Statistical study on ultralow‐frequency waves in the magnetotail lobe observed by Cluster

Based on Cluster data, we investigate 263 waves with periods between 40 and 150 s (Pi2 band events) and 161 waves with periods between 150 and 600 s (Pc5 band events) in the magnetotail lobe. Our findings are as follows: (1) 90% of the mean wave amplitudes within 40–150 s (150–600 s) are below ~0.25 (0.36) nT for the transverse components and ~0.16 (0.39) nT for the compressional components; (2) 69.6% (35.4%) of the compressional ratios of the waves with periods 40–150 s (150–600 s) are less than 0.5 with the maximum occurrence at ~0.3 (0.8); (3) waves within 40–150 s are more likely to occur in the lobe region close to the plasma sheet; (4) the wave amplitudes and the AE index are weakly correlated; however, the amplitudes tend to be larger when the AE index is larger; and (5) the amplitudes also tend to be larger when the solar wind velocity and the solar wind dynamic pressure or its variations (∆PSW) are larger; the correlation coefficient between the wave amplitudes with periods between 150 and 600 s and ∆PSW is up to ~0.58. We suggest that both dynamic processes in the plasma sheet boundary layer or plasma sheet (inner sources) and solar wind conditions (outer sources) can contribute to the generation of the lobe ULF waves; waves within 40–150 s are affected more by inner sources, while strong ∆PSW can drive compressional waves within 150–600 s in the magnetotail lobe.