Auroral kilometric radiation (AKR) is generated at high latitudes and can propagate down to low latitudes. Due to the lack of direct observations, the characteristics of AKR in the middle and low latitudes of two hemispheres have not been studied so far. Here, using observations of the Arase satellite from 23 March 2017 to 31 July 2019, we present the first statistical study of AKR distribution in the northern (Magnetic latitude Mlat = 0°–40°) and southern (Mlat = −40°–0°) hemispheres. Results (totally 30,353 samples) show that relatively high occurrence rates (>30%) of AKR in the northern (southern) hemisphere primarily stay in the region of magnetic local time MLT = 17–24 (MLT = 21–05). About 60% of wave samples in the northern (southern) hemisphere are observed in the frequency range of ≤300 kHz (>300 kHz). The asymmetric distribution in two hemispheres can further enrich our understanding of AKR.

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.

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.

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<p>Electron cyclotron harmonic (ECH) waves which mainly observed in the first harmonic band are believed to play a part in scattering diffuse aurora electrons in the Earth's magnetosphere. Here we report two enhanced ECH emission events with nominal wave magnitudes exceeding 1mV/m in higher bands ( up to the 4<sup>th</sup> harmonic bands) observed from Van Allen Probes mission during geomagnetic disturbance periods in the low density area. Based on actual measurements sampled by Van Allen Probes, we model the electron distribution using a superposition of bi-Maxwellian functions and then numerically evaluate the local growth rates and diffuse coefficients. These differences in frequency distribution for two events can be explained by differences in the loss cone feature of hot electron components. The scattering properties in the first four bands for two events are debated, these results suggest ECH waves in higher band can still cause efficient pitch angle scattering which are similar to ECH waves in the first band. This work broadens our understanding in the formation of diffuse aurora contributed by ECH waves.</p>