Welcome back to Launch Pad, I’m
Christian Ready, your friendly neighborhood astronomer. In our
last video on ESA’s Solar Orbiter, we talked briefly about how it
compares to the Parker Solar Probe. But come to think of it,
it’s been a while since we last checked in on Parker.
It turns out it’s been busy! For a quick recap, Parker was launched
in August 2018 to investigate the Sun’s outer atmosphere or corona. There’s two
really weird things about the corona. First, it’s millions of degrees
hotter than the Sun’s surface, despite being well above the Sun’s
surface. Even though its temperature was determined in 1940, no explanation
for the coronal heating problem has ever been tested experimentally.
Another problem is that the solar wind is accelerated in the corona
to millions of kilometers per hour. But, it’s not clear how the
wind is accelerated, either. In order to understand both of these
phenomena, you need to trace the flow of energy and make measurements in the
region where all the action happens. But that requires being there. In the Sun’s
corona. Which is really, really hot. That’s why Parker is protected by a
heat shield made of carbon composite, along with solar arrays
that are cooled by water, and retract as Parker
swings about the Sun. During its closest approach,
Parker will fly to within 9 solar radii of the Sun’s
“surface”. That’s about 3.83 million miles (6.16 million km)
or just 0.05 astronomical units. At that distance, the heat shield
will reach temperatures near 2500 °F (1400 °C), but the spacecraft’s
payload will be near room temperature, at about 85 °F (29 °C). However, it’s going to be a while
before Parker makes its closest approach because it has to get rid of some of the
orbital energy it inherited from Earth. So, Parker was launched into an
elliptical orbit around the Sun. And, its launch was timed in such a way as
to make the occasional flyby of Venus. During each of these flybys, some of
Parker's orbital energy is transferred to Venus, and the spacecraft
descends closer with each orbit. As it approaches the Sun,
Parker samples the solar wind, while spacecraft like STEREO A and B, SDO, and SOHO see the overall
environment Parker flies through. This allows the science team to put
Parker’s observations in their proper context, and understand what the Sun was
doing when the measurements were made. Now, even though it won’t make its
first really close approach until 2025, Parker's already come much closer than
any spacecraft in history. And, Parker’s already made some really interesting
discoveries about the Sun! So, here now, are 5 Really Cool Things Parker
Learned About the Sun, plus something else it learned that’s really cool
that’s not about the Sun at the end. #1 Evidence for a Dust-Free Zone The Sun is surrounded by a thin disk of
dust spread throughout the inner solar system. Some of it comes from comets,
while some are the remains of collisions that formed the planets, asteroids, and
comets billions of years ago. This dust scatters sunlight. If you can get to
a dark enough site on a clear night, you’ll see the faint glow of this dust
in a phenomenon called Zodiacal light. It’s long been thought
that close to the Sun, the radiation there should
be high enough to either vaporize the dust particles entirely, or
push them away with radiation pressure. But from our vantage point on Earth, we
cannot discern the disk’s interior edge. But as it made its way around the
Sun during its first three orbits, It noticed a thinning out of the
dust starting at about 7 million miles (11 million km) from the Sun.
This decrease in dust continued steadily to the limits of WISPR's
measurements, which at the time could see to a little over 4 million
miles (6 million km) from the Sun. At the current rate of thinning, it’s
possible the truly dust-free zone may start around 2-3 million
miles (3-4.6 million km) from the Sun's surface. That means
Parker might see the dust-free zone as early as September 2020,
when it makes its sixth flyby of the Sun and closes to within 11.5
million miles (18.7 million km). But if that weren’t cool enough, Parker
also “heard” the dust striking the spacecraft as it passed through it at
250,000 miles per hour (402,000 kph)! At those speeds, the spacecraft
doesn't just crash into these particles — it obliterates them
into a plasma that Parker's FIELDS instrument can - for
lack of a better word - “hear.” [static] Pretty cool, huh? However, each collision chips away a
tiny bit of the spacecraft. And this was something the Parker team
expected to happen. But until now, they could only guess at the rate of
these collisions using models based on remote observations. It turns out
the dust is denser than expected, but not enough to pose a concern
for the mission. Still, it’s just one of those things you
can’t tell for sure until you’re there. Not only did Parker hear the dust
impact the spacecraft, but... #2 Parker hears the
turbulence of the solar wind The solar wind is the stream of charged
particles - or plasma - blowing from the corona in all directions. But
weirdly, the solar wind actually speeds up as it leaves the Sun.
Not only that, but it stays hot, even as it travels away from the Sun! By the time the solar
wind reaches Earth, it mostly flows at a fairly steady
rate. Any trace of the mechanisms that heated and accelerated
the wind has been smoothed out. But closer to the Sun, Parker’s
FIELDS instrument revealed a much more chaotic and turbulent system
even when the Sun is “quiet”. FIELDS detected thousands
of waves of plasma rippling through the solar
wind. Such waves would be driven by fluctuations in the
electric and magnetic fields. It’s kinda like how fluctuations in
air pressure on Earth create the winds that drive rolling waves on the ocean. In the solar wind, particles can ride
these plasma waves and are propelled to higher speeds. But as they surf those
waves, the particles interact with each other, creating fluctuations in the
frequency and amplitude of the waves. Parker recorded these waves as they
passed by, and they sound really cool. These are whistler-mode waves. They're
caused by energetic electrons bursting out of the Sun’s corona. These electrons
follow magnetic field lines that stretch from the Sun out to the solar system’s
farthest regions. But as they do so, the electrons spiral around the
lines. When a plasma wave’s frequency matches the frequency of the spinning
electrons, they amplify each other. It’s thought that part of
the solar wind’s acceleration may be due to these escaping
electrons. It’s also possible they may play a role in
heating the solar wind. As the plasma waves move
through the solar wind, they quickly shift from one
frequency to another, creating other waves called dispersive
waves that FIELDS detects. [windy, chirping sounds] These dispersive waves are
rarely detected near Earth, so they weren’t thought to be a
significant driver of the solar wind. But near the Sun, dispersive
waves are everywhere. It’s yet not clear what causes
the changes in frequency that creates these waves, or how they may
heat the solar wind. But this is a really cool finding that’s going to
be followed up on with future orbits. And speaking of funky electromagnetic
phenomena, Parker also discovered… #3 Magnetic Switchbacks During its first couple of orbits, Parker’s FIELDS instruments detected
sudden reversals in the magnetic fields. At first, it was thought the
spacecraft might have been passing across a series of magnetic field
lines with alternating polarities. But the Solar Wind Electrons Alphas and
Protons - or SWEAP experiment - made measurements of the particles flowing
along the magnetic field lines. SWEAP showed that the outflowing
wind particles were in fact reversing their direction, and then reversing
again to their original outflow with twice the kinetic energy
of the background wind! The science team dubbed these
reversals “switchbacks”. Nothing like them have been detected
before. At Earth’s distance, only the occasional wiggle of some magnetic
field lines have ever been detected. But these are full 180 ° reversals that
pack a ton of energy. It’s like trying to do the same thing with a bungee
cord that’s already pulled taut. The more tightly drawn it is, the
more energy is required to reverse it. These switchback reversals
last anywhere from a few seconds to several minutes. When field
reverses, it’s like cracking a whip, and the particles pack twice
their original kinetic energy. Parker measured clusters of
switchbacks during its first two flybys. But as it flew closer
to the Sun on subsequent orbits, it measured an increase in both the
number and energy of the switchbacks. The exact source of the switchbacks
isn't yet understood, but with each new set of measurements, scientists
can narrow down the possibilities. It’s thought that as Parker gets
closer to the Sun, the switchbacks should become more common and stronger.
If that turns out to be the case, they may turn out to be one of the energy
sources that’s heating the corona! Parker doesn’t have a camera that faces
the Sun, because the radiation it faces would fry any camera we could put on it.
But ESA's Solar Orbiter may be able to image new features on the Sun that
can be linked to these switchbacks. Solar Orbiter already spotted small
bursts of energy dubbed “campfires”. As Solar Orbiter gets closer to the Sun,
it’ll determine whether or not these campfires are the long-theorized “nano
flares” and if so, are they providing the energy needed to generate the
switchbacks that accelerate the wind? But the atmosphere and solar wind
behave differently than we thought in a very significant
way, in particular... #4 The atmosphere and solar wind rotate
farther from the Sun than we thought. The solar wind emerges from the
corona. Near Earth, the wind streams more or less radially from the
Sun, going out in all directions. But the Sun rotates, carrying the corona
along with it. That means the solar wind is initially traveling in curved path
before switching to a straight one. It’s a little bit like riding
a merry-go-round. The farther you are from the center,
the faster you’re moving. If you jump off, you would then be
moving in a straight line outward. Somewhere between the Sun and
Earth, the solar wind does the same thing and transitions from
a rotational to a radial flow. Exactly where this happens
has implications for how the Sun - and how stars in
general - slow down over time. Parker’s SWEAP instrument measured
this rotational flow for the first time when it was still 20 million miles
(32 million km) from the Sun. That’s considerably father
from the Sun than predicted. Not only that, but as Parker
approached perihelion, the speed of the rotation increased to more
than 10 times faster than predicted! Not only does this tell us something new
about our star, but it has implications for understanding the lifecycles
of stars in general, and the formation - and even habitability
- of their planetary systems. You see, where and how the wind
transitions affects how rapidly the star slows its rotation. The farther
away the wind transitions, the more rotational energy it carries away and
the more quickly the star slows down. In general, the slower a star rotates,
the less magnetically active it is, and the more habitable its planets
are. Perhaps one of the reasons we’re here in the first place is because the
Sun slowed its rotation quickly enough to give life a chance to evolve into
more complex, sophisticated creatures. Of course, that doesn’t mean the
Sun is completely without activity. Even during the quiet
times, Parker observed... #5 Small flares and space weather. We know that electrons and ions in
the solar wind are accelerated by explosive solar activity. Exactly how
this happens is not yet well understood, but under the right conditions, it can
create storms of energetic particles. Major events on the Sun, such as
flares and coronal mass ejections, can send these particles racing out
into the solar system at nearly the speed of light. That means they can
reach Earth in under a half an hour. These particles carry a lot of
energy, enough to damage spacecraft electronics and endanger astronauts.
When astronauts eventually do return to the Moon and head out for Mars,
they won’t have the protection of Earth’s magnetic field and won’t
have much advance warning, either. However, Parker’s Integrated
Science Investigation of the Sun - or ISIS - there, I said
it, I'm taking it back - detected several high-energy particle events
that have never been seen before. These events proceeded a
small coronal mass ejection that unleashed a burst of material
with as much mass as Lake Michigan. ISIS detected high-energy particles
rushing ahead of the ejected mass. If we can learn more about the nature of
these phenomena, it may be possible to use them as part of an early-warning
system for future explosions. Meanwhile, the CME ejecta formed
structures in the corona and solar wind that were captured by the Wide-field
Imager for Solar PRobe, or WISPR. High-energy particles slammed into
WISPR’s cameras, creating these brief flashes of radiation “snow" as they
bombard the detectors. Previously, radiation “snow” had only been
detected by spacecraft like the Solar Heliospheric Observatory - or SOHO
- during major solar eruption events. But now WISPR is detecting
them on a much smaller scale inside the corona, while ISIS is
detecting the kinds of high-energy particle events that
may be causing them. All of these observations demonstrate
just how active the Sun is, even during so-called “quiet” times.
But Parker, and its sideways-facing WISPR cameras in particular, are
in a position to see some really cool things in our solar system that
aren’t the Sun. Which brings me to #6 Cool Things Parker detected in
our solar system that aren’t the Sun. During Parker’s first solar encounter in
November 2018, its WISPIR camera picked up a really faint structure that’s never
been seen before: a 60,000 mile- (97-161 million km-) wide dust trail in the
orbit of the asteroid Phaethon. Phaethon is one of the closest Sun-approaching
asteroids in our solar system. Its orbit is highly elliptical, and reaches
all the way out past Mars' orbit. A couple of thousand years ago,
Phaethon approached the Sun and something happened to it. We’re
not sure what happened to it, but it released a long
debris trail into its orbit. Every December, Earth passes
through this debris field, and we see them as the
Geminid meteor shower. Phaethon comes about as close
to the Sun as Parker does. Every time it does so, it heats
up and bits of the crust break off to form a dust trail. It’s like
Phaethon is a kind of “rock comet”. So, was this particular trail of
dust created in the same event that created the Geminid meteor shower thousands of years ago, or
was it created more recently? To find out, scientists estimated the
mass of the dust in the Parker images. They found that the trail weighs about
a million tons (1 billion kg). However, Phaethon is currently losing
mass at a rate that is way too low to create the amount
of dust seen in this trail. However, the amount of dust measured
is comparable to the estimated amount of dust that enters our atmosphere
every December during the Geminids. And that’s pretty solid evidence that
Parker is looking at dust created in the same event that created
the Geminids. In other words, Parker saw what a meteor
shower looks like from space! Only, one that’s caused by an
asteroid instead of a comet. And speaking of comets, Parker got a
really nice look at the newly-discovered comet NEOWISE on July 5th. On the
right (correction: left) side of the image there is sunlight
being scattered by dust. In fact, there’s a little black
structure on the lower left of the image that's actually a grain of dust
resting on the camera lens. The comet’s broad dust tail is easily
visible. But, after some processing to remove the excess brightness from
scattered sunlight, the comet’s straight ion tail pointing directly away
from the away from the Sun can be seen. So yeah, Parker's been busy! And
now it’s been joined by ESA’s Solar Orbiter which orbits a little
further away. But, unlike Parker, it will be just close enough to take
the closest images of the Sun. In fact, ESA just released the first set
of images from Solar Orbiter, and I made a video about them
and the Solar Orbiter mission, so if you haven’t seen it yet, I’ll meet
you over there when we’re done here. My thanks as always to my Patreon
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