The Uranian aurora and its relationship to the magnetosphere

Abstract

About 32 h of Voyager Ultraviolet Spectrometer (UVS) observations of Uranus H2 band airglow emission (875 ≤ λ ≤ 1115 Å) have been analyzed using the singular value decomposition (SVD) approach to inversion, producing an intensity map showing aurora at both magnetic poles. An H Lyman α aurora may also be present but is difficult to separate from scattered solar and local interstellar medium components. SVD analysis of variance shows that the intensity estimate is significantly larger than the error estimate over both Uranographic poles and part of the equatorial region, fortuitously including both magnetic polar regions. The Goddard Space Flight Center Q3 magnetic field model correctly predicts that the aurora should be larger in area and emit more power at the weaker N magnetic pole than at the stronger S magnetic pole. However, the auroral emissions are quite localized in magnetic longitude and so do not form complete auroral ovals. The brightest auroral emission at each magnetic pole is confined to a range of ≈ 90° of magnetic longitude centered on the magnetotail direction, at moderate magnetic L parameter (5 ≤ L ≤ 10), but some emission at each pole is distributed over a range of more than 180° of longitude. The S polar auroral intensity maximum is coincident with the source of the broadband bursty and broadband smooth Uranian kilometric radio emission (UKR), while the N polar auroral intensity maximum may coincide with the dayside UKR source. The N and S auroral intensity maxima also lie at the conjugate magnetic footprints of the maximum intensities of whistler‐mode plasma wave emission and 22‐ to 35‐keV electron fluxes observed by Voyager. The magnetic longitudes of the aurora are completely inconsistent with the “windshield wiper” effect for either ions or electrons, indicating that some other effect, such as rapid depletion of the population of precipitating particles or highly localized strong pitch‐angle diffusion, may be acting to localize emission. The low apparent L of the precipitating particles indicates that their energies may be ≤ 10 keV. Hence magnetospheric convection is likely to be important, and thus particles exciting the aurora may not remain on constant L shells. The precipitating particles may be a relatively low‐energy population at high L that is heated to aurora‐exciting energy by adiabatic compression during convection to low L. We estimate that the total auroral power output at H Lyman α and shorter wavelengths is about 3 × 109 to 7 × 109 W, requiring about 10 times that much power for excitation.