## 1 Introduction  
The aurora is a natural light phenomenon mostly observed around magnetic polar regions in both hemispheres. The energetic particles precipitating from solar wind interact with geomagnetic field and accelerate along magnetic field lines towards Earth [1]. These particles collide with neutral constituents in the upper atmosphere, which generates the most impressive wonder of Auroral Borealis and Aurora Australis [2]. More importantly, the structure of aurora is a mirror which reflects the physical process in the magnetosphere and assists the scientists to research the intrinsic mechanism in the solar wind-magnetosphere-ionosphere coupling process [3]. The aurora observation methods can be mainly divided into two categories. One is the ground-based observation system composed of all-sky imagers (ASI), and the other is the space borne observation system represented by the ultraviolet imager (UVI). At first, the ASI was used to monitor the local auroral structure. During the international geophysical year (IGY) (1957-1958), space scientists all around the world coordinated their efforts to record the aurora from many places at the same time. Using the data from the IGY-ASI, [PERSON], was one of the first to understand that the global aurora in the North was an oval of light surrounding the north magnetic pole: the auroral oval [4]. With the development of the aerospace technology, space scientists are able to observe the aurora through the UVI cameras aboard the spacecraft. The aurora images captured by these two instruments are shown in Figure 1. Different from the ground-based observation, auroral images captured by UVI instruments in the space mainly focus on providing global information, e.g., the overall configuration of the aurora and the spatial distribution of the auroral intensity [5; 6]  
In the last few years, the extraction of auroral oval attracts huge interests of geophysicists due to its usefulness to monitor the geomagnetic activity and research the physical mechanism of the solar wind-magnetosphere-ionosphere coupling process [2; 3; 7; 8; 9; 10; 11]. The exact location of the auroral oval's equatorward boundary depends on the magnetospheric electric and magnetic fields. The poleward boundary of the auroral oval is taken to separate the closed magnetic field lines (field lines connected at both ends to the earth) from the polar cap covered by open magnetic field lines (field lines connected from the earth to the solar wind) [12]. Besides, because the interplanetary environment can modulate the structure of the magnetosphere, it influences the morphology of the auroral oval remarkably [13]. For instance, different interplanetary magnetic field (IMF) conditions make the auroral oval drift to different directions [14; 15], and the IMF \(B_{z}\) is widely known to control the location of the auroral oval [16]. Therefore, the extraction of the auroral oval is of considerable significance to understand the interplanetary and geomagnetic environment. The aurora extraction mission can be seen as a segmentation task to divide the UVI image into two groups: the aurora region and the background region. Because of the tediousness of manually detecting auroral oval from UVI images, many techniques have been applied to extract the auroral oval automatically. However, the UVI images are low in contrast and often corrupted by intense noise and interference, such as cosmic ray tracks, bright stars, and dayglow contamination, which make it difficult to distinguish the auroral oval from the background. Existing auroral oval segmentation methods can be broadly classified into two categories, active contour-free and active contour-based models [17]. Active contour-free models include adaptive minimum error thresholding (AMET) [18], linear randomized Hough transform (LLSRHT) [19], maximal similarity-based region merging (MSRM) [20], quasi-elliptical fitting with fuzzy local information C-means clustering result (FCM + QEF) [8], etc.