The effect of Doppler broadening on D region negative ion ratio measurements at Arecibo

Abstract

AbstractWe present an analysis of the ratio of negative ions to electron concentration in the D region. The analysis is based on measurements made by the Arecibo (18.3°N) incoherent scatter radar (ISR) during 10:00 LT to 17:30 LT, 4 July 2006 and 11:00 LT to 15:00 LT, 3 September 2016. Previous ISR studies inferred the negative ion ratio without considering the broadening effect that nonthermal fluctuations have on the power spectra. Our results show that the Doppler effect from high‐frequency nonthermal fluctuations plays a significant role in broadening the ISR power spectra in the lower D region. A 10 Hz power spectrum can be Doppler broadened by 50% when the vertical velocity varies continuously within ±5 m/s, which is often observed. Hence, the ratio of the negative ion to electron concentration (λ) inferred in previous ISR studies is likely overestimated. Using the measured Doppler velocities, the broadening effect on the ISR power spectra due to Doppler shift is eliminated and λ is reevaluated. The results show that λ is largest at around 69 km. Although λ stays mostly below 1, a 3 km layer with λ greater than 1 is observed in the time period of 12–13 LT, 3 September 2016. Our results are largely consistent with the model study presented by Kull et al. (1997).