Life on earth is sustained within a relatively narrow range of well-defined environmental parameters such as oxygen concentration, atmospheric pressure, temperature, water vapor, and light. The proper conditions are met only within a thin, film-like shell of atmosphere immediately adjacent to the surface of the earth. Deleterious factors such as ionizing radiation from the sun and other sources in space are screened out by charged particles trapped in the outermost reaches of the earth's magnetic field and by the ionized outer layers of the atmosphere itself. For life to be maintained these factors must be kept within their "normal" limits; even minor deviations produce immediate physiological and sensory effects.
There are other factors, which we can only sense with specially designed instruments, that constitute our electromagnetic environment. While the existence of the earth's magnetic field has been known for centuries, only within the past few decades has its true complexity been revealed. Far from being static and unvarying, the magnetic field exhibits variations ranging from catastrophic polarity reversals, in which the north and south poles exchange position, to a small but definite, cyclic variation in its magnitude at a "circadian" (about a day) rate. The interaction between the earth's field and those of the sun and the galaxy impose other cycles with periods ranging from several weeks to centuries. Magnetohydrodynamic factors, arising in part from the resonant cavity formed between the earth's surface and the ionosphere, produce "micropulsations" in the magnetic field at frequencies ranging from 0.01 to 20 hz. Transients (magnetic storms) occur in the total field in response to major solar events such as flares, injecting large numbers of charged particles into the earth's field. Lightning discharges in the atmosphere produce radio-frequency energy in the kilocycle range which propagates along the lines of force of the magnetic field, literally "bouncing" back and forth between the northern and southern hemispheres many times before dying out. A complex electrostatic field exists between the surface of the earth and the ionosphere within which atmospheric atoms are ionized. Large electrical currents flow within the earth itself (telluric currents), as well as within the ionosphere. All of these factors are naturally present, and have been since the formation of the planet. The earth's electromagnetic environment is rich and complex, with its interrelated factors continually varying in a dynamic fashion. Our understanding of it is far from complete.
Long before the existence of the earth's magnetic field was known man had postulated "cosmic" influences that affected all life. Visible astronomical events such as comets, the aurora, and the position of the planets were associated with catastrophes such as plagues. Life was thought to be dominated by unseen forces-generated in the stars. As knowledge of the magnetic and electric fields was acquired, these similarly invisible forces began to be presented as the scientific basis for the older beliefs. Mesmer, for example, in his use of magnetic fields, was actually trying to place the astrological theories of Paracelsus (a truly great physician of the fifteenth century) on an acceptable scientific basis by relating them to magnetism. Variations in the earth's magnetic field were believed to explain the "obvious" influence of the moon on human life and behavior (lunatics, moon madness, etc.). The infamous "ill winds" such as the Foehm and Simoon, producing physiological and psychological changes in humans, were thought to act through their abnormal air ion composition.
It appeared highly unlikely, if not downright impossible, for such weak forces (the earth's normal magnetic field averages half a gauss) to have any effect upon living things, particularly since the generally accepted view of biology in the early years of this century completely excluded such effects. However, as knowledge of the complexities of the earth's fields grew, so did the evidence for some subtle, but extremely important relationships with living organisms. While changes in the natural field do not produce the same kind of immediate, obvious biological effects as changes in the atmospheric oxygen concentration, they do have profound effects upon the most basic functions of life.