Morphological evolution and spatial profile changes of poleward moving auroral forms

Abstract

We investigated the morphology of poleward moving auroral forms (PMAFs) qualitatively by visual inspection of all sky camera (ASC) images and quantitatively using the arciness index. The PMAFs in this study were initially identified with a meridian scanning photometer located at Kjell Henriksen Observatory (KHO), Svalbard, and analyzed using ASC images 10 taken by cameras at KHO and in Ny-Ålesund, Svalbard. We present a detailed six-step evolution of PMAF morphology in two dimensions. This evolution includes (1) an equatorward expansion of the auroral oval and an intensification of auroral brightness at the open-closed boundary, (2) the appearance of an arc-like structure in the oval, (3) poleward and possible west/eastward propagation, (4) azimuthal expansion events, (5) rebrightening of the PMAF and eventual (6) fading away. While there have been previous studies commenting on PMAF morphology this is the first work dedicated to the morphological 15 evolution of PMAFs and it includes more detailed discussion and novel aspects, such as the observation of initial merging of 557 . 7nm auroral patches to form a PMAF. Moreover, the morphology of PMAFs is quantified using the arciness index, which is a number describing how arc-like auroral forms appear in ASC images. We present the results of a superposed epoch analysis of arciness in relation to PMAF occurrence. This analysis uncovered that arciness increases suddenly during the onset of a PMAF event and decreases over the PMAF lifetime to return to its baseline value once the event has concluded. This 20 behavior may be understood the identify Additionally, the auroral emission strengths in the 557 . 7nm and 630 . 0nm emission lines at high latitudes were observed to be different in the pre-noon and post-noon sectors. Pre-noon PMAFs (PMAF2s) were observed to have strong 557 . 7nm emissions throughout their entire event lifetime, while post-noon events (PMAF1s) appeared to be fading more quickly in 557 . 7nm emission. 40 Another pre-/post-noon asymmetry that was discovered by Sandholt relates to auroral oval intensifications at the onset of PMAF events. Under eastward IMF pre-noon events would lead to equatorward boundary intensifications, while post-noon events would not. Instead, Sandholt observed poleward boundary intensifications during the onsets of post-noon PMAF events under duskward IMF. Their findings also confirmed the dependence of the magnetic local time of PMAFs on the IMF B y direction. 45 This paper expands upon the previous work and presents a more detailed description of the morphological evolution of PMAFs. We used Meridian Scanning Photometer (MSP) data from the Kjell Henriksen Observatory (KHO) (78.15 ◦ N, 16.04 ◦ E) in Longyearbyen, Svalbard, to initially identify PMAF events. We made use of All Sky Camera (ASC) images taken in Longyearbyen and Ny-Ålesund (78.92 ◦ N, 11.91 ◦ E) to analyze the structure and evolution of PMAFs. Furthermore, we quantified the morphology of PMAFs using an index first introduced by Partamies et al. (2014) known as the arciness 50 index. The motivation behind the use of the arciness index was to (1) have a quantitative and objective measure of auroral morphology, and (2) to enable the development of an automated algorithm capable of identifying PMAFs based on arciness data.

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ORCID for Daniel Whiter: orcid.org/0000-0001-7130-232X

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