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Bright, discrete, thin auroral arcs are a typical form of auroras in nightside polar regions. Their light is produced by magnetospheric electrons, accelerated downward to obtain energies of several kilo electron volts by a quasi-static electric field. These electrons collide with and excite thermosphere atoms to higher energy states at altitude of ~ 100 km; relaxation from these states produces the auroral light. The electric potential accelerating the aurora-producing electrons has been reported to lie immediately above the ionosphere, at a few altitudes of thousand kilometres1. However, the highest altitude at which the precipitating electron is accelerated by the parallel potential drop is still unclear. Here, we show that active auroral arcs are powered by electrons accelerated at altitudes reaching greater than 30,000 km. We employ high-angular resolution electron observations achieved by the Arase satellite in the magnetosphere and optical observations of the aurora from a ground-based all-sky imager. Our observations of electron properties and dynamics resemble those of electron potential acceleration reported from low-altitude satellites except that the acceleration region is much higher than previously assumed. This shows that the dominant auroral acceleration region can extend far above a few thousand kilometres, well within the magnetospheric plasma proper, suggesting formation of the acceleration region by some unknown magnetospheric mechanisms.

The plasmasphere within Earth's magnetosphere plays a crucial role in space physics, with its electron density distribution being pivotal and strongly influenced by solar activity. Very Low Frequency (VLF) waves, including whistlers, provide valuable insights into this distribution, making the study of their propagation through the plasmasphere essential for predicting space weather impacts on various technologies. In this study, we evaluate the performance of different deep learning model sizes for lightning whistler detection using the YOLO (You Only Look Once) architecture. To achieve this, we transformed the entirety of raw data from the Arase (ERG) Satellite for August 2017 into 2736 images, which were then used to train the models. Our approach involves exposing the models to spectrogram diagrams—visual representations of the frequency content of signals—derived from the Arase Satellite's WFC (WaveForm Capture) subsystem, with a focus on analyzing whistler-mode plasma waves. We experimented with various model sizes, adjusting epochs, and conducted performance analysis using a partial set of labeled data. The testing phase confirmed the effectiveness of the models, with YOLOv5n emerging as the optimal choice due to its compact size (3.7 MB) and impressive detection speed, making it suitable for resource-constrained applications. Despite challenges such as image quality and the detection of smaller whistlers, YOLOv5n demonstrated commendable accuracy in identifying scenarios with simple shapes, thereby contributing to a deeper understanding of whistlers' impact on Earth's magnetosphere and fulfilling the core objectives of this study. [ABSTRACT FROM AUTHOR]

We report an Arase‐all sky imager (ASI) conjugate event in which the pulsating aurora (PsA) has a one‐to‐one correspondence with chorus bursts. Wavelet analysis displayed three peaks at ∼0.3 Hz, 4 Hz, and >10 Hz, corresponding to the main pulsation, internal modulation, and fast modulation, respectively. These correspond to the old terms of ∼5–15 s pulsations, chorus risers/elements and subelements/subpackets, respectively. Electron "microbursts" correspond to the 4‐Hz peak. The internal and fast modulations are further verified by the analysis based on fast Fourier transform analyses. Moreover, the spatial distributions of the Fourier spectral amplitude show that the internal and fast modulations are well‐structured within auroral patches. The above results indicate a paradigm shift away from quasilinear theory which implicitly assumes diffuse wave generation. The three time‐scale modulations are consistent with coherent chorus which has been theoretically argued to lead to pitch angle transport three orders of magnitude faster. Plain Language Summary: Pulsating aurora exhibit irregular patches of brightness with quasiperiodic on‐off transitions (∼2–20 s). More rapid modulations, such as internal modulation (∼3–4 Hz) or fast modulation (>10 Hz), have been detected within the pulsation "on" time. However, due to the measurement limitations, the simultaneous observation of three time‐scale modulations has never been reported. In this study, we analyze the conjugate observations of the pulsating aurora (PsA) and chorus recorded by the ground‐based Arase‐all sky imager and the Arase satellite, which demonstrates the coexistence of three time‐scale modulations in the PsA. The spatial distributions of Fourier spectral amplitude show that the internal and fast modulations are well‐structured within the aurora patches. The three time scales of chorus modulation have been previously called chorus 5–15 s pulsations, risers/elements and subelements/subpackets corresponding to the three aurora peaks. This study verifies the existence of internal and fast modulations in PsA, implying an extremely rapid electron loss mechanism. Quasilinear theory cannot explain any of the three time‐scale modulations. The discovery that chorus is coherent and will lead to pitch angle transport 1,000 times faster than diffuse waves is consistent with the fast modulations shown in this paper. A new theory of chorus generation is needed to update that of quasilinear theory. Key Points: We report a conjugate event in which the pulsating aurora (PsA) has a one‐to‐one correspondence with chorus burstsThe frequency spectra of auroral intensities obtained by wavelet analysis and fast Fourier transform (FFT) show the coexistence of ∼0.3 Hz, 4 Hz, and >10 Hz modulationsSpatial distributions show that the internal and fast modulations are well‐structured within aurora patches [ABSTRACT FROM AUTHOR]

Many studies have been conducted about the impact of energetic charged particles on the atmosphere during geomagnetically active times, while quiet time effects are poorly understood. We identified two energetic electron precipitation (EEP) events during the growth phase of moderate substorms and estimated the mesospheric ionization rate for an EEP event for which the most comprehensive dataset from ground-based and space-born instruments was available. The mesospheric ionization signature reached below 70 km altitude and continued for ~15 min until the substorm onset, as observed by the PANSY radar and imaging riometer at Syowa Station in the Antarctic region. We also used energetic electron flux observed by the Arase and POES 15 satellites as the input for the air-shower simulation code PHITS to quantitatively estimate the mesospheric ionization rate. The calculated ionization level due to the precipitating electrons is consistent with the observed value of cosmic noise absorption. The possible spatial extent of EEP is estimated to be ~8 h MLT in longitude and ~1.5° in latitude from a global magnetohydrodynamic simulation REPPU and the precipitating electron observations by the POES satellite, respectively. Such a significant duration and spatial extent of EEP events suggest a non-negligible contribution of the growth phase EEP to the mesospheric ionization. Combining the cutting-edge observations and simulations, we shed new light on the space weather impact of the EEP events during geomagnetically quiet times, which is important to understand the possible link between the space environment and climate.