The Earth is the only one of the four inner planets (Mercury, Venus, Earth and Mars) that has a substantial magnetic field. Shaped very much like a bar magnet, and powered by enormous currents of electricity in the molten core of the Earth, this field extends millions of miles into space to form the magnetosphere. Outside this region, charged particles from the Sun and deep space, may be deflected or may leak into the interior of this region to form the Van Allen Belts, or produce auroral activity.
This field changes in complex ways as CMEs find their way to the Earth and impact the magnetic field. Observatories on the ground have kept track of the strength and direction of the Earth's magnetic field for over a century. Their records show that rapidly changing field conditions are common, especially when the Sun is active.
The most dramatic of these episodes are called the geomagnetic storms which can last several days. Less intense changes can last hours or minutes and are called geomagetic sub-storms. Navigation by compass is especially difficult during either of these magnetic storms because compass bearings can change by 10 degrees or more during the course of a few hours. As anyone familiar with using a map and compass can tell you, without knowing the 'magnetic deviation', it is impossible to use a compass to determine where geographic north is located. As a result, surface navigation can become dangerously imprecise.