SEPARATION OF THE FLAPPING OSCILLATIONS MODES IN THE PRESENCE OF NOISE

Flapping oscillations observed in the current sheet of the Earth's magnetotail, represent rather slow waves propagating from the center to the flanks with a typical speed ~20-60 km/s, amplitude ~1-2Re and quasiperiod ~2-10 minutes (2,4,5,7). The relevant model is based on double gradient of magnetic field: gradient of tangential (Bx) component along the normal (zGSM) direction and gradient of the normal component (Bz) along the x-direction. In the framework of this model the rotation of the vector of magnetic field in the plane Z-Y as well as vector of plasma velocity is investigated to find differences between kink and sausage modes of the flapping oscillations. It is also shown that the speed of the rotation of the vector (v or B) gives the fundamental parameters of the model including double gradient frequency. The theoretical results are compared to the flapping oscillations observed by space mission Themis on 03.05.2008 in the morning sector of the magnetotail. The rotation of the velocity vector simultaneously observed on two spacecrafts of Themis mission corresponds to the kink mode of the flapping oscillations. Obtained mechanism of the modes separating were analyzed on ability to withstand noise and also we analyzed behavior in case of combined modes. It was shown that behavior of the speed vector is more invariable than magnetic field's one. By this fact we can explain why theoretical prediction of magnetic field vector rotation was not found out in the experimental data.