December 3, 2023

As we enjoy the long days of summer in the northern hemisphere, few realize that there is still a lot we don’t understand about the sun. For billions of years, our star has provided the kind of constant energy production that has allowed not only the evolution of life as we know it, but also the development of civilization as we know it. However, the exact points of the astrophysics that control our deceptively simple yellow dwarf star remain a mystery.

We know the basics of how the sun’s energy comes from fusing hydrogen into helium and that the sun’s complex chemical makeup largely stems from previous generations of stars that seeded their elemental makeup. But there are major gaps in our understanding of the sun’s internal mechanics and chemical makeup.

To learn more about what we don’t know about the physics of the Sun, I reached out to astronomer and heliophysicist Jason Jakiewicz, at New Mexico State University in Las Cruces, in order to tackle five of the top solar mysteries below.

— The secret of why the Sun’s outer atmosphere (or corona) is hotter than its surface.

Jackiewicz told me that the corona is located above the surface of the Sun, and therefore above the heat source, yet it is 1,000 times higher. The surface of the Sun provides enough energy to flow away from it to keep the corona hot, he says. However, this type of heating requires energy to be deposited there in some way.

But because this outer corona is so weak, Jackiewicz and his colleagues can’t figure out why this energy doesn’t travel to outer space. Heliophysics are still puzzled over how the corona relates to this amount of heat.

—- How and where does the sun generate its magnetic fields?

Magnetic fields are observed at the surface (as sunspots) and in the Sun’s atmosphere (as coronal rings), but they are likely to be generated indoors, Jakevich says. Such processes require plasma (a charged gas) and motion (a possible spin), he says.

Many researchers believe this occurs most strongly at about 70 percent of the sun’s radius, which could be where the fields are “terminated” and strengthened, Jackiewicz says. And since the magnetic fields are floating, they rise to the surface and emerge as sunspots, he says.

But since the Sun is a gaseous body, it does not rotate at the same constant speed, as an Earth-mass object. For example, at the surface, its poles rotate more slowly than at the equator.

The equator rotates once in about 25 days, and the polar regions about 30-32 days, Jakevich says. He points out that this certainly does not happen on Earth, or that the planet will split itself into pieces.

The different layers under the sun also rotate at different speeds.

So, if you go to the Sun from a surface about 50,000 kilometers away, you’ll be in a region that rotates faster than the surface, Jackiewicz says. Then if you go a little deeper than that, it slows down again, he says.

At a depth of about 200 million kilometers, we find the Sun spinning as a solid object, more like the Earth, says Jackiewicz, and we know all this by studying solar earthquakes that probe the interior.

—- What determines and regulates the Sun’s 11-year cycles?

Jackiewicz says that sunspots and magnetism in general dwindle and fade in the sun over an 11-year period.

Sunspots appear as darker areas on the surface of the sun due to the strong magnetic fields that appear through the surface of the sun. This results in a slight cooling of the region causing dark spots to appear when compared to the photosphere (or surface) surrounding the Sun.

At the start of the solar cycle, sunspots tend to be at mid-latitudes, about plus or minus 30 degrees latitude in each hemisphere, Jakevich says. As the cycle continues, it gets closer and closer to the equator, he says. When the next cycle begins, it then reappears in mid-latitudes, even though the polarity has reversed in the northern and southern hemispheres from the previous cycle, Jakevich says.

As for the 11 year cycle?

Every day or so, Jackiewicz says, surface features on the sun come and go. This is a strange timeline.

— What generates the Sun’s super-explosions and supermassive blasts?

It’s connected to magnetic fields, which happen when magnetic energy needs to be released due to the twisting and stretching of the fields, Jackiewicz says. The main difference between flares and coronal mass ejections (CMEs) is that the flares mainly emit X-rays and ultraviolet radiation, but in fact coronal mass ejections raise mass from the sun, he says.

Major solar events cause billions in damage each year, due to power outages, communication failures, and damage to electrical systems, Jackiewicz says. If we go back to the human-centered space exploration program, with humans on the Moon or Mars, the consequences of such space weather would become even more significant, he says.

—- The secret of the chemical composition of the sun

In the beginning, there was only hydrogen and helium, Jakowitz says, with all the other elements on the periodic table made in the cores of stars. Our sun formed when the universe was about two-thirds old or its current age, and therefore is much richer in these other elements than those early stars, he says.

“The Sun is the reference star for all the hundreds of billions of other stars in our galaxy, and for the trillions and trillions of stars in other galaxies,” Jackiewicz said.

Jackiewicz says: We know all the elements that make up the Sun, but we don’t know their relative abundance. Therefore, the chemical composition of the sun is still a matter of debate, he says. It’s hard to measure, says Jackiewicz, even for our closest star. Observations and models have to work together to produce consistent results, and they don’t always do that, he says.

What puzzles you the most about the sun?

Jackiewicz says it’s fair to say we understand the Sun’s mass, age, size, and total irradiance very well. We know how it evolved, and how it will evolve billions of years into the future, in a general way, he says. But the higher-order things are very puzzling; Its deep internal structure, magnetic fields, solar cycle differences, and volcanic events, says Jackiewicz.

As for his personal quest to understand the sun?

I want to know what the interior looks like, says Jackiewicz. Like an ultrasound that shows a fetus inside the mother, we try to take pictures under the sun’s surface, but we can only explore a little of the way, he says.

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