Morphological evolution and spatial profile changes of poleward moving auroral forms
<p>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 (MSP) located at the K...
Main Authors: | , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2023-03-01
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Series: | Annales Geophysicae |
Online Access: | https://angeo.copernicus.org/articles/41/115/2023/angeo-41-115-2023.pdf |
Summary: | <p>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 (MSP) located
at the Kjell Henriksen Observatory (KHO), Svalbard, and analyzed using
ASC images taken by cameras at the 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 (OCB), (2) the appearance of an arc-like structure in the oval,
(3) poleward and possible west/eastward propagation,
(4) azimuthal expansion events, (5) re-brightening of the PMAF and
eventual (6) fading away. This is the first work dedicated to the
morphological evolution of PMAFs and it includes more detailed discussion
and novel aspects, such as the observation of initial merging of 557.7 nm
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. This allows an unbiased
statistical investigation of auroral morphology. 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 behavior
may be understood based on changes in the morphology of PMAFs and
validates our understanding of PMAF morphology. Furthermore, our findings
relating to arciness may enable automatic identification of PMAFs,
which has been found to be notoriously difficult.</p> |
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ISSN: | 0992-7689 1432-0576 |