The exact time lightning develops is still unclear. A team on international researchers, based on high-resolution data of the LOFAR radio telescope, has now found needle-shaped structures that might shed light on the discharge processes. Foundations that are essential for measuring lightning with the world's most extensive antenna array were laid at KIT. The researchers published the results of their study in Nature.
"Lightning needles" that were for a long time mysterious might aid the explanation of why lightning does not always discharge at once, as was thought for a long time, but can strike several times within seconds.
The collision of the ice crystals with a thundercloud will make them charge electrically. Winds may separate the crystals such that parts of the clouds are charged positively, whereas other parts are charged negatively.
When there are too large results from the electric field, a violent discharge takes place which we perceive as lightning and thunder. The release begins with a small volume of air whereby electrons separate from air molecules. This ionized air, also called plasma, is electrically conductive. The plasma propagates in the form of branched channels until it hits the Earth and charge stored in the clouds is discharged in the mode of lightning. Information about the exact processes taking place in these channels derived from lightning flashes. Scientists used the Dutch radio telescope LOFAR (Low-Frequency Array) to collect these data.
The recent observation of the LOFAR research team made revealed that positively and negatively charged plasma channels behave differently during lightning initiation. This effect is due to the needle-shaped structures that have now become visible for the first time. The researchers lead away from the positively charged channels in the perpendicular direction, are about 100m long, and have a diameter of less than 5m.
The scientist assumed that the charge of a positive plasma channel is not discharged entirely during a lightning strike, but that part of the charge returns to the thundercloud via the needles. The stored charge could then initiate further discharges. This dynamics would explain why lightning does not discharge at once, as was thought for a long time, but can strike many times within seconds.
The first author of the publication in Nature, Dr. Brian Hare of Groningen University said that the LOFAR data allowed them to detect lightning propagation at a scale where, for the first time, they can distinguish the primary processes. LOFAR has thousands of antennas distributed across Europe, an array primarily developed for astronomical observations, which meanwhile is also being used to measure cosmic rays. LOFAR reaches a spatial accuracy of up to 1 meter and typically records one signal per microsecond when measuring lightning flashes. The result of the high-resolution 3D films allows studies of lightning initiation at unprecedented levels of detail.
