Miguel Guimaraes

Lightning Response of Grounding Grids: Simulated and Experimental Results

The response of grounding grids subjected to currents with typical waveforms of first and subsequent lightning return strokes is analyzed, based on experimental and simulated results. Parameters, which characterize this response, such as the grounding potential rise, impulse impedance, impulse coefficient, and effective grid area are determined for grids buried in soils of low, moderate, and high resistivity, varying the grid dimension and considering the frequency dependence of soil parameters. Based on such results, expressions to obtain the impulse impedance from the low-frequency resistance are derived.

M-component currents of first return strokes in natural negative cloud-to-ground lightning

[1]xa0Records of M-component currents of first return strokes in natural negative cloud-to-ground lightning measured in a short instrumented tower are shown along with correlated video records, electric field changes, and relative luminosity. The synchronized fast-camera records clearly match the increase of luminosity of the faint lightning channel. The parameters of these M components are all consistent with those of triggered lightning. According to records of electric field change of lightning events near Morro do Cachimbo Station and of currents measured at the instrumented tower, the frequency of first return strokes exhibiting M components is within the range of tens of percent and not of a few percent, as usually accepted. This expectation of a few percent is based on a very low number of first return strokes expected to be followed by continuing currents. It was found that most of the analyzed first-return-stroke M components of natural lightning occur at the final phase of the return-stroke process and not during the flow of continuing currents. Apparently, this also holds true for most of the first M components of subsequent return strokes.

Unusual features of negative leaders’ development in natural lightning, according to simultaneous records of current, electric field, luminosity, and high‐speed video

The development of downward and upward leaders that formed two negative cloud-to-ground return strokes in natural lightning, spaced only about 200u2009µs apart and terminating on ground only a few hundred meters away, was monitored at Morro do Cachimbo Station, Brazil. The simultaneous records of current, close electric field, relative luminosity, and corresponding high-speed video frames (sampling rate of 20,000 frames per second) reveal that the initiation of the first return stroke interfered in the development of the second negative leader, leading it to an apparent continuous development before the attachment, without stepping, and at a regular two-dimensional speed. Based on the experimental data, the formation processes of the two return strokes are discussed, and plausible interpretations for their development are provided.

Lightning Measurements at Morro do Cachimbo Station: new results

The results of measurements of lightning quantities at MCS are updated. Simultaneous records of current, electric field, optical sensor data and high-speed video of some events are considered in the analysis presented in this work. The main parameters of the lightning currents are summarized. Two sets of interesting and rare results consisting of first return stroke M-events and upward lightning currents measured in tropical regions are addressed and their main features are discussed.

Assessing events of upward lightning measured at Morro do Cachimbo Station

Some data of currents of lightning events measured in Morro do Cachimbo Station during the period 2008-2012 are presented, comprising 9 downward negative first strokes (4 of them pertaining to multiple-stroke flashes), 8 unconnected upward leaders developed in response to downward negative leaders approaching the ground and 5 negative upward lightning. The current waves of all negative upward lightning are presented along with an example of representative waves for first stroke and unconnected upward leader currents. Parameters of such currents are summarized.

Features of Upward Positive Leaders Initiated From Towers in Natural Cloud‐to‐Ground Lightning Based on Simultaneous High‐Speed Videos, Measured Currents, and Electric Fields

Original simultaneous records of currents, close electric field and high-speed videos of natural negative CG lightning striking the tower of Morro do Cachimbo Station are used to reveal typical features of upward positive leaders before the attachment, including their initiation and mode of propagation. According to the results, upward positive leaders initiate some hundreds of microseconds prior to the return stroke, while a continuous uprising current of about 4 A and superimposed pulses of a few tens amperes flow along the tower. Upon leader initiation, the electric field measured 50 m away from the tower at ground level is about 60 kV/m. The corresponding average field roughly estimated 0.5 m above the tower top is higher than 0.55 MV/m. As in laboratory experiments, the common propagation mode of upward positive leaders is developing continuously, without steps, from their initiation. Unlike downward negative leaders, upward positive leaders typically do not branch off, though they can bifurcate under the effect of a downward negative leaders secondary branch approaching their lateral surface. The upward positive leaders estimated average two-dimensional propagation speed, in the range of 0.06 × 106 to 0.16 × 106 m/s, has the same order of magnitude as that of downward negative leaders. Apparently, the speed tends to increase just before attachment.