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Vector field data products include the following quantities (and at least 23 others, including uncertainties)
 . |B| - total flux
 . Inclination (gamma) to the line of sight
 . Azimuth (phi) relative to 'up' on the CCD
 . Magnetic line-of sight velocity

Data will be exported in a variety of coordinate systems: [''Bx,By,Bz''], [''B_total,gamma,phi''], and [''Br,Btheta,Bphi'']

HMI Vector Magnetic Field Data Products

Overview of Vector Field Pipeline

The HMI instrument collects 4096*4096 filtergrams in various wavelengths and polarizations sufficient to determine the Stokes parameters, I, Q, U, & V, as rapidly as every 90 seconds. Currently a 135-second base sequence is collected using the HMI side camera. The sequence measures I+-Q, I+-U, I+-V at 6 wavelengths. Level1 filtergrams are interpolated in time and space, correcting for solar rotation, cosmic rays, distortion, and other instrumental effects. Every 12 minutes data collected over a 16-minute tapered interval are averaged to reduce noise to produce the Stokes IQUV parameters in a 720-second data series, [http://jsoc.stanford.edu/ajax/lookdata.html?ds=hmi.S_720s hmi.S_720s]

From many of the same filtergrams are determined the line-of-sight magnetic field ([http://jsoc.stanford.edu/ajax/lookdata.html?ds=hmi.M_720s hmi.M_720s]hmi.M_720), continuum intensity ([http://jsoc.stanford.edu/ajax/lookdata.html?ds=hmi.Ic_720s hmi.Ic_720s]), line width ([http://jsoc.stanford.edu/ajax/lookdata.html?ds=hmi.Lw_720s hmi.Lw_720s]), and line depth ([http://jsoc.stanford.edu/ajax/lookdata.html?ds=hmi.Ld_720s hmi.Ld_720s]). The other HMI camera produces a time series of 45-second data. The pipeline produces near real time version of several of these data series.

The Stokes parameters are inverted to determine the prime magnetic field parameters: B-Total, Inclination, and 180-degree Azimuth relative to the line of sight along with uncertainties on each quantity. Several other [http://hmi.stanford.edu/doc/magnetic/hmi.ME.Segments.doc hmi.ME plasma parameters and co-variances] are also determined. The Milne-Eddington inversion code is called VFISV, the Very Fast Inversion of the Stokes Vector (VectorDataReferences).

Definitive, science-grade, HMI vector magnetic field data are available as of May, 2012 via JSOC under the series name [http://jsoc.stanford.edu/ajax/lookdata.html?ds=hmi.ME_720s_fd10 hmi.ME_720s_fd10]. The polarimetric filtergram data are processed with the Milne-Eddington inversion code VFISV (Borrero et al. 2010) to produce full-disk vector magnetic field maps every 12 minutes (i.e. 720 seconds) starting 26 April 2012. Data products are available, on average, a week after observation. We are retroactively processing selected data from 2010 onwards. Currently the processed database includes at least 4 full-disk images per day since May 1 2010. The interval Feb 1- Mar 10 2011 has the full disk inverted data available every 720 s. Click [http://jsoc.stanford.edu/doc/data/hmi/coverage_maps/ Monthly Coverage Maps] and select the fd10/ folder for specific times. The inversion code is operating with a fixed magnetic filling factor of unity, and the azimuth has not been disambiguated (it ranges between 0 and 180 degrees). Recent updates to the inversion code include the addition of a regularization term to the merit function in order to bias the solution towards lower eta0 values. The aim of this modification is to reduce spatial discontinuities due to the presence of double minima in the chi-square surface. Other updates focus on the convergence criteria and speed optimization.

Be aware that some magnetic field values in strong umbrae and complex regions are erroneous in HMI vector data. The dynamic range of the HMI measurements goes up to ~3500 Gauss. Strong magnetic fields leads to a complete split of the spectral line (Zeeman saturation). In our case, stronger values than 3500 Gauss extend the spectral line outside of the observed HMI spectral range. High Doppler velocities can also shift the spectral line partially outside of the HMI spectral range. When either one of these facts or their combination happens, the inversion code does not have enough information to constrain the field strength, and occasionally it hits the preset upper limit of 5000G. Only a very, very small fraction of the pixels exhibits such problems, which are easily seen 'by eye'. These and other minor issues will be described in detail at ReleaseNotes3.

The method of computation may differ for quick-look and final data products, for full-disk and selected patches, and for strong and weak field regions. Routines are available to export the data in useful heliographic coordinates, either in rectilinear {Bx, By, Bz} for patches or for full-disk observations in spherical coordinates, either {Br, Btheta, Bphi} or {Br, inclination, and azimuth}. Contact an HMI team member for assistance. Data can also be exported after remapping to standard synoptic or synchronic map coordinates. Averaging vector field maps in time and/or space can also be accomplished. A module will exist to merge the various methods.

The first release of disambiguated data for science analysis took place in November, 2011. That release included 5-days of 12-minute vector magnetograms of AR 11158, the X-class flare producing region from February 12-16, 2011. The processing is not completely standard because we compute longer. A skeleton of the VectorPaper "Measuring the Vector Magnetic Field of AR 11158 with HMI" describing the processing is available. Presently the data for this a few other regions are available for science analysis; however we consider this a 'beta' release and look forward to receiving your feedback. See ReleaseNotes and ReleaseNotes2 for more information. These data will be combined to produce Carrington charts, synoptic maps, and synchronic frames of the vector field components. A long-term estimate of the polar field will be available for applications requiring global coverage. Some notes about the noise mask used in the disambiguation are available at MagneticNoiseMask.

From the photospheric data various products and models will be computed, including local and global harmonic coefficients, global potential field - source surface models, global and local magnetostatic models (including NLFFF on patches), global and local MHD models of various types.

Methods for tracking surface features will include local correlation tracking, ILCT, and DAVE4M. Interfaces to other models will be provided as practical.

Near real time SpaceWeatherProducts will be produced from quick look data. These will include indices calculated for the full disk and patches of interest.

Older Documentation

Vector field data products include the following quantities (and at least 23 others, including uncertainties)

  • |B| - total flux
  • Inclination (gamma) to the line of sight
  • Azimuth (phi) relative to 'up' on the CCD
  • Magnetic line-of sight velocity

Data will be exported in a variety of coordinate systems: [Bx,By,Bz], [B_total,gamma,phi], and [Br,Btheta,Bphi]

The following are from 2010 and many items are outdated. Use with care and only if newer documentation is unavailble.

Status of various modules can be found in the ["HMIModuleTable"]

Vector Magnetic Field Modules

Associated HMI Pipeline Modules

Line of Sight Magnetic Field Modules


JsocWiki: VectorMagneticField (last edited 2014-05-14 13:38:18 by XudongSun)