Wednesday, February 23, 2011
A geomagnetic storm is a temporary disturbance of the Earth's magnetosphere caused by a disturbance in space weather. Associated with solar flares and resultant solar coronal mass ejections (CME), a geomagnetic storm is caused by a solar wind shock wave and/or cloud of magnetic field which typically strikes the Earth's magnetic field 3 days after the event. The solar wind pressure on the magnetosphere and the solar wind magnetic field will increase or decrease depending on the Sun's activity. The solar wind pressure changes modify the electric currents in the ionosphere, and the solar wind's magnetic field interacts with the Earth's magnetic field causing the entire structure to evolve. Magnetic storms usually last 24 to 48 hours, but some may last for many days. In 1989, an electromagnetic storm disrupted power throughout most of Quebec and caused aurorae as far south as Texas.
When magnetic fields move about in the vicinity of a conductor such as a wire, a geomagnetically induced current is produced in the conductor. This happens on a grand scale during geomagnetic storms (the same mechanism also influences telephone and telegraph lines, see above) on all long transmission lines. Power companies which operate long transmission lines (many kilometers in length) are thus subject to damage by this effect. Notably, this chiefly includes operators in China, North America, and Australia; the European grid consists mainly of shorter transmission cables, which are less vulnerable to damage.
The (nearly direct) currents induced in these lines from geomagnetic storms are harmful to electrical transmission equipment, especially generators and transformers — induces core saturation, constraining their performance (as well as tripping various safety devices), and causes coils and cores to heat up. This heat can disable or destroy them, even inducing a chain reaction that can overload and blow transformers throughout a system.
At any given time there are about 2000 thunderstorms around the globe. Producing ~50 lightning events per second, these thunderstorms create the background Schumann resonance signal.
Lightning discharges are considered to be the primary natural source of Schumann resonance excitation; lightning channels behave like huge antennas that radiate electromagnetic energy at frequencies below about 100 kHz. These signals are very weak at large distances from the lightning source, but the Earth–ionosphere waveguide behaves like a resonator at ELF frequencies and amplifies the spectral signals from lightning at the resonance frequencies.
In an ideal cavity, the resonant frequency of the n-th mode fn is determined by the Earth radius a and the speed of light c.
The earth is surrounded by a layer of charged particles called the ionosphere. This layer of active charged particles is able to reflect low frequency electromagnetic radiation in a similar way as that of a mirror which is able to reflect light. Electromagnetic radiation is like radio waves or light. It travels at close to the speed of light, 300,000,000 m/s.
Both the earth's surface and the ionosphere are conductive surfaces able to reflect low frequency electromagnetic radiation. Together they create a resonant cavity. This cavity then is what makes the earth into a "resonant chamber" for electromagnetic radiation.
The earth is approximately 6,371 km in diameter or 40,030 km in circumference. An electromagnetic or radio signal travels at close to the speed of light. It, therefore, takes close to 0.1334 seconds to travel around the earth. This time would correlate with about 7.49 Hz. Notice that this is less than the 7.83 Hz that is commonly quoted. If you take into account that the ionosphere is approximately 300 km up and adjust the radius by 150 km, the calculation results in a round trip taking 0.136 seconds, which correlates to a frequency of approximately 7.32 Hz.
The Earth behaves like an enormous electric circuit. The atmosphere is actually a weak conductor. There is a 'cavity 'defined by the surface of the Earth and the inner edge of the ionosphere 55 kilometers up. At any moment, the total charge residing in this cavity is 500,000 Coulombs. There is a vertical current flow between the ground and the ionosphere of 1 - 3 x 10^-12 Amperes per square meter. The resistance of the atmosphere is 200 Ohms. The voltage potential is 200,000 Volts. There are about 1000 lightning storms at any given moment worldwide. Each produces 0.5 to 1 Ampere and these collectively account for the measured current flow in the Earth's 'electromagnetic' cavity.
The Schumann Resonances are quasi standing wave electromagnetic waves that exist in this cavity. Like waves on a spring, they are not present all the time, but have to be 'excited' to be observed. They seem to be related to electrical activity in the atmosphere, particularly during times of intense lightning activity. They occur at several frequencies between 6 and 50 cycles per second; specifically 7.8, 14, 20, 26, 33, 39 and 45 Hertz, with a daily variation of about +/- 0.5 Hertz. So long as the properties of Earth's electromagnetic cavity remains about the same, these frequencies remain the same. Presumably there is some change due to the solar sunspot cycle as the Earth's ionosphere changes in response to the 11-year cycle of solar activity.
Two factors determine the structure and behavior of the magnetosphere: (1) The internal field of the Earth, and (2) The solar wind.
1. The internal field of the Earth (its "main field") appears to be generated in the Earth's core by a dynamo process, associated with the circulation of liquid metal in the core, driven by internal heat sources. Its major part resembles the field of a bar magnet ("dipole field") inclined by about 10° to the rotation axis of Earth, but more complex parts ("higher harmonics") also exist. The dipole field has an intensity of about 30,000-60,000 nanoteslas (nT) at the Earth's surface, and its intensity diminishes like the inverse of the cube of the distance, i.e. at a distance of R Earth radii it only amounts to 1/8 of the surface field in the same direction. Higher harmonics diminish faster, like higher powers of 1/R, making the dipole field the only important internal source in most of the magnetosphere.
2. The solar wind is a fast outflow of hot plasma from the sun in all directions. Above the sun's equator it typically attains 400 km/s; above the sun's poles, up to twice as much. The flow is powered by the million-degree temperature of the sun's corona, for which no generally accepted explanation exists yet. Its composition resembles that of the Sun—about 95% of the ions are protons, about 4% helium nuclei, with 1% of heavier matter (C, N, O, Ne, Si, Mg...up to Fe) and enough electrons to keep charge neutrality.
Data showing live geomagnetic activity from the sun: