Auroral alert version 1.0: two-step automatic detection of sudden aurora intensification from all-sky JPEG images

Abstract

Abstract. A sudden and significant intensification of the auroral arc with expanding motion (we call it “local-arc breaking” hereafter) is an important event in many aspects but easy to miss for real-time watching due to its short rise time. To ease this problem, a real-time alert system for local-arc breaking was developed for the Kiruna all-sky camera (ASC) using ASC images in the JPEG format. The identification of the local-arc breaking is made in two steps using the “expert system” in both steps: (1) explicit criteria for classification of each pixel and simple calculations afterward are applied to each ASC image to obtain a simple set of numbers, or the “ASC auroral index”, representing the occupancy of aurora 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 for Level 6 as clear local-arc breaking and Level 4 as a precursor for it (reserving Levels 1–3 for less active aurora and Level 5 for less intense sudden intensification). The first step is further divided into two stages. Stage (1a) uses simple criteria for R (red), G (green), and B (blue) values in the RGB color code and the H (hue) value calculated from these RGB values, each pixel of a JPEG image is classified into three aurora categories (from brightest to faintest, “strong aurora”, “green arc”, and “visible diffuse (aurora)”) and three non-aurora light source categories (“cloud”, “artificial light”, and “Moon”). Here, strong aurora means that the ordinary green color by atomic oxygen's 558 nm emission is either nearly saturated or mixed with red color at around 670 nm emitted, by molecular nitrogen. In stage (1b), the percentage of the occupying area (pixel coverage) for each category and the characteristic intensity of the strong aurora pixels are calculated. The obtained ASC auroral index is posted in both an ASCII format and plots in real time (https://www.irf.se/alis/allsky/nowcast/, last access: 11 April 2023). When Level 6 (local-arc breaking) is detected, an automatic alert email is sent out to the registered addresses immediately. The alert system started on 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 in the ASC during the rest of the aurora season of the Kiruna ASC (i.e., all images during a total of 5 months until April 2022 were examined and occasionally double-checked in the sky). Unless the Moon or the cloud blocks the brightened region, a nearly one-to-one correspondence between Level 6 and eye-identified local-arc breaking in the ASC images is achieved with an uncertainty of under 10 min.