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Sushant Mahajan presents about uncovering the enigmatic and awe-inspiring secrets of the sun through the lens of solar astrophysics. Part of the “Discover Our Universe” public lecture series at KIPAC, this event was filmed on Halloween 2023.

Total solar eclipses happen when the moon passes directly in front of the sun, plunging narrow bands of the Earth into darkness as the moon’s shadow travels along the “path of totality” on Earth. The next one in the United States will happen on April 8 and will be most visible from parts of the Northeast, the Midwest, and Texas. (After that, the next total solar eclipse visible from the U.S. will be on Aug. 23, 2044.)

Solar astrophysicist Sushant Mahajan will travel to Dallas, Texas, next week for an annual meeting of solar astronomers scheduled to coincide with the April 8th eclipse. “On that day of the meeting, there is no agenda except looking at the eclipse,” said Mahajan.

Mahajan is a postdoctoral fellow in physics in the School of Humanities and Sciences. He is member of the Solar Group at the Hansen Experimental Physics Laboratory (HEPL) and a member of the Kavli Institute for Particle Astrophysics and Cosmology. He explains the science behind total solar eclipses and some lesser-known solar phenomena.


1. What’s something that most people don’t know about solar eclipses?

Sushant Mahajan (Image credit: Lori White)

People are usually unaware of how rare our place in the universe is for us to be able to see total solar eclipses. If you look at the rocky planets in our solar system, which are the first four (Mercury, Venus, Earth, and Mars), only Earth has a sizable moon. Mercury and Venus don’t have moons at all, and Mars has these two tiny rocks orbiting it, but they’re not big enough to cover the sun fully.

Astronomers measure the size of objects in the sky with angles because that’s how big they appear to someone on Earth. The sun being the size it is, at the distance that it is at, makes it span about half a degree in the sky. And the moon being 400 times smaller than the sun but also about 400 times closer to the Earth means that it appears to be about the same size. And that creates this interesting phenomenon of total eclipse, where the moon blocks the sun’s light. It’s the only time that you can see, with your naked eyes, features that are in the sun’s atmosphere.

It’s one of the most fortunate coincidences that we have. And it’s not going to last forever, because the moon is drifting away from Earth at a rate of 3.8 centimeters every year. In about 316 million years, the moon will always be smaller than the sun. So we will stop having total solar eclipses.

Total solar eclipses are rare already. The next one after April 8 that will be visible from the U.S. will be on August 23, 2044. So, better catch it when you can.


2. There’s always a lot of excitement over total solar eclipses. What solar phenomenon doesn’t get the excitement it deserves?

There are a few solar activities that lead to interesting phenomena on Earth, including solar flares and Coronal Mass Ejections (CMEs). These are dynamic magnetic phenomena triggered by the sun’s magnetic field in and around sunspots. A sunspot on the sun is essentially a region of a very high magnetic field of the strength of MRI machines (2 Tesla) spread over an area twice the size of the Earth. Since these spots are located on a giant bubbling hot ball of gas, the magnetic field in sunspots can become unstable and erupt into a sudden burst of x-ray radiation, creating a solar flare. Sometimes, along with a flare, the sun might burp out some hot magnetized plasma – that is a CME.

If a CME is traveling toward Earth, it brings along a lot of charged particles that get trapped in Earth’s magnetic field, which then redirects the flow of those charged particles toward the North and South poles. There, the particles collide with oxygen and nitrogen in the Earth’s atmosphere, exciting the molecules. And, when the oxygen and nitrogen molecules try to relax back down to their ground states, they emit that excess energy in green and red light, which we call the northern or southern lights (aurorae).

Another event also usually associated with solar flares is a Solar Energetic Particle (SEP) event, which sends a very intense burst of high-speed charged particles out from the sun.


3. How do variations in the sun’s behavior affect our lives on Earth?

The high-energy particles radiated by solar flares or SEP events are harmful to electronics in outer space, as well as to astronauts who could be out on a spacewalk. So, one of the goals of our community is to be able to predict these eruptive events at least 24 hours in advance.

A CME can create geomagnetic storms on Earth. If a flight flies close to the North or South Pole during a geomagnetic storm, the plane could have a radio frequency blackout. And so agencies like the U.S. Air Force and airlines around the world use these predictions, especially for long-distance flights.

The strongest geomagnetic storm on record happened in 1859 when astronomer Richard Carrington observed a sudden brightening on the sun. Eighteen hours later, there were these very strong, bright northern lights almost everywhere in the world. Miners in Colorado woke up at 3 a.m. and started preparing breakfast because they thought it was daytime. Aurorae were seen from Cuba as well. Several telegraph operators experienced electric shocks.

If that strong of an event happened today, it’s estimated it would cost the U.S. economy about $10 trillion. Large parts of the U.S. could be out of electricity for four to six months. So in 2016, President Obama signed an executive order called the National Space Weather Strategy and Action Plan, which put into motion communication between power distributors and scientists to try to separate parts of the power grid and make it less susceptible to events like this.


4. What are some unanswered questions in solar research?

One question that I research is how magnetic fields inside the sun are generated, and how they come up to the surface of the sun in the form of sunspots. There are theories about this, and most are similar to how the Earth’s magnetic field is explained. Complicated flows caused by convection and varying rotation in the sun likely twist and amplify the magnetic field. But because we cannot directly see inside the sun, we don’t yet have conclusive proof yet.

With the invention of a field called helioseismology in the 1980s, we are working toward understanding the internal structure of the sun better and better. Helioseismology essentially analyzes how sound waves propagate inside the sun to understand the sun’s internal structure. It’s the same way that we know the internal structure of the Earth by studying how sound waves produced by earthquakes propagate from one location to the other. Last year, NASA funded a Drive Science Center called Consequences of Flows and Fields in the Interior and Exterior of the Sun (COFFIES), which is a large collaboration of scientists who are trying to understand the dynamics of magnetic fields in the sun, and I am excited that I’m a part of it.