Auroral emissions of the giant planets

Abstract

Abstract. Auroras are (generally) high-latitude atmospheric emissions that result from the precipitation of energetic charged particles from a planet's magnetosphere. Auroral emissions from the giant planets have been observed from ground-based observatories, Earthorbiting satellites (e.g., International Ultraviolet Explorer (IUE), Hubble Space Telescope (HST), and R6entgensatellit (ROSAT)), flyby spacecraft (e.g., Voyager 1 and 2), and orbiting spacecraft platforms (e.g., Galileo) at X-ray, ultraviolet (UV), visible, infrared (IR), and radio wavelengths. UV, visible, and IR auroras are atmospheric emissions, produced or initiated when ambient atmospheric species are excited through collisions with the precipitating particles, while radio and X-ray auroras are beam emissions, produced by the precipitating species themselves. The emissions at different wavelengths provide unique and complementary information, accessible to remote sensing, about the key physical processes operating in the atmospheric and magnetospheric regions where they originate. This paper reviews the development of our current understanding of auroral emissions from Jupiter, Saturn, Uranus, and Neptune, as revealed through multispectral observations and supplemented by plasma measurements.

A Panchromatic View of Brown Dwarf Aurorae

“…Most of the remaining ∼15% emerges as part of the UV emission, with less than ∼1% of the energy going into optical aurorae (Bhardwaj & Gladstone 2000). Additionally, the radio contribution only represents 0.1% of the total auroral energy, and the X-rays represent even less (Bhardwaj & Gladstone 2000).…”

“…These different multiwavelength auroral emission processes are the consequence of the energy dissipation from the electrodynamic engine operating in the planetary magnetosphere. In the Jovian system, the bulk of the energy, ∼85%, goes into atmospheric heating and thermal radiation (Bhardwaj & Gladstone 2000). Most of the remaining ∼15% emerges as part of the UV emission, with less than ∼1% of the energy going into optical aurorae (Bhardwaj & Gladstone 2000).…”

X-rays from Saturn: a study withXMM-NewtonandChandraover the years 2002–05

“…However, the magnetospheric particle density at these planets is some 10 3 times lower than at Jupiter ( Table 3), suggesting that we should expect much lower FUV emitted powers than this initial extrapolation implies. The observed FUV powers, reported by Bhardwaj & Gladstone (2000), are at most 40 and 0.1 GW: the latter value is of the order which we would expect for the low plasma content of Neptune's magnetosphere, but Uranus does not seem to scale down in the same way; perhaps the very large tilt (59 • ) between its magnetic dipole and rotation axis and the 30% offset of the dipole from the planet's centre, causing an order of magnitude difference in surface magnetic field strength between day-and night-side, have also the effect of enhancing the power output in the aurora. In any case, taking these estimates together with the vaste distances from Earth of these two planets excludes the possibility of detecting their X-ray auroral emissions with current instruments.…”

“…Naively we would expect to observe a similar dichotomy on Saturn, given the strength of its magnetic field and its fast rotation (as for Jupiter) and its powerful UV aurorae (Gérard et al 2004(Gérard et al , 2005, but this is not the case. While the characteristics of Jupiter's UV aurorae are dictated by the rotation of plasma internal to its magnetosphere (Bhardwaj & Gladstone 2000;Clarke et al 2004), those of Earth are solar wind driven ); Saturn's UV aurorae appear to be at variance with both these scenarios, rather than being intermediate between the two, as originally expected (Bhardwaj & Gladstone 2000). In Saturn's case the UV aurora responds strongly to the solar wind dynamic pressure (Crary et al 2005), whose compression of the magnetosphere may induce reconnection leading to auroral brightenings (Cowley et al 2005); compared to Earth, though, where aurora brightenings are very rapid (∼tens of minutes), those on Saturn can last for days .…”

Jupiter and Io:The unique celestial couple

“…These auroral emissions are generally produced by the excitation of the upper atmospheric atoms and molecules by energetic electrons and ions precipitating down from the planet's magnetosphere. The occurrence of aurora is a topic of its own and a meticulous review on the aurora of the outer planets is given in Bhardwaj and Gladstone (2000).…”