Energy transport by kinetic‐scale electromagnetic waves in fast plasma sheet flows

We report observations from the THEMIS spacecraft characterizing the nature and importance of low frequency electromagnetic fluctuations on kinetic scales embedded within fast flows in the Earth's plasma sheet. A consideration of wave property variations with frequency and flow speed suggest that for spacecraft frame frequencies satisfying | v f |/ñ i ≤ ù sc ≤ 100| v f |/ñ i (or 0.2 ≲ f sc ≲ 20 Hz) these fluctuations can generally be described as kinetic Alfvén waves. Here v f is the flow speed, ñ i the ion gyroradius, and ù sc and f sc are the angular and cyclical frequencies respectively in the spacecraft frame. The statistics of energy transport via Poynting flux ( S ) in these fluctuations and ion energy flux (å) in the flow follow log normal distributions with mean values of 〈 S 〉 = 10 1.1 ± 0.7 and 〈 ε 〉 = 10 2.4 ± 0.4 mW/m 2 respectively where the values are ‘mapped’ to a reference magnetic field at 100 km altitude. Here the indices following ‘ ± ’ correspond to one standard deviation. We find that 〈 S / ε 〉 = 10 −1.3 ± 0.7 or that kinetic Alfvén waves on average transport ∼5% of the total energy transport in the flow but note that the values larger than 25% are within one standard deviation of the mean. Our observations show that these waves are continually radiated outward from the flow toward the auroral oval, low latitude boundary layer or lobes and that over several Earth‐radii the integrated energy loss from the flow channel can be comparable to the total energy content of the flow itself. We find that this plasma sheet energy loss process is particularly effective within |X GSE | ≤ 15 R E .