We know that space has an above zero temperature so an empty volume of space must contain some massless energy that could be magnetic. As free space has magnetic permeability then E=1/2*L*I^2 per metre cube.

We know that space has an above zero temperature so an empty volume of space must contain some massless energy that could be magnetic.

 

If it were empty, then it couldn't have a temperature.

 

What is "massless energy"?

 

What is the connection between "massless energy" and magnetism?

The temperature is that of the photons comprising the cosmic microwave background.

The temperature is that of the photons comprising the cosmic microwave background.

 

Ah, good point. I was thinking of the temperature of the interstellar medium (which is much warmer).

We know that space has an above zero temperature so an empty volume of space must contain some massless energy that could be magnetic. As free space has magnetic permeability then E=1/2*L*I^2 per metre cube.

This has been measured, the universe on large scales is magnetically neutral

Energy creates gravitation too, so "massless" is a dangerous idea.

 

If the temperature of said vacuum is for instance that of a black body, and say at 2.7K, then the microwave power passing through a surface is known (as well as the energy contained in a volume). Better than 0.5*LI² which doesn't apply easily here (the currents are polarization rather than conduction), the Poynting vector gives a mean value for the electric and magnetic fields.

 

1m² of black body at 2.7K radiates 3µW and receives as much from its surroundings. This, and vacuum's 377 ohm impedance, makes an average E=34mV/m spread over hundreds of GHz and H=90µA/m, B=0.1nT.

Enthalpy,

A splendid answer at last and sensible attempt at the maths. Does your calculation take into account that the voltage is at right angles to the polarized current field.

BTW I once calculated the solar wind voltage to be 500 volts per metre from the average speed at which the H+ ions must travel from sun to earth; so your E=34mV would seem reasonable.