Low Frequency ULF Waves in the Earth's Inner Magnetosphere: Statistics During Coronal Mass Ejections and Seeding of EMIC Waves

Here we perform a statistical analysis of low frequency ultra‐low‐frequency (ULF) waves (∼ mHz ‐ Hz) in the Earth's inner magnetosphere excluding electromagnetic ion cyclotron waves concurrently observed. We use the magnetic field data from the two Van Allen Probes during their first magnetic local time (MLT) revolution that cover the periods of coronal mass ejections. The major results of our analysis are as follows. (a) Spectra of both the transverse and compressional ULF waves are well approximated by the power‐laws in the ∼ mHz ‐ Hz frequency range. (b) There are two sources of the low frequency ULF waves: an internal magnetospheric source and an external source outside of the magnetosphere. (c) The average transverse power in the 6–24 h MLT sector dominates that in the 0–6 h sector, whereas the compressional power in the 12–24 h sector dominates that in the 0–12 h sector. (d) The average powers of transverse and compressional ULF waves in the plasmasphere dominate the average powers in the high L shell region of 5.5≲L≲6.5, and there is a deep power minimum in the intermediary region of 3.5≲L≲4.5. (e) The compressional ULF wave power has a maximum in the near equatorial region, whereas the transverse power has a minimum there. (f) A wave energy cascade from low frequency ULF waves into the higher frequency range of EMIC waves (∼ Hz) supplies the nonthermal seed fluctuations from which EMIC waves can then grow due to instabilities of the energetic magnetospheric ions. Key Points Power‐laws approximate well the spectra of transverse and compressional ultra‐low‐frequency (ULF) waves in the Pc 1–5 frequency range Statistics of low frequency ULF waves in the Earth's inner magnetosphere during coronal mass ejection conditions in the upstream solar wind are presented Cascade from low frequency ULF waves into the higher frequency range supplies seed fluctuations for electromagnetic ion cyclotron wave growth due to instabilities