Oxygen Ion Butterfly Distributions Observed in a Magnetotail Dipolarizing Flux Bundle

Abstract

Cluster observed two intermittent oxygen ion (O⁺) flux enhancements with energy dispersions in a dipolarizing flux bundle, which is known as a region of enhanced northward magnetic field (B_z) embedded in the earthward high-speed flow. The flux enhancements of O⁺ show clear pitch angle dependences, which are termed as butterfly distributions. Two corresponding flux enhancements of field-aligned protons (H⁺) are also shown in its spectrum, but they are weaker and emerge later (~10 s) than those of O⁺. Simulation shows that both enhanced ion species are the counterstreaming populations. They originated from the lobe region and were driven into the center plasma sheet by the dawn-dusk electric field (E_y). Backward tracing test-particle simulations reproduce the butterfly O⁺ and the counterstreaming H⁺ distribution. The differences between O⁺ and H⁺ are because of their different gyroradii. The lobe O⁺ can arrive at the magnetic equatorial plane in less than one gyromotion due to its large gyroradius, and O⁺ with a larger field-aligned velocity can arrive at the equatorial plane earlier, leading to the energy and pitch angle dependence. While H⁺ with similar energy can drift into dipolarizing flux bundle through electric field drift (E × B motion) and arrive at the equatorial plane through adiabatic motion, which consequently forms the field-aligned flux enhancements in dipolarizing flux bundle, that is, the B_z-dominant region. The simulation further confirms that intermittent increases of E_y component can produce the two intermittent flux enhancements, as indicated in the in situ observation.