Heliospheric current sheet
Encyclopedia
The heliospheric current sheet (HCS) is the surface within the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 where the polarity
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, and attracts or repels other magnets.A permanent magnet is an object...

 of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

's magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

 changes from north to south. This field extends throughout the Sun's equatorial plane in the heliosphere
Heliosphere
The heliosphere is a bubble in space "blown" into the interstellar medium by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself...

. The shape of the current sheet
Current sheet
A current sheet is an electric current that is confined to a surface, rather than being spread through a volume of space. Current sheets feature in magnetohydrodynamics , the study of the behavior of electrically conductive fluids: if there is an electric current through part of the volume of such...

 results from the influence of the Sun's rotating magnetic field
Rotating magnetic field
A rotating magnetic field is a magnetic field which changes direction at a constant angular rate. This is a key principle in the operation of the alternating-current motor. Nikola Tesla claimed in his autobiography that he identified the concept of the rotating magnetic field in 1882. In 1885,...

 on the plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

 in the interplanetary medium
Interplanetary medium
The interplanetary medium is the material which fills the solar system and through which all the larger solar system bodies such as planets, asteroids and comets move.-Composition and physical characteristics:...

 (Solar Wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

). A small electrical current flows within the sheet, about 10−10 A
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...

/m². The thickness of the current sheet is about 10,000 km.

The underlying magnetic field is called the interplanetary magnetic field
Interplanetary Magnetic Field
The interplanetary magnetic field is the term for the solar magnetic field carried by the solar wind among the planets of the Solar System....

, and the resulting electric current forms part of the heliospheric current circuit. The heliospheric current sheet is also sometimes called the interplanetary current sheet.

Ballerina's skirt shape

As the Sun rotates, its magnetic field twists into a Parker spiral
Parker spiral
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...

, a form of an Archimedean spiral
Archimedean spiral
The Archimedean spiral is a spiral named after the 3rd century BC Greek mathematician Archimedes. It is the locus of points corresponding to the locations over time of a point moving away from a fixed point with a constant speed along a line which rotates with constant angular velocity...

, named after its discovery by Eugene Parker
Eugene Parker
Eugene N. Parker is an American solar astrophysicist who received his B.S. degree in physics from Michigan State University in 1948 and Ph.D. from Caltech in 1951. In the mid 1950s Parker developed the theory on the supersonic solar wind and predicted the Parker spiral shape of the solar magnetic...

. Parker's spiral magnetic field was divided in two by a current sheet, a mathematical model first developed in the early 1970s 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. 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 cause of the ballerina spiral shape has sometimes been called the "garden sprinkler effect" or "garden hose effect", 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.

The magnetic field at the surface of the Sun is about 10−4 tesla
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...

. If the form of the field were a magnetic dipole
Magnetic dipole
A magnetic dipole is the limit of either a closed loop of electric current or a pair of poles as the dimensions of the source are reduced to zero while keeping the magnetic moment constant. It is a magnetic analogue of the electric dipole, but the analogy is not complete. In particular, a magnetic...

, 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 has a radial component (directed inward) as well as an azimuthal 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 3×109 ampere
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...

s. As a comparison with other astrophysical electric currents, the Birkeland current
Birkeland current
A Birkeland current is a set of currents which flow along geomagnetic field line connecting the Earth’s magnetosphere to the Earth's high latitude ionosphere. They are a specific class of magnetic field-aligned currents. Lately, the term Birkeland currents has been expanded by some authors to...

s that supply the Earth's aurora
Aurora (astronomy)
An aurora is a natural light display in the sky particularly in the high latitude regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere...

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

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK