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 equipped with a custom-built telescoping weather instrument that extended from the roof. Their goal was to track summer thunderstorms that occur almost daily in Florida and observe a phenomenon never seen before outside the laboratory.
This phenomenon, known as corona discharge, involves the generation of small bursts of electricity at the tips of leaves. These weak electrical pulses can cause the treetops to emit a subtle glow in the ultraviolet (UV) range. For more than 70 years, scientists have suspected that abnormal electric field activity causes these effects in forests during storms, but direct evidence in nature has remained elusive.
A long-standing mystery is finally verified in the field.
The research team included William Brune, distinguished professor of meteorology and atmospheric science; Patrick McFarland, a PhD student in the same field; Jenna Jenkins, assistant professor; David Miller, a former associate research professor who now works at the Penn State Applied Research Institute; Their aim was to record a naturally occurring corona discharge for the first time.
Florida was chosen because it experiences frequent thunderstorms and was considered ideal for the study. However, the weather did not turn out as expected. For three weeks, McFarland and Brune tracked a short-lived storm that yielded no useful data.
Groundbreaking observations in North Carolina
Things changed as the team began returning to Pennsylvania. A large, long-lasting storm developed just west of I-95. The researchers took advantage of the opportunity to stop by the University of North Carolina at Pembroke and set up equipment in the parking lot. They aimed their instruments at the upper branches of a sweetgum tree about 100 feet away from the van.
The thunderstorm lasted for nearly two hours, bringing heavy rain and frequent thunderstorms. During this time, the team recorded corona discharges in sweetgum trees and observed similar activity in nearby long-coniferous pines as the storm weakened. These observations marked the first confirmation of the detection of corona discharge in a natural environment. The discovery was later published in Geophysical Research Letters.
“This shows that discovery science is still happening,” said McFarland, lead author of the paper. “For more than half a century, scientists have theorized that corona exists, and this proves it.”
How corona discharges form in storms
According to the researchers, corona discharges are caused by strong electrical imbalances during storms. Thunderclouds generate large negative charges that attract positive charges on the ground. This positive charge moves upward through the trees and is concentrated at the highest points, such as the tips of the leaves.
In these tiny hair-like structures, the electric field becomes strong enough to produce a faint glow visible in both visible and ultraviolet light. The ultraviolet light produced by this process can break down water vapor molecules and cause the formation of hydroxyls.
Atmospheric chemistry and air purification effects
Hydroxyl plays an important role as the main oxidant in the atmosphere. Oxidizing agents help remove pollutants by reacting with chemicals in the air and converting them into substances that are easier to remove. These reactions involve compounds emitted by trees and human-produced pollutants such as methane, a potent greenhouse gas.
Previous work by the research team has shown that corona discharges may be an important source of these air purifying agents within the forest canopy. This makes this phenomenon potentially important for air quality and climate processes.
Laboratory insights and field confirmation
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 ultraviolet radiation from corona discharges and hydroxyl production. Both in these experiments and in recent field observations, we also noticed mild damage to the leaves where the corona occurred.
To observe this phenomenon outdoors, the team created a coronal observation telescope system. The instrument is a Newtonian telescope connected to a UV-sensing camera. This includes geolocation capabilities, sensors that measure atmospheric electricity, and calibration using a mercury lamp. The system blocks the sun’s UV wavelengths, allowing only the corona, lightning, and fire to generate detectable signals.
Capturing hundreds of corona events
Using this system in North Carolina, the team recorded 859 corona events in sweetgum trees and 93 corona events in loblolly pine. McFarland said each incident lasted anywhere from a few seconds to a few seconds. Additional observations were made across four different tree species during four other thunderstorms.
“Although barely visible to the naked eye, our instruments give us a vision of glowing coronal bands as thunderstorms pass overhead,” McFarland said. “Such a widespread corona can have far-reaching impacts on tree, forest, and atmospheric health, affecting the removal of hydrocarbons emitted by trees and subtle damage to tree leaves.”
Unanswered questions about trees and the environment
Although the research team confirmed that corona discharges occur in nature, many questions remain. Researchers want to know whether these electrical phenomena harm trees or provide some benefit. They are also investigating whether trees tolerate or are adapted to take advantage of this process, and whether the resulting air purification benefits forest ecosystems.
To investigate these questions, scientists have begun collaborating with tree ecologists and biologists. Their research could lead to new insights into how forests interact with the atmosphere and how those interactions affect the health of the environment.
The study was supported by the National Science Foundation and co-authored by Brune, Jenkins, and Miller.
