The Spatial Energy Spectrum of Magnetic Fields in Our Galaxy

Interstellar magnetic fields exist over a broad range of spatial scales,extending from the large Galactic scales ($\sim 10$ kpc) down to the very smalldissipative scales ($\ll 1$ pc). In this paper, we use a set of 490 pulsarsdistributed over roughly one third of the Galactic disk out to a radius $R\simeq 10$ kpc (assuming $R_\odot = 8.5$ kpc) and combine their observedrotation and dispersion measures with their estimated distances to derive thespatial energy spectrum of the Galactic interstellar magnetic field over thescale range $0.5 - 15$ kpc. We obtain a nearly flat spectrum, with a 1Dpower-law index $\alpha=-0.37\pm0.10$ for $E_{\rm B}(k)=C k^{\alpha}$ and anrms field strength of approximately $6 \mu$G over the relevant scales. Ourstudy complements the derivation of the magnetic energy spectrum over the scalerange $0.03 - 100$ pc by \citet{ms96b}, showing that the magnetic spectrumbecomes flatter at larger scales. This observational result is discussed in theframework of current theoretical and numerical models.Comment: 7 pages, 6 figures, ApJ accepte