Onset of nonadiabatic particle motion in the near‐Earth magnetotail

Abstract

Abstract. The onset of nonadiabatic proton motion is studied using direct integration of the Lorentz The net A/x is extremely sensitive to initial gyrophase and it is shown that for/Sa > 0.01 differences in gyrophase diverge exponentially with repeated equatorial crossings. Because the equatorial gyrophase determines the/x scattering, this implies that the/x scattering is chaotic so that no gyrophase-averaged invariant exists for the nonadiabatic drift motion. Despite this, the average nonadiabatic drift paths are fairly well defined. The resulting hybrid drift consists of dayside adiabatic and nightside nonadiabatic drift. A single nonadiabatic nightside drift path is associated with a family of adiabatic dayside drift paths. If some of the adiabatic drift paths are open to the magnetopause, all of the particles on the family of hybrid drift trajectories will be subject to loss on a timescale comparable to the drift period. Because the nonadiabatic behavior is due solely to field line curvature, the same behavior will be present with a nonzero convection electric field with the important difference that the lower-energy particles will be on open convection drift paths. The hybrid drift path-induced loss effects are therefore most important for higher-energy particles, > 50 keV, whose adiabatic drift paths are closed in the presence of a convection electric field. The implications of nonadiabatic effects for ring current modeling based on Liouville's theorem apply equally well in the zero and finite electric field cases.

Ring Current Energy Input and Decay

“…Other losses (Coulomb collisions and wave-particle interactions) contribute only a small (∼ few percent) amount of the decay but are extremely important to thermal plasma dynamics. Potential new loss processes in the literature are associated with the interchange instability (Sazykin et al, 2002), magnetopause compressions (c.f., Kozyra et al, 1998b), and scattering of ions in stretched magnetic fields (c.f., Anderson et al, 1997).…”

“…However, loss processes not included in these simulations may increase the decay of the ring current. For instance, a static dipole magnetic field was assumed in the simulation results presented above, but a stretched field configuration will cause some ions (especially those at high energies) to violate the first adiabatic invariant as they cross the low-field equatorial plane region (Anderson et al, 1997;Lyatsky, 1999). This will cause pitch angle isotropization and subsequent loss to the atmosphere.…”

Butterfly pitch angle distribution of relativistic electrons in the outer radiation belt: Evidence of nonadiabatic scattering

“…Thus, magnetotail-like configurations with stretched field lines can emerge (see results of empirical models [Tsyganenko et al, 2003;Sitnov et al, 2008;Kubyshkina et al, 2009]). Violation of adiabaticity of charged particle motions in such a magnetic field configuration has been thoroughly investigated both for energetic ions and relativistic electrons [e.g., Anderson et al, 1997;Delcourt and Belmont, 1998, and references therein]. The main effect of this motion is due to nonadiabatic jumps of the first adiabatic invariant near the equatorial plane.…”

“…This improved adiabatic invariant is equal to in certain points along the trajectory. For example, calculated at the mirror points conserves with higher accuracy [e.g., Mozer, 1966;Anderson et al, 1997]. This property of the magnetic moment is important for understanding the particle trajectory calculated for = 2.1 and a large initial 0 (see Figure 4).…”

“…In this case, each change is calculated on a small time step when is approximately constant, while from step to step the evolution of is taken into account [Slutskin, 1964]. Therefore, the preexponential factor in equation (B5) can be correct, but this correctness should be tested numerically [see, e.g., Anderson et al, 1997;Young et al, 2002] or with the more accurate analytical approach. …”