11:00 AM - 1:00 PM
[PEM11-P03] Duct propagation of Whistler mode chorus near the high latitude plasmapause observed by Arase satellite
Keywords:whistler mode chorus, duct propagation, Arase satellite, plasmapause
Whistler mode chorus is a type of plasma waves generated near the geomagnetic equator by wave-particle interactions. Chorus scatters the pitch angle of energetic electrons and precipitates the electrons into the atmosphere. The resonance energy of chorus becomes higher as the wave propagates at higher latitudes, resulting in the scattering of relativistic electrons into the loss cone. Thus, the propagation of chorus is of great importance for the generation of microbursts and resultant loss of radiation belt electrons. The loss of radiation belt electrons results in significant ionization of the upper atmosphere, which has an impact on the middle atmosphere.
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, Moullard et al. 2002, Haque et al. 2011). 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 >20° .
During the period from June to July in 2018, we identified 22 cases of chorus propagating in the duct-like structure near the plasmapause with Plasma Wave Experiment (PWE) / Onboard Frequency Analyzer (OFA). We examined properties of the chorus propagating in the duct-like structure characterized by density enhancement at geomagnetic latitude of ~-23° and MaIlwain L of ~4.0.
First, we calculated the wave normal angles (WNAs) using the background magnetic field observed by Magnetic Field Experiment (MGF) and the spectral matrix of the magnetic field observed by OFA. The WNAs of the observed chorus in the duct (about 30 degrees) are smaller than those outside the duct (about 80 degrees). We calculated the wave number parallel to the ambient magnetic field k_para using the WNAs and the dispersion relation and confirmed that the parallel wave number was constant at the same frequency in the duct, which is consistent with Snell's law. On the other hand, the wave number perpendicular to the magnetic field changes with the electron density in the duct. Based on the theory of Chen et al. the propagation of the chorus is confined to the region where the electron density is higher than the critical density (k_perp = 0). From the dispersion relation, the critical density increases with the wave frequency. It is expected that the regions where the chorus can propagate in the duct become narrow with increasing frequency. We confirmed that the observations were consistent with the prediction despite the limited amount of available data.
Second, we examined the lower and upper limits of the observed chorus frequency. The lower frequency limit was higher than the lower hybrid resonance (LHR) frequency. This suggests that the lower limit reflects the lowest frequency at the source region or is determined by the local LHR frequency along the propagation path. We compared the upper frequency limit with half of the electron cyclotron frequency at the magnetic equator. We used the magnetic field models (TS04, T89, and IGRF) to estimate magnetic field strength at the equator. We confirmed that the observed chorus is in the frequency band of Lower Band Chorus (LBC).
We confirmed that properties of the chorus propagating in the density duct observed by Arase near the plasmapause was consistent with the dispersion relation. It is concluded that the LBC keeps small WNAs during the propagation inside the duct and propagates toward higher latitude of > 20°.
We plan to analyze other events such as (1) series of chorus alternately propagating in ducts generated by density enhancement and depression, and (2) ducted chorus which is accompanied by pitch angle scattering of high energy 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, Moullard et al. 2002, Haque et al. 2011). 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 >20° .
During the period from June to July in 2018, we identified 22 cases of chorus propagating in the duct-like structure near the plasmapause with Plasma Wave Experiment (PWE) / Onboard Frequency Analyzer (OFA). We examined properties of the chorus propagating in the duct-like structure characterized by density enhancement at geomagnetic latitude of ~-23° and MaIlwain L of ~4.0.
First, we calculated the wave normal angles (WNAs) using the background magnetic field observed by Magnetic Field Experiment (MGF) and the spectral matrix of the magnetic field observed by OFA. The WNAs of the observed chorus in the duct (about 30 degrees) are smaller than those outside the duct (about 80 degrees). We calculated the wave number parallel to the ambient magnetic field k_para using the WNAs and the dispersion relation and confirmed that the parallel wave number was constant at the same frequency in the duct, which is consistent with Snell's law. On the other hand, the wave number perpendicular to the magnetic field changes with the electron density in the duct. Based on the theory of Chen et al. the propagation of the chorus is confined to the region where the electron density is higher than the critical density (k_perp = 0). From the dispersion relation, the critical density increases with the wave frequency. It is expected that the regions where the chorus can propagate in the duct become narrow with increasing frequency. We confirmed that the observations were consistent with the prediction despite the limited amount of available data.
Second, we examined the lower and upper limits of the observed chorus frequency. The lower frequency limit was higher than the lower hybrid resonance (LHR) frequency. This suggests that the lower limit reflects the lowest frequency at the source region or is determined by the local LHR frequency along the propagation path. We compared the upper frequency limit with half of the electron cyclotron frequency at the magnetic equator. We used the magnetic field models (TS04, T89, and IGRF) to estimate magnetic field strength at the equator. We confirmed that the observed chorus is in the frequency band of Lower Band Chorus (LBC).
We confirmed that properties of the chorus propagating in the density duct observed by Arase near the plasmapause was consistent with the dispersion relation. It is concluded that the LBC keeps small WNAs during the propagation inside the duct and propagates toward higher latitude of > 20°.
We plan to analyze other events such as (1) series of chorus alternately propagating in ducts generated by density enhancement and depression, and (2) ducted chorus which is accompanied by pitch angle scattering of high energy electrons