6.2.1 Observations
In this study, we concentrate on the simultaneous observation obtained by the Interface Region Imaging Spectrograph (IRIS; De Pontieu et al. 2014) and Hinode (Tsuneta et al.
2008). We focus on a quiet Sun area with small-scale structures in the transition region and investigate their relation with the underlying magnetic field.
In our study, we use IRIS level-2 spectra and slit-jaw-images. We analyse large dense raster with 400 steps and a step size of 0.3500, a spatial scale along the slit of 0.1700pixel−1, and a spectral resolution of 40 mÅ. The raster covers a 14000×17000field-of-view. We fo-cus on the Si iv line at 1393.8 Å, which corresponds to the transition region emission at logT=4.9, and the Lyman continuum of Si i that is formed near the temperature min-imum. To investigate the temporal evolution of the small-scale structures we use IRIS slit-jaw-images at 1400 Å (thereafter SJI1400) with a spatial scale of 0.1700pixel−1, a ca-dence of 63.5 s and an exposure time of approximately 60 s. The SJI1400 covers the 17500×17500field-of-view around the slit.
To investigate the evolution of the photospheric magnetic field, we use Hinode SOT data obtained in the FGSIV200 mode. We use the level-0 data: maps of the Stokes pa-rameter I (intensity image) and V. We use them to create the line-of-sight magnetograms and the procedure is described in the next subsection. Hinode provides observations with 61.400×81.900field-of-view at a spatial scale of 0.1600per pixel and at a cadence of 35 s. For more details about our regions-of-interest, we refer to Table 6.1.
Table 6.1: Overview of the observations
instrument observable start end exp. time [s] X[00] Y[00] Hinode SOT Stokes V, I 00:21:07(+1d) 02:58:36(+1d) 35 -82 -28
IRIS SJI 1400 Å 23:27:28 02:59:15(+1d) 60 -120 -41
IRIS raster Siiv, continum 23:27:28 02:59:15(+1d) 30 -120 -41
6.2.2 Data reduction
6.2.2.1 Hinode data
For the Hinode data we apply the six-step preparation procedure that is described schemat-ically in Fig. 6.1.
First, we convert the raw data to unsigned 16-bit integers. The next step is to pro-duce level-1 data. For this, we use the Hinode reduction software which is available in SolarSoft1. We want to create the magnetogram and to this end, the map of the Stokes pa-rameter V is divided by the map of the Stokes papa-rameter I. We obtain two magnetograms which correspond to the negative and positive part of the profile of the Stokes parameter V respectively, from which the average magnetogram is calculated. The data are used in our analysis were recorded by two parts of the CCD detector separately, therefore the data are merged to obtain the final magnetogram. In the next step, we select magnetograms that are closest in time to the slit position of the raster image. The selected magnetograms are co-aligned and rebinned to the IRIS SJI1400 resolution.
From this procedure, we obtain a time series of spatially aligned magnetograms at the same spatial scale as the IRIS SJI1400 images.
6.2.2.2 IRIS slit-jaw images
For the IRIS SJI level-2 data, we apply three-step procedure that is also shown schemati-cally in Fig. 6.1.
First, we define pairs of the Hinode magnetograms and almost simultaneous obtained slit-jaw-image as well as the corresponding IRIS raster maps. Then, slit-jaw-images are corrected for artifacts, such as dust spots, using the software which is available in the SolarSoft2.
The next and main step is the co-alignment of the SJI1400 images. This step is critical to the further analysis of the spatial and temporal evolution of small-scale structures.
Therefore, we use a cross-correlation technique and the sub-pixel precision alignment.
Using this procedure, we obtain a dust-corrected and a spatially aligned series of SJI1400.
6.2.2.3 IRIS raster map
The Fig. 6.1 shows schematically the preparation procedure of the raster map. We use the level-2 data obtained by the FUVL camera.
1fg_prep.pro available at SolarSoft,http://darts.jaxa.jp/pub/solar/ssw/.
2iris_dustbuster.pro available at SolarSoft,https://darts.isas.jaxa.jp/pub/ssw/.
IRIS SJI IRIS raster
rescale rescale
co-alignment Hinode
co-alignment temporal series of data
co-alignment temporal series of data
co-alignment temporal series of data
co-aligment series of Hinode data
co-aligment series of IRIS SJI data
artifical raster creation dust correction
co-alignment creation
of level 1 data
co-alignment temporal series of data magnetogram creation (V/I)
merge data from two part of CCD detector
merge data from two part of CCD detector
calibration
map creation
Figure 6.1: Flowchart of data preparation process. See Sect. 6.2.2.
In the first and main step, we calibrate the wavelength scale using the Fe ii line at 1392.82 Å as a reference.
Then, we prepared the intensity map of Siivat 1393.76 Å. To this end, we integrated over the whole line covering a range of 0.76 Å around the line center. In this analysis, the continuum is subtracted. We also created the continuum map. For this the contin-uum intensity is integrated from 1394.35 Å to 1395.76 Å. For the density calculation, we prepared intensity maps of Oiv lines at 1399.77 Å and 1401.16 Å. We calculate the line intensity integrated around the lines center over a range of 0.6 Å, for this map we subtract the continuum emission. We fit a gaussian function to the Si iv line profiles. The fitting parameters are used to create a map of the Doppler velocity (line center position) and a map of the full width at half maximum (FWHM, this is the Gaussian width multiplied by 2√
ln 2≈1.67).
Finally, the raster maps are rescaled to the proper aspect ratio.