Observations of Asymmetric Lobe Convection for Weak and Strong Tail Activity

Abstract

In this study we use high-quality convection data from the Electron Drift Instrument on board Cluster to investigate how near-Earth tail activity affects the average convection pattern in the magnetotail lobes when the interplanetary magnetic field has a dominating east-west (B y ) component. Two different proxies have been used to represent different levels of reconnection in the near-Earth tail: The value of the AL index and the substorm phases identified by the Substorm Onsets and Phases from Indices of the Electrojet algorithm. We find that the convection changes from a dominantly Y GSM direction, but opposite in the two hemispheres, to a flow oriented more toward the plasma sheet, as the north-south component of the convection increases when reconnection enhances in the near Earth tail. This result is consistent with recent observations of the convection in the ionosphere, which suggest that the nightside convection pattern becomes more north-south symmetric when tail reconnection increases. This is also supported by simultaneous auroral observations from the two hemispheres, which shows that conjugate auroral features become more symmetric during substorm expansion phase. The reduced asymmetry implies that the asymmetric pressure balance in the lobes is altered during periods with strong reconnection in the near-Earth tail.

Plasma Convection in the Terrestrial Magnetotail Lobes Measured Near the Moon's Orbit

“…In general, the lunar ion data from ARTEMIS and the upstream condition data from the OMNI data set appear very consistent with previous studies of the convection of the tail lobes closer to the Earth (Haaland et al, 2008, 2009; Ohma et al, 2019). This suggests that measurements of lunar ions provide a useful tool to investigate the convection in the deep lobes near the orbital distance of the Moon in the magnetotail.…”

“…10.1029/2020GL090217 (Haaland et al, 2008(Haaland et al, , 2009Ohma et al, 2019). This suggests that measurements of lunar ions provide a useful tool to investigate the convection in the deep lobes near the orbital distance of the Moon in the magnetotail.…”

“…Panel (b) shows that lunar ions during some events move toward the + Y direction and others toward the − Y direction, which indicates the direction of the dawn‐dusk convection flow in the magnetotail lobes (Haaland et al, 2008). This convection pattern has been suggested to be produced by the asymmetric pressure distribution caused by different mechanisms in open and closed magnetospheres, which result from the magnetospheric interaction with the upstream solar wind (Cowley, 1981; Cowley & Lockwood, 1992; Khurana et al, 1996; Ohma et al, 2019; Tenfjord et al, 2015). Panel (c) shows that the lunar ion velocities far from the Moon surface are primarily northward for three events, in contrast to the predominately southward lunar ion velocity in the other 13 events.…”

“…Specifically, the dawn‐dusk convection in different lobes strongly depends on the dawn‐dusk component of the IMF direction. Ohma et al (2019) showed that the magnitude of the asymmetry of the convection flow could also be affected by tail reconnection, which is linked to magnetospheric activity. McCoy et al (1975) and Lin et al (1977) showed the magnitude of field lines in the magnetotail lobes moving toward the plasma sheet could be ~15 km/s typically in the Apollo era.…”

Convection in the Magnetosphere‐Ionosphere System: A Multimission Survey of Its Response to IMF By Reversals

“…Yet when one specifically looks for these B y ‐related flows in the tail, for example, Pitkänen et al (2016), the reconnection event must have already taken place for the flows to be observed and thus the control is clear. Importantly, we also note that too much tail activity, particularly substorms, can inhibit the asymmetry observed in ionospheric flows (e.g., Ohma et al, 2018, 2019; Reistad et al, 2018) and so we have attempted to address this by filtering by Kp and AL in the SuperDARN plots.…”