On Earth, lightning flashes are followed by low-frequency plasma waves called whistlers. Because of this well-known relationship, the pace of whistlers on Venus has been used for decades to argue that lightning frequently crackles through its atmosphere. Now, new measurements have revealed that at least one Venus whistler traveled in the opposite direction than it would have if lightning had generated it, suggesting a different cause of the wave.
“We aren’t trying to say, ‘Lightning doesn’t exist on Venus,’” said Harriet George, a magnetospheric physicist at the University of Colorado Boulder and lead researcher on the project. However, the new discovery “really calls into question the validity of the assumption that underlies the [lightning] occurrence rates from whistler waves.”
A Stormy Debate
For decades, scientists have hotly debated whether lightning strikes on Venus. In the 1970s, the Soviet Venera missions detected whistlers, optical flashes, and other lightning-related radio emissions.
“The best evidence for lightning was obtained by [Soviet] instruments that landed on Venus and had optical and acoustic sensors,” explained Christopher Russell, a planetary scientist at the University of California, Los Angeles, who was not involved with the new research. The Pioneer Venus Orbiter in 1980, the Galileo spacecraft in 1991, and the Venus Express mission in 2007 also detected possible signals of lightning.
The few flashes seen by ground- or space-based cameras “are so infrequent that they could be meteors.”
“Based on previous whistler detections, some scientists argued that lightning is more common on Venus than on Earth,” explained Joseph O’Rourke, a planetary scientist at Arizona State University in Tempe. “However, searches for optical flashes from Venusian lightning have mostly failed to detect anything.”
NASA’s Cassini mission, which flew by Venus twice, and Japan’s Venus Climate Orbiter “Akatsuki,” in orbit at Venus since 2015, were equipped with instruments designed for detecting lightning. Neither craft found any. What’s more, the few flashes seen by ground- or space-based cameras “are so infrequent that they could be meteors,” said O’Rourke, who was not involved with the new research.
Russell, a self-described “pro–Venus lightning adherent,” maintains that the detections of optical flashes from the surface of the planet are incontrovertible proof of lightning. “Optical flashes are not rare on Venus,” he said. “They are just rarely seen on Earth because of the clouds and the distance of Venus from Earth.”
Where Was That Whistler Going?
On Earth, all lightning produces whistlers, but not all whistles are produced by lightning. The same could be true on Venus.
Reports of optical flashes remain controversial, so scientists have turned to radio detections to try to solve the mystery. George and her team sought to understand where Venus’s whistlers come from. On Earth, all lightning produces whistlers, but not all whistles are produced by lightning. The same could be true on Venus, the team postulated.
The researchers gathered data taken by NASA’s Parker Solar Probe during its fourth flyby of Venus in February 2021. During the roughly 1-hour flyby, the spacecraft traveled to within about 2,400 kilometers of the planet’s surface and measured electric and magnetic fields along the craft’s trajectory. It detected several whistlers but no other obvious lightning signals.
The team homed in on one whistler and used the electric and magnetic field measurements to calculate the direction that the plasma wave traveled: toward the planet, not away from it.
“If it was generated by lightning and we observed it out there, the shape of the magnetic field around the planet would have meant that the wave [traveled] off into the magnetotail,” George explained. By moving toward the planet instead, “the wave was traveling the opposite direction that it should have been going in if it was generated by lightning.”
“These new observations are the first to determine the direction the whistler waves are moving in,” said Karen Aplin, a space scientist at the University of Bristol in the United Kingdom. Aplin, who was not involved with this study, added that a whistler’s directionality is key to determining whether it was generated from the planet, for example, by lightning, or above its atmosphere through another process.
The researchers published these results in Geophysical Research Letters in September and will present this research at AGU’s Annual Meeting 2023 on 14 December.
Whistling Without Lightning
“These observations demonstrate that an alternative, nonlightning interpretation for the whistler signals observed at Venus exists,” said Marc Pulupa, a plasma astrophysicist at the University of California, Berkeley, who was not involved with the new study. “The observations also highlight the importance of analyzing electric and magnetic observations together, which is a capability we are fortunate to have on Parker Solar Probe,” he said.
“A definitive answer requires a mission dedicated to Venus.”
The team suggested that a reconnection of magnetic field lines in Venus’s magnetosphere could have generated the whistler. Reconnections can excite electron beams within a plasma and generate waves like whistlers. Parker Solar Probe also detected a different type of plasma wave, called a Langmuir wave, in Venus’s magnetosphere at the same time as the whistler. The timing suggested that the two types of plasma waves might have been generated by the same beam of electrons.
“That just gives us another clue at what must be going on to generate these whistlers,” George said. The team hopes that the next and final flyby of the Parker Solar Probe, in November 2024, will detect more whistlers and allow the team to narrow down possible explanations.
“I would bet a few months’ salary that lightning does not exist at Venus, but I wouldn’t yet bet my house.”
Aplin said that if Venus’s whistlers aren’t generated by lightning, it might call into question whistler-based claims of lightning on other planets, like Neptune. “This is a classic case of ‘absence of evidence is not evidence of absence,’” Aplin said, “so the question of Venus lightning remains open.”
“The time limitations of flyby observations preclude any categorical statement that Venus lacks lightning,” Pulupa said. “A definitive answer requires a mission dedicated to Venus.”
O’Rourke, too, thinks a long-term Venus orbiter could answer the question, but he has a strong suspicion of what that answer will be. “I would bet a few months’ salary that lightning does not exist at Venus,” he said, “but I wouldn’t yet bet my house.”
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer
