4.4 Diurnal Variation

The term Diurnal simply means ‘daily’ and for many centuries it has been noticed that the magnetic field seems to follow a rough sine wave during the course of the day. Here is a graph of field strength observations taken in Colorado over a 2 day period.

It can be seen that the field strength is following a 24 hour period sine wave.
See ‘Interpolated In Field Referencing’, a method of correcting for diurnal variation in the field.

These variations may be small but for high accuracy MWD work especially at high latitudes, they may need to be corrected for. We now know that this effect is due to the rotation of the Earth and a varying exposure to the solar wind. The sun is constantly emitting ionized plasma in huge quantities across the solar system. These winds intensify during magnetic storms and the material can be seen on a clear night at high or low latitudes, being concentrated at the magnetic poles and forming the ‘Aurora Borealis’ and the ‘Aurora Australis’.

*During such storms the measurements taken from magnetometers and compasses are unlikely to be reliable but even in quiet times, the diurnal variation is always present.