1. Rutherford Appleton Laboratory, Chilton, Oxfordshire OX11 0QX, UK

Correspondence to: C. J. Davis¹ Correspondence and requests for materials should be addressed to C.J.D. (Email: C.J.Davis@rl.ac.uk).

A connection between thunderstorms and the ionosphere has been hypothesized since the mid-1920s¹. Several mechanisms have been proposed to explain this connection^{2, 3, 4, 5, 6, 7}, and evidence from modelling⁸ 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 events¹⁵, 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.

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