Dynamics of the Inner Radiation Belt Near the Trapping Limit

该提案计划研究在1-3个地球半径高度的一个被称为内辐射带的环面中捕获高能质子的过程。这些被捕获的质子对穿过内磁层的卫星来说是一种危险，会损坏太阳能电池、传感器和集成电路，而卫星电子设备的小型化和数字化进程会加剧这种危险。由于该区域的磁场结构类似于磁瓶，高能质子在内部带中被捕获。高能质子沿着磁力线向磁极旋转，在那里磁场强度的增加使它们被反射。结果，它们在这些“镜像点”之间来回反弹很长一段时间，并有效地被困住了。为了形成危险的内带，必须向磁瓶区域提供高能质子。宇宙射线与大气粒子碰撞产生的副产物质子出现在磁层深处，并立即被困在磁瓶中。太阳耀斑释放的太阳质子或被行星际冲击波加速的太阳质子必须穿透保护地球的磁泡（称为磁层）。这些高能质子沿着绕行星际磁场线的弯曲路径运动，如果弯曲半径足够大，它们就能穿透磁层。这种穿透的程度是用一个叫做地磁截止的参数来描述的。与太阳风的相互作用不断扰乱磁场，改变地磁截止点的位置。在太空风暴期间，地磁中断向地球移动，使太阳质子能够穿透更深的磁层，当活动平静下来时，地磁中断后退，将质子留在磁瓶中。内带的形成取决于地磁截止点位置的变化、磁瓶的结构和高能质子源之间的动力学相互作用。本建议使用模型和观察来更好地理解这种相互作用。从更广泛的影响来看，这项工作将有助于达特茅斯学院培养一名研究生和一名女性博士后。为了更深入地挖掘调查的细节，该提案对太阳风驱动的地球磁场变化以及由此产生的地磁截止点位置变化对内辐射带生长和衰减的影响进行了数值研究。在这些变化的磁场中，内带的高能质子被及时地跟踪，以确定它们是否起源于地球磁层外未被捕获的太阳质子。将模型与宇宙飞船（最著名的是美国宇航局的范艾伦探测器）在磁暴期间捕获的内带质子通量的变化进行比较，将作为模型真实性的严格检验。数值模型的日益复杂以及范艾伦探测器的更详细的观测是这项研究的新元素，有望使我们对内辐射带动力学的理解取得进展。

This proposal plans to investigate the processes that trap high-energy protons in a torus called the inner radiation belt at altitudes of 1-3 Earth radii. These trapped protons represent a hazard for satellites traveling through the inner magnetosphere, damaging solar cells, sensors and integrated circuits, exacerbated by the progress in miniaturization and digitization of satellite electronics. The trapping of high-energy protons in the inner belt results because the magnetic field configuration in this region is analogous to a magnetic bottle. The energetic protons spiral along magnetic field lines towards the magnetic poles, where the increase in magnetic field strength causes them to be reflected. As a result they bounce back and forth between these "mirror points" for long periods of time and are effectively trapped. For a dangerous inner belt to form, high-energy protons must be supplied to the region of the magnetic bottle. Protons that are created as a by-produce of collisions between cosmic rays and atmospheric particles appear deep within the magnetosphere and are immediately trapped in the magnetic bottle. Solar protons released in solar flares or accelerated by interplanetary shock waves must penetrate into the magnetic bubble (called the magnetosphere) that shields the Earth. These energetic protons follow curved paths around interplanetary magnetic field lines and if the radius of curvature is large enough they are able to penetrate into the magnetosphere. The extent of this penetration is described by a parameter called the geomagnetic cutoff. The interaction with the solar wind disturbs the magnetic field continually changing the location of the geomagnetic cutoff. During space storms, the geomagnetic cutoff moves earthward allowing solar protons to penetrate deeper into the magnetosphere and as the activity calms, the geomagnetic cutoff retreats leaving the protons behind in the magnetic bottle. The formation of the inner belt depends on the dynamical interplay between the changing location of the geomagnetic cutoff, the configuration of the magnetic bottle, and the sources of energetic protons. The present proposal uses models and observations to better understand this interplay. As a broader impact, this work will contribute to the training of a graduate student and a female postdoc at Dartmouth College.Digging more deeply into the details of the investigation, the proposal undertakes a numerical study of the effects of solar wind driven changes in the Earth's magnetic field and the resultant changes in the location of geomagnetic cutoffs on the growth and decay of the inner radiation belt. Energetic protons in the inner belt are followed backwards in time in these changing magnetic fields to determine whether or not they originated in the un-trapped solar protons outside of Earth's magnetosphere. Comparison of the models to changes in the trapped inner belt proton flux during magnetic storms by spacecraft (most notably NASA's Van Allen Probes) will be used as stringent tests of the model realism. The increased sophistication of the numerical models along with the more detailed observations by the Van Allen Probes are the new elements in this investigation that are expected to produce advances in our understanding of the dynamics of the inner radiation belt.