Modulation of magnetospheric substorm frequency: Dipole tilt and IMF By effects

Abstract:

Using five substorm onset lists based on independent onset signatures, we show that substorms occur more frequently when the Interplanetary Magnetic Field (IMF) B_y component and the dipole tilt angle Ψ have opposite compared to equal sign. These results confirm that the magnetospheric response depends explicitly on the sign of B_y for non-zero Ψ. This contradicts the traditional assumption that IMF B_y introduce mirror symmetric inter-hemispheric differences in high latitude electrodynamics, and implies a variation in the dayside reconnection rate and/or the magnetotail response for the different B_y and Ψ combinations. We additionally observe more frequent onsets for positive compared to negative B_y in a list based on identifying negative bays in the auroral electrojet, regardless of season. Taking into account all five onset lists, we conclude that this apparent phenomenon is not real. It is rather a consequence of the particular substorm identification method, which is affected by local ionospheric conditions that depend on IMF B_y and Ψ, and hence the result of an inter-hemispheric asymmetry.