木星非热硬X射线的观测与起源
Observation and origin of non-thermal hard X-rays from Jupiter
AI总结：
本研究利用NuSTAR首次观测到木星极光8-20 keV的硬X射线，结合Juno飞船数据，证实其源于电子随机加速产生的非热轫致辐射，揭示了地球与木星硬X射线极光机制的相似性。
摘要：
在地球上，由丰富多样的波散射或随机过程加速的电子产生高达约1 MeV的硬非热X射线轫致辐射，并为地球各类极光提供能量。尽管木星的磁场比地球大一个数量级，但天基望远镜此前仅探测到高达约7 keV的X射线。基于木星极光X射线产生的理论模型，2-7 keV波段的X射线发射被解释为热（源于以Maxwell-Boltzmann分布为特征的电子）轫致辐射。在此，我们报告了利用NuSTAR X射线天文台观测到的来自木星极光的8-20 keV波段硬X射线。这些X射线拟合为斜率为0.60+/-0.22的平幂律模型——这是非热硬X射线轫致辐射的光谱特征。我们利用Juno飞船的JADE和JEDI仪器进行的同期原位测量，确定了keV到MeV能量范围内的电子通量和光谱形状。我们观察到的木星电子光谱形式，此前被解释为产生于随机加速，而非电场的相干加速。通过在木星大气模型中模拟经历沉降电子能量损失、次级电子产生和轫致辐射发射的非热电子群，我们重现了NuSTAR观测到的X射线光谱形状和近似通量，并解释了Ulysses未探测到硬X射线的原因。结果突出了地球和木星上产生硬X射线极光的过程之间的相似性，这在土星上可能也在发生。
英文摘要：
Electrons accelerated on Earth by a rich variety of wave scattering or stochastic processes generate hard non-thermal X-ray bremsstrahlung up to >~ 1 MeV and power Earth's various types of aurorae. Although Jupiter's magnetic field is an order of magnitude larger than Earth's, space-based telescopes have previously detected X-rays only up to ~7 keV. On the basis of theoretical models of the Jovian auroral X-ray production, X-ray emission in the ~2-7 keV band has been interpreted as thermal (arising from electrons characterized by a Maxwell-Boltzmann distribution) bremsstrahlung. Here we report the observation of hard X-rays in the 8-20 keV band from the Jovian aurorae, obtained with the NuSTAR X-ray observatory. The X-rays fit to a flat power-law model with slope 0.60+/-0.22 - a spectral signature of non-thermal, hard X-ray bremsstrahlung. We determine the electron flux and spectral shape in the keV to MeV energy range using coeval in situ measurements by the Juno spacecraft's JADE and JEDI instruments. Jovian electron spectra of the form we observe have previously been interpreted to arise in stochastic acceleration, rather than coherent acceleration by electric fields. We reproduce the X-ray spectral shape and approximate flux observed by NuSTAR, and explain the non-detection of hard X-rays by Ulysses, by simulating the non-thermal population of electrons undergoing precipitating electron energy loss, secondary electron generation and bremsstrahlung emission in a model Jovian atmosphere. The results highlight the similarities between the processes generating hard X-ray auroras on Earth and Jupiter, which may be occurring on Saturn, too.
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