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 throughout the Sun's equatorial plane in the heliosphere.^[1][2] 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).^[3] A small electrical current flows within the sheet, about 10⁻¹⁰ A/m². The thickness of the current sheet is about 10,000 km.

The underlying magnetic field is called the interplanetary magnetic field, and the resulting electric current forms part of the heliospheric current circuit.^[4] The Heliospheric current sheet is also sometimes called the Interplanetary Current Sheet.

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Characteristics

Ballerina's skirt shape

  As the Sun rotates, its magnetic field twists into a Parker spiral,^[5] a form of an Archimedean spiral, named after its discovery by Eugene Parker. Parker's spiral magnetic field was divided in two by a current sheet^[6], a mathematical model first developed in the early '70's by Schatten. As the spiraling magnetic sheet changes polarity, it warps into a wavy spiral shape that has been likened to a ballerina's skirt.^[7][8] 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".^[9]

The cause of the ballerina spiral shape has sometimes been called the "garden sprinkler effect" or "garden hose effect"^[10][11], and likening to holding a lawn sprinkler, and moving it in your hand vertically up and down, while your body rotates. The stream of water represents the Solar Wind, and moves radially outwards at all times.

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 5×10⁻⁶ T.^[4]

The magnetic field at the surface of the Sun is about 10^-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^-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 3×10⁹ amperes.^[4] 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^-10 A/m² (10^-4 A/km²).

History

The heliospheric current sheet was discovered by John M. Wilcox and Norman F. Ness, who published their finding in a 1965.^[12] Hannes Alfvén and Per Carlqvist speculate^[13] on the existence of a galactic current sheet, a counterpart of the heliospheric current sheet, with an estimated galactic current of 10¹⁷ to 10¹⁹ amperes, that might flow in the plane of symmetry of the galaxy.

References

See also

• Interplanetary Magnetic Field