On July 11, 2020, NASA’s Parker Solar Probe made its third flyby of Venus. During 7 min around the closest approach, one of the spacecraft’s scientific instruments detected low-frequency radio emission of a type naturally generated by planetary ionospheres. By measuring the frequency of this radio emission, a team of NASA researchers directly calculated the density of the ionosphere around the spacecraft, finding it to be far less dense than previous missions encountered. The results support the theory that the ionosphere of Venus varies substantially over the 11 year solar cycle.
Venus in real colors, processed from Mariner 10 images. Image credit: Mattias Malmer / NASA.
“I was just so excited to have new data from Venus,” said study lead author Dr. Glyn Collison, a researcher at NASA’s Goddard Space Flight Center.
One of Parker Solar Probe’s instruments is FIELDS, named for the electric and magnetic fields it measures in the Sun’s atmosphere.
For just seven minutes — when Parker Solar Probe was closest to Venus — FIELDS detected a natural, low-frequency radio signal.
The thin frown in the data caught the attention of Dr. Collison and colleagues. The shape and strength of the signal seemed familiar, but they could not place it.
They recognized the signal from a previous work with NASA’s Galileo orbiter, which explored Jupiter and its moons before the mission ended in 2003. A similar frown appeared whenever the spacecraft passed through the ionospheres of Jupiter’s moons.
Like Earth, Venus sports an electrically charged layer of gas at the upper edge of its atmosphere, called the ionosphere.
This sea of charged gases, or plasma, naturally emits radio waves that can be detected by instruments like FIELDS.
“When we identified that signal, they realized Parker Solar Probe had skimmed Venus’ upper atmosphere — a pleasant surprise, though one we might have expected based on previous data,” Dr. Collison said.
The researchers used this radio emission to calculate the density of the ionosphere that Parker Solar Probe flew through.
Astronomers last obtained direct measurements of Venus’ ionosphere from Pioneer Venus Orbiter in 1992. Then, the Sun was near solar maximum, the stormy peak of the solar cycle.
In the years that followed, data from ground-based telescopes suggested big changes were taking place as the Sun settled into its calm phase, solar minimum.
While the bulk of the atmosphere remained the same, the ionosphere was much thinner during solar minimum. Without direct measurements, it was impossible to confirm.
The observations from Parker Solar Probe’s recent flyby, which occurred six months after the latest solar minimum, verify the puzzle in the Venusian ionosphere.
Indeed, the planet’s ionosphere is much thinner compared to previous measurements taken during solar maximum.
“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 study co-author Dr. Robin Ramstad, a postdoctoral researcher at the Laboratory of Atmospheric and Space Physics at the University of Colorado, Boulder.
The study was published in the journal Geophysical Research Letters.
Glyn A. Collinson et al. Depleted Plasma Densities in the Ionosphere of Venus near Solar Minimum from Parker Solar Probe Observations of Upper Hybrid Resonance Emission. Geophysical Research Letters, published online May 3, 2021; doi: 10.1029/2020GL092243