Ducted propagation of whistler mode chorus waves observed by the Arase satellite near the plasmapause at high latitudes

Whistler mode chorus is a type of plasma waves generated near the geomagnetic equator by wave-particle interactions. The propagation of chorus is of great importance for the generation of microbursts and resultant loss of radiation belt electrons. The mechanism of the chorus propagation from the equator to higher latitudes has not been clarified. One of the most promising theories is the propagation along a field-aligned density duct. However, only a few observations of chorus propagating in duct structures have been reported [Chan et al. 2021, Haque et al. 2011, Moullard et al. 2002]. In this study, we report on observations of chorus propagating in duct structures observed by the Arase satellite near the plasmapause at the geomagnetic latitude of >10o.

During the period from April to July in 2017 and 2018, we identified 23 cases of chorus propagating in the duct-like structure with Arase. We estimated the wave normal angles (WNAs) using the Singular Value Decomposition (SVD) method [Santolik et al., 2003] with the assumption on the presence of a single plane wave. We obtained a theoretical relation between the wave frequency and the electron density which satisfies the Snell's law and the dispersion relation with the quasi-longitudinal approximation using the WNAs. We compared the theoretical relation with the observed electron density and the chorus frequency range.

The observational results were classified into three categories. (1) was found in a duct structure characterized by density enhancement, (2) and (3) were in density depression. The WNAs were distributed (1) around 0o, (2) from 0o to Gendrin angle (θG) and (3) around θG. The observed chorus was LBC that was consistent with the dispersion relation (1) with WNA=0o, (2) with WNA=0o–θG and (3) between WNA=0o and WNA=θG. We identified 10, 11, and 2 events which were categorized as (1), (2), and (3), respectively. We interpreted each category as the propagation of a chorus based on a different sense of ducting theory. In our presentation, we will also discuss our interpretation of planarity of the observed chorus waves derived from the SVD method.

To investigate characteristics of the low planarity in the density depression duct, we plan to perform detailed analysis for ducted chorus events for which waveform data observed by PWE/Waveform Capture (WFC) is available.