The Spontaneous Nature of Lightning Initiation Revealed

Abstract

The basic principle of radio interferometry is that radio signals measured by separate antennas from a single source add coherently when adjusted for propagation time delays, while pulses from different sources or from random noise add incoherently (Taylor et al., 1999). For a lightning source, the combined signals will result in a received power approximately proportional to the square of the number of antennas and inversely proportional to the square of the distance from each antenna to the source. In contrast, signals from random noise will result in received power approximately proportional to the number of antennas. LOFAR is comprised of thousands of VHF antennas that are distributed all over Europe. For lightning studies, antennas are selected from the Netherlands to provide both large and small antenna separations (also known as baselines). The combination of the low-noise antennas and long baselines provides outstanding image resolution due to the fact that the maximum achievable angular resolution is proportional to λ/d, where λ is the wavelength of the radiation and d is the largest baseline length. Interferometers previously used to study lightning typically consisted of 3-4 antennas separated by a few hundred meters resulting in a resolutions on the order of 1.6° azimuth and 3.5° in elevation with no sensitivity along the radial axis (Tilles et al., 2019). In many cases, the algorithm used is closer to a time-of-arrival technique where only the location of the peaks are extracted from the result of the cross-correlations (Rison et al., 2016;Stock et al., 2014). The LOFAR impulsive imager uses a similar technique to the time-of-arrival, but has the advantage of hundreds of antennas and large baselines (Scholten et al., 2021b). As a result, the impulsive imager achieves source densities of over 200 sources per millisecond (Scholten et al., 2021b). Within this work and the previous impulsive imager, we achieve angular resolutions up to 1 arc sec in azimuth and 2 arc sec in elevation. This results in submeter resolution along both the horizontal (azimuth) and elevation axes, while also achieving 5 m resolution along the radial axis. LOFAR beamforming combines hundreds of antennas and selects baselines of up to 100 km, resulting in images with remarkably high signal to noise ratios and resolutions

Close Observation of the Evolution Process during Initial Stage of Triggered Lightning Based on Continuous Interferometer

“…Some studies have attempted to observe the initial process of natural lightning. Particularly in recent years, researchers have been able to image the development of lightning flashes with meter-scale accuracy and unprecedented detail with the low-frequency array (LOFAR) [33,34]. These results further shed light on the initial process of natural lightning.…”

Interferometric imaging of intensely radiating negative leaders

“…[6]. Only when performing interferometry, adding the signals of 200 antennas to suppress background, at a time in a lightning flash when the background is still small, a positive leader can be seen to develop [7]. The basic physics behind either of these modes is not understood.…”

Quantitative prediction of streamer discharge branching in air

“…Electron impact ionization in this enhanced field causes non-linear growth with velocities of 10 5 -10 7 m/s. Streamers are precursors of sparks and lightning leaders, they can be observed directly as sprites high above thunderclouds [2][3][4], and they play a prominent role in lightning inception [5,6]. They are also widely used in plasma and high voltage technology [1,[7][8][9].…”