The IMF originates in regions on the Sun where the 
magnetic field is "open"--that is, where field lines 
emerging from one region do not return to a 
conjugate region but extend virtually indefinitely 
into space. The direction (polarity, sense) of the 
field in the Sun's northern hemisphere is opposite 
that of the field in the southern hemisphere. (The 
polarities reverse with each solar cycle.) Along the 
plane of the Sun's magnetic equator, the oppositely 
directed open field lines run parallel to each other 
and are separated by a thin current sheet known as 
the "interplanetary current sheet" or "heliospheric 
current sheet." The current sheet is tilted (because 
of an offset between the Sun's rotational and 
magnetic axes) and warped (because of a 
quadrupole moment in the solar magnetic field) and 
thus has a wavy, "ballerina skirt"-like structure as 
it extends into interplanetary space. Because the 
Earth is located sometimes above and sometimes 
below the rotating current sheet, it experiences 
regular, periodic changes in the polarity of the IMF. 
These periods of alternating positive (away from 
the Sun) and negative (toward the Sun) polarity are 
known as magnetic sectors.