Coronal Hole recognition by He 1083 nm imaging spectroscopy

1. Coronal Hole recognition by He 1083 nm imaging spectroscopy O. Malanushenko (NSO) and H.P.Jones (NASA's GSFC) Tucson, Arizona, USA Solar Image Recognition Workshop, Brussels, Belgium, 23- 24 October, 2003

2. Coronal Hole recognition by He 1083 nm imaging spectroscopy. O. Malanushenko (NSO) and H.P.Jones (NASA's GSFC) The location of a coronal hole (CH) in the upper chromosphere is usually based on equivalent width (EqW) images in the He 10830 line. A CH is seen on these images as bright areas, which represent low values of EqW. But it is difficult to differentiate a CH from the bright centers of chromospheric network (CN) without complementary data and the skill of an experienced of observer . To remove the above ambiguity we apply a new spectral analysis technique to compare parameters of a He I 1083 nm line in CH and CN. We used imaging spectroscopy data obtained with the NASA/NSO spectromagnetograph 00/04/17 (Malanushenko and Jones, 2002, BAAS 33, 700). We fit a Gaussian profile to the main component of the He line and deduce the parameters of central intensity (I), half width at halfmaximum (Hw) and line shift (V). On the Hw-images, CHs are distinguished from the surrounding regions as bright areas; similarly, they are also seen as bright on the I-images. Chromospheric network is seen on Hw-images as opposite in contrast to the I-images, and this distinction is the basis for our CH identification method. We normalize the I- and Hw-images by subtracting their respective quiet-sun means and dividing by the corresponding standard deviations. The sum of the normalized I- and Hw-images shows increasing contrast of the CH and a depression of contrast in the network, and can be used as an independent CH diagnostic.

3. Imaging spectroscopy data at He I 1083 nm NSO/KP Vacuum Telescope with the NASA/NSOspectromagnetograph The location of a CH in the upper chromosphere is usually based on Intensity or EqW images in the He 10830 line. CH Q 3D imaging spectroscopy: 2D space & 1D spectral dimension The date of observation: 17, April, 2000 Space resolution: 1.10"/pix (averaged to 2.68 "/pix) Spectral resolution: 0.083 Å/pix (2.3 km/s)

4. Coronal Hole recognition on intensity images a c b d Image of CH at He line (a), and the same image with contours2%, 1%, 0% (b,c,d) above average value of quiet sun as an example of unsuccessful attempts to contour CH area (b - too small, d - too large, c – good size of CH, but contour outlines cells of network also). It is difficult to differentiate a bright CH from the bright centers of chromospheric network.

5. Spectral data reduction - preliminary For dark and flat-field correction we used a special technique for synoptic observations (Jones, 2003, in preparation) Alignment of spectra to solar lines: shift and scale wavelength to fixed position of Si I 1082.71 nm and Na I 1083.49 nm -to minimize variability due to solar rotation - to correct for instrumental variations of dispersion Na I 1083.49 Normalization by linear fit to continuumas seen in Atlas spectrum. Problem: Comparison SPM spectra and Atlas spectrum show that linear interpolation for continuum is not adequate for analysis of He line in our data 5

6. Spectral data reduction - continuum Normalization to non-linear continuum: Idea: getting a right continuum from a well calibrated reference spectrum {V.Malanushenko et el, 1992, A&A, 259, 567}. We used NSO Atlas as reference spectrum {Wallace et el: 1993, N.S.O. Technical Report, #93-001}. • Main steps of procedure : • Calculate ratios between individual spectra and reference spectrum • Select spectral zones where ratio is less sensitive to solar and instrumental variations • Define a non-linear continuum as fitting function trough the zones. • Normalize our spectrum to continuum

7. Reduction: de-blending of spectra and Gaussian profile fitting De-blendingof He line: We perform a multi-profile fitting of average spectrum to get average spectrum without He lines, and we subtract it from each individual spectrum. He line parameters: We select a spectral zone at central part of He line and fit a Gaussian function to each spectrum to define: central intensity(I), HWHM (Hw) and line shift (V).

8. Parameters of fitted Gaussian profiles CH Q Intensity (I): CH (red strip) and the center of CN (blue strip) have resembling values of intensity I and it is easy to misinterpret them. Half width (Hw): Hw in CH is bigger than the Hw in average spectrum, but Hw in the center of CN is smaller. This reflects a difference in their physical conditions. Hw-I: Hw and I have positive correlation for CH and anti-correlation in Q. Line shift (V): The centers of CH and CN reveal blue line shifts. The line shift in CH (as well as the “I”) is bigger than in CN. I-V: I and V variations correlate for both CH and CN. To see a sum and difference between parameters we normalize them by subtracting their respective quiet-sun means and dividing by the corresponding standard deviations. Parameters of the Gaussian profiles for a single row of data. The horizontal solid lines represent the values for average spectrum.

9. Hw normalization and Criterion for CHrecognition Inrm=(I-Iavr)/s I nrm+Hwnrm :level=2s c a c Hwnrm=(Hw-Hwavr)/s I nrm- Hwnrm b d The both normalized I(a) and Hw (b) images show bright CH. Borders and centers of CN are in opposite contrast on I(a) and Hw (b) images . The sum I+Hw(c) shows a double contrast of the CH and depresses the contrast of CN and we can objectively outline CH. The difference I-Hw(d) cancels the contrast of CH and increases the contrast of CN.

10. Line shift normalization Inrm=(I-Iavr)/s I nrm+Vnrm a c Vnrm=(V-Vavr)/s I nrm-Vnrm b d Normalized I (a) and V(b) images show an opposite contrast for both CH and CN features. The sum I+V (c) cancelcontrast of both CH and CN.The difference I–V (d) shows double contrast of the CH and CN. Smallincrease of V in center of CH is similar to increasing V in center of good developed centers of CN.

11. Summary • We applied a new method for the analysis of He 1083 nm imaging spectroscopy data to study a CH in the upper chromosphere. We have found: • He line profiles in CH have not only larger intensity relative nearby Q, but their Hw are 1.5-2.0 km/s broader and they show blue line shift up to 4 km/s. • Hw of the He profile in CHs and in chromospheric network are different. We use this difference to distinguish a CH from a quiet sun. The sum of normalized I and Hw images shows increasing contrast of CH and a depression a contrast in the network, and can be used as an objective coronal hole diagnostic. Intensity Hw & Intensity Acknowledgments: This research was partially supported by NASA Supporting Research and Technology task 344-12-52-14 and 344-12-52-19. NSO/Kitt Peak data used here were produced cooperatively by AURA/NSO, NASA/ GSFC, and NOAA/SEC.