Parker spiral

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The heliospheric current sheet is a three-dimensional form of the Parker spiral that results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium.^[1]

The Parker spiral is the shape of the Sun's magnetic field as it extends through the solar system. Unlike the familiar shape of the field from a bar magnet, the Sun's extended field is twisted into an arithmetic spiral by the magnetohydrodynamic influence of the solar wind. The shape is named after Eugene Parker,^[2] who predicted the solar wind and many of its associated phenomena in the 1950s.

The influence of the Sun's spiral-shaped magnetic field on the interplanetary medium (solar wind) creates the largest structure in the Solar System, the heliospheric current sheet.

The Parker spiral shape of the solar wind changes the shape of the Sun's magnetic field in the outer solar system: beyond about 10-20 astronomical units from the Sun, the magnetic field is nearly toroidal (pointed around the equator of the Sun) rather than poloidal (pointed from the North to the South pole, as in a bar magnet) or radial (pointed outward or inward, as might be expected from the flow of the solar wind if the Sun were not rotating). The spiral shape also greatly amplifies the strength of the solar magnetic field in the outer solar system.

The spiral shape is similar to the pattern produced by a spinning lawn sprinkler, for similar reasons: the solar wind travels outward from the Sun at a uniform rate, but an individual jet of solar wind from a particular feature on the Sun's surface rotates with the solar rotation, making a spiral pattern in space. Unlike the jet from a sprinkler, the solar wind is tied to the magnetic field by MHD effects, so that magnetic field lines are tied to the material in the jet and take on an arithmetic spiral shape.

The Parker Spiral

The Parker spiral may be responsible for the differential solar rotation, in which the Sun's poles rotate more slowly (about a 32-day rotation period) than the equator (about a 27-day rotation period). The solar wind is guided by the Sun's magnetic field and hence largely emanates from the polar regions of the Sun; the induced spiral shape of the field causes a drag torque on the poles due to the magnetic tension force.