Effects of VLF transmitter waves on the inner belt and slot region

Signals from VLF transmitters can leak from the Earth-ionosphere wave guide into the inner magnetosphere, where they propagate in the whistler mode and contribute to electron dynamics in the inner radiation belt and slot region. Observations show that the waves from each VLF transmitter are highly localised, peaking on the nightside in the vicinity of the transmitter. In this study we use ~5 years of Van Allen probe observations to construct global statistical models of the bounce-averaged pitch angle diffusion coefficients for each individual VLF transmitter, as a function of L*, Magnetic Local Time (MLT) and geographic longitude. We construct a 1D pitch-angle diffusion model with implicit longitude and MLT dependence to show that VLF transmitter waves weakly scatter electrons into the drift loss cone. We find that global averages of the wave power, determined by averaging the wave power over MLT and longitude, capture the long-term dynamics of the loss process, despite the highly localised nature of the waves in space. We use our new model to assess the role of VLF transmitters waves, hiss waves, and Coulomb collisions on electron loss in the inner radiation belt and slot region. At moderate relativistic energies, E~ keV, waves from VLF transmitters reduce electron lifetimes by an order of magnitude or more, down to the order of 200 days near the outer edge of the inner radiation belt. However, VLF transmitter waves are ineffective at removing multi-MeV electrons from either the inner radiation belt or slot region.

Funding was provided by the NERC grant NE/P01738X/1.

甚低频（VLF）发射机的信号可从地球-电离层波导泄漏至内磁层，在此处以哨声模式（whistler mode）传播，并对内辐射带和槽区的电子动力学过程产生影响。观测结果表明，每台甚低频发射机产生的波具有高度局域化特征，峰值出现在发射机附近的夜侧区域。本研究利用约5年的范艾伦探针（Van Allen Probe）观测数据，构建了针对每台独立甚低频发射机的反弹平均投角扩散系数（bounce-averaged pitch angle diffusion coefficients）全局统计模型，该模型以L*、磁地方时（MLT, Magnetic Local Time）和地理经度作为自变量。我们构建了一个具备隐式经度与磁地方时依赖关系的一维投角扩散模型，用以证明甚低频发射机产生的波可将电子弱散射至漂移损失锥（drift loss cone）中。研究发现，尽管空间中的波具有高度局域化特性，但通过对磁地方时与地理经度范围内的波功率取平均得到的全局波功率平均值，仍可捕捉该损失过程的长期动力学特征。我们利用新构建的模型，评估了甚低频发射机波、嘶声（hiss）波以及库仑碰撞对内辐射带和槽区电子损失过程的作用。在中等相对论性能量（约千电子伏特（keV）量级）下，甚低频发射机产生的波可将电子寿命缩短一个数量级甚至更多，在内辐射带外边缘附近，电子寿命可降至约200天。然而，甚低频发射机产生的波无法有效从内辐射带或槽区中移除兆电子伏特（MeV）量级的高能电子。

本研究由NERC资助项目NE/P01738X/1提供支持。