Effects of particle precipitation on the polar mesospheric summer echoes observed by EISCAT VHF 224 MHz radar

Abstract

We present the results from investigation of spectra of polar mesospheric summer echoes (PMSE) observed with EISCAT VHF 224 MHz radar during energetic particle precipitation events in the summer of 2019. We used the sudden enhancements in electron densities derived from the VHF observations above 90 km as indicators of particle precipitation. We find that the altitude extent of the PMSE increased along with an enhancement of the strength of the pre-existing PMSE. On some occasions, PMSE suddenly appeared during particle precipitation events. After the particle precipitation subsided, the PMSE intensities continued to be stronger for a few minutes and then decreased. At the altitudes where the maximum enhancement in PMSE backscatter occurred, there is no corresponding broadening in the spectral widths. Interestingly, the most intense PMSE echoes almost always coincided with the lower values of spectral widths both during particle precipitations and other times. In some cases, the spectral widths show well separated regimes between the upper and lower parts of the PMSE layers and this also happened irrespective of particle precipitation. The frequency Doppler shifts showed alternating upward and downward motions without much difference before and after the particle precipitation. Based on these observations, we conclude that the sudden enhancement in the electron densities during particle precipitation intensifies PMSE structures that match the Bragg scale of the incident radio wave thereby enabling an enhancement in the backscatter strength. Similarly, PMSE backscattering may get initiated in the altitudes in which the structures were already present but where electron density was not sufficient before the onset of precipitation. Spectral widths and Doppler velocities does not show sudden variations with particle precipitation because they depend on the neutral dynamics.