JEFF BRIDGES' sci-fi sequel Tron: Legacy has leapt to the top of the US box office grid with a 43.6 million dollar opening weekend, according to studio estimates. The Disney release reboots the story line started in the 1982 tale Tron, in which Jeff's character is hurtled into a deadly virtual reality known as the Grid. The movie co-stars Garrett Hedlund and Olivia Wilde. Though quaint by today's standards, the computer-graphic effects in the original Tron were cutting-edge at the time. Yet the movie was a box office underachiever whose following somehow swelled in the intervening decades in a way that perplexed even the studio's executives. "I sure wish I knew, because there is a very, very committed core group of people who just love that movie, and they have fanned the opening-weekend grosses," said Chuck Viane, head of distribution for Disney.Other newcomers premiered with modest to poor receipts, continuing a sluggish end to Hollywood's year. Agencies North American bOX office 1. Tron: Legacy - $43.6m 2. Yogi Bear - $16.7m 3. The Chronicles of Narnia: The Voyage of the Dawn Treader - $12.4m 4. The Fighter - $12.2m 5. The Tourist - $8.7m The weekend proved no picnic for Dan Aykroyd's family flick Yogi Bear, which fell flat at a weak number two with 16.7 million dollars. The Warner Bros release features the voices of Aykroyd and Justin Timberlake in an adaptation of the TV cartoon about the picnic-basket-thieving bear. With children out of school over the holidays, Warner Bros executives hope Yogi Bear will hold up well through Christmas and New Year. "We wish it had been a bit higher, but we'll catch up as we get going," said Jeff Goldstein, the studio's general sales manager. The previous weekend's top movie, 20th Century Fox's The Chronicles of Narnia: The Voyage of the Dawn Treader, fell to number three with 12.4 million dollars, raising its total to 42.7 million. Agencies