Measuring the Vector Magnetic Field of AR 11158 with HMI

The first disambiguated vector field data released by the HMI project for science analysis is a 5-day interval of 12-minute active region patches encompassing AR 11158. The region developed rapidly from February 12-16, 2011 and produced the first X-class flare of Solar Cycle 24 at 01:56 UT on February 15.

The vector magnetic field is derived from the standard HMI IQUV 720-second data series. Patches on the Sun that include AR 11158 were identified using the HMI Active Region Patch (HARP) code and then enlarged to provide a surrounding buffer of weak field. The VFISV inversion code was run with very strict convergence criteria to determine the optimal values, this required additional computing resources. The disambiguation was computed with spherical geometry in a limited region with fairly generous thresholds on field strength to optimize the annealing process. It takes about 9 hours on an 8-processor cluster to disambiguate a 600 pixel square region.

Data are provided in two formats. The 10 FITS files for each time include the three vector field components, error estimates for each quantity, the heliographic latitude and longitude of each point, and two quality maps providing one estimate of the quality of the data at each pixel. Keywords provide other important information. Maps of additional quantities and covariances can be retrieved using JSOC. Details can be found below.

1. Cut outs of the data are provided in a tracked 650 * 600 CCD-pixel window. Components are the total flux, the inclination of the field relative to the local line of sight, and the azimuth measured counter-clockwise from 'up' on the CCD. Keywords provide information about the solar geometry. Note that solar north is down in the movie. The [http://hmi.stanford.edu/doc/magnetic/hmi_test.B_720s_cutout.keywords.pdf hmi_test.B_720s_cutout keywords] and the data can be found in JSOC.

2. Remapped images computed with a Lambert cylindrical equal area projection centered on the uniformly tracked region. The vector field components are heliographic Br, Btheta, and Bphi. Error and quality estimates are for the nearest original pixel. The [http://hmi.stanford.edu/doc/magnetic/hmi_test.B_720s_CEA.keywords.pdf hmi_test.B_720s_CEA keywords] and the data can be found in JSOC.

Vector field data must be used with care. There are several issues with vector magnetic field data that make inversions fail in some pixels and make definitive disambiguation problemmatic. Additionally various systematic errors must be considered when using the data. These include daily, 12-hour, and other long-period variations associated with SDO's orbital velocity that cause large-scale spatial and temporal variations in sensitivity and cross-talk due to limitations in the knowledge of instrument characteristics. Various confidence and error estimates are provided, but they do not always indicate which pixels are suspect.

AR 11158 emerged quickly in a complex way with three major centers of flux emergence. During the build-up to the flare, mixed-polarity regions intensified and horizontal flows were apparent. At the time of the flare significant long-lasting changes in the horizontal magnetic field occurred.

The following paper describing the analysis of the vector field for this region will be submitted to Solar Physics shortly. Sections will be filled in as they become available. Pointers to the data and more detailed treatments of some aspects of the processing follow.

Measuring the Vector Magnetic Field of AR 11158 with HMI

• by the VectorFieldTeam

• 1 Intro – Vector magnetic fields
  • 1.1 HMI Vector Field Overview
  • 1.2 AR 11158 – X-class Flare
• 2 Instrument and Data Processing
  • 2.1 HMI instrument
  • 2.2 Framelist for vector field
  • 2.3 Relevant calibration of HMI instrument
  • 2.4 Specifics of vector data camera, filtergrams
  • 2.5 Processing of filtergrams – dark, flat, cosmic rays, gap filling, distortion
  • 2.6 Pipeline and special HMI processing

• 3 IQUV calculation [http://hmi.stanford.edu/doc/magnetic/observables_code.pdf Section 3]

  • 3.1 Computation of IQUV
  • 3.2 720 calculation – interpolation in time and space, averaging
  • 3.3 Other 720s quantities (very briefly)

• 4 VFISV inversion [http://hmi.stanford.edu/doc/magnetic/section4.pdf Section 4 ]

  • 4.1 ME method and VFISV summary
  • 4.2 Specifics of implemented VFISV for this data series
    • 4.2.1 Weighting
    • 4.2.2 Filter Profile Approximation - Wavelength Hack
    • 4.2.3 Model Initialization
    • 4.2.4 Convergence Criteria, multiple minima
  • 4.3 Formal Error estimation

• 5 Disambiguation [http://hmi.stanford.edu/doc/magnetic/disambig_longer.pdf Section 5]

  • 5.1 General method, issues, and uncertainties - planar and spherical
    • 5.1.1 Selection of Pixels to Anneal
    • 5.1.2 Potential Field Calculation
    • 5.1.3 Minimum Energy Methods
    • 5.1.4 Treatment of Weak Field Areas
  • 5.2 Computation Parameters
• 6 HMI.B Data series and access
  • 6.1 Organization of data series

  • 6.2 Details of HMI.B contents - [http://hmi.stanford.edu/doc/magnetic/hmi.ME.Segments.doc hmi.ME data series]

  • 6.3 Description of HARPs
  • 6.4 Export – general

  • 6.5 HMI Vector Data Versions and Referencing Data in publications - VectorDataReference

• 7 Projections - [http://hmi.stanford.edu/doc/magnetic/VectorTransformation.pdf Remapping and Vector Transformation]

  • 7.1 Native format – CCD coordinates etc.
  • 7.2 Coordinate Transformation and Vector Components on Export

    • 7.2.1 Mapping of pixels to regular coordinate system - [http://hmi.stanford.edu/doc/magnetic/sec7.FieldFormats.pdf Native CCD Cut-outs and Remapped CEA Coordinates ]

    • 7.2.2 Vector field components/directions - [http://hmi.stanford.edu/doc/magnetic/VectorCoordinates.pdf Transformation of Vector Components when Projected ]

    • 7.2.3 Handling of computed error quantities
• 8 Known and unresolved issues with HMI data and processing

  • [http://hmi.stanford.edu/doc/magnetic/hmivectorissues.pdf Vector Field Issues]

  • 8.1 Periodic Errors
  • 8.2 Residual Cross-talk
  • 8.3 Disambiguation issues
  • 8.4 Other systematic and instrumental issues that affect data
• 9 Discussion of HMI measurements of AR 11158 - Validation (KD et al.)
  • 9.1 General comments on determination of vector magnetic field
  • 9.2 Time Evolution of AR 11158 – features both real and otherwise
  • 9.3 Validation and limitations of HMI processing and data
  • 9.4 Qualitative comparison with B_LoS, Hinode, and other field measures
• 10 Summary (Todd et al.)
  • 10.1 Remaining work for pipeline modules
  • 10.2 Uses of data – NLFFF, DAVE4VM, other

Access to AR 11158 Vector Field Data

Movies of the Vector Field

• [ftp:pail.stanford.edu/pub/HMIvector/movie/Cutout_Br.mov Br Cutout]	[ftp:pail.stanford.edu/pub/HMIvector/movie/bvec_600g.mov Vector Field]	[ftp:pail.stanford.edu/pub/HMIvector/movie/surface_vectorline_west.mpg Vector Hedgehog]
  [ftp:pail.stanford.edu/pub/HMIvector/movie/Ic_Bl.mov Intensity + Blos]	[ftp:pail.stanford.edu/pub/HMIvector/movie/track-movie.2011-feb-NEW_1.mp4 Full Disk Regions 1-16 Feb (HARP 0883)]	[ftp:pail.stanford.edu/pub/HMIvector/movie/track-movie.2011-feb-NEW_2.mp4 Full Disk Regions 16-28 Feb]

Vector Field Data - One-Day Tar Files of 12-minute Cadence

• Data are currently only available for testing and evaluation.

  • ClickHereForDataRelease

One-Per-Day Sample Images and ~-FITS-~ Files

• Data are currently only available for testing and evaluation.

  • ClickHereForDataRelease

A collection of topical discussions of the processing

Details of some aspects of the processing and some older documentaiton can be found below. Watch for updates:

• [http://hmi.stanford.edu/doc/magnetic/observables_code.pdf IQUV Computation] {updated 9/2011}

• [http://hmi.stanford.edu/doc/magnetic/section4.pdf VFISV Most complete] {updated 10/2011}

  • [http://hmi.stanford.edu/doc/magnetic/VFISVdescription.doc VFISV Inversion Method Overview] {updated 7/2011},

  • [http://hmi.stanford.edu/doc/magnetic/weights.pdf Weighting Scheme for IQUV in VFISV Inversion] {updated 8/2011}

  • [http://hmi.stanford.edu/doc/magnetic/hack.pdf Filter / Line Profile Approximation (the Hack)] {updated 8/2011}

• [http://hmi.stanford.edu/doc/magnetic/hmi.ME.Segments.doc hmi.ME inverted data detailed description] {updated 7/2011}

• [http://hmi.stanford.edu/doc/magnetic/disambig_overview.pdf Disambiguation Overview] {updated 8/2011}

• [http://hmi.stanford.edu/doc/magnetic/disambig_longer.pdf Disambiguation Detail (incomplete)] {updated 8/2011}

  • Notes on [http://www.cora.nwra.com/~graham/HARP.html Determining Reliability of Disambiguation] {GB - 8/2011}

• [http://hmi.stanford.edu/doc/magnetic/hmivectorissues.pdf Systemic Effects] {updated 10/2011}

  • A rough collection of [http://sun.stanford.edu/~yliu/hmi/doc/hmivector_summaryV00.html Vector Field Systematic Issues ] {updated 7/2011}

• [http://hmi.stanford.edu/doc/magnetic/VectorTransformation.pdf Remapping & Coordinate Transformation] {updated 9/2011}

• [http://hmi.stanford.edu/doc/magnetic/sec7.FieldFormats.pdf Native CCD Cut-outs and Remapped CEA Coordinates ]

  • Note on [http://hmi.stanford.edu/doc/magnetic/VectorCoordinates.pdf Transformation of Vector Components when Projected ]

• [http://hmi.stanford.edu/doc/magnetic/ Vector Field Documentation Directory] {updated 10/2011}

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