What causes natural electromagnetic fields?

[stochastic]

What event creates and sustains natural electromagnetic fields? And what do they mean when they say a photon mediates the electromagnetic field?

 

[Lucretius]

Well, I just learned in my EM class that what sustains the EM wave is fact that changing electric and magnetic fields continually produce one another; that is; an electric field that changes produces a changing magnetic field, and vice versa.

 

[ranger]

Natural electromagnetic fields are electromagnetic fields originating mainly from lightning discharges in the Earth-ionosphere cavity. At some distance from their source, these fields propagate as a plane wave with respect to the horizontally directed vector of the magnetic field. There are two components of the field each having different properties.

The first component of natural electromagnetic fields is caused by global thunder activity. The signal produced by electromagnetic fields of this source are relatively stable. A record of this signal received in a manner that is known using a coil as a sensor appears to be random noise. It has almost a homogeneous spectral density in the range between 8 and 500 Hz. Horizontal components of this signal have spectral density of 0.05-0.15 nV/(Hz .sup.1/2.times.m.sup.2). The signal can change gradually several times a day depending on such factors as time of day, time of year and geographical location depending on changes in the conditions of the conductive layers in the ionosphere. Generally the worst time for measurements is winter midday because of the relatively low ambient electromagnetic fields.
The second component of natural electromagnetic fields is caused by local thunderstorms generally happening within a radius 700-1000 km. This signal is more intermittent and appears on a known sensor as separate pulses of oscillations of 1-5 ms each. The spectrum of the field is most intensive in the 2000 to 20000 Hz range. Its intensity can change significantly in a matter of hours.

System, method and computer product geological surveying utilizing natural electromagnetic fields; United States Patent 20030094952
http://www.freepatentsonline.com/20030094952.htm

As for the latter part of your question, I believe that it simply means that mediates the fundamental force between matter particles. Photon - EM waves, graviton - gravitational force. Mayby someone else can give a more thorough intro to this.

 

[waht]

And what do they mean when they say a photon mediates the electromagnetic field?

That means a photon is a force carrier for electromagnetic fields, also known as a virtual particle or virtual photon.

Similarly a graviton would be a force carrier for a gravitational field, a gluon for strong nuclear force, all were never detected. But for weak nuclear force a mediator W Bosons were detected.

 

[damgo]

When you quantize the electromagnetic field, the resulting particle is the photon. At first you deal with electromagnetism as a classical field theory, where the electric and magnetic fields obey Maxwell's equations. You can derive Maxwell's equations from a standard Langrangian or Hamiltonian. We can quantize this system, in very roughly the same manner we'd quantize eg the harmonic oscillator or square well, and the resulting theory has photons in it.

The language "mediated" comes from quantum field theory. In most cases, we can view electromagnetic interactions between particles as caused by the exchange of virtual photons.