On the velocity distribution of ion jets during substorm recovery

The velocity distribution of earthward jetting ions that are observed principally during substorm recovery by satellites at ∼15–35 R E in the magnetotail is compared quantitatively with two different theoretical models, the ‘adiabatic deformation’ of an initially flowing Maxwellian moving into higher magnetic field strength (model A) and the field‐aligned electrostatic acceleration of an initially nonflowing isotropic Maxwellian including adiabatic deformation effects (model B). It is assumed that the ions are protons or, more generally, consist of only one species. Both models can explain the often observed concave‐convex shape of isodensity contours of the distribution function. Model A, however, gives a somewhat better overall quantitative agreement with the observed distributions, leading us to conclude that field‐aligned electrostatic acceleration may not play the fundamental role in the energizing of the jetting ions that was suggested earlier (Frank et al., 1978; DeCoster and Frank, 1979). The two models imply different plasma conditions in the tailward source region. Model A requires a source in the far tail with plasma of kT ≈ 1 keV and density less than that at the observing satellite. Model B requires a source, probably closer to the satellite position, with plasma of kT ≈ 2–5 keV and density greater than that at the observing satellite. The conditions at the source region implied by model A are consistent with the plasma's having originated in the tail lobes and having been accelerated by a reconnection process in the far tail.