The Spatial Energy Spectrum of Magnetic Fields in Our Galaxy

Interstellar magnetic fields exist over a broad range of spatial scales, extending from large Galactic scales (~10 kpc) down to very small dissipative scales (≪1 pc). In this paper, we use a set of 490 pulsars distributed over roughly one-third of the Galactic disk out to a radius R ≃ 10 kpc (assuming R ☉ = 8.5 kpc) and combine their observed rotation and dispersion measures with their estimated distances to derive the spatial energy spectrum of the Galactic interstellar magnetic field over the scale range 0.5-15 kpc. We obtain a nearly flat spectrum, with a one-dimensional power-law index α = -0.37 ± 0.10 for E B ( k ) = Ck α and an rms field strength of approximately 6 μG over the relevant scales. Our study complements the derivation of the magnetic energy spectrum over the scale range 0.03-100 pc by Minter & Spangler, showing that the magnetic spectrum becomes flatter at larger scales. This observational result is discussed in the framework of current theoretical and numerical models.