Magnetogram and variometer:
The purpose of this memo is to explain what is magnetogram and how to obtain it. To know the method of magnetic measurement would be helpful for avoiding misuse of magnetograms served from our data center.
A magnetogram means a graphic presentation of magnetic field variation. Therefore a plot of geomagnetic data measured with a digital magnetometer such as a fluxgate magnetometer is also a magnetogram. However, a 'magnetogram' normally means an analog recording obtained by a torsion type magnetic variometer as shown in Figure 1 Until recently this type of magnetometer has been used and the data have been obtained as magnetograms. Magnetograms have been copied on microfilms or microfiches and collected at World Data Centers.
Torsion type magnetic variometer:
Figure 1 explains how geomagnetic variations are measured and recorded by a set of the torsion type magnetic variometer. The method is very simple. A small magnet is hung with a string of crystal or metal which is used for spring. A small rotation of the magnet by geomagnetic field variation is magnified by reflecting the ray with a small mirror attached to the magnet. The reflected ray makes a spot on photo paper which is put on a slowly rotating drum.
As seen from the figure, the measurement of declination (D) is most easy. That is, when the magnet is hung without torsion, the magnet points the geomagnetic north at the point. Therefore the magnet is most sensitive to the force orthogonal to the geomagnetic north, i.e., the variation in declination.
To measure the horizontal component (H), i.e., variation in geomagnetic north-south direction, the magnet is rotated 90 degree with torsional force of the string. Then the magnet points (geomagnetic) east-west direction and it is most sensitive to the field variation in geomagnetic north-south direction. The vartical component (Z) is measured by keeping the magnet in the horizontal plain as seen in the figure.
By adjusting the relative location and direction of the light source, variometer and the drum, three rays are focused on the drum and the magnetic variation of the three components is recorded as a magnetogram. For normal-run magnetogram, the drum rotates once a day. For rapid-run magnetogram, the drum rotates many times a day, and the drum shifts slowly in the direction of the rotation axis drawing many lines on a magnetogram. The time mark is recorded by shutting the rays or putting another ray.
The sensitivity depends on the magnetic moment of the small magnet, torsional elasticity of the string and the distance between the mirror and the drum. The sensitivities per unit length (normally nT/mm on the record) are found at the beginning of the month. For Russian magnetograms, they are usually written on the magnetogram of the first day of the month. For most of other magnetograms, they are normally given on a separate sheet.
The system is very simple and hence stable in general. However, new photo paper has to be set every day. At a severe geomagnetic storm, the three traces sometimes cross each other. We also have to be careful on the distortion caused by the microfilm camera, because the records kept at WDCs are on microfilm.