Observations of Uranus in the near-infrared by ground-based telescopes from 1992 to 2018 have shown that the planet's upper atmosphere (thermosphere) steadily cooled from ∼700 to ∼450 K. We explain this cooling as due to the concurrent decline in the power of the solar wind incident on Uranus' magnetic field, which has dropped by ∼50% over the same period due to solar activity trends longer than the 11-year solar cycle. Uranus' thermosphere appears to be more strongly governed by the solar wind than any other planet where we have assessed this coupling so far. Uranus' total auroral power may also have declined, in contrast with the power of the radio aurora that we expect has been predominantly modulated by the solar cycle. In the absence of strong local driving, planets with sufficiently large magnetospheres may also have thermospheres predominantly governed by the stellar wind, rather than stellar radiation.

中文翻译：

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太阳能风能可能控制天王星的热层温度


1992 年至 2018 年用地面望远镜在近红外中对天王星的观测表明，这颗行星的上层大气（热层）从 ∼700 K 稳步冷却到 ∼450 K。我们将这种冷却解释为由于太阳风对天王星磁场的强度同时下降，由于太阳活动趋势超过11年太阳周期，该磁场在同一时期下降了约50%。天王星的热层似乎比我们迄今为止评估过这种耦合的任何其他行星都更强烈地受到太阳风的控制。天王星的总极光功率也可能有所下降，相比之下，我们预计射电极光的功率主要受到太阳周期的调节。在没有强大的本地驱动力的情况下，具有足够大磁层的行星也可能具有主要由恒星风而不是恒星辐射控制的热层。