In some daring flybys that took it closer to the Sun than Mercury, NASA’s Parker Solar Probe has uncovered features that will fundamentally change our understanding of the Sun say the scientists who have been analysing the mission data from the closest-ever look inside our star’s atmosphere, the corona.
Launched in August 2018, NASA's Parker Solar Probe that is packed with cutting-edge scientific instruments to measure the environment around the spacecraft, has completed three of 24 planned passes through never-before-explored parts of the Sun's corona.
Even with just a few flyby’s under its belt, the probe is already unveiling processes that no one has ever seen before, that will not only help scientists understand the sun's activity but could ultimately provide an early warning for solar storms.
Among the findings are new understandings of how the Sun's constant outflow of solar wind – the constant outflow of hot, ionised gas that streams outward from the Sun – and how the solar wind couples with solar rotation.
Seen near Earth, the solar wind plasma appears to be a relatively uniform flow with an occasional turbulent tussle going on. But from an earthly point of view 145 million kilometres (90 million miles) away, the traces of the exact mechanisms for heating and accelerating the solar wind have long since dissipated.
From the close-up position of the spacecraft however, Parker has revealed that the sun's rotation impacts the solar wind much farther away than previously thought.
Researchers already knew that close in to the star, the sun's magnetic field pulls the solar wind in the same direction as the star's rotation.
Farther from the sun however, scientists had expected to see this phenomenon trail off and just a weak signature of that rotation would be evident.
This appears not to be the case. The probe's solar wind instrument detected rotation starting more than 32 million kilometres (20 million miles) from the Sun, and as Parker approached its perihelion point (the point in an orbit that is nearest to the sun), the speed of the rotation increased.
Although it transitioned more quickly than predicted to an outward flow, the strength of the circulation was stronger than many scientists had anticipated.
"To our great surprise, as we neared the sun, we've already detected large rotational flows – 10 to 20 times greater than what standard models of the sun predict," said Justin Kasper, a professor of climate and space sciences and engineering at the University of Michigan who serves as principal investigator for Parker's Solar Wind Electrons Alphas and Protons (SWEAP) instrument suite.
"So we are missing something fundamental about the sun, and how the solar wind escapes. This has huge implications. Space weather forecasting will need to account for these flows if we are going to be able to predict whether a coronal mass ejection will strike Earth, or astronauts heading to the moon or Mars," Kasper said.
Meanwhile, the probe’s FIELDS instrument suite, which studies the scale and shape of electric and magnetic fields, showed quick reversals in the direction of the magnetic field and sudden, faster-moving jets of material – all characteristics that make the solar wind more turbulent.
These reversals – dubbed "switchbacks" – is where the magnetic field whips back on itself until it is pointed almost directly back at the Sun.
They last anywhere from a few seconds to several minutes but release lots of energy, accelerating the solar wind away in long tubes that are approximately the diameter of the Earth.
Together, FIELDS and SWEAP, the solar wind instrument suite led by the University of Michigan and managed by the Smithsonian Astrophysical Observatory, measured clusters of switchbacks throughout Parker Solar Probe's first two flybys.
"Waves have been seen in the solar wind from the start of the space age, and we assumed that closer to the Sun the waves would get stronger, but we were not expecting to see them organize into these coherent structured velocity spikes," Kasper said.
These bursty 'spikes' of solar wind were observed to originate in holes in the Sun's outer atmosphere near its equator. They are then accelerated by the switchbacks as they flow away into deep space and past our planet.
This data, suggests Kasper and team, is that the switchbacks are kinks in the magnetic field – localised disturbances travelling away from the Sun, rather than a change in the magnetic field as it emerges from the Sun.
The switchbacks are expected to grow even more common as the spacecraft gets closer to the Sun and although the process is still not fully understood, Parker’s measurements are allowing scientists to narrow down the possibilities.
"We usually think of the fast solar wind as very smooth, but Parker Solar Probe saw surprisingly slow wind with a large number of these little bursts and jets of plasma, creating long tubes of fast wind containing plasma with around twice the energy of the background solar wind,” says Professor Tim Horbury, a co-investigator on Parker Solar Probe's FIELDS instrument based at Imperial College London.
"We are detecting remnants of structures from the Sun being hurled into space and violently changing the organisation of the flows and magnetic field. This will dramatically change our theories for how the corona and solar wind are being heated,” added Kasper.
The solar probe also helped confirm another theory regarding dust. Our solar system is awash with the tiny leftovers of rocky bodies that have been obliterated in cosmic collisions over the eons.
Scientists have long suspected that, close to the Sun, this dust would be heated to high temperatures by powerful sunlight, turning it into a gas and creating a dust-free region around the Sun. But no one had ever observed it.
With the help of WISPR, Parker Solar Probe's only imaging instrument, scientists have for the first time seen a thinning of the dust a little over 12 million kilometres (7 million miles) from the Sun.
The dust continues to steadily decrease over the next 4 million miles which is at the current limit of WISPR's measurements.
Parker Solar Probe has its next solar encounter on 29 January, 2020, where it will get nearer to the Sun than ever before. With many new insights already gleaned from the data so far, scientists will no doubt be hoping for many more before the mission ends.
Next year, the probe will have a visitor as it is joined by the European Space Agency’s Solar Orbiter, a mission which aims to study the Sun up close by taking high-resolution images of the Sun's poles for the first time. With a wingspan of 18 metres and 10 instruments on board, Solar Orbiter is due to launch in February 2020 from Cape Canaveral.
