The hot plasma and radiation environment of the Uranian magnetosphere

Abstract

We report on the hot plasma and particle radiation environment of the magnetosphere of Uranus as diagnosed with the low‐energy charged particle investigation on the Voyager 2 spacecraft (measuring electrons and ions with energies ≳22 keV and ≳28 keV, respectively). The encounter of the inbound bow shock (at ∼24 RU; 1 RU = 25,600 km) was immediately preceded (∼30–24 RU) by intense upstream proton events characterized by bulk streaming pointing approximately tangentially to the magnetospheric boundaries (flowing away from the magnetospheric “nose” regions). Just inside a somewhat disturbed magnetopause, convective boundary layer flows were observed. Inside the magnetosphere proper the higher‐energy particle channels (≳200 keV) show dramatic evidence of losses associated with the planetary satellites. The positions of “flux minima” signatures within the electron channels are reasonably well ordered (but not exactly) by the predicted minimum L shell positions of the satellites. In contrast the proton signatures show larger deviations from such ordering, due at least in part to dynamical time variations. Excluding the magnetotail regions, the maximum observed hot plasma β parameter (β is particle pressure/magnetic pressure) was ∼0.13, low by comparison with other visited magnetospheres. While the β parameter indicates that this magnetosphere is relatively “empty,” the energetic electron fluxes in the inner regions nevertheless exceed the whistler mode stably trapped limit by 1 order of magnitude. The energy spectral properties of both ions and electrons within the “core” or hard radiation magnetospheric regions are unusual and complex and are often not well characterized using the so‐called kappa distribution or a simple, one‐slope power law. The low‐energy portions of the electron distributions (E < 200 keV) generally have a power law shape (j ∼ E−γ) with γ between 1 and 3. However, in the vicinity of the minimum L shell position of the satellite Miranda (Lmin ∼ 4.9 RU) the distribution dramatically thermalizes to a Maxwellian with a temperature kT ∼ 30 keV. The low‐energy proton spectral shapes (E < 200 keV) vary between power law (γ ∼ 1.5‐4) and Maxwellian shapes (kT ∼ 35–50 keV). Most unusual is the distinctly Maxwellian shape of the high‐energy ions (E ∼ 200‐3500 keV) within several regions, with temperature parameters between 125 and 250 keV. At other locations a high‐energy power law suffices (with γ ∼ 2.5–4). The shapes of the core magnetospheric pitch angle distributions generally show well‐developed trapped or “pancake” forms at both low and high energies. In the outbound trajectory between the minimum L shell locations of the satellites Miranda and Ariel there is evidence of a population confined particularly close to the magnetic equator, perhaps explaining an apparent dramatic inbound/outbound asymmetry in the flux profiles in this region of the magnetosphere. The magnetotail is very active for particles at low (∼30 keV) energies. Substantial “plasma sheet” ion and electron enhancements were observed at the previously reported “neutral sheet” crossings. In addition, there were sporadic ion enhancements well separated in time and space from the neutral sheet. Also, dramatic field‐aligned streaming of energetic particles was observed at the boundaries of these ion enhancements, reminiscent of the energetic particle streaming observed at the boundaries of the plasma sheet of the Earth's magnetotail. The streaming is sometimes in the planetward and sometimes in the tailward directions. These observations, together with the special properties of one particular plasma sheet encounter, are highly suggestive that substorm processes analogous to those occurring within the Earth's magnetotail are occurring within the Uranian magneotail. Furthermore, at least one energetic particle event observed within the quasi‐dipolar regions was suggestive of substorm injection phenomena. We suggest that time‐stationary convection models of particle transport be viewed with caution.