6 HMI.B Data series and access

The HMI data series hmi.B_720s [currently hmi_test.B_720s_cutout] contains the disambiguated vector field quantities, plasma parameters, uncertainties, covariances, and quality information. In all 24 4096*4096 mapped quantities are saved for each 720s time step. Keywords provide additional data needed to identify and interpret the observations. An export program associates the keywords and data and, depending on the parameters selected, applies the appropriate disambiguation and remaps the data and error quantities into the desired coordinate system. For the current release of data for AR 11158, only a portion of the full disk has been populated with data.

6.1 Organization of hmi.B Data Series

The data for each 12 minutes are provided in hmi.B_720s [currently hmi_test.B_720s_cutout] in 26 arrays. The include the total field, inclination relative to the line of sight, and azimuth measured counterclockwise relative to 'up' on the CCD. Additional plasma parameters provided by the VFISV module are eta_0 (the line to continuum opacity ratio), damping of the spectral line (currently not inverted), Doppler width, magnetic plasma velocity, source function continuum, source function gradient, and magnetic alpha (filling factor, currently always set to 1). Errors determined during the inversion are provided for the field, azimuth, magnetic velocity, and alpha; covariances are provided for field-inclination, field-azimuth, inclination-azimuth, field-alpha, inclination-alpha, and azimuth-alpha. We also provide maps of the chi^2 of the VFISV inversion, the convergence code at each pixel of the inversion, a disambiguity flag, and an overall confidence in the result. For the AR 11158 patch data, a bitmap of the field associated with the active region is also available. These can all be accessed through the SDO JSOC. A table listing all of the keywords and data segments can be found at this location.

For the purposes of this release we have exported the data and bundled a subset into one-day tar files. The tar files contain for each 12 minutes in a 650 by 600 cutout window ten files with the three disambiguated field parameters, the errors in each component, the correlations (square root of the covariances) of the three field components, and a quality map that tells about each processing step. A second set of tar files contains the data remapped into the Cylindrical Equal Area (CEA) coordinate system. The CEA set comprises 7 files at each time step with the three field vector components (Br, Bt, Bp), uncertainties in each, and the quality map. Both sets of data are computed for the time range from 2011:02:12_00:00:00 TAI to 2011:02:16_23:48:00 TAI. The files are Rice compressed FITS files. A sample data series with data from 0:00 TAI at each day from February 12 - 16 is also available. The data are described at ReleaseNotes and available at ClickHereForDataRelease.

Movies that provide context can be found at Movies

6.2 Details of Milne-Eddington Inversion Results

The Milne-Eddington VFISV inversion was described in Section 4. ME results before disambiguation can be found in hmi.ME through the JSOC. More detail about each of the VFISV inversion parameters can be found at hmi.ME data series.

6.3 Description of HARPs

Active region masks are identified in HMI line-of-sight magnetic field data every 12 minutes. Regions are tracked in time and data are collected in a data series, hmi.MHARP_720s, whose two prime-keys are HARP number and time. The HARP time series includes geometrical information about the size and location of the box that encloses the active region and a mask that indicates which pixels in the patch are part of the identified region. The near real time identification will be used to identify regions of interest for determining space weather quantities. After disk passage is complete the definitive HARP will be determined and it will be possible to follow an interesting region of constant heliographic size across the disk. The data from February 2011 were analyzed and the HARP region 0883 was associated with AR 11158. The largest extent of the region during the disk passage was used to determine the center position and heliographic boundary of the region. For this data release the cutout was expanded by an additional 150 pixels in each direction to provide a healthy buffer of low field around the active region.

A more detailed description of HMI Active Region Patches (HARPs) is available at HARPDataSeries.

6.4 Export – general

The export process has an important function in making the vector field data usable. The first is the completion of the disambiguation. The disambiguation can be determined in several ways with several initial assumptions that result in different large-scale patterns. On export the selection is made between methods using the initial potential field, radial field, or random seed method. The appropriate bit is used to assign the appropriate 360 degree value to the azimuth of the field. Internally the data are stored in native CCD coordinates with angles defined relative to the orientation of the CCD and the observers line of sight. On export the vectors are transformed to a useful coordinate frame, usually Br, Btheta, Bphi or Bx, By, and Bz in a heliographic frame. In addition the data are remapped onto a regular grid in the new coordinate system. To accomplish this without adding noise the data are highly oversampled when converting to the new coordinate system to avoid aliasing and sampling problems. The transformation and remapping are described in Section 7. For the present the uncertainties associated with the individual output pixels are those of the nearest observed CCD pixel. For this data release the prepared series provide the data in the native CCD pixel cutouts and in a standard Lambert Cylindrical Equal Area projection that is centered on the middle of the active region.

6.5 HMI Vector Data Versions and Referencing Data in publications

See VectorPaper for links to the AR 11158 HMI Vector Field data.

See VectorDataReference for the proper citations to be used when referencing the data.

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