#15.     Energy

Energy can be loosely defined as anything able to turn a machine.

Electricity moves the wheels of both watches and railroad engines, heat turns the steam turbines in power stations, the chemical energy of gasoline runs a car--all these are forms of energy. Light is energy, too--a solar cell converts it to electricity, and green plants convert it to chemical energy.

A grandfather clock is powered by weights descending from a higher level--that is "potential energy" (Thomas Jefferson at his home in Monticello had a 7-day clock run by hanging cannonballs). Speed of motion is "kinetic energy", like the energy of a turning flywheel or of the wind hitting a windmill, both of which can move machinery. And it takes energy to set up a magnetic field--even when the source is an electric current which meets no resistance, like the Earth's ring current.

The Flow of Energy to the Magnetosphere

Energy is the universal currency in which any physical process in nature must be paid for. A fundamental law of physics asserts that it is never lost or gained, only changed from one form to another. Both the fast-moving ions of the outer radiation belt and the fast electrons which produce the aurora contain appreciable kinetic energy, which must be supplied from somewhere.

The ultimate source is the Sun: nuclear processes in its core convert hydrogen to helium and produce heat, which flows to the Sun's surface. Most of the heat of the surface layers is then radiated away as sunlight, the sustainer of life on Earth, but a small part ends up heating the outermost layer of the Sun, the corona, to a million-degree temperature. As the hot gas of the corona--a plasma, really--expands upwards, its heat is converted to the kinetic energy of a fast outwards-moving flow: the solar wind. Four or five days after leaving the Sun, solar wind plasma hits the magnetosphere, an obstacle in its path. As it flows around that obstacle, some kinetic energy is withdrawn, ultimately powering the aurora and a variety of other processes.

The above is the basic chain by which energy reaches the magnetosphere. Many links however remain obscure: Why is the corona so hot? How is solar wind energy transmitted to the magnetosphere? How are particles injected into the inner magnetosphere during magnetic storms? There exist clues, theories and observations, but only partial understanding.

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Last updated March 13, 1999