Satellite-borne Far-ultraviolet Aurora Imager with Large Dynamic Range and Low Stray Light

Intense solar activities （such as coronal mass ejections and solar flares） release a large number of high-energy charged particles. When these particles reach Earth， they interact with the magnetosphere and trigger magnetic storms through processes like particle precipitation. The duration of magnetic storms can range from several hours to days. During this period， they cause ionospheric disturbances and drastic changes in the geomagnetic field， leading to disruptions in on-orbit satellite communications and shortwave broadcasting， significant degradation or even loss of lock in the positioning accuracy of global navigation satellite systems， and an increased risk of accelerated orbital decay for low-orbit satellites due to sudden increases in atmospheric drag.During magnetic storms， precipitating particles collide with the upper atmosphere at the poles along magnetic field lines， exciting the generation of auroras. The radiation characteristics of auroras are directly related to the flux and energy of the incident particles. Therefore， detecting the morphology of auroras in specific wavelength bands has become a key method for studying solar activities.During magnetic storms， precipitating high-energy particles collide with neutral atmospheric components （such as atomic oxygen and nitrogen molecules） above Earth's poles. Through excitation and ionization processes， these collisions produce auroral radiation covering wavelengths from visible light to the far ultraviolet. Among this radiation， the emission intensity of the LBH band of nitrogen molecules （N₂）—located in the far ultraviolet range of 160~180 nm—has a direct and sensitive quantitative relationship with the energy flux of the precipitating particles.Far ultraviolet radiation is effectively absorbed by the upper atmosphere， enabling its observation even in sunlight. However， achieving high-precision imaging of N₂ LBH band auroral radiation faces two core technical challenges. Firstly， the dynamic range of auroral brightness under natural conditions is extremely wide. Imaging across a range from approximately 100 Rayleigh （R） to over 30 000 Rayleigh （30 kR） during intense activity periods requires the instrument to have an extremely high linear dynamic range. Secondly， in the far ultraviolet band， stray light interference from non-target sources （such as the Sun and geocoronal background） is extremely strong， which can easily overwhelm weak auroral signals. This places extremely stringent requirements on the system's internal stray light suppression capability.To capture the full dynamic range （100 R~30 kR） of N₂ LBH band （160~180 nm） auroral radiation and overcome the challenges of weak far ultraviolet signals and strong background interference， this study developed an optical system based on a coaxial four-reflection structure. The core of the instrument's performance lies in its large dynamic range detection capability and high stray light suppression level. Through targeted optical design， simulation， and testing， the system ultimately achieved a signal-to-noise ratio （SNR） > 1.5 for extremely weak signals （100 R）， ensuring the reliable extraction and detectability of weak signals. For strong signals （30 kR）， the SNR reached > 30， guaranteeing measurement accuracy and the upper limit of dynamic range during intense auroral activity. Meanwhile， the stray light suppression level reached 10-3 within the 160~180 nm operating band and exceeded 10-9 in the non-operating band of 200~1 000 nm， thereby effectively extracting real auroral signals from complex optical backgrounds.This instrument can continuously monitor the spatiotemporal evolution of the position， morphology， scale， and intensity distribution of the auroral oval. Through inversion and analysis of these high-quality image data， we can quantitatively obtain core parameters such as information on precipitating particles and their energy flux. This deepens the scientific understanding of magnetic storm triggering mechanisms， energy coupling processes， and evolution laws. It provides crucial data support and technical backing for accurate magnetic storm forecasting， space weather modeling， and ensuring the stable operation of on-orbit satellites and ground communication systems， boasting significant scientific value and broad application prospects.