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The morphology and motion of auroras have been widely studied due to their indications on magnetospheric processes. Here, we report a new kind of "auroral curls," which have wavelengths in the mesoscale (∼100 km) and propagate azimuthally. Utilizing data from the Chinese Antarctic Zhongshan Station (the all‐sky imager and the high‐frequency radar), the Active Magnetosphere and Planetary Electrodynamics Response Experiment and the Defense Meteorological Satellite Program, we analyze an event occurred on 23 April 2019. We find these curls are fine structures in the poleward boundary of multiple arcs. Corresponding field‐aligned currents manifest as a series of longitudinally arranged pairs, while ionospheric flow velocities nearby oscillate with periods in the Pc 5 band. Observational evidence suggests these curls are connected with ultra‐low frequency (ULF) waves, which opens the possibility of using auroras to globally image ULF waves. Plain Language Summary: Auroras caused by precipitation of magnetospheric particles contain information about physical processes happened in the magnetosphere. In this letter, we report a new kind of auroral dynamic forms observed in Antarctica. These structures present both spatial and temporal periodic characteristics, which have similar scales with those of magnetospheric ultra‐low frequency (ULF) waves. We propose these auroral forms are connected with ULF waves, which provides a potential method to globally image ULF waves by analyzing properties of these auroras. Key Points: Azimuthally propagating "auroral curls" with mesoscale wavelengths were observed in AntarcticaThese curls are fine structures in the poleward boundary of multiple arcs formed by longitudinal‐arranged field‐aligned current pairsIonospheric flow velocities nearby oscillate with periods in the Pc 5 band, indicating connections with ultra‐low frequency waves [ABSTRACT FROM AUTHOR]

In Saturn's magnetosphere, the radially‐inward transport of magnetic fluxes is usually carried by localized flux tubes with sharply‐enhanced equatorial magnetic fields. The flux tubes also bring energetic particles inward, which are expected to drift azimuthally and produce energy‐dispersive signatures. Spacecraft observations, however, indicate the occurrence of energy‐dispersionless signatures for perpendicular‐moving particles. These unexpected features are attributed to the sharp magnetic gradient at the edge of the flux tubes, which significantly modifies the drift trajectories of perpendicular‐moving particles to enable their trapping motion within the flux tubes. The bouncing particles are less affected by the gradient, and therefore, still display energy‐dispersive signatures. It is the distinct particle behavior, together with different spacecraft traversal paths, that underlies the observational diversity. The results improve our understanding of particle dynamics in the magnetospheres of giant planets and indicate that pitch‐angle information should be considered in the extraction of flux‐tube properties from energetic particle observations. Plain Language Summary: The conservation of magnetic fluxes in Saturn's magnetosphere requires that the outward convection is compensated by a return process of magnetic fluxes, which has been observed in the form of localized flux tubes associated with sharply‐enhanced equatorial magnetic field and hot plasma population. The azimuthal drift of energetic particles within the flux tubes produces energy‐dispersive signatures, which have been utilized to estimate the age and starting position of the returning flux tubes. In this paper, we are motivated by Cassini observations of energy‐dispersionless signatures for perpendicular‐moving particles, to demonstrate that their drift paths can be significantly modified by the sharp magnetic gradient to cause their trapping within the flux tubes. The bouncing particles, on the other hand, are less affected by the gradient and, therefore, can leave the flux tubes to continue their drift around the planet. We further construct the magnetic configuration associated with the flux tubes, to illustrate the origin of the diverse observational signatures depending on particle pitch angle, spacecraft traversal path, and the trapping extent. These results have important implications for the interpretation of observational data in the injection flux tubes, and therefore improve our understanding of giant planet's magnetosphere and the associated particle dynamics. Key Points: In Saturn's injection flux tubes, charged particles often show distinct energy‐dispersive or dispersionless signals depending on pitch angleThe unexpected, energy‐dispersionless features for perpendicular‐moving particles are formed by their trapping motion within the flux tubesThe bouncing particles can hardly be trapped, and therefore, exhibit the characteristic energy dispersion and particle leaking signatures [ABSTRACT FROM AUTHOR]

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]