A Brief Guide for Using the HMI Vector Magnetogram

Transcription

1 A Brief Guide for Using the HMI Vector Magnetogram v0.0 Nov Xudong Sun and the HMI Vector Team 1. Read Rice compressed HMI fits images Most HMI fits images (including the vector data) are stored in Rice compressed format, in which floats are scaled into ints. The scaling is determined by the BSCALE and BZERO keywords, which are present in the headers. When reading an image via a fits reader, the ints are converted back to floats. The HMI team suggests using the IDL module fitsio_read_image.pro to deal with the Rice compressed data. The module requires a share object fitsio.so. To read in the field strength for the cutout image of the Feb 16 12:00 UT record, IDL> data = fitsio_read_image(file, hdr) IDL> help, data DATA DOUBLE = Array[650, 600] IDL> print, data[0,0] NaN The SSW module read_sdo.pro can also be used to read Rice compressed data. Be warned that it does not treat missing values properly 1. The missing values are 1<<31 as indicated by the BLANK keyword. While fitsio_read_image converts them into NaNs, read_sdo reads them as a regular number, e07. One simple work-around is to find any values less than -1.e07 (and set them to NaNs if one wishes), since no magnetic field has such value. IDL> print, data[0,0] e+07 IDL> nan_pos = where(data le - 1.e07) IDL> if (nan_pos[0] ne - 1) then data[nan_pos] =!values.f_nan For a more detailed description and alternatives, see access.htm 2. Keyword examples Keywords in the fits header follow the WCS standard. A few ad-hoc keywords are provided for additional information. T_OBS: center time of the averaging window for the 720 s cadence data. 1 For this first release, only part of the disk is processed according to the automatic feature identification result. Thus, there are missing values in the cutout images.

2 CROTA2: negative solar p-angle. CRPIX1/CRPIX2: reference pixel address. For a cutout image, it refers to the disk center with regard to the lower-left corner of the cutout, which has an address of (1, 1). For a remapped image, it refers to the patch center with regard to the lower-left corner. CRVAL1/CRVAL2: For a cutout image, it is always 0 arcsec (for disk center). For a remapped image, it s the Carrington longitude/latitude (for patch center). CRLN_OBS/ CRLT_OBS: Carrington longitude and latitude of disk center. For a cutout image: IDL> print, index.t_obs _11:59:54_TAI IDL> print, index.crota IDL> print, index.crpix1, index.crpix IDL> print, index.crval1, index.crval For a remapped image: IDL> file = hmi_test.b_720s_cea _120000_tai.br.fits IDL> print, index.crota IDL> print, index.crpix1, index.crpix IDL> print, index.crval1, index.crval Using the geometry information: co-alignment with AIA image Based on the keywords in the header we can determine the position of any pixel in the cutout with regard to the solar disk. This information can then be used to co-align with other solar images. We again use the Feb 16 12:00 frame as example. Let s determine the position of pixel (475, 450), with lower-left (1,1). IDL> cosa = cos (index.crota2 *!dtor) & sina = sin (index.crota2 *!dtor) IDL> cdelt = index.cdelt1 IDL> crval1 = index.crval1 & crval2 = index.crval2 IDL> ; (475, 450) relative to disk center IDL> pix1 = index.crpix1 & pix2 = index.crpix2 IDL> ; to the west of disk center:

3 IDL> wx = (pix1 * cosa - pix2 * sina) * cdelt + crval1 IDL> ; to the north of disk center: IDL> wy = (pix2 * cosa + pix1 * sina) * cdelt + crval2 IDL> print, wx, wy Let s extract a 300x300 HMI cutout with its upper-right corner at (475, 450). After 180 rotation, this pixel becomes the lower-left corner. Since we have (wx, wy) for this pixel, we may get the corresponding AIA cutout easily. These two cutouts have their lower-left corner registered to the nearest full pixel. IDL> field = data[175:474, 150:449] IDL> ; Approximate treatment for 180 degree p- angle IDL> field = rotate(field, 2) IDL> ; Get AIA 193 cutout IDL> file_aia = AIA _115919_0193.fits IDL> read_sdo, file_aia, index_a, data_a IDL> ; Get lower- left pixel address IDL> llx = round (index_a.crpix1 + wx / index_a.cdelt1) - 1 IDL> lly = round (index_a.crpix2 + wy / index_a.cdelt2) - 1 IDL> ; Get subregion dimension IDL> nx = round (300 * cdelt / index_a.cdelt1) IDL> ny = round (300 * cdelt / index_a.cdelt2) IDL> print, llx, lly, nx, ny IDL> aia = data_a[llx:llx+nx - 1, lly:lly+ny- 1] Below is a plot of the cutouts. 4. Transformation of field vectors (basic) The cutout images present the field vectors in three components: field strength 2, inclination, azimuth. The convention of angles are as follows. Inclination is with regard to the line-of-sight (LOS) direction ranging 2 HMI has a fixed unity filling-factor, thus all field are essentially mean flux density.

4 from 0 to points towards Earth, 180 into the plane of sky. Azimuth is defined with regard to the +y direction of the native image (up), ranging from 0 to 360, increasing counter-clockwise (CCW). Note that the HMI images usually has a p-angle of about 180, so the 0 azimuth is actually point toward south. We convert the cutout data into LOS, EW and NS components. Be careful about the sign of EW and NS components, when p-angle is 180. IDL> file_field = hmi_test.b_720s_cutout _120000_tai.field.fits IDL> file_inclination = hmi_test.b_720s_cutout _120000_tai.inclination.fits IDL> file_azimuth = hmi_test.b_720s_cutout _120000_tai.azimuth.fits IDL> field = fitsio_read_image(file_field) IDL> inclination = fitsio_read_image(file_inclination) IDL> azimuth = fitsio_read_image(file_azimuth) IDL> ; LOS field IDL> b_los = field * cos (inclination *!dtor) IDL> ; Transverse field IDL> b_trans = field * sin (inclination *!dtor) IDL> ; EW component, E positive for 180 degree p- angle IDL> b_ew = - b_trans * sin (azimuth *!dtor) IDL> ; NS component, S positive for 180 degree p- angle IDL> b_ns = b_trans * cos (azimuth *!dtor) 5. Transformation of field vectors (advanced) Field vectors can be transformed into LOS/transverse components easily, as shown above. One may want to transform them into the more useful vertical/horizontal decomposition. The remapped series provided in this release uses equation (1) provided in Gary and Hagyard (1990). In that convection, the field vector is denoted and B x, B y and B z. B z is normal to the solar surface, B x and B y form a plane tangent to the surface, with +y pointing north. Note that B z is essentially the radial component, or B r in the remapped series in this release. Similarly, B x is B p, B y is -B t. For each pixel, one will need the following information to perform the transformation: the latitude and longitude of disk center, the solar p-angle, and the latitude and longitude of the very pixel. The first three can be found in the fits header. The last two can be computed from the header information. For this particular release, latitude and longitude values at each point are provided as images in the DRMS series. One can get them from the JSOC lookdata webpage, from the hmi_test.b_720s_cutout data series. The data segments are named lat and lon respectively. Alternatively, one may use the SSW routine xy2lonlat.pro to compute the latitude and longitude of a pixel. IDL> ; We are using pixel (475, 450) in example 2 IDL> ; We already computed the needed distance from disk center wx and wy IDL> ang = [ , ] IDL> ; date0 is obsolete in this case IDL> date0 = IDL> ; b- angle is just the latitude of the disk center

5 IDL> b0 = index.crlt_obs *!dtor IDL> ; Solar radius is provided in arcsec IDL> rsun = index.rsun_obs IDL> ; Use xy2lonlat IDL> position = xy2lonlat (ang, date0, b0=b0, radius=rsun) IDL> print, position IDL> ; the longitude is relative to disk center, the disk center Carrington longitude is IDL> print, index.crln_obs IDL> ; Carrington longitude of the pixel IDL> print, The value obtained from the HMI pipeline result lon and lat images are and , the longitude is Carrington longitude. The difference with SSW result is ~ degree.