The role of drift orbit bifurcations in energization and loss of electrons in the outer radiation belt

Abstract

[1] Radiation levels in Earth's outer electron belt (L^2.5) vary by orders of magnitude on the time scales ranging from minutes to days. Multiple acceleration and loss processes operate across the belt and compete in defining its global variability. One such process is the drift orbit bifurcation effect. Caused by coupling of the drift and bounce motions, it breaks the second adiabatic invariant of radiation belt electrons producing their transport in radius and pitch angle. In this paper we investigate implications of drift orbit bifurcations to the global state and variability of the outer electron belt. For this purpose we use three-dimensional test particle simulations of electron guiding center motion in a realistic magnetic field model. We show that even at most quiet solar wind conditions bifurcations affect a broad range of the belt penetrating inside the geosynchronous orbit. This has an important practical implication for the analysis of experimental particle data: since the third adiabatic invariant is undefined for bifurcating orbit, the electron phase space density cannot be expressed in terms of three adiabatic invariants. We show that long-term transport of electrons due to drift orbit bifurcations is a complex combination of large ballistic jumps and small-amplitude diffusion in the second invariant and radial location. To model long-term transport, we derive an empirical map of the second invariant and radial jumps at bifurcations. The map can also be implemented by other radiation belt models, which cannot directly account for the physics of drift orbit bifurcations. Drift orbit bifurcations can produce electron losses through the magnetopause escape and through pitch angle scattering into the atmospheric loss cone. Most electrons, however, can stay quasi-trapped in the bifurcation regions for very long time periods. The pitch angle and radial transport due to drift orbit bifurcations lead to their meandering back and forth across the region producing mixing and recirculation of particle populations with different initial conditions. We show that this recirculation can greatly amplify electron energization by radial diffusion. Compared to the diffusion alone, the combined action of radial diffusion and drift orbit bifurcations can double electron energization at each recirculation cycle. Our results suggest that drift orbit bifurcations can play an important role in the buildup of increased electron fluxes in the storm recovery phase.

Quantifying the effect of magnetopause shadowing on electron radiation belt dropouts

“…Reductions of energetic electron flux in the outer radiation belt can generally be attributed to (a) adiabatic motion (i.e., Dst effect) (Kim and Chan, 1997) that radially transports particles adiabatically following a configuration change in the magnetosphere to conserve the three adiabatic invariants (µ, K, φ) and (b) nonadiabatic processes, such as the loss caused by pitch-angle scattering via various cyclotron wave-particle interaction, which leads to electron precipitation to the lowaltitude atmosphere (e.g., Lyons et al, 1972;Thorne et al, 2005;Summers et al, 2007a, b;Millan et al, 2007) as well as the loss across the magnetopause (i.e., magnetopause shadowing) into the interplanetary space (e.g., Desorgher et al, 2000;Ohtani et al, 2009;Ukhorskiy et al, 2006Ukhorskiy et al, , 2011. Magnetopause shadowing is often caused by either an inward motion of the magnetopause that opens up the previously closed particle drift shells and depletes the particles or by outward motion of the particles that subsequently encounter the magnetopause boundary.…”

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

“…In presence of the deformation of the magnetic field configuration at the dayside, due to action of a solar wind dynamic pressure, relativistic electrons can be scattered by the mechanism involving the bifurcation of drift orbits [see Shabansky, 1971]. This mechanism destroys the second adiabatic invariant of electrons providing jumps of equatorial pitch angles (it is especially effective for small pitch angles, see Ukhorskiy et al [2011b]). Particles drifting azimuthally are scattered at the dayside magnetosphere: each pitch angle jump corresponds to one period of azimuthal electron motion; thus, this mechanism is much slower than the destruction of magnetic moment due to electron scattering at nightside (in latter case each jump corresponds to one bounce period).…”

A novel technique for rapid L* calculation using UBK coordinates

“…Violation of the adiabatic invariants can occur due to Coulomb collisions between particles, due to wave-particle interaction, or due to significant variations in the fields on short time scales. It can also occur on the dayside where a bifurcation of the drift trajectory due to dayside compression breaks the second adiabatic invariant and hence the third adiabatic invariant is not defined [e.g., Ukhorskiy et al, 2011]. In the UBK method the bifurcating region is easily identified by comparing the equatorial magnetic field magnitude with the magnitude at the mirror point.…”