Post by cjm on Jan 4, 2016 5:12:22 GMT
McCarthy & Rubidge: Earth & Life Struik, (2005), pp 312-315
...
IS THE EARTH’S MAGNETIC SHIELD
IN JEOPARDY?
The Earth is one of several planets that possess a
magnetic field ...,
behaving as if it had a bar magnet inside aligned
more or less along the axis of rotation. The field is
actually very weak; in fact it is several hundred
times weaker than that exerted by a toy horseshoe
magnet. Nevertheless it is very important to us. The
field extends about 60 000 km into space on the
sunlit side of the Earth, and much further on the
dark side. It is believed to arise in the liquid outer
core of the Earth some 3 000 km below the surface
from flowing streams of molten metal.
The Earth is bathed in a constant stream of
high-energy particles emanating from the Sun,
known as the Solar Wind. Many of these particles
are electrically charged; in fact, most consist of
protons (positively charged) and electrons (nega-
tively charged). This stream of particles deforms the
Earth’s magnetic field into the shape of a teardrop,
the tail pointing away from the Sun. A magnetic
field exerts a force on a moving charged particle.
You can demonstrate this by bringing a magnet up
to a TV or computer screen (CRT or cathode-ray
tube); the presence of the magnet causes distortion
of the image on the screen. This happens because
the screen is illuminated by a stream of electrons
fired from an electron gun at the back of the tube:
the magnetic field exerts a force on the electron
stream, altering its path and causing distortion.
The Earth’s magnetic field similarly influences
charged particles in the Solar Wind, deflecting them
away from the Earth. In effect, the Earth's magnetic
field acts as a giant shield. Every now and then, activ-
ity on the surface of the Sun increases dramatically
..., causing a sudden increase in the
intensity of the Solar Wind. This in turn can cause dis-
turbances in the Earth’s magnetic field, so-called
magnetic storms. These are rapid variations in the
strength of the field as it changes shape in response
to gusts in the Solar Wind. Minor disturbances pro-
duce aurorae in the sky at high latitudes. More severe
storms cause disturbances in radio communications,
while very severe storms can cause major disruption.
The worst case on record occurred in March
1989, when a severe magnetic storm cut
off electricity to six million people in
Quebec, Canada, as a result of its effect on
power transmission lines. Such storms can
also cause problems with radio and tele-
phone communications and with radar and
radio navigation systems. But only extreme
magnetic storms have this effect, because
the Earth’s magnetic field shields us.
We know from studies of rocks that the
Earth’s magnetic field has periodically
switched around ...
in events known as magnetic rever-
sals. During these events, the Earth’s mag-
netic field changes direction: the magnetic
North Pole becomes the South Pole, and
vice versa. It is not that the Earth flips
around; rather it is as if the imaginary bar
magnet inside the Earth has flipped around.
If such a reversal were to occur now, the
North Pole of compasses would point south.
The last such reversal occurred about
780 000 years ago and there is growing evi-
dence to suggest we may be on the_brink of
another such reversal.
It has been known for many years that the
strength of the Earth’s magnetic field is not the
same everywhere: in particular, there is a region
over the South Atlantic where the field is very
weak - essentially a hole in the magnetic shield.
According to Pieter Kotze of the Hermanus
Magnetic Observatory in the Western Cape,
charged particles approach Earth much closer in
this region than anywhere else on Earth, resulting
in intense radiation. Although this does not affect
the Earth’s surface, it could potentially cause
severe problems for astronauts and satellites in
low orbits, and as a result it is the subject of
intense study.
The cause of the hole in the magnetic field has
now been pinned down: the Earth’s magnetic field
in a localised patch below the southern Cape and
the adjacent Atlantic has actually reversed, and
has become a north pole. Compasses in the
southern Cape still point in the right direction,
though, because the overall magnetic field of the
Earth is still dominant over this small patch. But
the patch is growing and the strength of the Earth’s
magnetic field at Hermanus has decreased by
20% in the last 60 years ...
World-wide, there has been a steady decrease
in magnetic field strength, but less than that meas-
ured locally. American earth scientist Peter Olsen
reported that at the current rate of decrease, the
Earth's magnetic field would vanish early in the next
millennium. It is thought that the decrease in field
strength may be a reflection of an incipient mag-
netic reversal. It is hypothesised that reversals come
about as patches of reversed magnetism such as that
below the southern Cape form and grow, weaken-
ing the overall field. Ultimater they coalesce and
form a new field, but in the reverse direction. The
time required to form the new field is not known. Of
course, this decrease in field strength may simply be
a natural oscillation in the field, and the field may
recover. We have insufficient data to be able to pre-
dict which way events are likely to turn out.
Effects of a weakened magnetic field
As matters stand, we are faced with the prospect of a
steady weakening of the Earth’s magnetic field, espe-
cially in South Africa. What are the consequences of
this likely to be? We know that there have been many
reversals in the past, and the palaeontological record
tells us that life was not unduly affected - there were
no mass extinctions associated with these reversals.
The atmosphere will in all probability provide some
protection from the solar radiation.
Weakening and disappearance of the field may
result in the loss of the ozone shield. This possibility
arises from observations made during intense solar
activity in October and November 2003. At that
time, an intense flux of electrons from the Solar Wind
was funnelled down into the upper atmosphere over
the North Pole by the Earth’s magnetic field lines. The
electrons ionised nitrogen, which reacted with oxy-
gen to produce nitrous oxides. These in turn
destroyed ozone, resulting in a massive ozone hole
over the northern hemisphere. A globally weakened
magnetic field will allow widespread penetration of
electrons from the Solar Wind into the upper atmos-
phere where nitrous oxide formation may occur,
resulting in extensive loss of the ozone shield.
But loss of the ozone layer and an increase in
solar radiation may be the least of our worries. Ours
is a very technological world, and we have become
dependent on sophisticated electronics. How will
this technology shape up as the protective magnetic
field steadily weakens? Will power grids become
increasingly sensitive to even mild magnetic storms?
How will communications and computers be affect-
ed? Will satellites still be able to operate effectively
over our region? Compasses will be useless for navi-
gation, but will the Global Positioning System (GPS)
still be able to function? How will high-flying aircraft
be affected as the hole in the magnetic field deepens?
These are issues that will need to be addressed
in the near future because of the possible risks to
our way of life. At the same time we need to inten-
sify our research on the magnetic field: study of the
earth's magnetic field, especially in South Africa,
should be on the agenda alongside or even ahead of
global warming and the ozone hole_We need to know
what is going to happen because our very survival may
depend on it. If we understand the dangers we can
take the necessary evasive action.
...
...
IS THE EARTH’S MAGNETIC SHIELD
IN JEOPARDY?
The Earth is one of several planets that possess a
magnetic field ...,
behaving as if it had a bar magnet inside aligned
more or less along the axis of rotation. The field is
actually very weak; in fact it is several hundred
times weaker than that exerted by a toy horseshoe
magnet. Nevertheless it is very important to us. The
field extends about 60 000 km into space on the
sunlit side of the Earth, and much further on the
dark side. It is believed to arise in the liquid outer
core of the Earth some 3 000 km below the surface
from flowing streams of molten metal.
The Earth is bathed in a constant stream of
high-energy particles emanating from the Sun,
known as the Solar Wind. Many of these particles
are electrically charged; in fact, most consist of
protons (positively charged) and electrons (nega-
tively charged). This stream of particles deforms the
Earth’s magnetic field into the shape of a teardrop,
the tail pointing away from the Sun. A magnetic
field exerts a force on a moving charged particle.
You can demonstrate this by bringing a magnet up
to a TV or computer screen (CRT or cathode-ray
tube); the presence of the magnet causes distortion
of the image on the screen. This happens because
the screen is illuminated by a stream of electrons
fired from an electron gun at the back of the tube:
the magnetic field exerts a force on the electron
stream, altering its path and causing distortion.
The Earth’s magnetic field similarly influences
charged particles in the Solar Wind, deflecting them
away from the Earth. In effect, the Earth's magnetic
field acts as a giant shield. Every now and then, activ-
ity on the surface of the Sun increases dramatically
..., causing a sudden increase in the
intensity of the Solar Wind. This in turn can cause dis-
turbances in the Earth’s magnetic field, so-called
magnetic storms. These are rapid variations in the
strength of the field as it changes shape in response
to gusts in the Solar Wind. Minor disturbances pro-
duce aurorae in the sky at high latitudes. More severe
storms cause disturbances in radio communications,
while very severe storms can cause major disruption.
The worst case on record occurred in March
1989, when a severe magnetic storm cut
off electricity to six million people in
Quebec, Canada, as a result of its effect on
power transmission lines. Such storms can
also cause problems with radio and tele-
phone communications and with radar and
radio navigation systems. But only extreme
magnetic storms have this effect, because
the Earth’s magnetic field shields us.
We know from studies of rocks that the
Earth’s magnetic field has periodically
switched around ...
in events known as magnetic rever-
sals. During these events, the Earth’s mag-
netic field changes direction: the magnetic
North Pole becomes the South Pole, and
vice versa. It is not that the Earth flips
around; rather it is as if the imaginary bar
magnet inside the Earth has flipped around.
If such a reversal were to occur now, the
North Pole of compasses would point south.
The last such reversal occurred about
780 000 years ago and there is growing evi-
dence to suggest we may be on the_brink of
another such reversal.
It has been known for many years that the
strength of the Earth’s magnetic field is not the
same everywhere: in particular, there is a region
over the South Atlantic where the field is very
weak - essentially a hole in the magnetic shield.
According to Pieter Kotze of the Hermanus
Magnetic Observatory in the Western Cape,
charged particles approach Earth much closer in
this region than anywhere else on Earth, resulting
in intense radiation. Although this does not affect
the Earth’s surface, it could potentially cause
severe problems for astronauts and satellites in
low orbits, and as a result it is the subject of
intense study.
The cause of the hole in the magnetic field has
now been pinned down: the Earth’s magnetic field
in a localised patch below the southern Cape and
the adjacent Atlantic has actually reversed, and
has become a north pole. Compasses in the
southern Cape still point in the right direction,
though, because the overall magnetic field of the
Earth is still dominant over this small patch. But
the patch is growing and the strength of the Earth’s
magnetic field at Hermanus has decreased by
20% in the last 60 years ...
World-wide, there has been a steady decrease
in magnetic field strength, but less than that meas-
ured locally. American earth scientist Peter Olsen
reported that at the current rate of decrease, the
Earth's magnetic field would vanish early in the next
millennium. It is thought that the decrease in field
strength may be a reflection of an incipient mag-
netic reversal. It is hypothesised that reversals come
about as patches of reversed magnetism such as that
below the southern Cape form and grow, weaken-
ing the overall field. Ultimater they coalesce and
form a new field, but in the reverse direction. The
time required to form the new field is not known. Of
course, this decrease in field strength may simply be
a natural oscillation in the field, and the field may
recover. We have insufficient data to be able to pre-
dict which way events are likely to turn out.
Effects of a weakened magnetic field
As matters stand, we are faced with the prospect of a
steady weakening of the Earth’s magnetic field, espe-
cially in South Africa. What are the consequences of
this likely to be? We know that there have been many
reversals in the past, and the palaeontological record
tells us that life was not unduly affected - there were
no mass extinctions associated with these reversals.
The atmosphere will in all probability provide some
protection from the solar radiation.
Weakening and disappearance of the field may
result in the loss of the ozone shield. This possibility
arises from observations made during intense solar
activity in October and November 2003. At that
time, an intense flux of electrons from the Solar Wind
was funnelled down into the upper atmosphere over
the North Pole by the Earth’s magnetic field lines. The
electrons ionised nitrogen, which reacted with oxy-
gen to produce nitrous oxides. These in turn
destroyed ozone, resulting in a massive ozone hole
over the northern hemisphere. A globally weakened
magnetic field will allow widespread penetration of
electrons from the Solar Wind into the upper atmos-
phere where nitrous oxide formation may occur,
resulting in extensive loss of the ozone shield.
But loss of the ozone layer and an increase in
solar radiation may be the least of our worries. Ours
is a very technological world, and we have become
dependent on sophisticated electronics. How will
this technology shape up as the protective magnetic
field steadily weakens? Will power grids become
increasingly sensitive to even mild magnetic storms?
How will communications and computers be affect-
ed? Will satellites still be able to operate effectively
over our region? Compasses will be useless for navi-
gation, but will the Global Positioning System (GPS)
still be able to function? How will high-flying aircraft
be affected as the hole in the magnetic field deepens?
These are issues that will need to be addressed
in the near future because of the possible risks to
our way of life. At the same time we need to inten-
sify our research on the magnetic field: study of the
earth's magnetic field, especially in South Africa,
should be on the agenda alongside or even ahead of
global warming and the ozone hole_We need to know
what is going to happen because our very survival may
depend on it. If we understand the dangers we can
take the necessary evasive action.
...