Magnetic fields

Until 1821 magnetism was known only from natural magnets, the so-called lodestones. In this year the Danisch scientist Christian Oersted discovered that an electric current also generates a magnetic field. In France, Andrè Marie Ampére laid the foundations for the electromagnetic theory. He realized that the magnetic field is a force between charged particles. James Clark Maxwell later formulated the equations for the electromagnetic field. Until today, these are one of the corner stones of modern physics.

Already in clasical Greece rare rocks that attracted iron were known from a place called `Magnesia'. These were probably iron ores that were struck by lightning and made magnetic by its electric current, the so-called lodestones. Around the year 1000 the Chinese discovered that freely suspended magnetized steel needles oriented themselves in a North-South direction. The magnetic compass had been invented.

An aid in visualizing a magnetic field is due to Michael Faraday, known for many fundamental experimental findings about magnetic and electric fields. He imagined a compass needle that can rotate freely in all three directions. If you always follow the direction indicated by such a needle, your path will trace out one of the so-called field lines of a magnetic field.

Just as an electric current, that is, a flow of electric charges, causes magnetic field, a magnetic field exerts a force on moving charges. In this way a current is induced in a coil that moves in a magnetic field. In this way, a bicycle dynamo produces the current for a light bulb. Under suitable conditions this interaction between current and magnetic field can cause an intially weak magnetic field to grow. In this way power stations generate electricity, and cosmic dynamos also operate on this principle. The image on the left shows how the differential rotation of the Sun amplifies a magnetic field by stretching of the field lines.

The Northern light is caused by reconnection processes in the Earth's magnetotail. Not all the light produced in an aurora can be seen with the naked eye, like the blue light, for example , that is produced by Nitrogen ions at 150 km altitude. Green light from Oxygen at 100 km dominates over the red light from Oxygen in higher layers and the pink light from Nitrogen molecules in lower layers.

Auroras in the north and south can be nearly mirror images of each other. Such mirroring had been suspected for centuries but dramatically confirmed only last month by detailed images from NASA's orbiting Polar spacecraft. Pictured on the left, a time-lapse movie shows simultaneous changes in aurora borealis, at the top, and aurora australis, at the bottom. A cloud of electrons and ions moving out from the Sun on October 22, 2001 created the auroras. The solar explosion that released the particles occurred about three days earlier.

Many galaxies, like 3C219 shown here, have an active galactic nucleus with a massive black hole in the center. The black hole swallows gas and stars in its neigbourhood. Two collimated gas streams form, which can propagate at a velocity very close to the speed of light. These jets extend, perpendicular to the plane of the galaxy, several hundred thousand light years into intergalactic space, visible by their intensive radio emission. The propagation of astrophysical jets is studied at the MPA by means of computer simulations. Gas enters through a nozzle the computational domain, which is homogeneously filled with a denser gas. The velocity of the injected gas is six times the speed of sound. Because of the supersonic propagation of the jet a bow shock forms. Some gas in the beam is deflected sideways at the head of the jet and accumulates in the turbulent coccoon, which encloses the beam. Shock waves and expansion waves within the beam are the cause for the remarkable collimation of the jet.