Active Region Electron Density and Dimensions from Fe XVII X-Ray Lines

The Fe XVII 2p⁶¹S₀-2p⁵3d¹P₁ X-ray line at 15.015 Å is resonance-scattered in solar active region and flare spectra, as has been deduced by comparing theoretical intensities of lines in the Fe XVII 2p⁶-2p⁵3l (l = s, d) arrays with intensities observed by the flat crystal spectrometer (FCS) on the Solar Maximum Mission. We show that the amount of resonance scattering is strongly dependent on the heliocentric distance of the emitting plasma, disk regions showing much larger scattering than limb regions. Average values for electron density N_e and path length l can be derived for various heliocentric distances using the X-ray line intensities. We find N_e ≈ 10⁹ cm^-3 and l ≈ 100,000 km (region on the disk) to N_e ≈ 10¹¹ cm^-3 and l ≈ 1000 km (region on the limb) for active region AR 4787, as it rotated across the solar disk to the limb. These lengths appear to be consistent with FCS images. We note, from our findings, that the approximate extent of stellar X-ray active regions could be determined, if high-resolution Fe XVII or equivalent observations were available.