In 1999, the first sprites were observed above European thunderstorms using sensitive cameras. Since then, Eurosprite campaigns have been conducted to observe sprites and other transient luminous events (TLEs), expanding into a network covering large parts of Europe and coastal areas. In 2009 through 2013, the number of optical observations of TLEs reached a peak of 2000 per year. Because of this unprecedented number of European observations, it was possible to construct 5 a climatology of 8394 TLEs observed above 1018 thunderstorm systems, and study for the first time their distribution and seasonal cycle above Europe and parts of the Mediterranean Sea. The number of TLEs per thunderstorm was found to follow a power law, with less than 10 TLEs for 801 thunderstorms and up to 195 TLEs above the most prolific one. The majority of TLEs were classified as sprites, 641 elves, 280 halos, 70 upward lightning, 2 blue jets and 1 gigantic jet. The climatology 10 shows intense TLE activity during summer over continental areas, and in late autumn over coastal 1 vised manuscript with changes highlighted Click here to access/download;attachment to manuscript;Arnone2019_SG_revised_manuscript_RM_changes_ lick here to view linked References areas and sea. The two seasons peak respectively in August and November, separated by March and April with almost no TLEs, and a relative minimum around September. The observed TLE activity, i.e. mostly sprites, is shown to be largely consistent with lightning activity, with a 1/1000 of observed TLE to lightning ratio in regions with most observations. The overall behavior is consistent among individual years, making the observed seasonal cycle a robust general feature of TLE activity above Europe.
Article Highlights• Detailed climatological study of TLEs and lightning above Europe produced for the first time • Seasonal cycle of TLEs and lightning with two active seasons in summer over land and autumn over sea and coastal areas • Power law distribution of number of TLEs per thunderstorm Keywords Thunderstorms, lightning, transient luminous events, ground-based observations, Europe, climatology. 1 Introduction 1.1 Transient luminous events Three decades ago, a test low-light camera recorded a sprite (Franz et al., 1990), a spectacular discharge extending for tens of km above a thunderstorm. It was the first discovery of a whole family of upper atmosphere electrical processes, collectively known as transient luminous events (TLEs -see
