Auroras

FAS Astronomers Blog, Volume 32, Number 10.

If you live way up north, you might look up at the night sky from time to time and see shimmering lights stretch across the horizon. These are what we call the northern lights. They are also known as auroras.

Auroras occur all the time. We don’t see them this far south because they are usually restricted to the far north (and are called Aurora Borealis) or the far south (and are called Aurora Australis). They are the result of charged particles (protons and electrons) that travel from the Sun and are deflected by the Earth’s magnetic field either north or south toward the poles. At the poles these particles sometimes interact with the Earth’s atmosphere. These interactions excite the electrons in the atmosphere’s molecules as they absorb energy. In turn, the electrons release energy in the form of light. This is what we see as an aurora.

Solar particles are carried by the solar wind, which is continuously traveling our way. However, the Sun, from time to time, kicks out more particles than normal via something called a Coronal Mass Ejection (CME). CMEs occur in the vicinity of solar flares and sunspots, which are visible on or near the surface of the Sun. These waves of intense particles overwhelm the Earth’s magnetic field and result in auroras that are more intense than usual and ones that are visible farther to the south (for the Aurora Borealis) or to the north (for the Aurora Australis). 

If you’ve seen an aurora, you might have noticed that they come in different colors. This is because the Earth’s atmosphere is made up of different molecules. The colors may vary, but here are the most common. 

• Particles interact with oxygen at high elevations (above 150 miles) producing a reddish color. 
• At lower altitudes (150-60 miles) they interact with oxygen resulting in green auroras. 
• Lower still (60 miles), the particles interact with nitrogen creating bluish or purple auroras.
• Yellowish/Pinkish auroras can sometime occur due to the mixing of the other colors.

The Sun goes through an eleven-year cycle during which time the Sun cycles between two solar minimums or between two solar maximums. We are currently experiencing cycle 25 and are approaching the solar maximum, which is predicted to occur in 2025. Because of this, the Sun is quite active, and we see more intense auroras as well as several sunspots on its surface.

This high level of solar activity treated those of us who live down here in North Carolina to two very rare occurrences of the northern lights. For more on this see our previous post, The Southern Lights. 

The folks at NOAA and the National Weather Service have an Aurora Dashboard, which will provide you with the forecast of upcoming auroras. They also provide measurements of Space Weather Conditions.