Solar storms powerful enough to wreak havoc on electronic equipment strike Earth every 25 years, according to a new study. And less powerful - yet still dangerous - storms occur every three years or so.
This conclusion comes from a team of scientists from the the University of Warwick and the British Antarctic Survey.
These powerful storms can disrupt electronic equipment, including communication equipment, aviation equipment, power grids, and satellites.
The team identifies two types of powerful magnetic storms: 'great super storms' are the most powerful and occur every 25 years on average. The weaker but still dangerous 'severe super storms' occur every three years on average.
The new paper presenting these results is titled "Using the aa index over the last 14 solar cycles to characterize extreme geomagnetic activity." It's published in the journal Geophysical Research Letters. The lead author is Dr. S.C. Chapman from the University of Warwick.
Solar storm are also called geomagnetic storms. They're caused by disturbances in the Sun that send charged particles into space. When those particles strike Earth's magnetosphere, they cause the storm.
The particles can come from coronal mass ejections (CME), co-rotating interaction regions (CIR), and coronal holes that emit a high-speed stream of solar wind that can travel twice as fast as normal solar wind.
The most famous geomagnetic storm is the Carrington Event of 1859. The Carrington Event is also the most powerful geomagnetic storm ever recorded. That storm knocked out some telegraph systems in different parts of the world, started some fires, and even shocked some telegraph operators.
More recently, a 1989 storm in Quebec disrupted the power distribution system, and created powerful auroras that were seen as far south as the state of Texas.
Solar storms pose an increasing risk as our world becomes more linked electronically. Not just our power distribution systems, but our global communications systems, too.
Our satellites might be the most vulnerable, and modern society relies on them more than many people realize. It's been calculated that a storm as powerful as the Carrington Event, if it were occur today, would cause billions, possibly even trillions of dollars worth of damage.
Scientists are interested in these storms because of the need to predict them. This new paper is based on magnetic field data going back 150 years. The authors say they can detect how many powerful storms there were in that time period, and how often they occurred.
In a press release, lead author Professor Sandra Chapman, from the University of Warwick's Centre for Fusion, Space and Astrophysics, said: "These super-storms are rare events but estimating their chance of occurrence is an important part of planning the level of mitigation needed to protect critical national infrastructure."
In their paper, the authors show that 'severe' magnetic storms occurred in 42 out of the last 150 years, or about every three years. The more powerful 'great' super-storms occurred in 6 years out of 150, or about every 25 years. Usually these storms only last a few days, but they can still be very disruptive to modern technology.
Super-storms can cause power blackouts, disrupt or damage satellites, disrupt aviation and cause temporary loss of GPS signals and radio communications. (GPS is not just for navigation. Believe it or not, the modern banking system relies heavily on GPS to synchronize financial transactions.)
"This research proposes a new method to approach historical data, to provide a better picture of the chance of occurrence of super-storms and what super-storm activity we are likely to see in the future," said Chapman.
The Carrington Event was not part of the study, because the data the researchers looked at doesn't go back that far. Their magnetic field data is from the opposite ends of the Earth, from stations in the UK and Australia. It covers the last 14 solar cycles, dating back to well before the space age.
Their analysis shows that super storms as powerful as the Carrington Event may be more common than thought, and that they can happen at any time, with very little warning.
Professor Richard Horne, who leads Space Weather at the British Antarctic Survey, said: "Our research shows that a super-storm can happen more often than we thought. Don't be misled by the stats, it can happen any time, we simply don't know when and right now we can't predict when."
These storms are born in the Sun, but space weather can be monitored by observing changes in the magnetic field at the earth's surface. There's high quality data from multiple stations on Earth going back to the start of the space age, around 1957.
Scientists know that the sun has an approximately 11-year cycle of activity, and during that cycle the Sun varies in intensity. The problem is that there's not enough of this data. It only covers five solar cycles.
A better understanding of powerful solar storms and their rate of occurrence requires a larger data set spanning more solar cycles. In this new study, the researchers went back further in time.
They looked at the aa geomagnetic index, which comes from sites in the UK and Australia, at opposite ends of the Earth. The aa index cancels out Earth's background field, and reaches back 150 years, or 14 solar cycles. It's the longest, almost continuous record of changes in magnetic fields across the earth's surface.
The team used annual averages from the top few percent of the aa index to reach their conclusion. That's how they found that a 'severe' super-storm occurred in 42 years out of 150, and the rarer but more powerful 'great' super-storm occurred in 6 years out of 150.
That means these extreme storms occur once in every 25 years. As an example, the 1989 storm that caused a major power blackout of Quebec was a great storm.
A few years ago there was a near miss. In 2012, the Sun unleashed a powerful burst from an exceptionally large and strong coronal mass ejection. Luckily for us, Earth was not in its path. But data showed that it would have been a super storm if it had struck us.
There's more and more interest in the Sun and the space weather it sends our way. As our economy and way of life become more and more reliant on satellites, communications, and power grids, governments and agencies have made understanding and predicting space weather a priority.
There are several spacecraft studying the Sun right now, including SOHO (Solar Heliospheric Observatory), SDO (Solar Dynamics Observatory), and the Parker Solar Probe. These spacecraft are growing our understanding of the Sun, and our ability to predict these dangerous storms.