ACTIVE-REGION ELECTRON-DENSITY AND DIMENSIONS FROM FE-XVII X-RAY-LINES

The Fe XVII 2p(6) S-1(0)-2p(5)3d P-1(1), X-ray line at 15.015 Angstrom is resonance-scattered in solar active region and Rare spectra, as ha s been deduced by comparing theoretical intensities of lines in the Fe XVII 2p(6)-2p(5)3l (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 muc h larger scattering than limb regions. Average values for electron den sity N-e and path length l can be derived for various heliocentric dis tances using the X-ray line intensities. We find N-e approximate to 10 (9) cm(-3) and l approximate to 100,000 km (region on the disk) to N-e approximate to 10(11) cm(-3) and l approximate to 1000 km (region on the limb) for active region AR 4787, as it rotated across the solar di sk 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.