Collaborative Research: Direction-Finding Measurements of LF/MF/HF Auroral Radio Emissions at South Pole

拟议项目的重点是改进目前在南极使用的实验技术，以便更好地定向探测和了解观测到的由太阳和地磁活动在极光电离层产生的低、中、高频（50 - 5000千赫）发射（特别是中频爆发）的来源和传播。近地电磁环境的研究是重要的，因为电离层等离子体波和不稳定性在带电粒子群之间的能量交换中起着直接作用，提供了遥感电离层和磁层等离子体过程和边界的有效手段，并代表了研究也发生在遥远的行星或天体物理等离子体中的发射机制的“局部实验室”。南极站在上述频段的低噪声电磁环境允许统计地研究两种极光无线电发射：(a)极光咆哮，一种靠近电子回旋谐波的窄带发射；(2)极光中频暴，这是一种与地磁亚暴发生有关的宽带发射。一个多功能波形接收器（自2002年以来部署在南极）以全分辨率捕获了大量的极光咆哮发射，现在有许多完全分辨率的低频极光嘶嘶事件的例子可供研究。建议先完成对高分辨率“极光咆哮”和“嘶嘶”数据的分析，然后把重点放在极光中频（MF）爆发上——这是极光的一个突出的谜，因为它的产生机制仍然未知。该项目计划在南极部署一个新的数字测向接收器，并利用它进行一些重要的观测实验：(1)在亚暴演变过程中，通过跟踪脉冲到达的方位角作为时间函数来识别与中频暴相关的亚暴扩展特征；(2)通过比较超低频和光学数据与中频暴的时序，比较和验证确定亚暴爆发的不同方法；(3)分析多波长光学和成像测量数据，确定引起中频暴的极光电子能谱特征；(4)利用垂直交叉天线的数字数据确定中频突发和极光嘶嘶发射之间的详细关系，以及中频突发事件延伸至1500 kHz以下的极化的手性（表明传播模式）。前两个实验将有助于发现亚暴研究的新实验方法，其余的将集中于确定中频暴的源机制及其与其他无线电发射的关系。这些实验将为理论家提供重要的实验材料，以显著提高我们对极光射电发射及其与各种极光现象的宏观物理关系的认识。

The proposed project is focused on improving the experimental technique currently used at South Pole for better directional detection and understanding of the sources and propagation of observed low, medium, and high frequency (50 - 5000 kHz) emissions (and specifically medium-frequency bursts) generated in the auroral ionosphere by the solar and geomagnetic activity. The studies of near-Earth electromagnetic environment is important because ionospheric plasma waves and instabilities play a direct role in mediating exchange of the energy between charged particle populations, provide effective means of remote sensing of the ionospheric and magnetospheric plasma processes and boundaries, and represent a "local laboratory" for studying emissions mechanisms which also occur in remote planetary or astrophysical plasmas. The low-noise electromagnetic environment of the South Pole Station in the above-mentioned frequency band allows to study statistically two auroral radio emissions: (a) auroral roars, a narrowband emission near the electron cyclotron harmonics; and (2) auroral MF bursts, a broadband emission associated with geomagnetic substorm onsets. A versatile waveform receiver (deployed at South Pole since 2002) captured scores of auroral roar emissions at full resolution, and numerous examples of fully resolved low-frequency auroral hiss events are now available for the study. It is proposed to complete the analysis of the high-resolution "auroral roar" and "hiss" data, but then focus on the auroral medium-frequency (MF) bursts - an outstanding mystery of the aurora because its generation mechanism is still unknown. The project plans to deploy a new digital direction-finding receiver at South Pole and use it to perform a few significant observational experiments: (1) identify features of the substorm expansion correlated with the MF bursts by tracking the azimuth of bursts' arrival as a function of time as the substorm evolves; (2) compare and validate different methods of determining substorm onsets by comparing the timing from ultra-low-frequency and optical data and MF bursts; (3) analyze multi-wavelength optical and imaging riometer data to determine characteristics of the auroral electron spectra that cause the MF burst; and (4) determine the detailed relationship between MF burst and auroral hiss emissions using digital data from perpendicular crossed antennas and the handedness of polarization (indicative of the propagation mode) of MF burst events extending below 1500 kHz. The first two experiments will help discovering new experimental methods for the substorm studies, and the rest - to focus on determining the source mechanism of the MF bursts and its relation to other radio emissions. These experiments will provide significant experimental materials for theorists to markedly advance our knowledge of the auroral radio emissions and their relation to macrophysics of various auroral phenomena..