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For over a century, numerous undocumented reports have appeared about unusual large-scale luminous phenomena above thunderclouds and, more than 80 years ago, it was suggested that an electrical discharge could bridge the gap between a thundercloud and the upper atmosphere. Since then, two classes of vertically extensive optical flashes above thunderclouds have been identified-sprites and blue jets. 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. 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. 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.

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The vertical motion of ionospheric sporadic layers in the 80‐ to 140‐km altitude region is examined by using a numerical solution for ion trajectories in a wind system composed of diurnal and semidiurnal tides similar to those observed at Arecibo. The confluence of the two tides in the 100‐ to 110‐km altitude region is shown to produce a variety of layer motion possibilities including ‘quasi‐stagnation’ of the layer as well as rapid upward or downward motions. Experimental layer trajectories are presented which confirm the basic theoretical results and which indicate the continuous opportunity for layers to form in the tidal wind system. The availability of metallic ions for layer formation is discussed as is the observed presence of layers below 90‐km altitude.