NASA's Parker Solar Probe offers first evidence of how the sun flings electrons toward Earth

May 12, 2021
A new solar probe is documenting what the sun is sending our way. (Pixabay/Dimitris Vetsikas)

A new solar probe is documenting what the sun is sending our way. (Pixabay/Dimitris Vetsikas)

The same star that pleasantly brightens each day also ejects fiery plasma into the universe, catapulting particles toward Earth and endangering satellites and astronauts in orbit, and researchers recently provided the first direct evidence of how the sun's solar wind scatters these electrons in space.

According to the study, published April 22 in The Astrophysical Journal Letters, the motion of electrons cast out in this solar wind form a halo-like emanation that depends on the type of electromagnetic wave nearby and the energy involved. 

The research details the properties of the solar wind's electron-scattering mechanism using measurements taken from NASA's Parker Solar Probe. Launched in 2018, this historic spacecraft dipped into the sun's corona, the outermost part of its atmosphere, and provided humanity with its "first visit to a star."

Study author Cynthia Cattell, a professor in the School of Physics and Astronomy at the University of Minnesota, explained to The Academic Times that the potential dangers of the solar wind are part of the reason her team aims to understand how it works.

"Understanding how the properties of the solar wind evolve as it travels out to the Earth — and beyond — is important for being able to predict space weather," she said. "It is also relevant for understanding whether certain types of exoplanets around certain types of stars might be habitable."

Space weather occurs when the solar wind pushes particles from the sun, namely electrons, toward Earth. That often impacts GPS satellites in orbit, for example, and threatens astronauts who may be traveling to the moon or spacewalking outside the International Space Station. Storms can become so strong that even the average smartphone can pick them up.

Uncovering the processes of Earth's very own star may also shed light on how other stars in the universe operate, perhaps to one day help scientists identify planets beyond the solar system that are viable for human colonization.

Although seemingly spherical and calm from Earth's point of view, the sun is more like a sea of plasma. Because of how incredibly hot its surface is — topping 1 million degrees Celsius — it rips apart atoms, meaning isolated particles swim along the star's magnetic fields. Due to the strength of those magnetic fields, their waves exude into the rest of the solar system, too.

The waves carry ions and electrons to Earth, generating a sort of electron rain. That movement of electrons toward Earth is what Cattell and her colleagues examined. They determined that rather than the electrons moving in a single-file manner, the particles move at an angle to each wave, becoming scattered by what is called a whistler wave.

"Electromagnetic waves called whistlers … were first identified by radio operators during World War I," Cattell said. "Those whistlers were generated by lightning — a very different process than what happens in the solar wind."

Humans are protected from the solar wind by Earth's own magnetic fields, which redirect it, and when the planet's fields trap some electrons raining down, beautiful displays called the northern lights are created. 

But things beyond Earth's magnetic fields have a different experience. Human technologies including satellites, GPS and power grids, as well as astronauts and personnel on polar-route aircraft, are all affected by space weather and solar energetic particles, Cattell explained.

The physicist conveyed that her team's findings regarding the scattering of electrons via the solar wind relate to the wind having a few stages of life. Within each stage, electrons behave differently. 

"The 'young' solar wind," she said, "is a term coined by professor Jasper Halekas at the University of Iowa — who also provided the electron data for our scattering study — to describe the solar wind close to the sun, before it has had time to excite waves and interact with waves."

The average electron has what is called a magnetic moment, meaning it gyrates around a magnetic field in a particular way, Cattell explained. However, the information the researchers received from the Parker Solar Probe indicated that some electrons ride the sine-curve-shaped whistler waves — kind of like surfers — rather than taking a standard route.

"We showed that the electrons were very strongly scattered when simultaneously observed [with] the very sinusoidal whistler waves," Cattell said, "but were not scattered when these waves were not present. The amount of scattering was correlated with the size of the waves."

She explained that the solar wind can be modeled with two types of electrons. There are core electrons, which are low-energy and isotropic, or the same in all directions of motion. There are also strahl electrons, which are more energetic and anisotropic, meaning they travel in a narrow beam. By the time the solar wind travels farther and becomes older, a third population called the halo is created. The halo is more energetic than the core but also isotropic, and it has fewer strahl electrons.

"What our research showed," Cattell said, "is that these almost sinusoidal whistler waves could scatter the strahl electrons to make the halo."

The study, "Parker Solar Probe evidence for scattering of electrons in the young solar wind by narrowband whistler-mode waves," published April 22 in The Astrophysical Journal Letters, was authored by C. Cattell, A. Breneman, J. Dombeck, B. Short and  J. Wygant, University of Minnesota; J. Halekas, University of Iowa; Tony Case and Mike Stevens, Smithsonian Astrophysical Observatory; J. C. Kasper, BWX Technologies, Inc. and University of Michigan; D. Larson, S. D. Bale, M. Pulupa, K. Goodrich and P. Whittesley, University of California, Berkeley; T. Dudok de Wit, University of Orléans; R. MacDowall, NASA/Goddard Space Flight Center; M. Moncuquet, Sorbonne Université; and D. Malaspina, University of Colorado.

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