D region reflection height modification by whistler-induced electron precipitation

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.

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