Kliore A J, Hinson D P, Flasar F M, Nagy A F, Cravens T E
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA.
Science. 1997 Jul 18;277(5324):355-8. doi: 10.1126/science.277.5324.355.
The Galileo spacecraft performed six radio occultation observations of Jupiter's Galilean satellite Europa during its tour of the jovian system. In five of the six instances, these occultations revealed the presence of a tenuous ionosphere on Europa, with an average maximum electron density of nearly 10(4) per cubic centimeter near the surface and a plasma scale height of about 240 +/- 40 kilometers from the surface to 300 kilometers and of 440 +/- 60 kilometers above 300 kilometers. Such an ionosphere could be produced by solar photoionization and jovian magnetospheric particle impact in an atmosphere having a surface density of about 10(8) electrons per cubic centimeter. If this atmosphere is composed primarily of O2, then the principal ion is O2+ and the neutral atmosphere temperature implied by the 240-kilometer scale height is about 600 kelvin. If it is composed of H2O, the principal ion is H3O+ and the neutral temperature is about 340 kelvin. In either case, these temperatures are much higher than those observed on Europa's surface, and an external heating source from the jovian magnetosphere is required.
伽利略号航天器在环绕木星系统运行期间，对木星的伽利略卫星木卫二进行了6次无线电掩星观测。在这6次观测中的5次里，这些掩星现象揭示了木卫二存在一个稀薄的电离层，其表面附近的平均最大电子密度接近每立方厘米10⁴个，从表面到300千米处的等离子体标高约为240±40千米，在300千米以上则为440±60千米。这样一个电离层可能是由太阳光电离以及木星磁层粒子撞击产生的，其所在大气层的表面密度约为每立方厘米10⁸个电子。如果这个大气层主要由O₂构成，那么主要离子是O₂⁺，由240千米标高所暗示的中性大气温度约为600开尔文。如果它由H₂O构成，主要离子是H₃O⁺，中性温度约为340开尔文。在任何一种情况下，这些温度都远高于在木卫二表面观测到的温度，因此需要来自木星磁层的外部加热源。
