Alternating North-South Brightness Ratio of Ganymede's Auroral Ovals: Hubble Space Telescope Observations Around the Juno PJ34 Flyby

We report results of Hubble Space Telescope observations from Ganymede’s orbitally trailing side which were taken around the flyby of the Juno spacecraft on June 7, 2021. We find that Ganymede’s northern and southern auroral ovals alternate in brightness such that the oval facing Jupiter’s magnetospheric plasma sheet is brighter than the other one. This suggests that the generator that powers Ganymede’s aurora is the momentum of the Jovian plasma sheet north and south of Ganymede’s magnetosphere. Magnetic coupling of Ganymede to the plasma sheet above and below the moon causes asymmetric magnetic stresses and electromagnetic energy fluxes ultimately powering the auroral acceleration process. No clear statistically significant time variability of the auroral emission on short time scales of 100s could be resolved. We show that electron energy fluxes of several tens of mW m−2 are required for its OI 1356 °A 38 emission making Ganymede a very poor auroral emitter.

我们报告了哈勃空间望远镜（Hubble Space Telescope）于2021年6月7日朱诺号探测器（Juno spacecraft）飞掠期间，对木卫三（Ganymede）轨道后缘侧的观测结果。我们发现木卫三的南北极光椭圆（auroral ovals）亮度会交替变化，面向木星磁层等离子体片（magnetospheric plasma sheet）的椭圆比另一侧的更亮。这表明，驱动木卫三极光的能量来源是木卫三磁层南北两侧木星等离子体片（Jovian plasma sheet）的动量。木卫三与该卫星上下方等离子体片的磁耦合会产生不对称磁应力和电磁能量通量，最终为极光加速过程提供能量。在100秒的短时间尺度上，未观测到极光发射具有明显的统计显著性时间变异性。我们发现，其OI 1356埃（Å）发射线需要数十毫瓦每平方米（mW m⁻²）的电子能量通量，这使得木卫三成为一个极弱的极光发射体。