Lightning Observatory In Gainesville Research Articles

Lightning Observatory in Gainesville (LOG), Florida: A review of recent results

A review of several recent studies conducted at the Lightning Observatory in Gainesville (LOG), Florida, is given, including (a) evaluation of field-to-current conversion equations, (b) lower positive charge in the cloud and lightning type, (c) positive lightning, (d) compact intracloud lightning discharges, (e) lightning interaction with the ionosphere, and (f) X-rays produced by first and subsequent strokes in natural lightning. The results serve to improve our understanding of the physics of lightning with important implications for lightning detection and lightning protection areas.

On remote measurements of lightning return stroke peak currents

Return-stroke peak current is one of the most important measures of lightning intensity needed in different areas of atmospheric electricity research. It can be estimated from the corresponding electric or magnetic radiation field peak. Electric fields of 89 strokes in lightning flashes triggered using the rocket-and-wire technique at Camp Blanding (CB), Florida, were recorded at the Lightning Observatory in Gainesville, about 45km from the lightning channel. Lightning return-stroke peak currents were estimated from the measured electric field peaks using the empirical formula of Rakov et al. (1992) and the field-to-current conversion equation based on the transmission line model (Uman and McLain, 1969). These estimates, along with peak currents reported by the U.S. National Lightning Detection Network (NLDN), were compared with the ground-truth data, currents directly measured at the lightning channel base. The empirical formula, based on data for 28 triggered-lightning strokes acquired at the Kennedy Space Center (KSC), tends to overestimate peak currents, whereas the NLDN-reported peak currents are on average underestimates. The field-to-current conversion equation based on the transmission line model gives the best match with directly measured peak currents for return-stroke speeds between c/2 and 2c/3 (1.5 and 2×108m/s, respectively). Possible reasons for the discrepancy in the peak current estimates from the empirical formula and the ground-truth data include an error in the field calibration factor, difference in the typical return-stroke speeds at CB and at the KSC (considered here to be the most likely reason), and limited sample sizes, particularly for the KSC data. A new empirical formula, I=−0.66–0.028rE, based on data for 89 strokes in lightning flashes triggered at CB, is derived.

A study of X‐ray emissions from thunderstorms with emphasis on subsequent strokes in natural lightning

X‐ray emissions associated with leaders of natural cloud‐to‐ground lightning are examined. For 23 (8 first and 15 subsequent) strokes within 2 km of the Lightning Observatory in Gainesville, the occurrence of detectable X‐rays was 88% and 47% for the first and subsequent strokes, respectively. Some subsequent‐stroke leaders, retracing previously formed, low‐density channels, are more prolific producers of detectable X‐rays than their corresponding first‐stroke leaders, developing in the virgin air. The energy of some photons was in the MeV range (in one case possibly in excess of 5 MeV); that is, in the gamma‐ray range. There was a significant difference between first and subsequent leaders in terms of the distribution of estimated X‐ray source heights. For first leaders, the maximum source height did not exceed 800 m, whereas for subsequent leaders the source height distribution appeared to extend to about 3.6 km. Not all leaders within a flash produced detectable X‐rays. For the same leader near ground, some steps were accompanied by detectable X‐ray emissions, while others were not. One possible explanation is that electric field enhancements (>30 MV/m or so for the case of normal air density), needed for the cold runaway breakdown, are very brief and highly localized, so that in many cases a sufficiently energetic electron from the tail of the bulk distribution may be unavailable to start the runaway process. This implies that the cold runaway breakdown is not a necessary feature of lightning leaders, even if the required fields do occur.