Full-Disk Disambiguated Vector Magnetic Field from HMI
The hmi.B_720s Data Series
HMI full-disk disambiguated vector field observations available in the data series hmi.B_720s have been computed routinely 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 only as a definitive data product, thus the data 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 HMI vector 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. Two newly added full-disk images give the results from the disambiguation process in disambig and conf_disambig. Disambiguated vector magnetic field observations from HMI 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 indicating the applied algorithm is contained 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' is off the 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 different 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 nearly radial is selected. For the random solution, whether the azimuth should be flipped is decided randomly.
The result from the disambiguation determination 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 or 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, the user must pick one of the three bits in the disambig image. Where a pixel bit has value=1, 180 degrees 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. Field is the total field strength; inclination is the angle of the field relative to the line of sight; azimuth is the ccw angle between 'up' on the image and the transverse field before disambiguation; and the bits in disambig indicate whether the azimuth should be increased by 180 degrees.
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 using other geometry keywords. Headers provide an explanation of usage and examples.
These IDL 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 have been 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 the earlier data where all pixels were annealed and the weaker pixels smoothed). The default algorithm for the weak field is radial-acute in the SHARP series. 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