Earth and Mineral Sciences

Weather phenomenon that eluded scientists for decades captured in nature as corona discharges glow on tips of leaves

The positive, left, and negative corona discharges are shown on a spruce branch in a nearly pitch-black environment of a meteorology and atmospheric sciences lab at Penn State. Credit: William Brune / Penn State. Creative Commons

UNIVERSITY PARK — In a converted 2013 Toyota Sienna affixed with a hand-built telescopic weather device protruding from the roof, Penn State experts in meteorology and atmospheric science made their way down the nation’s eastern coast in June 2024 in search of Florida’s famed near-daily summer thunderstorms.

They were hoping to catch corona discharges, a long-hypothesized atmospheric weather phenomenon where miniscule pulses of electricity dance at the tips of tree leaves, causing the canopy to glow in the ultraviolet (UV). For more than 70 years, scientists have suspected treetops might emit these corona electrical discharges because of odd electric field activity in and over forests during storms, yet they have never been documented outside the lab.

The team, consisting of William Brune, distinguished professor of meteorology and atmospheric science; Patrick McFarland, a doctoral candidate in meteorology and atmospheric science; Jena Jenkins, assistant research professor; and David Miller, a former associate research professor who is now at the Penn State Applied Research Lab; worked to be the first to document this effect.

They chose the Sunshine State because of its propensity to produce frequent thunderstorms. However, as is often the case during research endeavors, the typical weather proved atypical.

For three weeks in Florida, McFarland and Brune chased pop-up storms that left as quickly as they formed.

The researchers had little to show for their efforts until, as they made their way back to Penn State, massive and sustained storms began cropping up just west of Interstate 95. The team caught an exit, nestled in a parking lot at the University of North Carolina at Pembroke, and trained their instruments to the top branches of a sweetgum tree that the rangefinder logged as 100 feet from their van.

The thunderstorm flashed lightning and poured rain for nearly two hours, giving them time to also observe corona on a nearby long needle loblolly pine tree as the storm waned. The results, which were the first directly-observed corona discharges occurring in nature, were recently 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.”

Corona discharges take shape during storms, the researchers said, because clouds build up strong negative charges that attract the opposite positive charge on the ground below. Opposites attract and this positive electrical ground charge rises up through the trees to the highest point, causing an electric field on the tiny, hair-like tips of leaves that is great enough to create the weak corona glow in both visible and UV form. This UV from the corona breaks apart water vapor, producing hydroxyl.

Hydroxyl is the atmosphere’s main oxidizer. Oxidizers clean the air by reacting with chemicals emitted into the air, making other chemicals that are easier to remove. These chemicals include volatile organic compounds emitted by trees or human activities and the greenhouse gas methane. The team’s prior research found corona discharges to be a substantial source of atmospheric cleansers in the forest canopy.

The chemical conversion is what researchers keyed in on. Several years ago, the team applied high-voltage, low-current electrical impulses to tree branches and found a strong correlation between the UV emissions from corona discharges and the creation of hydroxyl compounds. In that project and the more recent observations, researchers noted leaf damage at the point corona was emitted.

To capture the phenomena in nature and make use of this correlation, the team developed the Corona Observing Telescope System, a Newtonian telescope that feeds into a UV camera. It’s geolocated, equipped with a device for measuring atmospheric electricity and calibrated for UV emissions using a mercury lamp. The solar UV wavelength band is completely blocked, leaving corona, lightning and fire as the only sources of UV in the field.

In North Carolina, this system captured 859 coronae events on the sweetgum tree and 93 on the loblolly pine. Events ranged from a blink to several seconds, McFarland said. During the field campaign, researchers observed coronae in four additional thunderstorms and on four additional 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.”

While the researchers have confirmed the phenomena, they said they still don’t know much about the potential impacts of these corona discharges and have more questions, such as: Are trees harmed during this process? Or do they benefit in some way? Have they evolved to withstand it? Does the atmospheric cleansing have a benefit to the forest? The researchers are beginning collaborations with interested tree ecologists and biologists to answer these questions, thus blazing new paths of discovery into the natural world around us.

This work was supported by the U.S. National Science Foundation. Brune, Jenkins and Miller were co-authors on the research.