Global structure of ULF waves during the 24-26 September 1998 geomagnetic storm

The geomagnetic storm event occurring during the 24-26 September 1998 interval was characterized by significant ULF wave activity in the inner magnetosphere. Here we undertake an analysis of the ULF wave activity occurring in global MHD simulations of the event, paying particular attention to the azimuthal mode structure characterizing the waves.We find that most of the power in the fluctuation components of the electric and magnetic fields can be described by low (m < 3) mode numbers during the recovery phase of the storm but that there is appreciable activity at finer azimuthal spatial scales in the electric fields during the main phase of the storm. The results of this work have particular implications for the common m = 1 assumption frequently used in studies of the interactions of radiation belt particles with global ULF waves.