Implications of the Drift Orbit Bifurcations to Variability of the Outer Electron Belt

The outer radiation belt (L>3) consists of energetic electrons trapped by magnetic field onto the drift-bounce trajectories closed around the Earth. Radiation levels in the outer belt can vary by orders of magnitude on the timescales ranging from minutes to days. Multiple acceleration and loss processes operate across the belt and compete in sculpting its flux levels. One of such processes, which was insufficiently investigated, is the drift orbit bifurcation effect (DOB). In the region adjacent to the dayside magnetopause the magnetic field profiles along the field lines have two minima off the equator on both its sides north and south. A particle with a large pitch angle bouncing on the nightside in the vicinity of the equatorial plane will branch off the plane into one of the minimum-B pockets after drifting into this region and will return back to the equatorial vicinity after drifting out of it. Such DOBs are non-adiabatic: they violate both the second and the third invariants and therefore drive radial transport even in the absence of dynamical variations of the field. The goal of this paper is to investigate implications of DOBs to transport, acceleration, and loss of the outer belt electrons. For this purpose we use three-dimensional test-particle simulations in the guiding center approximation based on the most recent Tsyganenko-Sitnov magnetic field model. To describe long-term statistical properties of electrons affected by DOBs we derive an algebraic map which can be implemented directly into global diffusion-like models of the outer belt to account for the radial transport driven by these bifurcations. We show that DOBs can produce both radial transport and loss of the outer belt electrons with particles being lost to the magnetopause as well as in the magnetotail due to the pitch-angle scattering at sharp inhomogeneities in the magnetic field. It is shown that DOBs can produce butterfly pitch-angle distributions due to a combination of increased transport and loss rates at largest pitch angle values. While the radial transport rates are larger or comparable to empirically determined rates of radial diffusion due to ULF fluctuations in the magnetic field, transport properties are different; most of the electrons in the DOB region remain quasi-trapped recirculating over a confined range of radial distances. DOBs by themselves do not change particle energy, however, the recirculation they produce can effectively amplify energization by other mechanisms operating in this region.