earth's magnetic field seems to reverse orientations about every million years.
earth's magnetic field seems to reverse orientations about every?
million years
the main part of earth's magnetic field is generated in the core, the site of a dynamo process that converts the kinetic energy of thermally and compositionally driven convection into electrical and magnetic field energy. the field extends outwards from the core, through the mantle, and up to earth's surface, where it is, approximately, a dipole. the poles of the dipole are located close to earth's geographic poles. at the equator of the magnetic field, the magnetic-field strength at the surface is , with a magnetic dipole moment of at epoch 2000, decreasing nearly 6% per century. the convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. this causes secular variation of the main field and field reversals at irregular intervals averaging a few times every million years. the most recent reversal occurred approximately 700,000 years ago.
earth
the solar magnetic field extends well beyond the sun itself. the electrically conducting solar wind plasma carries the sun's magnetic field into space, forming what is called the interplanetary magnetic field. in an approximation known as ideal magnetohydrodynamics, plasma particles only move along the magnetic field lines. as a result, the outward-flowing solar wind stretches the interplanetary magnetic field outward, forcing it into a roughly radial structure. for a simple dipolar solar magnetic field, with opposite hemispherical polarities on either side of the solar magnetic equator, a thin current sheet is formed in the solar wind. at great distances, the rotation of the sun twists the dipolar magnetic field and corresponding current sheet into an archimedean spiral structure called the parker spiral. the interplanetary magnetic field is much stronger than the dipole component of the solar magnetic field. the sun's dipole magnetic field of 50–400 μt (at the photosphere) reduces with the inverse-cube of the distance to about 0.1 nt at the distance of earth. however, according to spacecraft observations the interplanetary field at earth's location is around 5 nt, about a hundred times greater. the difference is due to magnetic fields generated by electrical currents in the plasma surrounding the sun.
sun