Glowing particle may reveal how lightning is sparked

Around 2,500 people die in lightning strikes around India each year. — © AFP

By deploying a precisely aligned pair of laser beams, scientists from Austria can now hold a single aerosol particle in place and monitor how it charges up. The particle’s glow signals each step in its changing electrical state, revealing how electrons are kicked away and how the particle sometimes releases sudden bursts of charge. Such behaviors mirror what may be happening inside storm clouds. The technique could help explain how lightning gets its initial spark.

Aerosols are tiny droplets or solid particles suspended in the air, and they surround us constantly. Some are large enough to see, such as springtime pollen, while others, like viruses that circulate during flu season, are far too small for the human eye. A few can even be sensed by taste, including the fine salt particles carried on ocean winds.

Researchers have been studying the behaviour of ice crystals that form within clouds. To better understand how these crystals gather charge, the scientists have modelled aerosols made from very small, transparent silica spheres.

This is based on a technique that uses two intersecting laser beams to trap, stabilise, and electrically charge a single silica particle. This setup opens the door to new investigations into how cloud electrification begins and how lightning is sparked.

A lone particle held by lasers may reveal the hidden spark that starts lightning

Using green laser beams to cross a space and measuring the bouncing from mirror to mirror, the scientists began to detect a slow, steady hissing noise coming from an anti-vibration table. It was observed that the beams travel through a series of aligned parts before converging into two narrow streams that enter a sealed container.

Where they meet, the beams create a concentrated point of light that can hold small particles in place. These “optical tweezers” keep drifting aerosols suspended long enough to study them. When a particle is caught, a bright green flash appears, confirming that the trap has successfully grabbed a glowing, perfectly round aerosol particle.

It was later discovered that the particles gain charge through a “two-photon process.” What is special here relates to aerosol particles usually carrying almost no net charge, with electrons (negatively charged entities) orbiting within each atom. Laser beams are made of photons (particles of light travelling at the speed of light).

When two photons strike the particle at the same moment and are absorbed together, they can remove a single electron. Losing that electron gives the particle one unit of positive charge, and with continued exposure, the particle becomes progressively more positively charged.

As such, scientists can now precisely observe the evolution of one aerosol particle as it charges up from neutral to highly charged and adjust the laser power to control the rate. As the charge builds, the particle also begins to lose charge again in sudden, short bursts. These spontaneous discharges hint at behaviours that may occur naturally in the atmosphere.

High above, cloud particles may undergo similar cycles of charge build-up and release. Thunderstorm clouds contain a mix of ice crystals and larger chunks of ice. As these collide, they trade electrical charges. Over time, the cloud becomes so electrically imbalanced that lightning forms.

It is thought that the earliest spark of a lightning bolt could arise directly from charged ice crystals. Yet the exact mechanism behind lightning formation remains unresolved. The new research supports the idea that an electric field inside clouds appears too weak to initiate lightning on its own.

The research appears in the journal Physical Review Letters, titled “Using Optical Tweezers to Simultaneously Trap, Charge, and Measure the Charge of a Microparticle in Air.”