With data from its closest pass of the Sun yet, the ESA/NASA Solar Orbiter spacecraft has found compelling clues as to the origin of magnetic switchbacks, and points towards how their physical formation mechanism might help accelerate the solar wind.
Solar Orbiter has made the first ever
remote sensing observation consistent with a magnetic phenomenon called a solar
switchback – sudden and large deflections of the solar wind’s magnetic field.
The new observation provides a full view of the structure, in this case
confirming it has an S-shaped character, as predicted. Furthermore, the global
perspective provided by the Solar Orbiter data indicates that these rapidly
changing magnetic fields can have their origin near the surface of the Sun.
While a number of spacecraft have flown
through these puzzling regions before, in situ data only allow for a
measurement at a single point and time. Consequently, the structure and shape
of the switchback has to be inferred from plasma and magnetic field properties
measured at one point.
When the German-US Helios 1 and 2
spacecraft flew close to the Sun in the mid 1970s, both probes recorded sudden
reversals of the Sun’s magnetic field. These mysterious reversals were always
abrupt and always temporary, lasting from a few seconds to a number of hours
before the magnetic field switched back to its original direction.
These magnetic structures were also
probed at much larger distances from the Sun by the Ulysses spacecraft in the
late 1990s. Instead of a third the Earth’s orbital radius from the Sun, where
the Helios missions made their closest pass, Ulysses operated mostly beyond the
Earth’s orbit.
How a solar switchback
is formed
Their number rose dramatically with the
arrival of NASA’s Parker Solar Probe in 2018. This clearly indicated that the
sudden magnetic field reversals are more numerous close to the Sun, and led to
the suggestion that they were caused by S-shaped kinks in the magnetic field.
This puzzling behaviour earned the phenomenon the name of switchbacks. A number
of ideas were proposed as to how these might form.
On 25 March 2022, Solar Orbiter was just
a day away from a close pass of the Sun – bringing it within the orbit of
planet Mercury – and its Metis instrument was taking data. Metis blocks out the
bright glare of light from the Sun’s surface and takes pictures of the Sun’s
outer atmosphere, known as the corona. The particles in the corona are
electrically charged and follow the Sun’s magnetic field lines out into space. The electrically charged particles themselves are
called a plasma.
At around 20:39 UT, Metis recorded an
image of the solar corona that showed a distorted S-shaped kink in the coronal
plasma. To Daniele Telloni, National Institute for Astrophysics – Astrophysical
Observatory of Torino, Italy, it looked suspiciously like a solar switchback.
Comparing the Metis image, which had
been taken in visible light, with a concurrent image taken by Solar Orbiter’s
Extreme Ultraviolet Imager (EUI) instrument, he saw that the candidate
switchback was taking place above an active region catalogued as AR 12972.
Active regions are associated with sunspots and magnetic activity. Further
analysis of the Metis data showed that the speed of the plasma above this
region was very slow, as would be expected from an active region that has yet
to release its stored energy.
Daniele instantly thought this resembled
a generating mechanism for the switchbacks proposed by Prof. Gary Zank,
University of Alabama in Huntsville, USA. The theory looked at the way
different magnetic regions near the surface of the Sun interact with each
other.
Close to the Sun, and especially above
active regions, there are open and closed magnetic field lines. The closed
lines are loops of magnetism that arch up into the solar atmosphere before
curving round and disappearing back into the Sun. Very little plasma can escape
into space above these field lines and so the speed of the solar wind tends to
be slow here. Open field lines are the reverse, they emanate from the Sun and
connect with the interplanetary magnetic field of the Solar System. They are
magnetic highways along which the plasma can flow freely, and give rise to the
fast solar wind.
Daniele and Gary proved that switchbacks
occur when there is an interaction between a region of open field lines and a
region of closed field lines. As the field lines crowd together, they can
reconnect into more stable configurations. Rather like cracking a whip, this
releases energy and sets an S-shaped disturbance traveling off into space,
which a passing spacecraft would record as a switchback.
According to Gary Zank, who proposed one of the
theories for the origin of switchbacks, “The first image from Metis that
Daniele showed suggested to me almost immediately the cartoons that we had
drawn in developing
the mathematical model for a switchback. Of course, the first image was just a
snapshot and we had to temper our enthusiasm until we had used the excellent
Metis coverage to extract temporal information and do a more detailed spectral
analysis of the images themselves. The results proved to be absolutely
spectacular!”
Together with a team of other
researchers, they built a computer model of the behavior, and found that their
results bore a striking resemblance to the Metis image, especially after they
included calculations for how the structure would elongate during its
propagation outwards through the solar corona.
“I would say that this first image of a magnetic
switchback in the solar corona has revealed the mystery of their origin” says
Daniele, whose results
are published in a paper in
The Astrophysical Journal Letters.
In understanding switchbacks, solar
physicists may also be taking a step toward understanding the details of how
the solar wind is accelerated and heated away from the Sun. This is because
when spacecraft fly through switchbacks, they often register a localised
acceleration of the solar wind.
“The next step is to try to
statistically link switchbacks observed in situ with their source regions on
the Sun,” says Daniele. In other words, to have a spacecraft fly through the
magnetic reversal and be able to see what’s happened on the solar surface. This
is exactly the kind of linkage science that Solar Orbiter was designed to do,
but it does not necessarily mean that Solar Orbiter needs to fly through the
switchback. It could be another spacecraft, such as Parker Solar Probe. As long
as the in-situ data and remote sensing data is concurrent, Daniele can perform
the correlation.
“This is exactly the kind of result we
were hoping for with Solar Orbiter,” says Daniel Müller, ESA Project Scientist
for Solar Orbiter. “With every orbit, we obtain more data from our suite of ten
instruments. Based on results like this one, we will fine-tune the observations
planned for Solar Orbiter’s next solar encounter to understand the way in which
the Sun connects to the wider magnetic environment of the Solar System. This
was Solar Orbiter’s very first close pass to the Sun, so we expect many more
exciting results to come.”
Solar Orbiter’s next close pass of the Sun – again within the orbit of Mercury at a distance of 0.29 times the Earth-Sun distance – will take place on 13 October. Earlier this month, on 4 September, Solar Orbiter made a gravity assist flyby at Venus to adjust its orbit around the Sun; subsequent Venus flybys will start raising the inclination of the spacecraft’s orbit to access higher latitude – more polar – regions of the Sun.
Journal article: https://iopscience.iop.org/article/10.3847/2041-8213/ac8104
Source: Solar
Orbiter solves magnetic switchback mystery – Scents of Science
(myfusimotors.com)
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