NSWC Pt. Barrow Geomagnetic Observatory

DANIEL S. LENKO, JOHN F. SCARZELLO, AND DAVID TAYLOR

The purpose of the observatory is to measure and characterize geomagnetic field variations using a sensitive magnetometer array for a period of at least one sunspot cycle.

The system was first installed in the spring of 1991. The three magnetometer sensor suites were placed on Air Force Long Range Radar Site Pt. Barrow property and cables connected the sensors to a PC in the CMDL Barrow Observatory building that controls the array and stores the magnetic information onto transportable media that is mailed to Maryland for analysis.

Configured to form two orthogonal gradiometer axes, magnetic N-S and E-W, with a 150 m baseline, each of the three magnetic sensor sites consists of actually two types of magnetometers. The first and most sensitive is the He-3 total field magnetometer which is an optically pumped, nuclear magnetic resonance sensing device. It provides a very accurate "absolute" measurement of the ambient magnetic field in the bandwidth from DC to about 0.1 Hz. In terms of sensitivity, the He-3 sensor can resolve submilligamma magnetic data when two sensors are configured as a gradiometer; here, a second He-3 sensor is used to cancel out earth's magnetic field. The dynamic range accommodates magnetic fields over 100,000 gamma, but the He-3 sensor is gradient sensitive which can shorten the measurement time between pumpings. The second magnetic sensor included in each location is a triaxial fluxgate magnetometer. The three orthogonal axes measure magnetic fields in a bandwidth from DC to about 1 Hz. The digital data is appended to each of the He-3 sample updates (2.34 times a second) and the fluxgate sensor can resolve about 0.1 gamma.

The geomagnetic noise collected by the previously mentioned system has been characterized and used to develop and test noise reduction algorithms in an effort to enhance advanced detection methods. Specifically, Pt. Barrow ambient magnetic data has been "played" into detection software as a training data set. Also, on occasions, data from the observatory was used to correlate magnetic noise events at other locations throughout the world. In terms of the observatory hardware, the arctic environment is quite harsh and, therefore, cabling and sensor maintenance is in order. Also, computer and/or storage media upgrades will be performed as funds permit.

In order to complete the current sunspot cycle, NSWC plans to support the Pt. Barrow Observatory through the end of the decade.

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