学位の種別: 課程博士審査委員会委員 : （主査）東京大学准教授 齋藤 義文, 東北大学准教授 坂野井 健, 東京大学教授 吉川 一朗, 国立極地研究所教授 宮岡 宏, 東京大学教授 星野 真弘University of Tokyo(東京大学

Fukuda Yoko

福田 陽子

UTokyo Repository

論文の内容の要旨 
 
 
Generation mechanism of  
flickering aurora 
（フリッカリングオーロラの発生メカニズム） 
 
福田 陽子 
 
 
At the altitude of the ionosphere, the brightest auroral phenomena are driven by a substorm 
triggered in the Earth’s magnetosphere through field-aligned currents. The spatiotemporal auroral 
variation reflects not only the magnetospheric disturbance but also dynamic physical processes at 
the magnetosphere and ionosphere (M-I) coupling region. Auroras formed at the M-I coupling 
region are characterized by fine-scale structures on the order of a few kilometers, and it is often 
suggested that dispersive Alfvén waves (DAWs) play an important role in the formation 
mechanism. Flickering auroras, which are known to have the fastest temporal variation among 
the auroral phenomena, may be generated by electromagnetic ion cyclotron (EMIC) waves, which 
are one type of DAWs, and they often appear before or during the auroral breakup. Open questions 
about flickering auroras are a upper limit of the flickering frequency and what conditions are 
needed to form flickering auroras. This thesis has the purpose of answering these questions by 
determining their generation mechanisms using high-speed fine-scale imaging observations. 
First, we designed a continuous observational system with a high spatiotemporal resolution, 
which was automatically controlled by a real-time auroral auto-detection system based on a 
machine learning technique. It was found that the accuracy of the auroral auto-detection was 
approximately 80% by comparison with the results judged by auto-detection and eye. It was also 
found that the amount of data was decreased by 75% using the auroral auto-detection system. We 
were finally able to obtain a continuous observational system with a spatial resolution of ~50 m 
at the 100 km altitude and a temporal resolution of 320 frames per second (fps) over three winter 
seasons.  
In order to clarify the necessary conditions for flickering auroras, we statistically investigated 
the basic property of flickering auroras using the automatic detection of flickering auroras. It is 
found that the occurrence rate is basically proportional to the background non-flickering auroral 
intensity and that the bright auroras without the flickering modulation occasionally appears as the 
isolated arc. These results indicate that it is hard to excite EMIC waves within a weak and narrow 
acceleration region. It is also found that the flickering frequency is narrowband and has no 
correlation with the background auroral intensity and the substorm phase. These signatures 
suggest that the flickering aurora occurs at the low-altitude acceleration region. This result shows 
that the low-altitude acceleration region would play an important role in the excitation of EIMC 
waves. For the first time, it was also found that the flickering amplitude (%) decreases with the 
background non-flickering intensity. This variation would be formed by the difference between 
the electron velocity determined by the parallel potential drop and the phase velocity of EMIC 
waves. 
We also found the first evidence that flickering auroras may be modulated by H+-band EMIC 
waves, based on 160 fps optical observations. The fastest flickering aurora of 40–80 Hz 
transiently coexisted with the typical 10 Hz flickering as a result of O+-band EMIC waves within 
the brightest aurora around the magnetic zenith. This result is strong evidence that flickering 
auroras are generated by multi-ion EMIC waves.  
As an application of the high-speed imaging observations, we focused on the rapidly moving 
features of the pulsating aurora. A pulsating aurora has modulations of a few hertz embedded in 
the main pulsation. During a pulsation ON-phase, repetitive expansions are often observed around 
the edges of pulsating patches. Approximately 80% of all the deduced expansion speeds were less 
than 70 km s−1 at ionospheric altitudes, which is less than the projected Alfvén speed from the 
magnetospheric equator to the ionosphere. The rapid motions with speeds of tens of kilometers 
per second are unlikely to be explained by obliquely propagating chorus elements, which are 
known to cause 3 Hz modulation, because the perpendicular speed of the oblique chorus waves 
is faster than the Alfvén speed. One of candidates to generate the rapid spatial motions is the slow-
mode Alfvén wave.  
Our high-speed fine-scale imaging observation is helpful to visualize wave-particle 
interactions and to monitor plasma environments in the M-I coupling region.  
 


フリッカリングオーロラの発生メカニズム

https://repository.dl.itc.u-tokyo.ac.jp/?action=repository_action_common_download&item_id=52325&item_no=1&attribute_id=14&file_no=2

フリッカリングオーロラの発生メカニズム

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