The Role of Solar Flare Activity in Northern and Southern Lights

The Sun will be entering the peak of its 22-year cycle in 2013, so the next few years are certain to produce some memorable solar flares. Extreme solar flares have produced northern lights as far south as Hawai’i. While auroras are also lit by the constant solar wind, solar flares produce the most spectacular northern and southern lights.

Solar flares and Earth

The Earth is constantly bombarded by the atomic particles contained in the solar wind. When these charged particles encounter the Earth’s magnetic field, the field lines deflect them toward the polar regions of the Earth, where they interact with atmospheric gases and cause those gases to lose or gain electrons. The polar regions are the only place where solar particles commonly interact with atmospheric gases.

Each time an atom of oxygen or nitrogen gains or loses electrons, it gives off photons of visible light. When many photons of visible light are given off at the same time, the result is the northern and southern lights, or auroras.

During a solar flare, the solar wind becomes much stronger, to the point where it can overwhelm the Earth’s magnetosphere and interact with atmospheric gases far outside the polar regions. Even a moderate solar flare often causes northern lights which are visible as far south as latitude 50 degrees. A strong solar flare can cause auroras which are visible over most of the continental U.S.

The aurora is magnetic in nature, so it also induces electrical currents on Earth. Usually these currents pass mostly unnoticed except as tiny spikes in grid power or an occasional radio crackle, similar to what a thunderstorm might cause. However, the auroral currents generated by the interaction of a strong solar flare with the Earth’s magnetosphere can be strong enough to disrupt the electrical grid.

Strongest solar flare in history

The strongest confirmed solar storm in human history happened on September 1 and 2, 1859. This solar storm started with a solar flare which was so intense that the amount of sunlight briefly doubled. When the solar particles from the flare reached Earth, they were in opposite alignment with the Earth’s magnetic field and were strong enough to overwhelm it.

The auroras from that solar flare were some of the brightest ever seen. They were also seen as far south as Hawai’i. Further north in Boston, the New York Times reported that the aurora was “so brilliant that about one o’clock ordinary print could be read by [its] light.”

The auroral current associated with that solar flare was so strong that a few telegraph operators were able to cut off their battery power and communicate using only the geomagnetically induced current. The same auroral currents also shorted out other telegraph wires, and even caused a few fires.

If a similarly strong solar flare were to strike today, a few fires is the least amount of damage that can be expected. The world is dependent on power grids and communication satellites which are vulnerable to extreme solar flares. The next massive solar flare in Earth’s direction might even be stronger than the 1859 solar flare.

It will happen again. The only question is when.