The term "gravity wave" (not to be confused with the space-time distorting gravitational wave) is a bit confusing -- in the atmosphere, it can be thought of as vertical wave. The most common way to trigger a vertical wave is by putting something in the way of surface winds, like a mountain. That forces it to rise, and if the surrounding air is stable enough, gravity will make it sink right back down, kicking off the wave cycle. On our planet, this can cause long, thin clouds to appear at high elevations over mountainous regions at the "peaks" of the gravity wave (as shown in the GIF below).
On Venus, it's the same phenomenon, but in a much more extreme setting. The "morning star" is like Bizarro Earth, if every single person drove diesel Volkswagens for the next billion years or so. The planet's sulfuric acid atmosphere is 90 times denser than ours, and those greenhouse gases produce surface temperatures of 850 degrees F, while winds constantly whip the planet at up to 250 mph. (Venus is about 72 percent of Earth's distance to the Sun and nearly the same size.)
Using its UV and infrared cameras, the Akatsuki team examined temperature variances on the bow wave, with known topography from the planet overlaid (below). The center of the bow is located at the western slope of "Aphrodite Terra," an Africa-sized highland region stretching up to 5 km (15,000 feet) high. Mission scientists figure that the winds whipped across these highlands, rapidly pushing a column of air into the upper reaches of the 65km thick atmosphere.
At a certain elevation, conditions were right to form a stable, bow-shaped gravity wave in the atmosphere, scientists believe. "It further shows that such stationary gravity waves can have a very large scale -- perhaps the greatest ever observed in the solar system," the team writes.
Simulations show such a feature should be theoretically possible, but it doesn't fit known data about the planet's surface weather, so climatologists may have to rethink what they know about Venus. "Winds in the deep atmosphere may be spatially or temporally more variable than we previously thought," the team posits. That's understandable, given that it's not exactly easy to set a probe down on the planet's metal-melting surface.
After several days of observation, Akatsuki crossed to the other side of the planet and when it returned a few months later, the structure was gone. As of yet, the probe has yet to spot a similar disturbance, so it may be a rare event and "beginner's luck" for the mission. Either way, it could prove helpful to future experiments or be an aid if we ever decide to terraform our nearest neighbor.