Aurora and magnetic field of Uranus

Abstract

[1] Resolution of the details of a planetary magnetic field from magnetometer measurements made during a single flyby can be severely limited because of the incomplete geometrical sampling of the planetary neighborhood by the flyby trajectory. This problem was especially severe for the only spacecraft encounter with Uranus, that of Voyager 2 in 1986. Fortunately, auroras at the magnetic field line footprints serve as additional constraints that may be used to determine the higher multipole moments of planetary fields (Connerney et al.'s (1998) VIP-4 model of for Jupiter). In the present work, this approach is applied to improving the resolution of the magnetic field of Uranus. The auroral emission distribution at Uranus is determined from scans by the Voyager 2 Ultraviolet Spectrometer (UVS), enhancing an earlier analysis by Herbert and Sandel (1994) by incorporating more observations and by using more powerful analysis techniques. The resulting new determination of the auroral ovals is well correlated with the field lines associated with the strongest plasma wave and radio emissions but differs from model ovals computed by Connerney et al. (1987) from the Q 3 magnetic field model for Uranus. Consequently, a search has been initiated for model coefficients of the planetary magnetic field that agree both with the magnetic field observations and also with the reasonable assumption that the newly determined auroral emissions lie at the magnetic footprints of an equidistant circum-Uranian region of the magnetosphere. The dipole and quadrupole terms of the new field model, termed AH 5 , are similar to those of the dipole + quadrupole Q 3 model, but the AH 5 higher multipole terms diverge from the dipole + quadrupole + octupole I3E1 model of Connerney et al. (1987), from which the Q 3 model was derived. Inasmuch as the I3E1 octupole terms were not resolved, the AH 5 model derived here comprises a first estimate of the higher multipole moments of Uranus's magnetic field.

The science case for an orbital mission to Uranus: Exploring the origins and evolution of ice giant planets

“…Modelling of Uranus' internal magnetic field, and observations of radio emissions (Uranian Kilometric Radiation (UKR)) and atmospheric motions all provide independent estimates of the rotation rate of the planet, although not always from the same region of the planet. Analyses of Voyager 2 data have yielded three estimates of the rotation rate of Uranus, from 17 h 12 min 36 s ( 772 s) (Herbert, 2009) to 17 h 17 min 24 s (7 36 s) (e.g., Ness et al, 1991). New measurements of Uranus' magnetic field and UKR will enable us to significantly improve the accuracy on the determination of the planetary period (to a few parts in 10 À 5 ), and check if second order effects (e.g., Saturn displays different radio periods in both magnetic hemispheres, each varying with time) are present.…”

“…Models of the internal field can also be greatly improved by the use of auroral images which provide additional highlatitude constraints. Herbert (2009) combined the Voyager observations of the internal field and assumed magnetically conjugate southern and northern UV auroral emissions to derive such a higher order model. Better-quality images of auroral emissions than are possible from Earth (e.g., Lamy et al, 2012) are paramount for improving the accuracy of the planetary field model.…”

“…There has only been one spatially resolved observation of the UV aurora of Uranus (Herbert, 2009), using a mosaic of Voyager 2 UV observations mapping emission from H Lyman-α and EUV H 2 band emission (Fig. 12a).…”

“…3. Comparison of Uranus' highly asymmetrical field with Saturn's symmetrical field using the models of Herbert (2009) and Davis and Smith (1990) data from which to test these competing models. This will lead to significant changes in our understanding of field generation in Ice Giant planets and of planetary magnetic field generation in general.…”

The comparative exploration of the ice giant planets with twin spacecraft: Unveiling the history of our Solar System

“…Neptune and Uranus have strong non-axial multipolar magnetic field components compared with the axial dipole component (Connerney et al, 1991;Herbert, 2009). The magnetic fields of both planets are generated in the deep, electrically conducting regions of their interiors, i.e.…”