Dawn‐Dusk Asymmetry of Energetic Electron at LEO During a Storm: Observation by FY3E.

Electrons in Low Earth Orbit (LEO) have been a popular research topic since the early days of space exploration. However, due to the limited pitch angle coverage and energy resolution of past in‐situ measurements, there is still much to uncover about the radiation environment. Fortunately, the recently launched satellite FY3E, which is equipped with the Medium‐Energy Electron Detector, is changing this. FY3E is the world's first early‐morning‐orbit meteorological satellite for civil use. With the ability to provide full pitch angle coverage and discrete energy measurements of 30–600 keV electrons, we now have an unprecedented opportunity to investigate the dynamics of space radiation at LEO and Magnetosphere‐Ionosphere coupling. Using the newly available measurements, we have discovered that during storms, electron precipitation exhibits dawn‐dusk asymmetry up to 100 keV. At dawn sector downward electrons can be similar to trapped electrons, and the loss cone ratio (precipitating flux over trapped flux) can be as high as unity at L‐shell greater than 6. While at dusk barely precipitation can be seen. The flux of trapped, upward, downward and net precipitation are calculated, and separate energy flux spectra of downward, upward, and trapped electrons are presented for the first time. With significant upward electrons up to 200 keV, asymmetric local pitch angle is observed at dawn within intense precipitation. In light of these findings, FY3E reveals critical information for understanding the Magnetosphere‐Ionosphere Coupling process. Plain Language Summary: The world's first early‐morning‐orbit meteorological satellite for civil use, FY3E, is equipped with new‐generation of energetic electron detectors, Medium‐Energy Electron Detector (MEED). MEED is able to provide the most thorough radiation distribution at LEO for now, which is vital for understanding magnetosphere and ionosphere physics. With the advantage of FY3E measurements, we found the severe asymmetry of the precipitation between dawn and dusk sector. Our analysis reveal details of the dawn‐dusk asymmetry, including precipitation intensity, precipitation region, pitch angle distribution and electron spectrum. Key Points: The first early‐morning‐orbit meteorological satellite FY3E provides near‐complete pitch angle distribution of 30–600 keV electrons at Low Earth OrbitThe downward electron flux can be comparable to trapped electrons up to 100 keV only at dawn and L‐shell >6 during the 2021.1.15 stormSeparate energy flux spectra of downward, upward, and trapped electrons have been presented for the first time [ABSTRACT FROM AUTHOR]