Autogenous and efficient acceleration of energetic ions upstream of Earth’s bow shock,Nature

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Autogenous and efficient acceleration of energetic ions upstream of Earth’s bow shock
Nature ( IF 50.5 ) Pub Date : 2018-09-01 , DOI: 10.1038/s41586-018-0472-9
D L Turner ₁ , L B Wilson ₂ , T Z Liu ₃ , I J Cohen ₄ , S J Schwartz ₅ , A Osmane _{6,

7} , J F Fennell ₁ , J H Clemmons ₁ , J B Blake ₁ , J Westlake ₄ , B H Mauk ₄ , A N Jaynes ₈ , T Leonard ₉ , D N Baker ₉ , R J Strangeway ₃ , C T Russell ₃ , D J Gershman ₂ , L Avanov ₂ , B L Giles ₂ , R B Torbert _{10,

11} , J Broll _{11,

12} , R G Gomez ₁ , S A Fuselier _{11,

12} , J L Burch ₁₁

Affiliation

Space Sciences Department, The Aerospace Corporation, El Segundo, CA, USA.
NASA Goddard Space Flight Center, Greenbelt, MD, USA.
Department of Earth, Planetary, and Space Science, University of California, Los Angeles, CA, USA.
Applied Physics Laboratory, Laurel, MD, USA.
Imperial College London, London, UK.
School of Electrical Engineering, Aalto University, Espoo, Finland.
Rudolf Peierls Centre of Theoretical Physics, University of Oxford, Oxford, UK.
Department of Physics and Astronomy, University of Iowa, Iowa City, IA, USA.
Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA.
Institute For the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH, USA.
Southwest Research Institute, San Antonio, TX, USA.
Departoment of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX, USA.

Earth and its magnetosphere are immersed in the supersonic flow of the solar-wind plasma that fills interplanetary space. As the solar wind slows and deflects to flow around Earth, or any other obstacle, a ‘bow shock’ forms within the flow. Under almost all solar-wind conditions, planetary bow shocks such as Earth’s are collisionless, supercritical shocks, meaning that they reflect and accelerate a fraction of the incident solar-wind ions as an energy dissipation mechanism1,2, which results in the formation of a region called the ion foreshock3. In the foreshock, large-scale, transient phenomena can develop, such as ‘hot flow anomalies’4–9, which are concentrations of shock-reflected, suprathermal ions that are channelled and accumulated along certain structures in the upstream magnetic field. Hot flow anomalies evolve explosively, often resulting in the formation of new shocks along their upstream edges5,10, and potentially contribute to particle acceleration11–13, but there have hitherto been no observations to constrain this acceleration or to confirm the underlying mechanism. Here we report observations of a hot flow anomaly accelerating solar-wind ions from roughly 1–10 kiloelectronvolts up to almost 1,000 kiloelectronvolts. The acceleration mechanism depends on the mass and charge state of the ions and is consistent with first-order Fermi acceleration14,15. The acceleration that we observe results from only the interaction of Earth’s bow shock with the solar wind, but produces a much, much larger number of energetic particles compared to what would typically be produced in the foreshock from acceleration at the bow shock. Such autogenous and efficient acceleration at quasi-parallel bow shocks (the normal direction of which are within about 45 degrees of the interplanetary magnetic field direction) provides a potential solution to Fermi’s ‘injection problem’, which requires an as-yet-unexplained seed population of energetic particles, and implies that foreshock transients may be important in the generation of cosmic rays at astrophysical shocks throughout the cosmos.Observations of a hot flow anomaly accelerating solar-wind ions suggest a mechanism for such acceleration—a Fermi acceleration trap caused by Earth’s bow shock interacting with the solar wind.

中文翻译：

地球弓形激波上游高能离子的自生和高效加速

地球及其磁层沉浸在充满行星际空间的太阳风等离子体的超音速流动中。当太阳风减速并偏转以绕地球或任何其他障碍物流动时，流动中会形成“弓形冲击”。在几乎所有的太阳风条件下，像地球这样的行星弓形冲击都是无碰撞的超临界冲击，这意味着它们会反射和加速一小部分入射的太阳风离子作为能量耗散机制1,2，从而导致形成区域称为离子前震3。在前震中，可能会发展出大规模的瞬态现象，例如“热流异常”4-9，这是在上游磁场中沿着某些结构引导和积累的冲击反射的超热离子的浓度。热流异常呈爆炸式发展，通常会导致沿其上游边缘形成新的冲击5,10，并可能导致粒子加速11-13，但迄今为止还没有观察到限制这种加速或确认潜在机制。在这里，我们报告了对太阳风离子从大约 1-10 千电子伏特加速到近 1,000 千电子伏特的热流异常的观察。加速机制取决于离子的质量和电荷状态，并且与一阶费米加速一致 14,15。我们观察到的加速度仅是地球弓形激波与太阳风相互作用的结果，但与弓形激波加速度在前震中通常产生的能量相比，产生的高能粒子数量要多得多。

更新日期：2018-09-01

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