Processes at the magnetotail boundary: Comments on 'On hot tenuous plasmas, fireballs, and boundary layers in the Earth's magnetotail' by L. A. Frank, K. L. Ackerson, and R. P. Lepping

E. W. Hones, Jr.

University of California, Los Alamos Scientific Laboratory, Los Alamos, New Mexico 87545

Introduction:

Frank et al. [1976] recently reported on measurements of plasmas, magnetic field, and energetic particles made with the Imp 8 satellite in and near the earth's magnetotail. Several of the conclusions they derived from the measurements were not, we believe, supported by the reported data. In this paper we shall discuss their claim to have acquired evidence that magnetic merging takes place in the boundary layer along the flanks of the magnetotail. It now appears that the boundary layer field lines at low latitudes are swept from the front of the magnetosphere to the tail as closed lines [e.g., Eastman et al., 1976]; thus merging in the boundary layer seems unlikely to occur, unless possibly at relatively great distances where 'pinching off' of greatly stretched closed boundary layer field lines may occur. Nevertheless, we do not say that it does not occur; we simply show that Frank et al. [1976] (hereafter referred to as FAL) have not provided what we would regard as valid evidence for its occurrence.

Evidence that FAL cite for magnetic merging in the boundary layer is the occurrence of southward oriented magnetic field in the boundary layer and the occasional appearance of hot plasma in conjunction with rotations of the magnetic field there from southward to northward or vice versa. We shall show in this paper that in all likelihood, Imp 8 was in the magnetosheath (not the boundary layer) when the southward oriented magnetic fields were observed and that the rotations of the field and associated hot plasma simply signified entries of the satellite into the plasma sheet. Our discussion will be supported by data from the Los Alamos Scientific Laboratory (LASL) plasma analyzer on Imp 8. The relevant features of the instrument were described by Hones [1977] and will not be repeated here. We shall also use magnetic field data from the Goddard Space Flight Center magnetometer on Imp 8.

J. Geophys. Res., 83, No. A5, 2216-2227, May 1978