Response of quasi-adiabatic ions to magnetotail reconfigurations
Résumé
Particles traveling in sharp field reversals like in the Earth's magnetotail may not conserve their magnetic moment (first adiabatic invariant) due to significant variation of the magnetic field on the length scale of their Larmor radius. Although their motion is non-adiabatic per say and differs from a regular helical one, some particles may experience negligible net change of magnetic moment, a behavior that is referred to as quasi-adiabatic [Büchner and Zelenyi, 1989] like in the well-known Speiser orbit [Speiser, 1965]. Such a behavior is more pronounced at specific values of the adiabaticity parameter kappa (square root of the minimum curvature radius to maximum Larmor radius ratio) due to resonance between the slow gyromotion in the tail midplane and the fast oscillation in the direction perpendicular to it. On the other hand, during rapid reconfigurations of the magnetotail as observed during substorms, the impulsive electric field induced by the time-varying magnetic field may lead to non-adiabatic behaviors as well, with large variations of the magnetic moment for particles that have cyclotron periods comparable to the field variation time scale. In this case, the kappa parameter that is used to characterize spatial non-adiabaticity cannot be used since magnetic field lines are rapidly evolving in time. We examine the response of quasi-adiabatic ions in the presence of such short-lived reconfigurations of the magnetic field lines using single particle calculations. We demonstrate that quasi-adiabatic ions may remain quasi-adiabatic while experiencing an impulsive energization under the effect of the induced electric field ; hence, their faster oscillations about the tail midplane and their higher resonance order. Systematic acceleration up to about 3VE (where VE is the peak ExB drift speed during field line reconfiguration) is found for the lowest energy particles. We show that, altogether, impulsive transport and energization may be responsible for short-lived earthward injections of ions and for the Time-of-Flight dispersed ion structures that are observed at low altitudes in the plasma sheet boundary layer.