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

In the plasma sheet of the earth's magnetotail, at geocentric distances of 15-20 R_E and greater, the plasma is quite frequently observed to flow at high speed (sometimes 1000 km/s or faster), often in the sunward or antisunward direction. This phenomenon is generally associated with geomagnetic activity and has, in many instances, been clearly related to individual magnetospheric substorms [e.g., Hones et al., 1974, 1976; Hones, 1977]. It has been our experience, in observing the rapidly flowing plasmas with Los Alamos Scientific Laboratory (LASL) plasma analyzers on various Vela and Imp spacecraft, that the thermal energy of the plasma protons is at least equal to, and often greater than, the kinetic energy of their bulk motion and always much greater than the thermal energy of the accompanying plasma electrons.

In a recent paper, however, Frank et al. [1976] reported, on the basis of plasma measurements they made with the University of Iowa plasma analyzer on the Imp 8 satellite, that the thermal energy of the protons in such rapidly flowing plasmas, specifically those that they identified with 'fireballs,' is much less than the bulk flow energy. For example, they state, 'Characteristic energies of the jetting protons are small (~1 keV) in relation to their convectional energies of 10-15 keV' [Frank et al., 1976, p. 5878]. Frank and Ackerson [1977] reaffirm this view, stating, 'Temperatures of electrons and protons in the jetting plasmas of the fireballs are approximately equal, and the kinetic energy of the protons is dominated by that associated with the high speeds of bulk flow...As these jetting plasmas flow into the plasma sheet, they are believed to be thermalized, thus yielding the higher proton temperatures relative to those for electrons as are normally found within the plasma sheet.'

In this paper we report evaluations of proton and electron temperatures and proton bulk flows that we have made for several time intervals on April 18, 1974. Three of these intervals, during a magnetospheric substorm, were encompassed by one of the 'fireball encounters' reported by Frank et al. [1976], and a fourth, also during a substorm, was during a period of rapid plasma flow that Frank et al. deemed not to be a fireball. We find, in agreement with our observations on other occasions and contrary to the report of Frank et al., that proton thermal energies exceeded their flow energies and greatly exceeded the electron thermal energies. This was true both in the presence and in the absence of fireballs.

We also report here a remarkable disagreement between the cold proton distribution function reported by Frank et al. for ~ 1530 UT October 8, 1974 [Frank et al., 1976, Figure 8], and cited by them as typical of plasma jetting from fireballs and the proton distribution measured at the same time by the LASL analyzer on the same satellite (Imp 8). The LASL analyzer revealed a very hot distribution at that time.

Before proceeding further, however, we must provide a few words of explanation to minimize the reader's confusion with nomenclature. Major attributes of a fireball as it was defined by Frank et al. [1976] are tailward flowing plasma threaded by southward oriented magnetic field and earthward flowing plasma threaded by northward oriented field. But these are precisely those plasma and magnetic phenomena in the magnetotail that had earlier been identified with the occurrence of magnetospheric substorms and shown to be consistent with the occurrence of magnetic reconnection at a substorm neutral line formed in the near-earth portion of the plasma sheet [Hones et al., 1976]. Indeed, those very fireballs described by Frank et al. have also been shown to be clearly correlated in this way with substorms [Hones, 1977]. Thus in actuality the 'fireball phenomenon' simply comprises a combination of several time-variable features of the magnetotail's behavior during a substorm. Therefore in the following discussion, when it is necessary for us to use the term fireball, it is to be understood in this context.

J. Geophys. Res., 83, No. A7, 3358-3364, July 1978