle Ψ have different signs as opposed to when they have the same sign. These results confirm that for Ψ ≠ 0 the magnetosphere exhibits an explicit dependence on the polarity of By, as other recent studies have suggested, and imply variation in the dayside reconnection rate and/or the magnetotail response. On the other hand, we find no clear relationship between substorm intensity and this explicit By effect. We additionally observe more frequent onsets for positive By in an onset list based on identifying negative bays in the auroral electrojet, regardless of season. Taking into account all five onset lists, we conclude that this phenomenon is not real, but is rather a consequence of the particular substorm identification method, which is affected by local ionospheric conditions that depend on By and Ψ.