The azimuthal wavenumber of ultra low-frequency (ULF) waves in themagnetosphere is a required parameter in the calculations of the diffusionrates of energetic electrons and protons in the magnetosphere, as electronsand protons of drift frequency have been shown to radiallydiffuse due to resonant interaction with ULF waves of frequency = . However, there are difficulties in estimating , due tolack of multipoint measurements. In this paper we use magnetic fieldmeasurements at geosynchronous orbit to calculate the cross-spectrogram powerand phase differences between time series from magnetometer pairs.Subsequently, assuming that ULF waves of a certain frequency and would beobserved with a certain phase difference between two azimuthally alignedmagnetometers, the fraction of the total power in each phase difference rangeis calculated. As part of the analysis, both quiet-time and storm-timedistributions of power per number are calculated, and it is shown thatduring active times, a smaller fraction of total power is confined to lower than during quiet times. It is also shown that in the dayside region,power is distributed mostly to the lowest azimuthal wavenumbers = 1 and2, whereas on the nightside it is more equally distributed to all thatcan be resolved by the azimuthal separation between two spacecraft.
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