Using data collected by NASA's Parker Solar Probe during its closest approach to the sun, a University of Arizona-led research team has measured the dynamics and ever-changing "shell" of hot gas from where the solar wind originates.
Published in Geophysical Research Letters , the findings not only help scientists answer fundamental questions about energy and matter moving through the heliosphere – the volume of space controlled by the sun's activity – which affects not just the Earth and moon, but all planets in the solar system, reaching far into interstellar space. These effects include significant space weather events.
"One of the things that we care about as a technologically advancing society is how we are impacted by the sun, the star that we live with," said Kristopher Klein, associate professor in the U of A Lunar and Planetary Laboratory who led the research study.
For example, during a coronal mass ejection, the sun flings chunks of its atmosphere – highly energetic, charged particles – out into the solar system, where they interact with Earth's magnetic field, with varying impacts on satellites, radio communications and even the radiation airplane passengers are exposed to when they fly over the poles, Klein explained.
"If we can better understand the sun's atmosphere through which these energetic particles are moving, it improves our ability to forecast how these eruptions from the sun will actually propagate through the solar system and eventually hit and possibly impact the Earth," he said.
While the idea of the sun having an atmosphere may seem difficult to imagine, since our star is essentially a roiling ball of plasma – hot, ionized hydrogen gas – with no appreciable surface, a century of studying its properties has led to a more nuanced picture. The core, where hydrogen undergoes nuclear fusion into helium, is the furnace driving the sun's activity, causing it to constantly radiate energy out into space.
Several layers wrap around the core, with the outermost ones forming the sun's atmosphere. The photosphere, where sunspots are located, is surrounded by a thin "peel" known as the chromosphere, from which flares may sprout and that forms the blotchy "surface" one may see when looking at the sun through a telescope equipped with special filters to allow for safe viewing. The sun's outermost atmospheric layer, the corona, is a fuzzy halo of plasma hidden from view at all times by the star's intense brilliance except for brief moments during a total solar eclipse.
Launched in 2018, Parker Solar Probe has approached the sun closer than any spacecraft mission before. Orbiting the sun in a complex orbit, involving seven passes by Venus, the probe reached its first closest approach on Christmas Eve 2024, and these close approaches have allowed the science team to map the sun's "outer boundary" in a way not possible until now.
In a counterintuitive twist, as the plasma bubbles up from the sun's core, it cools from 27 million degrees to about 10,000 degrees Fahrenheit in the visible photosphere, but as it fans out into the corona, it heats up again, to temperatures in excess of 2 million degrees.
The processes driving these strange dynamics involve complex interactions of the sun's charged particles with powerful magnetic fields that bend, twist and even snap back on themselves – with poorly understood details that have vexed heliophysicists to this day.
"We know there's this constant heat that's being input into the solar wind, and we want to understand what mechanisms are actually leading to that heating," Klein said. "We have made simplified models, we've run computer simulations, but by launching Parker Solar Probe, and by doing these detailed calculations of the structure of the velocity distribution of the particles, we can improve those models and calculate actually how the heating occurs at these at these extremely close distances where we have never measured before."
Before sending a robotic spacecraft capable of "kissing the sun," as the Parker team has referred to the probe's closest flyby, taking it to within 3.8 million miles above the sun's surface, researchers could only describe this heating using simple models for the charged particle distributions.
"One of the pressing questions we seek to answer is how the solar wind is heated as it is accelerated from the sun's surface," he said. "With these new measurements and calculations, we're rewriting our understanding of how energy moves through the sun's outer atmosphere."
A numerical code developed by Klein's team, dubbed Arbitrary Linear Plasma Solver, or ALPS, allowed the researchers to analyze the actual measured distribution rather than using a simplified model to determine how waves move through the plasma Parker is measuring, and – importantly – how the heating changes as the particles hurtle away from the sun. At the point of no return, where the solar wind is born, they begin to cool, but much more slowly than would be expected for a gas that is simply expanding, Klein explained – a process known as damping and yet another mystery waiting to be fully understood.
With ALPS and Parker's observations, the team can measure in detail how much energy is imparted onto the different species of charged particles in the solar wind, said Klein, explaining that this ability changes researchers' understanding of that process not just for the sun, but for all astrophysical objects involving heated plasma and magnetic fields.
"If we can understand the damping in the solar wind, we can then apply that knowledge of energy dissipation to things like interstellar gas, accretion disks around black holes, neutron stars and other astrophysical objects."
