How do weak shocks accelerate high energy particles?

冲击波在宇宙中随处可见，是加速质子和电子等粒子的最有效方法之一。然而，产生这些冲击波和加速粒子所需的条件是如此极端，以至于它们不可能在地球上重现。因此，我们仍然不知道这些激波是如何加速粒子的，或者它们是如何受到密度或磁场等因素的影响的。产生这些高能粒子的大多数激波也都在其他星系中非常遥远的地方，这使得对它们进行适当的研究变得困难。例如，虽然我们可以用天文望远镜看到超新星冲击，但确实很难识别和研究它所加速的粒子。然而，地球靠近一个密度、温度和磁场变化极大的自然实验室，它经常产生大规模的冲击，向我们喷洒高能粒子--太阳。我们有一队航天器返回对太阳的持续观测，使我们能够近乎实时地看到太阳大气中存储的磁能(也称为日冕)的突然释放。这种能量释放可以产生我们称为太阳耀斑的辐射爆发，向太阳系向地球抛出被称为日冕物质抛射的巨大等离子体气泡，并发出巨大的全球冲击波，这些冲击波可以在不到一小时的时间内穿过太阳。尽管这些冲击波比超新星弱得多，它们不应该能够加速任何粒子，但它们经常产生数十亿个高能粒子，我们几乎可以立即在地球上检测到这些粒子。这些粒子对绕地球运行的卫星可能是致命的，会使它们失明并导致它们失败，对GPS和电信产生连锁反应。通过我的研究，我试图理解这些非常微弱的冲击为什么会发生，它们如何将粒子加速到令人难以置信的高能量，以及这些高能粒子如何影响地球和近地环境。太阳提供了一个独特的机会，可以以前所未有的细节研究极端冲击及其同时加速的粒子；我们可以看到发生了什么，并“触摸”产生的粒子，这是你在任何其他天体物理学领域都做不到的。关于这种情况的一切也都非常违反直觉；太阳是一颗相当普通的恒星，产生的冲击波非常弱，不应该能够加速任何粒子，但它却设法将粒子加速到令人难以置信的高能量。这是如何发生的仍然是一个悬而未决的问题，这不仅对我们对太阳的理解有影响，而且对基本的等离子体物理学和太空天气也有影响。如果我们知道这个过程是如何工作的，我们就可能预测到它，这将有助于我们保护地球上脆弱的航天器和基础设施。然而，在更个人化的层面上，研究这个话题确实揭示了从地面上看太阳时的平静程度与我们在太空中看到的剧烈活动的太阳爆发之间的差异，我只是认为这很吸引人。

Shock waves are found everywhere in the Universe and are one of the most efficient ways of accelerating particles like protons and electrons. However, the conditions required to produce those shocks and accelerate particles are so extreme that they're impossible to recreate on Earth. As a result, we still don't know a lot about how these shocks accelerate particles or how they're affected by things like density or magnetic field. Most of the shocks that produce these very high energy particles are also incredibly far away in other galaxies, making them difficult to study properly. For example, while we can see a supernova shock using astronomical telescopes, it's really hard to then identify and study the particles it accelerates.However, the Earth is located close to a natural laboratory with extreme density, temperature and magnetic field variations which regularly produces large-scale shocks that shower us with energetic particles; the Sun. We have a fleet of spacecraft returning constant observations of the Sun, allowing us to see in near-real-time the sudden release of stored magnetic energy in the solar atmosphere (also called the corona). This energy release can produce bursts of radiation that we call solar flares, hurl massive bubbles of plasma called coronal mass ejections into the solar system towards the Earth and launch vast global shock waves that can travel across the Sun in under an hour. Although these shocks are so much weaker than supernovae that they shouldn't be able to accelerate any particles, they regularly produce billions of energetic particles that we can almost immediately detect at Earth. These particles can be fatal for satellites orbiting the Earth, blinding them and causing them to fail, with knock-on effects for GPS and telecommunications. With my research, I'm trying to understand why these really weak shocks occur, how they accelerate particles to incredibly high energies and how those energetic particles affect the Earth and the near-Earth environment.The Sun offers a unique opportunity to study both extreme shocks and the particles that they accelerate at the same time in unprecedented detail; we can see what happens and "touch" the resulting particles, which is something that you can't do in any other field of astrophysics. Everything about this situation is also very counterintuitive; the Sun is a pretty average star producing very weak shocks that shouldn't be able to accelerate any particles yet it manages to accelerate particles to incredibly high energies. How this happens is still an open question, and one that has implications not just for our understanding of the Sun, but also for fundamental plasma physics and space weather. If we know how this process works we might be able to predict it, which will help us to protect vulnerable spacecraft and infrastructure on Earth. On a more personal level though, working on this topic really hammers home the differences between how calm the Sun is when you look at it from the ground versus the violently active Sun producing solar eruptions which we see from space, which I just think is fascinating.