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The Origin of Magnetism of the SunObservations and Understanding of Solar Magnetism and Phenomena
Sep 12, 2010 © Harry P. Schlanger
The Sun's variable rotation at its surface is the source of small- and large-scale, dynamic magnetic fields that create the familiar solar phenomena.
A previous article outlined the
structure of the Sun.
The Sun's magnetism originates inside the sun and extends to the surface, corona, and across the
entire solar system. A wide variety of phenomena are observed as a result - from sunspots and solar flares
on the Sun to the Aurora Borealis and interference of geomagnetic fields on Earth.
This article summarises basic knowledge about the magnetic Sun - how visual observations and
measurement are closely linked to magnetism and a solar dynamo concept has been proposed.
Observed Rotation of the Sun
The Sun has a North and South magnetic pole and rotates in the same direction as the Earth. However,
unlike the Earth, the Sun rotates once every 25 days at its equator but slower, up to 35 days, at
its poles.
This difference in rotational speed profile extends on roughly radial lines through the solar convection
zone to the interior. The transition between the convection and radiative zones is called the
tachocline (Tobias, 2002). The Sun's rotation changes abruptly from:
- Differential rotation at the surface, including the convection zone, to
- Solid body rotation in the radiative zone, where the inner Sun rotates at the same rate everywhere.
Effects of Differential Rotation on Magnetic Lines
The figure below shows a model of the Sun's magnetic lines and stages of the 22-year magnetic cycle.
In (a), a static Sun would cause magnetic lines
running from South to North. However, the Sun rotates differentially, causing
magnetic lines to be dragged along the direction of rotation as shown in (b).
As years pass, the magnetic lines become more and more tangled, producing magnetic loops that protrude from
the surface of the Sun as shown in (c) and (d). This is thought to give rise to the observed sun's surface
phenomena.
Model of the Sun's magnetic lines
(From a video courtesy SOHO).
The Sun rotates differentially, dragging magnetic field lines until they tangle, make complex loop
patterns and finally break after 11 years, reversing the magnetic pole.
Magnetic Trapping of Plasma
The Sun's corona is threaded with these complex
networks of magnetic fields.
It is believed that most of the solar activity observed on the surface of the sun is caused by such magnetic
fields due to the important property of magnetic of trapping and containing hot gases, as is shown below:
solar flares and prominences are ejections of plasma into the corona by the magnetic field.
Magnetic fields also contribute to the so-called
solar wind,
formed as the Sun's top layer blows off into space, carrying magnetic fields still attached to the Sun.
Solar Winds perturb Earth's geomagnetic field and affect radio transmission and the power grid.
Sunspots - Observations and Measurement
Sunspots are prime evidence of solar magnetic activity. They are sites of darker colour due to strong
magnetic activity on the surface layer of the sun called the photosphere. When sunspots reach a maximum
after each 11 years, magnetic poles reverse.
Existence of an 11-year
solar sunspot cycle
is known to relate to the 22-year magnetic cycle.
The Magnetic Butterfly (modified from Hathaway)
Hathaway has presented magnetic activity related to the sunspot cycle in his
magnetic
butterfly trace (above) - it is a view of the developing solar sunspot cycle. Sunspots can be seen to
progressively move towards the Sun's equator at the sunspot maximum. Magnetic poles have been measured
to reverse or flip at each maximum of the sunspot cycle. Each second pole reversal marks the start
of a new magnetic cycle.
The Solar Dynamo
Solar scientists believe that the origin of the Sun's magnetism is at the tachocline. Circulating solar
plasma (charged particles) give rise to a solar current, with consequent induced magnetic fields. This area
of study is called the "The Solar Dynamo" and has been well presented by
Joseph Werne
from North West Research Associates, and NASA's
Solar Physics at
Marshall Space Flight Center.
References:
- "
The Solar Cycle
," David H. Hathaway. Living Rev. Solar Phys., 7, (2010), 1
(pdf)
- "The Solar Dynamo",
S. M. Tobias, Philosophical Transactions of the Royal Society, London, A, pp 2741-2756, 2002.
(pdf)
- "Solar Rotation",
Dragan Roša and Darije Maricic, Zagreb Astronomical Observatory.
- "Pictures of the Solar Atmosphere",
Online Astronomy eText: The Sun.
The copyright of the article The Origin of Magnetism of the Sun: Observations and Understanding of Solar Magnetism and Phenomena is owned by Harry P. Schlanger. Permission to republish in print or online must be granted by the author in writing.
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