An academic study using satellite imagery suggests that the drop in violence in Baghdad is due to ethnic redistribution and not the US troop surge.

The study by researchers from the University of California in Los Angeles used levels of light pollution to track the movements of social groupings within the city.

Superimposing satellite images over neighborhood maps showed that light levels had dimmed by more than 20% more in Sunni-dominated west and south-western regions of the city suggesting that Sunnis had "cleared out".

Light levels in Shia areas and the Green Zone increased during the same period.

The researchers believe this is evidence of a demographic shift which concluded before the arrival of 30,000 extra US troops in February 2007, the date which supporters of the surge say was the beginning of a decline in sectarian violence in the Iraqi capital.

"By the launch of the surge, many of the targets of conflict had either been killed or fled the country, and they turned off the lights when they left," UCLA geography professor John Agnew said in a statement.

"Essentially, our interpretation is that violence has declined in Baghdad because of intercommunal violence that reached a climax as the surge was beginning," said Agnew, who studies ethnic conflict.

The study adds weight to the argument that it was not the US troop surge alone that reduced sectarian violence in Iraq but a range of factors.

Sunni Arabs were driven out of many Baghdad neighborhoods following the bombing of the Shia Samarra mosque in February 2006 which sparked a wave of sectarian violence.

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