The interplanetary magnetic field (IMF) is a part of the Sun's
magnetic field that
is carried into interplanetary space by the
solar wind. The interplanetary
magnetic field lines are said to be "frozen in" to the solar wind
plasma. Because of the Sun's rotation, the IMF, like the solar
wind, travels outward in a spiral pattern that is often
compared to the pattern of water sprayed from a rotating
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.)
The heliospheric current sheet
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" (see the figure above).
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
(see the figure on the left). 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.
The IMF is a vector quantity with three directional
components, two of which (Bx and By) are oriented parallel
to the ecliptic. The third component--Bz--is perpendicular to
the ecliptic and is created by waves and other disturbances
in the solar wind.
When the IMF
and geomagnetic field lines
are oriented opposite or "antiparallel" to each other,
they can "merge" or "reconnect,"
resulting in the transfer of
energy, mass, and momentum from the solar wind flow to
coupling --with the most dramatic magnetospheric
effects-- occurs when the Bz component is oriented southward.
The IMF is a weak field, varying in strength near the Earth
from 1 to 37 nT, with an average value of ~6 nT.
The cartoon showing the tilted current sheet and open solar magnetic field
adapted from Smith et al., Observations of the interplanetary sector
structure up to heliographic latitudes of 16 degrees: Pioneer 11, J. Geophys.
Res., 83, 717, 1978. The artist's conception of the heliospheric current
sheet was obtained from J. Todd Hoeksema, Stanford University.