Europa’s Optical Aurora
Europa’s Optical Aurora
复制标题
欧罗巴的光学极光
作者：
K. de Kleer;M. Brown
极光发射为研究太阳系卫星的稀薄大气层提供了机会，揭示了分子和原子种类的存在和丰度及其时空变异性。几十年来，远紫外极光一直被用来研究伽利略卫星的大气层。在这里，我们首次探测到了欧罗巴可见波长的原子氧极光，其波长为6300/6364 nm，由亚稳态产生，这是在2018年2月至4月期间，当欧罗巴被木星日食时，用凯克一号和哈勃太空望远镜观测到的。盘积分的O（1D）亮度在不同日期之间的变化范围从<500 R到超过2 kR，比O i 1356的平均亮度高出15倍。在6300/5577的发射比是母体分子的特征;在我们的数据集中没有检测到5577的发射，这有利于O2作为主要的大气成分，并排除了O/O2混合比高于0.35。在木卫二轨道上的O2大气和典型的等离子体条件下，测得的表面亮度范围对应于（1-9）× 1014 cm−2的柱密度。
Auroral emissions provide opportunities to study the tenuous atmospheres of solar system satellites, revealing the presence and abundance of molecular and atomic species as well as their spatial and temporal variability. Far-UV aurorae have been used for decades to study the atmospheres of the Galilean satellites. Here we present the first detection of Europa’s visible-wavelength atomic oxygen aurora at 6300/6364 Å arising from the metastable ) state, observed with the Keck I and Hubble Space Telescope while Europa was in eclipse by Jupiter on six occasions in 2018 February–April. The disk-integrated O(1D) brightness varies from <500 R up to more than 2 kR between dates, a factor of 15 higher than the O i 1356 Å brightness on average. The ratio of emission at 6300/5577 Å is diagnostic of the parent molecule; the 5577 Å emission was not detected in our data set, which favors O2 as the dominant atmospheric constituent and rules out an O/O2 mixing ratio above 0.35. For an O2 atmosphere and typical plasma conditions at Europa’s orbit, the measured surface brightness range corresponds to column densities of (1–9) × 1014 cm−2.