The Havre Police Department was inundated with hundreds of phone calls late last night after residents were awakened by a sonic boom.  The boom was caused by pilots from the 120th Fighter Wing Montana Air National Guard F-15s participating in night flying operations.  The operations will continue through Friday, with the final take off time at 9:30 p.m. each night and approximate flying times of 90 minutes.  Training exercises occur every day, and night training is necessary to maintain pilot proficiency in the dark hours.  It includes the use of night vision goggles and the enhanced radar system the jets are equipped with.  The training is for pilots to perfect their "air-to-air combat capability in a night-time environment," Maj.  Tim Crowe with the Montana Joint Force headquarters in Helena said this morning.  Crowe extended apologies for causing concern.  The Air National Guard "sincerely apologizes to the community of Havre for alarming them last night," he said.  "It's not our intention to startle communities with our training operations," he added. The Guard out of Great Falls is in the final stages of switching to combat ready with F-15s from F-16s, and the conversion is expected to be complete in the next month or so, Crowe said.  Once the process is complete, the 120th Fighter Wing will be prepared to respond to any deployment, he said.  Pilots using the overland military operating area Havre is located under must adhere to height requirements of 18,000 feet above Havre and 30,000 feet above ground for any super sonic flight.  Those limits were adhered to during last night's operations, Crowe said, but weather conditions were such that the boom was heard on the ground.  For the remaining days of the exercises, sonic speed will not be reached over Havre, he added.  "So there will be further restrictions on those aircraft," he said.  Other than the high volume of calls, the flights caused no problems, Gabe Matosich, assistant chief of operations for the Havre police, said.  Likewise, no problems were caused in the county, Sheriff Don Brostrom said.  "We've heard that for years," he said about the booms.