Tag Archives: magnetism

The Kp Index

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The Kp index is a way of quantifying the level of geomagnetic activity, and the chance of observing the aurora borealis. The higher the Kp index the higher the chance of observing the aurora, and the further south the aurora may be visible.

kpmap

Source

At Kp = 5 the aurora can be seen from the very north of mainland Scotland, and at Kp = 7 it can be seen in London (assuming, in both cases, ideal observing conditions).

The Kp index is calculated every three hours by taking the average of the K-index as measured at thirteen different measuring stations. The K-index is a logarithmic scale that measures disturbances of the Earth’s magnetic field caused by solar activity, but it is adjusted so that the regularity of occurrences of each K-index value is the same at each station: that is, the frequency of Kp = 3 events is the same at Lerwick in Scotland as it is at Witteveen in the Netherlands, even though Lerwick is far more northern than Witteveen.

aurora-australis

The disturbances in the Earth’s magnetic field that the K-index measures are important because it is these disturbances that push the particles into the atmosphere, where they ionise the particles there, causing the emission of light that make up aurorae.

Three different types of magnetism

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When you think of magnetism the chances are that you’re only thinking of one type of magnetism: ferromagnetism. But there are two other types of magnetism: paramagnetism and diamagnetism, that are less well known.

Ferromagnetism is the only type of magnetism that produces forces large enough to be easily felt, and ferromagnetic materials are the only ones that demonstrate spontaneous magnetism – magnetism outside of an applied magnetic field. The most common ferromagnetic materials are those that contain iron, cobalt and nickel but other elements such as dysprosium and gadolinium and compounds such as chromium oxide and manganese bismide also demonstrate ferromagnetic properties.

Paramagnetic and diamagnetic effects only exist in the presence of an applied magnetic field: paramagnetic materials such as tungsten and aluminium create an attractive force when exposed to magnetic fields and diamagnetic materials such as pyrolytic carbon and mercury create a repulsive one.

A small sheet of pyrolytic carbon levitates above an array of neodymium-iron-boron magnets.

Water is weakly diamagnetic, about forty times less diamagnetic than the pyrolytic carbon shown above, but this is enough that light objects which contain a large amount of water can be levitated if placed in a very strong magnetic field.

This frog was levitated using a 16 tesla Bitter electromagnet at the High Field Magnetic Laboratory at the Radboud University Nijmegen in the Netherlands.