Image of the Week: Celebrating the Solstice with Solar Activity

A medium-sized Coronal Mass Ejection (CME) occurred on June 20th, 2013, the Northern Hemisphere summer solstice. It is the explosive release of material seen on the left. Credit: NASA

Last week, on June 20^th, people across the Northern Hemisphere celebrated the summer solstice. In Anchorage, Alaska, they held a midnight festival and a marathon in honor of their 24 hours of sunlight. Bonfires burned around Europe, a tradition long thought to ward off evil spirits on a day full of magic. Even the sun celebrated: in the excitement, it emitted a spectacular coronal mass ejection (CME) just after 11 pm EDT — an unbeatable finale to an evening of revelry.

But what exactly is a solstice and what is a coronal mass ejection? (By the way, they have nothing to do with each other except their recent cosmic coincidence). Most people have probably heard of the solstice, but many (including the 1968 class of Harvard graduates) don’t know why it happens. It’s tempting to think that long days and warm weather arise because the Earth swings closer to the sun in its orbit. But that wouldn’t explain why it’s now winter in the Southern Hemisphere. In fact, the seasons stem from the tilt of the Earth. The solstice marks the day when the planet’s axis points straight at the sun, tipping the Northern Hemisphere toward its heat while the Southern Hemisphere tips away. This makes the sun appear highest in the sky to those of us here on the northern half Earth, and leads to the longest hours of light.

And a CME? As the name implies, these mesmerizing events occur on the surface of the sun (the corona), a searing hot plasma layer of charged particles which contains “ropes” of magnetically connected material. The corona rotates, but unlike on the solid Earth where every location on the planet must complete one revolution per day, the material in the sun rotates at different rates at different solar latitudes. This twists the ropes, sometimes causing loops to leap out from the surface (solar flares). When these ropes are warped so much that they break, they release huge amounts of plasma and energy into space — more than the combined energy of 20 million atomic bombs (don’t worry, they don’t harm the Earth, but they can interfere with communication systems and energy grids). Happy belated solstice!

LEARN MORE

Learn why the charged particles on the sun’s surface create a magnetic field in our module on Light and Electromagnetism and how fusion fuels the sun in our module on Nuclear Chemistry.

Watch this YouTube video showing how the tilt of the Earth causes the seasons!

See more amazing pictures of CMEs and learn why they happen at EarthSky.org!

Written by Julia Rosen

Julia Rosen is a freelance science writer and PhD student at Oregon State University. She received a Bachelor’s degree in Geological and Environmental Sciences from Stanford University before beginning her doctoral research on polar ice cores and climate change. In between, she did her “Master's” in backpacking around the world and skiing. Julia is a periodic contributor to Oregon State’s research magazine, Terra, and helps write blog content and develop learning modules for Visionlearning.