SDO HMI-AIA Joint Science Operations Center (JSOC)
There are two types of Carrington synoptic maps. One is from the near-real-time (NRT) 720-s line-of-sight magnetograms; the other from the definitive 720-s line-of-sight magnetograms. The series names for radial field are hmi.Mrsynop_720s_nrt (NRT) and hmi.Mrsynop_720s (Definitive), and for line-of-sight field hmi.Mlossynop_720s_nrt (NRT) and hmi.Mlossynop_720s (Definitive). The size of map is 3600x1440 with longitude in degree in x-axis and Sin(latitude) in y-axis. The resolution is 0.1 degrees in longitude.
Smaller size maps are also produced by applied a box-car averaging over the large-size maps. The size of the small maps is 720x360. The series names are hmi.Mrsynop_small_720s_nrt, hmi.Mrsynop_small_720s, hmi.Mlossynop_small_720s_nrt, and hmi.Mlossynop_small_720s. The resolution is 0.5 degrees in longitude.First, HMI 720-s line-of-sight magnetograms are remapped to a Carrington coordinate grid. Each point in the grid has been adjusted to the time of its central meridian crossing in order to minimize additional smearing when carrying out averaging attribute to the differential rotation, as suggested by Ulrich et al. (2002). Then the line-of-sight field is converted to radial field by divided by cosine of the center-to-limb angle (Alternatively, this line-of-sight to radial fields converting can be carried out on the original full-disk line-of-sight magnetograms first before doing remapping. A detailed discussion between these two approaches is given in next paragraph). Initially, the remapped grid retains the spatial resolution of the disk-center pixel, i.e., 0.03 degrees. It reduces to 0.1 degrees by convolving a two-dimensional Gaussian function. The width of the Gaussian is three pixels. The field strength measured at each synoptic grid point is averaged from all of the contributing remapped magnetograms. Currently the average is done with the measurements from the twenty 720-s magnetograms that contribute pixels observed closest to the central meridian. Outliers are excluded if their values depart the median by more than three times of rms. If it happens, pixels next closest to the central meridian are added. The effective temporal width of the HMI synoptic-map contribution is about four hours at each Carrington longitude, i.e. within two hours of central meridian passage. This means that the pixels used are roughly within ± 1.2 degrees of central meridian. It is the procedure for producing Carrington synoptic charts of radial field. The Carrington synoptic charts of line-of-sight field are derived from the radial field charts by multiplying cosine of the latitude for each grid pixel.
Radial field Br in remapped images can be remapped directly from Br data on the solar disk (converted from line-of-sight field Blos) or remapped from Blos on the solar disk and then converted to Br. A cubic convolution is involved in this remapping process that is applied to Br in the former case and to Blos in the latter case. Would these two approaches produce the same (or similar) Br? An analysis is carried out and reported here in detail. The conclusion is that the difference of Br in the remapped maps from these two methods is ignorable, and it increases toward the limb. At a location of 84.3 degrees from the disk center, the difference is less than 0.1% on average.