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Many times we see these originating from southern Nevada (home of Nellis AFB Test Range -- and to some, Area 51 ;-). This probably means they are testing one of their still secret aircraft.
Occasionally it appears when two radars are pointing directly at each other. The received power greatly exceeds the normal value.
Finally, note the long, narrow streaks in southern California in his example image. Those are the signatures from chaff dropped by military aircraft as part of their ECM.
It also appears at sunrise/sunset as mentioned in an earlier reply. David -- +---------------------------------------------------------------------+| David O. Blanchard david_blanchard@xxxxxxxxxxxxx | | Flagstaff, Arizona http://www.dblanchard.net/ |
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