We report the occurrence of X-rays at ground level due to cloud-to-ground flashes of\nupward-initiated lightning from Gaisberg Tower, in Austria, which is located at an altitude of 1300 m.\nThis is the first observation of X-ray emissions from upward lightning from a tower top located\nat high altitude. Measurements were carried out using scintillation detectors installed close to the\ntower top in two phases from 2011 to 2015. X-rays were recorded in three subsequent strokes of\nthree flashes out of the total of 108 flashes recorded in the system during both phases. In contrast to\nthe observations from downward natural or triggered lightning, X-rays were observed only within\n10 �¼s before the subsequent return stroke. This shows that X-rays were emitted when the dart leader\nwas in the vicinity of the tower top, hence during the most intense phase of the dart leader. Both the\ndetected energy and the fluence of X-rays are far lower compared to X-rays from downward natural\nor rocket-triggered lightning. In addition to the above 108 flashes, an interesting observation of X-rays\nproduced by a nearby downward flash is also presented. The shorter length of dart-leader channels\nin Gaisberg is suggested as a possible cause of this apparently weaker X-ray production.
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