Observations of Lyman-alpha Emission in Solar Flares

离我们最近的星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越来越依赖于太空技术，太阳的影响可能会变得具有破坏性。每隔一段时间，太阳的辐射输出增加，由于巨大的能量释放称为太阳耀斑。这些爆炸事件会在整个电磁频谱上发出辐射，从无线电波到X射线，在极端情况下，还会发出伽马射线。对地球大气层特别重要的是紫外线和X射线辐射，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。在我们的太阳系之外，随着对潜在可居住系外行星的搜索加剧，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发射可能很难检测到。因此，对太阳耀斑发射的研究扩展到一系列天体物理学研究领域。太阳耀斑辐射的一个组成部分被称为莱曼α辐射，它是由中性氢原子产生的。由于太阳几乎完全由氢组成，莱曼α的发射线是整个太阳光谱中最亮的发射线之一。然而，尽管在这个波长上进行了数十年的太阳观测，我们直到最近才有能力在太阳耀斑期间观察太阳输出的莱曼α光的快速变化。虽然以前报告的几项研究要么没有结论，要么相互矛盾，但最近的一项统计研究现在提供了莱曼α耀斑测量的基线，并证实了莱曼α发射与电离层扰动之间的联系。然而，这些测量是宽带的，并在整个太阳盘上进行了综合（“太阳作为恒星”观测），因此提供的空间或光谱信息很少或根本没有。因此，本项目的主要目标是利用最近公布的在耀斑期间记录的莱曼α光谱数据，对先前的研究采取后续行动，以了解谱线轮廓本身的变化。这些信息对于了解太阳大气中的排放是在什么条件下产生的至关重要。这将有助于详细的比较与国家的最先进的数值模拟耀斑加热的预测。这项建模工作正在与美国宇航局戈达德太空飞行中心的合作者平行进行。该项目的第二个要素是了解为什么发生在太阳边缘附近的太阳耀斑与太阳圆盘上的太阳耀斑相比，产生的莱曼α发射增强较少。这可能是由于从地球上看，在太阳大气层中，莱曼α光子沿着沿着视线的散射增加了，也可能是视角变化带来的光学效应。为了解决这个问题，将从地球和火星同时以不同的分离角度对太阳耀斑进行立体观测，因为目前在这两颗行星的轨道上都有莱曼-阿尔法仪器。这个项目也是及时的，因为它恰逢新的太阳周期的黎明，这将在未来几年带来一系列太阳活动。下一代太阳卫星也将观测即将到来的太阳活动周期。其中包括NASA、ESA、Jaxa和中国最近和即将发射的任务，所有这些任务都将配备莱曼-阿尔法仪器。

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.