Lightning is so much more than simply a big spark. The million-dollar question in atmospheric electricity is how lightning starts inside thunderstorms in the first place. The conventional understanding of how electrical discharges are formed indicates that an electric field of 30 kV/cm (at equivalent ground-level pressure) is required to create a spark. Such high electric field has never been measured inside a thunderstorm, cultivating this long-standing puzzle. Some researchers suggest that lightning initiation is facilitated by thundercloud ice and water particles, while some others believe that the initiation threshold is alleviated by the presence of high-energy cosmic-ray secondary particles. Since the deep interior of thunderclouds is not accessible to the naked eye, we researchers rely on analysis of its radio electromagnetic emissions in various wavelengths to address this question.
The Plasma Nature of Lightning Channels
Lightning channels – called leaders – are made of plasma. Plasmas are also known as the 4th state of the matter. The lightning plasma consists of ionized air that exhibits collective behavior. The plasma nature of lightning channels is responsible for the asymmetric features of leaders carrying positive and negative charges, including: their propagation mechanisms in the Earth's atmosphere (positives propagate smoothly, while negatives step and emit X-rays), the contrasting charge transfer to the ground (positives have long continuing current transferring much more charge to the ground ), occurrence rates an multiplicity of ground flashes (negatives are 90% of ground flashes, with each flash consisting of several strikes to the ground), only positive cloud-to-ground flashes create sprites in the mesosphere, only positive intracloud discharges produce terrestrial gamma-ray flashes, and several more features.
Lightning Effects in the Upper Atmosphere
Thunderstorm and Lightning byproducts in the middle and upper atmosphere include: sprites, halos, jets, and elves. Sprites are filamentary discharge channels created in the mesosphere (50-90 km altitude), which grow out of small ionospheric electron density inhomogeneities. They are induced by the penetration of lightning quasi-electrostatic fields in the lower ionosphere, following the occurrence of strong positive cloud-to-ground lightning. Halos are direct evidence of lightning quasi-electrostatic fields penetrating in the lower ionosphere and inducing optical emissions. Blue jets and gigantic jets are formed from the upward channels of intracloud lightning that manage to escape the thundercloud top and propagate through the stratosphere (blue jets typically terminate at 40 km altitude) and the mesosphere (while gigantic jets stop at 90 km). Elves are expanding rings of luminosity induced in the interface between the lower ionosphere and the neutral atmosphere (at ~90 km altitude, the so-called edge of space), formed by the interaction of a lightning electromagnetic pulse (EMP) with the free electrons available at this interface.
Production of Energetic Radiation by Lightning and Laboratory Discharges
Perhaps one of the most fascinating discoveries in atmospheric sciences is the ability thunderstorms and lightning to act as particle accelerators and produce energetic radiation, i.e., high-energy electrons (tens of keV to tens of MeV), X-rays, and gamma rays. In the tip of negative-polarity lightning channels there are strong electric fields that can accelerate electrons to high energies. Nonlinear plasma waves called streamers drive the acceleration of this runaway electrons, which produce bremsstrahlung X-ray radiation. A similar process has been shown to happen in laboratory discharges, from 1 m length down to a few cm. It is still to be determined whether the level of energetic radiation emitted is a health hazard for airplane passengers flying nearby electrically-active thunderstorms.
Waves in Space Plasmas
The Earth's geomagnetic field traps energetic electrons arriving from the Sun forming a region called the Van Allen radiation belts. In such collisionless plasma the electron population balance is governed by wave-particle interactions. Right-hand circularly polarized waves, in the so-called whistler mode, can efficiently interact with trapped electrons, and depending on the resonance condition they can cause either pitch-angle scattering and losses to the atmosphere (electron precipitation), or energization. In the inner belt (2-3 Earth radii) lightning electromagnetic emissions are a key source of whistler waves, while in the outer belt (4-9 Earth radii) whistler-mode waves are excited by temperature-anisotropy plasma instabilities. Geosynchronous satellites have an orbital radius of about 6.6 Earth radii, placing them in heart of the outer radiation belt. Therefore, understanding the population balance of the Earth's radiation belts is crucial for extending the lifetime of satellites assets.
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