Magnetometer Viby
Viby (Sollentuna), Sweden · geographic coordinates: 59°27' N 17°54 E · geomagnetic coordinates (2017): 57.69°N, 106.22°E

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The device

SAM Viby facilitates Speake & Co Llanfapley FGM-3 fluxgate sensors for measuring the local magnetic field. The magnetometer is located in Viby (Sollentuna), about 15 km North of Stockholm.

Geographic coordinates: 59o27min N 17o54min E
Geomagnetic coordinates (2017): 57.69oN, 106.22oE

Magnetic declination: 5 57.60' East (IGRF-12 Model , 2015)
Annual Change (minutes/year): 10.3 '/y East

Construction and installation of the instrument

After a successful maintenance of SAM Haimhausen, a magnetomete which I am running 20 km north of Munich, Germany, I decided to install a magnetometer close to my residence north of Stockholm. Just some kilometers off the 60th parallel, this location with about 10 northern light occurrences every seasoa seems well suited for geomagnetic measurements.

Ralf Pitscheneder generously donated his 2-axis SAM magnetometer for further use as SAM Viby. This magnetometer was operated from 2004-2010 in Munich as well. We have brought it to Sweden and installed it end of 2016. The device now takes measurements in a preliminary, yet stable configuration.

The two magnetic field sensors were reused and installed in a 50-liter cartboard box in my storage, which features reasonable temperature stability (the magnetic field sensors are quite sensitive to temperature changes on the 0.1 degree Centigrade level). The location is as magnetically quiet as a suburban neighborhood can be, but SAM Viby is powerful software tools to counteract effects of man-made disturbances on the measurements. Recently, the observatory was fitted with temperature sensors, and I have plans to improve temperature insulation of the observatory using styrofoam detector holders and embedding the setup in an aluminum shield filled with styrofoam spheres (which are produced by a local chap).

On the scientific side, the observatory currently reads the two horizontal components of the Earth's magnetic field. The accuracy of the relative measurements is given by ±1-2 nT (sensor accuracy), and probably ≈5 nT man-made disturbances. Relative measurements are relevant for producing alerts for northern lights observations. SAM Viby also does absolute magnetic field measurements. These are currently calibrated using the WMM2015 model which is evaluated on daily basis. The model bears uncertainties of ≈135 nT in each coordinate.

An immediate upgrade will be to equip SAM Viby with a third fluxgate sensor as to measure also the horizontal component of the magnetic field.

Some pictures of the magnetometer. More thorough documentation to come soon.

Some diagnostic plots

A first trial to assess the temperature dependence of the measurements: This plot shows the magnetic flux vs. the ambient outdoor temperature. Dedicated temperature sensors directly at the fluxgate sensors will be installed soon.

Darrel Emerson did exhaustive tests on the charateristics, thermal dependencies and thermal and long-term effects

Another interesting plot contains the raw measurements without compensating for offsets, as caused by passing cars, slamming fridge doors, operating the washing machine, getting skies out of the room where the magnetometer is measuring or just dropping a nail ;-)

A 300-hr correlation with a DS18B20 sensor close to the FGM3 sensors:

A first, 27-day correlation vs planetary Kp:

Calibrating the local K scale: In his Geomagnetism Tutorial, Whitham Reeve describes a method suggested by Dirk Lummerzheim to develop a K-index scale for a particular station:

  1. Determine the Kp-index values for several 3-hour time periods over several days.
  2. Determine the field values (nT) at a nearby magnetometer station corresponding to the same time periods and days.
  3. For each Kp-index value, compare the range of corresponding field values to the range limits, or multiples of the range limits, described above (the multiple is the same for each Kp-index value and does not have to be an integer)
  4. Assign the appropriate range limits to each local K-index value so they resemble the Kp- index to the extent possible
When using local K-indices, also artificial disturbances will enter. Nevertheless, for illustration the plot below collects locally measured disturbances and shows their distibutions for individual Kp indices. The yellow bars represent the full dataset, the green bars a particularly undisturbed subset taken during 00 UTC and 06 UTC.