Sun’s
massive explosion upgraded
 
By Dr David Whitehouse 
BBC News Online science editor  

Match 17, 2004

The massive
solar flare that erupted from the Sun last November was far bigger
than scientists first thought. 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. 
   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. 
     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. 
     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. 
    “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.  
    “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.