The greatest mystery of the Sun is an odd one, but this centuries-old enigma may now have an answer. If you build a campfire, the greatest amount of heat will be felt when you get close to the flame. As you walk away from the fire, the amount of heat reaching you is reduced. However, the atmosphere of the Sun, known as the corona, is significantly hotter than the surface of our local star. The reason behind this paradox is still unknown, but a new theory suggests an answer. Testing of this new idea may be confirmed, or refuted, by a spacecraft set to come closer to the Sun than any other observatory in history.
The Parker Space Probe launched on August 12, 2018, on a mission to explore the Sun at distances closer than ever before attempted by any previous spacecraft. At its closest approach, Parker will come within 6.4 million kilometers (four million miles) of the surface of the Sun. In doing so, Parker will be able to explore, firsthand, how the corona can become superheated, forming filaments of plasma, which can affect our technological civilization here on Earth.
“Whatever the physics is behind this superheating, it’s a puzzle that has been staring us in the eye for 500 years. In just two more years Parker Solar Probe will finally reveal the answer,” said Justin Kasper of the University of Michigan (U-M) and principal investigator for the Parker mission.
You Know this Space is Getting Hot…
Within the atmosphere surrounding the Sun, known as the corona, heating operates in a manner unlike that seen elsewhere. Astronomers, however, are uncertain how far from the Sun superheating of the corona ends. In 2021, Parker is set to cross into the solar atmosphere, revealing exactly where this region of superheating lies, relative to our parent star. Once there, Parker will analyze particles and magnetic fields, attempting to unravel one of the greatest mysteries of our parent star.
Kasper and his team at U-M believe magnetic fields within the corona bounce around the Sun, “stirring” the corona, driving the superheating measured by astronomers. The corona of the Sun extends out to many times the diameter of our local star.
Adding to this conundrum of the solar corona is the fact that certain elements within the structure are heated to different degrees than others. Some heavier elements can be heated until they are hotter than the core of the Sun.
No, Alfvén is NOT a chipmunk!
Within the corona, hydromagnetic Alfvén waves bounce back-and-forth between the edge of the solar atmosphere and the surface of the Sun. Outside this region, at a distance known as the Alfvén point, the solar wind travels faster than Alfvén waves, and these oscillations can no longer return to the solar surface. This line is not a clear boundary, as there appears to be a more gradual transition than originally postulated by astrophysicists.
“What we conclude is that there isn’t a clean Alfvén surface. There’s a wide ‘no-man’s land’ or ‘Alfvén zone’ where the solar wind gradually disconnects from the Sun, rather than a single clear boundary,” Craig DeForest, solar physicist at the Southwest Research Institute, stated.
Using data from the Wind spacecraft, researchers were able to generate estimates of the extent of the superheating zone, which they calculated to lie between 10 and 50 times the diameter of the Sun. A more precise figure is not possible, since some variables in the calculation had to be estimated.
The Alfvén point is known to expand and contract with solar activity, and this region seems to mark the limits of superheating within the corona. Why this happens remains a mystery, but the Parker Solar Probe may provide some insights when it passes the boundary, and begins its study of that region of the solar atmosphere.
One Ring to Rule them All…
“In the midst of all dwells the Sun. For who could set this luminary in another or better place in this most glorious temple, than whence he can at one and the same time brighten the whole.”
― Nicolaus Copernicus
The corona of the Sun can be seen during solar eclipses as a ring around our parent star. This ring glows with energy from superheating, allowing the atmosphere of the Sun to be seen by amateur skygazers here on Earth.
Superheating within the solar corona can send charged particles racing toward the Earth, potentially disrupting radio communications from, and between, satellites. Understanding the nature of the corona could help solar scientists better predict how these solar phenomenon could affect life on our own planet.