Origin and dynamics of multi-component (H +/He ++/He +/O +) ion flows in the lobe/mantle regions

Abstract

Over a wide range of tailward distances in the lobe/mantle regions of the Earth's magnetotail (<210 R E ), the Geotail spacecraft sometimes observed multi-component ion flows (MCIF) consisting of both ionospheric (H +/He +/O +) and solar wind ( H + He ++ ) ions. All ion species in the MCIFs are streaming tailward with nearly the same velocity to each other. A positive correlation between proton density and the flow velocity ∥ v∥ suggests that most of protons have come from the solar wind. The existence of He ++ further supports this suggestion. The lack of positive correlation between O + as well as He + density and ∥ v∥ is consistent with the idea that these ions have a different origin from protons, i.e., the ionosphere. Another remarkable feature is that the He + and O + beams often appear to exist nearly exclusively of each other on a short time scale, and even when He + and O + coexist, their densities are sometimes anti-correlated. When the ionospheric ions have undergone an energization which leads to a different velocity for different ion species, the observed alternating appearance and anti-correlation are explicable only if a velocity filter is applied after the energization. A mechanism leading to similar velocities, on the other hand, needs an alternating enhancement of He + and O + fluxes in a source region to explain an anti-correlation. Statistical properties of the MCIFs show that when they include O + and He + components, ν ∥ is higher than usual, and the enhanced field-aligned velocity enables these ionospheric ions to reach the distant lobe/mantle despite their E×B drift toward the plasma sheet. While the frequency of He + detection (correspondent to ∼3% of all lobe/mantle observations) is only about a quarter of that of O + (corr. ∼13%), the He + and O + beams are observed under similar conditions to one another: These ion beams of ionospheric origin tend to exist during geomagnetically active periods on the “loaded” quadrants of the tail plasma asymmetry caused by IMF orientation, i.e., in the mantle-like regions where the plasma entry from the magnetosheath is enhanced. The results suggest the existence of extra energization of cusp/cleft originating ions that is closely related to geomagnetic activity and IMF orientation. They may also imply that the observed ions in the lobe/mantle have originated on closed field lines as energetic upward flowing ion (UFI) beams or equatorially trapped ions.