| Worldwide Magnetograms with Geomagnetic Components D, H, Z, or X, Y, and Z |
| The NOAA National Centers for Environmental Information (formerly National Geophysical
Data Center) receives magnetograms from over 200 geomagnetic observatories. The Center
archives data from 1867 to the present, however the primary holdings are from 1957
to the present. The archives contain over 5,000 reels of 35 mm microfilm copies of
magnetograms. Magnetograms are records obtained from an instrument known as a magnetometer
which is designed to measure small variations in the components of the geomagnetic
field. There are two types of magnetometers, the La Cour and the Ruska. The magnetogram
consists of one or more (usually 3) variometers, one for each element usually D, H,
Z (less common X, Y, Z) and a recorder. The standard magnetogram usually has all three
components recorded simultaneously on one sheet of paper. In addition to the 3 traces,
reference lines (baselines) are recorded. The baselines are used in absolute calibration
of the magnetogram. The normal recording speed is 20 mm/hour with the drum turning
once in 24 hours. |
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