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 6-day interval of 12-minute active region patches encompassing AR 11158. The region developed rapidly from February 12-17, 2011 and produced the first X-class flare of Solar Cycle 24.

The 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 was run with very strict convergence criteria to provide 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 annealing.

Data Files are provided in two formats.

1. Cut outs of the data are provided in a tracked 650 * 600 CCD-pixel window. The inclination of the field is relative to the local line of sight and the azimuth is measured counter-clockwise from up on the CCD. Keywords provide information about the solar geometry. Note that solar north is down in the movie.
2. Remapped images computed with a Lambert equal area projection centered on the tracked region. The vector field components are heliographic Br, Btheta, and Bphi.

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 definitive disambiguation problemmatic. Additionally several 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 do not always indicate which pixels are suspect.

AR 11158 emerged quickly in a complex way with several centers of flux emergence. During the build-up to the flare mixed-polarity regions strengthened 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. More detailed treatments of some aspects of the processing follow.

Movie of the Radial Field: [ftp:pail.stanford.edu/pub/HMIvector FTP Directory]

Measuring the Vector Magnetic Field of AR 11158 with HMI

• by the VectorFieldTeam

• 1 Intro – Vector magnetic fields (Todd et al.)
  • 1.1 HMI Vector Field Overview
  • 1.2 AR 11158 – X-class Flare

• 2 Instrument and Data Processing (Sebastien & Jesper)

  • 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 (Sebastien) [http://hmi.stanford.edu/doc/magnetic/observables_code.pdf IQUV Computation]

  • 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 (Rebecca) - [http://hmi.stanford.edu/doc/magnetic/VFISVdescription.doc VFISV Inversion Method - Details]

  • [http://hmi.stanford.edu/doc/magnetic/section5.RCE.pdf VFISV Inversion - for paper, incomplete]

  • 4.1 ME method and VFISV summary
  • 4.2 Specifics of implemented VFISV for this data series

    • 4.2.1 Weighting - [http://hmi.stanford.edu/doc/magnetic/weights.pdf Weighting Scheme]

    • 4.2.2 Filter Profile Approximation - [http://hmi.stanford.edu/doc/magnetic/hack.pdf The Hack]

    • 4.2.3 Convergence Criteria, multiple minima
  • 4.3 Formal Error estimation

• 5 Disambiguation (Graham+KD) - [http://hmi.stanford.edu/doc/magnetic/disambig_overview.pdf Brief Overview]

  • 5.1 General method, issues, and uncertainties - planar and spherical [http://hmi.stanford.edu/doc/magnetic/disambig_longer.pdf Disambiguation Details]

  • 5.2 HMI specifics, thresholds, mapping, annealing
  • 5.3 Bitmap and confidence index
• 6 HMI.B Data series and access (Todd)
  • 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 (Xudong and Yang) - [http://hmi.stanford.edu/doc/magnetic/VectorTransformation.pdf Remapping and Coordinate 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
    • 7.2.2 Vector field components/directions
    • 7.2.3 Handling of computed error quantities

• 8 Known and unresolved issues with HMI data and processing (Yang, et al.) - [http://sun.stanford.edu/~yliu/hmi/doc/hmivector_summaryV00.html Vector Field Discussion]

  • [http://hmi.stanford.edu/doc/magnetic/hmivector.YLiu.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

A collection of topical discussions of the processing

Details of some aspects of the processing 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/section5.RCE.pdf VFISV Most complete] {updated 09/2011},

  • [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}

• [http://hmi.stanford.edu/doc/magnetic/hmivector.YLiu.pdf Systemic Effects] {updated 9/2011}

  • 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/ Vector Field Documentation Directory] {updated 10/2011}

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