1. CSSL Laboratory, Penn State University, University Park, Pennsylvania 16802, USA
2. Langmuir Laboratory, New Mexico Tech, Socorro, New Mexico 87801, USA
3. STAR Laboratory, Stanford University, Stanford, California 94305, USA
4. These authors contributed equally to this work
5. Present address: Space and Atmospheric Sciences, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.

Correspondence to: Victor P. Pasko^1,2Mark A. Stanley^2,3,5 Correspondence and requests for materials should be addressed to V.P.P. (e-mail: Email: vpasko@psu.edu) or M.A.S. (e-mail: Email: stanleym@lanl.gov).

For over a century, numerous undocumented reports have appeared about unusual large-scale luminous phenomena above thunderclouds^{1, 2, 3, 4, 5, 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, 10, 11} and blue jets^{12, 13, 14}. Sprites initiate near the base of the ionosphere, develop very rapidly downwards at speeds which can exceed 10⁷ m s^-1 (ref. 15), and assume many different geometrical forms^{16, 17, 18, 19}. In contrast, blue jets develop upwards from cloud tops at speeds of the order of 10⁵ m s^-1 and are characterized by a blue conical shape^{12, 13, 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.