Measuring the Sun’s electric field

To the delight of solar physicists, the Parker Solar Probe is taking a page from the story of Icarus and flying too close to the Sun.

Physicists led by the University of Iowa have made the first definitive measurements of our star’s electric field, based on observations made by the NASA probe as it flew within a record-breaking 0.1AU (14.5 million kilometres) from the Sun.

This is closer than any spacecraft has ever dared to go before. It allowed the scientists to calculate how electrons are distributed within its electric field and therefore understand its size and scope.

“You can’t make these measurements far away from the Sun. You can only make them when you get close,” says Jasper Halekas, co-researcher from the University of Iowa.

“It’s like trying to understand a waterfall by looking at the river a mile downstream. The measurements we made at 0.1 AU, we’re actually in the waterfall. The solar wind is still accelerating at that point. It’s really just an awesome environment to be in.”

The results are published in The Astrophysical Journal.

The electric field of the Sun is generated by the interactions between the protons and electrons formed during the fusion process. While electrons are blown outwards, the heavier protons are left behind – but their positive charge still exerts some control, pulling some electrons back that are not quick enough to overcome the force and escape.

“We are measuring the ones that come back and not the ones that don’t come back,” Halekas says. “There’s basically a boundary in energy there between the ones that escape and the ones that don’t, which can be measured.

Researchers measured electrons streaming from the sun, a main constituent of the solar wind, to determine the boundary in energy between electrons that escape the sun’s clutches and those that don’t. Credit. Jasper Halekas lab, University of Iowa

“Since we’re close enough to the Sun, we can make accurate measurements of electrons’ distribution before collisions occur further out that distort the boundary and obscure the imprint of the electric field.”

Measuring the Sun’s electric field can help physicists understand the solar wind, the fast-flowing stream of charged particles emitted by the Sun which cause auroras on Earth, and can affect satellites and communications systems.

“We can now put a number on how much of the acceleration [of the solar wind] is provided by the Sun’s electric field,” Halekas concludes “It looks like it’s a small part of the total … That then points to other mechanisms that might be giving the solar wind most of its kick.”

Launched in 2018, NASA’s Parker Solar Prove is currently orbiting the Sun, making periodic fly-bys of Venus in order to gradually shrink its orbit and dive ever closer to our star.

Solar physicist are hoping to use the probe’s measurements to solve a stellar mystery: why is the Sun’s surface temperature only 6000 degrees Celsius, while the temperature of its corona exceeds one million degrees?

The probe is expected to make its closest approach to the Sun in 2025 – a blistering 6.9 million kilometres away.

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