Articles | Volume 35, issue 3
Regular paper
10 May 2017
Regular paper |  | 10 May 2017

Geomagnetic activity and local time dependence of the distribution of ultra low-frequency wave power in azimuthal wavenumbers, m

Theodore E. Sarris and Xinlin Li

Abstract. The azimuthal wavenumber m of ultra low-frequency (ULF) waves in the magnetosphere is a required parameter in the calculations of the diffusion rates of energetic electrons and protons in the magnetosphere, as electrons and protons of drift frequency ωd have been shown to radially diffuse due to resonant interaction with ULF waves of frequency ω = mωd. However, there are difficulties in estimating m, due to lack of multipoint measurements. In this paper we use magnetic field measurements at geosynchronous orbit to calculate the cross-spectrogram power and phase differences between time series from magnetometer pairs. Subsequently, assuming that ULF waves of a certain frequency and m would be observed with a certain phase difference between two azimuthally aligned magnetometers, the fraction of the total power in each phase difference range is calculated. As part of the analysis, both quiet-time and storm-time distributions of power per m number are calculated, and it is shown that during active times, a smaller fraction of total power is confined to lower m than during quiet times. It is also shown that in the dayside region, power is distributed mostly to the lowest azimuthal wavenumbers m = 1 and 2, whereas on the nightside it is more equally distributed to all m that can be resolved by the azimuthal separation between two spacecraft.

Short summary
In this paper we describe a novel way to approximate the decomposition of magnetospheric ultra low-frequency (ULF) wave power in key azimuthal wavenumbers m, which is a parameter describing the number of azimuthal wavelengths that fit within a particle drift orbit. This is a critical parameter that is required in estimates of the rates of radial diffusion, and we show for the first time that there is a local time and geomagnetic activity dependence in the distribution of power in wavenumbers m.