ASTRONOMY AND ASTROPHYSICS. The hot prominence periphery in EUV lines. C.R. de Boer 1, G. Stellmacher 2, and E. Wiehr 3

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1 Astron. Astrophys. 334, (1998) ASTRONOMY AND ASTROPHYSICS The hot prominence periphery in EUV lines C.R. de Boer 1, G. Stellmacher 2, and E. Wiehr 3 1 Max-Planck Institut für Aeronomie, D Katlenburg-Lindau, Germany (cboer@osf1.mpae.gwdg.de) 2 Institut d Astrophysique, 98 bis Boulevard d Arago, F Paris, France (stell@iap.fr) 3 Universitäts-Sternwarte, Geismarlandstrasse 11, D Göttingen, Germany (ewiehr@uni-sw.gwdg.de) Received 26 November 1997 / Accepted 6 February 1998 Abstract. Two sets of He I and metallic lines were observed with the EUV spectrograph SUMER in a quiescent prominence. H, He, and Ca II lines were observed simultaneously with both German telescopes on Tenerife. The visible lines from elements with different atomic weights yield thermal and nonthermal broadening parameters of 7500 T kin 8000 K and 2.5 <ξ<5.0 km/s for the cool prominence body. The EUV lines, however, show line widths which correspond to much higher temperatures and non-thermal velocities. If the calculated formation temperature for every individual ion is assumed, the observed line widths require non-thermal velocities of km/s. The narrowest reduced widths of the EUV He I 584 and He I 537 lines are 3.1 and 2.9 times broader than those of the visible He D 3 and He 3888 lines. If this is due to optical depth effects in the EUV lines, one obtains τ and τ , respectively. The emission ratios of the Ca II to Balmer lines vary little inside the prominence, indicating a largely constant gas pressure. The ratios of the visual He to Balmer lines as well as those of the EUV He to metallic lines show a significant branching between peripheral and central prominence regions. The total emissions in the main prominence body amount to 13, 0.3, and 4 [Watt/ (m 2 ster)] for the 584, 537, and D 3 lines, respectively. The observed emission ratio E(He 584)/E(He 537)= 45 agrees with model calculations whereas their total emissions are about 37 times higher than calculated. The observed ratio E(He D 3 )/ E(He 584) 0.3 is about 15 times smaller than model predictions. The observations indicate that the emissions of different ions originate from individual (isothermal) threads with different temperatures between 10 4 and 10 5 K. Key words: Sun: prominences Sun: UV radiation 1. Introduction Solar prominences are cool, dense clouds embedded in much hotter and less dense coronal gas. They are commonly supposed to be supported by a weak magnetic field (a few Gauss). Their kinetic temperatures typically amount to 6000 <T kin < 8500 K. The emission lines are additionally broadened by non-thermal velocities of 2 <ξ<6 km/s. In the extreme UV, emission lines are observed which require formation temperatures much in excess of the T kin deduced from the visible lines. The model calculations by Heasley and Milkey (1978) show that a judicious choice of a restricted number of H, He, and Ca II lines may be sufficient for the determination of all relevant atmospheric parameters of the cool prominence matter. Systematic measurements of such prominence emission lines at high spatial resolution have been made since 1990 at the solar telescopes on Tenerife using modern CCD-sensors. From these observations, well-calibrated spectroscopic data were obtained for various quiescent prominences. These have firmly established previously existing, and also a few new relations: a) Bright and unstructured prominences with E max tot (H β ) 40[Watt/m 2 ster] are optically thick in the Hα line center (τα max 8) and cool (T kin 6500 K). In contrast, faint and structured prominences with E max tot (H β ) 10 [Watt/(m 2 ster)] are hot (T kin 8000 K) and optically thin (τα max < 2) (Stellmacher & Wiehr 1994b). b) The Balmer decrement reaches values of E α tot/e β tot 12 in faint prominences (or faint prominence regions), and decreases with brightness (Stellmacher & Wiehr 1994b, 1995). c) The He-to-H emission ratio shows a characteristic value for each prominence depending on T kin and on the structuring. Deviations from this mean ratio occur in fainter peripheral regions into which the He ionizing and exciting UV radiation can freely penetrate (Stellmacher & Wiehr 1995). d) The pressure sensitive emission ratio Ca II 8542/Hβ is found surprisingly constant across prominences indicating small spatial fluctuations of the gas pressure. The rather low mean value of P g pa is supported by a low density of n H [m 3 ], derived from the Helium singlet-to-triplet emission ratio (Stellmacher & Wiehr 1997). The transition between cool prominence and hot coronal matter has not yet been satisfactorly explained. Simple radiative cooling (similar to a cooling trap, Unsöld 1956), would partly rarefy the surrounding corona which is not observed. The transition layer can be studied from EUV emission lines (cf. Orrall & Schmahl, 1976; Engvold 1997, Chiuderi-Drago et al. 1992) using the SUMER instrument. Simultaneous ground based observations allow a comparison with existing prominence data.

2 C.R. de Boer et al.: The hot prominence periphery in EUV lines Observations On June 28, 1996, a quiescent hedgerow type prominence at E20 o S was observed with both German solar telescopes on Tenerife and with the ultraviolet spectrometer SUMER located on the ESA/NASA spacecraft SOHO (cf. Wilhelm et al 1997). With the evacuated Gregory Coudé telescope (GCT) the emission lines He I 3888Å and H Å were observed in the 14 th grating order simultaneously with Ca II 8498Å inthe6 th order with two CCDs integrated over 12 s. With the Vacuum Tower Telescope (VTT), Hβ in the 48 th, He-D 3 in the 35 th, and Ca II 8542Åinthe27 th order were observed simultaneously with three CCDs integrated over 3 s. A fourth CCD took Hα slit yaw images. The slits of both telescopes of correspondingly 1.5 arcsec width were oriented perpendicular to the horizon, i.e. in the direction of the (wavelength dependent) atmospheric refraction. This assures that the slit equally covers the same prominence structures for all wavelengths. At the same time, SUMER took in a first scan across the prominence the He I 584 Å emission in the 2 nd grating order simultaneously with O I 1152 Å, C I 1158 Å, and C III 1176 Å in the 1st order. In a second series, SUMER took He I 537 Å in the 2 nd, simultaneously with N II 1086 Å, S III 1077 Å, and S IV 1063 Å in the 1st order. The SUMER slit width corresponded to 1 arcsec, the integration time was 75 s. The 60 slit positions of each series took 75 min. Fig. 1 shows a Hα slit jaw image from the VTT together with a composed He I 584 image from SUMER. All CCD images were corrected for the dark and the gain matrices. For the ground-based spectra, the straylight aureole spectrum was subtracted, and the emissions were finally calibrated using the disc center intensities from Labs & Neckel (1970). The EUV emission calibrations are described by Wilhelm et al (1997). The emission lines were fitted by Gaussian profiles. The fine-structure of C I , C III , and N II was considered by fitting only the line centers of the strongest components (Fig. 2). Finally, shifts [km/s], widths [må], and integrated total emissions [Watt/(m 2 ster)] were determined. The shifts were normalized to the median wavelengths in the main prominence body. The narrowest lines, occurring for the He I emissions, show full widths at half central intensity of FWHM=70 må. This corresponds to 3.1 pixels and thus exceeds the spectral resolution limit of SUMER. Fig. 1. Composed He I 584 emission from SUMER together with the Hα slit jaw image from the VTT (upper) showing a much better spatial resolution than the spectra from space and ground 3. Results and their discussion 3.1. Ground based data Examples of simultaneous spectra of Hβ,HeD 3, and Ca II 8542 (Fig. 3) show (i) similar shifts of the three lines and (ii) that in fainter regions the He D 3 emission is relatively brighter than that of the other two lines. The widths of the simultaneously observed emission lines from ground were used to determine thermal and non-thermal line broadening parameters, making use of the different weights of the emitting atoms (cf. Bendlin et al., 1988). The ratio of the reduced 1/e-widths, λ D /λ, ofhβ and Ca II 8542 (cf. Fig. 4), indicates a narrow range of temperature, 7500 < T kin < 8000 K and non-thermal velocities 2.5 <ξ<6.0 km/s. The observed widths of He D 3 are compatible with these values for profiles where the red triplet component is well resolved. They show a mean Doppler width D =0.137 Å which fits to T kin = 8000 K and ξ =4km/s obtained from Hβ and Ca II Those He D 3 profiles, where the red triplet component is not resolved, are affected by limited spatial resolution and by superpositions of prominence structures along the line-of-sight. This effect also manifests itself in systematic

3 282 C.R. de Boer et al.: The hot prominence periphery in EUV lines Fig. 3. CCD-spectra of the simultaneously observed prominence emissions Hβ,HeD 3, and Ca from VTT; total height 60, wavelength increasing upwards over 2.2, 2.7, and 4.0 Å for the three lines, respectively Fig. 2. Spectral scans of the EUV emissions from the main prominence body (full lines) and from the adjacent solar disc (dashed lines); the spectral fine-structures are well separated for the narrow lines from the prominence excess widths of He triplet over singlet lines, observed by Stellmacher & Wiehr (1997). The pressure-sensitive emission relation of Ca II 8542 versus Hβ (Fig. 5) shows an almost constant ratio across the whole prominence indicating only small spatial variations of P g. Following the calculations by Heasley & Milkey (1978), the observed mean ratio of 0.43 corresponds to P g = pa. The temperature sensitive emission relation of He D 3 versus Hβ (Fig. 6) shows the known branching (Stellmacher & Wiehr 1995): High He to H emission ratios are found in peripheral regions, where the He ionizing and exciting UV radiation is more effective. The two violet lines He 3888 and H , observed simultaneously with Ca II 8498 at the Gregory telescope, yield an emission ratio He/H This value, together with the maximum emission E(H β )=14[Watt/(m 2 ster)], fits the relation given by Stellmacher & Wiehr (1994a) for T kin 8000 K and faint, hot, structured prominences (also established in Stellmacher & Wiehr, 1994b, 1995). The mean emission ratio of HeD 3 (VTT) to He 3888 (GCT) is found to be 15 for the central part of the prominence. This is about twice the value of 7.8 calculated by Heasley et al. (1974) for T kin = 8000 K. The corresponding emission ratio of Ca II 8498 (GCT) to Ca II 8548 (VTT) amounts for the main prominence body to 0.15, in good agreement with Landman & Illing (1977) SUMER data The spatial variations of the EUV emissions along the slit (Fig. 7) show smallest structures near 2 arcsec. The same is found for the line shifts. At the prominence borders the emission is

4 C.R. de Boer et al.: The hot prominence periphery in EUV lines ( D/ ) /( D/ ) Ca T Kin = 8000K T Kin = 7500K V tu (Km/s) E(HeD3) [10 watt m -2 ster -1 ] R = 0.7 R = E(H ) [10 watt m -2 ster -1 ] ( D/ ) Ca Fig. 4. Ratio of the reduced width of Hβ to Ca II 8542 as a function of the reduced Ca II 8542 width together with calculated kinetic temperatures and non-thermal line broadenings E(Ca ) [10 watt m -2 ster -1 ] R = E(H ) [10 watt m -2 ster -1 ] Fig. 5. Total emission of Ca II 8542 versus Hβ; the small scatter indicates largely constant gas pressure over the prominence enhanced in higher ionized lines whereas it rapidly drops for the cold lines O I, He I, and N II. The composed two-dimensional images in Fig. 8a show that these enhancements generally occur at the peripheral prominence regions and are much broader than the spatial resolution achieved. We find no indication for an increase of the typical spatial width with ionization state Line shifts The line shifts show a certain over-all agreement for the different EUV lines, but significant differences in the spatial details. Fig. 6. Total emission of He-D 3 versus that of Hβ full dots are data from the main body, open circles from the prominence periphery The clustering of velocity elements at a spatial scale near 2 arcsec is similar to results Engvold (1981) obtained from Ca II K Doppler filtergrams. This spatial scale for macro shifted velocity elements may explain the good over-all agreement of simultaneously observed line shifts in Fig. 9 which would not have been expected from the spatial details of the shift variations. It indicates that even lines which correspond to very different formation temperatures are emitted in regions with similar macro shifts (in agreement with Wiik et al. 1993). The scatter plots of the line shifts versus the total emissions (Fig. 10) show for the hot lines C III and S IV velocities of about 8 km/s at locations with relatively high total emissions, E tot > 1, respectively, > 0.03 [Watt/(m 2 ster)] which originate from the bright prominence periphery. The largest velocities (> 20 km/s) show a slight preference for a direction toward the observer, best seen in He I 584. Such high blue shifts are also seen in the visible lines in outer, fainter regions. They resemble the lateral motions often seen in Hα time series (e.g. Engvold 1981) Total emissions Like the line shifts, the total emissions also show a coarse agreement but significant differences in the detailed spatial distribution for lines of different ionization state. A comparison between the E tot values of the whole set of EUV lines is only reasonable for mean values from the main prominence body (cf. Table 2) since the integrated total line emissions vary significantly over the prominence (cf. Figs. 7 and 8a). Moreover, the two EUV observing series were taken 75 min apart during which the prominence borders exhibited noticeable changes. For simultaneously measured lines, however, a strict comparison is possible. The corresponding scatter plots (Fig. 11) show simple relations only for emissions corresponding to similar formation temperatures: E(SIV) / E(SIII) = 1.8 and (less tight) E(O I) / E(C I) 3 (cf. Table 1.

5 284 C.R. de Boer et al.: The hot prominence periphery in EUV lines Fig. 8a. Two-dimensional distribution of the integrated total emissions, E tot, of the EUV lines from series 1: C I 1158, O I 1152, He I 584, C III 1176 (left column from top to bottom), and series 2 (75 min later): He I 537, N II 1086, S III 1078, S IV 1063 (right column from top to bottom) Fig. 7. Spatial variation (along the slit) of the total emissions of series 1 (upper) and series 2 (lower panel) indicating bright emissions for hot lines at peripheral prominence regions; the slit positions corresponds to a horizontal line at the very center of Fig. 1b The other line pairs do not show simple relations. Here, the majority of data points from the main prominence body gives emission ratios which differ significantly from those of hot branches originating from the peripheral prominence regions (Table 1). This branching is most pronounced in the scatter plot of C III versus O I, less pronounced for C III versus He I, and only marginally indicated for S IV versus N II. In addition, the scatter plots show lower boundaries, indicating that the hotter lines are not emitted at spatial locations where the cooler lines have faintest emissions. This may be due to their different formation temperatures or to a different influence of the filling by threads. Fig. 8b. Two-dimensional distribution of line shifts from series 1: O I 1152, He I 584, C III 1176 (left column from top to bottom), and series 2: N II, S III, S IV (right column from top to bottom); black = blue-, white = red-shifts, for maximum values see Fig Line widths The reduced line widths (Fig. 12) from atoms of different weight do not fit a unique pair of kinetic temperature and non-thermal broadening, as do the visible lines. This might be due to different temperatures in volume elements where the EUV lines of different ionization are emitted. In addition, the broader lines may well originate from a superposition of different unresolved emitting structures in our spectra of 2 arcsec spatial resolution. Considering then only the narrower lines (at 0.1 maximum of the corresponding histogram) and assuming formation tem-

6 C.R. de Boer et al.: The hot prominence periphery in EUV lines 285 Fig. 9. Scatter plots of the Doppler shifts [km/s] of simultaneously observed EUV lines Fig. 11. Scatter plots of the total emissions [Watt/(m 2 ster)] of simultaneously observed EUV lines Table 1. Observed ratios of the mean total line emissions in different parts of the prominence lines body periphery lower limit CIII1176/OI C III 1176 / He I SIV1063/NII HeI5876/Hβ HeI3888/H Ca II 8542 / Hβ Fig. 10. Scatter plots of total emissions [Watt/(m 2 ster)] versus Doppler shifts [km/s] for He I 584, N II 1086, C III 1176, and S IV 1063 peratures of the NLTE calculations by Arnaud & Rothenflug (1985) and small optical thickness, we obtain for these widths non-thermal broadenings between 14 and 25 km/s (see Table 2). These values are similar to those found by Dere & Mason (1993) for the solar transition region. The majority of observed broader lines would yield much higher non-thermal velocities up to 42 km/s. When comparing the widths of different lines, one has to consider a certain dependence on the brightness, since narrowest lines occur preferably for faint emissions. As a consequence, C I, S III, and He 537 may actually be narrower than indicated in Table 2. Although the corresponding temperatures of the neutral O I, C I, and He I lines are rather similar, even these line widths do not give a consistent pair of T kin and ξ. Scatter plots of the reduced FWHM as a function of the total emission (Fig. 13) give no indication of an increase with brightness, in contrast to Wiik et al. (1993)

7 286 C.R. de Boer et al.: The hot prominence periphery in EUV lines Fig. 12. Scatter plots of the reduced half line widths of simultaneously observed EUV lines 3.3. Comparison of ground based and SUMER data Both neutral lines He D 3 and He 3888 yield reduced Doppler widths λ D /λ = This value is in accordance with the broadening parameters T kin 8000 K and ξ 5 km/s obtained from the Hβ and Ca II 8542 emissions. For He I 584, the narrowest observed line widths amount to λ D /λ = (FWHM/λ = in Fig. 13). This value is 3.1 times larger than that of the optically thin visible He lines. Interpreting the excess widths of these strong EUV He I resonance lines by optical depths effects (rather than by T> 8000 K), one can roughly estimate τ0 584 from the line profile at 1/e of the central intensity, I 0. At a wavelength distance of λ = λ e from the line center, it is: I e /I 0 =1/e =(1 exp( τ 0 e ( λe/ λ D) 2 )/(1 e τ0 ) which yields ( λ e / λ D ) 2 = ln[ 1 τ 0 ln(1 + e (1+τ0) 1/e)] For the optically thin case, one can take as reduced width that of the HeD 3 line, following Hirayama and Nakagomi (1974) who observed even for the He II 4686 line the same broadening as for the He I 4713 line. From the above formula we obtain τ0 584 = which is slightly below the range of τ calculated for T=8000 K and n H = [m 3 ]by Heasley et al. (1974) and Heasley& Milkey (1976). The majority Fig. 13. Dependence of the line half width, FWHM, on the emissions E tot for He I 584, N II 1085, C III, and S IV of He I 584 line widths is, of course, much broader and would thus yield larger τ 0 values. However, we suppose that these widths are affected by superpositions (see above). The fainter He I 537 line yields sufficient counts only for brighter prominence regions; its smallest widths are below the spectral resolution limit of 2 pixels. We thus reverse the analysis performed for He I 584: since both EUV He lines have the same lower excitation level, their τ 0 ratio is given by the ratio of their oscillator strength times the wavelengths and amounts to a factor of 4.1. The optical thickness of He I 537 should then be τ0 537 = This value yields 2.9 times larger width than that of the optically thin case, i.e. a reduced width of λ e /λ = or λ 537 e =36mÅ, in fair agreement with our observations. Comparing the EUV with the visible He lines, we find in the central prominence parts typical emissions of 13, 0.3, and 4 [Watt/(m 2 ster)] for the 584, 537, and D 3 lines, respectively. The ratio between the EUV emissions E(He 584)/E(He 537) = 45 as well as the absolute He D 3 emission agree well with the values calculated by Heasley et al. (1974) for T kin = 8000 K and n H =10 16 [m 3 ]. However, the observed absolute He EUV emissions are about 37 times higher than the model predictions and, correspondingly, the observed ratio between the visible and the EUV He emission, E(He D 3 )/E(He 584) = 0.3, is about 15 times smaller than that calculated.

8 C.R. de Boer et al.: The hot prominence periphery in EUV lines 287 Table 2. Observed quantities of the visual and the EUV lines: total emission, 1/e-widths, corresponding non-thermal broadening, ξ, for given formation temperatures, T kin ; [widths in brackets may be underestimated due to the faint emission]; the EUV lines are assumed to be broadened by optical depth effects line E tot 1/e width ξ T kin [W/m 2 ster] [må] [km/s] [K] H β Ca II He I He I He I (36) 8000 CI (65) OI C III NII S III (70) SIV According to Andretta & Jones (1997), the emission ratio E(He 584)/E(He 537) remains remarkably constant in a variety of atmospheric structures; their total emissions (originating in a self-field, generated within the slab) require a solution of the full NLTE radiative transfer problem. Calculations of uniform models (Heasley et al. 1974) show that the He emissions depend rather sensitive on the density. The strong EUV emissions may be formed in much hotter threads of low density, a case which has not been modeled. Although a detailed consideration of the filling by threads can not be deduced from our 2 arcsec resolution, our observation of a rapidly decreasing He I 584 emission toward the prominence periphery (where the emission of higher ionized lines increases; cf. Fig. 7) indicate that the He 584 emissions are related more to volume elements emitting He D 3 than to elements emitting the higher ionized lines. Consistent observations and model calculations reproducing simultaneously observed He I emissions are needed and may eventually lead to a reliable determination of the solar He abundance. 4. Conclusions Our observations indicate various manifestations of the transition between the cool prominence regions and their coronal surroundings. The enhanced emission of the hot lines at the prominence periphery seen in Figs. 7, 8a, and 11, indicate different physical conditions in the prominence periphery. The nonthermal broadening of the EUV lines exceeds that of the visible lines at least by a factor 6 (Tab. 1). An increase of non-thermal broadening towards peripheral regions was already found from visible observations (see Engvold et al. 1989a). This may reflect a highly dynamical state of the prominence plasma, possibly due to rapid variations in temperature and/or density of the threads, implying that the prominence is not in a stationary radiative state (Engvold 1997). The enhanced line shifts at the prominence periphery (Fig. 8b) may be a signature of energy fed into the prominence (Engvold et al. 1989b). The lack of detailed spatial coherence in Fig. 7 then shows that the emissions of different ions may originate from different isothermal threads with very different temperatures in the range K. This may also explain the branching in the emission relation between ions of different formation temperatures (Fig. 11). The larger spatial widths of emission regions at the prominence periphery may indicate a clustering of hotter threads. From these considerations we can draw the following picture: the central part of the prominence mainly consists of clusters of cool threads and is surrounded by regions where more and more hotter threads cluster. This seems to be supported by eclipse observations with a 3.6 m telescope which show significant structuring of the void around a prominence (November & Koutchmy 1996). The dimensions of the clusters of hotter threads can be estimated from our observed spatial variation of the total emissions along the slit (Fig. 7). The typical modulation in the SUMER spectra is about 2 arcsec which is nearly the same as in the VTT spectra. In contrast to these clusters, single emitting elements show widths of 300 km in our best resolved Hα slit jaw images (Fig. 1) in agreement with Engvold (1980, 1997) who finds dimensions down to 200 km width. The true sizes of threads may, however, be much smaller since frame selected Hα images show most spatial structures at the 150 km resolution limit of the VTT. Acknowledgements. Dr.P.Sütterlin kindly assisted with the ground based VTT observations. Drs. E. Marsch and K. Wilhelm contributed interesting comments. The GCT and the VTT on Tenerife are operated by the Universitäts Sternwarte, Göttingen and the Kiepenheuer Institut für Sonnenphysik, Freiburg, respectively, at the Spanish Observatorio del Teide of the Instituto de Astrofísica de Canarias. 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