IRIS Measurement Data At Last!

The first set of IRIS MgII h and k spectral feature data is available here.  The ZIP file is here.

The input calibrated data can be accessed here.

Measurements of MgII h and k lines

The singly ionized Magnesium h and k lines are a source of strong ultraviolet emission in the spectrum of the sun.  Each of these lines has features in their profiles that have been identified as k1r, k2r, k3, k2v, k1v, h1r, h2r, h3, h2v, and h1v.  The measurement of their wavelengths, intensities, and spacial locations is the purpose of this project.  The k1r, k1v, h1r, and h1v features are not included in this current project.

We start with a portion of the sun that is being observed in 'Sit-and-Stare' mode (see Figure 1) at a particular time.  Different data sets may have a different time interval between successive images/spectra.  In this figure, the thin black vertical line is the entrance slit for the spectrograph.  Notice that the slit intersects a sunspot about 2/3 of the way from the bottom of the image.

Figure 1

The spectrograph produces spectra at many different wavelength bands of interest, one of them being a band that covers the Mg II k and h lines, which are at approximately 2796.4A and 2803.5A respectively.  Figure 2 shows a typical h spacial-spectrum.  The x-axis corresponds to wavelength and the y-axis correponds to spacial location along the slit.

Figure 2

Figure 3 shows the identified spectral line features.  The red, green, and blue points identify the h2r, h3, and h2v features, respectively.  The x-axis is wavelength (in angstroms) and the y-axis is the number of vertical arcseconds away from the center of the sun.  Compare this to the input data (Figure 2) and you'll see that the algorithm is generally doing a fairly good job at feature identification.

Figure 3

Figure 4 shows a typical line profile for the h line, identifying the three features that will be measured.  Figure 5 is the same data, but zoomed in to show just the spectral features and their calculated pixel positions.

Figure 4

Figure 5

The algorithm scans the profile and calculates the difference in intensities between adjactent points.  When the difference flips between a positive difference and a negative difference, a peak has been found.  When the difference flips from negatice to positive, a trough has been found.  The exact location where the difference has a value of zero is the location of the peak or the trough.  Figure 6 shows the differences.  The black dots indicate where the differences are zero.  There is a 1/2 pixel offset between Figure 5 and Figure 6 values because the intensity differences are measured from the "centers" of the pixels while the intensities themselves are measured from the "edges" of the pixels.

Figure 6

Because of various factors, not all line features can be measured.  These features disappear, for example, in the location of the sunspot.  This algorithm is looking for a peak-trough-peak sequence and will fail proper identification if this sequence is not encountered (for instance if it finds a trough before the first peak, or if just a single peak is found).

Once the pixel location of the peak or trough has been calculated, it is translated into an spacial offset (in arcseconds away from the center of the sun) and wavelength (in angstroms), by using the "CRVAL1" and "CDELT1" metadata values provided in the FITS headers.  The intensity of the peak or trough is also measured and reported.

This is done separately for the k and h lines.

Currently, the software will only work with 'Sit-and-Stare' input data.

Data Example

The data is organized into separate text files corresponding to spectra taken at each raster epoch.  For example, if there are 144 raster images, then there will be 144 separate text files with the following type of data:

6912.522659 30 120.884 144.357 2803.386 1519.708 2803.537 1114.734 2803.614 1178.763 31539.500
6912.522659 31 120.884 144.523 2803.389 1504.800 2803.530 1235.172 2803.615 1337.240 31642.000
6912.522659 32 120.884 144.690 2803.406 1400.550 2803.518 1242.861 2803.607 1341.820 31097.500
6912.522659 33 120.884 144.856 2803.397 1384.531 2803.487 1195.102 2803.596 1341.308 30745.000
6912.522659 34 120.884 145.022 2803.394 1409.741 2803.474 1225.024 2803.580 1337.178 30260.750
6912.522659 35 120.884 145.189 2803.391 1395.913 2803.494 1173.014 2803.596 1392.362 29817.250
6912.522659 36 120.884 145.355 2803.389 1359.357 2803.485 1160.290 2803.597 1352.109 29595.250
6912.522659 37 120.884 145.521 2803.372 1312.136 2803.493 1140.555 2803.593 1333.213 29225.250
6912.522659 38 120.884 145.688 2803.358 1268.717 2803.488 1131.498 2803.601 1314.044 29207.000
6912.522659 39 120.884 145.854 2803.354 1316.627 2803.457 1084.482 2803.596 1335.140 29375.250
6912.522659 40 120.884 146.020 2803.350 1356.074 2803.476 991.637 2803.601 1320.630 28961.250
6912.522659 41 120.884 146.187 2803.342 1408.039 2803.470 968.719 2803.603 1325.508 29083.750
6912.522659 42 120.884 146.353 2803.343 1409.933 2803.469 967.708 2803.612 1352.689 29782.750
6912.522659 43 120.884 146.519 2803.344 1391.391 2803.465 1012.583 2803.611 1395.858 30408.750
6912.522659 44 120.884 146.686 2803.334 1380.299 2803.457 1033.055 2803.612 1392.879 30696.000
6912.522659 45 120.884 146.852 2803.355 1393.129 2803.467 1131.581 2803.593 1397.156 31680.500
6912.522659 46 120.884 147.018 2803.350 1509.836 2803.470 1211.112 2803.609 1413.840 32370.000
6912.522659 47 120.884 147.185 2803.350 1448.126 2803.471 1135.210 2803.612 1331.672 31034.500
6912.522659 48 120.884 147.351 2803.341 1351.033 2803.499 1047.807 2803.615 1278.129 29607.000
6912.522659 49 120.884 147.518 2803.350 1321.970 2803.485 1096.239 2803.618 1293.102 29079.750
6912.522659 50 120.884 147.684 2803.356 1274.051 2803.460 1117.601 2803.462 1130.555 28155.250
6912.522659 51 120.884 147.850 2803.358 1190.512 2803.453 1041.789 2803.478 1055.543 26629.000
6912.522659 52 120.884 148.017 2803.368 1084.548 2803.455 971.335 2803.467 978.182 24838.750
6912.522659 53 120.884 148.183 2803.399 1035.587 2803.500 984.569 2803.561 1060.511 24682.750
6912.522659 54 120.884 148.349 2803.383 1124.145 2803.387 1111.945 2803.420 1152.975 25633.500
6912.522659 55 120.884 148.516 2803.395 1224.889 2803.485 1129.833 2803.529 1163.413 26514.750
6912.522659 56 120.884 148.682 2803.393 1250.542 2803.437 1184.945 2803.467 1204.757 27016.500
6912.522659 57 120.884 148.848 2803.368 1227.481 2803.479 1162.977 2803.587 1244.195 26974.500
6912.522659 58 120.884 149.015 2803.378 1213.820 2803.453 1178.464 2803.480 1184.556 26797.500

The files will be named in the following format:

obs_AAAAAAAAAA_BBBBB[h/k].txt

where the A's correspond to the IRIS observation tag, and the B's correspond to the raster image number starting at '00000'.  'h' or 'k' corresponds, of course, to the spectral line.

Column Descriptions:

1:  Julian Date (JD - 2450000) of observation
2:  Raster image row number
3:  X position (arcsec) from center of Sun
4:  Y position (arcsec) from center of Sun
5:  Measured k/h2v position (angstroms)
6:  Measured k/h2v intensity (data units)
7:  Measured k/h3 position (angstroms)
8:  Measured k/h3 intensity (data units)
9:  Measured k/h2r position (angstroms)
10: Measured k/h2r intensity (data units)
11: Total power

IRIS is a NASA small explorer mission developed and operated by LMSAL with mission operations executed at NASA Ames Research center and major contributions to downlink communications funded by the Norwegian Space Center (NSC, Norway) through an ESA PRODEX contract.