For an indication of what could happen, researchers look to the solar storm of 1989 when a flare knocked out power to the entire province of Quebec for nine hours. The blackout didn't extend into the southern reaches of the grid, on the US Eastern seaboard, but it did concentrate observers' attention on this potentially vulnerable area.
The biggest solar flare in recorded history, known as the Carrington Event after Richard Carrington who saw it and was the first to make the connection between solar activity and its magnetic effects on our planet, happened in September 1859. It was so strong that the auroras, both borealis (north) and australis (south), were seen far beyond their usual zones. In fact, the northern lights were seen as far south as Cuba and were said to be so bright that people could read the newspaper by their glow. Some telegraph operators were injured by sparks and flames leaping from their equipment, and when they shut off the power, they could still send messages by the power from the auroras alone.
All solar flares start as "sun spots," areas of activity visible from earth. The filaments push out from the surface of the sun, rather like the motion of a rubber band stretching or a bubble expanding. Occasionally, their power is so strong that they erupt out of the sun's magnetic field, in a Coronal Mass Ejection (CME), breaking the surface tension. If they happen to be on the side of the sun facing us, they speed towards our atmosphere, causing magnetic disruption. Flares are classified as A,B,C, M or X, depending on their power, and in the first four classes there are also strengths numbered from 1 to 9. In class X, there is no upper limit.
The solar activity in 1859 (theorized to have measured X40) was as strong as we are expecting in this current cycle, but the difference is that we now have research facilities dedicated to observing and reporting on upcoming solar flares. Since 2010, Solar Dynamics Observatory, launched from Cape Canaveral in the USA, has been collecting data on the sun. Scientists in California and Colorado analyse it for trends in solar activity.
There is a 12 per cent chance of a catastrophically severe flare shooting off in our direction, but unlike in 1859, we have at least 20 hours' notice of such eruptions. While an unexpected flare could burn out power grids in Europe, North America, and Asia, plans exist to temporarily shut down and re-route power apparatus, paying with a short black out to avoid years of technological and infrastructure destruction.
Our "envelope" of satellite and wireless communication is so crucial to everything we need for modern life, from transportation to food security to our financial systems, that we simply cannot afford to leave its protection to chance.
It is to be hoped that we don't see another Carrington Event any time soon, but on 23rd October, we will be able to watch a more benign solar event: a partial eclipse. The best view will be from the Arctic, but at lower latitudes at sunset, we will see the moon move slowly across the face of the sun. Do not make the mistake of looking at it with your naked eyes or with an unfiltered telescope, please! Use a solar filter or a pinhole viewer.