Abstract
Energetic electrons and ions that are trapped in Earth's magnetosphere can suddenly be accelerated towards the planet1,2,3,4,5. Some dynamic features of Earth's aurora (the northern and southern lights) are created by the fraction of these injected particles that travels along magnetic field lines and hits the upper atmosphere4. Jupiter's aurora appears similar to Earth's in some respects; both appear as large ovals circling the poles and both show transient events6,7,8,9,10,11. But the magnetospheres of Jupiter and Earth are so different—particularly in the way they are powered—that it is not known whether the magnetospheric drivers12 of Earth's aurora also cause them on Jupiter. Here we show a direct relationship between Earth-like injections of electrons in Jupiter's magnetosphere and a transient auroral feature in Jupiter's polar region. This relationship is remarkably similar to what happens at Earth, and therefore suggests that despite the large differences between planetary magnetospheres, some processes that generate aurorae are the same throughout the Solar System.
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The injections within Earth's magnetosphere (Fig. 1) involve particles with kilo-electron volt (keV) to mega-electron volt (MeV) energies4. Often occurring at radial distances of 6 to 10 Earth radii, injections are one component of global dynamic events called ‘magnetospheric substorms’. Substorms represent, in part, the transient release of energy stored in the magnetosphere with stressed magnetic fields13. The energy source for Earth's substorms is the solar wind of charged gases, or plasmas, emanating from the Sun. Substorms create dramatic brightening of the aurora at high geographic latitudes and a substantial expansion of the regions where auroral emissions occur.
The recent discovery of Earth-like charged particle injections within Jupiter's magnetosphere14,15 is surprising because Jupiter's magnetosphere is powered mostly from the inside by the rapid but steady planetary rotation rather than from the outside by the variable solar wind. The role of injections in generating auroral emissions at Jupiter has been heretofore unknown, to our knowledge.
A unique opportunity to address dynamics in Jupiter's space environment was made available by a Jupiter joint observation campaign in late 2000 and early 2001. It involved the fly-by of the Cassini spacecraft, headed toward Saturn, the Galileo spacecraft orbiting Jupiter, and remote imaging by the Hubble Space Telescope (HST). During the campaign, Galileo recorded energetic electron injection signatures at radial distances of ∼10 to ∼13 Jupiter radii (Fig. 2). A simple model (Fig. 3) explains the energy-dispersed character of these signatures (different particle energies arrived at Galileo at different times). The model is closely analogous to models derived from injections at Earth16,17,18. Quantitative analysis (Fig. 4) reveals the temporal relationship between the injections and the signatures. At the times of the injections, around 15 h before the dispersed signatures were observed, Galileo was at a radial distance of about 20 Jupiter radii and recorded no obvious signature of the injections occurring closer to Jupiter.
Ultraviolet HST auroral images, similar to those in previous reports7, were taken during the Galileo operations and remapped to polar coordinates (Fig. 5). The images reveal a distinct auroral emission patch in eight consecutive images (100-s exposures) obtained over a 36-min HST operation period (Fig. 5, extreme right and close to the yellow square labelled ‘15 h’). The auroral patch moved with Jupiter as Jupiter rotated, and thus maintained its latitude–longitude position. The last image of the set was taken at about 1230 UT, about 30 min before the beginning of the first injection signature in Fig. 2.
We believe the patch was generated by the first of the injections observed by Galileo (Fig. 4). In Fig. 5, the yellow squares and the yellow dashed line show two different calculations of the motion of the Galileo spacecraft trajectory when mapped along magnetic field lines from the spacecraft to Jupiter's upper atmosphere. The yellow dashed line is the better estimate because the magnetic field model used to map Galileo's position includes contributions both from currents internal to Jupiter and from up-to-date estimates of average currents flowing external to Jupiter within its magnetosphere19. Adequate approximation to the magnetic field requires that external currents be included because the actual magnetic field configuration beyond about 9 Jupiter radii is distorted away from the nearly dipolar configuration expected from internal currents alone19,20. Projections derived using a field model with only internal currents (yellow squares in Fig. 5; VIP421) are shown to provide a sense of the uncertainties involved with mapping. With the better estimate, the Galileo trajectory appears to have crossed into the isolated patch between ∼14.7 and ∼15.3 h, centred on the maximum of the higher-energy portion of the injection feature (Fig. 4). This result does not depend on detailed timing because, as mentioned, the auroral patch maintained its latitude–longitude position as Jupiter rotated.
The auroral patch characteristic of rotating with Jupiter is expected because the injected electrons also rotate with Jupiter (within several per cent at ∼12 Jupiter radii15). Although the auroral patch of interest here was the brightest observed during the joint observation campaign, such patches are common within HST images. Likewise, the occurrence of jovian electron injections is also common15. The other energetic electron injections revealed in the Galileo data (Figs 2 and 4) map to regions (Fig. 5) that also show measurable auroral emissions. However, those emissions do not appear patch-like and may have been active even in the absence of injections.
Studies of Earth's magnetosphere13 suggest two different ways that injected energetic particles can generate auroral emissions. (1) The particle energy distributions are modified during injection and become unstable to exchanges of energy with magnetospheric wave modes. The waves scatter particles so that some travel narrowly along the magnetic field lines until they encounter the atmosphere. (2) The injected particle cloud is a high-pressure region and so electric current flows along its boundary. This pressure-driven (diamagnetic) current diverges along the leading and trailing edges of the rotating cloud because the magnetic field strength changes with radial distance. Currents are driven along the magnetic field lines towards and away from the planet and can interact strongly with plasmas close to the planet. At Jupiter, that interaction would yield downward accelerated electrons, and atmospheric auroral emissions, at the trailing edge of the rotating plasma cloud. Although there is substantial uncertainty in the magnetic mapping, the position of the auroral patch does match best with either the trailing edge of the electron cloud (the second mechanism) or the centre of that portion of the cloud that contained the higher-energy electrons measured (the first mechanism).
For an aurora resulting from the scattering process, the maximum power density that the measured (>20 keV) electron cloud can provide to the aurora is 60 ± 30 erg cm-2 s-1. Higher power densities are possible with the electric current generation mechanism. Models of interaction between electrons and Jupiter's atmosphere22, recalculated with the energy distribution shapes measured in Fig. 2, yield about 3 erg cm-2 s-1 for the electron input needed to explain the auroral emissions. Thus, measured electrons can supply the requisite energy with scattering efficiencies of only 3% to 10% of the maximum. Auroral optical emission spectra were not available for this event to estimate independently the electron energies involved. However, recent Galileo auroral observations measured the tangent altitude (above the 1-bar atmospheric pressure level) of peak auroral emissions at 245 ± 30 km, with some emissions extending to an altitude of 120 ± 40 km (ref. 23). Atmospheric penetration of electrons modelled for diffuse aurorae require the involvement of over 48 keV electrons to explain even the peak auroral emissions24,22.
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Acknowledgements
We gratefully acknowledge help from and discussions with R. W. McEntire and T. Choo, and the support of the Space Telescope Institute.
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Mauk, B., Clarke, J., Grodent, D. et al. Transient aurora on Jupiter from injections of magnetospheric electrons. Nature 415, 1003–1005 (2002). https://doi.org/10.1038/4151003a
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DOI: https://doi.org/10.1038/4151003a
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