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Magnetosphere & Solar Protons

The magnetosphere

The magnetosphere is the region around the Earth where the influence of the Earth's magnetic field is to be felt. This magnetic field emanates from the magnetic north and south poles of the Earth, forming loops of magnetic force that are, away from the poles, high up beyond the earth's atmosphere. These magnetic force lines can strongly effect the distribution of charged particle radiation in the environment of the Earth, and in part they provide some degree of protection from the cosmic-rays and solar particles impacting the Earth from space. The Earth's magnetic field is also responsible for Van Allen Belts.

At low altitudes, the Earth's geomagnetic field is similar in distribution to a simple bar magnet - it has the form of an electromagnetic dipole. The points where the axis of the dipole cuts through the surface of the Earth are the Earth's magnetic poles and these are offset from the physical poles, or geo-centre, of the Earth by more than 300 km. The geomagnetic axis is also tilted by approximately 11° with respect to the geographical axis - the north pole being at approximately 78.5° N, 291° E).

The actual form of the geomagnetic field is composed of three distinct components, each associated with a different type of naturally-induced electric current that flows within the Earth. These currents are too small to be measured directly, but are large enough to generate measurable geo-magnetic fields. The core field is thought to be due to the convective motion of conducting (ferrous) liquid in the Earth's interior. The crustal field is associated with the distribution of ferromagnetic material near to the Earth's surface and can be detected as magnetic anomalies associated with geological features. Both these comprise the main or internal field which represents approximately 99% of the field strength at the Earth's surface. The remaining 1% is due to the external field, which results partly from the solar-wind (a plasma originating from the Sun and "blowing" out into the solar system).

The core geomagnetic field is not constant, but changes slowly over time. Geophysical evidence, particularly from observed development of the ocean-ridges, shows that the geomagnetic field has reversed many times in the past. For example there have been at least nine major polarity changes of the Earth's magnetic field over the last 3.6 million years. The last reversal took place 730,000 years ago, and currently the field is decaying (7% over the last 160 years). At this rate, the next reversal will probably occur within 2,000 years of present. In contrast, the external component of the Earth's magnetic field is very variable depending largely on solar activity, and changes on all time scales from milliseconds to the 22-years of the solar cycle.

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Inner Van Allen belt
 

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