Dynamics of the Inner Radiation Belt Near the Trapping Limit

这项提议计划研究在地球半径1-3的高度被称为内辐射带的环状结构中捕获高能质子的过程。这些捕获的质子对穿越内磁层的卫星来说是一种危险，破坏太阳能电池、传感器和集成电路，卫星电子设备小型化和数字化的进展加剧了这种危害。高能质子被困在内带是因为这个区域的磁场构型类似于一个磁瓶。高能质子沿着磁力线向磁极盘旋，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”之间来回反弹了很长一段时间，有效地被困住了。为了形成危险的内带，必须向磁瓶的区域提供高能质子。作为宇宙射线和大气粒子碰撞的副产品而产生的质子出现在磁层深处，并立即被困在磁瓶中。太阳耀斑中释放的或被行星际冲击波加速的太阳质子必须穿透到屏蔽地球的磁泡(称为磁层)中。这些高能质子沿着绕行星际磁力线的曲线运动，如果曲率半径足够大，它们就能够穿透到磁层中。这种穿透的程度由一个称为地磁截止的参数来描述。与太阳风的相互作用会扰乱磁场，不断改变地磁截止的位置。在空间风暴期间，地磁截止点向地球移动，允许太阳质子深入磁层，随着活动的平静，地磁截止点后退，将质子留在磁瓶中。内带的形成依赖于地磁截止位置的变化、磁瓶的形状和高能质子源之间的动力相互作用。本提案使用模型和观察来更好地理解这种相互作用。作为更广泛的影响，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。该提案更深入地挖掘调查的细节，对太阳风驱动的地球磁场变化以及由此导致的地磁截止位置变化对内辐射带生长和衰变的影响进行了数值研究。在这些变化的磁场中，内带中的高能质子在时间上被倒退，以确定它们是否起源于地球磁层外未被捕获的太阳质子。将这些模型与航天器(最著名的是NASA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，将被用作对模型真实性的严格测试。数值模型的日益复杂，以及范艾伦探测器更详细的观测，是这次调查中的新元素，有望促进我们对内辐射带动力学的理解。

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.