The heliospheric current sheet, or interplanetary current sheet, is a surface separating regions of the

heliosphere The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstell ...

where the

interplanetary magnetic field The interplanetary magnetic field (IMF), now more commonly referred to as the heliospheric magnetic field (HMF), is the component of the solar magnetic field that is dragged out from the solar corona by the solar wind flow to fill the Solar Sy ...

points toward and away from the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

. A small

electrical current Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described b ...

with a

current density In electromagnetism, current density is the amount of charge per unit time that flows through a unit area of a chosen cross section. The current density vector is defined as a vector whose magnitude is the electric current per cross-sectional a ...

of about 10⁻¹⁰ A/m² flows within this surface, forming a current sheet confined to this surface.Israelevich, P. L., ''et al.'',
MHD simulation of the three-dimensional structure of the heliospheric current sheet
(2001) ''Astronomy and Astrophysics'', v.376, p.288–291 The shape of the current sheet results from the influence of the Sun's

rotating magnetic field A rotating magnetic field is the resultant magnetic field produced by a system of coils symmetrically placed and supplied with polyphase currents. A rotating magnetic field can be produced by a poly-phase (two or more phases) current or by a singl ...

on the plasma in the

interplanetary medium The interplanetary medium (IPM) or interplanetary space consists of the mass and energy which fills the Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding ...

. The thickness of the current sheet is about near the orbit of the Earth.

Characteristics

Ballerina's skirt shape

As the Sun rotates, its magnetic field twists into an

Archimedean spiral The Archimedean spiral (also known as the arithmetic spiral) is a spiral named after the 3rd-century BC Greek mathematician Archimedes. It is the locus corresponding to the locations over time of a point moving away from a fixed point with a ...

, as it extends through the Solar System. This phenomenon is often called the Parker spiral, after

Eugene Parker Eugene Newman Parker (June 10, 1927 – March 15, 2022) was an American solar and plasma physicist. In the 1950s he proposed the existence of the solar wind and that the magnetic field in the outer Solar System would be in the shape of a Pa ...

's workParker, E. N.,
Dynamics of the Interplanetary Gas and Magnetic Fields
, (1958) ''Astrophysical Journal'', vol. 128, p.664 that predicted the structure of the interplanetary magnetic field. The spiral nature of the heliospheric magnetic field was noted earlier by

Hannes Alfvén Hannes Olof Gösta Alfvén (; 30 May 1908 – 2 April 1995) was a Swedish electrical engineer, plasma physicist and winner of the 1970 Nobel Prize in Physics for his work on magnetohydrodynamics (MHD). He described the class of MHD waves now ...

, based on the structure of comet tails. The influence of this spiral-shaped magnetic field on the

(

solar wind The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sol ...

) creates the largest structure in the Solar System, the heliospheric current sheet. Parker's spiral magnetic field was divided in two by a current sheet, a mathematical model first developed in the early 1970s by Schatten. It warps into a wavy spiral shape that has been likened to a ballerina's skirt.Rosenberg, R. L. and P. J. Coleman, Jr., Heliographic latitude dependence of the dominant polarity of the interplanetary magnetic field, ''J. Geophys. Res.'', 74 (24), 5611–5622, 1969.Wilcox, J. M.; Scherrer, P. H.; Hoeksema, J. T.,
The origin of the warped heliospheric current sheet
(1980) The waviness of the current sheet is due to the magnetic field dipole axis' tilt angle to the solar rotation axis and variations from an ideal dipole field. Unlike the familiar shape of the field from a bar

magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nicke ...

, the Sun's extended field is twisted into an arithmetic spiral by the magnetohydrodynamic influence of the

. The

travels outward from the Sun at a rate of 200-800km/s, but an individual jet of solar wind from a particular feature on the Sun's surface rotates with the

solar rotation Solar rotation varies with latitude. The Sun is not a solid body, but is composed of a gaseous plasma. Different latitudes rotate at different periods. The source of this differential rotation is an area of current research in solar astronomy. ...

, making a spiral pattern in space. Unlike the jet from a sprinkler, the solar wind is tied to the

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

by MHD effects, so that magnetic field lines are tied to the material in the jet and take on an arithmetic spiral shape. The cause of the ballerina spiral shape has sometimes been called the "garden sprinkler effect" or "garden hose effect",Louise K. Harra, Keith O. Mason, ''Space Science'' 2004, Imperial College Press, Smith, E.,
The Sun, Solar Wind, and Magnetic Field
", Jul 1999, Proceedings of the International School of Physics Enrico FERMI Varenna, Italy because it is likened to a

lawn sprinkler An irrigation sprinkler (also known as a water sprinkler or simply a sprinkler) is a device used to irrigate (water) agricultural crops, lawns, landscapes, golf courses, and other areas. They are also used for cooling and for the control of airbo ...

with nozzle that moves up and down while it spins. The stream of water represents the solar wind. The Parker spiral shape of the solar wind changes the shape of the Sun's magnetic field in the

outer Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...

: beyond about 10–20

astronomical units The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits ...

from the Sun, the magnetic field is nearly

toroid In mathematics, a toroid is a surface of revolution with a hole in the middle. The axis of revolution passes through the hole and so does not intersect the surface. For example, when a rectangle is rotated around an axis parallel to one of its ...

al (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 Radial is a geometric term of location which may refer to: Mathematics and Direction * Vector (geometric), a line * Radius, adjective form of * Radial distance, a directional coordinate in a polar coordinate system * Radial set * A bearing f ...

(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 Parker spiral may be responsible for the differential

, in which the Sun's poles rotate more slowly (about a 35-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. During

solar maximum Solar maximum is the regular period of greatest solar activity during the Sun's 11-year solar cycle. During solar maximum, large numbers of sunspots appear, and the solar irradiance output grows by about 0.07%. On average, the solar cycle tak ...

the entire magnetic field of the Sun flips, thus alternating the polarity of the field every

solar cycle The solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a nearly periodic 11-year change in the Sun's activity measured in terms of variations in the number of observed sunspots on the Sun's surf ...

Magnetic field

The heliospheric current sheet rotates along with the Sun with a period of about 25 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 . The magnetic field at the surface of the Sun is about . If the form of the field were a

magnetic dipole In electromagnetism, a magnetic dipole is the limit of either a closed loop of electric current or a pair of poles as the size of the source is reduced to zero while keeping the magnetic moment constant. It is a magnetic analogue of the electric ...

, the strength would decrease with the cube of the distance, resulting in about 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 has a radial component (directed inward) as well as an

azimuthal An azimuth (; from ar, اَلسُّمُوت, as-sumūt, the directions) is an angular measurement in a spherical coordinate system. More specifically, it is the horizontal angle from a cardinal direction, most commonly north. Mathematically, ...

component, the radial circuit being closed by outward currents aligned with the Sun's magnetic field in the solar polar regions. The radial current in the circuit is on the order of . As a comparison with other astrophysical electric currents, the

Birkeland current A Birkeland current (also known as field-aligned current) is a set of electrical currents that flow along geomagnetic field lines connecting the Earth's magnetosphere to the Earth's high latitude ionosphere. In the Earth's magnetosphere, the curr ...

s that supply the Earth's

aurora An aurora (plural: auroras or aurorae), also commonly known as the polar lights, is a natural light display in Earth's sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic). Auroras display dynamic patterns of bri ...

are about a thousand times weaker at a million amperes. The maximum current density in the sheet is on the order of ().

History

The heliospheric current sheet was discovered by John M. Wilcox and Norman F. Ness, who published their finding in 1965.

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 10¹⁷ to 10¹⁹ amperes, that might flow in the plane of symmetry of the galaxy.

References

External links

The Heliospheric Current SheetA Star With Two North Poles
(features animation)
NASA Astrophysics Data System article referencesOnline full text articles
{{The Sun Dynamics of the Solar System Solar phenomena Spirals Plasma physics Sun Space plasmas Magnetism