DOI: 10.7910/dvn/yuquch
title: SPMG Kitt Peak Vacuum Telescope Corrected Magnetograms

description: &lt;h1&gt;Citation and Acknowledgements&lt;/h1&gt;

Please cite this database (see above), as well as adding the following acknowledgement:

"SPMG Kitt Peak Vacuum Telescope Corrected Magnetograms were downloaded from the solar dynamo dataverse (https://dataverse.harvard.edu/dataverse/solardynamo), maintained by Andrés Muñoz-Jaramillo."

&lt;h1&gt;Main Limitations&lt;/h1&gt;

The main limitations of these data are:

&lt;ul&gt;
 &lt;li&gt;Cadence is one image per day, but it is common to have missing days&lt;/li&gt;
 &lt;li&gt;Some magnetograms may display reading errors (they look like repeated vertical rows/patterns).&lt;/li&gt;
 &lt;li&gt;Some magnetograms may have slight variations due to cloud cover.&lt;/li&gt;
&lt;/ul&gt;

Overall, the data is in great shape, especially compared to the original state. We have used them to create a catalog of active regions &lt;a href="https://doi.org/10.7910/DVN/QEMSZ2"&gt;https://doi.org/10.7910/DVN/QEMSZ2t&lt;/a&gt;
However, it is worth visually inspecting the magnetograms before using them.

&lt;h1&gt;File Structure&lt;/h1&gt;

The database is comprised of one tar.gz file per month, each containing all available magnetograms for that given month. A month without data will not have an associated tar.gz file. &lt;br&gt;
 
Each magnetogram is stored using fits format with header information that includes a new set of parameters related to the correction. They are relatively large files because we have kept the original data untouched. The fits table has 5 layers (across the 3rd dimension):&lt;br&gt;

&lt;pre&gt;
Layer 1: Original untouched data (Mx/cm^2)
Layer 2: 868.8nm wing intensity (intensity units; gives a sense of cloud coverage)
Layer 3: Fully corrected magnetogram (Mx/cm^2)
Layer 4: New X pixel position to transform original into corrected geometry
Layer 5: New Y pixel position to transform original into corrected geometry
&lt;/pre&gt;

&lt;br&gt;

&lt;h1&gt;Description&lt;/h1&gt;

The SPMG instrument operated by doing four scanning passes, each of a quarter if the solar disk, a process that would take a total of ~48 minutes (~12 minutes per scan). These would happen as the Sun moved in the sky. In order to assemble the 4 scans into a single full disk image, it is necessary to fit the angle inclination of the slit with respect to the scan, atmospheric refraction, as well as taking limb information into consideration to deal with offsets in each of the scans. &lt;br&gt; &lt;br&gt; 

Jack Harvey and I (Andrés Muñoz-Jaramillo) worked in 2016 on improve on this process as well as making the identification of the zero point of the magnetograph much more stable. This has resulted in a significantly improvement on the quality of the majority of the SPMG magnetograms. &lt;br&gt;

While we never got around to make a publication documenting these changes, we feel that it is important for us to ensure that these magnetograms are open and available to the public so we are releasing them as they are. If possible, we will document better the process of improvement and will update this description with more information. &lt;br&gt;

Current corrections to the data include:


&lt;pre&gt;
1. Remove the slit curvature.
2. Correct for atmospheric refraction.
3. Find the boundaries of the first two isophotes (circles of constant 868.8nm wing intensity) and find the center of each swath and its offset with respect of a perfectly centered Sun. During this step several things are done to try to remove pathologic deviations from an almost circular Sun. 
4. Find the optimum values that minimize the difference between both isophotes and a circular sun with the size of the ephemeris radius that is perfectly centered in the image array. 
5. Create the corrected image by remaping it using natural neighbor interpolation.
6. Remove pixels outside the ephemeris radius and assign them a NaN value.
7. Intensity adjustment based on previous and next observation (to fix drifting amplitudes on days of strong cloud coverage).
&lt;/pre&gt;