Modulation of Magnetospheric Substorm Frequency: Dipole Tilt and IMF <i>B</i> _<i>y</i> Effects

Abstract

Abstract Substorm activity is heavily influenced by the Interplanetary Magnetic Field (IMF) B z component and magnetospheric substorms occur most frequently when B z is strongly negative. The substorm occurrence rate is also affected by the magnitude of the B y component, but it is usually presumed that this contribution is independent of the sign of B y . Using five independent substorm onset lists, we show that substorm activity does depend on the sign of B y near the solstices. Specifically, we show that substorms occur more frequently when B y and the dipole tilt angle Ψ have different signs as opposed to when they have the same sign. These results confirm that the magnetosphere exhibits an explicit dependence on the polarity of B y for nonzero Ψ, 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 B y regardless of Ψ. Last, for the onset list based on identifying negative bays at auroral latitudes, we observe an overall trend of more frequent onsets for positive B y , regardless of season. However, substorm frequency in the other four substorm lists does not exhibit an overall preference for positive B y . We show that this phenomenon is very likely a consequence of the particular substorm identification method (i.e., identification of negative bays), which is affected by local ionospheric conditions that depend on B y and Ψ.

Topics & Concepts