By Dr David Whitehouse
BBC News Online science editor
The massive solar flare that erupted from the Sun last November was far bigger than scientists first thought.
The flare overloaded detectors
At the time, satellite detectors were unable to record its true size because they were blinded by its radiation.
But University of Otago physicists say they have now estimated the probable scale of the huge explosion by studying how X-rays hit the Earth's atmosphere.
They tell Geophysical Research Letters the X45 class event was more than twice as big as the previous record flare.
Fortunately, the Earth did not take a direct hit from this immense blast of radiation and matter.
Had it done so, several orbiting satellites would almost certainly have been damaged and there could have been considerable disruption of radio communications and power grids on the planet's surface.
The big one
Last October and November, the Sun underwent an extraordinary surge in activity, producing a series of big flares from the most active sunspot region ever seen.
Day after day, gas was being explosively heated to millions of degrees, sending radiation and billions of tonnes of charged particles streaming into space.
SOLAR FLARE CLASSES
Flares classified by brightness at X-ray wavelengths
X-class is biggest; can trigger radio blackouts on Earth
M-class is medium-sized; radio interference at polar regions
C-class is smallest; few noticeable effects on Earth
But it was on 4 November, as Active Region 486 was being carried out of sight around the Sun's western limb by solar rotation, that the most extraordinary flare let rip.
Between 1929 and 1950 GMT, the enormous explosion sent an intense burst of radiation towards the Earth.
Even before the storm had peaked, X-rays had overloaded the detectors on solar-monitoring satellites, in particular those on the Geostationary Operational Environmental Satellites, which usually provide data allowing scientists to estimate the size of such events.
Later study suggested the flare was an X28.
Right place and time
The biggest previous solar flares on record were rated X20, on 2 April 2001 and 16 August 1989. So 4 November's explosion certainly set a new mark. But only now do scientists understand the probable true power of the event.
The New Zealand researchers in Otago looked at the effect the flare's radiation had on the Earth's upper atmosphere and used that to judge its strength.
The gas cloud starts on its way
"So when this event overloaded the satellite detectors, we were in a unique position to make this measurement," they said.
Detailing their work in their journal paper, the team report that at the time of the big solar explosion they were probing the ionosphere with radio waves as part of a long-term research programme.
They noticed that X-rays from the flare changed the properties of the ionosphere, an effect that has been observed many times before.
The changes the Otago researchers saw allowed them to produce a new estimate of the flare's intensity, increasing its rating from X28 to X45.
"This makes it more than twice as large as any previously recorded flare," said Associate Professor Neil Thomson.
Luckily the radiation from the flare only struck a glancing blow to the Earth.
Refers to region of Earth's upper atmosphere
Normally extends 70 to 400 km above the surface
Atmosphere location where solar radiation can affect transmissions of radio waves
"If the accompanying particle and magnetic storm had been aimed at the Earth, the damage to some satellites and electrical networks could have been considerable," said Thomson.
The New Zealanders say their calculations show the flare's X-ray radiation bombarding the atmosphere was equivalent to that of 5,000 suns, though none of it reached the Earth's surface, they stress.
"Given that any future flares are unlikely to be large enough to overload the ionosphere, we believe that our new method has great advantages in determining their size in the event of satellite detector overloads," said Thompson.