<P> Some of the charged particles do get into the magnetosphere . These spiral around field lines, bouncing back and forth between the poles several times per second . In addition, positive ions slowly drift westward and negative ions drift eastward, giving rise to a ring current . This current reduces the magnetic field at the Earth's surface . Particles that penetrate the ionosphere and collide with the atoms there give rise to the lights of the aurorae and also emit X-rays . </P> <P> The varying conditions in the magnetosphere, known as space weather, are largely driven by solar activity . If the solar wind is weak, the magnetosphere expands; while if it is strong, it compresses the magnetosphere and more of it gets in . Periods of particularly intense activity, called geomagnetic storms, can occur when a coronal mass ejection erupts above the Sun and sends a shock wave through the Solar System . Such a wave can take just two days to reach the Earth . Geomagnetic storms can cause a lot of disruption; the "Halloween" storm of 2003 damaged more than a third of NASA's satellites . The largest documented storm occurred in 1859 . It induced currents strong enough to short out telegraph lines, and aurorae were reported as far south as Hawaii . </P> <P> The geomagnetic field changes on time scales from milliseconds to millions of years . Shorter time scales mostly arise from currents in the ionosphere (ionospheric dynamo region) and magnetosphere, and some changes can be traced to geomagnetic storms or daily variations in currents . Changes over time scales of a year or more mostly reflect changes in the Earth's interior, particularly the iron - rich core . </P> <P> Frequently, the Earth's magnetosphere is hit by solar flares causing geomagnetic storms, provoking displays of aurorae . The short - term instability of the magnetic field is measured with the K - index . </P>

The process by which magnetic fields of the earth move