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Published online by Cambridge University Press: 12 April 2016
During September 24-29, 1987, the longitudinal magnetic fields of a decaying active region (AR 4855) were observed almost continuously at Big Bear and Huairou Solar Observatories. The target consisted of an αp sunspot and surrounding enhanced network. We achieved a 75-hour coverage with 4 night-time gaps of six to eight hours. The images were then combined to made a continuous movie.
The BBSO Videomagnetograph (VMG) System was originally developed by Leighton and Smithson and has been improved in recent years (Zirin, 1985). This system has made it possible to study very weak photospheric magnetic structures.