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 since 19 December, 2013. As currently planned, full-disk data from earlier times will be processed only a few times per day and for selected campaign events and upon request (See the coverage maps). The full-disk disambiguated field is available in definitive version only, thus they become available a few days after the observation is made.
The hmi.B_720s data series mostly inherits the quantities from the VFISV ME inversion (hmi.ME_720s_fd10), with added keywords and segments (images) related to disambiguation. For the ME data series, please refer to the pipeline overview paper and the VFISV ME inversion paper. The newly added keywords in hmi.B_720s contain information for 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. Disambiguated data are already available for nearly the entire mission in Space-weather HMI Active Region Patches (SHARPs) as described in the SHARP paper.
Algorithm and Disambiguation Solution
The full-disk disambiguation differs from the SHARP (Space-weather HMI AR patch) version in that only the strong field pixels are subject to the minimum-energy algorithm in order to speed up the processing. Certain other parameters have also been adjusted to find a balance between quality and speed.
The information indicated the applied algorithm is retained in the conf_disambig image, where a value of 90 indicates the pixel is above the noise threshold, 60 indicates pixels peripheral to the strong field, 50s are weak-field pixels, and 0 off disk. The "90" and "60" pixels are "annealed" using the minimum-energy algorithm, and the "60" pixels are subsequently smoothed. The weaker "50" pixels are not "annealed". Three algorithms are used instead: potential-acute; random, and radial-acute.
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 decided randomly.
The result from the disambiguation is a 3-bit mask, where a 1 indicates that 180 degrees should be added to the azimuth at that pixel. The three results are stored in the disambig image. Only the lowest three bits are valid. For weak-field pixels (conf_disambig 50), the three bits correspond to solutions from potential-acute (lower bit), random (middle), and radial-acute (higher) algorithm, respectively. That is, 011 means 0 from radial acute and 1 from potential-acute and randome. For stronger-field pixels (conf_disambig 90 and 60), the three bits are all 1's or all 0's.
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 of the three bits in the disambig image. Where a pixel has value 1, a 180 degree should be added the azimuth. The randomly assigned disambiguation is safest because the radial-acute and potential-acute methods can show systematic large-scale patterns in weak regions.
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 onto spherical coordinate unit vectors. Headers provide an 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 by looking at 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 only in the square HMI AR patches and all pixels are annealed.
Due to time constraints, some higher-noise non-optimal-quality data are not disambiguated as of now. This averages 1-2 frames per day, typically when one or more filtergrams are missing due to calibrations. The QUALITY keyword indicates when data are not optimal.
References
If you use the vector field data, please reference the HMI Vector Field Pipeline paper and the HMI Instrument paper