17:15 〜 19:15
[PEM13-P05] ULF波動によるホイッスラーモード波動のダクト伝搬：事例・統計解析
キーワード：ホイッスラーモード波動、ダクト伝搬、ULF波動、磁気圏
Whistler-mode chorus waves have long been studied for their role in electron acceleration and atmospheric precipitation through wave-particle interactions. Particularly, chorus waves propagating to high latitudes can reach relativistic resonance energies, thus influencing the generation and loss of radiation belt electrons. Duct propagation has been recognized for over half a century as a key mechanism for guiding chorus waves to high latitudes. While density ducts formed by density variations have been extensively studied, recent research highlights the existence of magnetic ducts associated with magnetic field variations. Despite extensive research, the generation mechanism of ducts remains quantitatively underexplored. Suggestions include the formation of ducts by localized density structures resulting from secondary electron outflows linked to high-energy electron precipitation, or plasmapause disturbances, but quantitative assessments are lacking.
In this study, we investigate the hypothesis that ULF waves can generate ducts, presenting results from event analyses, simulations, and statistical studies. An event study using Cluster satellite data reveals chorus and concurrent ULF waves at L=6.5, MLT=3.8, and magnetic latitude 20 degrees. Enhancements in chorus intensity correlate with refractive index increases driven by ULF wave-induced density and magnetic field variations. Observed wave normal angles are less than 40 degrees, consistent with theoretical maximum angles for duct propagation. Ray-tracing simulations using observation-based duct models reproduce ducted propagation and the frequency dependency of maximum wave normal variation, agreeing with observations and theoretical predictions. These results demonstrate that ULF waves can create ducts, guiding chorus waves to high latitudes and modulating their intensity.
To further investigate this mechanism, we performed a statistical analysis using Arase satellite data from March 2017 to May 2024, identifying 76 events of high-latitude chorus modulated by ULF waves with field line resonance (FLR) characteristics. These events predominantly occur in the MLT 3-6 sector and L=6-7 range, exceeding a 5% occurrence rate. The occurrence rate is enhanced during periods of high solar wind speed and elevated AL index. Solar wind-related events are observed over a broader L-shell range (L=4-9), while AL index-related events are concentrated at L=6-8. These statistical trends suggest ULF wave ducting is associated with both high-speed solar wind and substorm activity, known drivers of ULF wave and chorus excitation. Furthermore, we identify events with proton flux modulation in the ULF range or high solar wind dynamic pressure, indicating that ULF waves generated by both external and internal processes can create ducts. The statistical analysis reveals that high-latitude chorus propagation is frequently associated with solar activity and substorms.
This study proposes ULF waves as a potential solution to the long-standing question of duct generation and demonstrates a connection between high-latitude chorus propagation and solar activity or substorms, which relates to the excitation of ULF waves and chorus. By bridging phenomena across different spatial and temporal scales, this research contributes to a deeper understanding of wave-particle interactions in the radiation belts and magnetospheric disturbance.
In this study, we investigate the hypothesis that ULF waves can generate ducts, presenting results from event analyses, simulations, and statistical studies. An event study using Cluster satellite data reveals chorus and concurrent ULF waves at L=6.5, MLT=3.8, and magnetic latitude 20 degrees. Enhancements in chorus intensity correlate with refractive index increases driven by ULF wave-induced density and magnetic field variations. Observed wave normal angles are less than 40 degrees, consistent with theoretical maximum angles for duct propagation. Ray-tracing simulations using observation-based duct models reproduce ducted propagation and the frequency dependency of maximum wave normal variation, agreeing with observations and theoretical predictions. These results demonstrate that ULF waves can create ducts, guiding chorus waves to high latitudes and modulating their intensity.
To further investigate this mechanism, we performed a statistical analysis using Arase satellite data from March 2017 to May 2024, identifying 76 events of high-latitude chorus modulated by ULF waves with field line resonance (FLR) characteristics. These events predominantly occur in the MLT 3-6 sector and L=6-7 range, exceeding a 5% occurrence rate. The occurrence rate is enhanced during periods of high solar wind speed and elevated AL index. Solar wind-related events are observed over a broader L-shell range (L=4-9), while AL index-related events are concentrated at L=6-8. These statistical trends suggest ULF wave ducting is associated with both high-speed solar wind and substorm activity, known drivers of ULF wave and chorus excitation. Furthermore, we identify events with proton flux modulation in the ULF range or high solar wind dynamic pressure, indicating that ULF waves generated by both external and internal processes can create ducts. The statistical analysis reveals that high-latitude chorus propagation is frequently associated with solar activity and substorms.
This study proposes ULF waves as a potential solution to the long-standing question of duct generation and demonstrates a connection between high-latitude chorus propagation and solar activity or substorms, which relates to the excitation of ULF waves and chorus. By bridging phenomena across different spatial and temporal scales, this research contributes to a deeper understanding of wave-particle interactions in the radiation belts and magnetospheric disturbance.