Global heating of Jupiter’s upper atmosphere by auroral energy circulation

Giant planet upper atmospheres have long been observed to be signiﬁcantly hotter than 1 expected 1–4 . Magnetosphere-atmosphere coupling processes give rise to auroral emissions 2 and enormous energy deposition near the magnetic poles, explaining high temperatures for 3 narrow regions of the planet. However, global circulation models have difﬁculty redistribut- 4 ing auroral energy globally due to the strong Coriolis forces and ion drag 4–6 . Heating by 5 solar photons is insufﬁcient at giant planets, and yet other proposed processes, such as heat- 6 ing by waves originating from the lower atmosphere, also fail to explain the warm equatorial 7 temperatures 7 . There remains no self-consistent explanation for measured non-auroral tem- 8 peratures at present, mostly due to a lack of deﬁnitive observational constraints. Here, using 9 high-resolution maps capable of tracing global temperature gradients at Jupiter, we show 10 that upper-atmosphere temperatures decrease steadily from the aurora to the equator. Dur- 11 ing a period of enhanced auroral activity, likely driven by a coincident solar wind compres- 12 sion event, we also ﬁnd a global increase in temperature accompanied by a high temperature 13 planetary-scale structure that appears to emanate from the auroral region. These observa- 14 25 2017 10:22 UTC 11:36 16:28 UTC, long 0.432 (cid:48)(cid:48) wide and each along the slit had a angular resolution of 0.144 (cid:48)(cid:48) per pixel. The spectral resolution was λ / δλ ∼ 25,000. On 14 April, each of the 115 recorded spectral images of Jupiter were 30 seconds long and formed by six integrations each 240 ﬁve seconds long, while on 25 Jan. the 80 recorded spectral images of Jupiter were 60 seconds long 241 and formed by six integrations each ten second long. The process of saving spectral images and 242 nodding the between positions results in overhead time which led to an average elapsed time between Jupiter spectra of 2.4 minutes (14 April) and 3.4 minutes (25 Jan.), so Jupiter rotated a respective 1.4 ◦ and ◦ in longitude during this time.

oval traces on average to 30 R J in Jupiter's equatorial plane (1 R J is Jupiter's equatorial radius of markers, mapping out from the planet to 5.9 R J and 2.54 R J in the equatorial plane, respectively.

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Temperatures generally decrease from 1000 to 600 K between auroral latitudes and the equa-68 tor, as shown by Fig. 2 and 3. H + 3 densities, which are enhanced by aurorally-driven charged with it. This must then be facilitated principally by equatorward propagating meridional winds.

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The Jovian magnetosphere, which is subjected to the solar wind, compresses in response to 78 high solar wind dynamic pressure 20, 23 . One model has shown that magnetospheric compression 79 events could lead to propagation of heat away from the main auroral oval towards the equator and 80 polar cap, introducing a temporary, local temperature increase of 50 -175 K 6, 24 . Temperatures 81 were higher planet-wide on 25 Jan., as were main oval H + 3 densities, so a solar wind propagation corresponds to the magnetic footprint of Amalthea (as described in the main text). A visible computer-generated globe of Jupiter is shown underneath the H + 3 temperature projection. Note that Jupiter is tilted differently on each date in order to reveal different features. The longitude and latitude gridlines shown are spaced in 60 • and 10 • increments, respectively. Median (and maximum) uncertainty percentiles are 2.2% (5%) for 14 April 2016 and 1.6% (5%) for 25 Jan. 2017. The Methods section describes the mapping process and Table S1 show the spatial bin sizes that were used in each projection.  percentiles for 14 April 2016 are: temperature 2.2% (5%), density 9.4% (15%) and radiance 2.2% (5%). Median (and maximum) uncertainties for 25 Jan. 2017 are: temperature 1.6% (5%), density 5.8% (15%) and radiance 1.8% (5%).
The Methods section describes the mapping process and Tables S1, S2 and S3 show the bin sizes that were used in each parameter map.
observations reported here (relative to 14 April), we expect that auroral energy deposition was larger on 25 Jan. Factoring in the uncertainty of the modelled solar wind arrival time at Jupiter, 91 ±1 days on 14 April and ±1.5 days on 25 Jan., we conclude that Jupiter was observed to be in the 92 midst of a global heating event owing to solar wind compression of the Jovian magnetosphere on 93 25 Jan.

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An unusual high temperature structure was found on 25 Jan. equatorward of the main au-95 roral oval, extending for 160 • longitude and straddling the fiducial footprint of Amalthea. Here, 96 relatively cold ∼800 K atmosphere is surrounded by hot auroral and sub-auroral atmosphere at 97 ∼1000 K. We propose that this structure is a 'snapshot' of a large region of heated upper atmo-98 sphere caught propagating equatorward away from the main auroral oval, after a 'pulse' in solar 99 wind pressure was exerted on the magnetosphere 6 . An accurate calculation of the wave propaga-100 tion speed is difficult as we lack knowledge of the longitudinal and latitudinal velocity vectors,    190-192 (2016       3. b and e by bin size. Each cell of Fig. 3. is of dimension 2 • ×2 • longitude × latitude, with the majority of data in each of them being sourced from the 2 • ×2 • bin size   Fig. 3. c and f by bin size. Each cell of Fig. 3. is of dimension 2 • ×2 • longitude × latitude, with the majority of data in each of them being sourced from the 2 • ×2 • bin size      05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24   14 April 2016   05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 January 2017   16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 01 02 03 04 Tao-MHD Solar Wind model uncertainty Tao