Auroral diagnosis of solar wind interaction with Jupiter's magnetosphere

Content Provider	Semantic Scholar
Author	Yao, Zhen Han Bonfond, B. Grodent, D. Chan'e, E. Dunn, William R. Kurth, William S. Connerney, John E. P. Nichols, J. D. Palmaerts, B. Hospodarsky, G. B. Mauk, Barry H. Kimura, Tsuyoshi Bolton, S. J.
Copyright Year	2020
Abstract	Although mass and energy in Jupiter’s magnetosphere mostly come from the innermost Galilean moon Io’s volcanic activities, solar wind perturbations can play crucial roles in releasing the magnetospheric energy and powering aurorae in Jupiter’s polar regions. The systematic response of aurora to solar wind compression remains poorly understood. Here we report the analysis of a set of auroral images with contemporaneous in situ magnetopause detections. We distinguish two types of auroral enhancements: a transient localized one and a long-lasting global one. We show that only the latter systematically appears under a compressed magnetopause, while the localized auroral expansion could occur during an expanded magnetopause. Moreover, we directly examine previous theories on how solar wind compressions enhance auroral emissions. Our results demonstrate that auroral morphologies can be diagnostic of solar wind conditions at planets when in situ measurements are not possible. Introduction Jupiter has the brightest aurorae of all the planets in our solar system, facilitating the remote observation of energy dissipation across vast distances1. The auroral power can significantly vary by orders of magnitude in time scales ranging from tens of seconds2 to several hours3, and can be observed at different wavelengths4-7. Hubble Space Telescope (HST) provides high-resolution ultraviolet (UV) images of Jupiter’s aurora for more than 20 years, and have resolved key auroral components of the aurora, which consists of a main auroral oval, a dark region on the dawnside, a polar swirl region and a polar active region8. The main auroral oval is traditionally suggested driven by a magnetosphere-ionosphere coupling current system due to the breakdown of rigid corotation of plasma in the middle magnetosphere9-11. Observations of multiple light bands show auroral enhancements during solar wind compressions4,12-15, contradicting the theoretical predictions based on steady-state assumptions9,16. New interpretations from time-varying modeling17 and numerical simulations18 were thus proposed to mitigate the proliferating conflict between observation and classical steady-state theoretical prediction. Recent study revealed that solar wind shocks and auroral brightening are coupled by very complicated relations19. Moreover, Kita, et al. 19 indicated that it requires substantial time for Jupiter’s magnetosphere to response to solar wind shock arrival at the upstream magnetosphere boundary, i.e., the magnetopause. Juno’s first 7 apojove periods provided direct examination of magnetopause compression20, which could eliminate uncertainty in solar wind propagation models and could mostly exclude the response time to the solar wind compression at the magnetopause. Meanwhile, HST was planned to regularly monitor Jupiter’s UV aurora during these orbits. Therefore, we could perform a systematic determination on the relation between the magnetopause compression and auroral activities, which is pivotal to assess the proposed interpretations from modeling and simulation investigations. Results One of the regular sequences of HST UV imaging observations in coordination with the Juno spacecraft21 was planned from January 22 to 27 2017. Fig. 1 shows the projections of auroral images onto Jupiter’s northern pole, and each image over about 40 minutes. On January 22, there was an auroral brightening around the dawn arc (Fig. 1a), which was not found in the successively available HST image in Fig. 1b (~29 hours later). Similar auroral enhancements on the dawn local times with significant expansions in latitudes have been identified as auroral dawn storms (ADS)22. The auroral image shown in Fig. 1c was obtained ~19 hours after Fig. 1b, which shows a global enhancement in all local times within HST’s field of view. The dawn arc enhancement is relatively narrow in width, i.e., the direction perpendicular to the average main auroral oval (the white curve, indicated by the pink arrow in Fig. 1c), which is named main auroral brightening (MAB) in this letter. If we take the power in Fig. 1b (i.e., 1045 GW) as the baseline of quiet Jovian aurorae, the total auroral power in Fig. 1c is a factor of two higher than the total power in Fig. 1b. This auroral morphology remained similar and the power further increased to 2430 GW in the following HST visit (Fig. 1d, ~1.5 hour later). The thin enhanced auroral arc on the dawn to noon local times remained in Fig 1e while with significantly decreased power and return to almost quiet time auroral power in Fig 1f. Therefore, the MAB event likely lasted for about 2 to 3 days, consistent with previous reports on main auroral enhancements during solar wind compression based on the analysis of observations from HST14 and Hisaki23. Figure 1| Polar projections of six auroral images from January 22 to 27 2017. Each image was averaged over ~40 minutes. The main oval (indicated by the pink arrow in panel c) is an average main auroral oval location. In addition to the timescales for which they exist, two different types of morphologies also allow us to distinguish the ADS in Fig. 1a from the MAB in Fig. 1c-e: (1) the MAB’s enhanced dawn arc extended to near-noon local times, while the ADS in Fig. 1a was limited to the dawn local times before 9h; (2) the dawn auroral arc along the reference main oval is thinner and smoother for the MAB, but thicker and more variable along the reference oval for the ADS. We therefore define two parameters, i.e., mean arc width of the dawn aurora and the variation of auroral width along the main oval reference, for characterizing the two types of auroral morphologies. The quantitative analysis is provided in the Methods section. The mean arc width and the variation of the ADS in Fig. 1a are 1468 km and 548 km. The two values are 626 km and 262 for the MAB in Fig. 1c. The mean arc width and variation in the ADS are a factor of two larger than for the MAB. A key unsolved question is whether or not the two auroral morphologies correspond to fundamentally different drivers. Previous studies have suggested that brightening of the aurora is associated with solar wind conditions14,18,24, however, whether solar wind conditions drive both ADS and MAB is unclear. Disentangling this solar wind influence from other drivers is critical for auroral interpretation. Since the two auroral events were successively observed separated by two days, it indicates that a complete transition between the two types of auroral morphologies could be shorter than two days. Therefore, it is insufficient to apply a modeling solar wind prediction to assess whether or not the two auroral events happened under different solar wind conditions, since the modeling prediction of solar wind condition usually involves an uncertainty of more than two days even in ideal conditions25. Here we directly identify magnetopause crossings using Juno’s Waves instrument26 and MAG instrument27 in coordination with HST’s auroral context. Using the magnetopause model by Joy, et al. 28, we know that the magnetopause for a compressed magnetosphere in the dawn sector is at ~90 RJ, whereas for an expanded magnetosphere it will at ~130 RJ. We can therefore identify intervals when the magnetosphere is compressed, so that we accurately assess the influences of solar wind compressions on auroral activities, and provide key information to answer two questions: (1) how does the solar wind modulate Jupiter’s main aurora? (2) ADS have previously been observed during solar wind compressions24,29, is there a physical causality or was this coincidence? Interestingly, the hectometric radio emission with frequency of several MHz (Fig. 2a) was enhanced since January 24 when the MAB auroral event was observed, but not significantly enhanced for the ADS event on January 22. We note that the hectometric emission remained enhanced for at least two days after the MAB auroral event (e.g., Fig. 1e). Below we analyze Juno’s in situ measurements to determine the extent to which solar wind conditions control the different auroral morphologies. The intense emissions with frequencies between about 200 Hz and 2 kHz (Fig. 2b) are the trapped continuum radiation30,31. The appearance (or disappearance) of the emission serves as a good indicator of entry into the magnetosphere (or exit into the magnetosheath)20,32,33. During ultraviolet auroral observations in Fig. 1, Juno traveled inbound from >110 RJ to ~70 RJ to the planet center (1 RJ = 71,492 km) in the sector (near 05:00 Magnetic Local Time), and encountered an inward moving magnetopause on January 24 at ~78 RJ, so that Juno was exposed to the magnetosheath thereafter. The nominal magnetopause location on the dawnside is at > 100 RJ but it can move to ~70 80 RJ during strong compressed situations, as suggested by both models and Juno’s statistical results20,28. On January 26th Juno returned to the magnetosphere (evidenced by the reappearance of the trapped continuum radiation), which is likely due to the recovery of magnetopause to a more probable location. Based on the wave feature, we could determine that Juno was in the magnetosheath during the period marked by the green bar on the top of panel (b) in Fig. 2, and in the magnetosphere during the rest of this period. The strongly perturbed magnetic field between January 25 and 26 also confirms that Juno was in the magnetosheath. The wave frequencies, which reflect the plasma number density32, has significantly increased shortly before (after the first vertical dashed purple line) Juno’s entry into the magnetosheath. The density increase is a naturally expected consequence of magnetopause compression20,32,33, confirming our determination of compression from the appearance (or disappearance) of the trapped continuum radiation. Juno rapidly encountered the magnetopause after compression (marked by the first vertical dashed purple line), and the spacecraft remained in a compressed region for ab
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