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 720-second Stokes-parameter data series, hmi.S_720s. 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 compared to the expected standard pipeline processing. The disambiguation was computed with spherical geometry in a limited region with fairly generous thresholds on field strength and very gradual annealing; this also required significant additional computing resources.

The HMI team is working to document the reduction pipeline and the uncertainties in the data a processing. We are reasonably confident in the results for this time series and are releasing this first HMI data series for test purposes now. Rather than wait longer, the HMI vector team asks that you treat these preliminary data with care and not publish results until the documentation is complete. The data series may change and the descriptions provided below will definitely improve as we move toward publication planned for early December, 2011. We welcome your feedback on the data and documentation.

You are strongly encouraged to read the ReleaseNotes that provide details and helpful information.

For this specific release, data have been exported into two special formats - direct cut outs and remapped images.

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 problematic. 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 in cross-talk in the Q and U components of the Stokes vector; these arise in part 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. To summarize:

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 strong horizontal flows were apparent. At the time of the flare significant long-lasting changes in the horizontal magnetic field occurred.

The following outlines the analysis of the vector field for this region; a paper based on this outline will be submitted to Solar Physics shortly. Sections will be updated as they become available. Pointers to the data and more detailed treatments of some aspects of the processing follow. We strongly advise that you become familiar with the strengths and weaknesses of the data outlined below. Please contact a member of the VectorFieldTeam if you have questions about the data.

Measuring the Vector Magnetic Field of AR 11158 with HMI

Access to AR 11158 Vector Field Data

Movies of the Vector Field

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

One-Per-Day Sample Images and FITS Files

Other HMI data are available

A range of other HMI data pipeline products are available for this time period through the JSOC. These include the original Stokes parameters (the data series hmi.S_720s), the line-of-sight magnetic field (hmi.M), Doppler velocity (hmi.V), continuum intensity (hmi.Ic), line depth (hmi.Ld), and line width (hmi.Lw).

A longer synoptic time series of inverted (but not disambiguated) data are described at ME720se15w1332

SHARP data are available

The SHARP, or Spaceweather HMI Active Region Patch, data set are a collection of FITS header keywords with the HARP series included as DRMS linked segments. In the HARP data set, the vector magnetic field is determined for each patch and is available as a time series of FITS files. Many spaceweather quantities are calculated per patch. These quantities are known as SHARPs.

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:

JsocWiki: VectorPaper (last edited 2013-05-01 04:35:25 by localhost)