Low‐frequency waves in the near‐Earth plasma sheet

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Abstract
Using magnetometer and plasma instrument data of the Active Magnetospheric Particle Tracer Explorers/Ion Release Module satellite obtained during 3 months of plasma sheet passes, we compute power spectra of magnetic fluctuations in the frequency ranges 0.1–25 mHz, 1–270 mHz, and 0.03–8 Hz and statistically survey their dependence on location, ambient magnetic field, plasma moments, and magnetospheric activity. Average power spectra, S, of the field vectors increase monotonically with decreasing frequency. Whereas for frequencies between 0.03 and 2 Hz power laws, S ∼ ƒ−ϵ with 2 < ϵ < 2.5 hold, the slopes ϵ of the average power spectra fall below 1.5 for frequencies below 1 mHz. In the range 0.03–2 Hz the right‐hand spectrum Sr and the left‐hand spectrum Sl are equal. Being much less than the transverse wave spectra Sr and Sl in the plasma sheet boundary layer, the compressional wave spectrum S∥ surpasses Sr and Sl in the vicinity of the neutral sheet. As expected, the waves are enhanced during substorms. The highest values of all wave spectra Sr, Sl, and S∥ in the range 0.03–2 mHz are connected with high‐speed flows and strong changes in density and temperature in the central plasma sheet as well as with strong changes of the magnetic field in both the central plasma sheet and the plasma sheet boundary layer. Magnetic variations in the range 0.15–1 mHz (periods between 17 min and 2 hours) reflect the substorm dynamics of the near‐Earth plasma sheet and flapping motions of the tail. They have root‐mean‐square amplitudes of a few nanoteslas, the amplitude along XGSM being about twice as high as the amplitudes along YGSM and ZGSM. Close to the neutral sheet we find three events exhibiting compressional narrow‐band fluctuations with spectral peaks at periods of 1–2 min. These fluctuations occur at substorm onset and are accompanied by perturbations of the thermal pressure that are in antiphase with the perturbations of the magnetic pressure.