Full-Disk Disambiguated Vector Magnetic Field from HMI
The hmi.B_720s Data Series
HMI full-disk disambiguated vector field observations, hmi.B_720s, are routinely available from 19 December, 2013. As currently planned, full-disk data from early times will be processed for campaign events and upon request (coverage). The full-disk disambiguated data are available in definitive version only, thus becomes available a few days after the observation.
The hmi.B_720s data series mostly inherits from the ME inversion (hmi.ME_720s_fd10), with added keywords and segments (images). For the ME data series, please refer to the pipeline overview paper and the ME inversion paper. The added keywords contain information of the disambiguation module parameters, and will be detailed in a forthcoming pipeline paper. The two added full-disk images are the results from disambiguation process: disambig and conf_disambig.
Algorithm and Disambiguation Solution
The full-disk disambiguation differs from the AR patch version in that only the strong field pixels are subject to the minimum-energy algorithm in order to speed up the processing. Other parameters have been adjusted too to find a balance between quality and speed.
The information of the applied algorithm is retained in the conf_disambig image, where a value of 90 indicates the pixel is above the noise threshold, 60 pixels peripheral to the strong field, 50 weak field, and 0 off disk. The 90 and 60's are "annealed" using the minimum-energy algorithm, whereas the 60 pixels are subsequently smoothed. The 50's are not "annealed". Three algorithms are used instead: potential-acute; radial-acute; random.
For the potential-acute method, a potential field is extrapolated from the vertical field component; the azimuth that makes the field vector more similar to the potential field vector (i.e. a larger dot product) is selected. For the radial-acute method, the azimuth that makes the field more radial is selected. For the random solution, whether the azimuth should be flipped is randomly decided.
The result from the disambiguation is a bit mask containing 0 or 1, where 1 indicates that a 180 degree should be added to the azimuth at that pixel. They are stored in the disambig image. The lowest three bits are set. For weak-field pixels (conf_disambig 50), the three bits correspond to solutions from potential-acute (lower bit), radial-acute (middle), and random (higher) algorithm, respectively. That is, 011 means 1 from potential-acute and radial-acute, 0 from random. For strong field pixels (conf_disambig 90 and 60), the three bits are all 1's or all 0's (7 or 0 as integer).
Note that the azimuth image in hmi.B_720s is NOT DISAMBIGUATED and has a value between 0 to 180. To include the disambiguation solution, one should pick one from the three bits in the disambig image. Where a pixel has value 1, a 180 degree should be added the azimuth.
Using the Data
Generally speaking, one needs to download at least four images to study the vector data: field, inclination, azimuth, and disambig.
We provide two sample SSWIDL modules for data processing: hmi_disambig.pro and hmi_b2ptr.pro. The first combines disambig and azimuth; the second generates Br, Btheta, Bphi, i.e. the field vector components projected on to spherical base unit vector. Headers give explanation of usage and examples.
The modules are not thoroughly tested, so please use with care. We plan to distribute them through SolarSoft later.
Miscellaneous
The full-disk azimuthal disambiguation solutions are now used to produce the definitive SHARP data (hmi.sharp_720s, hmi.sharp_cea_720s) starting from Jan 15, 2014. They can be distinguished from the AMBPATCH keyword (0 for full disk disambiguation, 1 for early data). The default algorithm for the weak field is radial-acute. NRT SHARP data are still disambiguated on the AR patches.
Due to time constraints, some non-optimal quality data are not disambiguated as of now. This averages 1-2 frames per day.