The main geomagnetic field, which is slowly varying, originates within the Earth. More rapid variations, with periods from seconds to days, are produced by processes above the Earth. At the top of the atmosphere, solar radiation creates an ionised region called the ionosphere. Above this is the magnetosphere where the Earth's magnetic field acts as a shield against the solar wind, the stream of charged particles coming from the Sun. Electric currents within the ionosphere and magnetosphere produce magnetic fields that are observed at the Earth's surface along with the Earth's own magnetic field.
The sequences of phenomena that give rise to geomagnetic disturbances originate on the Sun. The simplest starts with the electromagnetic radiation given off by the Sun. As well as illuminating and heating the day-side of the earth, this radiation also heats the ionosphere causing convection. The convection moves charged particles through the earth's magnetic field creating a dynamo action that drives ionospheric electric currents above the equator and up to mid latitudes. These currents produce a magnetic field that, viewed from space, appears fixed on the day side of the earth. The rotation of the earth carries a site on the surface in and out of this magnetic field creating a 12-hour variation.
The sun also radiates particles out into space. This solar wind compresses the magnetosphere on the sunward side and draws it out into a comet-like tail on the nightside. Eruptions on the sun produce high-speed streams of particles within the solar wind. These high speed particles further compress the magnetosphere causing a sudden change in the magnetic field observed at the earth's surface. This often heralds the start of a magnetic storm and is called a "storm sudden commencement (SSC)".
The continuing interaction between the solar wind and the magnetosphere increases the number of charged particles trapped within regions of the magnetosphere called the "radiation belts", 25,000 to 40,000 km above the Earth. These particles drift around the Earth creating a ring current that produces a depression of the horizontal magnetic field, seen at lower latitudes around the world as the "main phase" of a magnetic storm. This is followed by the "recovery phase", lasting one day or more, during which the ring current subsides and the magnetic field returns to normal.
During disturbed times, charged particles are guided down the magnetic field lines into the upper atmosphere where they create the aurora borealis (northern lights) and, in the southern hemisphere, the aurora australis (southern lights). Strong electric currents are also generated and flow down to the ionosphere in the auroral zones where intense east-west currents, called the auroral electrojets, are produced. The magnetic disturbances observed at high latitudes are due to the magnetic fields of these auroral electrojets.
Sometimes, sinusoidal wave trains with periods from fractions of a second to a few minutes are seen riding on the magnetic traces. These are magnetic pulsations. In a simplified way, they can be thought of as the "twanging" of magnetic field lines in a manner analogous to the vibrations of a guitar string.