| Popis: |
Recent R-matrix calculations of electron impact excitation rates in N-like Si VIII are used to derive theoretical emission line intensity ratios involving 2s22p3–2s2p4 transitions in the 216–320 A wavelength range. A comparison of these with an extensive dataset of solar active region, quiet-Sun, sub-flare and off-limb observations, obtained during rocket flights of the Solar EUV Research Telescope and Spectrograph (SERTS), indicates that the ratio R1= I(216.94 A)/I(319.84 A) may provide a usable electron density diagnostic for coronal plasmas. The ratio involves two lines of comparable intensity, and varies by a factor of about 5 over the useful density range of 108–1011 cm−3. However R2= I(276.85 A)/I(319.84 A) and R3=I(277.05 A)/I(319.84 A) show very poor agreement between theory and observation, due to the severe blending of the 276.85 and 277.05 A lines with Si VII and Mg VII transitions, respectively, making the ratios unsuitable as density diagnostics. The 314.35 A feature of Si VIII also appears to be blended, with the other species contributing around 20% to the total line flux. |
