Geomagnetism

Earth's magnetic field

The Earth's magnetic field is generated by electric currents deep in the interior and high above the surface of the planet. The field extends far into space where it encounters the moving plasma of the solar wind. The solar wind flows around the magnetic field, compressing it on the day-side of the planet and stretching it out into a long tail on the night-side.

The geomagnetic field shields the Earth’s surface from the solar wind by deflecting high energy particles emitted by the Sun. During magnetic storms, vast amounts of the Sun’s energy and plasma are dumped into the Earth's upper atmosphere affecting satellites, electricity supplies, radio communication and producing expanded auroral displays.

The Earth's magnetic field is a vector quantity, meaning it has both a magnitude (size) and direction. It can be described by combinations of components or ‘elements’. In geomagnetism the elements most commonly referred to are: X, Y, Z, F, H, D and I.

A compass needle points to magnetic north, which is defined by the direction of the horizontal component of the geomagnetic field (H). The geomagnetic declination (D), sometimes called variation, is the angle between true north and magnetic north.

The inclination (I), sometimes called dip, is the angle the field vector is inclined to the horizontal plane.

Geomagnetic observatories in Australia monitor four of the elements of the geomagnetic field, the true north (X) component, true east (Y) component, vertical (Z) component and the total intensity of the magnetic field (F).

The X, Y, Z, F and H elements of the magnetic field are measured in units of nanotesla (nT). Declination (D) and inclination (I) are measured in angular degrees.

All other elements of the magnetic field can be derived from X, Y and Z. For example, F is calculated using the following equation: F = ⟌X² + Y² + Z²

More information about the components of the magnetic field.

Magnetic storms

Sunspots are areas on the surface of the Sun where the solar magnetic field becomes contorted, preventing the normal flow of heat, resulting in cooler, darker areas. Related to sunspots are flares on the Sun. These are violent explosions usually lasting a few minutes. A solar flare emits vast amounts of high-energy electromagnetic (UV and X-rays) and ionized particle radiation (protons) into the solar system.

When a solar flare is directed towards Earth, the electromagnetic radiation, travelling at the speed of light, arrives in a little over eight minutes, synchronized with the visual observation of the event. Much of the energy of this radiation is absorbed in the ionospheric D-region (around 65km), spontaneously increasing the ionization density there, and resulting in short-wave fadeouts (SWF) and signifying the beginning of a magnetic storm. A small indicator of this type of event is a solar flare effect, immediately detectable during daylight by magnetic observatories.

The particle radiation from a solar flare, travelling at speeds of approximately 1000km per second, takes a day or two to reach Earth. Upon arrival it becomes compressed and generates a shockwave in Earth's magnetic field which is recorded by magnetic observatories around the globe. A sudden step in the magnetic field is observed, followed by large excursions in the field's intensity and orientation. Such magnetic storms can last from a few hours to several days.

During magnetic storms huge electric currents flow in Earth's ionosphere. They may cause inaccuracies in compass readings, radio broadcasts to be received in areas far from where they were intended and auroras to be seen at latitudes much nearer the equator than usual. People may report lost homing pigeons or experience GPS problems and currents may be induced in long conductors such as pipelines and powerlines.

Auroras

Magnetic storms often result in the sighting of auroras, colourful displays that appear in the night sky, at places much nearer to the equator than where they are usually seen. Auroras are commonly seen in areas around Earth's polar regions. They are often referred to as the Southern Lights or Aurora Australis in the Southern Hemisphere, and the Northern Lights or Aurora Borealis in the Northern Hemisphere. Auroras are a dynamic and visually striking manifestation of magnetic storms on Earth.

Auroras happen when charged particles from the Sun enter the magnetosphere. Once inside, the geomagnetic field directs them toward the north and south magnetic poles. Travelling at high speeds the particles collide with gas molecules and atoms in the atmosphere, which energises them. A visible glow appears when they release the energy and return to their ground states, much like the way a fluorescent light works.

When a magnetic storm occurs, the auroral zones expand towards the equator from the polar regions, sometimes providing spectacular displays to residents of mid-latitude regions. During intense magnetic activity auroral displays have been reported from as far north as Queensland.

The different colours seen in auroras are produced by different gases in the atmosphere. At high altitudes, light gases like hydrogen and helium, create blue and violet auroras and high-altitude oxygen (about 320km) is the source of the red emission. At lower altitudes (about 100km) oxygen produces a brilliant yellow-green - the brightest and most common auroral colour. Ionized nitrogen produces blue light and neutral nitrogen produces a red glow (but a different hue to high altitude oxygen). Nitrogen can also create the purplish-red lower borders and rippled edges of the aurora.

Related information

• Calibration of compasses and magnetometers
• Australian Geomagnetism Report
• Geomagnetic data applications