Choosing emission lines for EIS studies

v2.0, Peter Young

Young et al. (2007a) discuss the major lines and density diagnostics seen in the EIS spectrum. Some of this information is summarised below.

Core lines

Currently there are three core lines that must be in every EIS study. These are:

Ca XVII 192.82
Fe XII 195.12
He II 256.32

Transition region lines

The following transition region lines are recommended:

Mg V 276.54
Mg VI 268.99
Mg VII 278.44/280.74
Si VII 275.35

They are all weak lines, but they can become strong in certain conditions, such as in the footpoints of active region loops. Exposure times of at least 20 seconds are recommended.

The Mg VII ratio is a density diagnostic. Note that the 278.44 line is blended with a Si VII line, but the Si VII 275.35 line can be used to correct this, so make sure this line is also included in your raster. See Young et al. (2007b) for more details.

There are a number of weak O IV, V and VI lines throughout the spectrum. The most useful O V line is at 192.9, lying just next to Ca XVII 192.82, and so will automatically be in most studies. The best O VI line is at 184.12.

Coronal lines

Taking a complete set of iron lines is recommended, and the best individual lines from each ion are:

Fe VIII 185.21  (warning: blended with Ni XVI line at high temps.)
Fe IX 197.86
Fe X 184.54
Fe XI 188.23 (actually two Fe XI lines at 188.23 and 188.30)
Fe XII 195.12
Fe XIII 202.04
Fe XIV 274.20
Fe XV 284.16
Fe XVI 262.99
Fe XVII  254.35
Fe XXIII 263.78
Fe XXIV 192.03

The Fe XXIII and Fe XXIV lines can probably be omitted for quiet Sun studies.

One warning for Fe XII: if you pick an exposure time of 30 seconds or more for an active region, you are likely going to saturate the Fe XII 195 line in parts of the raster simply because EIS is so sensitive at this wavelength. You thus may want to choose one of the two weaker Fe XII lines at 192.39 or 193.51 as well as the core 195.12 line in this case.

Recommended density diagnostics

The following diagnostics involve strong lines and have good sensitivity to density:

Fe XIII 203.83/202.04
Fe XIII 196.54/202.04
Fe XII 186.88/195.12

The first of these is the best for most conditions. If the density is expected to get very high (flares) then it is good to also take the 196.54 line which is sensitive to higher densities. These diagnostics are discussed in Young et al. (2009).

Other density diagnostics

Some other useful diagnostics are:

Fe X 257.26/184.54
Si X 258.37/261.04
Fe XI 182.20/188.23
Fe XIV 264.78/274.20

Note that the Fe X 257.26 line lies close to several other lines. A large wavelength window is recommended to allow the continuum level to be assessed properly for this line.


This page mantained by Dr.P.R.Young, last revised on 26-Aug-2013