In June 2024, a group of Penn State meteorology and atmospheric science researchers set out on a road trip along the East Coast in a modified 2013 Toyota Sienna. The van was outfitted with a custom-built telescopic weather instrument extending from the roof. Their goal was to track down Florida’s near-daily summer thunderstorms and observe a phenomenon that had never been confirmed outside a laboratory.
That phenomenon, known as corona discharge, involves tiny bursts of electricity forming at the tips of leaves. These faint electrical pulses can cause treetops to emit a subtle glow in the ultraviolet (UV) range. Scientists have suspected for more than 70 years that forests might produce these effects during storms due to unusual electric field activity, but direct evidence in nature had remained elusive.
A Long-Standing Mystery Finally Tested in the Field
The research team included William Brune, a distinguished professor of meteorology and atmospheric science; Patrick McFarland, a doctoral student in the same field; Jena Jenkins, an assistant research professor; and David Miller, a former associate research professor now working at the Penn State Applied Research Lab. Their objective was to document corona discharges occurring naturally for the first time.
Florida was chosen because of its frequent thunderstorms, which seemed ideal for the study. However, the weather did not cooperate as expected. For three weeks, McFarland and Brune followed short-lived storms that dissipated quickly, leaving them without useful data.
Breakthrough Observation in North Carolina
As the team began heading back to Pennsylvania, conditions changed. Large, long-lasting storms developed just west of Interstate 95. Seizing the opportunity, the researchers stopped at the University of North Carolina at Pembroke and set up their equipment in a parking lot. They aimed their instruments at the upper branches of a sweetgum tree located about 100 feet from their van.
A thunderstorm persisted for nearly two hours, bringing heavy rain and frequent lightning. During this time, the team recorded corona discharges on the sweetgum tree and also observed similar activity on a nearby long needle loblolly pine as the storm weakened. These observations marked the first confirmed detection of corona discharges in a natural setting. The findings were later published in Geophysical Research Letters.
“This just goes to show that there’s still discovery science being done,” said McFarland, lead author on the paper. “For more than half a century, scientists have theorized that corona exists, but this proves it.”
How Corona Discharges Form in Storms
According to the researchers, corona discharges occur because of strong electrical imbalances during storms. Thunderclouds develop large negative charges that attract positive charges on the ground. This positive charge travels upward through trees, concentrating at the highest points such as leaf tips.
At these tiny, hair-like structures, the electric field becomes intense enough to produce a faint glow that can be seen in both visible light and UV. The UV radiation generated by this process can break apart water vapor molecules, leading to the formation of hydroxyl.
Atmospheric Chemistry and Air Cleaning Effects
Hydroxyl plays a key role in the atmosphere as its primary oxidizer. Oxidizers help remove pollutants by reacting with airborne chemicals and transforming them into substances that are easier to eliminate. These reactions involve compounds released by trees as well as human-generated pollutants, including methane, a potent greenhouse gas.
Earlier work by the team showed that corona discharges could be a significant source of these atmospheric cleansing agents within forest canopies. This makes the phenomenon potentially important for air quality and climate processes.
Laboratory Insights and Field Confirmation
The researchers had previously studied this effect in controlled experiments. By applying high-voltage, low-current electrical pulses to tree branches, they found a strong link between UV emissions from corona discharges and the production of hydroxyl. In both those experiments and the recent field observations, they also noted minor damage to leaves at the points where corona formed.
To observe the phenomenon outdoors, the team created the Corona Observing Telescope System. This instrument is a Newtonian telescope connected to a UV-sensitive camera. It includes geolocation capabilities, sensors to measure atmospheric electricity, and calibration using a mercury lamp. The system blocks solar UV wavelengths so that only corona, lightning and fire can produce detectable signals.
Hundreds of Corona Events Captured
Using this system in North Carolina, the team recorded 859 corona events on the sweetgum tree and 93 on the loblolly pine. Each event lasted anywhere from a fraction of a second to several seconds, according to McFarland. Additional observations were made during four other thunderstorms and across four different tree species.
“It’s nearly invisible to the naked eye but our instruments give rise to a vision of swaths of scintillating corona glowing as thunderstorms pass overhead,” McFarland said. “Such widespread coronae have implications for the removal of hydrocarbons emitted by trees, subtle tree leaf damage and could have broader implications for the health of trees, forests and the atmosphere.”
Open Questions About Trees and the Environment
Although the team has confirmed that corona discharges occur in nature, many questions remain. Researchers want to know whether these electrical events harm trees or provide some benefit. They are also investigating whether trees have adapted to tolerate or even take advantage of this process, and whether the resulting atmospheric cleansing benefits forest ecosystems.
To explore these questions, the scientists are beginning collaborations with tree ecologists and biologists. Their work could lead to new insights into how forests interact with the atmosphere and how these interactions influence environmental health.
The study was supported by the U.S. National Science Foundation, with Brune, Jenkins and Miller serving as co-authors.