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EIS data analysis guide#
This guide describes how to access, calibrate and derive scientific results from the EIS instrument on Hinode. For a description of EIS and its observing modes please consult the instrument paper, Culhane et al. (2007)
, and the MSSL Science Centre webpage.
Tutorials#
The sections below describe individual steps in going from finding data to deriving scientific results. For new users tutorials are provided that show how to process example data-sets and derive results.
Tutorial for narrow slit data (1", 2" slits)
Tutorial for slot data (40", 266" slits)
Searching and downloading EIS data#
Each country contributing to the Hinode mission provides access to the data through web portals. Each one offers different capabilities and so it is recommended that you try each to familiarize yourself with the methods for searching for data. The five data access points are given below and we also list an IDL-based tool for accessing data.
Hinode Science Data Centre (Europe)
Hinode Data Centre, DARTS (Japan)
Virtual Solar Observatory (US)
The large quantity of EIS data coupled with the large number of free parameters (raster size, wavelength windows, slit choice) means that it can be quite difficult to find the perfect data-set if you don't already know which study you want. Check out Tips for finding the perfect data-set to get some hints on searching for data.
Calibrating EIS data#
Calibration of EIS data is performed with a single IDL routine, EIS_PREP, which converts a level-0 FITS file (raw data) to a level-1 FITS file (calibrated data). Note that the data archives listed above do not distribute level-1 files, so the user needs to perform the calibration. It is highly recommended that the user perform calibration before browsing the data as detector artifacts called warm pixels make browsing the raw data difficult.
The standard call for eis_prep is:
IDL> eis_prep, filename, /quiet, /retain, /default, /save
an explanation of these keywords and a description of the calibration steps performed by eis_prep are available in EIS Software Note No. 1.
Taking a quick-look at your data#
For browsing a data-set there are several 'quicklook' tools available that can be accessed by first calling a single IDL widget called XFILES:
IDL> xfiles
The widget allows you to choose an EIS file by manually typing in the path to the EIS file. Make sure to change 'eis_l0_*' to 'eis_l1_*' in the 'Set search filter' box to look for level-1 files rather than level-0 files. After confirming your selection a new widget called XCONTROL pops up which allows the user to access the five quicklook tools:
Control - find basic info on your data, run eis_prep, access QL tools.
Detector - displays detector images.
Browser - allows user to browse the 3D data cubes (wavelength-X-Y) obtained from narrow slit observations.
Spectroheliogram - for a specified line shows a solar-Y vs. wavelength plot for each exposure.
Whisker - for a specified line shows a solar-X vs. wavelength plot for each exposure.
Intensity map - for a specified line shows an intensity image.
EIS objects and structures#
A complete data-set is read into IDL as an object as follows:
IDL> data=obj_new('eis_data',filename)
where 'filename' is the name of the level-1 file (a level-0 file can be read in the same way). The contents of the object can be read and manipulated through a large number of methods and an overview of some of the most useful methods is given below.
The routine EIS_GETWINDATA is also available if you want to extract a single EIS data window into an IDL structure. Some routines (e.g., EIS_AUTO_FIT) require as input the structures produced by EIS_GETWINDATA.
Narrow slit data (1", 2" slits)#
The 1" and 2" slits return emission line spectra and a basic task for such data-sets is to fit Gaussians to the emission lines to return intensity, centroid and width information. Gaussian fitting routines available to EIS users are described below. The routines correct for most instrumental effects, but users should familiarize themselves with some of these effects through the software notes listed below:
- Warm and hot pixels (Software Notes 6 and 13)
- Orbital drift of line centroids (Software Note 5)
- EIS slit tilts (Software Note 4)
- EIS grating tilt (Software Note 3)
Gaussian fitting routines#
Fitting a Gaussian function to an emission line yields measures of the line intensity, centroid (velocity) and width. Single Gaussian fits to individual lines are possible using EIS object methods, while more sophisticated fitting options are available through the EIS_AUTO_FIT and SPEC_GAUSS_EIS routines available in Solarsoft. Documents describing these routines and giving examples are listed below.
Gaussian fitting for the Hinode/EIS mission
Gaussian fitting examples using eis_auto_fit
Users should be aware that EIS line profiles can often take on non-Gaussian shapes which may contain valuable scientific information (e.g., Chifor et al. 2008, De Pontieu et al. 2009).
Line ratio maps and density diagnostics#
Some emission line ratios are sensitive to the electron density, and software is available to take the line fit structures output by EIS_AUTO_FIT and convert them to electron density maps. The density then allows the emitting column depth of the plasma to be determined which in turn allows the filling factor of the plasma to be estimated. The document below describes how this done, making use of atomic data from the CHIANTI database.
Deriving densities from Hinode/EIS data
Emission measure#
When line intensities are available from several different ions, then an emission measure (EM) or differential emission measure (DEM) distribution can be constructed. No specific EIS software is available for doing this and so users are referred to software in the CHIANTI database, in particular:
integral_calc.pro: computes an EM value from a single line intensity
chianti_dem.pro: computes a DEM distribution from multiple line intensities
Another popular DEM code is available within the PINTofALE software package.
Slot data (40", 266" slits)#
The EIS slots produce images rather than spectra, and the most common application will be to produce movies from the slot rasters in a range of emission lines. Diagnostics are possible with the slots, however, by taking ratios of images and temperature and density diagnostics are described below.
Making movies#
Temperature diagnostics#
Density diagnostics#
Pointing and co-alignment#
Most EIS users will be analyzing data-sets from multiple instruments and so it is necessary to determine where an EIS raster is positioned relative to the other instruments. Usually the pointing information within an EIS file will not be good enough to accurately co-align data-sets and so cross-correlation will be necessary.
- Obtaining pointing information from EIS data
- Creating an IDL map from EIS data
- Cross-correlating EIS and SOT images
- Cross-correlating EIS and XRT images
EIS Software Notes#
A series of Software Notes have been written that give details about various technical aspects related to the EIS mission.
Software Note No. 1 - eis_prep
Software Note No. 2 - Radiometric calibration
Software Note No. 3 - Grating/Detector tilt
Software Note No. 4 - Slit tilts
Software Note No. 5 - Orbital variation of EIS wavelength scale
Software Note No. 6 - Hot and Warm pixels
Software Note No. 7 - Variation of line width with slit position
Software Note No. 8 - EIS Point Spread function (coma)
Software Note No. 9 - EIS Pointing
Software Note No. 10 - Dark Current
Software Note No. 11 - JPEG Compression
Software Note No. 12 - Stray Light
Software Note No. 13 - Interpolation of missing EIS data
Software Note No. 14 - The EIS Bright Point Trigger