The Earth acts like a great spherical magnet, in that it is surrounded by a magnetic field. This magnetic field changes both with time and with location on the Earth and resembles, in general, the field generated by a dipole magnet (i.e., a straight magnet with a north and south pole) located at the center of the Earth. The axis of the dipole is offset from the axis of the Earth's rotation by approximately 11 degrees. This means that the north and south geographic poles and the north and south magnetic poles are not located in the same place. At any point and time, the Earth's magnetic field is characterized by a direction and intensity which can be measured. Often the parameters measured are the magnetic declination, D, the horizontal intensity, H, and the vertical intensity, Z.
From these elements, all other parameters of the magnetic field can be calculated using Geomagnetic Field Calculator.

The rate of change of declination at Lerwick, Eskdalemuir and Greenwich-Abinger-Hartland observatory series in the UK have been a number of changes in the general trend of secular variation in the past, in particular at about 1925, 1969, 1978 and 1992. These sudden changes are known as jerks or impulses and, at the present time, are not well understood and are certainly not predictable. Some researchers have found evidence for a correlation with length-of-day changes.

Regular small variation.

The geomagnetic field has a regular small variation with a fundamental period of 24 hours. This variation is easiest to observe during periods of low solar activity when large irregular disturbances are less frequent. For this reason it is often referred to as the Solar quiet or Sq variation. In reality, this type of variation in the geomagnetic field would affect the direction of a compass needle by no more than a few tenths of a degree.
This regular fluctuation is caused by electrical currents high in the ionosphere, a region that begins at an altitude of about 100 km. All currents, like those in wires, can only flow in materials that conduct.

As well as the regular daily variation the Earth's magnetic field also exhibits irregular disturbances, and when these are large they are called magnetic storms. These disturbances are caused by interaction of the solar wind, and disturbances therein, with the Earth's magnetic field. The solar wind is a stream of charged particles continuously emitted by the Sun and its pressure on the Earth's magnetic field creates a bounded comet-shaped region surrounding the Earth called the magnetosphere. When there is a disturbance in the solar wind the current systems existing within the magnetosphere are enhanced and cause magnetic disturbances and storms.
A geomagnetic storm has three phases:
- An initial phase, a main phase and a recovery phase. The initial phase is characterized by Dst (or its one-minute component SYM-H) increasing by 20 to 50 nT in tens of minutes. The initial phase is also referred to as a storm sudden commencement (SSC). However, not all geomagnetic storms have an initial phase and not all sudden increases in Dst or SYM-H are followed by a geomagnetic storm.
- The main phase of a geomagnetic storm is defined by Dst decreasing to less than -50 nT. The selection of -50 nT to define a storm is somewhat arbitrary. The minimum value during a storm will be between -50 and approximately -600 nT. The duration of the main phase is typically between 2 and 8 hours.
- The recovery phase is the period when Dst changes from its minimum value to its quiet time value. The period of the recovery phase may be as short as 8 hours or as long as 7 days.
The size of a geomagnetic storm is classified as moderate ( -50 nT >minimum of Dst > -100 nT), intense (-100 nT > minimum Dst > -250 nT) or super-storm ( minimum of Dst < -250 nT).
Although irregular, magnetic disturbances exhibit some patterns in their frequency of occurence. The main pattern is the correlation with the 11-year solar cycle. Another important pattern is the 27-day recurrence of some storms related to the 27-day rotation of the Sun as seen from Earth.