The Sun is the most
prominent feature in our solar system. For most of the time that
humans have been on earth, the sun has been regarded as a celestial object of special
significance. Aside from its calenderical or positional importance, the sun is a unique
source for the study of stellar phenomena. Since the last half century, the hottest topic
of
discussion among the world’s top physicists has been the elusive and the enigmatic
magnetic field of the sun. A number of theories have been advanced to explain the wealth
of observations that have been accumulated over the years.
The question arises as to how the scientists have been able to make these scientific
observations. The answer lies in the advancement of technology which has enabled the
scientists to make new or improved observational instruments. The instruments of
particular importance include the following:
The sun photographed using various techniques.
1-Spectroheliograph
This measures the spectrum of individual solar features and
was invented as far back as 1889 by the American
Astronomer George Hale. In conjunction with a telescope, it
photographs the sun in monochromatic light using the
spectrum.
2-Coronograph
This permits the study of solar corona without an eclipse. It
was invented by the French Astronomer, Bernard Lyot in 1931.
The solar corona during an eclipse.
3-Magnetograph
Magnetograph was invented by the American Astronomer
Horace W. Babcock in 1948. It measures the magnetic field
strength over the solar surface.
Besides these , space instruments have also revolutionised the study of the sun. Special
spectrographs , telescopes and coronographs sensitive to extreme ultraviolet radiation
and to X-rays have been developed. The Hubble space telescope is also providing better and
breathtaking images of the sun. Besides, more powerful ground telescopes are
being developed and sophisticated technological innovations, such as Lasers, are being
introduced to minimise the distorting atmospheric effects.
SCIENTIFIC DISCOVERIES ABOUT THE SUN’S MAGNETISM
The facts that scientists have uncovered about the sun are awe-inspiring. One of the
earliest findings was the discovery of numerous extended regions on the solar disk ,
where fields could be detected with intensities ranging from a fraction of a guass to
several tens of guass or more. A great majority of the magnetic regions are bipolar and
are divided into two parts of opposite polarity. The magnetic lines of force are
envisioned as forming arches between them that loop upwards into the solar atmosphere;
with the feet of the arches anchored in two parts of the bipolar region.
A rare type of solar magnetic area is the so- called uni-polar magnetic region. Very
little
is known about these regions but scientists assume that their field lines extend upwards
radially from the surface to great distances and eventually return to the sun diffusely in
areas that have not been identified as yet.
Scientists have also detected a widespread weak magnetism that may be termed as the
poloidal field of the sun. An interesting and remarkable fact about this field is that not
only
does its intensity change and its distribution shift, but its polarity reverses.
Various solar phenomena have also been found to have deep links with the sun’s
magnetism e.g. faculae, sunspots, prominances and solar flares. Faculae are bright
luminous hydrogen clouds which form regions where sun spots are about to be formed .
Sunspots are dark depressions on photosphere ( sun’s outer visible layer) with a
typical
temperature of 4000 degrees centigrade. Faculae are related to the bipolar regions
having a maximum field intensity above 20 guass while sunspots are linked to bipolar or
multi-polar regions where line of force are crowded specially close together.
Prominances appear in the corona ( outer part of the sun’s atmosphere). These are
immense clouds of glowing gas that erupt from the upper chromosphere (region above
the photosphere ). They also owe their support to the magnetic fields that arch above
bipolar regions.
Amongst the known solar phenomena, sunspots are extremely interesting. A typical
sunspot has a magnetic field strength of 2500 guass. Sunspots tend to occur in pairs;
with the two spots having magnetic fields that point in opposite directions- one into and
the other out of the sun. The number of spots have been found to be systematically larger
and smaller in alternate 11 year periods. Of sunspot pairs in the sun’s northern
hemisphere , the spot that leads its partner in the direction of rotation has a magnetic
field direction opposite to that of a leading sunspot in the southern hemisphere. As a new
11 year cycle begins, the magnetic field direction of leading sunspots in each
hemisphere reverses. Thus a full solar cycle, including the magnetic field polarity, takes
approximately 22 years. In addition , sunspots on the surface of the sun at any given time
tend to occur at the same latitude in each hemisphere.This moves from about 45 to 5
degrees during the sunspot cycle . The 22 year cycle is not just a property of the
individual sunspots. It reflects deep- seated and long lasting processes in the sun and
therein lies their importance. Interestingly, unipolar regions have so far not been
identified to produce faculae , sunspots or other surface disturbances.
THEORETICAL PROPOSITIONS
From all the evidence accumulated so far , the scientists have been able to construct
theories to explain features and phenomena of the sun mass. It now appears that the
general magnetic field of the sun has two components. One is the poloidal field and the
other is submerged toroidal ( ring-shaped) field running parallel to the equator in the
northern and the southern hemispheres. This toroidal component presumably accounts
for most of the localised surface magnetic effects and their accompanying visible
features as well as for arching lines of force that loop far out above the surface..
We notice a new pair of rings forming in the higher mid-latitudes in the beginning of each
11 year half cycle. These then migrate slowly towards the equator and disappear at the
end of the period. Once the toroidal fields break through the surface, their interaction
with
matter can produce a variety of visible effects. For instance, the interaction between the
strong magnetic field and the material composing the sunspot results in the interference
of the magnetic field with the normal flow of convective energy
Convection currents on the solar surface
transportation from within the sun . A direct consequence is that the material within the
spot is somewhat cooler and less energetic which in turn makes the spot appear as a
dark region i.e. a spot. Similar explanations have been advanced to explain faculae,
prominances and solar flares.
PHYSICAL BASIS OF SUN’S MAGNETISM
The physical conditions that underlie sun’s magnetism are sadly not that well
understood.
The sun’s magnetic field is thought to be produced by fluid motions within and just
below
the convection zone through a two-step process. In the first step, the interior magnetic
field is stretched out and strengthened due to earth’s differential rotation.
Material at the
equator rotates more rapidly than material at higher latitudes. So a magnetic field line
threading through the interior gets wound up around the sun. Superimposed on the
differential rotation is a very slow torsional oscillation. When combined with the
spinning
motion , the effect is to cause a small cyclic change in the rate of rotation. It is
thought that such a change may explain the alternate 11 year sunspot cycle.
The sun’s meridional flow also plays an important role in producing the magnetic
cycle. A pole-ward flow is observed in the surface layers. Material rises up from inside
the sun at the equator and spreads out towards the poles in the surface layers. An
equator-ward
return flow must exist below the surface and this may also contribute to the equator-ward
movement of the sunspots and magnetic activity. However, this theoretical picture is still
a bit fragmented and a lot of important questions still remain unanswered. Scientists are
hopeful of finding an adequate theory as observed data increases day by day.
In conclusion it must be pointed out that the benefits
to be gained from the explanation
of the sun’s magnetism are enormous. Magnetic studies provide clues to the nature of
the turbulent activity in the sun’s interior. Solar winds and sunspots affect the
earth’s
magnetism and the communication systems on earth.. By learning more about them ,
we can anticipate and prepare for their impact. The sun is the ultimate controller of the
world weather and we need to understand the mechanism of the sun in order to
understand and predict its effects on the planet we inhabit. The problem is intricate , to
put it mildly, and is presently engaging some of finest minds in the scientific
community world-wide. |
