Closest-ever approach to the Sun gives new insights into the solar wind

'Switchbacks' of faster solar wind emerge from coronal holes near the Sun's equator. Credit: Ronan Laker/GONG/NASA/HelioPy/PFSSPy

The Parker Solar Probe spacecraft, which has flown closer to the Sun than any mission before, has found new evidence of the origins of the solar wind.

NASA’s Parker Solar Probe was launched in August 2018. Its first results are published today in a series of four papers in Nature, with Imperial College London scientists among those interpreting some of the key data to reveal how the solar wind is accelerated away from the surface of the Sun.

We could see what might be ‘spikes’ of faster solar wind, and now we have been able to confirm their existence in striking detail with Parker Solar Probe Professor Tim Horbury

The solar wind is a stream of charged particles released by the Sun that fills our Solar System. It is responsible for the North and Southern lights, but can also cause disruption during violent episodes like solar flares and coronal mass ejections, knocking out power grids and satellites.

Now, an international team have shown that bursty ‘spikes’ of solar wind originate in holes in the Sun’s outer atmosphere near its equator, and are accelerated by magnetic phenomena as they flow away into deep space and past the Earth.

The new research suggests that the spikes are generated by ‘magnetic reconnection’ near the Sun, a process that pulls on the tense lines of the Sun’s magnetic field creates folds or ‘switchbacks’. These events last only a couple of minutes but release lots of energy, accelerating the solar wind away in long tubes that are approximately the diameter of the Earth.

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Parker Solar Probe flew through several ‘switchbacks’ – tubes of fast solar wind emerging from coronal holes in the Sun’s upper atmosphere. Credit: NASA

Fast, energetic wind

The finding builds on data from the HELIOS missions, launched in the 1970s, the previous record-holders for the closest approach to the Sun.

Professor Tim Horbury from Imperial’s Department of Physics is a co-investigator on Parker Solar Probe’s FIELDS instrument, which is led by the University of California, Berkeley. He said: “From HELIOS data we could see what might be ‘spikes’ of faster solar wind, and now we have been able to confirm their existence in striking detail with Parker Solar Probe.

“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.”

Closest approach

Parker Solar Probe is studying the Sun’s outer atmosphere, called the corona, directly flying through it to better understand the origins of the solar wind.

For the new study, Parker Solar Probe took data at a distance of 24 million kilometres from the Sun, inside the orbit of Mercury. It will fly successively closer to the Sun in the coming years, eventually reaching a distance of less than six million kilometres from its surface and far closer than the Earth’s average distance of 150 million kilometres.

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A coronal hole (black region) at a slanted angle nearly centred on the face of the Sun. Coronal holes are magnetically open areas from which solar wind particles speed into space. Credit: NASA

Scientists know the properties of the solar wind change as it travels from the Sun to the Earth, so studying the solar wind closer to its origin should reveal more about how it is created and evolves.

Pairing with Solar Orbiter

Parker Solar Probe will also be joined next year by Solar Orbiter, a European Space Agency mission with Imperial kit onboard.

Professor Horbury added: “Although Parker Solar Probe will get even more accurate measurements of the young solar wind at its closest approach, it’s too close for telescopes, so it won’t be able to see what features on the surface of the Sun may be creating the structures of the solar wind.

“This is where Solar Orbiter comes in. It will not go as close to the Sun, but will have sophisticated telescopes and instruments on board that will be able to see from a distance what might be causing phenomena Parker Solar probe is detecting up close, forming a fuller picture of what creates and accelerates the solar wind.”

Other results from the first data include measurement of the speed the solar wind, which does not flow radially away from the Sun, but has a sideways speed of 15-25 times faster than predicted; and a ‘snowplow’ effect where charged particles bunch up before being accelerated by a coronal mass ejection event.

Undergraduate contributions

Two undergraduates in the Department of Physics were involved in studying the data from Parker Solar Probe, and are named as authors on one of the papers, as well as now continuing their work as PhD students.

It has been exciting to see how something we contributed to as part of our degree has made it into one of the most prestigious science journals. Ronan Laker

Ronan Laker helped map which magnetic field lines from the Sun Parker Solar Probe detected, helping lead to the idea that the solar wind they saw was coming from a small coronal hole.

He said: "It has been exciting to see how something we contributed to as part of our degree has made it into one of the most prestigious science journals.

“These first results are really exciting, as, whilst there is evidence of these spikes in the magnetic field, their origin and nature is still open for discussion. We hope to contribute to this future area of research through our respective PhDs at Imperial.”

Thomas Woolley investigated the durations, deflections, occurrence rates and periodicity of the spikes of fast solar wind, looking to answer for example whether one spike occurring meant another one was likely to follow shortly after.

He said: "We are both very happy to have been given the opportunity to work on Parker Solar Probe during our MSci project. At the start, we didn't know where the project would lead, which can often be the case with new space missions. We are however pleased with how the project progressed and glad that we were able to contribute to the wider scientific community."

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'Highly structured slow solar wind emerging from an equatorial coronal hole' by S. D. Bale et al. and 'Alfvénic velocity spikes and rotational flows in the near-Sun solar wind' by J. C. Kasper et al are published in Nature.

See the press release of this article