The term magnetic energy spectrum can perhaps have more than one meaning in astronomy. One usage is the spatial spectral density (like the power spectrum of the cosmic microwave background, but an energy spectral density) regarding the energy associated with magnetic fields over a sphere.
The sphere could be associated with a planet, i.e., centered on the planet, and encompass a given range of radii, or it could be the celestial sphere, with some similar kind of specification of distance.
A magnetic field's energy (magnetic energy) is essentially the minimal amount of work sufficient to construct the magnetic field. Magnetic energy density is a measure of magnetic energy per unit volume. This is equal to the magnetic pressure, the pressure implied by the internal pressure of plasma compressed by the magnetic field.