Electrical discharge from a thundercloud top to the lower ionosphere

For over a century, numerous undocumented reports have appeared about unusual large-scale luminous phenomena above thunderclouds(1-6) and, more than 80 years ago, it was suggested that an electrical discharge could bridge the gap between a thundercloud and the upper atmosphere(7,8). Since then, two classes of vertically extensive optical flashes above thunderclouds have been identified-sprites(9-11) and blue jets(12-14). Sprites initiate near the base of the ionosphere, develop very rapidly downwards at speeds which can exceed 10(7) m s(-1) (ref. 15), and assume many different geometrical forms(16-19). In contrast, blue jets develop upwards from cloud tops at speeds of the order of 10(5) m s(-1) and are characterized by a blue conical shape(12-14). But no experimental data related to sprites or blue jets have been reported which conclusively indicate that they establish a direct path of electrical contact between a thundercloud and the lower ionosphere. Here we report a video recording of a blue jet propagating upwards from a thundercloud to an altitude of about 70 km, taken at the Arecibo Observatory, Puerto Rico. Above an altitude of 42 km- normally the upper limit for blue jets and the lower terminal altitude for sprites-the flash exhibited some features normally observed in sprites. As we observed this phenomenon above a relatively small thunderstorm cell, we speculate that it may be common and therefore represent an unaccounted for component of the global electric circuit.