HMI Data Series hmi.Synoptic_Mr_720s - Synoptic Charts - Radial Field
Data Series General Description
The HMI radial field synoptic maps in the data series hmi.Synoptic_Mr_720s provide magnetic field for each Carrington Rotation starting with CR 2096 in April, 2010.
The maps are constructed from HMI 720s line-of-sight magnetograms (hmi.M-720s-info). Typically central meridian data from 20 magnetograms collected over a 4-hour interval contribute to each longitude in the synoptic map. The magnetograms collected over a ~27.27-day interval are combined to make a synoptic map with 3600 longitudes and 1440 steps in sine latitude. For example,CR 2106 was constructed from observations taken from January 20 - February 16, 2011.
The data in hmi.Synoptic_Mr_720s is the basis for computation of all other HMI synoptic maps.
Steps in Constructing an HMI Synoptic Map
Each 4096*4096 magnetogram is first transformed into a 'radial' field map by dividing by the cosine of the angle from disk center, i.e. we assume that the measured field is the line-of-sight component of a strictly radial field. This is done for high-quality magnetograms, viz. those with the QUALITY keyword tested for bits 0xfffefb00 and keyword TINTNUM>9, see Quality Definitions for more details.)
Next, the radial image is transformed into a high resolution map in heliographic coordinates. We exclude about 12 pixels near the limb (r < 0.994 Rs) to avoid the noisiest observations. To preserve something close to the disc-center resolution, we interpolate onto a 5403 * 4320 grid with columns corresponding to 1/30 of a degree in Carrington Longitude and rows corresponding to equal steps of sine latitude. Then each magnetogram is smoothed using a two-dimensional Gaussian with a full width at half max of 9 pixels that is truncated at a full width of 13 pixels. This results in a 1801 by 1440 heliographic map with the resolution of the final synoptic chart where the columns are aligned to 0.1 degree Carrington longitude centers and rows are equally spaced in sine latitude.
Each point in the final synoptic chart combines data from 20 transformed radial magnetograms. Good data observed nearest central meridian are averaged. Typically this includes magnetogram data collected within 2 hours of central meridian passage. We perform an additional quality test at each point by excluding observations that are more than 3 standard deviations from the mean of the 30 perfect-quality magnetograms observed nearest central meridian. Data values observed farther from central meridian are used to replace bad values. The number of magnetograms contributing to each point is recorded in the epts segment of the data series. Except at high latitude where the poles are not visible, it is rare for there to be other than 20 contributing values.
The nominal photon noise in a 720s magnetogram pixel is 10-15G. Each synoptic maps combines 20 magnetograms averaged to about 1/3 the spatial resolution, so the low-latitude noise level should be well below 0.5 G. To estimate the radial field in this series the line-of-sight observations are divided by cosine of the angle from disk center, so the noise level will increase accordingly at higher latitudes. Systematic errors are not included in the estimate.
Final synoptic data are in the synopMr data segment. One record is available for each Carrington rotation. The prime keyword for the data series is the rotation number, CAR_ROT.
A note on Carrington longitude, Carrington Time, and alignment. The Carrington coordinates are a sometimes confusing mix of time and space, because we link a virtual physical location on a inertially rotating astronomical body whose parameters are described in sidereal coordinates with the central meridian passage time as observed by a spacecraft circling a planet in a moon-perturbed, elliptical, inclined orbit. We've done our very best to account for all of these complications. The Carrington longitude of a particular column in these high-resolution maps refers to the longitude of the center of the pixel. Thus the final column in the fits file, which is also the far right pixel in a plot, is observed first (temporally) in the synoptic map; that is because time goes from right to left to match the direction of solar rotation. The longitude of that far right, last column of pixels is 360.0, thus it includes data from either side of that meridian at the time the observation was made. Similarly, the first column of pixels (observed last, temporally) is labelled with longitude 0.1 and is representative of data from 0.05 to 0.15 Carrington degrees for that particular Carrington rotation. But, recall that about three disc-center pixels in the original magnetograms fit into one longitude bin in the final synoptic map and that we've applied a 9x9 FWHM 2-D Gaussian smoothing to the transformed magnetograms.
A more complete description of the data series keywords is given in the next section.
Data Series Keywords
The complete list of keywords for hmi.Synoptic_Mr_720s is available as a pdf
The Keywords for any JSOC data series as defined in the JSD can be inspected using the Series Content tab of the lookdata web access tool.
A helpful description of WCS coordinate systems as implemented by HMI can be found here.