Abstract

The EUV Imaging Spectrometer observes the Sun in the 170-292 Angstrom wavelength range, which is dominated by coronal emission lines that are mainly due to iron ions. Valuable science can still be obtained from transition region ions formed in the 0.1-0.8 MK temperature range, and some recent results are presented here.

Mg VII and Si VII (formed at 0.6 MK) produce a number of lines that are widely used for diagnostics of coronal loops. However, the standard reference wavelengths for the ions’ lines are clearly discrepant with the wavelengths measured from EIS , leading to erroneous Doppler velocity measurements. I demonstrate how EIS (which does not have an absolute wavelength calibration) can be used to obtain new reference wavelengths for the lines in combination with Astrophysical infrared measurements. The new wavelengths for Mg VII and Si VII will be added to the CHIANTI database. In addition, the EIS data show that the two ions are formed at almost exactly the same temperature and the Mg/Si relative abundance is enhanced over photospheric values by about 50%.

The coolest ion observed by EIS (other than He II) is O IV , formed at 0.1 MK. It has a rich spectrum in the EIS wavelength range with more than 20 emission lines, however most of the lines are so weak they cannot be measured in normal solar conditions. A flare spectrum from 9 March 2012 is presented where the lines can be measured, and new diagnostics are presented that yield an O IV temperature of log(T/K)=5.10 and a density of log(Ne/cm^-3)=12.55 for the flare ribbon. The results are interpreted in terms of ionization and flare models.