Friday, September 08, 2006

Ballerina's Skirt

Heliospheric current sheet. Image credit: NASA artist Werner Heil Enlarge Image

Ballerina's skirt
As the Sun rotates, its magnetic field twists into a Parker spiral, a form of an Archimedean spiral, named after its discovery by Eugene Parker. As the spiraling magnetic sheets changes polarity, it warps into a wavy spiral shape that has been likened to a ballerina's skirt.
Further dynamics have suggested that
"The Sun with the heliosheet is like a bashful ballerina who is repeatedly trying to push her excessively high flaring skirt downward".

The Parker Spiral
The Parker spiral is the shape of the Sun's extended 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 influence of the solar wind and magnetohydrodynamics. The shape is named after Eugene Parker, 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 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.

The Heliospheric Current Sheet
The Heliospheric current sheet (HCS) is the surface within the Solar System where the polarity of the Sun's magnetic field changes from north to south. This field extends from the Sun's equatorial plane throughout the entire Solar System, and can be considered its largest structure.
The shape of the current sheet results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium (Solar Wind).

A small electrical current flows within the sheet, about 10 to the -10 amps/m2. The thickness of the current sheet is about 10,000km. The underlying magnetic field is called the interplanetary magnetic field, and the resulting electric current forms part of the heliospheric current circuit.

The Heliospheric current sheet is also sometimes called the Interplanetary Current Sheet.

Magnetic field
The heliospheric current sheet rotates along with the Sun once every 27 days, during which time the peaks and troughs of the skirt pass through the Earth's magnetosphere, interacting with it. Near the surface of the Sun, the magnetic field produced by the radial electric current in the sheet is of the order of 5 x 10 to the -6 T.

The magnetic field at the surface of the Sun is about 10 to the -4 tesla. If the form of the field were a magnetic dipole, the strength would decrease with the cube of the distance, resulting in about 10 to the -11 tesla at the Earth's orbit. The heliospheric current sheet results in higher order multipole components so that the actual magnetic field at the Earth due to the Sun is 100 times greater.

Electric current
The electric current in the heliospheric current sheet is directed radially inward, the circuit being closed by outward currents aligned with the Sun's magnetic field in the solar polar regions. The total current in the circuit is on the order of 3×10 to the 9 amperes.

As a comparison with other astrophysical electric currents, the Birkeland currents that supply the Earth's aurora are about a thousand times weaker at a million amperes. The maximum current density in the sheet is on the order of 10 to the -10 A/m2 (10 to the -4 amps/km2).

The heliospheric current sheet was discovered by John M. Wilcox and Norman F. Ness, who published their finding in a 1965. Hannes Alfvén and Per Carlqvist speculate on the existence of a galactic current sheet, a counterpart of the heliospheric current sheet, with an estimated galactic current of between 10 to the 17 and 10 to the 19 Amps, that might flow in the plane of symmetry of the galaxy

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