Statistical Study on Spatial Distribution of Frequency‐Chirping ECH Elements by Van Allen Probes

• Published in: Geophysical research letters 2023-11, Vol.50 (22), p.n/a
• Main Authors: Liu, Si, Chen, Yadan, Yang, Qiwu, Yang, Hongming, Xiao, Fuliang, Wang, Bowen, Gao, Zhonglei
• Format: Article
• Language: English
• Subjects: Atmosphere; Auroras; Coefficients; Cyclotrons; Diffuse aurora; Diffusion coefficient; Diffusion coefficients; ECH wave; Electrons; Elements; Equatorial regions; Field strength; Frequency dependence; frequency‐chirping fine structure; Geomagnetic activity; Geomagnetism; Magnetic field; Magnetic fields; Magnetospheres; Magnetospheric dynamics; Probes; Sensors; Shells; Spatial distribution; Statistics; Wave frequency
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL106371

Electron cyclotron harmonic (ECH) waves can scatter electrons into the atmosphere causing diffuse aurora with diffusion coefficients depending on the wave frequency. Here we report the Van Allen Probe observations of chirping ECH elements, which are fine structures with a frequency‐chirping rate of ∼kHz/s. 1,834 samples are identified in the 51‐month database and over 93 percent of them exhibit a falling tone pattern. ECH elements cover a broad region from 18 Magnetic Local Time (MLT) through dawn to 14 MLT with L = 4 − 7, and mainly occur in the region of 00–07 MLT with L = 5 − 6. More ECH elements are observed with the occurrence region extending to lower L‐shells during the geomagnetic activity period. Chirping ECH elements can propagate to |MLAT| ∼ 20°, but be confined by the contour of magnetic field strength. The statistical results can be applied on the global simulation of ECH‐induced electron penetration. Plain Language Summary Electron cyclotron harmonic (ECH) wave plays a critical role in the magnetospheric dynamics by diffusing energetic electrons into the atmosphere. Previous studies show that the diffusion coefficients are very sensitive to the wave frequency. Here based on the Van Allen Probes data, we report the presence of ECH frequency chirping structures (so‐called chirping ECH elements) in the magnetosphere. The duration of ECH elements is comparable with the well‐known chorus elements. Adopting a 51‐month database, we study the statistical properties of such ECH elements. All of the ECH elements occur in the first harmonic band and more than 93 percent of them are falling tones. ECH elements cover a broad range of L = 4 − 7 from 18 MLT through dawn to 14 MLT, and mainly appear in the region L = 5 − 6 at the midnight to dawn sector. It is widely accepted that ECH waves generally occur in the equatorial region. Here the ECH elements can propagate to |MLAT| ∼ 20°. Moreover, when the geomagnetic activities get more intense, more ECH elements can be observed and they can extend to lower L‐shells. The current results could be important for investigating their generation mechanism and quantifying the ECH‐electron resonance process. Key Points All chirping electron cyclotron harmonic (ECH) elements (1,834 samples) occur in the first harmonic band, showing a distinct preference to falling tone structures (>93%) The chirping ECH elements cover a broad region of radiation belts, with a higher occurrence in the region of 00–07 Magnetic Local Time