Determination of auroral electrostatic potentials using high‐ and low‐altitude particle distributions

Abstract

The Dynamics Explorer (DE) pair of spacecraft provide a unique opportunity to search for the presence of electric fields aligned parallel to magnetic field lines by sampling, nearly simultaneously, the velocity‐space distribution functions of ions and electrons at two points on auroral field lines: DE 1 at high altitudes (9000–15,000 km in this study) and DE 2 at low altitudes (400–800 km). Three independent techniques are used to infer the auroral electrostatic potential difference from the particle distributions: (1) the energy of the precipitating electrons at DE 2 (compared to that at DE 1), (2) the energy of the upgoing ions at DE 1, and (3) the widening of the loss cone for electrons at DE 1. The three estimates are in general agreement, confirming the long‐standing, but not fully accepted, hypothesis that parallel electrostatic fields of 1–10 kV potential drop at 1–2 RE altitude are an important source for auroral particle acceleration. The upflowing ion distribution typically can be characterized by a sharp peak and a falloff at high energies of the form exp‐{(E‐Epeak)/Eo}, with Epeak being the peak energy and Eo the characteristic energy. This is the functional dependence one expects if a Maxwellian of thermal energy Eo is accelerated upward by a parallel electric field with eΦ=Epeak. The fact that the peak energies and not the flow velocities of the various ion species are in agreement also lends strong credence to the parallel electric field hypothesis. The acceleration mechanism cannot be a simple parallel electric field, however, for two reasons: first, the characteristic energy Eo is considerably larger than the ionospheric thermal energy (Eo is typically hundreds of electron volts and 20–30% of Epeak), and second, the energy Epeak is typically 30–50% smaller than that inferred by the two other independent techniques. The distribution does appear to be consistent with an ionospheric source, heated within (or above) the acceleration region, since the ion average energy is comparable to eΦ. The average energies of O+ and He+ are comparable to, but typically somewhat larger than, that of H+, indicating that a two‐stream instability may be the heating mechanism. Electron heating is also detected within the auroral acceleration region, with gains in characteristic energies of 10–15% of eΦ. From the high‐altitude electron measurements, we can determine a minimum potential distribution as a function of altitude in order to overcome the mirror force. We find that in one case at least 9M V of the 2300‐V potential drop must occur above 7000 km, and at least 960 V above 2000 km.