Lightning-induced intensification of the ionospheric sporadic E layer

Above the storm Cloud-to-ground lightning is a familiar feature of the global electric circuit through which the upper and lower atmosphere interact. Less well studied is the current flow above thunderclouds, upwards to the electrified region of the Earth's atmosphere, the ionosphere. Measurements here are scarce because it is above the heights at which aircraft fly and below spacecraft, but a remote method that detects the reflection of radio pulses in the ionosphere can now bridge that gap. Lightning-induced intensification of the sporadic ionospheric E layer was seen when this system was used during thunderstorms, providing a glimpse of the charge-transfer mechanisms in operation. A connection between thunderstorms and the ionosphere has been hypothesized since the mid-1920s 1 . Several mechanisms have been proposed to explain this connection 2 , 3 , 4 , 5 , 6 , 7 , and evidence from modelling 8 as well as various types of measurements 9 , 10 , 11 , 12 , 13 , 14 demonstrate that lightning can interact with the lower ionosphere. It has been proposed, on the basis of a few observed events 15 , that the ionospheric ‘sporadic E’ layer—transient, localized patches of relatively high electron density in the mid-ionosphere E layer, which significantly affect radio-wave propagation—can be modulated by thunderstorms, but a more formal statistical analysis is still needed. Here we identify a statistically significant intensification and descent in altitude of the mid-latitude sporadic E layer directly above thunderstorms. Because no ionospheric response to low-pressure systems without lightning is detected, we conclude that this localized intensification of the sporadic E layer can be attributed to lightning. We suggest that the co-location of lightning and ionospheric enhancement can be explained by either vertically propagating gravity waves that transfer energy from the site of lightning into the ionosphere, or vertical electrical discharge, or by a combination of these two mechanisms.