SOT quick analysis guide


This document contains some notes about using SOT data. The official SOT analysis guide is held at Lockheed.

Downloading files

SOT files can be downloaded from the Oslo SDC archive. On your computer you should make sure that you have the environment variable $HINODE_DATA defined, pointing to the top-level of your Hinode data directory. For example:

setenv $HINODE_DATA = /user/home/data/hinode

If you choose the gzipped tar file option for the download, then put the file in the $HINODE_DATA directory and unpack it. This will ensure that your SOT data are correctly archived.

For SP data it's best to access the level-2 files from the Lockheed level-2 site.

Reading and prepping

The first thing to do is to query the SOT catalog for the times you are interested in:

IDL> time0='16-feb-2011 06:30'
IDL> time1='16-feb-2011 09:30'
IDL> sot_cat, time0, time1, /level0, cat, files

The particular filters that are required can then be selected with

IDL> ss=sot_umodes(cat,/interactive)

which is a widget-based routine.

The subset of files is then selected and read with:

IDL> cfiles=files[ss]+'.gz'
IDL> read_sot, cfiles, index, data

(Note that the Oslo archive gives the user gzipped files so it's necessary to manually add the gzip file extension.)

The data can be directly prepped using:

IDL> fg_prep, cfiles, -1, index, data

Archiving the level-1 files

After running fg_prep you can store the level-1 files in your data archive with:

IDL> cfiles_out=str_replace(cfiles,'level0','level1])
IDL> for i=0,n-1 do write_sot,index[i],data[*,*,i],outfile=cfiles_out[i]

Note that sot_cat doesn't accept a /level1 keyword, so to read the files you will have to follow the steps described above but use str_replace again to swap 'level0' for 'level1'.

Pointing

The image frames returned by fg_prep should be co-aligned relative to each other, and so extracting a sub-region from the 3D array should yield a smooth movie sequence. I notice, however, that the xcen,ycen values seem to have some jitter in them. So if you try to interpolate the frames based on the xcen,ycen values then you'll actually introduce jitter into the movie. It is best just to extract a specific pixel range from the 3D data-cube to get a smooth movie.

However, the movie will only be steady for the feature being tracked with the correlation tracker. If you're interested in another feature in the field-of-view then you may find a systematic drift with time (an active region with emerging flux is an example where this may occur). There is no specific correction for this in the software at present.

When you use fg_prep, it seems it modifies the pointing information from the coordinates that would be obtained by just doing read_sot.  This is consistent with Appendix C of the SOT analysis guide.

SP data

The best SP data to get are the level-2 files, which have 2D maps of the various quantities. These can be downloaded from the Lockheed level-2 site by cutting-and-pasting the appropriate line into Solarsoft. The level-2 files don't seem to be available at the Oslo site. The level-1 files consist of many individual FITS files for each exposure of the SP scan. If you use sot_cat to search for SP scans, it will return a list of these individual FITS files.

The level-2 file can be read from Solarsoft with:

IDL> read_sotsp, l2file, index, data

Note that the type of each image is given by index.ftype. There doesn't seem to be any special software for plotting the vector field.

Notes

  • The SOT magnetograms have a reversed polarity compared to HMI.

Page maintained by Dr Peter R. Young