Heliospheric current sheet
Theheliospheric current sheet,orinterplanetary current sheet,is a surface separating regions of theheliospherewhere theinterplanetary magnetic fieldpoints toward and away from theSun.[1]A smallelectrical currentwith acurrent densityof about 10−10A/m2flows within this surface, forming acurrent sheetconfined to this surface.[2][3][4]The shape of the current sheet results from the influence of the Sun'srotating magnetic fieldon theplasmain theinterplanetary medium.[5]The thickness of the current sheet is about 10,000 km (6,200 mi) near the orbit of the Earth.
Characteristics[edit]
Ballerina's skirt shape[edit]
As the Sun rotates, its magnetic field twists into anArchimedean spiral,as it extends through the Solar System. This phenomenon is often called theParker spiral,afterEugene Parker's work[6]that predicted the structure of the interplanetary magnetic field. The spiral nature of the heliospheric magnetic field was noted earlier byHannes Alfvén,[7]based on the structure of comet tails.
The influence of this spiral-shaped magnetic field on theinterplanetary medium(solar wind) creates the largest structure in the Solar System, the heliospheric current sheet. Parker's spiral magnetic field was divided in two by acurrent sheet,[8]a mathematical model first developed in the early 1970s by Schatten. It warps into a wavy spiral shape that has been likened to aballerina'sskirt.[9][10]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.[11]
Unlike the familiar shape of the field from a barmagnet,the Sun's extended field is twisted into anarithmetic spiralby themagnetohydrodynamicinfluence of thesolar wind.Thesolar windtravels 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 thesolar rotation,making a spiral pattern in space. The cause of this ballerina spiral shape has sometimes been called the "garden sprinkler effect" or "garden hose effect",[12][13]because it is likened to alawn sprinklerwith nozzle that moves up and down while it spins; the stream of water represents the solar wind. Unlike the jet from a sprinkler, however, the solar wind is tied to themagnetic fieldbyMHDeffects, so that magnetic field lines are tied to the material in the jet and take on an arithmetic spiral shape.
The Parker spiral shape of the solar wind changes the shape of the Sun's magnetic field in theouter Solar System:beyond about 10–20astronomical unitsfrom the Sun, the magnetic field is nearlytoroidal(pointed around the equator of the Sun) rather thanpoloidal(pointed from the North to the South pole, as in a bar magnet) orradial(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 differentialsolar rotation,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 themagnetic tension force.
Duringsolar maximumthe entire magnetic field of the Sun flips, thus alternating the polarity of the field everysolar cycle.[14]
Magnetic field[edit]
| Part of a series of articles about |
| Heliophysics |
|---|
|
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 of5×10−6T.[2]
The magnetic field at the surface of the Sun is about10−4T.If the form of the field were amagnetic dipole,the strength would decrease with the cube of the distance, resulting in about10−11Tat 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[edit]
The electric current in the heliospheric current sheet has a radial component (directed inward) as well as anazimuthalcomponent, 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 of3×109amperes.[2]As a comparison with other astrophysical electric currents, theBirkeland currentsthat supply the Earth'sauroraare about a thousand times weaker at a million amperes. The maximum current density in the sheet is on the order of10−10A/m2(10−4A/km2).
History[edit]
The heliospheric current sheet was discovered byJohn M. WilcoxandNorman F. Ness,who published their finding in 1965.[15]Hannes AlfvénandPer Carlqvistspeculate on the existence of agalactic current sheet,a counterpart of the heliospheric current sheet, with an estimated galactic current of 1017to 1019amperes, that might flow in the plane of symmetry of the galaxy.[16]
References[edit]
- ^"The heliospheric current sheet"Smith, E. J, Journal of Geophysical Research 106, A8, 15819, 2001.
- ^abcIsraelevich, P. L.,et al.,"MHD simulation of the three-dimensional structure of the heliospheric current sheetArchived2017-03-23 at theWayback Machine"(2001)Astronomy and Astrophysics,v.376, p.288–291
- ^A Star with two North PolesArchived2009-07-18 at theWayback Machine,April 22, 2003, Science @ NASA
- ^Riley, Pete; Linker, J. A.; Mikić, Z., "Modeling the heliospheric current sheet: Solar cycle variations",(2002)Journal of Geophysical Research(Space Physics), Volume 107, Issue A7, pp. SSH 8-1, CiteID 1136, DOI 10.1029/2001JA000299. (Full textArchived2009-08-14 at theWayback Machine)
- ^"Artist's Conception of the Heliospheric Current Sheet".Archived from the original on September 1, 2006.Retrieved2005-11-20.
{{cite web}}:CS1 maint: bot: original URL status unknown (link) - ^Parker, E. N., "Dynamics of the Interplanetary Gas and Magnetic Fields",(1958)Astrophysical Journal,vol. 128, p.664
- ^"On the theory of comet tails",H. Alfvén, Tellus 9, 92, 1957.
- ^"Current Sheet Magnetic Model for the Solar Corona",K. H. Schatten, Cosmic Electrodynamics, 2, 232–245, 1971.
- ^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)
- ^Owens, M. J.; Forsyth, R. J. (2013)."The Heliospheric Magnetic Field".Living Reviews in Solar Physics.10(1): 11.arXiv:1002.2934.Bibcode:2013LRSP...10....5O.doi:10.12942/lrsp-2013-5.S2CID122870891.
- ^Louise K. Harra, Keith O. Mason,Space Science2004, Imperial College Press,ISBN1-86094-361-6
- ^Smith, E., "The Sun, Solar Wind, and Magnetic FieldArchived2008-02-05 at theWayback Machine",Jul 1999, Proceedings of the International School of Physics Enrico FERMI Varenna, Italy
- ^Barbier, Beth."NASA's Cosmicopia – Sun – Sun's Magnetic Field".Archived fromthe originalon 1998-12-02.
- ^Wilcox, John M.; Ness, Norman F. (1965). "Quasi-Stationary Corotating Structure in the Interplanetary Medium".Journal of Geophysical Research.70(23): 5793–5805.Bibcode:1965JGR....70.5793W.doi:10.1029/JZ070i023p05793.hdl:2060/19660001924.S2CID121122792.
- ^Alfvén, Hannes; Carlqvist, Per (1978)."Interstellar clouds and the formation of stars".Astrophysics and Space Science.55(2): 487–509.Bibcode:1978Ap&SS..55..487A.doi:10.1007/bf00642272.S2CID122687137.
External links[edit]
- The Heliospheric Current Sheet
- A Star With Two North Poles(features animation)
- The interplanetary magnetic fieldArchived2012-04-29 at theWayback Machine
- 3-Dimensional View of the Heliospheric Current Sheet
- NASA Astrophysics Data System article references(Online full text articles)
