Erin H. Lay

Reduction of electron density in the night-time lower ionosphere in response to a thunderstorm

Tropospheric thunderstorms have been reported to disturb the lower ionosphere, at altitudes of 65–90 km. The use of lightning signals from a distant mesoscale storm to probe the lower ionosphere above a small tropospheric thunderstorm reveals a reduction in ionospheric electron density in response to lightning discharges in the small storm. Tropospheric thunderstorms have been reported to disturb the lower ionosphere, at altitudes of 65–90 km, by convective atmospheric gravity waves1,2,3,4,5 and by electric field changes produced by lightning discharges6,7,8,9,10,11,12,13,14,15. Theoretical simulations suggest that lightning electric fields enhance electron attachment to O2 and reduce electron density in the lower ionosphere7,8. Owing to the low electron density in the lower ionosphere, active probing of its electron distribution is difficult16,17, and the various perturbative effects are poorly understood. However, it is now possible to probe the lower ionosphere in a spatially and temporally resolved manner by using remotely detected time waveforms of lightning radio signals4,5,18,19. Here we report such observations of the night-time ionosphere above a small thunderstorm. We find that electron density in the lower ionosphere decreased in response to lightning discharges. The extent of the reduction is closely related in time and space to the rate of lightning discharges, supporting the idea that the enhanced electron attachment is responsible for the reduction. We conclude that ionospheric electron density variations corresponding to lightning discharges should be considered in future simulations of the ionosphere and the initiation of sprite discharges.

Ionospheric acoustic and gravity waves associated with midlatitude thunderstorms

Acoustic waves with periods of 2–4 min and gravity waves with periods of 6–16 min have been detected at ionospheric heights (250–350 km) using GPS total electron content measurements. The area disturbed by these waves and the wave amplitudes have been associated with underlying thunderstorm activity. A statistical study comparing Next Generation Weather Radar thunderstorm measurements with ionospheric acoustic and gravity waves in the midlatitude U.S. Great Plains region was performed for the time period of May–July 2005. An increase of ionospheric acoustic wave disturbed area and amplitude is primarily associated with large thunderstorms (mesoscale convective systems). Ionospheric gravity wave disturbed area and amplitude scale with thunderstorm activity, with even small storms (i.e., individual storm cells) producing an increase of gravity waves.

D region electron profiles observed with substantial spatial and temporal change near thunderstorms

Broadband lightning signals are used to probe the D region ionosphere with a temporal resolution of 5 min and a spatial resolution of ~50 × 50 km. Together with a full wave propagation model, this technique allows determination of the reference height, h′, and steepness parameter, β, of an exponential electron density profile sensitive to the range of 106–108 electrons/m3. Daytime and nighttime background electron profiles away from thunderstorms are presented, as well as profiles from three regions nearby and atop thunderstorms. The average daytime profile parameters are found to be h′ = 67.7 km with a standard deviation of 0.9 km and β = 0.7 km−1 with a standard deviation of 0.1 km−1. Average nighttime parameters are h′ = 80.9 km with a standard deviation of 1.3 km and β = 2.8 km−1 with a standard deviation of 0.2 km−1. Nighttime electron profiles nearby and atop thunderstorms show slightly higher values of h′ (82.5–84.2 km) and significantly lower values of β (0.9–1.5 km−1). These findings indicate that there is significant electron depletion above ~80 km near and atop thunderstorms during the nighttime. Detailed analysis also shows substantial profile variations in space and time related to lightning discharges due to localized electron enhancement at high altitudes and reduction at lower altitudes. Nevertheless, the general depletion at higher altitudes appears to be related to the overall electrical behavior of the thunderstorm but not directly to lightning activity.

Temporal‐spatial modeling of electron density enhancement due to successive lightning strokes

[1] We report results on the temporal-spatial modeling of electron density enhancement due to successive lightning strokes. Stroke rates based on World-Wide Lightning Location Network measurements are used as input to an axisymmetric Finite Difference Time Domain model that describes the effect of lightning electromagnetic pulses (EMP) on the ionosphere. Each successive EMP pulse interacts with a modified background ionosphere due to the previous pulses, resulting in a nonlinear electron density perturbation over time that eventually reaches a limiting value. The qualitative ionospheric response to successive EMPs is presented in 2-D, axisymmetric space. Results from this study show that the nonlinear electron density perturbations due to successive lightning strokes must be taken into account and varies with altitude. The limiting maximum electron density is reached earlier in time for higher altitudes, and the most significant effect occurs at 88 km. The limiting modeled electron density profile in the 83-91 km altitude range does not depend on the initial electron density.