Lightning and the magnificent luminous events it generates are still poorly understood phenomena. Because lightning begins in the midst of towering black opaque thunderstorm clouds, it is particularly difficult to get a clear glimpse of the moment before it is triggered. Using data from the Low Frequency Interferometer Network (LOFAR), however, an international team has managed to unravel part of the mystery surrounding lightning.
Lightning is a strong electrostatic discharge that occurs between two regions of the atmosphere of opposite charge; it can occur within the same cloud, between several different clouds, or between a cloud and the earth's soil. The discharge begins with the ionization of a small area of air, which develops into a channel of tree plasma extending for several kilometers. Lightning produces a large amount of very high frequency radio pulses at the ends of the negative channels; positive channels only show broadcasts along the channel (not at its end).
The LOw Frequency Array (LOFAR) - a network of interferometers spread across Europe - can detect these signals in the very high frequency radio band. It is therefore able to detect and track the spread of lightning on an unprecedented scale, allowing scientists to "observe" the entire process. "The LOFAR measurements give us the first really clear picture of what's going on inside the storm," Joseph Dwyer, a physicist at the University of New Hampshire and co-author of the new study published in Geophysical Research Letters, told Quanta Magazine.
An event caused by electrically charged ice crystals
The study is based on data collected by LOFAR in 2018, when an intense lightning strike initiated at an altitude of approximately 6 kilometers streaked the sky above the observation instruments in the Netherlands. This is not the first time that radio detectors have been used to observe lightning, but LOFAR is much more efficient than the instruments used before - its imaging frequency is 200 times faster - and can even generate a 3D map of the phenomenon.
Analysis of the data showed that the radio pulses all originated from a single area about 70 meters wide in the heart of the storm cloud, confirming one of the hypotheses previously made to explain the conditions of lightning initiation. In 2019, researchers had already solved part of the mystery surrounding the phenomenon thanks to data from LOFAR; they notably mentioned needle-shaped structures to explain why lightning was likely to fall several times in the same place (contrary to popular belief).
The hypothesis is this: when ice crystals are gathered and collide in the storm cloud, they become electrically charged (the impacts eject some of their electrons and each crystal becomes a dipole). Therefore, the positive ends of crystals attract electrons from surrounding molecules; these attractive forces generate channels of ionized air (plasma), which extend and branch out several times. Each branch will heat the surrounding air, attracting more and more electrons from the air molecules and intensifying the current flowing to the ice crystals. When a plasma channel becomes hot enough and conductive, lightning propagates all the way.
Extremely rapid ramp-up
LOFAR data shows that the event intensifies exponentially: in 15 microseconds, the first lightning signal detected increases by two orders of magnitude! In total, the signal source traveled about 88 meters. "Initiation is probably caused by branched channels with an overall constant propagation velocity of 4.8 ± 0.1 × 106 m/s during the exponential ramp-up phase," summarize the study authors.
Another study confirms the role of ice crystals in the initiation of lightning: Another team of researchers recently highlighted a link between lightning and the COVID-19 pandemic . They noticed that lightning was less frequent at the start of the pandemic: lightning would have dropped from 10% to 20% in the first three months, their study concludes . This drop would be mainly due to a decrease in the concentration of aerosols in the atmosphere, inherent in containment measures, explain the researchers. When the population was confined to home, fewer pollutants were released into the atmosphere and as a result, ice crystals had fewer nucleation sites.
If this new study sheds light on the events that are at the origin of lightning, it does not however explain what happens when the ice crystals ionize the surrounding air, underlines Ute Ebert, physicist at Eindhoven University of Technology, which did not participate in the research. "Where does the first electron come from? How does the discharge start near an ice particle?", She wonders, according to her comments reported by Quanta Magazine.
On this point, cosmic rays - on which the other theory that can explain the origin of lightning is based - could ultimately play a role as well, by creating the first electrons triggering the plasma channels. Future LOFAR data could help lift the veil on these very first moments. The team also intends to be able to retrace the entire process, from the first spark leading to its initiation, to the impact of lightning on the ground, a series of stages whose unfolding remains poorly understood.
Reference:
The Spontaneous Nature of Lightning Initiation Revealed by C. Sterpka, J. Dwyer, N. Liu, B. M. Hare, O. Scholten, S. Buitink, S. ter Veen, A. Nelles