Astron. Astrophys. 346, 285–294 (1999) ASTRONOMYAND

ASTROPHYSICS

The NeVIII (λ770) resonance line:solar wavelengths determined by SUMER on SOHO

I.E. Dammasch1, K. Wilhelm 1, W. Curdt 1, and D.M. Hassler2

1 Max-Planck-Institut fur Aeronomie, Max-Planck-Strasse 2, D-37191 Katlenburg-Lindau, Germany2 Southwest Research Institute, 1050 Walnut St., Suite 426, Boulder, CO 80302, USA

Received 18 December 1998 / Accepted 1 February 1999

Abstract. The investigation of the source and acceleration ofthe solar wind was among the prime scientific goals of SOHO –ESA’s and NASA’s Solar and Heliospheric Observatory. For thecoronal regions closest to the Sun, the bright extreme ultravio-let (EUV) resonance lines of the Neviii spectrum at 770A and780A with a formation temperature of 630 000 K are promisingtargets for detecting the initial solar wind outflow. The investi-gations published up to now using data obtained by SUMER –the Solar Ultraviolet Measurements of Emitted Radiation spec-trometer – on SOHO gave conflicting results. They accepteda rest wavelength of 770.409A for the brighter Neviii lineas reported in the literature. We deduce the solar wavelengthof this line for several conditions on the Sun and derive a restwavelength of (770.428± 0.003)A and outflow speeds in coro-nal hole regions of≈ 9 km s−1. This is in agreement with otherrecent studies using SUMER data. The rest wavelength determi-nation is based on the assumptions of no average Doppler shiftabove the solar limb and a comparison with laboratory wave-lengths of cool chromospheric lines. Quiet Sun areas exhibit anaverage blue shift of (1± 1) km s−1 in this line.

Key words: atomic data – line: identification – Sun: corona –Sun: solar wind – Sun: UV radiation

1. Introduction

SOHO – ESA’s and NASA’s Solar and Heliospheric Observa-tory – was launched on December 2, 1995 and placed at itsobserving station near the first Sun-Earth Lagrange point, L1,in February 1996. In constant view of the Sun, many detailedstudies of the solar interior and the atmosphere have been con-ducted since then. Two of the main goals of SOHO’s investi-gations have been defined as understanding the heating of thesolar corona and the acceleration of the solar wind.

SUMER – the Solar Ultraviolet Measurements of Emit-ted Radiation spectrometer – on SOHO was designed for highspectral and spatial resolution measurements in the wavelengthrange from 500A to 1610A. The Neviii (λλ770, 780) reso-nance lines in this range are of particular interest for transition

Send offprint requests to: I.E. Dammasch(Dammasch@linmpi.mpg.de)

region and coronal studies. Solar observations of the Neviiidoublet (2s2 S1/2 − 2p 2P3/2,1/2) were first reported by Fried-man (1960), who gives wavelengths of 770.5A and 780.2A(misprinted as 880.2A). Violett & Rense (1959) do not list theNeviii lines in their table of solar emission lines obtained fromrocket data, although the less intense Oiv line near 780A is in-cluded. Burton & Ridgeley (1970) measured (770.42± 0.05)A(1 σ) for the stronger line of the doublet with a rocket spectro-graph having a spectral resolution of 0.3A. Soon it was recog-nized by many researchers that the Neviii lines with a forma-tion temperature of 630 000 K can provide an ideal link betweenobservations of the transition region and the corona. Emphasiswas put on Doppler shift determinations of the Neviii (λ770)line (2s 2S1/2 − 2p 2P3/2) in an attempt to detect the initialoutflow of the solar wind from the lower solar atmosphere. It isthen clear that any absolute Doppler shift measurement – andthus the line-of-sight (LOS) ion speed,vLOS, deduced – wouldcritically depend on both an accurate measurement of the solarwavelength and a good knowledge of the rest wavelength of thisline.

From the Doppler formula,vLOS = c (λ − λo)/λo,whereλo is the vacuum wavelength of the line in question fora source at rest,c is the speed of light andλ is the observedwavelength, it can be seen that the detection of bulk motionsdown to≈ 1 km s−1, which is the desired velocity sensitivity ofSUMER, requires not only a resolving power of the instrumentof λ/δλ ≈ 3 105, which would be adequate to detect relativemotions with speeds of this magnitude, but also the knowledgeof λo with an uncertainty of about± 3 mA at λo ≈ 770 A foran absolute measurement. We hope to demonstrate below thatthese demanding requirements can indeed be met, if we use theSun as a wavelength transfer standard and employ the SUMERspectrometer in sub-pixel mode.

2. Laboratory wavelength measurements

Measurements of the wavelengths of emission lines of highlyionized species are a considerable experimental challenge andhave been performed only at very few laboratories. We will firststudy the literature data available for the transitions2s 2S1/2 −2p 2P3/2,1/2 of the Ne7+ ion. Kelly (1987) lists wavelengths of770.409A and 780.324A for the Neviii resonance lines and

286 I.E. Dammasch et al.: The Neviii (λ770) resonance line

cites the references Edlen (1963) and Bockasten et al. (1963).The same rest wavelengths are given in the National Instituteof Standards and Technology catalogue (NIST, 1995). Edlen inhis report refers to Bockasten and co-workers, and thus no inde-pendent data points have been established. The first laboratoryobservations of the Neviii lines by Fawcett et al. (1961) gavewavelengths of 770.42A and 780.34A with uncertainties of30 mA. They were obtained with a normal-incidence vacuumspectrograph and a high-temperature (a few 100 eV) deuteriumplasma with neon as trace element. The large Doppler widthcaused by the high temperature limited the accuracy of thismeasurement. Bockasten et al. used the high-current toroidaldischarge apparatus SCEPTRE IV as light source and a modi-fied Hilger E 793 3-m normal-incidence vacuum spectrographwith a plate factor of 2.78A mm−1 in first order. The authorsestimate the uncertainty of their Neviii (λ770) wavelength de-termination to be± 5 mA. We will, although this is not men-tioned in the original publication, assume a 1-σ level for thisuncertainty. No justification is given for the uncertainty esti-mate, but the same uncertainty levels are given for the Ovi(λλ1031.945, 1037.627) and Fvii (λ890.762) lines. In the lat-ter three cases, Kelly (1987) quotes values of 1031.924A,1037.614A, and 890.786A, respectively, taken from the fol-lowing sources: Ryabtsev (1975), Edlen (1934), and Palenius(1971). Brown (1980) measured the wavelengths of the Ovilines from spectrograms taken with the 6.6-m vacuum spectro-graph at the Naval Research Laboratory (NRL), Washington,and found 1031.929A and 1037.617A with uncertainties of± 5 mA each. If we attach the same credibility to the later mea-surements as to the earlier ones, it follows that the uncertaintyestimates of Bockasten et al. were too optimistic in three cases,and, therefore, it can safely be assumed that the estimate forNeviii was also too confident. Based on the root-mean-square(RMS) values of the deviations established for the lines Oviand Fvii, we estimate an uncertainty of± 18 mA (1 σ) forNeviii (λ770.409). The wavelength of the Nv (λ1238) line,given with an uncertainty margin of 10 mA by Bockasten et al.(1963), was later reported to be 16 mA longer (Hallin 1966),another indication that the uncertainty levels of Bockasten et al.were too narrow.

Despite a careful literature search, no other calculations orlaboratory measurements have been found, which could im-prove our knowledge on the important Neviii wavelength. Thesituation is much more favourable for UV lines emitted by neu-tral species or ions not so highly ionized. Eriksson and Isberg(1968) estimated the accuracy for Oi UV line measurements tobe ± 2 mA and for calculations± 0.5 mA or better. We willuse 144 Sii and four Ci lines in the wavelength interval from1531.6A to 1552.2A given by Kelly (1987) in our comparisonand, in addition, the Feii (λ1550.260) and the Nii (λ775.965)lines. In particular, we will synthesize a spectrum in this rangefrom all these lines with the help of their relative intensities af-ter noting that the intensity values agree with our observationsfor isolated lines and after adjusting the Ci, Feii, and Nii in-tensity scales to the Sii one. All lines were assumed to haveGaussian profiles. A line width of 134 mA (FWHM) was cho-

Fig. 1.Spectral line shifts caused by thermal deformations of the opticalbench of SUMER. The periodic contribution is correlated with the cycleof the thermal control heater. The drift reflects the thermal adjustmentof the instrument to the specific mode of operation, which is slightlydifferent for each sequence of observations. The shift was determinedby averaging the line positions of Neviii (λ770) along the slit and over± 10 raster positions during the scan of September 21, 1996 (solid line)and of September 22, 1996 (dotted line). “Red” and “Blue” indicatethe direction of the effect, if interpreted as Doppler shift.

sen according to our measurements. The spectrum was built upin wavelength steps of 1 mA. Most of the Sii lines referencedby Kelly are taken from Brown et al. (1974) and Moore et al.(1977). The wavelength uncertainties quoted are± 5 mA, how-ever, in general, the wavelengths are in agreement with othersources to within± 3 mA. The corresponding Ci data stemfrom Kaufman & Ward (1966) with uncertainties of± 1 mA.The source for the Ci observations was a water-cooled cop-per hollow cathode. The spectra were obtained with a 10.7-mEagle-type vacuum spectrograph having a first-order reciprocaldispersion of 0.78A mm−1. Johansson (1966) calculated wave-lengths of Ci UV lines and estimated the uncertainties to be lessthan 2 mA.

3. The SUMER spectrometer

SUMER is described in detail by Wilhelm et al. (1995a). Firstresults together with operational performance characteristics arediscussed in Wilhelm et al. (1997) and Lemaire et al. (1997).The instrument has no inflight wavelength calibration systemand, moreover, no stable optical bench other than is achievablewith a light-weight aluminium structure adequate for space ap-plications. It has a spectral pixel size of≈ 42 mA in first orderat 1540A and≈ 21 mA in second order of diffraction at 770A.The Doppler formula then yields a velocity sensitivity at thesewavelengths of 8.2 km s−1 per pixel. With simulation stud-ies (Wilhelm et al. 1995b), it could be shown that line shifts ofstrong lines can be determined relative to the unshifted line witha precision of 0.1 of a pixel even with short integration times ofseveral seconds. Similar results have been obtained under oper-ational conditions (e.g., Warren et al. 1997; Brekke et al. 1997;Judge et al. 1998). The mechanical deformations of the opti-

I.E. Dammasch et al.: The Neviii (λ770) resonance line 287

Fig. 2. The SUMER spectrum in the wavelength range from 1531.6A to 1552.2A in first order (765.8A to 776.1A in second order) observedon September 22, 1996 around 06:00 UT. The spectrum of the quiet Sun has been averaged over 83 raster steps (the exposures in the westof the scan) and shows the bright lines of Siii, Neviii (in second order) and Civ, in addition to many weak Sii lines, some Ci lines plusthe Feii (λ1550.260) and Nii (λ775.965) (in second order) lines. Their spectral positions are marked by dots (except for the 144 Sii lines).The radiometric calibration has been performed with the standard SUMER software and is valid for the first-order lines. The radiances of thesecond-order lines have to be reduced by a factor of 3.38 to match this calibration. The horizontal bars indicate the ranges which will later beused for an absolute wavelength calibration of Neviii (λ770).

cal bench caused by residual periodic temperature variations of± 50 mK of the thermal control system led to spectral shifts of± 0.15 pixel within 2 hours, and also to a trend of≈ 0.5 pixelswithin 8 hours. This can be seen from Fig. 1 for the observationsdiscussed here. The drift is caused by a thermal adjustment to theconditions of the specific observational sequence. The thermalcontrol situation is, however, dependent on the specific mode ofobservation, and has to be studied on a case by case basis (see,e.g., Curdt et al. 1997).

Because of our accuracy requirements, we have to improveon the standard SUMER data analysis methods. We intend todo this by using information derived from the data directly asdescribed in Sect. 5.1. It is important to mention that Neviii(λ770) can be observed in second order with the SUMER de-tector A (with a wavelength range in first order from 780A to1610A). In this case the first order lines near 1540A are seenin the background of the intense neon line. This is essential forour arguments, as the grating formula,mλ = d (sinϑ + sinα)is strictly linear inm, wherem is the order of diffraction,d isthe grating spacing,ϑ is the angle of incidence on the grating,andα is the angle of reflection off the grating. Up to≈ 40A canbe exposed simultaneously on the detector in first order; 20Athereof on the potassium bromide (KBr) photocathode. The se-lection of a wavelength band for observation is done by puttinga certain wavelength on a specific reference pixel of the detec-tor. Without re-adjusting the grating position, one therefore canlook at the strong lines Neviii (λ770), Siii (λ1533) and Civ(λλ1548, 1550) together with numerous weaker lines, mainlyof the Sii and Ci spectra. By rastering the slit position acrossthe Sun, maps of certain solar areas have been built up.

4. Observations and data overview

In September 1996, a SUMER observational sequence was exe-cuted to determine the Doppler shifts of the Neviii (λ770) andthe Civ lines. The sequence was run twice. First on September21, 1996 from 00:16 to 07:31 UT near the north polar coronalhole; then on September 22, 1996 from 00:40 to 08:14 UT in amid-latitude region on the solar disk. In both cases, a wavelengthof 1541.5A was positioned on reference pixel 531 of detector A,and half of the detector array (512 spectral× 360 spatial pix-els) around that position was transmitted to the ground. Thisprovides us with a spectral window of≈ 20A on the KBr pho-tocathode as shown in Fig. 2 for a portion of the second run. Aslit with angular dimensions of 1′′× 300′′ was used, spectrallyresolved images of the slit were taken with an exposure timeof 150 s each, and the field of view was rastered with a stepsize of 8 elementary steps corresponding to 3.′′01 in east-westdirection. On the first day, exposures were performed at 173raster positions between 121′′ east and 397′′ west, with the cen-tre of the slit pointing to 780′′ north (from 630′′ to 930′′). On thesecond day, 181 exposures were taken between 476′′ east and66′′ west centred at 411′′ north (from 261′′ to 561′′). No onboardflat-field correction was used. The fields of view are shown inFig. 3 inserted in an EIT image (Extreme-ultraviolet ImagingTelescope, Delaboudiniere et al. 1995) taken between the twoSUMER rasters. The spectral window contains the strong linesof Si ii and Neviii as well as the Civ doublet. These lines andthe adjacent continuum radiation have been used to visualize inFigs. 4 and 5 the properties of the solar atmosphere at differenttemperature levels, namely at 14 000 K in the Siii (λ1533) line,at 100 000 K in the Civ (λ1548) line, and at 630 000 K in the

288 I.E. Dammasch et al.: The Neviii (λ770) resonance line

Fig. 3. The Sun seen in the wavelength band near 171A (Feix, Fex)by EIT/SOHO on September 21, 1996 at 19:00 UT (by courtesy of theEIT consortium). The raster scans of SUMER are shown as rectanglesnear the limb on September 21, 1996 and at mid-latitudes on September22, 1996. The areas indicated in the west of 83 raster steps each havebeen used to deduce the Neviii wavelengths.

Neviii (λ770) line. The carbon line is a typical transition re-gion line, whereas Siii and Neviii show the situations at thelower and higher temperature levels. The Li-like Ne7+ ion isparticularly well suited for observations over a wide tempera-ture range, because of the long tail towards high temperaturesof its contribution function,G(T ), (cf., Doschek et al. 1998;Laming et al. 1998).

Not only the line characteristics (intensity, Doppler shiftand line width) are of interest, but also the continuum radia-tion. It was estimated by determining “continuum” pixels in aspectral window around each line. The background correctionwas performed by subtracting the continuum value from pixelscontaining “line” information, and their remaining values wereintegrated to calculate the line intensity. Further details of thedata analysis methods will be discussed in the next section.

An exhaustive analysis of the diagrams shown in Figs. 4 and5 is beyond the scope of this contribution, but a few salient char-acteristics should be mentioned: (1) The chromospheric networkcan easily be recognized in the intensity maps of both the coro-nal hole and the quiet Sun regions for the Siii and Civ (λ1548)lines and the continua. (2) The network appears to be very dif-fuse for the quiet Sun in Neviii and disappears in the coronalhole. (3) The velocity maps confirm the intensity results. Thereis no significant difference between coronal hole and quiet Sunregions for Civ and Siii, but the Neviii line is shifted muchmore towards blue in the hole, however, less so in areas brightin this line. (4) The sizes of the upwards and downwards mov-ing structures increase with increasing temperatures. (5) Theline widths are, in general, positively correlated with the lineintensities for all lines.

These observations also form the basis of studies by Hassleret al. (1999) and Dammasch et al. (1999), which concentrateon the outflow speeds in quiet Sun areas as well as in coronalholes deduced from the Neviii and Siii lines and on a statisticalanalysis of the lines. In what follows, we will focus our attentionon the wavelength determinations for the Neviii (λ770) line anddefer a detailed investigation of the other lines.

5. Data analysis and results

5.1. Methods and corrections

All data were decompressed to reverse the onboard compres-sion scheme. Given the extreme accuracy we ultimately wantto achieve, many correction and calibration procedures have tobe applied. Basically, two approaches can be used to correct forpossible instrument biases: We can either employ external in-formation, such as a detector flat field to correct intensities or adetector grid image to correct geometric distortions, or exploitthe data itself to estimate and correct the effective biases of theparameters. We will use here the second approach, but, when-ever possible, a verification with additional information will beperformed.

In the first approach, a flat-field removal would correct themeasured counts with a known detector sensitivity bias factorbeforecalculating the line and continuum parameters. In thesecond approach, the influence of the detector bias is treated asa “black box”; its effect is estimatedafterwardsby evaluatingthe whole raster. For instance, the flat-field fluctuations wereremoved by correcting for sinusoidal variations of different pe-riodicities, which can be observed in averages over the wholeextension of the east-west raster.

An exposure of the detector at a raster step1 ≤ r ≤ R leadsto a matrix of intensity countsni,j,r, where0 ≤ i ≤ 359 is thespatial pixel address and0 ≤ j ≤ 1023 is the spectral pixeladdress. The calculations of line and continuum intensities, lineshift and widths are functions operating on subsets of this datacube, (i.e., on profiles):fi,r = f(ni,j′,r), wherej1 ≤ j′ ≤ j2mark the borders of a spectral window.

Let fi =∑R

r=1 fi,r/R be the average off applied to spatialpixel i across the raster. Leti1 ≤ i′ ≤ i2 be an interval of lengthP symmetrically aroundi, andFi,P =

∑i2i′=i1

fi′/P a movingaverage of lengthP . Thenci,P = fi/Fi,P measures the effectof the detector bias in the spectral windowj1 ≤ j′ ≤ j2 definedabove at spatial addressi on operatorf during the raster. If theraster is unbiased,ci,P will also estimate the real detector effectin an unbiased way.

ci,P is a correction factor close to 1.0, and the correctionvector along the slit is usually similar to a sinusoidal curveof spatial periodP . The operatorf is then corrected to bef∗

i,r = f(ni,j′,r)/ci,P for each pixel of the 2-dimensional fieldof view 0 ≤ i ≤ 359, 1 ≤ r ≤ R, andP = 2, 7, 15, 29;these spatial periods were detected empirically. This procedurewas, for instance, employed in producing Figs. 4 and 5 afterthe centroid position was deduced by integrating the line inten-sity across the spectral window and determining the 50% pointwith sub-pixel precision. This is adequate for an overview oftwo-dimensional velocity structures, if blends are of no majorimportance or have equal influence everywhere. The line widthwas estimated by using the relation between the full width at halfmaximum, FWHM, amplitude,A, and integral,I, of a Gaussian:FWHM = 2 I

√ln 2/(A

√π).

The determination of the detector curvature needed to cor-rect the effect of the geometric distortion on the position of the

I.E. Dammasch et al.: The Neviii (λ770) resonance line 289

Fig. 4. Diagrams showing theSeptember 21, 1996 results. Theflat-field corrections and detec-tor destretching routines outlinedin Sect. 5.1 have been applied tothese data. The first row gives lineintensities, the second continuumintensities, the third line shiftsand the fourth line widths. Thedifferent lines are arranged in threecolumns. Note the similarity of thecontinuum maps. Line radiances inunits of photon s−1m−2sr−1 arebetween 2.2 1016 and 6.5 1017

(Si ii), 2.0 1016 and4.6 1018 (C iv at1548A), and1.8 1015 and1.3 1017

(Neviii). Continuum intensitiesare between2.8 1016 and 3.7 1017

photon s−1m−2sr−1A−1. Line shiftsare between± 30 km s−1. Linewidths are between 181 mA and544 mA in FWHM of the Gaussfits (not corrected for instrumentaleffects) or 19 km s−1 and 63 km s−1,respectively, in corrected Dopplerwidth.

Fig. 5. Display of the data ofSeptember 22, 1996 for a quietSun region using the same ar-rangements and calibration fac-tors as in Fig. 4.

290 I.E. Dammasch et al.: The Neviii (λ770) resonance line

Fig. 6. Curvature of the detector in the spectral regime for the lineNeviii (λ770) as a function of the spatial pixel position from Septem-ber 22, 1996 data (solid line) and, for comparison, the correspondingcurve (dotted line) obtained from the standard SUMER geometric dis-tortion correction for the same detector position. (Note that the cur-vature is dependent on the detector position and therefore will varyfor different spectral lines.) The limit of applicability of the standardprocedure at position 339 (dashed-dotted line) is shown as well as theinterval (from spatial pixel 225 to 286; horizontal bar) used for theabsolute wavelength calibration in Sect. 5.2. It was placed at a pixelposition where the spectral correction is close to zero (dashed line).

Neviii line in the spectral regime was obtained with the helpof September 22, 1996 data, because this raster covered a ratheruniform area on the disk, i.e., without any possible bias fromcoronal hole or limb portions justifying the assumption that eachspatial pixel along the slit should have seen similar regions ofthe Sun across the raster scan. The line position as a functionof the spatial pixel is estimated by averaging the line centroids(obtained as outlined above) across the raster (i.e., the 83 expo-sures in the western portion) and is plotted in Fig. 6. Also shownis the spectral correction function resulting from the standardSUMER detector distortion correction (Moran 1997). The rangeof its application is recommended to be from pixel 19 to 339 inthe spatial regime of Fig. 6, which shows detector pixel numbersin reversed order (0: south; 359: north). Within these limits, bothdetector corrections do not differ by more than± 0.2 pixel. Thislends support to our detector curve, which was then subtractedin both data sets to deduce relative line shifts. Note, however,that a portion of the off-limb corona (spatial pixels> 339) isnot covered by the standard procedure.

The temporal variations, due to thermal influences (heatercycle and temperature drifts), have then been subtracted. Theyhave been estimated by averaging the residual line centroid po-sitions (i.e., after the detector correction) across the north-southextension of the slit. (They have been shown in Fig. 1). TheDoppler shift induced by the solar rotation speed of 2 km s−1 atthe equatorial limb has been taken into account by calculatingthe LOS velocities for all pixels as a function of their nominalmapping onto the solar disk. Without this correction a clear solarrotation signal would be discernible demonstrating the velocitysensitivity of SUMER (see also Fig. 5 of Peter 1998). All thesecorrections are, however, not sufficient to deduce absolute line

positions, because there may be systematic line shifts on thedisk (solar effects), and because the wavelength positions onthe detector may have moved between the observations (detec-tor effects).

5.2. Absolute wavelength calibration

For an absolute wavelength calibration, an extended spectralwindow is required around Neviii (in second order) containingcool chromospheric UV lines (in first order). It can reasonablybe assumed that these lines have no or very small solar Dopplershifts (Hassler et al. 1991; Brekke et al. 1997; Chae et al. 1998).The method for the detector curvature correction in the spec-tral regime along the slit as described above (cf., Fig. 6) is onlysuitable for single lines and has to be modified for a wide spec-tral window. To achieve this, the distortion correction will beperformed in such a way that the average correction is closeto zero. We selected an interval of 62 spatial pixels (approxi-mately± 10% of the detector around the spatial pixel 256 inFig. 6) to perform the calculation of an average profile for theextended spectral window. In this interval, the empirical detec-tor curvature employed here is also very close to the one usedby the standard SUMER destretching procedure. The last 83exposures of both raster scans are taken because (a) the coronalhole scan contains an off-limb region there, (b) the thermal drifteffects were stronger at the beginning of the scans than at theend. The spectra have simply been added in the spatial dimen-sion over the 62 pixel interval. Even if this will induce a certainincrease in line widths, it should not, with our parameter choice,lead to a net line shift.

For an identification of the numerous cold lines, a crudewavelength scale was first established for the mid-latitude scanon September 22 by fitting literature wavelengths of the peaks ofthe prominent lines Siii (λ1533.431), Neviii (λ770.409), Civ(λ1548.202) (a linear fit has been used, i.e., it has been assumedthat the spectral pixel size is constant). The resulting spectralpixel size in this first step is 42.4 mA. (The nominal pixel size,based on the optical design, was predicted to be 42.3 mA fordetector A in this wavelength range.)

With the help of our preliminary wavelength scale, in a sec-ond step, 51 cold lines or line blends could be identified in thesame data set. In the case of line blends, the expected wavelengthwas calculated as the average nominal position of the lines in-volved, weighted with their literature intensities. The pixel po-sitions were fitted against the literature or expected wavelengthsin a third-order polynominal fit, under the assumption that thepixel size is a quadratic function of the detector position. Theresult is a pixel size between 41.6 mA and 42.6 mA, with themaximum close to the middle of the detector. The differencesbetween expected and observed wavelengths of the lines havealways been smaller than 21 mA or 0.5 pixel and the standarddeviation is 10 mA or 0.25 pixel. The uncertainty estimate givenby the fitting routine for the wavelength range around 1540A isbetter than 3 mA. This defines a wavelength scale with variablepixel size against which the measured intensities can be plotted.

I.E. Dammasch et al.: The Neviii (λ770) resonance line 291

Fig. 7a and b.Comparison of observed spectra in coronal hole (solid line) and quiet Sun (dotted line) regionsa with a synthetic spectrum in thelower panelb. It was built up, as described in the text, by combining all Sii and Ci lines given by Kelly (1987) in this range. The interval markedby a horizontal bar covering the Neviii line was not considered when performing the cross-correlation analysis in Sect. 5.2. The syntheticspectrum is based on wavelength measurements in laboratories. By comparing the solar observations with this spectrum, the relativistic red shiftof 0.62 km s−1 has been taken care of. The spectral radiance is given for the first order lines as in Fig. 2.

In a third step, we have to determine the wavelength off-set which might be present between both days. The syntheticspectrum constructed in Sect. 2 and the spectra observed onSeptember 21 and 22 in our selected spatial interval of 62 pixels(with the wavelength scale defined above for the second day)have therefore been compared by a cross-correlation analysisin the wavelength range from 1538.16A to 1543.28A. For thisanalysis, the interval containing the Neviii line in second orderobviously has to be excluded (from 1540.38A to 1541.23A).The corresponding spectra are shown in Fig. 7. The highest cor-relation coefficient of the left portion of the spectrum of Septem-ber 21 is 0.80 and of the right portion 0.98 (with a mean dis-placement of + 13 mA). The corresponding coefficients for theSeptember 22 raster are 0.49 and 0.97, respectively, so only theshift corresponding to the right interval (+ 1 mA, thus essentiallyverifying the wavelength scale obtained in the second step) istaken into account. Consequently, both wavelength scales havebeen adjusted by a relative shift of 12 mA.

After applying these procedures, we have specified a wave-length for each spectral pixel of the detector transmitted onSeptember 21 and 22, and thus the wavelengths for the pixelsselected around the Neviii line (j1 ≤ j′ ≤ j2 as described inSect. 5.1) are uniquely identified. As a result we obtain rectifiedwavelength scales, which can be used for determining the spec-

tral position of the Neviii line under specific solar conditionsin the next section.

5.3. Doppler shifts and rest wavelength of Neviii (λ770)

The determination of the line centroids (measured in spectralpixel positions) and various corrections have been describedin Sect. 5.1. A unique wavelength has been defined for eachspectral pixel and both scans in Sect. 5.2. With this information,we are able to deduce the wavelength positions of the Neviiiline for each spatial pixel on both days. In order to determineDoppler shifts, we have to define the rest wavelength first.

The western portion of the solar area surveyed on Septem-ber 21, 1996 contains quiet Sun, coronal hole, and limb regions,as well as a small part of the off-limb corona. The line positionobserved off limb is assumed to have zero Doppler shift, be-cause any bulk plasma motion must, for reasons of symmetryand for optically thin media, average out along the line of sightthere. Optically thin conditions can be expected for Neviii inthe corona at altitudes higher than 20′′ above the photosphere(Doschek et al. 1998). This provides us with a wavelength scalewhich is independent of a Neviii (λ770) laboratory rest wave-length. The rest wavelength is thus defined as the average of the400 positions off the limb between 980′′ and 1000′′ distance

292 I.E. Dammasch et al.: The Neviii (λ770) resonance line

Table 1.Wavelengths obtained for Neviii (λ770) from mean line po-sitions on concentric circles and the resulting LOS speeds (cf., Fig. 8)

Region on Sun Wavelength,λ Speed,vLOS

(A) (km s−1)

Corona (off limb)a 770.428 ± 0.0Coronal holea 770.412 −6.2Quiet Sunb 770.426 −0.8

a September 21, 1996b September 22, 1996

from the disk centre. At greater altitudes, the spectra becomerather noisy and are dependent on a very small number of de-tector pixels near its edge, for which, moreover, the correctionroutines become unreliable. Hence they are disregarded for es-tablishing the rest wavelength.

LOS Doppler velocities for each spatial pixel of both scans(83 raster positions each) can then be deduced with the helpof the Doppler formula. The velocity structure across the Sunis obtained by averaging these velocities in concentric circleswith a width of 1′′ around the solar disk centre. The result shownin Fig. 8 is similar to that of Fig. 3(b) of Hassler et al. (1999),who calibrated the wavelength of the Neviii line with an elab-orate scheme using a Siii baseline. As for the off-limb corona,the mean wavelengths have also been calculated for two othertypical regions on the disk, namely the “coronal hole” regionbetween 880′′ and 900′′ (near the extreme of the blue shift), andthe “quiet Sun” region between 280′′ and 560′′ from the diskcentre (cf., Fig. 8). In Table 1, we list the wavelengths of theNeviii line for these regions.

With the assumptions of zero Doppler shift of Neviii in theoff-limb corona and no average shift of the atomic lines, we finda rest wavelength for the line Neviii (2s2 S1/2 – 2p 2P3/2) of770.428A. The line is then shifted predominantly towards bluein the other regions. By considering the uncertainties of the fitroutines, which give 3 mA, and taking into account the Gaussianfit and cross-correlation results as well as the uncertainties ofthe atomic line data, we estimate an overall uncertainty level of6 mA (1 σ) for our wavelength determinations in first order, or3 mA (1 σ) in second order.

The average wavelength measured on the disk (770.426A)would indicate a redshift of 6.6 km s−1 if the old literaturewavelength of 770.409A is assumed. This is, indeed, consistentwith the results obtained by Brekke et al. (1997) and Chae et al.(1998).

6. Discussion and conclusions

Several studies using SUMER data have been conducted withthe aim of determining the Doppler shift pattern of solar emis-sion lines (e.g., Brekke et al. 1997; Warren et al. 1997; Chae et al.1998; Brynildsen et al. 1998; Peter 1998; Hassler et al. 1999; Pe-ter & Judge 1999; Stucki et al. 1999). In many cases, the Neviii(λ770) line played an important role in these studies. However, itbecame increasingly clear that a consistent interpretation of the

Fig. 8.Average line shifts of the Neviii (λ770) line between 150′′ and397′′ west on September 21, 1996 (right data set), and between187′′ east and 66′′ west on September 22, 1996 (left data set). Dotsindicate averages calculated in concentric circles around the solar diskcentre of 1′′ width each. The left data set is characterized by quietSun conditions, whereas the right data set shows quiet Sun, coronalhole, and off-limb corona conditions. Note that the quiet Sun values onboth days are in agreement, thus confirming the relative positions ofthe rectified wavelength scales. Notice also the blue shift immediatelyabove the limb, probably caused by the opaqueness of Neviii there.Intervals used to determine typical average wavelengths and resultingline shifts (cf., Table 1) are indicated by horizontal bars. The limb ismarked at 960′′ (dashed-dotted line) as seen in the continuum. Withinthe pointing uncertainty, this is consistent with the photospheric limbat 965′′.

results met with difficulties under the assumption of a vacuumwavelength of the Neviii line at 770.409A. This motivated ourattempt to determine an accurate rest wavelength for this line.We findλo = (770.428± 0.003)A, where the uncertainty esti-mate is given at the 1σ level. The wavelength uncertainty trans-lates into a velocity uncertainty of± 1.2 km s−1. Note, however,that the LOS speeds relative to the baseline in the corona canbe determined with higher precision. Our value is in excellentagreement with the results of Peter (1998) who used a SUMERfull Sun Neviii image to estimateλo = (770.42± 0.01)A, aswell as Peter & Judge (1999) who analysed SUMER spectraobserved during a 360◦ roll manoeuvre of SOHO and obtainedλo = (770.428± 0.007)A. The new value is also consistentwith all rest wavelengths reported in the literature for Neviii, ifthe uncertainty assessment of Sect. 2 is taken into account. Thiscan be seen from Table 2 where the rest wavelengths availableare compiled with their uncertainty margins.

Assuming the new rest wavelength, a net blue shift forNeviii would prevail for the quiet Sun in the reports of Brekkeet al. (1997) and Chae et al. (1998). In general, all LOS speedsdeduced from this line would be shifted by an increment of7.4 km s−1 towards the outflow direction. At this stage, we can-not comment on other lines, more than what has been stated inrelation to Figs. 4 and 5. The data obtained for this study willallow us to deduce additional information on the Civ lines in

I.E. Dammasch et al.: The Neviii (λ770) resonance line 293

Table 2. Comparison of the rest wavelength determinations for thetransition Neviii (2s2 S1/2 – 2p 2P3/2)

Wavelength,λo Uncertainty Reference(A) (A)

770.5a – Friedman (1960)770.42 ± 0.030 Fawcett et al. (1961)770.409 ± 0.005b Bockasten et al. (1963)770.42a ± 0.050 Burton & Ridgeley (1970)770.42a ± 0.010 Peter (1998)770.428a ± 0.007 Peter & Judge (1999)770.428a ± 0.003 this work

a Solar wavelengthb Re-assessed in this work:± 0.018A

a future publication. But the Mgx line, in particular, will alsohave to be re-assessed before the issue of the average Dopplershifts of solar emission lines can be settled. In this context, itshould be mentioned that Peter & Judge (1999) came to the con-clusion that the rest wavelength of Mgx (λ625) also must beoffset by 18 mA or 8.6 km s−1, and Stucki et al. (1999) find blueshifts for Neviii and Fexii in coronal holes of≈ −4 km s−1

and≈ −5 km s−1, respectively. For the Fexii (λ1349) line, ablue shift of−6 km s−1 at disk centre with respect to the limbwas reported by Sandlin et al. (1977).

With a mean projection angle of 70◦ in the coronal holeand the assumption of an outflow direction according to thedirection seen in Neviii near the limb (cf., Fig. 2 of Wilhelmet al. 1998), which is≈ 25◦ off the radial direction, we get anaverage outflow speed of 9 km s−1 from the LOS velocity inTable 1.

The coronal hole profile of Neviii exhibits a significantshoulder on the red slope (cf., Fig. 7). Even after deducting theSi i contribution, a deviation from a Gaussian shape is evident inFig. 9(b). With a multi-Gauss fit, we have found that the Neviiiline is centred at 770.414A. Note that the coronal hole data setused in this figure is not identical with that of Fig. 8 and, conse-quently, the small difference in the corresponding wavelengthsof + 2 mA does not constitute a discrepancy. The red-wing line ispositioned at 1541.040A (in first order). The only line observedin solar spectra near this position is the H2 line at 1541.063A(Sandlin et al. 1986). We are not concerned with the line iden-tification, but have to demonstrate that such a blend does notinterfere with our arguments and the centroid method employedin Sect. 5.1. To this end, we have determined the wavelength ofNeviii in the coronal hole before and after deduction of the Siiand H2 (?) contributions and find differences of less than our un-certainty margin of± 3 mA. In any case, we would estimate theNeviii line position in the coronal hole to be too “red” and, con-sequently, our coronal outflow speed might be underestimatedby ≈ 1 km s−1. It should also be noted that this complicationis not present in the quiet Sun regions and, in particular, in theoff-limb corona, where the Neviii (λ770) rest wavelength isobtained (see Fig. 9(a) and (c)).

Fig. 9a–c.The spectral window containing the Neviii (λ770) line. Lineprofiles are plotted for the three representative solar regions defined inTable 1.a The profile in the off-limb corona (solid line) is of Gaus-sian shape (dotted line). Thelower panelsb andc display the Neviiiline profiles of Fig. 7 on a larger scale (solid lines) and demonstratethe effect of subtracting the Sii contributions (dashed-dotted lines)and the continuum radiation. The resulting profiles (dashed lines) arethen approximated by three Gaussians each (dotted lines). Radiometriccalibration as in Fig. 2.

It would be highly desirable if laboratory measurements andtheoretical calculations could be conducted to establish state-of-the-art rest wavelengths for Neviii and other lines in spectra ofhighly ionized species, such as Mgx and Fexii, for comparisonwith solar wavelength determinations.

294 I.E. Dammasch et al.: The Neviii (λ770) resonance line

Acknowledgements.The SUMER project is financially supported byDLR, CNES, NASA and the ESA PRODEX programme (Swiss con-tribution).

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