WASHINGTON (Reuters) - Satellite images taken at night show heavily Sunni Arab neighborhoods of Baghdad began emptying before a U.S. troop surge in 2007, graphic evidence of ethnic cleansing that preceded a drop in violence, according to a report published on Friday.

An image showing the proportion of change in night-lights in Baghdad between March 2006 and December 2007, released to Reuters on September 19, 2008. REUTERS/UCLA/Handout

The images support the view of international refugee organizations and Iraq experts that a major population shift was a key factor in the decline in sectarian violence, particularly in the Iraqi capital, the epicenter of the bloodletting in which hundreds of thousands were killed.

Minority Sunni Arabs were driven out of many neighborhoods by Shi’ite militants enraged by the bombing of the Samarra mosque in February 2006. The bombing, blamed on the Sunni militant group al Qaeda, sparked a wave of sectarian violence.

“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,” geography professor John Agnew of the University of California Los Angeles, who led the study, 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.

Some 2 million Iraqis are displaced within Iraq, while 2 million more have sought refuge in neighboring Syria and Jordan. Previously religiously mixed neighborhoods of Baghdad became homogenized Sunni or Shi’ite Muslim enclaves.

The study, published in the journal Environment and Planning A, provides more evidence of ethnic conflict in Iraq, which peaked just before U.S. President George W. Bush ordered the deployment of about 30,000 extra U.S. troops.

The extent to which the troop build-up helped halt Iraq’s slide into sectarian civil war has been debated, particularly in the United States, with supporters of the surge saying it was the main contributing factor, and others arguing it was simply one of a number of factors.

“Our findings suggest that the surge has had no observable effect, except insofar as it has helped to provide a seal of approval for a process of ethno-sectarian neighborhood homogenization that is now largely achieved,” Agnew’s team wrote in their report.

Agnew’s team used publicly available infrared night imagery from a weather satellite operated by the U.S. Air Force.

“The overall night light signature of Baghdad since the U.S. invasion appears to have increased between 2003 and 2006 and then declined dramatically from 20 March 2006 through 16 December 2007,” their report said.

They said the night lights of Shi’ite-dominated Sadr City remained constant, as did lights in the Green Zone government and diplomatic compound in central Baghdad. Lights increased in the eastern New Baghdad district, another Shi’ite enclave.

Satellite studies have also been used to help document forced relocations in Myanmar and ethnic cleansing in Uganda.