D region reflection height modification by whistler‐induced electron precipitation

Abstract

We examine the electron density profile expected in the lower ionosphere due to a 0.2‐s whistler‐induced electron precipitation (WEP) burst with experimentally determined properties. The ionization rate in the lower ionosphere due to a single such WEP event has a height variation with a rather broad maximum, leading to additional electron densities of ∼5 electrons cm−3 stretching over altitudes of ∼75–92 km. For ambient nighttime conditions a single WEP burst with these parameters will lead to a significant electron density changes only for altitudes below ∼85 km. We go on to consider the cumulative response of the nighttime D region to a sustained series of WEP bursts observed through Trimpi perturbation activity on one night in the Antarctic. For altitudes >70 km, significant long‐term changes in electron densities due to WEP bursts can occur. The additional WEP‐produced ionization leads to increases in the high‐altitude electron densities, until a new equilibrium level is reached. Peak changes in electron density are ∼16 times ambient at 85 km and ∼7 times ambient at 90 km, occurring in the ∼15‐min period during which the WEP rate is at its peak (∼4.5 per min). The simulation suggests that electron density levels “settle” into an new quasi‐equilibrium state during the ∼3‐hour period where the ionization at 85‐km altitude is 10–12 times ambient due to WEP bursts occur at ∼3 min−1. The ionization changes produced by WEP bursts lead to lower reflection heights for VLF and LF radio waves (in the Earth‐ionosphere waveguide). While significant short‐term changes in reflection heights are likely, realistic long‐term changes in WEP occurrence rates do not appear likely to be able to explain the reported ∼2 km decrease in LF reflection heights observed during the last 35 years.