# Neural Network Based Approach to Recognition of Meteor Tracks in the Mini-EUSO Telescope Data
## 3 Meteor and Background Signals  
Signals of meteors registered with Mini-EUSO have some features important for the presented analysis:  
* A signal produced by a meteor in a pixel has a shape resembling the bell-like curve similar to the probability density function of the normal distribution. * Meteor signals produce quasi-linear tracks in the focal surface. * The number of hit (\"active\") pixels in more than 75% of meteor tracks is \(\leq\)5, so that their footprints on the focal surface are small. * Peaks of a meteor signal shift from one pixel to another (except for arrival directions close to nadir). * There are multiple signals in the data with the shape similar to that of meteors but simultaneously illuminating large areas of the FS. * Meteors are often registered on strong and quickly varying background illumination. * The amplitude of a meteor signal is typically lower than amplitudes of some other signals in the FoV of Mini-EUSO registered simultaneously with the meteor. * In some cases, it is impossible to judge unequivocally if a signal originated from a meteor or another source. Let us discuss the most important of these features taking as an example signals demonstrated in Figures 1 and 2. Figure 1 provides an example of a bright and clearly pronounced meteor signal with numerous active pixels. The top row shows only the meteor signal, with the background illumination omitted. It can be seen that signals in every pixel have a typical shape resembling the bell curve (see the left panel). The peaks are shifted in time with respect to each other due to the meteor moving in the FoV of the instrument, resulting in a quasi-linear track on the focal surface (see the right panel). The task of recognizing meteor signals might look trivial after looking at these \"pure\" signals. However, the FoV of Mini-EUSO covers a huge area resulting in numerous different signals being registered simultaneously, with many of them being much brighter than those of meteors. This is demonstrated in the second row of Figure 1. The left panel shows shapes of all other signals recorded simultaneously with the meteor with the meteor signal shown in black. The right panel presents a snapshot of the FS made at the moment of the maximum of the meteor signal. The brightest pixel of the meteor has coordinates \((\mathrm{row},\mathrm{column})=(13,27)\) and can be seen as a small spot below a much brighter and extended area that appeared due to anthropogenic illumination (sine-like curves in the left panel). It is important to remark that the bottom rows of Figures 1 and 2 demonstrate signals that were flat-fielded during an offline analysis for the sake of clarity. Figure 1: An example of a clearly pronounced meteor signal registered by Mini-EUSO on 20 October 2019. (**Top left**): signals in pixels that constitute the meteor signal. Signals in different pixels are shown in different colors. (**Top right**): location of meteor pixels in the focal surface. Colors denote time shift of the peaks with respect to the first one (in units of D3 GTUs). (**Bottom left**): all signals registered by Mini-EUSO simultaneously with the meteor. The black curves show the meteor signal. (**Bottom right**): a snapshot of the focal surface made at the moment of maximum of the brightest meteor pixel (GTU 2874). However, all results presented below were obtained using raw data as they are recorded by the instrument. This was performed in order to understand how effectively is our method if implemented in onboard electronics. An example of a typical meteor is shown in Figure 2. It has only four active pixels, and it is so dim in comparison with other illumination registered simultaneously that it is hardly possible to find it by eye in the bottom right panel presenting a snapshot of the focal surface at the moment of the maximum brightness of the meteor.