Abstract
Keywords Lightning; Upward lightning; Initial stage; Current measurements; Current pulses; Gaisberg Tower Highlights * Upward-flash IS current is comprised of slowly varying and impulsive components. * 60% of IS current pulses were bipolar and 40% were unipolar. * Bipolar current pulses only occurred at early times (within first 15% of IS duration). We examined current waveforms for 58 upward flashes occurring in 2006--2014 initiated from the Gaisberg Tower located near Salzburg, Austria. The initial stage (IS) of these flashes comprised of relatively slowly varying "background" current, with faster, more impulsive current variations overlaid on this background current. In 46 of the 58 flashes (79%) the background continuing current was negative, and in the other 12 flashes (21%) it was bipolar. 1180 current pulses occurred during the IS of these 58 flashes, of which 708 (60%) were positive bipolar (positive initial polarity with a negative opposite polarity overshoot), 28 (2.4%) were positive unipolar (positive initial polarity with no opposite polarity overshoot), 440 (37%) were negative unipolar, and four (0.3%) were negative bipolar pulses. We found that bipolar current pulses only occurred in the IS at early times. We divided the IS current into two phases: (1) upward leader initiation and propagation phase (IPP) and (2) upward leader mature phase (MP). 901 or 76% (712 bipolar and 189 unipolar) pulses occurred during the IPP, and 279 or 21% (unipolar) pulses occurred during the MP. The median background-to-peak current was 134 A for IPP pulses and 687 A for MP pulses. Author Affiliation: (a) Department of Aerospace, Physics and Space Sciences, Florida Institute of Technology, Melbourne, Florida, United States of America (b) Austrian Lightning Detection and Information System, OVE Service GmbH, Vienna, Austria (c) Now at the Department of Chemistry and Physics, Gulf Coast University, Fort Myers, Florida, United States of America (d) Los Alamos National Laboratory, Los Alamos, New Mexico, United States of America * Corresponding authors. Article History: Received 2 February 2022; Revised 20 June 2022; Accepted 11 July 2022 Byline: Naomi Watanabe [nwatanabe@fgcu.edu] (a,c,*), Amitabh Nag [anag@fit.edu] (a,d,*), Gerhard Diendorfer (b), Hannes Pichler (b), Wolfgang Schulz (b), Hamid K. Rassoul (a)