It was an impressive sight that I remember clearly to this day; it was at once beautiful and frightening. In the cold and dark, and more than a little tired, I wondered if some of the energy might flick down from aloft and carelessly extinguish me.
The northern lights — or Aurora Borealis, as more technical-minded folks call them — are caused by a linking between the Earth and its magnetic field, and the sun and solar wind.
The solar wind is the debris, in the form of high-energy, sub-atomic particles that leak from the sun’s photosphere (the sun’s visible surface), where photons of light make their last stop before streaming through empty space to their ultimate destination — be it your cheek on a warm spring day or some other place millions of light-years away.
Electrically charged particles of the solar wind can become tangled in Earth’s magnetic field, which extends like the field of a bar magnet far into space. The field lines dip into Earth’s atmosphere and plunge to their source deep within Earth’s molten core in two places: in the Northern Hemisphere in extreme Northern Canada, north of Baffin Bay and just west of Greenland; and in the Southern Hemisphere in the Southern Ocean, just off the northern coast of Antarctica and south of Australia.
When the charged particles of the solar wind — trapped along Earth’s magnetic field lines — interact with molecules of nitrogen and oxygen high in Earth’s atmosphere, the molecules glow diffusely with characteristic colors: red and green from oxygen, and red and blue from nitrogen.
occur continuously above Earth’s northern and southern magnetic poles. But when conditions are right, the aurora can extend much farther to the south in the Northern Hemisphere (and farther to the north in the Southern Hemisphere) to be seen from less extreme, mid-latitude locations — like Michigan.
And, for reasons not fully understood, the conditions are more often “right” in the spring; and, to a lesser extent, the fall.
In addition, aurora activity tracks with solar activity in general, and energetic activity in the sun’s photosphere — in the form of solar-wind enhancing sun spots and solar flares — waxes and wanes in an 11-year cycle.
Presently, the sun is climbing out of an exceptionally deep and prolonged solar minimum.
All of this seems to be combining in recent weeks to bring aurora closer to home.
There have been reports of impressive northern light displays in Minnesota and even in central Lower Michigan over the past two weeks.
Northern lights don’t generally occur without warning, but forecasting them is a less accurate affair than forecasting the weather.
If you would like to see the northern lights sometime, there are websites that offer aurora activity alerts for free and by subscription. For starters, you might sign up for free e-mail alerts at http://spaceweather.com.
But beware that descriptions you might read online and in newspapers about memorable displays are much more common than the memorable displays themselves; and that to see the northern lights, the sky has to be really dark.
— By Doug Furton, a member of the physics faculty at GVSU. Send questions and suggestions to firstname.lastname@example.org. An archive of some of his “What’s up” columns is available online at http://gegenschein.wordpress.com.