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.
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