Mohawk Valley Astronomical Society

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Dark Innocence

by Perry Pezzolanella, MVAS

The Sun is the most amazing star in the sky because the light and heat it generates is vital to life here on Earth. It looks innocent enough in the sky, but a painful sunburn rudely reminds us of its power. To feel this effect ninety-three million miles away, we know there must be tremendous power there. Part of the Sun’s brute force can be credited to the most obvious feature, the sunspot.

Sunspots seem simple and yet can be larger than Earth, which means they can occasionally be seen with the unaided eye when the Sun is low on the horizon shining through a thick layer of haze. A history of sunspot sightings is found in ancient Chinese records, but it was not until the invention of the telescope that Galileo began to study them in 1610. Individual sunspots live for only a few weeks and the rate of appearance and disappearance is cyclical. This is known as the sunspot cycle where the number of sunspots increases and decreases every 11 years. The cycle is not exact, but comes close, and each peak in the sunspot cycle is different than the preceding ones.

The Earth’s climate may be affected in ways that are not yet fully understood. There was a period from 1640 to 1710 where the cycle ceased and there were hardly any sunspots. This period is known as the Maunder Minimum and it coincided closely with a period of intensely cold weather in northern Europe known as the “Little Ice Age”. It was so cold that rivers that never froze were suddenly frozen solid enough for skating and oxen could haul lumber across them. The summers were so chilly that there was frost every month including July. Any crops that managed to grow did not have long enough to reach harvest before an early freeze killed everything. Famine and disease were widespread and many plagues broke out resulting in unprecedented loss of life. Spacecraft have proven that the Sun’s energy output increases as the sunspots increase and therefore may have a direct relationship to Earth’s climate.

Sunspots appear dark because they are cool relative to the rest of the Sun. They are about 7000º F compared to 10,000º F for the Sun’s surface. If a sunspot could be placed by itself in the night sky it would appear like a reddish star, 10 times brighter than the full Moon. The relative darkness and coolness is due to the intense magnetic fields that generate them. The magnetic fields prevent convection that normally transports energy towards the surface. Sunspots form in pairs and each leading spot has the same polarity as all the other pairs in that hemisphere. The polarity remains the same throughout the 11-year cycle, but when a new 11-year cycle begins, the polarity of the two spots in the pair is reversed. After this cycle ends, the polarity is back to that of the initial cycle. So the sunspot cycle is actually part of a 22-year magnetic cycle. How/why the polarity switches is one of the Sun’s greatest mysteries.

Sunspots are a source of solar flares, the most violent explosions in the Solar System. Sunspots are strongly magnetic and if these fields become twisted too much, they will kink, and then reduce their twist abruptly. This sudden release of energy results in flares, which can pack a punch of more than a billion one-megaton thermonuclear explosions. A flare with this much power can propel high-energy particles up to one-third the speed of light. When these particles interact with the magnetic field of Earth, or any other planet, they can create an aurora, interrupt radio communications, induce currents in electrical power grids large enough to blow out transformers, and confuse birds’ navigation. Solar flares have destroyed and crippled satellites and spacecraft, and pose a radiation danger to astronauts, especially since they cannot readily be predicted.

Sunspots look innocent, but they are very complex and have a direct impact on our well being here on Earth. A peaceful night with an aurora flickering - a fire in the northern sky, radiating green and pink, born in the heart of the Sun - is an inspirational reminder of the power of our Sun.