Scientists have long known that lightning from thunderstorms can cause perturbations in the lower ionosphere, partially ionizing the layer and creating disturbances in radio frequency communications. These disturbances usually occur over timescales of less than 15 seconds. However, scientists have also noted ionospheric perturbations occurring over timescales of several minutes, coincident with thunderstorms. Because of the longevity of these signals, it’s unlikely they are caused by single lightning strikes; rather, they appear to result from the electrical activity of a thunderstorm as a whole.
Koh et al. provide the first analysis of this type of long-term, lower atmosphere–ionosphere energy coupling and propose that heating observed in the ionosphere results from charges in thunderclouds redistributing as updraft strength increases.
All thunderstorms are built on the upward convection of warm, moist air. This process helps create the separation of electrical charges in storms that can lead to lightning, but even when the spectacular fast discharges do not occur, convective forces still create strong electrical polarization in storm clouds. The new study shows that this type of convection-based charging can also cause heating in the lower ionosphere as the ions there are energized by the electrical activity in the storm below.
The researchers studied long-wavelength radio wave transmissions from four locations in the United Kingdom and Germany to a single receiver in Bath, United Kingdom, on 27 August 2016 at around noon local time. These radio signals are sensitive to charges in the atmosphere, including the lower ionosphere, and the team used them—along with independent electric field measurements from an antenna array in Portishead, United Kingdom—to monitor a strong convective system over south central England. The researchers observed disturbances in the lower ionosphere occurring on timescales too long to be associated with lightning strikes. The disturbances grew for approximately 1 minute, and then ionospheric conductivity took at least another 200 seconds to return to normal levels.
The researchers note several other mechanisms by which such ionospheric disturbances might occur, such as gravity waves or solar flares, but conclude that the updraft-related mechanism is the most likely. The results, they say, demonstrate a new type of energy coupling between thunderstorms and the ionosphere and offer a more complete picture of atmospheric geophysics. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1029/2019JA026863, 2019)
—David Shultz, Freelance Writer