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Recent studies have shown that auroral kilometric radiation (AKR) can propagate from the polar source cavity into the equatorial radiation belt but there is no direct evidence yet. By examining 19‐month data from Arase satellite and Van Allen Probes, we report a unique event that distinct AKR emissions are observed simultaneously at high and low latitudes, with the peak wave spectra in the frequency 300–400 kHz. Using the three‐dimensional ray tracing method, we simulate the AKR ray paths at three typical frequencies (220, 350, and 550 kHz), and show that more ray paths for 350 kHz can propagate downward through the locations of Arase satellite and Van Allen Probe B than those for other two frequencies. This result can successfully explain the observation and provides a direct evidence that AKR can propagate to low latitude radiation belt under suitable conditions. Plain Language Summary: Auroral kilometric radiation (AKR) is a widely existing electromagnetic wave in the magnetosphere and has potential for acceleration or precipitation loss of energetic electrons, yielding serious damage to orbiting space systems or ozone destruction in the atmosphere. Previous observation and numerical simulation have shown that AKR originated from the polar cavity region can be presented in the low latitude even in the equator. However, there is no direct observational evidence yet due to lack of the correlated data at higher and lower latitudes. Here, we examine observations of Arase satellite and Van Allen Probes from 25 March 2017 to 30 October 2018, and find a unique AKR event detected simultaneously by the Arase satellite (at high latitude) and Probe B (at low latitude). Using 3‐D ray tracing method, we simulate AKR paths for different frequencies (220, 350, and 550 kHz) with suitable wave normal angles and azimuthal angles. We show that more ray paths for 350 kHz can pass through the locations of Arase satellite and Probe B than those for other two frequencies, consistent with the observations. The current result provides a direct evidence that AKR can propagate to low latitude radiation belt under suitable conditions. Key Points: A unique auroral kilometric radiation (AKR) event is reported simultaneously by Arase satellite and Van Allen Probes during 19‐month observationsSimulations show that more AKR for 350 kHz pass the locations of two satellites, consistent with observationsAKR raypaths are highly dependent on wave frequency, initial wave normal angles and azimuthal angles [ABSTRACT FROM AUTHOR]

Previous studies have shown that auroral kilometric radiation (AKR) can play an important role in the magnetosphere‐atmosphere coupling and has the right‐handed extraordinary (R‐X), left‐handed ordinary (L‐O) and left‐handed extraordinary (L‐X) modes. However, the L‐X mode has not been directly observed in the lower latitude magnetosphere yet, probably because of its very limited frequency range. Here, using observations of the Arase satellite on 6 September 2018, we present an AKR event with two distinct bands (8–20 and 300–1000 kHz) around the location: L = 8 and latitude = −37°. The low (high) band is identified as the L‐X (R‐X) mode based on the polarization and frequency ranges. Simulations of 3‐D ray tracing show that most of ray paths with 14 (11 and 18) kHz pass (miss) the location of Arase, basically consistent with observations. Our study provides direct evidence that the L‐X mode can propagate from high latitudes downward to lower latitudes. Plain Language Summary: Auroral kilometric radiation (AKR) is a widely existing radio emission with kilometric wavelength at the Earth, contributing to the magnetosphere‐atmosphere coupling. Similar emissions have been observed on all magnetic planets of the solar system. Previous studies have shown that AKR primarily occurs in the R‐X mode, with a small contribution in the L‐O and L‐X modes. The L‐X mode at lower latitudes has not been directly observed so far, most likely due to its extremely limited frequency range. Here, we present an L‐X mode (peak frequency ∼14 kHz) in the lower latitude magnetosphere observed by the Arase satellite. Using the 3‐D ray tracing method, we simulate ray paths with different initial wave parameters and source locations. Simulations show that ray paths with 14 (11 and 18) kHz pass (miss) the location of the Arase satellite and are highly dependent on initial wave parameters and the location of source. Our results provide a direct evidence that the L‐X mode from high latitude source regions can propagate downward to lower latitudes under suitable conditions. This study enriches the understanding of AKR propagation characteristics in the magnetosphere. Key Points: An auroral kilometric radiation (AKR) event with two distinct bands (8–20 kHz and 300–1000 kHz) is observed around the location: L = 8 and latitude = −37°Based on the polarization and frequency ranges, the low (high) band AKR is identified as the L‐X (R‐X) mode3‐D ray tracing simulations show that L‐X mode can propagate downward to lower latitudes, basically consistent with observations [ABSTRACT FROM AUTHOR]

Electron cyclotron harmonic (ECH) waves, which are responsible for the diffuse aurora, are usually observed with the strongest intensity in the first band. Here, we present observations of enhanced higher‐band ECH with wave intensities above 10−3 mV2m−2 Hz−1 by Van Allen Probes. Fully thermal simulation results indicate that these waves can be locally excited by the observed electron distributions, and higher plasma density and lower magnetic strength are favorable for generating the enhanced higher‐band ECH. The pitch angle diffusion coefficient Dαα $\left\langle {D}_{\alpha \alpha }\right\rangle $ of the higher‐band ECH can exceed 10−5 s−1 inside the loss cone for energetic electrons (300 eV–∼15 keV), greater than the momentum diffusion coefficient Dpp $\left\langle {D}_{pp}\right\rangle $ by ∼100 times. It is suggested that the enhanced higher‐band ECH can also efficiently scatter these electrons into the loss cone and contribute to the diffuse aurora. Plain Language Summary: Electron cyclotron harmonic (ECH) waves are electrostatic emissions usually observed with the strongest intensity in the first band. Numerous works have demonstrated that the first‐band ECH waves can be responsible for diffuse auroral electron precipitation. However, so far the role of higher‐band ECH waves has not been confirmed. Here, we report an enhanced higher‐band ECH event with wave intensities above 10−3 mV2m−2 Hz−1 from Van Allen Probes. Using the realistic parameters, we conduct fully thermal simulations to investigate the generation of such ECH. The results indicate that growth rates can well reproduce the observed wave spectral characteristics in all four harmonic bands, and the higher density and lower magnetic field strength are favorable for generating enhanced higher‐band ECH. We calculate the diffusion coefficients to estimate the effect of the higher‐band ECH on electrons. The pitch angle diffusion coefficients induced by higher‐band ECH are ∼2 orders of magnitude greater than the momentum diffusion coefficients over a broad pitch angle range and exceed 10−5 s−1 inside the loss cone for energetic electrons. This indicates that the enhanced higher‐band ECH can also efficiently scatter these energetic electrons into the loss cone and potentially contribute to the diffuse aurora, similar to the first‐band ECH. Key Points: Enhanced higher‐band Electron cyclotron harmonic (ECH) waves with electric spectral density above 10−3 mV2m−2 Hz−1 were observed by Van Allen ProbesFully thermal simulations show that higher density and lower magnetic strength are favorable for generating enhanced higher‐band ECHEnhanced higher‐band ECH can cause efficient scattering loss of energetic electrons and contribute to the diffuse aurora [ABSTRACT FROM AUTHOR]