Multivariate analysis of dim elves from ISUAL observations

• Published in: Journal of geophysical research. Atmospheres 2015-08, Vol.120 (15), p.7454-7466
• Main Authors: Offroy, Marc, Farges, Thomas, Gaillard, Pierre, Kuo, Cheng Ling, Chen, Alfred Bing-Chih, Hsu, Rue-Ron, Takahashi, Yukihiro
• Format: Article
• Language: English
• Subjects: Atmospherics; dim elves; Elves (atmospherics); Emissions; Factor analysis; FUV emissions; Geophysics; ISUAL; Lightning; Lightning currents; Mathematical analysis; Multivariate analysis; Photometry; Principal components analysis; Records; Satellites; Selectivity; Spectrophotometers; Sprites; TARANIS; Transient luminous events; Ultraviolet
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1002/2014JD023012

The Imager of Sprites and Upper Atmospheric Lightning (ISUAL) on the FORMOSAT‐2 satellite, launched in 2004, records Transient Luminous Events (TLEs). ISUAL has an imager and a spectrophotometer that observe TLEs all over the globe. Among these phenomena, elves are particularly difficult to detect. ISUAL often records events that correspond to significant far ultraviolet (FUV) emissions in the spectrophotometer but have no discernible TLEs in the imager. These FUV events are called “dim” elves. Therefore, it is important to develop mathematical tools to analyze the data to obtain a better evaluation of the number of elves and their occurrence. Multivariate approaches are applied to characterize the unlabeled events. The first approach is the principal component analysis which distinguishes two different groups, one including elves and dim elves. The second approach is the PARallel FACtor analysis which provides a waveform model for each group. These methodologies confirm that FUV signal is the evidence of TLE presence. A crude classification method was then suggested taking into account these results. The proportion of elves, relatively to the considered ISUAL data set, is found to be about 40%. It is similar to previous results and confirms that relatively weak lightning peak current is sufficient to produce elves. This new strategy demonstrates the potential for discriminating between lightning and TLEs without prior knowledge within the selectivity of the FUV spectral band. Key Points The photometric signals of elves and lightning are strongly mixed Validation of FUV emissions as possible trigger for TLE detection on board satellite Detection of dim elves is possible without a priori information with multivariate analysis