Northern Lights Primer

Last week I presented a brief article about the recent increase in solar activity. Sunspot group AR1429 unleashed several solar flares and coronal mass ejections (CMEs) that eventually reached the Earth. While there were no reported power grid failures, GPS blackouts or mis-directions, or fried satellite electronics, the resulting geomagnetic storms that ensued did produce vivid displays of the aurora borealis (northern lights). Most of these displays were well north of the United States/Canadian border, while many extremely beautiful images were captured from Scandinavia.

There are many images available on the Internet, as well as some great videos. Here’s a great website (http://spaceweather.com/) where these displays have been documented and previous displays have been archived.

Contrary to popular belief, displays of the northern lights can become visible here in southern New England if the intensity of a geomagnetic storm is exceptionally high. Please read on to learn exactly what happens when the plasma (very energetic protons and electrons) expelled from the Sun reach Earth and interact with our magnetic field and atmosphere.

There is always a stream of charged particles being directed into space from the Sun. It’s called the solar wind. Earth’s magnetic field captures some of these particles and directs them toward the polar regions. These regions, called auroral ovals, sit within the upper atmosphere above both the north and south magnetic poles. Imagine the Earth wearing two halos or crowns. Those are the auroral ovals. Usually the solar wind is not energetic enough to trigger a display of the northern lights.

However, solar flares and CMEs greatly enhance the solar wind. The resulting geomagnetic storms often cause the auroral oval to expand southward (in the northern hemisphere). How far south the oval expands depends on how energetic the particles are. When we view a display that looks like an arc across and above the northern horizon, we are actually seeing the southern extent of the expanding oval.

Atoms and molecules of elements in our atmosphere get stimulated by the bombardment of these energetic and charged particles (electrons and protons) as they zip through the Earth’s magnetic field. The color of the northern lights display depends upon which atom or molecule is being stimulated, how energetic the particles are, and how high in our atmosphere the stimulation is occurring. A good analogy is a neon light. A specific gas in the vacuum of the glass tube gets excited by the electricity applied to it. The gas glows a color that is characteristic of the gas within the tube. Different gases produce different colors.

And the various forms, such as arcs, rays, draperies and spikes, are entirely dependent on the Earth’s changing magnetic field lines. (Think of a bar magnet and metal filings.) You are actually seeing the Earth’s field lines because the excited elements are glowing as the charged particles are accelerated down those magnetic field lines into our atmosphere. Auroral displays typically occur in the lower atmosphere at an altitude of about 75 miles. 

As the Sun’s activity continues to climb to solar maximum in 2013, you can expect the frequency and intensity of geomagnetic storms to increase as well, and with it the chance to witness a beautiful display of the northern lights from right here in southern New England.