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

The effort to achieve a clear understanding of the processes whereby solar wind energy and plasma are imparted to the earth's magnetosphere, and ultimately to the earth itself, depends heavily upon observations and interpretations of phenomena in the magnetotail. It is generally believed that the magnetotail constitutes a storehouse in which energy is deposited temporarily as stressed magnetic field and released at intervals, some of the energy being given to electric currents and energetic particles that produce auroras and other characteristic disturbances in the polar ionosphere. In one widely held view, stemming from the work of Dungey [1961], the magnetotail is a consequence of the merging of solar wind magnetic field lines with those of the earth, the latter then being stretched far downstream by the force imparted from the onflowing solar wind. Recent observations of the magnetospheric boundary layer by Eastman et al. [1976] suggest that a force akin to the 'viscous drag' proposed by Axford and Hine [1961], acting on closed field lines at the magnetosphere's surface, also shares in the generation of the magnetotail. The release of the tail's stored energy, which occurs primarily during sporadically occurring substorms, is believed by many to proceed by merging, across the tail's midplane, of oppositely directed magnetic field lines from the upper and lower halves of the tail. Evidence of various kinds has been presented that supports this belief in substorm merging. The evidence suggests further that the merging occurs at an X-type magnetic neutral line that forms across some fraction of the tail's width in its near-earth region (-10 R_E > X_SM > -20 R_E) at the substorm's onset [Hones et al., 1972, 1973, 1974, 1976; Hones, 1976a, b; McPherron et al., 1973; Nishida and Nagayama, 1973; Nishida and Hones, 1974; Nishida and Fujii, 1976; Teresawa and Nishida, 1976].

In their recent paper, Frank et al. [1976] (hereafter referred to as FAL) described measurements of plasma, energetic electrons, and magnetic fields made with Imp 8 in and near the magnetotail. A major claim of the paper is the discovery of a spatially limited region in the magnetotail where magnetic merging takes place; this region was given the name 'magnetotail fireball.' Outstanding observational features used to characterize the fireball and to identify it as the site of magnetic merging were rapid tailward flow of plasma associated with southward oriented tail magnetic field and rapid earthward flow of plasma associated with northward oriented tail field. These same features were reported by Hones et al. [1976] and were cited by them as further evidence for the substorm occurrence of merging at an X-type neutral line in the near-earth region of the magnetotail. Form a more detailed analysis of these data, Hones [1976a] concluded, more specifically, that the neutral line initially forms within the plasma sheet. FAL state that they 'found no evidence that supports the claim by Hones and co-workers that a neutral line, and hence magnetic merging, appears across the hot tenuous plasma sheet during magnetic substorms'. We suggest in this paper that on the contrary, the 'fireball encounters' of FAL constitute further nice examples of the phenomena reports by Hones et al. [1976] and Hones [1976a] and thus provide additional evidence for the formation of a neutral line across the near-earth plasma sheet near substorm onset. We feel that correct interpretation of such observations requires consideration of concurrent geomagnetic activity; FAL did not discuss that activity in the cases we examine below.

The discussion of fireball encounters in the present paper will be supported by data from the Los Alamos Scientific Laboratory (LASL) plasma probe and the Goddard Space Flight Center (GSFC) magnetometer on Imp 8.

J. Geophys. Res., 82, No. 35, 5633-5643, Dec. 1977