On the Lévy-Nature of Magnetic Field Fluctuations During Magnetospheric Tail Current Disruption

One of the most relevant phenomena occurring at the substorm onset is the development of a current wedge, which is responsible for the magnetosphere-ionosphere coupling during magnetic substorms. This current wedge is generally associated with the diversion or disruption of the near cross-tail current system [Lui, 1996]. In the last years this near-Earth dipolarization phenomenon has been the subject of several observation, as well as, simulation studies, which suggested a multiscale and a non-MHD nature of the phenomenon [Sitnov et al., 2000; Malova et al., 2000; Zelenyi et al., 2000; Miura, 2000]. Here, using magnetic field data relative to 3 current disruption (CD) events as observed by AMPTEE/CCE spacecraft, we investigate the statistical features of magnetic field fluctuations. In the kinetic domain (i.e. above the ion cyclotron frequency during CD) the distribution function of magnetic field fluctuations shows non Gaussian tails and the probability of return P_t(0) scales as t^-α with α !=q 1/2 which is compatible with a Lévy-statistics. Conversely, in the magnetohydrodynamic (MHD) region CD magnetic fluctuations are compatible with a classical Brownian motion. These findings seem to indicate that the near-Earth dipolarization process, associated with CD, is a non-MHD phenomenon, during which fast kinetic processes in collisionless plasmas take place.