Ionospheric O+ ion outflow and its impact on magnetospheric dynamics

Abstract

During geomagnetic activity periods, ionospheric heavy ions such as O+ ions have a transverse ion acceleration due to wave-particle interactions with the Alfvén waves from the magnetosphere. Accompanied by mirror force lifting, these non-thermal O+ ions can outflow to the magnetosphere, which plays an important role in the dynamics of the coupled solar wind-magnetosphere-ionosphere system. This manuscript mainly discusses the landing of O+ ion outflow in the magnetosphere and its effect on the magnetospheric convection mode. The spatial distribution of O+ ion outflow in the magnetosphere depends on the outflow velocity, which is known as the 'velocity filtering effect'. Hot fast O+ ion outflows (~50 km/s, ~100 keV) tend to land tailward of nighside reconnection sites (X-lines) and lost downtail without affecting magnetospheric dynamics significantly. Most of cold slow (~3 km/s, ~1 keV) O+ ion outflows land in the plasmasheet and inner magnetosphere, which significantly impacts the ring current, magnetic reconnection and the Dst index. Overall, these cold slow O+ ion outflows with a speed of the order of 3 km/s have a more significant effect on the dynamics of the magnetotail significantly than the hot fast ones. When ionospheric O+ ion outflow interacts with the nightside reconnection region, which can trigger an isolated substorm and induce a sawtooth convection mode with a period of 2～3 hours. Under ideal strong southward IMF conditions, the increasing hemispheric O+ outflow rate from ~1026 to ~1027 ions/s results in more outflows and energy entering the plasmasheet. Hence, the nightside field lines become more stretched, and nightside reconnection migrates. When the magnetic tension is unbalanced, a release of the stored energy is manifested as a substorm. In this induced substorm by O+ mass uploading, a large fraction of the released O+ fluid is lost downstream in the plasmoid and to the magnetopause during the convective surge associated with the substorm. After the substorm, the nightside ionospheric outflow continues to land Earthwards of the nightside reconnection region. The outflow refills the inner magnetosphere, stretching the field lines once again, resulting in another substorm within several hours. The periodic substorm process induced by the O+ ion outflow through the stretching of the magnetotail is known as sawtooth oscillations. In addition, the simulations with the O+ outflow effect reproduce a series of quasi-periodic sawtooth oscillations during the recovery phase of a magnetic storm on April 18, 2002. This indicates that besides periodic solar wind conditions, O+ ion outflow could also induce sawtooth oscillations. Finally, some scientific questions are put forward for future work.

Keywords

• ion outflow
• magnetospheric response
• velocity filtering effect
• magnetospheric convection
• sawtooth mode