Sky & Telescope - November 2020 - 26

Solar Close-up

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Perihelion

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0.2 a.u.
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Higher-energy protons detected per second (1 million - 1.8 million eV)
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Lower-energy protons detected per second (30,000 - 200,000 eV)

p SPED-UP PARTICLES IS IS's energetic particle instruments monitored particle energies and densities throughout the first pass around
the Sun, with few gaps between October 2018 and January 2019. Most
of the energetic particles are protons, at both low (30,000-200,000 eV)
and high (1 million-1.8 million eV) energies. Both the color and length of
the bars indicate how many particles per second the instruments were
detecting in their respective energy ranges.

NOVEMBER 2 02 0 * SK Y & TELESCOPE

Instead, Bale suggests, the switchbacks could be heralds
of Alfvén waves deep in the corona. Alfvén waves are a simple
feature of just about any magnetized plasma. As charged
particles move around, so do the magnetic ﬁelds tied to them,
wiggling like so many plucked guitar strings. "We're just
seeing Alfvén waves that have grown to be so big that they're
ﬂipping over on themselves," Bale speculates. But it's not
the only idea out there, he adds: "Reasonable people would
disagree with me."
Whatever switchbacks are, they're giving us information
about what heats the solar corona, whether that mechanism
involves magnetic reconnection, plain ol' Alfvén waves, or
something else entirely. Thinking over the possible scenarios is
half the fun. "The whole point of getting closer is that we'll see
the [switchbacks] in more of their original state," Case says.

Harbingers of Storms
Not all of what comes from the Sun is solar wind. Some tiny
fraction of charged particles in the corona somehow accelerate to near-light speed, following different paths than their
brethren. While the solar wind typically streams at 400 km/s
(almost 1 million mph), solar energetic particles can carry
anywhere from 10 to 100,000 times that energy.
When the Sun is active, these particles can serve as the
Paul Reveres of solar storms. But even during quiet times, as
now, these particles - though few in number - are constantly ﬂying out from the Sun.
Rather than blowing outward in bulk, the way most of
the solar wind does, these charged particles are more like
individuals, spiraling around the magnetic ﬁeld lines that
coil outward from the Sun. Because of their different paths,
"they're actually sampling quite different regions than the
[solar wind] plasma that you're measuring at the same time,"
explains David McComas (Princeton), principal investigator of the Integrated Science Investigation of the Sun (IS‫ݪ‬IS)
instruments. So connecting the particles to the processes that
created them can be tricky.
IS‫ݪ‬IS has two instruments that together detect energetic
particles across a wide range of energies, from thousands
to millions of electron volts. To maximize the number of
particles it can capture, IS‫ݪ‬IS sits right at the edge of the heat
shield. "It's completely out of the view of the Sun, but just by
a degree or two," McComas says.
From this vantage point, IS‫ݪ‬IS has access to details impossible to tease apart near Earth. So McComas can ﬁnally
start answering one of the many questions he has had since
the beginning of his career: "Why is it that some particular
proton ends up being the million-electron-volt particle and
almost none of the rest of them do?"
The answer, he says, has to do with the very ﬁrst processes,
the ones that accelerate particles a little bit, so that they can
then efﬁciently reach much higher energies later on. For the
ﬁrst time, IS‫ݪ‬IS can detect these tiny accelerating events,
and it's showing that they may be much more common than
previously thought.

G REGG DINDER M A N / S&T; SOURCE: MCCO M AS
E T A L. / NATURE 2019 DECEMBER 12

being carried out by the wind," Bale explains. In other words,
deep down in the corona, still beyond Parker's reach, something happens.
By the time it reaches Parker, the event appears as a huge
S-shape twist in the magnetic ﬁeld extending outward from
the Sun that's about 50 times longer than it is across. The
particles within this magnetic switchback are ﬂowing about
twice as fast as the particles outside.
Scientists had seen signatures of these switchbacks in
data collected by the Helios and Ulysses missions. But Parker
revealed that the events were both more transient and more
prevalent than previously thought - basically, whatever's
producing these events nearer the solar surface, it's happening everywhere, all the time. "The sheer number of them and
the size of them is surprising," says Case.
Some have suggested that the rogue waves come from
magnetic reconnection at or near the solar surface. When the
magnetic ﬁeld reorganizes - an open ﬁeld line jumping from
here to there - the process lets loose a burst of particles that
then, much later, zooms past Parker.
"But my own feeling is that they are not direct evidence
of reconnection," Bale says, "because it looks to us like the
plasma inside the switchbacks and the plasma outside the
switchbacks are basically the same." Reconnection, on the
other hand, would be heating particles in addition to accelerating them.

Sky & Telescope - November 2020

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