Observations of energetic ion enhancements and fast neutrals upstream and downstream of Uranus' bow shock by the Voyager 2 spacecraft

Abstract

The Low Energy Charged Particle (LECP) experiment on the Voyager 2 spacecraft performed measurements of energetic ions ( E i ≳ 28 keV) and electrons ( E e ≳ 22 keV) upstream and downstream from the planet Uranus. The results can be summarized as follows: (i) discrete ion increases associated with the bow shock were observed upstream at distances ≲ 140 R U ; lower intensity ion enhancements possibly associated with the bow shock may be present at ≳. 500 R U from the planet; (ii) ion energy spectra are described well by dj dE = KE −γ with γ ≳ 3–4, extend to ∼ 1 MeV and are similar to those measured inside the planet's magnetopause; (iii) the low energy (∼ 30 keV) ions exhibit anisotropies ≳ 40:1 and are typically field-aligned; (iv) a necessary (but not sufficient) condition for the appearance of these ions at Voyager is near-connection of the interplanetary magnetic field (IMF) to the bow shock; (v) the presence of ions is near-coincident in several cases with depression of the IMF magnitude, with energy densities in the energetic particles and the magnetic field approximately equal; (vi) ion events appear downstream from the planet, off the dawn bow shock, to distances of ∼ 1500 R U . We conclude that the measured characteristics of the upstream events strongly favor leakage from the magnetosphere as the source of the particles and do not support predictions of in situ Fermi acceleration theory. A search for energetic neutral particle emissions suggests an upper limit to the flux < 7.4 × 10 −3 (cm 2 sec keV) −1 at ≳ 40 R U , resulting in a volume-averaged neutral hydrogen density inside ∼ 5 R U of < 60 cm −3. The corresponding upper limit to the production rate of fast neutrals is < 10 21 (sec keV) −1, similar to that of Earth, and a factor ≳ 10 2 less than the observed rates at Jupiter and Saturn.