The higher regions of Venus's atmosphere are a natural source of radio waves, NASA has revealed. The Parker Solar Probe, fresh from having recently broken another speed record, delivered crucial observations that led to this discovery during its third flyby of Venus last year. The new insights provide a clearer picture of the Venusian atmosphere and how it's affected by the Sun's activity.
Reported in the Geophysical Research Letters, this is the first direct measurement of the ionosphere – the layer of electrically charged gas at the upper edge of a planet’s atmosphere – of Venus in 30 years and it showed something long suspected but not proven: in this region of the planet’s atmosphere changes in thickness are in response to the solar cycle.
The Sun's activity waxes and wanes over an 11-year cycle. The last direct measurement of Venus’s ionosphere was by the Pioneer Venus Orbiter in 1992, taken near the solar maximum when the Sun is most active. Parker’s measurement taken last July was near the solar minimum, during its quietest period.
The Parker Solar Probe detected a natural low-frequency radio signal for the seven minutes the probe was closest to Venus. Analyzing the signal the researchers realized the probe had skimmed the planet's upper atmosphere. To lead author Glyn Collison of NASA’s Goddard Space Flight Center, the signal seemed vaguely familiar but he couldn't place it.
“Then the next day, I woke up,” Collison said. “And I thought, ‘Oh my god, I know what this is!’”
The signal was similar to what the Galileo Probe, which orbited Jupiter two decades ago, saw in the ionosphere of Jupiter's moons. Thanks to that finding, the team was able to calculate the density of the atmosphere the probe flew through and establish that during a solar minimum Venus's ionosphere is much thinner than during a solar maximum, and so must shrink in thickness depending on the Sun's activity. This was suspected using ground-based observatories but a direct measurement was needed.
“When multiple missions are confirming the same result, one after the other, that gives you a lot of confidence that the thinning is real,” said Robin Ramstad, study co-author and post-doctoral researcher at the Laboratory of Atmospheric and Space Physics at the University of Colorado, Boulder.
Parker Solar Probe's main mission is to study the Sun by getting closer to our star than any other craft in history. It's not easy to get close, however, as anything from Earth will carry a bit of Earth’s momentum with it so the probe has to use flybys of Venus to essentially slingshot its way inwards towards the center of the Solar Sytsem. During these brief sojourns, it keeps seeing things that are unexpected.
“The goal of flying by Venus is to slow down the spacecraft so that Parker Solar Probe can dive closer to the Sun,” said Nour E. Raouafi, Parker Solar Probe project scientist at the Applied Physics Laboratory. “But we would not miss the opportunity to gather science data and provide unique insights into a mysterious planet such as Venus.”