09:15 〜 09:30
[PEM13-02] Comprehensive analysis of the shape of pulsating aurora, the energy of electron precipitation and the propagation characteristics of chorus waves
キーワード：脈動オーロラ、ホイッスラーモードコーラス波動、ダクト伝搬
Pulsating Auroras (PsA), which have quasi-periodic modulations in their luminosity, are one of the major classes of diffuse aurora associated with precipitation of a few to a few tens of keV electrons from the magnetosphere. The optical pulsations are believed to be generated by precipitating electrons scattered by lower-band chorus (LBC) waves, which are one of the whistler-mode waves in the magnetosphere, through the cyclotron resonance. Recent studies demonstrated that, during intervals of PsA, more energetic, i.e., sub-relativistic and relativistic, electrons precipitate into the ionosphere at the same time. Recent modeling studies suggested that such highly energetic electrons can be scattered at the higher latitude part of the magnetosphere by whistler-mode chorus waves propagating away from the magnetic equator. However, there have been no actual cases of simultaneous observations of precipitating electrons causing PsA (PsA electrons) and chorus waves propagating towards higher latitudes; thus, we still do not quite well understand under what conditions PsA electrons become more energetic and precipitate further down to lower altitudes.
To address this question, we have investigated an extended interval of PsA on January 12, 2021, during which simultaneous observations with the Arase satellite, ground-based all-sky imagers and the European Incoherent SCATter (EISCAT) radar were archived. In the first half of this PsA interval, the energy of precipitating PsA electrons was a few to a few tens of keV and the shape of PsA was not patchy. While, in the second half of this PsA interval, precipitating PsA electrons had larger energies and the PsA shape became patchy. At the same time, Arase detected intense chorus waves at magnetic latitudes above 20o. Considering what factors control the relationships between the PsA characteristics (e.g., spatial structure and energy of precipitating electrons) and characteristic of chorus wave propagation, we are speculating that, density ducts, which are tube-like regions where the electron density is lower/higher than the surrounding area in the magnetosphere, play an important role. Density ducts have a function to confine chorus waves within density ducts due to gradients in the refractive index as like optical fibers. As a result, chorus waves are allowed to propagate to higher latitudes along density ducts without attenuation and scatter relativistic/sub-relativistic electrons in a region where the resonance energy is higher. Furthermore, since the scattering should be confined within density ducts, the region, where the scattered electrons precipitate, should be patchy reflecting the cross-sectional shape of density ducts.
In order to test the "duct model," the magnetospheric electron density, that is an indicator of density ducts, was compared with the optical intensity at the magnetic footprint of the satellite, representing the spatial structure of PsA patches. Overall good correspondence between the irregularities of the electron density in the magnetosphere and the emission intensity of PsA patches at the footprint of the satellite suggests that the morphology of PsA and the energy of corresponding electrons are determined by the presence of "magnetospheric density ducts," which allow chorus waves to travel to higher latitudes and thereby precipitate more energetic electrons. Furthermore, during another event of simultaneous observations by EISCAT and Arase on September 7--8, 2023, we found that chorus waves did not reach higher latitudes and the energy of PsA electrons was moderate, which is an opposite result of the event on January 12, 2021, which further supports the "duct model" proposed in this study.
To address this question, we have investigated an extended interval of PsA on January 12, 2021, during which simultaneous observations with the Arase satellite, ground-based all-sky imagers and the European Incoherent SCATter (EISCAT) radar were archived. In the first half of this PsA interval, the energy of precipitating PsA electrons was a few to a few tens of keV and the shape of PsA was not patchy. While, in the second half of this PsA interval, precipitating PsA electrons had larger energies and the PsA shape became patchy. At the same time, Arase detected intense chorus waves at magnetic latitudes above 20o. Considering what factors control the relationships between the PsA characteristics (e.g., spatial structure and energy of precipitating electrons) and characteristic of chorus wave propagation, we are speculating that, density ducts, which are tube-like regions where the electron density is lower/higher than the surrounding area in the magnetosphere, play an important role. Density ducts have a function to confine chorus waves within density ducts due to gradients in the refractive index as like optical fibers. As a result, chorus waves are allowed to propagate to higher latitudes along density ducts without attenuation and scatter relativistic/sub-relativistic electrons in a region where the resonance energy is higher. Furthermore, since the scattering should be confined within density ducts, the region, where the scattered electrons precipitate, should be patchy reflecting the cross-sectional shape of density ducts.
In order to test the "duct model," the magnetospheric electron density, that is an indicator of density ducts, was compared with the optical intensity at the magnetic footprint of the satellite, representing the spatial structure of PsA patches. Overall good correspondence between the irregularities of the electron density in the magnetosphere and the emission intensity of PsA patches at the footprint of the satellite suggests that the morphology of PsA and the energy of corresponding electrons are determined by the presence of "magnetospheric density ducts," which allow chorus waves to travel to higher latitudes and thereby precipitate more energetic electrons. Furthermore, during another event of simultaneous observations by EISCAT and Arase on September 7--8, 2023, we found that chorus waves did not reach higher latitudes and the energy of PsA electrons was moderate, which is an opposite result of the event on January 12, 2021, which further supports the "duct model" proposed in this study.