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Synoptic Charts of the Photospheric Line-of-Sight Magnetic Field

Synoptic charts are maps of the entire Sun produced in Carrington coordinates. Synoptic maps are constructed by merging together solar observations taken over many days. Magnetic-field synoptic charts are produced using central meridian data from HMI full-disk magnetograms.

Daily Update Synoptic Frames use a much wider longitude range of data from the current day to provide a more simultaneous view of the field visible on the disk.

Carrington Maps of the Photospheric Magnetic Field

Synoptic maps are constructed from HMI 720s line-of-sight Magnetograms collected over a 27-day solar rotation. Near-central-meridian data from 20 magnetograms contribute to each point in the final map. Synoptic charts are presented in two projections in each of two sizes.

Algorithm For Calculation:

The 3600*1440 radial-field synoptic map is primary product from which the other synoptic maps are computed.

HMI 720s line-of-sight magnetograms are first converted to 'radial field magnetograms' by dividing by the cosine of the angle from disk center, i.e. for this purpose we assume that HMI measures the line-of-sight component of a purely radial magnetic field. Individual 'radial' magnetograms are then remapped and interpolated onto a very high-resolution Carrington coordinate grid. For each Carrington longitude the values from the 20 magnetograms obtained closest in time to the central meridian passage (CMP) of that longitude are averaged. By using a constant number of contributing magnetograms, the variation of the noise over the entire map is minimized. Generally all data are taken within about 2 degrees of CMP. The effective temporal width of the HMI synoptic map contribution is about three hours, i.e. 20 720-s magnetograms are obtained within about 90 minutes of central meridian passage. The final HMI synoptic maps have a size of 3600 x 1440, which means the resolution is lower than the disk-center resolution of a single HMI magnetogram. A two-dimensional Gaussian function is applied to high-resolution remapped data to reduce the spatial resolution before generating the 3600*1440 synoptic maps. The width of the Gaussian is 3 pixels. The upper limit of the noise is 2.3 Mx cm2

Daily Update Synchronic Frames

Synchronic frames replace a 120 degree longitude range of the data in a synoptic chart with data observed at a one time. As described above, the standard synoptic map is constructed from data observed within 2 degrees of central meridian, thus each longitude is both a different physical location on the surface and presents the field measured at a different time. Fast evolving features are poorly represented. The synchronic frame replaces 120 degrees in longitude from the original synoptic map with data visible on the disk at one time. In this way that part of the synoptic frame better represents the field observed simultaneously.

Frames are computed daily using data within 60 degrees of central meridian from the twenty 720s-magnetograms obtained between 10 and 14 UT. The magnetograms are registered to account for differential rotation. The averaged data in that 120 degree strip replace the corresponding data in the synoptic map. The origin of the frame is adjusted such that the new data appear at the leading (left) edge of the 360 degree map. In all other respects the processing is the same as described above. Because data at the extreme limb are excluded, some high latitude locations far from disk center in the replacement strip will contain missing values.

Synoptic Daily Frames are computed with both definitive data and HMI nrt data. The nrt data are available within a few hours, but should not be used for scientific analysis because they may be incomplete and can not be fully calibrated.

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