Reply on RC3

Abstract

A real-time alert system of sudden and significant intensification of auroral arc with expanding motion (we call it "Local-Arc-Breaking" hereafter) was developed for Kiruna all-sky camera (ASC) using ASC jpeg images. The identification is made in two steps: (1) Using an "expert system" in which a combinations of simple criteria is applied to each pixels with calculations afterward (expert system), each jpeg image of the ASC is converted into a simple set of numbers, or "ASC auroral index", representing the occupancy of auroral pixels and characteristic intensity of the brightest aurora in the image. (2) Using this ASC auroral index, the level of auroral activity is estimated, aiming Level 6 as clear Local-Arc-Breaking and Level 4 as precursor for it (reserving Levels 1–3 for less active aurorae). The first step is further divided into two stages: (1a) Using simple criteria for R (red), G (green), B (blue), and H (hue) values in the RGB and HLS colour codes, each pixel of a jpeg image is classified into several categories according to its colour as "visible diffuse", "green arc", "strong aurora" (which means saturated or mixed with N2 red line at 670 nm), "cloud", "artificial light", and "moon". (1b) The percentage of the occupying area (pixel coverage) for each category and the characteristic intensity of "strong aurora" are calculated. The obtained ASC aurora index is posted in both a ascii format and plots on a real-time bases at https://www.irf.se/alis/allsky/nowcast/. When Level 6 is detected, automatic alert E-mail is sent out to the registered addresses immediately. The alert system started 5 November, 2021, and the results (both Level 6 detection and Level 4 detection) were compared to the manual (eye-)identification of the auroral activity during the rest of the auroral season of Kiruna ASC (i.e., total five months until April 2022). Unless the Moon or cloud blocks the brightened region, nearly one-to-one correspondence between Level 6 and Local-Arc-Breaking judged by original ASC images is achieved within ten minutes uncertainty.