Geomagnetism
Earth's magnetic field and its foundations
Introduction to the Earth's magnetic field :
• Definition :
•
Earth's magnetic field (also known as
the geomagnetic field.) is the magnetic field that extends from the Earth's inner core to
where it meets the winds, a stream of energetic particles emanating from the Sun.
• Earth's magnetic field (and
the surface magnetic field) is approximately a magnetic dipole, with the magnetic field
S pole near the Earth's geographic north pole (see Magnetic North Pole) and
the other magnetic field N pole near the Earth's geographic south pole (see Magnetic
South Pole). This makes the compass usable for navigation. The cause of
the field can be explained by dynamo theory.
A magnetic field extends infinitely, though it weakens with distance from its source.
The Earth's magnetic field, also called the geomagnetic field, which effectively extends
several tens of thousands of kilometres into space, forms the Earth's magnetosphere.
A paleomagnetic study of Australian red dacite
and pillow basalt has estimated the magnetic field to be at least 3.5 billion years old
Importance :
Earth is largely protected from the solar wind, a stream of energetic
charged particles emanating from the Sun, by its magnetic field, which
deflects most of the charged particles. Some of the charged particles
from the solar wind are trapped in the Van Allen radiation belt. A
smaller number of particles from the solar wind manage to travel, as
though on an electromagnetic energy transmission line, to the Earth's
upper atmosphere and ionosphere in the auroral zones. The only time
the solar wind is observable on the Earth is when it is strong enough
to produce phenomena such as the aurora and geomagnetic storms.
Bright auroras strongly heat the ionosphere, causing its plasma to
expand into the magnetosphere, increasing the size of the
plasma geosphere, and causing escape of atmospheric matter into the
solar wind. Geomagnetic storms result when the pressure of plasmas
contained inside the magnetosphere is sufficiently large to inflate and
thereby distort the geomagnetic field.
The solar wind is responsible for the overall shape of Earth's
magnetosphere, and fluctuations in its speed, density, direction, and
entrained magnetic field strongly affect Earth's local space
environment. For example, the levels of ionizing radiation and radio
interference can vary by factors of hundreds to thousands; and the
shape and location of the magnetopause and bow shock
wave upstream of it can change by several Earth radii,
exposing geosynchronous satellites to the direct solar wind. These
phenomena are collectively called space weather. The mechanism of
atmospheric stripping is caused by gas being caught in bubbles of
magnetic field, which are ripped off by solar winds.[3] Variations in
the magnetic field strength have been correlated to rainfall variation
within the tropics.[4]
Magnetic poles and
magnetic dipole :
•
Geographic North and South poles are where lines of longitude
converge according to GIS Geography(opens in new tab). The
Geographic North Pole is located in the middle of the Arctic Ocean and
the Geographic South Pole is found in Antarctica.
•
Magnetic poles are located where the magnetic lines of attraction
enter Earth. The Magnetic North Pole is also known as the North Dip
Pole and is currently found on Ellesmere Island in Northern Canada.
When a magnetic compass points north it is aligning itself with Earth's
magnetic field and points to the Magnetic North Pole, not the
Geographic North Pole, which is actually about 310 miles (500
kilometers) away according to GIS Geography!
•
And just to make things that little more confusing, what we call the
North Magnetic Pole is actually a south magnetic pole… bear with me
on this. Magnetic field sources are dipolar, meaning they have a north
and south pole. And when it comes to magnets, opposite poles (N and
S) attract while other poles (N and N, S and S) repel. So when a
compass points north, it is actually attracted to the south magnetic
pole which lies close to the Geographic North Pole, according to
Physicist Christopher Baird's science FAQ website(opens in new
tab) "Surprising Questions with Surprising Answers."
•
Unlike the geographic poles, Earth's magnetic poles are not fixed and
tend to wander over time. British polar explorer James Clark Ross first
identified the Magnetic North Pole on the Boothis Peninsula in
Canada's Nunavut territory in 1831, according to the Antarctic travel
site Antarctic Logistics(opens in new tab). Since its discovery, the
magnetic north pole moves about 25 miles (40 kilometers) a year in a
northwest direction according to the Royal Museums
Greenwich(opens in new tab). Whatsmore, Earth's magnetic poles
have also 'flipped' whereby north becomes south and south becomes
north. These magnetic reversals occur at irregular intervals every
200,000 years or so.
HOW DOES THE MAGNETIC FIELD PROTECT EARTH?
• During particularly strong space weather events such as high solar winds or large CMEs, Earth's
magnetic field is disturbed and geomagnetic storms can penetrate the magnetosphere and lead to
widespread radio and power blackouts as well as endangering astronauts and Earth-orbiting
satellites.
• In 1859, a large solar storm known as the Carrington Event caused widespread telegraph system
failures and in 1989, a CME accompanied a solar flare and plunged the entire province of Quebec,
Canada into an electrical blackout that lasted around 12 hours according to a NASA
statement(opens in new tab).
• The degree of magnetic disturbance from a CME depends on the CME's magnetic field and Earth's.
If the CME's magnetic field is aligned with Earth's, pointing from south to north the CME will pass
on by with little effect. However, if the CME is aligned in the opposite direction it can cause
Earth's magnetic field to be reorganized,(opens in new tab) triggering large geomagnetic storms.
• A less destructive and far prettier side effect of magnetosphere disturbances is the aurora above
Earth's polar regions. The phenomenon is known as the northern lights (aurora borealis) in the
Northern Hemisphere and the southern lights (aurora australis) in the Southern Hemisphere.
• The disturbances in Earth's magnetic field funnel ions down towards Earth's poles where they
collide with atoms of oxygen and nitrogen in Earth's atmosphere, creating dazzling aurora light
shows.
•
Components of Earth’s Magnetic Field :
There are three components that
are responsible for the magnitude as
well as the direction of the earth’s
magnetic field:
• Magnetic declination
• Magnetic inclination or the angle of
dip
• Horizontal component of the earth’s
magnetic field
Geographic north
X
Magnetic north
Y
Geographic east
B
Z
down
Magnetic declination :
• he magnetic declination is defined as the angle between the true north and the magnetic north. On the
horizontal plane, the true north is never at a constant position and keeps varying depending upon the
position on the earth’s surface and time.
Magnetic Inclination :
• The magnetic inclination is also known as the angle of dip. It is the angle made by the horizontal plane on
the earth’s surface. At the magnetic equator, the angle of dip is 0°, and at the magnetic poles, the angle of
dip is 90°.
Horizontal Component of the Earth’s Magnetic Field :
• There are two components to explain the intensity of the earth’s magnetic field:
I. Horizontal component (H)
II. Vertical component (v)
• Declination
•
•
D = tan-1 (X/Y)
Inclination
Horizontal
North
East
Intensity
I = tan-1 (Z/H)
H2 = X2+Y2
X = H cos(D)
Y = H sin(D)
F2 = X2+Y2+Z2