DISTRIBUTION OF MAGNETIC DISCONTINUITIES IN THE SOLAR WIND AND IN MAGNETOHYDRODYNAMIC TURBULENCE

The statistical properties of magnetic discontinuities in the solar wind are investigated by measuring fluctuations in the magnetic field direction, given by the rotation Delta theta that the magnetic field vector undergoes during time interval Delta t. We show that the probability density function, P(Delta theta), can be described by a simple model in which the magnetic field vector purely rotates with a relative increment Delta B/B that is lognormally distributed. We find that the probability density function of increments, P(Delta B/B), has a remarkable scaling property: the normalized variable x = (Delta B/B) . (Delta t/Delta t(0))(-alpha) a has a universal lognormal distribution for all time intervals Delta t. We then compare measurements from the solar wind with those from direct numerical simulations of magnetohydrodynamic (MHD) turbulence. We find good agreement for P(Delta theta) obtained in the two cases when the magnetic guide field to fluctuations ratio B-0/b(rms) is chosen accordingly. However, the scale invariance of P(Delta B/B) is broken in the MHD simulations with relatively limited inertial interval, which causes P(Delta theta) to scale with measurement interval differently than in the solar wind.