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Juno observations of spot structures and a split tail in Io-induced aurorae on Jupiter

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Science  24 Aug 2018:
Vol. 361, Issue 6404, pp. 774-777
DOI: 10.1126/science.aat1450

Moons drive structure in Jupiter's aurorae

Like Earth, Jupiter has aurorae generated by energetic particles hitting its atmosphere. Those incoming particles can come from Jupiter's moons Io and Ganymede. Mura et al. used infrared observations from the Juno spacecraft to image the moon-generated aurorae. The pattern induced by Io showed an alternating series of spots, reminiscent of vortices, and sometimes split into two arcs. Aurorae related to Ganymede could also show a double structure. Although the cause of these unexpected features remains unknown, they may provide a way to examine how the moons produce energetic particles or how the particles propagate to Jupiter.

Science, this issue p. 774

Abstract

Jupiter’s aurorae are produced in its upper atmosphere when incoming high-energy electrons precipitate along the planet’s magnetic field lines. A northern and a southern main auroral oval are visible, surrounded by small emission features associated with the Galilean moons. We present infrared observations, obtained with the Juno spacecraft, showing that in the case of Io, this emission exhibits a swirling pattern that is similar in appearance to a von Kármán vortex street. Well downstream of the main auroral spots, the extended tail is split in two. Both of Ganymede’s footprints also appear as a pair of emission features, which may provide a remote measure of Ganymede’s magnetosphere. These features suggest that the magnetohydrodynamic interaction between Jupiter and its moon is more complex than previously anticipated.

Like Earth, Jupiter has aurorae. These permanent electromagnetic emissions are associated with the precipitation of magnetospheric plasma onto the planet’s ionosphere. Jupiter’s aurorae have emission features, absent at Earth, that are associated with its moons; bright spots appear in the ionosphere at the base of the magnetic field lines that sweep past the Galilean moons Io, Europa, and Ganymede. The discovery of auroral features at the magnetic field footprints of Io (1, 2) showed the electromagnetic interaction between Jupiter and its moons. In situ observations by the Galileo spacecraft of intense bidirectional, magnetic field–aligned electron beams, with energies up to 200 keV, within the wake and over the poles of Io showed that these interactions can lead to strong electron acceleration (35).

Infrared (IR) and ultraviolet (UV) observations of Jupiter’s auroral footprints led to theoretical understanding that the equatorial magnetospheric plasma roughly corotates with the planet, but the moons, which orbit more slowly, form an obstacle to this plasma (6, 7). This interaction leads to the formation of magnetohydrodynamic waves—specifically, Alfvén waves (7, 8)—that accelerate electrons toward the planet’s atmosphere, where they excite auroral emissions (9). Whereas electrons travel rapidly along the magnetic field lines, the slower Alfvén waves are convected downstream, resulting in a standing Alfvén wing that is slightly tilted away from the undisturbed magnetic field line (8). The speeds of the waves are highly variable, depending on the path toward Jupiter through different plasma environments (10). Waves generated at Io first travel a dense plasma torus around the moon and then into the low-density magnetosphere lobe regions (Fig. 1). Successive traversals of plasma discontinuities may lead to multiply reflected Alfvén waves, which has been suggested as a possible cause of the observed multiple footprint features (1113).

Fig. 1 Schematic illustration of the interaction between Io and the magnetosphere of Jupiter.

The figure shows Io, the Io plasma torus (yellow; simplified and not to scale), an unperturbed magnetic field line (cyan), an Alfvén wave (green; shape is simplified and not to scale), the main IFP (red), the footprint tail (cyan), the main auroral arc (blue), and the trajectory and position of the Juno spacecraft.

The observation of auroral H3+ emission in relation to the positions of the moons, where the interaction occurred, is a powerful tool for remote diagnostic of the jovian magnetosphere (9).

The Jovian Infrared Auroral Mapper (JIRAM) (14) on NASA’s Juno spacecraft (15, 16) has been observing the jovian aurora since 2016 (17) from a polar, highly elliptical orbit. From its position above the poles, JIRAM has obtained views of Jupiter’s auroral regions (18). JIRAM is an IR imager and spectrometer; one of the imaging channels operates in the range of 3.3 to 3.6 μm, at which some of the most intense H3+ auroral emission lines are observed. During Juno’s eighth close pass of the planet (1 September 2017), JIRAM targeted the predicted location of the Io footprint (IFP).

In Fig. 2, we show the IFP peak emissions observed by JIRAM on that occasion at a spatial resolution of ~20 km. Io orbits Jupiter rapidly, so its longitude in the reference frame of the planet was ~80°W during the northern observations and ~135°W during the southern observations. In both the northern and southern hemispheres, the main IFP is preceded by a ~1000-km-long, thin emission feature, interpreted as the IR signature of electron beams accelerated in both hemispheres [trans-hemispheric electron beam (TEB)] (13). However, on this occasion, the precursor feature is not detached from the rest of the footprint. The characteristic decay time of H3+ is a few hundred seconds (19)—longer than the almost instantaneous UV hydrogen-related time scale (12), which may explain the blurring and extent of the TEB signature in the IR, unlike previous UV observations (13).

Fig. 2 Images of the IFP (radiance) in polar orthographic projections with parallels and meridians overplotted.

(A) IFP on the south pole of Jupiter, as seen on 1 September 2017 at 22:50. The main spot (top left corner of the image) is followed by a series of regularly spaced secondary spots, which are alternate in displacement above and below the median track (blue). The thin blue arc with dots is the sequence of the predicted IFP positions (20), with dots showing its position every 100 s. Radiances are in mW m−2 sr−1. (B) As in (A), but showing a later image taken at 23:09. The main spot (indicated with a blue arrow) has an oval shape, with an angular offset from the average direction of the tail (median track). (C to E) The IFP in the north aurora, observed at 20:43, 20:48, and 20:53, respectively, on 1 September 2017. The model (20) is not shown in (C), (D), and (E) because it lies outside of the visible region. The black and white pixels [especially in (D)] are the effect of penetrating radiation affecting the detector. The results of Jacobsen et al.’s model is available in (23), figure 3, for comparison.

The southern main spot is followed by a series of regularly spaced secondary spots, which are alternately displaced poleward and equatorward of the median track (Fig. 2A). The transverse displacement is 100 ± 10 km. By tracing back to the orbit of Io by use of a magnetic field model (20), this distance corresponds to 4000 ± 1000 km, which is roughly the size of Io itself. The main spot location is exactly along the track predicted by the model (20), and the repetition step of the features is 350 ± 10 km.

Given the relative velocity of the main spot over the surface of Jupiter, the separation of secondary spots corresponds to ~60 s. The chemical lifetime of H3+ is longer (up to 1000 s) (19) and is expected to produce extensive smearing, which should hide the observed small-scale structure of the spots, as proposed for the leading TEB spot. A possible explanation is that the secondary spots are generated by higher-energy electrons. A model of inertial Alfvén waves acceleration (21) indicates that the electron energy spectrum might be different for the direct, main Alfvén wing and the conjugate (TEB) electron beams. High-energy electrons would precipitate deeper into the atmosphere, producing H3+ ions where their lifetime is shorter because of the high methane density (H3+ destruction occurs via the reaction H3+ + CH4 → CH5+ + H2) (22). An alternative explanation, for the blurring of the precursor feature, could be that the electron beam that creates the TEB spot, formed by up-going electrons from the opposite hemisphere, broadened on its way along the field line through the whole magnetosphere.

The 60-s separation between spots, and the similar intensities of subsequent spots, are difficult to explain with multiple specular reflections, neither inside nor outside the dense plasma torus (10). However, high-amplitude nonlinear waves are generally not reflected specularly. The multiplicity and shape of the spots can be explained by models (23), in which strong nonlinearities trigger wave-interference patterns that lead to the occurrence of multiple spots, depending on Io’s location in the torus. The observations suggest that the Alfvén waves might be broken up into smaller waves with different travel paths, leading to a turbulent emission pattern. The angular offset of the elongated main spot and the alternating displacements of the tail spots might be influenced by the Hall effect in Io’s ionosphere, which is expected to rotate the Alfvén waves (24). Alternatively, because this feature superficially resembles a von Kármán vortex street (a hydrodynamically observed phenomena) (25), this may result from Io-produced flow shears that generate vortices downstream in the plasma-atmosphere interaction region.

In the northern hemisphere (Fig. 2, C to E), the shape of the IFP is generally consistent with that observed in the south: An array of secondary spots is visible in the first part of the tail. Away from the main IFP, the tail becomes gradually more turbulent, likely because of a continuation of the Alfvénic interaction, similar to the main spot (26).

Each spot is ~200 km wide. Because JIRAM was observing from an angle of ~35° between the vertical direction and the line of sight, this means that the height of the main H3+ emission region cannot be taller than 200 km/tan (35°) ≈ 300 km. This short column height is consistent with generation by high-energy electrons (27).

Extrapolating the radiance collected by JIRAM filter to the full H3+ spectrum, and integrating it over the surface, the total radiated energy can be estimated at ~6 GW in the south and ~3 GW in the north, which is ~3 times less than in the UV (28). The north-south asymmetry of the emission, both in shape but especially in intensity, is a recurrent feature of Jupiter’s aurorae (29). Some hypotheses have been proposed (30) to explain this as a consequence of the north-south magnetic field asymmetries: The magnetic field controls the electron acceleration efficiency and the probability for an electron to reach the atmosphere. In our case, the difference may also be due to differing observation times.

The far tail of the IFP as observed on 27 August 2016 in the northern hemisphere is shown in Fig. 3. The main spot is ~100° upstream in longitude from this region. In this sector, we found that the tail consists of two separate parallel arcs. The poleward arc is thicker, more intense, and more turbulent, whereas the equatorward arc is fainter, thinner, and less turbulent. These features are visible in previous and subsequent images (taken within 1 hour) and extend for at least 70° along the tail. This double footprint tail has not been observed in UV images, probably because of insufficient spatial resolution. However, other JIRAM images with similar spatial resolution [(18), their figures 2 and 3] show a single tail, indicating that the double feature is not persistent or not present at all longitudes.

Fig. 3 The IFP tail, observed ~100° behind the main spot.

Image was taken on 27 August 2016 (contour plot). The tail consists of two separate arcs. The blue line is the prediction from a magnetic field model (20). The vertical artefacts on this image are produced by the noise-reduction routine. Local time is approximately dawn.

Compared with the cross-track displacement of the IFP tail close to Io shown in Fig. 2 (~100 km), the separation in Fig. 3 is larger: The parallel tail components are separated by 500 to 1000 km. The viewing angle in Fig. 3 is less than 10°, ruling out the possibility that JIRAM was observing a vertical (rather than horizontal) structure. The difference in the shape and intensity of the two arcs could be due to the Hall effect generating brighter aurorae in Io’s atmosphere on the anti-jovian side than on the jovian side (24). The poleward part of the corresponding footprint on Jupiter should then be brighter than the equatorward part, as observed. We suggest that the smooth tail might arise from trans-hemispheric electrons similarly to those of the TEB (13) feature in Fig. 2A, whereas the other tail might be related to the wave, which would produce a patchier tail. The splitting could then be due to a northern anomaly in Jupiter’s magnetic field (31), which might also explain why this bifurcation is not observed in the southern aurora.

The comparison of the IFP with the footprint of Ganymede (GFP) can provide insight into the mechanisms that generate these morphologies. Shown in Fig. 4 are several images of the GFP (taken 30 s apart), as observed on 11 July 2017 on Juno’s seventh close pass of the planet (north hemisphere); at this time, Ganymede was well south of the plasma sheet. The secondary, upstream spot could be a TEB spot, and the main spot-TEB distance of 1500 km is consistent with similar measurements in the far UV at the same longitude (28). However, the JIRAM images show that each spot consists of two identical peaks ~170 km apart. This separation corresponds to a tangential separation of ~10,000 km at the orbit of Ganymede, if magnetically mapped with a recent model (32) in a way similar to that of the IFP. Each spot is also followed by a very faint tail (26), which appears to be twisted.

Fig. 4 Five consecutive JIRAM images of Ganymede footprints.

(A) Diagram illustrating the main oval (dotted oval), Ganymede footprint (solid line oval) as predicted from a recent magnetic field model (33) and the approximate location of the images in (B) to (F). (B to F) Images are taken from 00:07:45 to 00:09:48 on 11 July 2017 on the north hemisphere (contour plots). The main Ganymede footprint (in the middle of each image, and moving down) is preceded by a dimmer precursor spot.

As with the IFP, the spatial separations of features in the Ganymede footprint are not consistent with the simple specular reflection of Alfvén waves. In the Ganymede case, however, there is no trailing array of spots, with only two visible. A possible explanation is that both the leading and trailing magnetic reconnection regions in Ganymede magnetosphere [(33), their figure 1] generate locally enhanced Alfvén waves. If so, the separation between the peaks is related to the size of Ganymede’s magnetosphere. Both the TEB and the main GFP display this double structure, perhaps suggesting that the cause is located at the origin of both the TEB and the main GFP Alfvén waves. However, other scenarios, such as reflections in the ionospheric resonator (34), could be invoked as well.

Auroral footprints of jovian moons are peculiar, but not unique; a similar footprint of Enceladus is observed at Saturn (35), and even tentative evidence of footprints of extrasolar planets on their central stars exists (36, 37). These auroral processes apply to any electrically conducting moon orbiting within a planet’s magnetosphere. The Juno/JIRAM high-resolution images of the satellite footprints in the IR show substructures, both in the spots and in the extended tail. The close-spaced, multiple spots require a process that occurs at smaller scales than would be produced by a single pair of Alfvén wings, in linear regime, bouncing between hemispheres, suggesting that they are produced by interactions occurring close to the moon, such as a train of vortices generated in Io’s wake. Alternatively, interference between Alfvén waves, rather than reflections, could be responsible. Our images show that the extended tail can occasionally form two parallel arcs. Because this bifurcation is not observed often, it is possible that it is triggered by the northern anomaly in Jupiter’s magnetic field. The Ganymede aurora expresses as twin spots, which may probe Ganymede’s distant magnetosphere using the auroral emission.

References and Notes

Acknowledgments: We thank Agenzia Spaziale Italiana (ASI) for support of the JIRAM contribution to the Juno mission. Funding: JIRAM is funded with ASI contract 2016-353 23-H.0. B.B., D.Gro., and J.-C.G. are supported by the PRODEX program managed by ESA in collaboration with the Belgian Federal Science Policy Office. T.G. is supported by the Juno Project under NASA grant NM06AAa75c to Southwest Research Institute. B.B. is funded by the Fund for Scientific Research (F.R.S.‐FNRS). Authors contributions: Preparation of the manuscript, figures, calculations, data analysis was performed by A.Mu. and A.A.; data interpretation was performed by A.Mu., A.A., J.E.P.C., S.B., F.A., F.B., B.B., B.M.D., J.-C.G., T.G., D.G., S.L., B.M., M.L.M., J.S., J.H.W., and D.T.; revisions of the manuscript were made by A.Mu., A.A., J.E.P.C., S.B., F.A., F.B., B.B., B.M.D., J.-C.G., T.G., D.Gro., S.L., B.M., M.L.M., J.S., and J.H.W.; S.B. is the Juno principal investigator; A.A. is responsible for the JIRAM instrument; preparation for measurements and data acquisition was performed by M.A., A.C., F.F., G.F., D.Gra., A.Mi., R.N., A.O., G.P., C.P., G.S., R.S., F.T., and D.T.; calibration was done by A.A., G.F., A.Mu., and G.P.; and georeferencing was performed by F.T., R.N., and A.Mu. Competing interests: The authors declare no competing interests. Data and materials availability: Data in this study is publicly available on the Planetary Data System (http://pds.nasa.gov) and can be downloaded from http://atmos.nmsu.edu:8080/pds. The individual datasets are available at https://atmos.nmsu.edu/PDS/data/jnojir_xxxx, where xxxx is 1001, 1002, or 1003 for EDR (Experiment Data Record; raw data) and 2001, 2002, or 2003 for RDR (Reduced Data Record; calibrated data) volumes. Noise reduction was performed by using the algorithm in (18); a 2- by 1-pixel filter was applied to remove the odd-even effect.
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