The magnetosphere of Neptune: Hot plasmas and energetic particles

Abstract

A comprehensive overview is provided of the hot plasmas and energetic particles (≳keV) observed in the vicinity of Neptune by the low energy charged particle (LECP) experiment on the Voyager 2 spacecraft. The LECP data are ordered with respect to magnetic field data and models derived from the Voyager magnetometer experiment. The findings include the following: (1) Weakly enhanced ion and electron fluxes were observed at the position of the subsolar bow shock. (2) Magnetic‐field‐aligned, antiplanetward streaming ions and electrons were sporadically observed within the inbound (subsolar) and outbound (tail flank) magnetosheaths, and within the unique “pole‐on” cusp region encountered during the inbound trajectory. (3) Tangential ion streaming was observed at the positions of both the inbound (dawnward streaming) and outbound (tailward streaming) magnctopauscs. (4) A distinct “trans‐Triton” ion population outside the minimumL shell of Triton is characterized by large angular anisotropics that show that heavy ions (presumably N+) are a likely constituent This population is at least partially corotating with Neptune out to at least L = 27 RNand is also characterized at times by cigar‐shaped (field‐aligned) pitch angle distributions, possibly indicative of an interaction with a neutral torus. (5) Within the middle magnetospheric regions (inside Triton), pitch angle distributions have well‐developed trapped or “pancake” shapes. Also, in contrast to Uranus, flux profiles show no evidence of substorm‐generated azimuthal asymmetries. (6) Triton (and/or Triton‐generated neutral gas) controls the outer bounds of the hot plasmas and energetic particles, although the mechanism of that control is unclear. Also, there are clear charged particle signatures of satellite 1989NI and of ring 1989N3R. However, the large number of calculated criticalLshell positions associated with all of the rings and satellites renders impractical at this time the unique determination of causal relationships between the many observed particle signatures and known material bodies. (7) Concerning the bulk (integral) and spectral parameters of the not plasmas, if it is assumed that the trans‐Triton population is dominated by N+, the plasma β parameter reaches ∼1 within the near‐planet magnetotail (L ∼28 RN; in conjunction with a magnetic field depression “tail event”), having only reached ∼0.2 in the more planetward regions. Integral electron energy intensities are such that the more localized Neptune UV aurora can be explained if loss cone intensities are ≲1% of trapped intensities. In contrast to the Uranian magnetosphere, the lower‐energy electron distributions appear generally to be at least as well characterized by hot Maxwellian distributions (kT = 10 to 30 keV) as by power law distributions inside L ∼20 RN, a characteristic generally exhibited at the other planets by the ions. At Neptune the ions have kT = 12 to 100 keV, and kTis strongly correlated with position relative to Triton's L shell. (8) Within the Neptunian magnetotail, planetward, magnetic‐field‐aligned streaming of ions and electrons is observed within the distant (∼67RN) plasma sheet and within a closer region thought to be a detached or striated portion of the plasma sheet population. Within the near‐planet magnetotail (L ∼28 RN), where the spacecraft crossed from the plasma sheet to the tail lobes, cigarlike electron distributions are observed, suggestive of shell‐splitting/magneto pause‐sweeping effects. Consistent with the middle magnetospheric observations, and in sharp contrast to the Uranian magnetotail, the Neptunian magnetotail shows no evidence of substorm processes.