Researchers measured the flow of electrons streaming from the sun as the Parker Solar Probe spacecraft made its closest approach to date to our home star.
From:
University of Iowa
July 14, 2021 -- As the Parker Solar
Probe ventures closer to the sun, we are learning new things about our home star.
In a new study, physicists led by the
University of Iowa report the first definitive measurements of the sun's
electric field, and how the electric field interacts with the solar wind, the
fast-flowing current of charged particles that can affect activities on Earth,
from satellites to telecommunications.
The physicists calculated the
distribution of electrons within the sun's electric field, a feat made possible
by the fact that the Parker Solar Probe jetted within 0.1 astronomical units
(AU), or a mere 9 million miles, from the sun -- closer than any spacecraft has
approached. From the electrons' distribution, the physicists were able to
discern the size, breadth, and scope of the sun's electric field more clearly
than had been done before.
"The key point I would make is you
can't make these measurements far away from the sun. You can only make them
when you get close," says Jasper Halekas, associate professor in the
Department of Physics and Astronomy at Iowa and the study's corresponding author.
"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 sun's electric field arises from the
interaction of protons and electrons generated when hydrogen atoms are stripped
apart in the intense heat generated by fusion deep within the sun. In this
environment, electrons, with masses 1,800 times less than that of protons, are
blown outward, less constrained by gravity than their weightier proton
siblings. But the protons, with their positive charge, exert some control,
reining in some electrons due to the familiar attraction forces of oppositely
charged particles.
"Electrons are trying to escape,
but protons are trying to pull them back. And that is the electric field,"
says Halekas, a co-investigator for the Solar Wind Electrons, Alphas, and
Protons instrument aboard the Parker Solar Probe, the NASA-led mission that
launched in August 2018. "If there were no electric field, all the
electrons would rush away and be gone. But the electric field keeps it all
together as one homogenous flow."
Now, imagine the sun's electric field as
an immense bowl and the electrons as marbles rolling up the sides at differing
speeds. Some of the electrons, or marbles in this metaphor, are zippy enough to
cross over the lip of the bowl, while others don't accelerate enough and
eventually roll back toward the bowl's base.
"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
the bowl and the ones that don't, which can be measured. 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."
From those measurements the physicists
can learn more about the solar wind, the million-mile-per-hour jet of plasma
from the sun that washes over the Earth and other planets in the solar system.
What they found is the sun's electric field exerts some influence over the
solar wind, but less than had been thought.
"We can now put a number on how
much of the acceleration is provided by the sun's electric field," Halekas
says. "It looks like it's a small part of the total. It's not the main
thing that gives the solar wind its kick. That then points to other mechanisms
that might be giving the solar wind most of its kick."
The paper, "The sunward electron
deficit: A telltale sign of the sun's electric potential," was published
online July 14 in The Astrophysical Journal.
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