Abstract
AbstractIn 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 (By) 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 YGSM 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.
Highlights
The plasma circulation in the magnetosphere is primarily controlled by the solar wind and the interplanetary magnetic field (IMF)
We find that the convection changes from a dominantly YGSM 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
For a purely southward IMF, this circulation is described by the Dungey cycle (Dungey, 1961): The IMF can reconnect with the terrestrial field at the dayside magnetopause, and the field lines opened by this process are dragged across the polar caps by the solar wind and added to the magnetotail lobes
Summary
The plasma circulation in the magnetosphere is primarily controlled by the solar wind and the interplanetary magnetic field (IMF). For a purely southward IMF, this circulation is described by the Dungey cycle (Dungey, 1961): The IMF can reconnect with the terrestrial field at the dayside magnetopause, and the field lines opened by this process are dragged across the polar caps by the solar wind and added to the magnetotail lobes. These field lines will eventually close again by reconnecting in the magnetotail and return to the dayside to repeat the cycle. Near-Earth reconnection, is associated with substorms and commences at closed field lines but progresses to open field lines as the substorm evolves (e.g., Hones, 1979)
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