Observations of Lyman-alpha Emission in Solar Flares

距离我们最近的恒星太阳对地球上的生命有着深远的影响。随着我们作为一个社会变得越来越依赖基于太空的技术，太阳的影响可能会变得具有破坏性。由于被称为太阳耀斑的巨大能量释放，太阳的辐射输出偶尔会增加。这些爆炸事件会在整个电磁波频谱上发射辐射，从无线电波到X射线，在极端情况下，还会发射伽马射线。对地球大气层特别重要的是紫外线和X射线的发射，这可能导致电离层的化学和动态变化。这可能会对卫星阻力、无线电通信和GPS精度产生连锁反应。了解这些辐射增加的原因和影响是空间气象研究的一个关键组成部分。在太阳系之外，随着寻找潜在宜居系外行星的力度加大，关于恒星耀斑发射的知识也至关重要。然而，由于星际介质的衰减，来自其他恒星的这种辐射可能很难检测到。因此，对太阳耀斑发射的研究扩展到了一系列天体物理研究领域。太阳耀斑辐射的一个组成部分被称为莱曼-阿尔法辐射，它由中性氢原子产生。由于太阳几乎全部由氢组成，因此莱曼-阿尔法的发射线是整个太阳光谱中最亮的之一。然而，尽管在这个波长进行了几十年的太阳观测，我们直到最近才有能力描述太阳在太阳耀斑期间输出的莱曼-阿尔法光的快速变化。虽然以前报道的少数几项研究要么没有定论，要么相互矛盾，但最近的一项统计研究现在提供了莱曼-阿尔法耀斑测量的基线，并确认了莱曼-阿尔法辐射和电离层扰动之间的联系。然而，这些测量是宽带的，并在整个太阳圆盘上进行综合(“太阳作为恒星”的观测)，因此提供的空间或光谱信息很少或根本没有。因此，这个项目的主要目标是通过使用最近发布的耀斑期间记录的莱曼-阿尔法光谱数据来跟踪这项先前的研究，以了解谱线轮廓本身的变化。这一信息对于理解太阳大气中的排放物是在哪里产生的以及在什么条件下产生至关重要。这将通过与最先进的耀斑加热数值模拟的预测进行详细比较而得到帮助。这项建模工作是与NASA戈达德太空飞行中心的合作者们并行进行的。该项目的第二个要素是了解为什么与太阳圆盘上的耀斑相比，发生在太阳边缘附近的太阳耀斑对莱曼-阿尔法辐射的增强作用较小。这可能是由于莱曼-阿尔法光子在太阳大气层内沿地球视线的散射增加，也可能是视角变化带来的光学效应。为了解决这个问题，将同时从地球和火星以不同的分离角度对太阳耀斑进行立体观测，因为目前这两个行星的轨道上都有莱曼-阿尔法仪器。这个项目也是及时的，因为它恰逢一个新的太阳周期的黎明，这将在未来几年带来一系列的太阳活动。下一代太阳卫星也将观测到即将到来的太阳周期。其中包括美国国家航空航天局、欧洲航天局、雅克萨和中国最近和即将发射的任务，所有这些任务都将以莱曼-阿尔法仪器为特色。

Our nearest star, the Sun, has a profound influence on life on Earth. As we as a society become evermore reliant on space-based technology, the Sun's influence can become disruptive. Every so often, the Sun's radiative output increases due to a colossal release of energy known as a solar flare. These explosive events emit radiation across the entire electromagnetic spectrum, from radio waves to X-rays, and in extreme cases, gamma-rays. Of particular importance to Earth's atmosphere are the ultraviolet and X-ray emission, which can lead to chemical and dynamic changes in the ionosphere. This can have knock-on effects on satellite drag, radio communication, and GPS accuracy. Understanding the cause and effect of these increases in radiation is a key component of space weather research. Outside our solar system, knowledge of stellar flare emission is also crucial as the search for potentially habitable exoplanets intensifies. However, this emission from other stars can be difficult to detect due to attenuation by the interstellar medium. The study of solar flare emission therefore extends to a range of astrophysical research areas. One component of solar flare radiation is known as Lyman-alpha emission, which is generated by neutral hydrogen atoms. As the Sun is comprised almost entirely of hydrogen, the emission line of Lyman-alpha is one of the brightest in the entire solar spectrum. However despite decades of solar observations at this wavelength, we have not had the capability to characterise the rapid variability in the Sun's output in Lyman-alpha light during solar flares until relatively recently. While the few studies that were previously reported were either inconclusive or contradicted one another, a recent statistical study has now provided a baseline of Lyman-alpha flare measurements, as well as confirming the link between Lyman-alpha emission and ionospheric disturbances. However, these measurements were broadband and integrated over the entire solar disc ("Sun-as-a-star" observations), and therefore provided little or no spatial or spectral information. The principle objective of this project is therefore to follow up this previous study by using recently released Lyman-alpha spectral data recorded during flares to understand changes in the line profile itself. This information is vital in understanding where in the solar atmosphere the emission is generated, and under what conditions. This will be aided by detailed comparison with predictions made by state-of-the-art numerical simulations on flare heating. This modelling effort is being carried out in parallel with collaborators at NASA's Goddard Space Flight Center. The second element of this project is to understand why solar flares that occur close the limb of the Sun produce less of an enhancement in Lyman-alpha emission compared to those on the solar disc. This may be due to increased scattering of Lyman-alpha photons within the Sun's atmosphere along the line of sight as viewed from the Earth, or it may be an optical effect brought about by the change in viewing angle. To address this question, stereoscopic observations of solar flares will be investigated from both Earth and Mars at different separation angles simultaneously as there are currently Lyman-alpha instruments in orbit around both planets. This project is also timely as it coincides with the dawn of a new solar cycle which will bring a flurry of solar activity over the coming years. The upcoming solar cycle will also be observed by the next generation of solar satellites. These included recently and soon-to-be launched missions by NASA, ESA, Jaxa and China, all of which will feature Lyman-alpha instruments.