By tracking the amount of light emitted by Baghdad neighborhoods at night, US geographers have uncovered fresh evidence that last year's US troop surge in Iraq may not have been as effective at improving security as some US officials have maintained.

Night light in neighborhoods populated primarily by embattled Sunni residents declined dramatically just before the February 2007 surge and never returned, the researchers from the University of California in Los Angeles (UCLA) said in a new study based on publicly available satellite imagery.

Until just before the surge, the night-light signature of Baghdad had been steadily increasing overall, said the report, which was released Friday.

The team reports its findings in the October issue of Environment and Planning A, a leading peer-reviewed academic journal that specializes in urban and environmental planning issues.

The night-light signature in four other large Iraqi cities -- Kirkuk, Mosul, Tikrit and Karbala -- held steady or increased between the spring of 2006 and the winter of 2007, the UCLA team found. None of these cities were targets of the surge.

Baghdad's decreases were centered in the southwestern Sunni strongholds of East and West Rashid, where the light signature dropped 57 percent and 80 percent, respectively, during the same period.

By contrast, the night-light signature in the notoriously impoverished, Shiite-dominated Sadr City remained constant, as it did in the American-dominated Green Zone. Light actually increased in Shiite-dominated New Baghdad, the researchers found.

"If the surge had truly 'worked,' we would expect to see a steady increase in night-light output over time, as electrical infrastructure continued to be repaired and restored, with little discrimination across neighborhoods," said co-author Thomas Gillespie, an associate professor of geography at UCLA.

"Instead, we found that the night-light signature diminished only in certain neighborhoods, and the pattern appears to be associated with ethno-sectarian violence and neighborhood ethnic cleansing."

"Essentially, our interpretation is that violence has declined in Baghdad because of intercommunal violence that reached a climax as the surge was beginning," said lead author John Agnew, a professor of geography at the University of California in Los Angeles (UCLA).

"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."

The effectiveness of the February 2007 deployment of 30,000 additional US troops has been a subject of debate.

In a report to Congress in September of that year, Gen. David Petraeus claimed "the military objectives of the surge are, in large measure, being met."

However, a report in the same month by an independent military commission headed by retired U.S. Gen. James Jones attributed the decrease in violence to areas being overrun by either Shiites or Sunnis.

The issue now figures in the US presidential race, with Republican presidential candidate John McCain defending the surge and Democratic hopeful Barack Obama having been critical of it.