Data description

(last modified 2-Nov-2010)

Summary of the current release

The YLA includes the following four levels of data products for the Yohkoh/SXT:

The total number of our main products (Level_1, 2, and 3) are summarized here. The Level 1 and 2 data products are in instrument units (DN/sec/pixel). Conversion to physical units requires knowledge or assumption about the solar differential emission measure and the use of software such as, e.g., The Level 3 images are scaled so that the solar structures are best viewed and the scaling varies with the Sun's activity level. Therefore, the Level 3 images best serve for browsing, and not suitable for the quantitative analysis.

Except Level 0 (raw) products, we have created SXT data products in both Yohkoh-specific (XDA) and FITS formats. The XDA format is convenient for analysis with existing SSW/Yohkoh software that runs under IDL. The more general FITS format enables use of advanced SSW applications including the SSW mapping software by Zarro et. al. FITS products do not specifically require IDL to read and view images.

To help users searching/downloading the YLA products, we have the several web services in our web site.

Level 0, 1, and 2 data products are also available via Virtual Solar Observatory. For Level 3 images, the SXT movie maker service is available to create the customized movies of the composite FFIs, of your own!

Cautions and disclaimers:
1.  Exposures comprising Level 2 and Level_3 composite images may be taken 
  up to +/- 11 minutes apart. Beware of solar changes between exposures.  
  For study of specific events or periods it is advisable to examine the 
  Level 1 data which often include many images that, for one reason or 
  another, didn't make it to level 2.
2.  Correction for CCD saturation in Level 2 products is not perfect in
  many highly saturated images.  Flare and eclipse images are not well
  corrected because of caution 0.
3.  There are uncorrected problems with the YOHKOH attitude database late
  in the mission, particularly in 2001.
4.  After 14-Nov-1992 straylight correction of the thin Aluminum images
  isn't perfect.  This has a minor effect for the analysis of bright
  features but causes intensity uncertainty of faint regions -- which is not
  reflected in the uncertainty arrays.  The best looking movies will be made
  from the AlMg filter, half-resolution, images.

Data details and links

Level_0 data

They are raw data files in the Yohkoh format, readable with a SolarSoft procedure. They were taken for each satellite orbit, and organized in the weekly directories.
An example of the file name: sfr920720.1502.
This file is included in the weekly direcotry, 1992_30/.
      sfr: A prefix for SXT Full Frame (whole sun) Images.  
      other prefixes --- spr : SXT Partial Frame Images
                         hda : HXT data
                         bda : BCS data
                         wda : WBS data
                         ada, cba : for calibration purpose. 
      920720.1502 : date and approximate time (yymmdd.hhmm) at which 
                    the satellite started observation in the orbit.

Entrance to the Level_0 data directories.

Level_1 data

Level 1 data include full frame images (FFI) and partial frame images (PFI) in floating point format. The Level 1 products include every usable solar image, even if some of the frame may be missing due to the telemetry dropout. All calibration images and detected problematic frames are eliminated. Basic calibration such as dark and leak(due to filter failure) corrections is done. The 'low-8bit images' taken with the optical filter are restored as far as the compressed counterparts are available. The observing regions taken in partial frame images are assembled to show the whole region of interest.

Level_2 data

SXT composite images combine 2 or 3 long and short exposures to increase dynamic range and remove detector saturation. Each composite image has a related uncertainty image which provides the statistical error for each pixel. The Level 2 data only use complete or nearly-complete images. The Level 2 images are not co-aligned, but may be readily co-aligned with existing SSW mapping tools.

Level_3 data

The SXT level-3, or movie, images are prepared from level-2 composite full-frame-images for convenience in viewing in movie mode to facilitate the study of coronal evolution and dynamic phenomena. This processing included co-alignment, rebinning 256x256 images to 512x512, logarithnmic and byte scaling, adjustment of effective exposure to minimize movie flicker and enhance coronal intensity around solar minimum, and deletion of images unduly contaminated by radiation-belt particles or straylight. As a result, level-3 SXT images sacrifice photometric accuracy for improved time-lapse viewing. They are not intended for quantitiative analysis.

The coaligned whole-sun white-light(WL) images are also categorized as level_3 images. They are obtained with the SXT Aspect Sensor, the optical telescope co-aligned with the SXT, and was in operation until 13-Nov-1992 when SXT had the entrance filter failure. The data have 12-bit or 8-bit intensity depth and archived in floating point. The images are co-aligned and have been cleaned to eliminate obvious hot pixels. Because the Aspect Sensor exponentially lost sensitivity after launch and because radiation damage to the CCD from excessive x-ray exposure left dark artifacts on the WL images, the WL images are not photometrically useful.

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