Solar Events of Special Interest
Information about data available for special events is available here.
Transit of Venus, 5 June 2012 and Transit of Mercury, 9 May 2016
Both SDO/HMI and SDO/AIA observed the transits with special onboard observing sequences. A set of full resolution image cutouts were made showing Venus ingression, egression, tracking across the solar disk, and stationary views of the Sun for the entire transit in 16:9 image format as well as whole Sun frames for several products. These images and the FITS files cutout from the full resolution images are all available. There were 11 "events" processed:
- HMI Continuum at 6173 Angstroms
- HMI Continuum with limb darkening removed
- AIA 1700 Angstrom
- AIA 1600 Angstrom
- AIA 304 Angstrom
- AIA 171 Angstrom
- AIA 131 Angstrom
- AIA 335 Angstrom
- AIA 211 Angstrom
- AIA 193 Angstrom
- AIA 94 Angstrom
The HMI special data was obtained by tuning the "Vector Camera", Camera #1, to the continuum at a 3.75s cadence. For the Venus transit the polarization was alternated between 4 states, basically I +- U and I +- Q Stokes parameters so the full linear polarization state could be obtained. For the Mercury transit a single linear polarization state is used. The "Line-of-Sight Camera", Camera #2, was operated in its normal 45s sequence throughout. Only the special sequence data was prepared for viewing during the transit since the LOS sequence takes image frames spanning 90s for each calculation of an "Observable" during which time Venus moved about 9 pixels. The HMI continuum intensity, "HMI_Ic," product is just the hmi.lev1_nrt near real time image sequence for Camera1. The limb-darkening removed product, "HMI_Ic_noLD", was made by multiplying the observed HMI_Ic data by the fit to limb darkening made from a pre-transit test of the onboard sequence made for 2 hours on 24 May 2012. The coefficients for the LD removal are in the include.csh files (see below). The special sequence was run for about an hour prior to first contact and about the same time after.
The AIA special data was obtained by running a special observing sequence for all 4 AIA cameras with an interval of darks first, then about an hour of 12s cadence of "normal" sequences but with no 4500A data and with no lossy compression. the effect of using only lossless compression meant that only about 60% of the disk could be fit into the AIA telemetry allocation. Each 96s a normal full disk set of images was obtained. All the EUV lines thus has sets of 6 partial images followed by one full image. the two UV lines alternated between 1600 and 1700with a result of 3 of each followed by the full size compressed image of 1700 only. This sequence was used for the start and end of the transit. In the middle AIA switched to a high-cadence sequence with only some EUV lines available but at a 6s cadence. There was a plan to also obtain a camera-cropped 3s cadence but that data was not obtained as planned. During the entire transit the 96s set of full disk images of all lines (but 4500) was preserved.
The AIA data was from the aia.lev1_nrt2 JSOC data series which means it had darks removed and the standard flat field correction. It did not have the "level-2" PSF correction which removes the diffraction pattern of the filter grids. This diffraction pattern is familiar to many as the diagonal patterns of repeated bright spots around flares. In the Transit case, this pattern brought some signal from adjacent areas onto the black Venus shadow giving an impression of a semi-transparent Venus. Post processing can remove this effect and may be done for some of the data products.
The directories (at this time) located at http://jsoc.stanford.edu/data/events/ contain not only the images but also the FITS files and the scripts used to generate the cutouts and the images. These scripts are csh scripts with an include.csh file "sourced" for each type of data to set the start and stop times, the cadence, the scaling, X and Y locations (arcsec) for the center of the cutout box, color table, min and max scaled values for the color table mapping, etc. The HMI limb darkening coefficients are also found there.