Gaussian fitting for IRIS (iris_auto_fit)
One or more Gaussians can be automatically fit to IRIS rasters
with the IDL routine IRIS_AUTO_FIT, available in Solarsoft. This page describes how
the routine is used and how the results can be viewed with
IRIS_AUTO_FIT is a wrapper to the routine EIS_AUTO_FIT that
was developed for the Hinode/EIS mission. For full details on how that
routine works and the options that are available, please check EIS
Software Note No. 16 and the examples in EIS
Software Note No. 17.
IMPORTANT: you will need to have the EIS Solarsoft directory
on your computer for IRIS_AUTO_FIT to work.
Calling iris_auto_fit for IRIS
The call is:
IDL> iris_auto_fit, windata, fitdata [,
The optional template and
wvl_select structures are created with eis_fit_template and
eis_wvl_select (see EIS
Software Note No. 16).
Viewing the results graphically
The fits can be viewed using the following GUI:
IDL> iris_fit_viewer, windata, fitdata
Again this is a wrapper to the EIS routine eis_fit_viewer. The
routine allows you to browse the fit results across the raster and
study individual profiles.
Extracting line parameters
To extract the intensity, line-of-sight velocity or width from
the FITDATA structure, use:
IDL> int=eis_get_fitdata(fitdata, /int)
IDL> vel=eis_get_fitdata(fitdata, /vel)
IDL> wid=eis_get_fitdata(fitdata, /wid)
You can give the /map keyword to put the images into an IDL
When working out the velocity, the routine uses the tag
FITDATA.REFWVL as the reference wavelength against which Doppler shifts
are determined. This tag is set to be the average centroid of the line
over the raster. One can manually set this to the reference wavelength
of the line (see my line list,
for example), however this assumes that the IRIS wavelength scale is
best approach is to measure the centroid of a very cool
line close to the line of interest as these lines generally have very
small velocities (< 1 km/s). Examples are S I 1401.51 (near Si
IV 1402.77) and O I 1355.60 (near Fe XXI 1354.08). You don't need to
measure these lines at every pixel in the raster, as you simply need
one measurement for an exposure. For example, you can sum all pixels
along the slit and measure the cool line centroid from this one
Suppose you measure the S I centroid, and it is +0.01 Å
compared to the reference wavelength. We can then manually set the
REFWVL value for Si IV 1402.77 to be:
IDL> fitdata.refwvl=1402.770 + 0.01
(1402.770 is the reference wavelength for the Si IV line) and
then the velocity map obtained with EIS_GET_FITDATA will be derived
using this value.
The WINDATA structure
IRIS_AUTO_FIT takes as input a WINDATA structure. A key point
windata structure is that it contains an error array for the
intensities, which is needed for Gaussian fitting (see Young et al.
2015 for a description of how the errors are computed). To create
windata structure, one does:
where wvl is the wavelength you're interested in (e.g., 1402.7
Si IV). The /CALIB keyword is recommended as it outputs intensities
(and errors) in calibrated units of erg cm-2 s-1
sr-1 pixel-1. If you do not use this keyword,
then the intensities are in units of DN pixel-1.
There are a set of routines for manipulating the windata
which are described in EIS
Software Note No. 21. For example one can
spatially bin data using EIS_BIN_WINDATA, or one can reduce the size of
arrays using EIS_TRIM_WINDATA.
IRIS_GET_WINDATA can run slowly for large files - see the tips
given below for how to speed up the routine.
The routine EIS_FIT_TEMPLATE is a GUI-based routine for
template for the fit. The user simply specifies the location and height
of the emission line (or lines) by clicking on a spectrum. The spectrum
is obtained by selecting a region in the displayed image. See EIS
Software Note No. 16 for more details.
The template is essential if you're attempting to doing
multi-Gaussian fitting. I also recommend it even if you're just doing a
EIS_FIT_TEMPLATE automatically detects if an IRIS spectrum has
input, and it adjusts the parameters that go into the template
accordingly. If you inspect the code you will see the lines:
Where width gives the initial value for the line's Gaussian
dcen gives the range over which the centroid can vary (i.e., +/- dcen
angstroms from the initial centroid), and wid_lim gives the allowed
range for the Gaussian width. These values enter into template.lines,
so you can manually edit this for different cases. For example, the
width of O I 1355.6 will never reach 0.55, so you can set a smaller
value to prevent silly fits. I set the parameters in order to
handle the very broad Si IV lines that are sometimes seen.
If the fits reach the parameter limits, then IRIS_AUTO_FIT
will let you know (see "Checking parameter limits..." text in output).
If many pixels are hitting the limits then you should manually change
the limits in the template.
Huge file problems
are two types of huge file for IRIS: (1) large rasters that take the
full-CCD spectra, and (2) long-duration sit-and-stare studies. Creating
the windata structures for these can take minutes, and so I give tips
below for speeding up this process.
If you are only interested in a sub-region of the spectrum,
then you can do:
IDL> wd=iris_getwindata(file, 1401, wrange=[1398,1407])
which will extract only the wavelength region 1398-1407
If there is a specific time period that you are interested in,
then note the pixels that correspond to this period (using
iris_raster_browser is a good way of doing this, as it shows the
X-pixel number in the top-left corner of the GUI). If the pixels are
[x0,x1], then do:
IDL> wd=iris_getwindata(file, 1401, ixrange=[x0,x1])
which will only extract this time range into the windata structure.
(Note that this can also be used to select a sub-range in the
X-direction for raster data.)
Fit the Si IV 1402.77 line across an active region raster.
IDL> files=iris_find_file('12-oct-2013 21:00')
IDL> eis_fit_template,wdx,template ; --> select the
Si IV with this widget
IDL> iris_auto_fit, wdx, fit, template=template
The fitting should take about 1-2 minutes. To view the results
IDL> iris_fit_viewer, wdx, fit
To extract the intensity array as an IDL map and plot, do:
Check the value of fit.refwvl (should be 1402.800), and try
to the Si IV rest wavelength of 1402.770. See how the velocity map
changes when you call iris_fit_viewer. This shows the transition region
is predominantly red-shifted. A more accurate wavelength calibration
needs to be done by using a cool line as reference (see above).
Page maintained by Dr P.R.