Earths magnetic field
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Electrical properties of Ionosphere and Earths Magnetic field
By Rohit Choudhury B.Tech EXTC C016
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Earths Magnetic Field
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Introduction• Earth's magnetic field, also
known as the geomagnetic field, is the magnetic field that extends from the Earth's interior to where it meets the solar wind, a stream of charged particles emanating from the Sun.
• Its magnitude at the Earth's surface ranges from 25 to 65 microteslas (0.25 to 0.65 gauss).
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• Roughly speaking it is the field of a magnetic dipole currently tilted at an angle of about 11.5 degrees with respect to Earth's rotational axis, as if there were a bar magnet placed at that angle at the center of the Earth.
• Unlike a bar magnet, however, Earth's magnetic field changes over time because it is generated by a geodynamo (in Earth's case, the motion of molten iron alloys in its outer core).
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Importance• Earth's magnetic field serves
to deflect most of the solar wind, whose charged particles would otherwise strip away the ozone layer that protects the Earth from harmful ultraviolet radiation.
• Humans have used compasses for direction finding since the 11th century A.D. and for navigation since the 12th century.
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• One stripping mechanism is for gas to be caught in bubbles of magnetic field, which are ripped off by solar winds. Calculations of the loss of carbon dioxide from the atmosphere of Mars, resulting from scavenging of ions by the solar wind, indicate that the dissipation of the magnetic field of Mars caused a near-total loss of its atmosphere.
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Characteristics
1. Intensity2. Inclination3. Declination
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Intensity• The intensity of the field is often
measured in gauss (G), but is generally reported in nanoteslas (nT), with 1 G = 100,000 nT.
• The tesla is the SI unit of the Magnetic field, B.
• The field ranges between approximately 25,000 and 65,000 nT (0.25–0.65 G). By comparison, a strong refrigerator magnet has a field of about 100 gauss (0.010 T).
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Inclination• Magnetic Inclination is the angle
made with the horizontal by the Earth's magnetic field lines.
• The inclination is given by an angle that can assume values between -90° (up) to 90° (down). In the northern hemisphere, the field points downwards.
• It is straight down at the North Magnetic Pole and rotates upwards as the latitude decreases until it is horizontal (0°) at the magnetic equator. It continues to rotate upwards until it is straight up at the South Magnetic Pole. Inclination can be measured with a dip circle.
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Declination• Magnetic declination is the
angle on the horizontal plane between magnetic north and true north.
• Declination is positive for an eastward deviation of the field relative to true north.
• It can be estimated by comparing the magnetic north/south heading on a compass with the direction of a celestial pole.
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Electrical properties of Ionosphere
• The ionosphere is the region where plasma density is maximum in the Earth's upper atmosphere.
• Its altitude ranges almost from 60 to 1000 km. It acts not only as the reflection or absorption layer of the radio wave, but as an electric current layer.
• The ionospheric currents cause large part of the variation of the geomagnetic field, although most of the geomagnetic field itself is generated by the dynamo action in the Earth's core.
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Parallel Conductivity• Parallel conductivity is for the direction parallel to the
magnetic field line and denoted as "σ0". • This is same as that when there is no magnetic field,
and much larger than Pedersen and Hall conductivities in the ionosphere.
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Pedersen Conductivity• Pedersen conductivity is for
the direction vertical to the magnetic field and parallel to the electric field. It is denoted as "σ1".
Hall Conductivity• Hall conductivity is for the
direction vertical to both the magnetic and electric fields. It is denoted as "σ2". In the ionosphere this conductivity is due to the drift motion of the electron (ExB drift) and maximum in the E region where only electron practically drifts to the direction of ExB.
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The conductivity depends on various parameters such as location, time, season and Solar activity. For example, the conductivity at night reduces to one several-tenths of that at noon.
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Thank you!