HARP - HMI Active Region Patches

HARP Overview

A HARP (short for HMI Active Region Patch) is an enduring coherent magnetic structure at the scale of a solar active region. The primary purpose of the HARP data series is to provide the practical geometric information needed to track a region as it crosses the solar disk. A HARP is initially identified in a sequence of HMI line-of-sight magnetograms. HARPs are typically observed over several days (possibly as long as a disk passage) and tracked from one image to the next. At each time step, the rectangular HARP bounding box is defined and a BITMAP that characterizes the pixels of the HARP is recorded.

Other summary parameters, such as total flux, are also determined. A more comprehensive data series, SHARPS, are descried at SpaceWeatherHARP.

• MHARPs

The definitive line-of-sight magnetic field HARPs are available in the data series hmi.Mharp_720s. The Mharp data series is indexed by two prime keys, an integer identifier, HARPNUM, and the time, T_REC. The HARP number is unique to the region during it's entire disk passage. A particular HARPNUM will often be linked with a NOAA Active Region (see the relevant keywords: NOAA_AR, NOAA_NUM, and NOAA_ARS). This data series provides pointers to information covering the entire disk passage for each HARP in the HMI catalog for each HARPNUM at each T_REC. Because we use 720s HMI data, some HARPs contain as many as 1500 distinct T_REC's, and some as few as three.

The HARPs are found by analyzing the active region masks in hmi.Marmask_720s. The masks are in turn derived from line-of-sight magnetograms, hmi.M_720s, and corresponding intensitygrams, hmi.Ic_noLimbDark_720s. All three of these data products are full-disk images in helioprojective-tangent coordinates (i.e., as projected on the focal plane in CCD coordinates, and not remapped to a latitude-longitude system). Links to the magnetogram and the masks are provided.

• HARP Bitmap

In addition to the keywords mentioned above, the HARP data series contains a 2D data segment, the BITMAP. Coded values in the BITMAP indicate exactly which pixels within the rectangular bounding box are part of the HARP at the time T_REC. The BITMAP is a rectangle with the size of the bounding box for the full-disk images referred to above, typically several hundred pixels in each dimension. The bounding box is larger than the HARP because it is sized to enclose the largest heliographic extent of the HARP during it's entire disk passage.

• The CCD pixel of the lower left corner of the bounding box is given in CRPIX1 and CRPIX2.

• The size of the box in pixels is given in CRSIZE1 and CRSIZE2.

• Note that the HMI fits images are rotated by the angle CROTA2 (about 180 degrees).

• Other keywords have information in heliographic coordinates.

The data series can be located by following the links to associated line-of-sight magnetograms. For diagnostic or summary purposes, plots of individual keywords, such as integrated flux or size of the HARP, are also useful.

Methodology

Once localized sites that are magnetically active have been found (i.e., building on the full-disk active region masks) the HARP identification problem consists of two pieces: spatially grouping magnetic activity into objects on the scale of active regions, and tracking these objects from image to image. The grouping problem is harder, because flux emergence can cause formerly isolated ARs to merge. This means that a given HARP cannot be declared complete until it has disappeared in view of the observer, or rotated off the visible disk. Consequently, final HARPs are delayed by about a month.

It is important to track HARPs up to the limb, so that all the history of the HARP can be taken in to account in making grouping decisions. Consequently, the grouping criterion takes the spherical geometry into account.

We expect it will be useful to have easy access to the precursors and successors of the HARP. So, we extrapolate the area containing the HARP backwards in time from where it was first detected, and forward from the time where it vanished, two days in each direction. (Or less, if the entire region would rotate off-disk in this time.) This has the effect of expanding the range of T_REC associated with each HARP.

The HARP identification component consists of two parts, a grouping/tracking component, implemented in Matlab, and a data ingestion component, implemented as a JSOC module.

HARP Feature Scale

The following images show how the convolution kernel used for spatial grouping of active regions compares to a small active region at disk center. The units on the top two plots are HMI pixels. The bottom plot shows the convolution kernel at the same scale.

[http://sun.stanford.edu/~turmon/jsoc-wiki/kernel-scale-ar-mag-small.png] [http://sun.stanford.edu/~turmon/jsoc-wiki/kernel-scale-ar-mask-small.png]

Example

The following images are approximate data segments extracted from the large February 2011 active region. The corresponding HARP contains hundreds of T_REC values; we only show five. The orange blob outlines the contents of the HARP. The black pixels, most of which are inside the HARP, are all pixels declared active in the mask. Some clumps of active pixels are not large enough to constitute a HARP.

The white rectangle is not part of the data segment. It is the bounding box in pixel coordinates which contains the entire HARP, and its coordinates are part of the keywords. There is a corresponding bounding box, not shown, in Stonyhurst latitude-longitude coordinates, which is also recorded in the keywords, as MINLON0, MINLAT0, etc.

[http://sun.stanford.edu/~turmon/jsoc-wiki/track-movie-2011-feb-066-zoom.png] [http://sun.stanford.edu/~turmon/jsoc-wiki/track-movie-2011-feb-126-zoom.png] [http://sun.stanford.edu/~turmon/jsoc-wiki/track-movie-2011-feb-186-zoom.png] [http://sun.stanford.edu/~turmon/jsoc-wiki/track-movie-2011-feb-246-zoom.png] [http://sun.stanford.edu/~turmon/jsoc-wiki/track-movie-2011-feb-306-zoom.png]

Anonther Example Showing HARPs on May 9, 2012:

ImageLink(MovieFrame_HARP,MovieFrame_HARP,width=512,height=512)

Bitmap

• BITMAP Pixel Values				Mask Value
  	Outside HARP	Inside HARP		Mar_mask
  Active	2	34		2
  Quiet	1	33		1
  Off-limb	0			0

Keywords

Besides the standard HMI keywords (observation geometry, time, and WCS), we have these keywords for HARP at each T_REC where it was observed:

Note

• This data product was published in April, 2012.

Related Data Series

• ["ARmaskDataSeries"]

• PatchesFound (obsolete)

• VectorPatches

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