A scenario for solar wind penetration of Earth's magnetic tail based on ion composition data from the ISEE 1 spacecraft

Energetic (0.1‐16 keV/ e ) ion data from the Plasma Composition Experiment on the ISEE 1 spacecraft show that Earth's plasma sheet (inside of 23 R E ) always has a large population of H + and He ++ ions. This population is the largest, both in absolute numbers and relative to the terrestrial ions, during periods of extremely weak geomagnetic activity and is then also the most “solar wind like”, in the sense that the He ++ /H + density ratio is at its peak (about 3% on average in 1978 and 79), and the H + and He ++ ions have mean (thermal) energies that are proportional to ionic mass and barely exceed the typical bulk flow energies in the solar wind. These quiet time conditions occur when the IMF is persistently northward and not excessively strong. At more active times the H + and He ++ ions are heated in the central plasma sheet, but they can still be found with lower solar wind like energies closer to the tail lobes, at least during plasma sheet thinning. In a number of cases examined here, such low‐energy H + and He ++ ions are found flowing antisunward close to the lobes, flowing roughly parallel (or antiparallel) to the tail magnetic field at speeds ranging from a few tens to a few hundred kilometers per second, as though they have recently entered the tail earthward of the ISEE 1. In some cases the flow vector appears to contain a significant E × B component directed away from the nearest tail flank. In order to explain these flows, as well as the solar wind like appearance of the plasma sheet as a whole during northward interplanetary magnetic field, it is argued that the solar plasma enters along slots between the tail lobes and the plasma sheet, even quite close to Earth, convected inward along the plasma sheet boundary layer, or adjacent to it, by the electric fringe field of the ever present low‐latitude magnetopause boundary layer (LLBL). The main point of the argument is that the required E × B drifts are produced by closing LLBL equipotential surfaces through the plasma sheet.