- 3381 - SUN - Parker Probing the Sun? For the first time in our history, a spacecraft has touched the Sun. NASA's “Parker Solar Probe” has now (2021) flown through the Sun's upper atmosphere - the corona - and sampled particles and magnetic fields there.
--------------------- 3381 - SUN - Parker Probing the Sun?
- Just as landing on the Moon allowed scientists to understand how it was formed, touching the very stuff the Sun is made of will help scientists uncover critical information about our closest star and its influence on the solar system.
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- As it circles closer to the Sun’s surface, Parker is making new discoveries that other spacecraft were too far away to see, including from within the solar wind, the flow of particles from the Sun that can influence us at Earth.
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- In 2019, Parker discovered that magnetic zig-zag structures in the solar wind, called “switchbacks“, are plentiful close to the Sun. But how and where they form remained a mystery. Halving the distance to the Sun since then, Parker Solar Probe has now passed close enough to identify one place where they originate: the solar surface.
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- Flying so close to the Sun, Parker Solar Probe now senses conditions in the magnetically dominated layer of the solar atmosphere - the corona - that we never could before. We see evidence of being in the corona in magnetic field data, solar wind data, and visually in images. We can actually see the spacecraft flying through coronal structures that can be observed during a total solar eclipse.
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- Parker Solar Probe launched in 2018 to explore the mysteries of the Sun by traveling closer to it than any spacecraft before. Three years after launch and decades after first conception, Parker has finally arrived in 2021.
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- Unlike Earth, the Sun doesn't have a solid surface. But it does have a superheated atmosphere, made of solar material bound to the Sun by gravity and magnetic forces. As rising heat and pressure push that material away from the Sun, it reaches a point where gravity and magnetic fields are too weak to contain it.
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- That point, known as the “Alfvén critical surface“, marks the end of the solar atmosphere and beginning of the “solar wind“. Solar material with the energy to make it across that boundary becomes the solar wind, which drags the magnetic field of the Sun with it as it races across the solar system, to Earth and beyond.
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- Beyond the Alfvén critical surface, the solar wind moves so fast that waves within the wind cannot ever travel fast enough to make it back to the Sun - severing their connection.
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- Researchers were unsure exactly where the Alfvén critical surface lay. Based on remote images of the corona, estimates had put it somewhere between 10 to 20 solar radii from the surface of the Sun - 4.3 to 8.6 million miles.
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- Parker's spiral trajectory brings it slowly closer to the Sun and during the last few passes, the spacecraft was consistently below 20 solar radii (91 percent of Earth's distance from the Sun), putting it in the position to cross the boundary .
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- On April 28, 2021, during its eighth flyby of the Sun, Parker Solar Probe encountered the specific magnetic and particle conditions at 18.8 solar radii ( 8,100,000 miles) above the solar surface that told scientists it had crossed the Alfvén critical surface for the first time and finally entered the solar atmosphere.
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- During the flyby, Parker Solar Probe passed into and out of the corona several times. This is proved what some had predicted - that the Alfvén critical surface isn't shaped like a smooth ball. Rather, it has spikes and valleys that wrinkle the surface. Discovering where these protrusions line up with solar activity coming from the surface can help scientists learn how events on the Sun affect the atmosphere and solar wind.
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- At one point, as Parker Solar Probe dipped to just beneath 15 solar radii ( 6,500,000 miles) from the Sun's surface, it transited a feature in the corona called a pseudo- streamer. “Pseudostreamers” are massive structures that rise above the Sun's surface and can be seen from Earth during solar eclipses.
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- Passing through the pseudostreamer was like flying into the eye of a storm. Inside the pseudostreamer, the conditions quieted, particles slowed, and number of switchbacks dropped. This was a dramatic change from the busy barrage of particles the spacecraft usually encounters in the solar wind.
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- The spacecraft found itself in a region where the magnetic fields were strong enough to dominate the movement of particles there. These conditions were the definitive proof the spacecraft had passed the Alfvén critical surface and entered the solar atmosphere where magnetic fields shape the movement of everything in the region.
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- The first passage through the corona, which lasted only a few hours, is one of many planned for the mission. Parker will continue to spiral closer to the Sun, eventually reaching as close as 8.86 solar radii (3.83 million miles) from the surface. Upcoming flybys, the next of which is happening in January , 2022, will likely bring Parker Solar Probe through the corona again.
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- The size of the corona is also driven by solar activity. As the Sun's 11-year activity cycle - the solar cycle - ramps up, the outer edge of the corona will expand, giving Parker Solar Probe a greater chance of being inside the corona for longer periods of time.
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- Even before the first trips through the corona, some surprising physics was already surfacing. On recent solar encounters, Parker Solar Probe collected data pinpointing the origin of zig-zag-shaped structures in the solar wind, called switchbacks. The data showed one spot that switchbacks originate is at the visible surface of the Sun - the “photosphere“.
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- By the time it reaches Earth, 93 million miles away, the solar wind is an unrelenting headwind of particles and magnetic fields. But as it escapes the Sun, the solar wind is structured and patchy.
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- In the mid-1990s, the NASA-European Space Agency mission Ulysses flew over the Sun's poles and discovered a handful of bizarre S-shaped kinks in the solar wind's magnetic field lines, which detoured charged particles on a zig-zag path as they escaped the Sun. For decades, scientists thought these occasional switchbacks were oddities confined to the Sun's polar regions.
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- In 2019, at 34 solar radii from the Sun, Parker discovered that switchbacks were not rare, but common in the solar wind. This renewed interest in the features and raised new questions: Where were they coming from? Were they forged at the surface of the Sun, or shaped by some process kinking magnetic fields in the solar atmosphere?
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- The new findings confirm one origin point is near the solar surface. The clues came as Parker orbited closer to the Sun on its sixth flyby, less than 25 solar radii out. Data showed switchbacks occur in patches and have a higher percentage of helium - known to come from the photosphere - than other elements.
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- The switchbacks' origins were further narrowed when the scientists found the patches aligned with magnetic funnels that emerge from the photosphere between convection cell structures called supergranules.
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- In addition to being the birthplace of switchbacks, the scientists think the magnetic funnels might be where one component of the solar wind originates. The solar wind comes in two different varieties - fast and slow - and the funnels could be where some particles in the fast solar wind come from.
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- The structure of the regions with switchbacks matches up with a small magnetic funnel structure at the base of the corona. Understanding where and how the components of the fast solar wind emerge, and if they're linked to switchbacks, could help scientists answer a longstanding solar mystery: how the corona is heated to millions of degrees, far hotter than the solar surface below.
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- While the new findings locate where switchbacks are made, the scientists can't yet confirm how they're formed. One theory suggests they might be created by waves of plasma that roll through the region like ocean surf. Another contends they're made by an explosive process known as magnetic reconnection, which is thought to occur at the boundaries where the magnetic funnels come together.
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- Now that researchers know what to look for, Parker's closer passes may reveal even more clues about switchbacks and other solar phenomena. The data to come will allow scientists a glimpse into a region that's critical for superheating the corona and pushing the solar wind to supersonic speeds. Such measurements from the corona will be critical for understanding and forecasting extreme space weather events that can disrupt telecommunications and damage satellites around Earth.
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- Astronomers have a new tool to help them understand giant stars. It’s a detailed study of the precise temperatures and sizes of 191 giant stars. It’ll also shed some light on what the Sun will go through late in its life.
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- This study began in 1997 when a group of astronomers started making high-precision measurements of giant stars with the Palomar Observatory’s Testbed Interferometer (PTI). It was built as a testbed for the upcoming Keck Interferometer in Hawaii.
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- The PTI closed down in 2008 and astronomers kept collecting data on giant planets until it closed. After that, astronomers used telescopes at the Lowell Observatory to keep collecting data, and amateur astronomers chipped in, too.
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- Giant stars are different from main-sequence stars or dwarf stars. All of the hydrogen available for fusion in their cores is depleted and they’ve left the main sequence. Compared to a main-sequence star or dwarf star with the same temperature, a giant star will be more luminous and have a larger radius.
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- Giant stars can be between tens and thousands of times more luminous than the Sun and have radii a few hundred times greater than the Sun’s. Stars more luminous than giant stars are called supergiants and hypergiants.
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- A detailed survey of giant stars in a 2003 study surveyed 85 giant stars with the Mark III Stellar Interferometer at the Mount Wilson Observatory. But this new one is noteworthy not only for the number of stars measured but also for its high precision.
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- The PTI was highly-efficient and partly robotic. This allowed it to collect large amounts of stellar fringe visibility data on any given night. It also gathered data in between other scheduled observing tasks. Every other study is only half this size, in terms of the number of stars.
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- The temperature measurements are particularly precise and are two to four times more accurate than previous studies. This study is valuable for a number of reasons, including in exoplanet studies. When astronomers find an exoplanet, nearly everything they can learn about it is in relation to the star it orbits.
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- The mass and luminosity and size of the star are used to infer the characteristics of the planet, like its mass, size, and density. So the more accurate star measurements are, the more accurate planet measurements are.
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- Astronomers know that eventually the Sun will become a red giant and will swell in size, engulfing Mercury and Venus, maybe Earth, too. But there’s a lot we don’t know about that process, and about giant stars in general.
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- The amount of swelling is unclear, with estimates ranging from 10 to 100 times its current size. The data in this study will help astronomers understand what will happen to the Sun when it swells, and will also explain some of our star’s current processes.
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December 17, 2021 - SUN - Parker Probing the Sun? 3381
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