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A Possible Interpretation of Cold Ion Beams in the Earth's Tail Lobe

The GEOTAIL spacecraft has previously observed cold ion beams of apparent ionospheric origin in the Earth's magnetotail lobes at large geocentric distances (r > 50 R E ). It is proposed here that ion beams originating in certain sections of the polar ionosphere may be on magnetic flux tubes that are not closely linked to the magnetosheath and solar wind beyond. Rather, these flux tubes extend far down the tail and have very weak convective electric fields on them. In other parts of the tail cross-section, it is suggested that there are recently reconnected field lines that are open and connected directly out to the solar wind. These are the flux tubes that represent rotational discontinuities at the magnetopause. ISEE-3, in earlier data, clearly observed parts of the lobe that were relatively devoid of plasma and parts that were densely loaded with plasma, probably due to local magnetosheath plasma entry. We suggest that the cold ionospheric ion streams seen by GEOTAIL could drift very slowly toward the plasma sheet in the regions of weak E-fields. Thus, the beams at large geocentric distances should tend to be seen in relatively pristine lobe conditions and would be on flux tubes that are connected to ionospheric regions of low convection (E × B) electric fields.