A geomagnetic storm is a temporary disturbance of the Earth’s magnetosphere caused by a disturbance in the interplanetary medium. A geomagnetic storm is a major component of space weather and provides the input for many other components of space weather. A geomagnetic storm is caused by a solar wind shock wave and/or cloud of magnetic field which interacts with the Earth’s magnetic field. The increase in the solar wind pressure initially compresses the magnetosphere and the solar wind magnetic field will interact with the Earth’s magnetic field and transfer an increased amount of energy into the magnetosphere. Both interactions cause an increase in movement of plasma through the magnetosphere (driven by increased electric field inside the magnetosphere) and an increase in electric current in the magnetosphere create magnetic force which pushes out the boundary between the magnetosphere and the solar wind. The disturbance in the interplanetary medium which drives the geomagnetic storm may be due to a solar coronal mass ejection (CME) or a high speed stream (co-rotating interaction region or CIR) of the solar wind originating from a region of weak magnetic field on the Sun’s surface. The frequency of geomagnetic storms is increases and decreases with the sunspot cycle. CME drive storms are more common during the maximum of the solar cycle and CIR driven storms are more common during the minimum of the solar cycle.
There are several space weather phenomena which tend to be associated with a geomagnetic storm or are caused by a geomagnetic storm. These include: Solar Energetic Particle (SEP) events, geo-magnetically incluced current (GIC), ionospheric disturbances which cause radio and radar scintillation, disruption of navigation by magnetic compass and auroral displays at much lower latitudes than normal. In 1989, a geomagnetic storm energized ground induced currents, which disrupted electric power distribution throughout most of Quebec province and caused aurora as far south as Texas.
In 1931, Chapman and others wrote in their article, a New Theory of Magnetic Storms, which sought to explain the phenomenon of geomagnetic storms [Chapman, et al. 1930]. They argued that whenever the Sun emits a solar flare it will also emit a plasma cloud. This plasma will travel at a velocity such that it reaches Earth within 113 days. The cloud will then compress the Earth’s magnetic field and thus increase this magnetic field at the Earth’s surface [Ferraro, 1933]. A geomagnetic storm is defined [Gonzalez, et al. 1994] by changes in the Dst [Tsurutani, et al. 2003] (disturbance-storm time) index. The Dst index estimates the globally averaged change of the horizontal component of the Earth’s magnetic field at the magnetic equator based on measurements from a few magnetometer stations. Dst is computed once per hour and reported in near-real-time. During quiet times, Dst is between +20 and -20 nano-Tesla (nT).
A geomagnetic storm has three phases [Gosling J.T., et al. 1987]: an initial phase, a main phase and a recovery phase. The initial phase is characterized by Dst (or its one-minute component SYM-H) increasing by 20 to 50 nT in tens of minutes. The initial phase is also referred to as a storm sudden commencement (SSC). However, not all geomagnetic storms have an initial phase and not all sudden increase in Dst or SYM-H are followed by a geomagnetic storm. The main phase of a geomagnetic storm is defined by Dst decreasing to less than -50 nT. The selection of -50 and approximately -600 nT. During of the main phase is typically between 2 and 8 hours. The recovery phase is the period when Dst changes from its minimum value to its quiet time value. The period of the recovery phase may be as 8 hours or as long as 7 days.
Unlike magnetic storms, substorms are magnetic disturbances occurring in a limited region of the earth, near the geomagnetic poles. The most obvious effect of a substorm is the occurrence of auroral displays near the north and south magnetic poles (Fig. 2.11). As was discovered in the last century, aurora are most frequent not at the magnetic poles themselves but in almost circular zones some 4000 km in diameter centred on the magnetic poles, known as the auroral ovals. The visible emission seen in an aurora occurs 90-130 km above the earth’s surface, and is due to the excitation of atoms in the atmosphere by energetic electrons. The characteristic red and green colours of strong auroral displays arise from spectral lines of neutral oxygen atoms in the earth’s atmosphere, with wavelengths of 630.0 nm (red) and 557.7 nm (green).
Interaction of IMF with Earth Magnetosphere
The solar wind, carrying with it the interplanetary magnetic field, flows out from the sun into the solar system and so encounters planets and other bodies orbiting the sun. How does the wind interact with them, particularly those planets like the earth that have magnetic field of their own?
Measurements from spacecraft and the ground have given us a very detailed picture in the case of the earth, one which can be broadly applied to the other magnetized planets that have been explored by spacecraft in recent years. What happens is that, as the solar wind moves through the lines of the earth’s field, electric currents are induced that alter the field’s configuration, compressing it on the sunward side and causing the wind flow to be diverted, avoiding a region surrounding the earth. The result is a cavity, known as a magnetosphere, having a boundary (the magnetopause) that is nearly spherical on the sunward side but drawn out into a long cylinder, the magnetotail, on the night side (see Fig.2.12). Expressed in units of the earth’s radius (1 RE=6371 km), the distance of the magnetopause from the earth along the sunward direction is 10 RE and the magnetotail’s diameter is 50 RE. Outside the magnetopause, the solar wind continues to flow, but within it the terrestrial magnetic field dominates. The magnetosphere is filled with plasma having a large range of densities and temperatures; the charged particles constituting the plasma, unlike the particles making up the other regions of the earth’s atmosphere, are bound by electromagnetic, i.e. non-gravitational, forces.
Near the earth’s surface, the terrestrial magnetic field is dipole-like, i.e. like that produced by a bar magnet near the earth’s core, and is tilted by about 20º to the earth’s rotation axis. On the sunward side of the magnetosphere, the terrestrial field lines are closed, as are those on the night side out to between 8 and 15 RE from the earth. The density of the plasma filling this, the inner magnetosphere, decreases sharply above a level corresponding to the magnetic field lines that are 4-5 RE distant from the earth along the sun-earth line. Inside this level, the plasmapause, is a region called the plasma-sphere where the gas has a density of 107-108 particles/m3 and originates from the ionosphere, a region of the earth’s atmosphere that is partly ionized by the sun’s radiation. Outside the plasma-pause the density is only106 particles/m3. Much of this plasma, too, is ionospheric, but particles are present with much higher energies that come from the solar wind, particularly via the Polar Regions, or cusps; the reason is that some field lines at the cusps connect with regions very near the magnetopause, allowing the solar wind particles to leak into the magnetosphere. In the magnetotail beyond about 8-15 RE, the field lines are open in their normal configuration and parallel to the earth-sun direction. The magnetotail has a northern lobe with field lines pointing towards earth, a southern lobe with field lines pointing away from earth, and an intermediate region called the plasma sheet where oppositely directed field lines lie close to each other forming a cross-tail current, in the form of a sheet. The plasma sheet eventually converges to a neutral line some 100 RE from the earth. It is the site of various plasma processes giving rise to geomagnetic and auroral phenomena. Because of the tilt of the earth’s magnetic axis to its rotation axis and the tilt of the rotation axis relative to the orbital plane, the plasma sheet is generally north or south of the ecliptic plane by up to 4 RE.
The magnetopause is actually a layer with a thickness of several hundred kilometers rather than a sharp boundary to the magnetosphere. There is slight motion in it according to the state of the solar wind, and in particular there is a rippling motion along the magnetotail boundary. Ahead, or ‘upstream’, of the magnetopause by about 3 RE on the sunward side is a standing shock front, known as the bow shock, whose presence is due to the fact that, relative to the solar-wind flow, the earth and its magnetosphere are moving faster than either the sound or Alfven speed. Its properties, like the magnetopause, depend on the solar wind, in particular the interplanetary magnetic field carried with it: when the direction of this field is almost parallel to the shock, the shock front is thin and well defined, but when nearly perpendicular it is rather wide and ill-defined. Some solar-wind particles incident on the shock are scattered back into a sunward direction. Between the bow shock and the nose of the magnetopause, a region known as the magnetosheath, the solar-wind flow continues at reduced speed, and is somewhat compressed and irregular.
There is a convection of plasma in the magneto tail as a combined result of a strong electric field set up across the magneto tail and magnetic field there. The convicted plasma moves towards the earth, then round it, leaving the magnetosphere on its sunward side. Some of the electron and ions in this convicted plasma become trapped in the field of the inner magnetosphere, following helical paths along the field lines and mirroring near the terrestrial magnetic poles. The motion between one mirror point and the other typically takes a fraction of a second. Owing to the decrease of field strength going away from the earth, there is a drift motion, with electrons drifting eastwards, ions westwards. This separation of charge sets up a current – the- ring current – flowing round the earth. The direction of an electric current is conventionally in the opposite direction to the flow of electrons, so the ring current flows westwards.