Ion motion in the current sheet with sheared magnetic field – Part 2: Non-adiabatic effects

Abstract

Abstract. We investigate dynamics of charged particles in current sheets with the sheared magnetic field. In our previous paper (Artemyev et al., 2013) we studied the particle motion in such magnetic field configurations on the basis of the quasi-adiabatic theory and conservation of the quasiadiabatic invariant. In this paper we concentrate on violation of the adiabaticity due to jumps of this invariant and the corresponding effects of stochastization of a particle motion. We compare effects of geometrical and dynamical jumps, which occur due to the presence of the separatrix in the phase plane of charged particle motion. We show that due to the presence of the magnetic field shear, the average value of dynamical jumps is not equal to zero. This effect results in the decrease of the time interval necessary for stochastization of trapped particle motion. We investigate also the effect of the magnetic field shear on transient trajectories, which cross the current sheet boundaries. Presence of the magnetic field shear leads to the asymmetry of reflection and transition of particles in the current sheet. We discuss the possible influence of single-particle effects revealed in this paper on the current sheet structure and dynamics.

Chaotic Charged Particle Motion and Acceleration in Reconnected Current Sheet

“…In this case, the main role is played by the dynamical jumps. As a result of the asymmetry of the phase portrait for s = 0, the averaged jump I z for the ensemble of separatrix crossings has a finite value (Artemyev, Neishtadt, and Zelenyi, 2013b). Thus, the estimate of the time of diffusive chaotization of the charged particle motion decreases as T st ∼ κ −2 .…”

“…Dynamical jumps of the invariant I z in the system with B y = 0 are proportional to κ ln κ (Neishtadt, 1986(Neishtadt, , 1987. Moreover, for B y = 0 the mean value of dynamical jumps is not equal to zero, I z = 0 (Artemyev, Neishtadt, and Zelenyi, 2013b). As a result, the rate of I z destruction increases with B y .…”

“…Particle trajectories in this parameter domain can have a highly complicated configuration. Particles moving along these trajectories spend much time far from the separatrix and cross it more rarely than in systems with s < 0.3 (Artemyev, Neishtadt, and Zelenyi, 2013b). Formally speaking, each separatrix crossing for this domain results in a dynamical jump of I z with the same amplitude ∼ κ ln κ as in the domain s < 0.3.…”

“…The coordinate z = z max is the mirror point of particle motion, and therefore at this pointẋ = 0 and p x,max = sz max (see Equation (6)). The position of the mirror point z max is about √ 2κx max , where x max is determined by the I z value (κx max ∼ 1/I 2 z for systems with s = 0 and s = 0; see Zelenyi, Galeev, and Kennel, 1990a;Artemyev, Neishtadt, and Zelenyi, 2013b). Thus, a change of energy is h = 2us √ 2κx max for s > κ y/2 √ 2κx max , where y is evaluated for s = 0.…”

“…However, because of the longer intervals between separatrix crossings, the final chaotization time of the charged particle motion can even increase while keeping the scaling T st ∼ κ −2 (i.e. there is a large numerical factor making T s>0.3 st /T s<0.3 st 1 (Artemyev, Neishtadt, and Zelenyi, 2013b)). We showed here that for all these three domains the particle dynamics become substantially different if there is a finite electric field u = 0.…”