LOW-FREQUENCY WAVES IN THE NEAR-EARTH PLASMA SHEET

Using magnetometer and plasma instrument data of the Active Magnetosph eric Particle Tracer Explorers/Ion Release Module satellite obtained d uring 3 months of plasma sheet passes, we compute power spectra of mag netic fluctuations in the frequency ranges 0.1-25 mHz, 1-270 mHz, and 0.03-8 Hz and statistically survey their dependence on location, ambie nt magnetic field, plasma moments and magnetospheric activity. Average power spectra, S, of the field vectors increase monotonically with de creasing frequency. Whereas for frequencies between 0.03 and 2 Hz powe r laws, S similar to f(-epsilon) with 2 < epsilon < 2.5 hold, the slop es epsilon 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 S-r and t he left-hand spectrum S-l are equal. Being much less than the transver se wave spectra S-r and S-l in the plasma sheet boundary layer, the co mpressional wave spectrum S-parallel to surpasses S-r and S-l in the v icinity of the neutral sheet. As expected, the waves are enl;anced dur ing substorms. The highest values of all wave spectra S-r, S-l, and S- parallel to in the range 0.03-2 mHz are connected with high-speed flow s 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 va riations in the range 0.15-1 mHz(periods between 17 min and 2 hours) r eflect the substorm dynamics of the near-Earth plasma sheet and flappi ng motions af the tail. They have root-mean-square amplitudes of a few nanoteslas, the amplitude along X(GSM) being about twice as high as t he amplitudes along Y-GSM and Z(GSM) Close to the neutral sheet we fin d three events exhibiting compressional narrow-band fluctuations with spectral peaks at periods of 1-2 min. These fluctuations occur at subs torm onset and are accompanied by perturbations of the thermal pressur e that are in antiphase with the perturbations of the magnetic pressur e.