<P> Data from THEMIS show that the magnetic field, which interacts with the solar wind, is reduced when the magnetic orientation is aligned between Sun and Earth--opposite to the previous hypothesis . During forthcoming solar storms, this could result in blackouts and disruptions in artificial satellites . </P> <P> Changes in Earth's magnetic field on a time scale of a year or more are referred to as secular variation . Over hundreds of years, magnetic declination is observed to vary over tens of degrees . A movie on the right shows how global declinations have changed over the last few centuries . </P> <P> The direction and intensity of the dipole change over time . Over the last two centuries the dipole strength has been decreasing at a rate of about 6.3% per century . At this rate of decrease, the field would be negligible in about 1600 years . However, this strength is about average for the last 7 thousand years, and the current rate of change is not unusual . </P> <P> A prominent feature in the non-dipolar part of the secular variation is a westward drift at a rate of about 0.2 degrees per year . This drift is not the same everywhere and has varied over time . The globally averaged drift has been westward since about 1400 AD but eastward between about 1000 AD and 1400 AD . </P>

The line of earth magnetic field run from