STATISTICAL ANALYSIS OF EUV AND UV LINES INSIDE ANDOUTSIDE OF SOLAR CORONAL HOLES

I. E. DAMMASCH1, D. M. HASSLER2, W. CURDT1 and K. WILHELM1

1 Max-Planck-Institut fur Aeronomie, D-37191 Katlenburg-Lindau, Germany2 Southwest Research Institute, 1050 Walnut St., Boulder, CO 80302, USA

Abstract. Two studies performed with SUMER (the Solar Ultraviolet Measurements of EmittedRadiation spectrometer on SOHO) in September 1996 are presented. Spatial scans, a long exposuretime and a broad spectral window provide reliable line profiles for various regions of the Sun (off-limb corona, coronal hole, quiet Sun, bright and dark regions on disk) and for different formationtemperatures (Si II, 14 000 K; C IV, 100 000 K; Ne VIII, 630 000 K). Line intensity, position and widthdistributions are presented together with the line positions relative to the wavelength seen above thelimb.

1. Introduction

The transition region of the Sun has been the object of many studies using remotesensing techniques in EUV and UV emission lines. The temperature of the solaratmosphere increases from chromospheric values of 10 000 K to coronal levels of1� 106 K across the transition region. Also, the entire mass flow of the solar windhas to cross this region and evidence of outflow should be discernible with sensitiveobservational methods.

We will use SUMER (the Solar Ultraviolet Measurements of Emitted Radiationspectrometer) observations obtained in September 1996 from the SOHO (Solarand Heliospheric Observatory) spacecraft to study the statistical properties of someemission lines and search for Doppler shifts indicative of the initial stages of solarwind acceleration.

2. Observations and Data Analysis

The dates and some of the SUMER observational parameters are compiled inTable I. For both sequences the 100� 30000 slit was used with a step size of 300 and asampling time of 150 s. The central portion of the detector A was read out coveringthe lines Si II (�1533), C IV (�1548) in first order and Ne VIII (�770) in second orderin the 512 spectral pixels telemetered to the ground.

The fields of view of SUMER are shown in Figure 1 superimposed on anEIT/SOHO image taken on 21 September 1996 at 19:00 UT. In addition to thestandard SUMER data handling methods, we have set the average Doppler shiftsjust above the limb to zero based on the assumption that for optically thin conditionsthere should be no line-of-sight component there. Near the limb, both C IV and

Space Science Reviews87: 161–164, 1999.c 1999Kluwer Academic Publishers. Printed in the Netherlands.

162 I. E. DAMMASCH ET AL.

Table ISummary of the observational parameters

Date Time Field of View Steps Remarks1996 (UT) (00)

21 Sept. 00:16 – 07:27 121 E – 397 W 173 off-limb, coronal hole,630 N – 930 N quiet Sun

22 Sept. 00:40 – 08:11 476 E – 66 W 181 quiet Sun262 N – 562 N

Figure 1. The Sun on 21 September 1996 at 19:00 UT at the EUV wavelength of 171 A (Courtesy ofEIT consortium) and the positions of the SUMER raster scans.

Ne VIII may exhibit some opacity effects, but the strong limb brightenings indicateno major deviation from the assumption of an optically thin medium.

3. Results

The statistical results of the quiet Sun scan on 22 September 1996 are summarizedin Figure 2, in which the probability of intensities are shown on a logarithmic-linearscale; the line shifts and line widths, however, on linear-linear scales. All curvesfollow Gaussian distributions, which are broadest in the transition region (C IV)in each case. From simulation studies (Wilhelm et al. 1995), we have estimatedthe widths resulting from count statistics and detector effects, in order to demon-strate that the observed distributions are significantly wider. This was verified byde-convolution calculations. Similar intensity distributions have been reported byWilhelm et al. (1998) and Griffiths et al. (1999). We show here that the line shiftsand the total line widths are also normally distributed.

STATISTICAL ANALYSIS OF EUV AND UV LINES 163

Figure 2. (a) Probability of intensity versus intensity for the lines Si II, C IV, Ne VIII and the continuumnear 1543 A; (b) Probability of line shifts versus shift; (c) Probability of line widths versus width. Allresults are for quiet Sun conditions. Note the different scales in (a), (b), and (c). The full widths at halfmaximum (FWHM) resulting from count statistics and detector effects are shown on the right-handside.

In Figure 3, we present the most relevant findings concerning the line shifts ofC IV and Ne VIII. C IV does not exhibit a strong difference in the quiet Sun and thecoronal hole, whereas Ne VIII does with relative blue shifts of about –4 km s�1.Moreover, the change of the slope has the opposite sign leading to an average redshift on the disk for C IV and a blue shift for Ne VIII.

4. Conclusions

Probabilities of intensities on a logarithmic scale, and line shifts and line widths onlinear scales follow Gaussian distributions. These distributions are always broadestfor C IV, i.e., the central transition region contains the widest variations with respect

164 I. E. DAMMASCH ET AL.

Figure 3. (a) The line shifts obtained for C IV under the assumption of zero shift just above the limb.Regions in which quiet Sun and coronal hole conditions prevail are shown as well as fits based on acosine dependence. (b) The corresponding line shift for Ne VIII.

to radiation, speed and effective ion temperature. C IV is redshifted on the solar disk(+ 5 km s�1), and Ne VIII is blueshifted on the solar disk (–2 km s�1). This is in goodagreement with results of Peter (1999). Inconsistencies with earlier findings may beresolved by a new determination of the Ne VIII rest wavelength, to be published byDammasch et al. (1999). A significant blueshift of Ne VIII in the coronal hole whichwas already detected by Warren et al. (1997) and Hassler et al. (1999) is confirmedhere. The upflow speed estimated for Ne VIII is at least –9 km s�1, probably morewhen effects of blends with Si I lines are removed quantitatively.

The SUMER project is financially supported by DLR, CNES, NASA, and theESA PRODEX program (Swiss contribution).

References

Dammasch, I. E., Wilhelm, K., Curdt, W., and Hassler D. M.: 1999, A&A, in press.Griffiths, N. W., Fisher, G. H., Woods, D. T., and Siegmund, O. H. W.: 1999, Ap.J., in press.Hassler, D. M., Dammasch, I. E., Lemaire, P., Brekke., P., Curdt, W., Mason, H. E., Vial, J. C., and

Wilhelm, K.: 1999, Science, in press.Peter, H.: 1999, Ap.J., in press.Warren, H. P., Mariska, J. T., and Wilhelm, K.: 1997, Ap.J., 490, L187.Wilhelm, K., et al.: 1995, SPIE, 2517, 2.Wilhelm, K., Lemaire, P., Dammasch, I. E., Hollandt, J., Schuhle, U., Curdt, W., Kucera, T., Hassler,

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