The Magnetosphere of Uranus: Hot Plasma and Radiation Environment

The low-energy charged-particle (LECP) instrument on Voyager 2 measured low-energy electrons and ions near and within the magnetosphere of Uranus. Initial analysis of the LECP measurements has revealed the following. (i) The magnetospheric particle population consists principally of protons and electrons having energies to at least 4 and 1.2 megaelectron volts, respectively, with electron intensities substantially exceeding proton intensities at a given energy. (ii) The intensity profile for both particle species shows evidence that the particles were swept by planetary satellites out to at least the orbit of Titania. (iii) The ion and electron spectra may be described by a Maxwellian core at low energies (less than about 200 kiloelectron volts) and a power law at high energies (greater than about 590 kiloelectron volts; exponent γ, 3 to 10) except inside the orbit of Miranda, where power-law spectra (γ approximately 1.1 and 3.1 for electrons and protons, respectively) are observed. (iv) At ion energies between 0.6 and 1 megaelectron volt per nucleon, the composition is dominated by protons with a minor fraction (about 10^-3) of molecular hydrogen; the lower limit for the ratio of hydrogen to helium is greater than 10⁴. (v) The proton population is sufficiently intense that fluences greater than 10¹⁶ per square centimeter can accumulate in 10⁴ to 10₅ years; such fluences are sufficient to polymerize carbon monoxide and methane ice surfaces. The overall morphology of Uranus' magnetosphere resembles that of Jupiter, as evidenced by the fact that the spacecraft crossed the plasma sheet through the dawn magnetosheath twice per planetary rotation period (17.3 hours). Uranus' magnetosphere differs from that of Jupiter and of Saturn in that the plasma β is at most 0.1 rather than 1. Therefore, little distortion of the field is expected from particle loading at distances less than about 15 Uranus radii.