1. Relationship between peak currents and the temporal separation between ground contacts Phenomenology and characterization of multiple ground contact strokes in natural lightning Leandro Z. S. Campos 1,*, Antonio C. V. Saraiva 1, Kenneth L. Cummins 2, Larissa Antunes 1, Osmar Pinto Jr. 1, Dailton G. Guedes 1 1 ELAT/CCST, INPE, National Institute for Space Research, São José dos Campos, São Paulo, Brazil 2 Department of Atmospheric Sciences, The University of Arizona, Tucson, Arizona, USA Financial support provided bythrough grants 08/56711-4 and 2010/01742-2, and scholarship number 2013/18785-4.* leandro.zanella@gmail.com Introduction and background During the development of a negative lightning channel to ground there is usually a large number of branches and, in some events, more than one of them reach ground virtually at the same time, leading to a Multiple Ground Contact Stroke (MGCS). First described by Schonland et al. [1935], the “forked strokes” typically create two (sometimes more) ground contact points separated by less than 1 ms. An apparently separate class of MGCS was recently reported by Stolzenburg et al. [2012, 2013], the upward illumination (UI) strokes. In these events the second ground termination appears to be cut off from the main leader channel and only its lowest (1-3 km) portion is illuminated. Instruments and dataset RAMMER network: Four digital high-speed cameras (3 black and white stationary, 1 colored mobile) operating at 2500 fps, 1200 x 504 pixels, GPS synchronized. BrasilDAT (EarthNetworks) and RINDAT (Vaisala): Lightning Locating Systems (LLS), provide estimates of time of occurrence, location and peak current for return strokes. BrasilDAT also allowed RAW data files of electric field waveforms from each sensor to be retrieved. PyRAW (open source Python library still under development): Generates false color images from the high-speed camera RAW data. Forked strokes Main observed characteristics: (i) Two or more temporally close ground terminations (ii) All ground terminations appear actively luminous at the same time for at least one frame (iii) They were all fully and uniformly illuminated after ground contact. The time interval between ground contacts ranged from 5 to 554 µs, with only two events with separations greater than 300 µs. Upward illumination (UI) strokes Main observed characteristics: Same (i) and (ii) as forked strokes (above) (iii) The upward-propagating luminosity enhancement in one of the branches is non- uniform (i.e., should not illuminate its entire branch, never reaching its forking point) The temporal separation between ground contacts ranged from 250 to 2600 µs, much higher than forked strokes. Additionally, unlike the dataset of Stolzenburg et al. [2012, 2013], no UI stroke presented a complete absence of channel luminosity between the UI branch and its main channel trunk. Selected references Schonland, B. F. J., et al. (1935), Progressive lightning, 2, Proc. Roy. Soc., A152, 595-625. Stolzenburg, M., et al. (2012), Strokes of upward illumination occurring within a few milliseconds after typical lightning return strokes, J. Geophys. Res., 117, D15203, doi:10.1029/2012JD017654. Stolzenburg, M., et al. (2013), Competing and cutoff leaders before “upward illumination”-type lightning ground strokes, J. Geophys. Res., 118, 7182-7198, doi:10.1002/jgrd.50512. Concluding remarks UI and forked strokes appear to belong to the same class of phenomena (MGCS) although separated by different conductivity régimes. Although being cut off from the channel main trunk is not a requirement for an UI stroke, its relatively lower peak current and the reduced channel luminosity in the connecting segments suggest that they are not perfectly coupled. OriginalInvertedFalse color No visually apparent cutoff For forked strokes: Secondary ground strike points may present peak currents almost as intense as the main ones, particularly for events with short intervals. For UI strokes: The relatively lower peak currents in UI channels indicate a smaller charge availability, possibly due to channel decoupling caused by longer intervals. Outlier possibly affected by the occurrence of a preceding attempted leader in the UI channel