ISEE 3 observations during the CDAW 8 intervals: Case studies of the distant geomagnetic tail covering a wide range of geomagnetic activity

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Abstract
The data obtained by the ISEE 3 spacecraft during the eight Coordinated Data Analysis Workshop 8 (CDAW 8) intervals provide an excellent opportunity to study the structure and dynamics of the distant geomagnetic tail under a wide range of geomagnetic activity ranging from intervals of magnetic quiet punctuated by isolated substorms to extended intervals of strong disturbance. By examining the properties of the plasma sheet, evidence has been found for the persistence of reconnection in the tail during long intervals of magnetic quiet, with the neutral line lying ≈100 to 200 RE or more downtail. The suggestion that the distant tail plasma sheet is populated exclusively by tailward moving closed flux tubes under quiet geomagnetic conditions is therefore not supported. However, a “slow plasma sheet” regime is also found during such conditions, in which closed flux tubes move slowly tailward in a thick region adjacent to the magnetopause, presumably due to some form of “viscous” momentum transfer from the magnetosheath. This process does not appear to simultaneously transfer mass into the tail, and there is some indication that the Kelvin‐Helmholtz instability is involved. The observations strongly suggest that the closed flux tubes originate from the closed field line plasma sheet region earthward of the neutral line rather than, for example, from the near‐Earth low‐latitude boundary layer. Plasmoids are observed in the distant tail following disturbance enhancements, the time of their appearance being generally consistent with disconnection from the near‐Earth region at the time of the enhancement. Their structure is entirely consistent with the “neutral line” model. However, not all enhancements in geomagnetic activity result in the observation of plasmoids. In particular, the CDAW 8 data suggest that during extended intervals of strong activity, the neutral line may reside in the near‐Earth tail (E from Earth) on an essentially continuous basis and that some disturbance enhancements may then relate to an increase in the reconnection rate at a pre‐existing neutral line, rather than to new neutral line, and plasmoid, formation. The plasma sheet downtail of the neutral line then thickens and may engulf the spacecraft, but no plasmoid signatures are present.