摘要：The intensity of the O iv 2s2 2p 2P–2s2p24P and S iv 3 s2 3p 2P–3s 3p24 P intercombination lines around
1400 Å observed with the Interface Region Imaging Spectrograph (IRIS) provide a useful
tool to diagnose the electron number density (Ne) in the solar transition region
plasma. We measure the electron number density in a variety of solar features observed by
IRIS, including an active region (AR) loop, plage and brightening, and the ribbon of the
22-June-2015 M 6.5 class flare. By using the emissivity ratios of O iv and
S iv lines, we find that our observations are consistent with the emitting
plasma being near isothermal (logT[K] ≈ 5) and iso-density (Ne ≈ 1010.6 cm-3) in the AR loop. Moreover,
high electron number densities (Ne ≈ 1013 cm-3) are obtained during the
impulsive phase of the flare by using the S iv line ratio. We note that the
S iv lines provide a higher range of density sensitivity than the O iv
lines. Finally, we investigate the effects of high densities (Ne ≳
1011 cm-3) on the ionization balance. In particular, the
fractional ion abundances are found to be shifted towards lower temperatures for high
densities compared to the low density case. We also explored the effects of a
non-Maxwellian electron distribution on our diagnostic method.
关键词：Sun: transition region;Sun: UV radiation;techniques: spectroscopic;atomic data
