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What Solar Activity Is on the Sun?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

Nuclear fusion is an amazing process that occurs in the Sun. Through particle physics, we take hydrogen and combine it to make heavier and heavier elements. While this is truly amazing, plenty of articles and books go into it. For this article, I want to explore other cool -er, hot - processes that happen in the Sun. They show us a dynamic star with complex mechanics that impact our lives here on Earth.

The solar wind encountering our magnetic field.

The solar wind encountering our magnetic field.

Solar Wind and the Heliosphere

Sometimes magnetic fields of opposite polarity meet, forcing magnetic recombination. If this happens in the low solar corona, it can cause a spike of energy to be released in the form of X-ray jets of plasma. These can then be fed into coronal funnels near the poles and allow it to leave the Sun, becoming the fast solar wind. Slow solar wind can arise from where magnetic fields enter and exit a star and mainly happen along the equatorial belt. Moving at about 300 to 800 kilometers pers second due to higher magnetic fields accelerating the winds, it causes a net mass loss of about 10,000,000 tons a second! This seems like a staggering amount, until you realist this is just 0.0000000000001% the mass of the Sun per year, so not really a big deal over the lifespan of the Sun. Amongst the electrons and photons in the wind, elements such as phosphorus, chlorine, potassium, titanium, chromium, nickel, and various isotopes of other elements have been seen in the wind (Seeds 123-4, Whitehead “Spacecraft”, Fleck, Harra, Marsch, “Five, “Alfven”).

Eventually, the solar wind interacts with the interstellar medium, or the area of space outside our solar system. First, the wind goes subsonic as it crosses the termination shock. Then it begins to shift its trajectory to match that of the interstellar medium forcing itself onto the wind. This is the heliosheath, which eventually will meet with the interstellar medium and form the heliopause. Once you pass this, you have exited the solar system. Exactly where this is remains unknown, due to solar activity as well as interstellar medium composition/movement (Hathaway “The Heliosphere”).

Some sunspots on the Sun.

Some sunspots on the Sun.

Sunspots

Activity on the outside of the Sun is by no means limited to the solar wind. Sunspots are one of the most fascinating exterior features of the Sun. In discerning the portions of a sunspot, we refer to the dark center as the umbra and the progressively lighter edges the penumbra. These terms can also be used to talk about the shadows of eclipsing celestial bodies because of the similar appearances, nothing to do with the actual mechanics at play. For sunspots, they are darker because they exist at a lower temperature (about 3700 to 4200 Kelvin) than their surroundings (about 5700 Kelvin) (Seeds 132, Hathaway “Photospheric features”).

After years of observations, scientists noticed that the sunspots undergo a cycle of 11 years, with their paths being traced out in what is known as a Maunder butterfly diagram (after the shape it takes upon unfolding the surface of the Sun). The sunspots start near the mid latitudes of each hemisphere and migrate towards the equator (Seeds 132-3, Hathaway “The Sunspot Cycle”).

To better understand what is going on here, scientists make use of the Zeeman effect. When an atom encounters a magnetic field, the electrons orbiting have either path altered. This opens the door for new wavelengths of light to be absorbed that otherwise would be impossible, opening up lines in our spectrums. How far apart the lines are gives an indication of the strength of the magnetic field. Using the Zeeman effect, scientists have seen great magnetic fields occurring at sunspots. These fields basically slow down our material, causing the motion to drop to the point where temperatures drop, causing the darker appearance (Seeds 133, Hathaway “Photospheric Features,” “SOHO Confirms”).

Sound waves have also revealed the powerful magnetic behavior happening here. As a magnetic field rises towards the surface, material is pushed upward, and a pressure wave forms. This causes sound waves already present to start speeding up, foretelling the coming magnetic field column. These field can trap material as they extend upward, until they branch off and form clusters of sunspots that break the surface. Temperature fluctuations cause material to move up or down, depending on how hot it is and where it is in relation to the magnetic column (“SOHO Reveals”)

Further Surface Activity

Sunspots are just one of many things that happen at the surface of the Sun. Faculae are similar to sunspots, but the magnetic activity here actually causes brighter areas to form. Prominences are where ionized gas travels in a magnetic loop, passing through the photosphere, the chromosphere, and the lower corona. They are easy to spot at eclipses and are pink in color due to the heavy hydrogen concentration. If a prominence lasts for a few days, its quiescent but if its only around for a few hours its eruptive. These release their material outward in the form of a solar flare and is a result of the magnetic field lines breaking, sending the material in the prominence out into space. With sufficient material inside one of these, we get a coronal loop. If these occur high enough int he corona, then we can have a great amount of material being released as a coronal mass ejection. No matter what event it is, tons of X-rays, UV rays, and other energetic particles are released into space (Seeds 136-7, Sharp, Hathaway “Photospheric features”).

A coronal mass ejection leaves the Sun.

A coronal mass ejection leaves the Sun.

Linking Flares and Rings

A surprising link has been shown between solar flare activity and oscillations of the Sun. For high frequencies above 5.3 MHz, these oscillations have been shown to strongly correlated with the solar flare activity on the surface. It seems to imply that when a solar flare goes off, the energy released causes reverberations throughout the Sun, though the exact mechanism behind this remains unclear (“SOHO Confirms”).

Falling Back

Another shocking finding was that gas around the Sun can actually fall back into it, despite all the activity seemingly pushing it outward. They can start at 2.7 million kilometers above the Sun’s surface, about twice the Sun’s diameter! At this point, the solar wind is moving at about 120 kilometers per second, while the infalling material moves back towards the Sun at 50 to 100 kilometers a second. This material usually falls back to about 700,000 kilometers from the surface before stopping its inward progress (Fleck “SOHO”, Sharp).

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What is going on there? Gravity isn’t enough to pull this stuff back in. Instead, it seems that magnetic forces are at play. While magnetic field loops carry material around in them, sometimes the solar wind can drag them far away from the Sun. When opposing field lines get too close, they can recombine to form new loops and send material towards the Sun, so long as the solar wind hasn’t yet achieved high speeds. Other work points to potentially different density mediums interacting and essentially creating dead zones of activity (Fleck “SOHO”, Whitehead “Scientists).

Works Cited

“Alfven Waves – Our Sun is Doing the Magnetic Twist.” Science20.com. ION Publications LLC, 18 Mar. 2009. Web. 17 Feb. 2022.

“Five Years of Discoveries with SOHO Have Made the Sun Transparent.” Sci.esa.int. ESA, 27 Apr. 2001. Web. 28 Feb. 2022.

Fleck, Bernhard. “Hinode: new insights on the origin of solar wind.” Esa.int. ESA, 12 Jul. 2007. Web. 28 Feb. 2022.

Harra, Louise. “Hinode: source of the slow solar wind and superhot flares.” Esa.int. ESA, 04 Feb. 2008. Web. 28 Feb. 2022.

Hathaway, David H. “Photospheric Features.” Solarscience.msfc.nasa.gov. NASA, 11 Aug. 2014. Web. 17 Feb. 2022.

---. “The Heliosphere.” Solarscience.msfc.nasa.gov. NASA, 11 Aug. 2014. Web. 28 Feb. 2022.

Marsch, Eckart. “Solar Wind Origin in Coronal Funnels.” Sci.esa.int. ESA, 22 Apr. 2005. Web. 28 Feb. 2022.

Seeds, Michael A. Horizons. Tenth Edition, Thomson Brooks/Cole, Belmont, CA. 2008. Print. 123-4, 132-3, 136-7.

Sharp, Tim. “The sun’s atmosphere: Photosphere, chromosphere, and corona.” Space.com. Future US, Inc., 01 Nov. 2017. Web. 17 Feb. 2022.

“SOHO Confirms 36 Year Old Solar Theory.” sci.esa.int. ESA, 17 Apr. 2008. Web. 03 Mar. 2022.

“SOHO Reveals How Sunspots Take a Stranglehold on the Sun.” sci.esa.int. ESA, 05 Nov. 2001. Web. 28 Feb. 2022.

Whitehead, Nadia. “Scientists Explain Mysterious Finger-like Features in Solar Flares.” pweb.cfa.harvard.edu. Harvard, 27 Jan. 2022. Web 03 Mar. 2022.

---. “Spacecraft Enters the Sun's Corona for the First Time in History.” Cfa.harvard.edu. Harvard University, 14 Dec. 2021. Web. 17 Feb. 2022.

© 2022 Leonard Kelley

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