A POSSIBLE INTERPRETATION OF COLD ION-BEAMS IN THE EARTHS TAIL LOBE

The GEOTAIL spacecraft has previously observed cold ion beams of appar ent ionospheric origin in the Earth's magnetotail lobes at large geoce ntric distances (r > 50 R(E)) It is proposed here that ion beams origi nating in certain sections of the polar ionosphere may be on magnetic Awe tubes that are not closely linked to the magnetosheath and solar w ind beyond. Rather, these flux tubes extend far down the tail and have very weak convective electric fields on them. In other parts of the t ail cross-section, it is suggested that there are recently reconnected field lines that are open and connected directly out to the solar win d. These are the flux tubes that represent rotational discontinuities at the magnetopause. ISEE-3, in earlier data, clearly observed parts o f the lobe that were relatively devoid of plasma and parts that were d ensely loaded with plasma, probably due to local magnetosheath plasma entry. We suggest that the cold ionospheric ion streams seen by GEOTAI L could drift very slowly toward the plasma sheet in the regions of we ak 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 convectio n (E x B) electric fields.