Abstract :
[en] This work focuses on the dynamics of one particular auroral feature in Jupiter’s polar region, which we call Jupiter’s polar auroral bright spot. The bright spot emission usually appears as a compact shape, which is very bright in UV aurorae. It was suggested to be the signature of the magnetospheric cusp and reconnection on the dayside magnetosphere, which is probably associated with the open magnetic field lines connecting to the solar wind. However, the importance of dayside reconnection to the Jovian magnetosphere is still a debated topic. Therefore, the dynamics and processes related to this feature are still unclear. The spacecraft of the recent NASA Juno mission was designed to fly on polar orbit around Jupiter. Juno’s observations thus allow us to make in situ observations with various types of instruments over both poles in very close detail. In addition, Juno can observe Jupiter’s nightside aurora, which cannot be observed by Earth-based instruments. We believe that Juno’s observations provide important information which help us to better understand the bright spot emission and Jupiter’s polar aurorae. In this work, the Juno observations provide detailed information on the bright spot feature.
Firstly, to analyze the bright spot characteristics, we used the data from the first 25 perijoves, observed by the Ultraviolet Spectrograph (Juno-UVS) from August 27, 2016, to May 29, 2019. Here we study their power emissions, sizes, positions, and local times in the ionosphere and their mapped positions and local times in the magnetosphere. The bright spot area is identified by the compact shape whose intensity is greater than twice the standard deviation above the background intensity. The elliptical fit was applied to determine the size of emissions. We found that the bright spot’s power is in the range of gigawatt, while some spots exhibit power up to a hundred gigawatts. The bright spots are found to be located in various SIII longitudes corresponding to various local times, which means that the bright spots do not appear at only noon magnetic local times. Moreover, the bright spots are generally located at the edge of the swirl region, where high-energy particles are usually found. In addition, the bright spots tend to appear in the region with a magnetic field strength greater than 8 Gauss. The mapped positions and local times in the magnetosphere vary as well. These results could exclude the relationship of the bright spot with the noon-magnetospheric cusp process. However, Jupiter’s magnetosphere is more complicated than we thought. The magnetic field can be twisted, implying that the noon magnetic local time might map to a position surprisingly distant from expectations based on a simple dipole assumption (Zhang et al., 2021). Therefore, the role of the magnetospheric cusp process needs further investigation to be confirmed or invalidated. The most interesting result is the reappearance of the bright spot with a time interval of 3 to 47 minutes between two consecutive brightenings. In addition, the bright spot in each perijove usually reappears at almost the same SIII position, implying the corotation of the plasma source with Jupiter. For the bright spot series found in PJ4 and PJ16, in which the observation sequence was long enough to see the bright spot recurring at the same SIII position, the analysis shows that the bright spots found in PJ4 and PJ16 reappear with the periods of 28 and 23 minutes, respectively. This periodicity is reminiscent of the periods observed in auroral radio and X-ray pulsations, which should be studied in deep detail in the future.
In addition, we found that there are three events in which the Juno positions according to the JRM09 magnetic model were above the bright spot positions. We assume that the spacecraft and bright spot emissions are connected by the same magnetic field line. Two cases were found from the first 25 perijoves, a northern spot observed during PJ3 and a southern spot observed during PJ15. An additional bright spot with the same characteristics is the southern spot found in PJ33. Juno carries many instruments for observing Jupiter’s magnetosphere and Jupiter’s aurorae. Considered together, the observations from such instruments provide crucial information for understanding the mechanics related to the bright spot emissions. The particle observations provide characteristics of particles related to auroral emissions. The data of the plasma waves and magnetic field-aligned currents suggest some ionospheric-magnetospheric dynamics related to auroral emissions. Hence, we perform a comparison between the data of bright spot emission observed by UVS remote sensing and the in situ measurements of particles, waves, and currents, observed with JEDI, Waves, and MAG, respectively. During the bright spot detection time, the waves are travelling upward and propagate in Whistler-mode. Particles are dominated by upgoing electrons, suggesting that Juno is above the acceleration region where the downgoing particles in the opposite direction probably propagate down and cause the aurora. Two of three cases (in PJ3 and PJ33) show that the intensification of upward Whistler-mode waves and upward electron enhancement simultaneously occurred, suggesting the presence of waves-particle interaction. The PJ3 spot was also detected during waves-particles enhancement, while the PJ33 spot was detected a few minutes later. The PJ15 event shows that the upgoing Whistler-mode was first detected, followed by bright spot detections and some particle enhancements at the end. However, the signature of magnetic field-aligned currents is less significant. We propose that the imperfection of the magnetic mapping model is the main source of the time mismatch between the wave, particle, and bright spot detection. Even though the times do not exactly match, which may simply be due to the JRM09 mapping errors, we believe that the wave-particle interactions play the main role in accelerating particles causing the bright spot emissions. However, other processes may also play a role in causing the bright spot, such as the magnetic reconnection near Jupiter’s polar magnetosphere (Masters et al., 2021) and the broadband acceleration due to the presence of an ionospheric Alfvén resonator (Lysak et al., 2021).
