The effects of extreme space weather may be larger than previously thought reveals research co-authored by Lancaster University in the journal Nature.
Space weather - caused by fluctuating electric fields in Earth's magnetic field and upper atmosphere - can affect technologies on and around Earth in several ways. Extreme geomagnetic storms make up some of the less frequent but extreme cases of space weather.
An example of space weather are extreme geomagnetic storms which are temporary disturbances in the plasma and magnetic field around the Earth causing disruptions in global satellite communication, extensive power outages, and even how much radiation astronauts and pilots are exposed to.
For decades, scientists have thought that there is an upper limit to how Earth responds to solar storms. Electric currents in the Earth's upper atmosphere are widely understood to reach an upper limit with increasing solar wind strength.
But now research suggests the upper limit is an illusion resulting from uncertainty in the measurement of the solar wind strength, as the true value regresses towards the mean. If so, this means solar storms could have far worse effects on our technology than previously thought.
The Nature paper entitled "Regression to the mean can explain saturation of geomagnetic storms" is led by Dr Nithin Sivadas of NASA's Goddard Space Flight Centre and co-authored by Dr Maria Walach from Lancaster's School of Physics and Astronomy.
Dr Walach said: "Our planet's magnetic field does a really great job of protecting us against many space weather effects and so they often just show up as glitches or beautiful aurora. There are however extreme cases, where satellites unexpectedly fall back to Earth, or we lose communication and GPS signals."
The solar wind is a never-ending stream of hot gases flowing from the Sun, which can strengthen during solar eruptions. Observations have suggested that, as the solar wind strengthens, electric currents in the Earth's upper atmosphere - which can affect satellites, communications, and navigation signals - increase to a certain point but then, on average, level off.
The team say this apparent limit is merely an effect of uncertainties in solar wind measurements.
They claim the issue is that most solar wind measurements of extreme events are taken by spacecraft at Lagrange point one, which is a million miles closer to the Sun than the Earth. Hence the solar wind that strikes the Earth is likely weaker due to a regression to the mean effect. Averaging observations from many events makes it look like strong solar winds do not produce equally strong currents because on average weaker solar winds arrive at Earth.
The team found evidence from more than a million solar wind measurements taken by Earth-orbiting NASA spacecraft, very close to our planet. Analysis of these observations showed a direct relationship between the strength of the solar wind and the currents in the upper atmosphere, suggesting there is no upper limit but rather Earth's response will continue to increase along with the solar wind strength, and impacts to technology can increase as well.
Dr Walach said: "If there is no upper limit to our planet's response to the solar wind, modelling for extreme cases needs to take this into account and we should be vigilant of space weather effects. Fortunately, these very extreme cases are rare, but this also means we have limited data to work with and only time will tell what happens at the very extreme one-in-a-thousand-year kind of event."
The lead author Dr Sivadas said: "We usually assume the truth may be around its measurement. But probability theory says it leans one way. That's why space weather risks appear underestimated."
