The Van Allen Probes mission provides unique measurements of the most energetic radiation belt electrons at ultrarelativistic energies. Simultaneous observations of plasma waves allow for the routine inference of total plasma number density, a parameter that is very difficult to measure directly. On the basis of long-term observations in 2015, we show that the underlying plasma density has a controlling effect over acceleration to ultrarelativistic energies, which occurs only when the plasma number density drops down to very low values (~10 cm^–3). Such low density creates preferential conditions for local diffusive acceleration of electrons from hundreds of kilo–electron volts up to >7 MeV. While previous models could not reproduce the local acceleration of electrons to such high energies, here we complement the observations with a numerical model to show that the conditions of extreme cold plasma depletion result in acceleration up to >7 MeV.

Van Allen Probes任务可对超相对论能量中的高能辐射带电子进行独特的测量。等离子体波的同时观察允许常规推断总血浆数密度，这是一个很难直接测量的参数。根据2015年的长期观察，我们显示出潜在的等离子体密度对超相对论能量的加速具有控制作用，这仅在等离子体数密度降至非常低的值（〜10 cm ^–3）时才会发生）。如此低的密度为电子的局部扩散加速（从数百千电子伏特到大于7 MeV）创造了优先条件。尽管先前的模型无法将电子的局部加速度重现到如此高的能量，但在这里我们用数值模型对观察结果进行了补充，以表明极端冷等离子体耗尽的条件会导致加速度高达> 7 MeV。