Statistical study on spatial distribution of frequency-chirping ECH elements by Van Allen Probes

Electron cyclotron harmonic (ECH) wave can scatter magnetospheric electrons into the atmosphere causing diffuse aurora with the diffusion coefficients depending on the wave frequency. Here we report the Van Allen Probe observations of ECH elements, which are fine structures with a frequency-chirping rate of <i>∼</i>kHz/s. 1834 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 MLT through dawn to 14 MLT with <i>L</i><i> </i>= 4 <i>−</i><i> </i>7, and mainly occur in the region of 00<i>−</i>07 MLT with <i>L</i><i> </i>= 5<i>−</i>6. The occurrence rate of ECH elements increases and the occurrence region extends to lower <i>L</i><i>−</i>shells during the geomagnetic activity period. Meanwhile, the ECH elements can propagate to <i>|</i>MLAT<i>| ∼</i>20° but be confined by the contour of corresponding equatorial magnetic field strength. The current results could be helpful for investigating the mechanism of generating such ECH elements.