Temporal Variations of the Earth's Magnetic Field - Overview
Like the gravitational field, the
magnetic field varies with time. When describing temporal variations of the
magnetic field, it is useful to classify these variations into one of three
types depending on their rate of occurrence and source. Please note explicitly
that the temporal variations in the magnetic field that we will be discussing
are those that have been observed directly during human history. However, the
best-known temporal variation, magnetic polarity reversals, while important
in the study of earth history, will not be considered in this discussion. We
will, however, consider the following three temporal variations:
- Secular Variations - These are long-term (changes in the field that occur over years)
variations in the main magnetic field that are presumably
caused by fluid motion in the Earth's Outer
Core. Because these variations occur slowly with respect to the time of completion of a typical
exploration magnetic survey, these variations will not complicate data reduction efforts.
- Diurnal Variations - These are variations in the magnetic field that occur over the
course of a day and are related to variations in the Earth's external magnetic field.
This variation can be on the order of 20 to 30 nT per day and should be accounted for when
conducting exploration magnetic surveys.
- Magnetic Storms - Occasionally, magnetic activity in the ionosphere
will abruptly increase. The occurrence of such storms correlates with enhanced
sunspot activity. The magnetic field observed during such times is highly
irregular and unpredictable, having amplitude changes as large as 1000 nt.
Exploration magnetic surveys should not be conducted during magnetic storms.
*In this context, "rapidly" means on the order of hundreds
to tens of years, down to minutes.
