The Gist of Science Reporter: October 2016
The Sun is an active star. It undergoes a cycle of 11 years
(solar cycle) which involves changes ill the Sun s activity (including changes
in the levels of solar radiation and ejection of solar material) and appearance
(changes in the number of sunspots, flares, and other manifestations). The
sunspot number is one of the first and most visible signs of the solar activity
About 400 years ago, Italian scientist Galileo observed spots
on the Sun and found that the Sun rotates around its own axis in -25 days. In
1621, he also observed mysterious lights in the night sky, which he termed as
“Aurora” after the Roman Goddess of Light. Solar Wind and Aurora depend on the
sunspot number. In a cycle of 11 years, the sunspot number goes from a minimum
to a maximum. These cycles are now numbered. The present cycle is the 24th.
When the sunspot number becomes very low (close to zero),
these periods are called “Minimum” and after about 5-6 years the sunspot number
becomes large, at times as high as 200 - these periods are called “Maximum”. The
light and total energy fr m the Sun also changes, but this variation is only
The brightest and most widely observed comet of the 20th
century is Comet Hale-Bopp. This was visible to the naked eye for nearly 18
months. Comet Hale-Bopp was discovered on 22nd July 1995. This is termed as the
Great Comet of 1997 when it was brightest and passed perihelion on 1st April
1997. It clearly shows-two tails - one white dust tail formed due the pressure
of light, and another one is blue ion tail. This is formed due to pressure of
solar wind and is separated from the dust tail by the solar magnetic field that
is carried along by the solar wind from the Sun.
In fact, many times the two tails are not separated from each
other. The main source of solar wind is related to the fusion reaction in the
core of the Sun. ill this reaction, four hydrogen nuclei fuse to form one helium
nucleus and produce a large amount of energy. This energy comes out to the outer
parts of the Sun and heats up the solar corona to a temperature of -one million
centigrade. The hydrogen present there gets ionized to form plasma. Some of this
plasma gets ejected from the corona and continuously flows out in the
interplanetary medium with very high speed in the range of 300-800 km/ s. This
flow is called solar wind.
Solar wind also carries with it solar magnetic field and high
energy charge particles. The energy and number of charged particles as well as
speed of solar wind vary with time depending on various activities of the Sun
(e.g., sunspot number, solar flares, etc.). This solar wind and associated
charged particles as well as magnetic field interact with all the solar system
bodies (e.g., interaction with comets to produce tail/s). The interaction with
the magnetosphere of the Earth is very interesting. The solar wind compresses
the day-side magnetic field of the Earth and expands night-side magnetic field.
Some of the outer magnetic field lines of the day-side break
and flow towards the night-side forming a Polar cusp wherin from solar wind
particles could enter into the Earth’s atmosphere. Disconnected field lines get
connected far away in the night-side and push part of the solar wind and high
energy charged particles into the Earth’s environment, forming a large plasma
sheet in the night-side magnetosphere. The energetic charged particles interact
with molecules and atoms in the upper atmosphere (100-300 km/s) of the Earth in
the oval- shaped region (called Auroral Oval). This interaction produces
mysterious lights in the night sky, which are termed as” Aurora”.
Solar wind and charged particles can enter only near the
North and South magnetic poles of the Earth. The Aurora is formed and seen only
in these regions (Polar Regions beyond 60° North and South). The Aurora of
northern hemisphere is called Borealis and that of the southern hemisphere is
called Australis. The large solar activity on the Sun produces more energetic
charged particles, which usually trigger a very large geomagnetic storm. During
such periods energetic particles penetrate into the Earth’s atmosphere well
below 60° of latitude. ill these rare events Aurora has been observed in lower
latitudes on the Earth as well.
There are three colours in which Aurora is produced - been, Blue and Red. These
are produced by the interaction of charged particles with molecules and atoms of
Oxygen and Nitrogen. Of these, Wheatish Green colour is the most commonly
observed, which is produced by molecular oxygen. Sometimes yellow and pink
colour Aurora is seen, which is formed by a combination of three colours - red,
green and blue.
These lights have been seen for a long time with naked eye observations. Now,
with the advent of electronic detectors and Satellites, the Aurora has been
observed and recorded in other parts of the electromagnetic spectrum i.e. in
X-ray, UV, Infrared and Radio.
There are cosmic rays whose origin is still largely unknown.
These are in the form of extremely large energetic particles. If these cosmic
rays could reach us directly, it would be a disaster. Solar winds and their
magnetic fields interact with these cosmic rays. The interaction converts them
into low energy particles in the interplanetary medium. The low energy cosmic
rays get largely absorbed by our upper atmosphere and hence do not cause any
adverse effect on us. Thus, solar wind also shield us from the harmful high
energy cosmic rays.