Artificial Auroras: The energy spectrum of accelerated electrons from wave-particle interactions

超过99%的可观测宇宙处于等离子体状态，这是一种由负电子和正离子组成的带电稀薄气体。星际空间介质中渗透着来自太阳的等离子体、太阳磁场和许多不同频率的电磁波。电磁波可以在磁化等离子体中转化为静电等离子体波，这是一个公认的事实，磁化等离子体可以通过几种不同的机制将带电粒子加速到高能。为了理解宇宙的基本组成部分，我们不仅要了解等离子体的行为，而且我们还需要了解粒子如何加速到足够高的能量，从而威胁到人类的太空旅行和卫星的生存。等离子体在地球上很罕见，但可以在闪电等放电中发现。它也可以在真空室中产生，但这种实验存在非线性的尺度尺寸问题和边界壁约束。然而，一种天然的无界等离子体在海拔100千米以上的高层大气——电离层中是自由存在的。尽管地处偏远，但利用雷达（例如SuperDARN和EISCAT）和光学对电离层进行遥感的长期和成功的跟踪记录。已经确定的是，向电离层发射高功率（1 MW）高频（4-8 MHz）电磁波（例如来自EISCAT加热器设施）会引起各种波等离子体相互作用，从而导致粒子加速，这可以通过雷达和光学远程诊断。例如，刺激Langmuir湍流加速电子并产生平行于磁场线的电子-Langmuir和离子声等离子体波，这可以被非相干散射雷达（例如EISCAT）检测到；刺激上层混合共振加速电子并产生垂直于磁场线的等离子体密度不规则，这可以通过相干散射雷达（例如SuperDARN CUTLASS）检测到；刺激低杂化空腔坍缩会加速电子和离子，并产生两种雷达都能探测到的等离子体空腔。有些等离子体波，例如电子-伯恩斯坦波，不会产生粒子加速。通过调节加热光束的偏振、功率、频率、指向方向和调幅周期，可以对不同的机制进行优先激发。此外，不同的机制具有不同的生长速率，例如上杂化共振为10 s，而其他机制通常小于1 s。加热器刺激的电子加速到高能量的一个明显症状是，这种传输到电离层产生与自然极光相同的人工光学发射，即人工极光。这些亚视觉辐射通常出现在200-300公里的高度范围内，在这个高度范围内，加热器可以有效地刺激各种等离子体共振，典型的尺寸为~20-50公里。这些人造极光只能来自于电子与上层大气中占主导地位的氧原子和氮分子的碰撞，就像自然极光一样。然而，对于人工极光来说，加速电子是由人工刺激的等离子体共振在局部产生的，而自然极光是由磁层外沉淀的电子产生的。在检测到的众多光发射中，对本研究最重要的4个标准波长分别为630.0、557.7、844.6和427.8 nm，其能量阈值分别为2、4.2、11和18.6 eV。利用标定的光学探测器测量每个波长的光子通量，并知道每个波长的光子发射率作为电子碰撞能量的函数，可以唯一地确定加速电子的定量能谱。这可以有选择地针对每个机制进行，这也是本建议的主要目标。

Over 99% of the observable universe is in the plasma state, a charged tenuous gas of negative electrons and positive ions. The interplanetary space medium is permeated by plasma emanating from the Sun, the solar magnetic field, and electromagnetic waves of many different frequencies. It is a well-established fact that electromagnetic waves can transform into electrostatic plasma waves in a magnetized plasma, which can accelerate charged particles to high energies via several different mechanisms. Not only is it vital that we understand the behaviour of plasma in order to understand a fundamental building block of the universe, but we also need to understand how particles become accelerated to high enough energies to threaten human space travel as well satellite survival. Plasma is rare on Earth, but can be found in electrical discharges such as lightening. It can also be produced in vacuum chambers, but there are non-linear scale size issues and boundary wall constraints with such experiments. However, a natural unbounded plasma is freely available in the upper-atmosphere above 100 km altitude, the ionosphere. Despite being remote, there is a long and successful track record of remote sensing of the ionosphere by radars (e.g. SuperDARN and EISCAT) and optics. It is well established that beaming high-power (1 MW) high-frequency (4-8 MHz) electromagnetic waves into the ionosphere (e.g. from the EISCAT Heater facility) causes various wave-plasma interactions resulting in particle acceleration, which can be diagnosed remotely by radars and optics. For example, stimulating Langmuir turbulence accelerates electrons and produces electron-Langmuir and ion-acoustic plasma waves parallel to the magnetic field line which can be detected by an incoherent scatter radar (e.g. EISCAT); stimulating upper-hybrid resonance accelerates electrons and produces plasma density irregularities perpendicular to the magnetic field line which can be detected by a coherent scatter radar (e.g. SuperDARN CUTLASS); and stimulating lower-hybrid caviton collapse accelerates electrons and ions and produces plasma cavitons which can be detected by both types of radars. Some plasma waves, e.g. electron-Bernstein waves, do not produce particle acceleration. The different mechanisms can be preferentially stimulated by adjusting the Heater beam polarization, power, frequency, pointing direction and amplitude modulation cycle. In addition, different mechanisms have different growth rates, e.g. 10 s for upper-hybrid resonance and typically <1 s for the other mechanisms. A clear symptom of Heater-stimulated electron acceleration to high energies is the fact that such transmissions into the ionosphere produce artificial optical emissions identical to the natural auroras, i.e. the artificial auroras. These sub-visual emissions typically appear in the height range 200-300 km, where the Heater can efficiently stimulate various plasma resonances, with a typical dimension of ~20-50 km. These artificial auroras can only come from electron collisions with the dominant atomic oxygen and molecular nitrogen constituents in the upper-atmosphere, as is the case for natural auroras. However, for the artificial auroras, the accelerated electrons are produced locally by the artificially stimulated plasma resonances, whereas natural auroras are produced by electrons precipitating out of the magnetosphere. Of the many optical emissions detected, the 4 standard wavelengths most important for the proposed research are 630.0, 557.7, 844.6 and 427.8 nm, with energy thresholds of about 2, 4.2, 11 and 18.6 eV. By measuring the photon flux at each wavelength using calibrated optical detectors, and knowing the photon emission rate for each wavelength as a function of electron collision energy, the quantitative energy spectrum of the accelerated electrons can be uniquely determined. This can be done selectively for each mechanism and is the primary goal of this proposal.