Surface of magnetic polarity change
The
heliospheric current sheet
, or
interplanetary current sheet
, is a surface separating regions of the
heliosphere
where the
interplanetary magnetic field
points toward and away from the
Sun
.
[1]
A small
electrical current
with a
current density
of about 10
?10
A
/m
2
flows within this surface, forming a
current sheet
confined to this surface.
[2]
[3]
[4]
The shape of the current sheet results from the influence of the Sun's
rotating magnetic field
on the
plasma
in the
interplanetary 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 an
Archimedean spiral
, as it extends through the Solar System. This phenomenon is often called the
Parker spiral
, after
Eugene Parker
's work
[6]
that predicted the structure of the interplanetary magnetic field.
The spiral nature of the heliospheric magnetic field was noted earlier by
Hannes Alfven
,
[7]
based on the structure of comet tails.
The influence of this spiral-shaped magnetic field on the
interplanetary 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 a
current 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 a
ballerina's
skirt.
[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 bar
magnet
, the Sun's extended field is twisted into an
arithmetic spiral
by the
magnetohydrodynamic
influence of the
solar wind
. The
solar wind
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
, 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 a
lawn sprinkler
with 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 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 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 Parker spiral may be responsible for the differential
solar 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 the
magnetic tension force
.
During
solar maximum
the entire magnetic field of the Sun flips, thus alternating the polarity of the field every
solar cycle
.
[14]
Magnetic field
[
edit
]
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
5
×
10
?6
T
.
[2]
The magnetic field at the surface of the Sun is about
10
?4
T
. 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
T
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
[
edit
]
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
×
10
9
amperes
.
[2]
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/m
2
(
10
?4
A/km
2
).
History
[
edit
]
The heliospheric current sheet was discovered by
John M. Wilcox
and
Norman F. Ness
, who published their finding in 1965.
[15]
Hannes Alfven
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 10
17
to 10
19
amperes, 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.
- ^
a
b
c
Israelevich, P. L.,
et al.
, "
MHD simulation of the three-dimensional structure of the heliospheric current sheet
Archived
2017-03-23 at the
Wayback Machine
" (2001)
Astronomy and Astrophysics
, v.376, p.288?291
- ^
A Star with two North Poles
Archived
2009-07-18 at the
Wayback Machine
, April 22, 2003, Science @ NASA
- ^
Riley, Pete; Linker, J. A.; Miki?, 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 text
Archived
2009-08-14 at the
Wayback Machine
)
- ^
"Artist's Conception of the Heliospheric Current Sheet"
. Archived from the original on September 1, 2006
. Retrieved
2005-11-20
.
{{
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. Alfven, 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
.
S2CID
122870891
.
- ^
Louise K. Harra, Keith O. Mason,
Space Science
2004, Imperial College Press,
ISBN
1-86094-361-6
- ^
Smith, E., "
The Sun, Solar Wind, and Magnetic Field
Archived
2008-02-05 at the
Wayback 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 from
the original
on 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
.
S2CID
121122792
.
- ^
Alfven, 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
.
S2CID
122687137
.
External links
[
edit
]
|
---|
Submagnetosphere
| |
---|
Earth's magnetosphere
| |
---|
Solar wind
| |
---|
Satellites
| |
---|
Research projects
| |
---|
Other magnetospheres
| |
---|
Related topics
| |
---|