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

Abstract

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.

Frequency Chirping of Electromagnetic Ion Cyclotron Waves in Earth's Magnetosphere

“…Such frequency chirping has also been observed in several other wave modes, including whistler mode chorus waves (Tsurutani & Smith, 1974), magnetosonic waves (Boardsen et al, 2014;Fu et al, 2014), electron cyclotron harmonic waves (S. Liu et al, 2023;Shen et al, 2021;Teng et al, 2021) in the magnetosphere and Alfvén waves (Heidbrink, 2008;Heidbrink & Sadler, 1994;McGuire et al, 1983) in fusion plasmas. These chirping waves generally consist of discrete packets that are narrowband and quasi-coherent (Tsurutani et al, 2009).…”

“…Although the linear instability of EMIC waves is well understood and typically related to temperature anisotropy in the inner magnetosphere (Gary et al., 1996; Gendrin et al., 1984), some EMIC waves exhibit nonlinear frequency chirping, in the form of spontaneous rising‐tone emissions (Mursula et al., 1994; Pickett et al., 2010). Such frequency chirping has also been observed in several other wave modes, including whistler mode chorus waves (Tsurutani & Smith, 1974), magnetosonic waves (Boardsen et al., 2014; Fu et al., 2014), electron cyclotron harmonic waves (S. Liu et al., 2023; Shen et al., 2021; Teng et al., 2021) in the magnetosphere and Alfvén waves (Heidbrink, 2008; Heidbrink & Sadler, 1994; McGuire et al., 1983) in fusion plasmas. These chirping waves generally consist of discrete packets that are narrowband and quasi‐coherent (Tsurutani et al., 2009).…”

Interplanetary shock induced intensification of electron cyclotron harmonic waves in the Earth’s inner magnetosphere

“…These emissions with quasi-perpendicular wave vectors are confined near their source regions [24]. While extensive event and statistical studies have focused on the strong dependence of ECH waves on geomagnetic activities [3,[25][26][27], none have directly established a link between ECH waves and solar wind disturbances. Interplanetary shocks, a subset of solar wind discontinuities frequently observed during active days [28][29][30][31], are highly geoeffective [32][33][34][35].…”