Definitive MDI Full-Disk 96-minute Synoptic Magnetic Field Images
Definitive MDI Full-Disk 96-minute Synoptic Magnetic Field Images
Definitive MDI Full-Disk 96-minute Synoptic Magnetic Field Images
Downlinked data are reconstructed as standard FITS files in Level 1.5. Numerous image characteristics and observing parameters are recorded in FITS header keywords. Level 1.8 processing goes on to provide corrections for plate scale, zero offset, sensitivity, and cosmic rays.
Many details can be found at
For questions or requests, contact MDI.
On October 15, 2007 newly calibrated versions of full-disk MDI magnetograms were made available.
The sensitivity of the improved Level 1.8 MDI magnetograms is more spatially uniform. The values reported have increased by a factor of approximately 1.7 when compared with previous MDI magnetic data. Visually the images appear nearly the same. The old uncorrected versions of MDI magnetograms will only be available for a limited time upon special request.
The new data have been corrected for spatially dependent sensitivity variations in the instrument as determined by Tran et al. [ApJS, 2005] in careful comparison with data obtained in several spectral lines at the Mt. Wilson Solar Observatory.
The new data have been corrected for instrumental zero level offset.
Some very severe cosmic ray contamination has been removed.
The known saturation in strong field regions above about 3500 G (2000G old values) has not been corrected. In strong field regions, such as sunspot umbrae, the MDI signal is saturated and actually becomes weaker. This is primarily a consequence of limitations of the on-board processing algorithms.
Newly recalibrated data are now routinely exported. Consolidated files are also available to download at our ftp site. These are 17 3-4Gb tar files. All exports in response to requests for lev1.8 magnetograms have been calibrated with the new process. The old-style magnetograms will be available through the end of 2007 by special request.
The spatially dependent MDI sensitivity correction derived from comparison with Mt. Wilson data has been applied to put the values in Gauss. More strictly, MDI is sensitive to the spatially averaged line-of-sight magnetic flux in each pixel, for convenience reported as Gauss. This correction increases the reported field values by a spatially non-uniform factor of about 1.7 relative to all earlier MDI magnetic data sets. The FITS keyword SCR_ADJ2 indicates the sensitivity map used in the calibration. This keyword did not exist in previous versions of Lev1.8.
These data have already been corrected for the uniform zero offset introduced by the MDI shutter noise by removing BFITZORG from all pixels. Note that as always your FITS reader must properly use the FITS keywords BSCALE and BZERO.
A few obvious cosmic rays have been removed; SPIKECNT gives the number of pixels above the SPIKETHReshold. Because a cosmic ray contaminates only one of many filtergrams that contribute to the magnetogram, most cosmic ray contamination cannot be simply and automatically detected in a single magnetogram.
These lev1.8 images have a time-dependent image scale correction applied as indicated in the keywords: FD_SCALE, IM_SCALE, XSCALE, YSCALE, R_SUN. The keyword FDSCALEQ indicates the quality of the correction: 0 means no correction was done, 1 means a correction using extrapolated coefficients was done and more up-to-date coefficients may result in a better correction, and 2 means the correction was done with the final set of coefficients. See the file scale_corrections for more details.
Of course the magnetic field at each pole is still observed only during the part of the year it is visible. Optional polar field corrections to the synoptic charts will be made available.
A new version of synoptic map showing the presumed radial field will also be produced.