An Investigation into Sources of Discrete Chorus Elements

Chorus waves, typically generated in the equatorial region, are a type of very low frequency waves. These waves can interact with electrons in the inner magnetosphere, resulting in the loss and acceleration of the electrons. The characteristics of chorus waves are discrete elements. Each element lasts a few hundred milliseconds and stops growing. If one element starts before the previous one ends, this element is called an overlapped element, and otherwise, it is called a non-overlapped element. Many people studied the propagation direction of chorus waves. Most of them focused on the statistics and local distributions of the propagation direction, reporting that not all the propagation directions are aligned with the magnetic fields and the aligning angle could be large for some cases. Therefore, the elements observed at the same place could be originated from difference sources. Minimum Variance Analysis (MVA) is a technique that helps determine the direction of a physical quantity that has minimum variance. Since chorus is a type of circularly polarized planed electromagnetic waves, the MVA can be used to find the minimum-variance direction of the magnetic fields, that is, the propagation direction of the chorus waves. In the proposed work, we will analyze high-resolution magnetic fields obtained from the THEMIS probes to determine the propagation direction of chorus waves with the MVA. Each chorus event contains multiple chorus elements and their sources may be different. If the determined propagation directions of the elements are the same, these elements are originated from the same source. Overlapped and non-overlapped elements can be determined according to the starting and ending times of elements. The proposed work can help us sort out the characteristics of chorus elements and understand the generation and development of chorus waves.