Horizontal magnetic field fluctuations measured in the Lincoln Sea

Many oceanic measurements require knowledge of the orientation of the measurement system or instrument with respect to true north; examples include current meters and acoustic arrays. Magnetic compasses are an attractive choice as a heading reference because they are low-power, small and relatively inexpensive sensors. However, in the Arctic, because of the proximity of the north magnetic pole, the magnetic field orientation is nearly vertical and the horizontal components of the field are relatively weak. Thus the headings obtained from magnetic compasses in the Arctic must be utilized carefully. A series of controlled tests of the behavior of magnetic compasses were conducted in the Lincoln Sea during the spring of 1993. At this location the magnetic field is oriented about 87° from the horizontal. Magnetic headings, recorded for 14 days from two compasses placed on the ice and within an autonomous weather station, were compared to similar recordings at two other Arctic locations. Ice floe orientation and other environmental parameters were monitored. The heading records from the two co-located systems were very similar and showed fluctuations of 4° to 6° every 24 hours. An empirical correction curve is determined which could be applied to measurement systems utilizing magnetic compasses for heading references