Pending change in SDO/JSOC metadata

some action needed by users

As of 30 September 2020

We will soon make a change in the calculation of Carrington and Stonyhurst longitude of the solar disk center as seen by SDO. This change effects both HMI and AIA data. This change corrects a long-standing error of about 0.0819222 degrees in the keyword CRLN_OBS and up to 0.015 degrees errors in Stonyhurst longitude which was not provided beyond level-1 data. If you use SDO HMI or AIA data and use the CRLN_OBS keyword, please read the following.

You may need to make software changes to accommodate the keyword changes which will be implemented on or shortly after 1 November 2020. The c. 0.08 degree change in the solar longitude of disk center as seen from SDO amounts to about 2.7 HMI pixels and 2.3 AIA pixels at disk center. If you use the CRLN_OBS FITS or DRMS keywords to select regions to track as the Sun rotates you will see a jump of 2.7 or 2.3 pixels in data for the time that the correction is made. If you use the AIA Stonyhurst HGLN_OBS keyword, it will change from always 0.0 degrees to correct value with a 24-hour period with amplitude about 0.015 degrees, a bit less than half a pixel. For HMI and AIA an appropriate calibration version keyword will contain an indication of the change.

More details are available (in draft version) at SDO_as_Observer_Location

JsocWiki: ObserverLocationCorrection (last edited 2020-10-01 08:24:35 by PhilScherrer)