www.elsevier.com/locate/asr

Advances in Space Research 39 (2007) 1876–1881

Quiet sun XUV and EUV spectroscopy

P. Lemaire *

Institut d’Astrophysique Spatiale, CNRS-Universite de Paris XI, 91405 Orsay, France

Received 29 October 2004; received in revised form 7 May 2007; accepted 9 May 2007

Abstract

The two XUV–EUV spectrometers on SOHO have collected a large amount of data in the 6000–106 K solar plasma temperaturerange. These data have allowed us to greatly enhance our knowledge of the processes acting in the solar atmosphere, from the chromo-sphere to the corona. Some results on the quiet Sun structure (network, quiet Sun versus coronal hole), on the dynamics (velocities,waves, transient events), and the main characteristics of the quiet Sun atmosphere are presented and discussed.� 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Sun; Chromosphere; Chromosphere–corona transition region

1. Introduction

In the 10–200 nm wavelength range most of the solaremission lines are tracers of the chromospheric to coronalplasma in the 6000–106 K temperature range. Since thebeginning of 1996 the two XUV–EUV spectrometers onSOHO (CDS – Coronal Diagnostic Spectrometer andSUMER – Solar Ultraviolet Measurement of EmittedRadiations) have allowed us to greatly enhance our knowl-edge of the processes acting in the solar atmosphere, fromthe chromosphere to the corona.

Previous solar UV spectrometers (Wilhelm et al., 2004)have limitations:

- spectral resolution of 0.16 nm (HCO/S055 on ATM/SkyLab, Reeves et al., 1977)

- few pixels detection (LASP/UVS on OSO8, Bruner,1977; LPSP/multichannel on OSO8, Bonnet et al.,1978; GSFC/UVSP on SMM, Miller et al., 1981)

- recording the data on photographic emulsion eitherwith limited stigmatism (NRL/S082B on ATM/Skylab, Bartoe et al., 1977) or short-duration observa-tions (NRL/HRTS on rocket or on SpaceLab-2,Brueckener et al., 1986).

0273-1177/$30 � 2007 COSPAR. Published by Elsevier Ltd. All rights reserv

doi:10.1016/j.asr.2007.05.027

* Tel.: +33 (0) 1 6985 8622.E-mail address: lemaire@ias.u-psud.fr

The data obtained by these spectrometers, combinedwith the EUV imagers (EIT on SOHO, TRACE) have per-mitted us to characterize the main structures of the transi-tion region (TR) and the low corona and to provide a firstlook at the processes acting in this part of the solaratmosphere.

After a brief presentation of the SOHO spectrometers,we report on some results on the quiet Sun’s atmosphericstructure and dynamics.

2. UV spectrometers to observe the solar disk on SOHO

The main characteristics of CDS and SUMER arereported in Table 1.

The solar beam collected by CDS grazing incidence tele-scope feeds the entrance slits of the two spectrometers(Harrison et al., 1995). The Normal Incidence Spectrome-ter (NIS) uses 2 toroidal gratings to focus the stigmaticspectra on two parts of the intensified CCD detector. TheGrazing Incidence Spectrometer (GIS) uses one sphericalgrating at grazing angle to focus astigmatic spectra on fourmicrochannel plate detectors.

The SUMER instrument is described in Wilhelm et al.(1995). An off-axis parabola (telescope mirror) feeds thespectrometer. Beyond the slit a collimator sends the paral-lel beam, after deflection onto a scanning flat mirror, to a

ed.

Table 1Spectrometer parameters

Range (nm) Pixel (nm) Slit (in.)

Spectral Angular Width Length

CDSNIS 31.8–38.1 51.3–63.1 0.01 2 2,4,90 4,240GIS 15.1–78.5 0.02 2,4,50 2,4,8 2,4,50

SUMER(detector)

80–160(A) 67–147(B) 0.0044 1 0.3,1,4 120,300

Table 3Quiet sun temperature, density and abundance

Temperature Quiet Sun Coronal hole Reference

Top of TR 106 K 8 · 105 K 1,2

Density ratio Temperature Cell/network Reference

Quiet & coronal hole 1.6 · 105 K 1.4–1.9 11.6 · 105 K 1.1–2.8 3P3 · 105 K 0.8–1.2 12 · 104 to 2 · 105 up to 4 4

Quiet/CH network 1.6 · 105 K 1.6 16.3 · 105 K 1.6 1

Abundance ratio Low FIP High FIP Reference

P. Lemaire / Advances in Space Research 39 (2007) 1876–1881 1877

spherical concave grating which focuses the stigmatic spec-tra to a microchannel plate detector.

Quiet cell 1 2 1Network 0.7 2 1CH cell 0.5 1.5–3 1Network 0.3 1–2 1

(1) Del Zanna and Bromage (1999); (2) David et al. (1998) (3) Teriacaet al. (2001); (4) Innes (2001).

3. Quiet Sun properties: chromosphere to corona

3.1. Overview

The chromospheric network is visible through the5000 K to 1 · 106 K temperature range (see Table 2).Above 3 · 105 K it begins to diffuse.

The quiet Sun TR seems very thin and from limb bright-ening in different lines (corresponding to TR temperatures)a thickness of a few arcseconds (a few Mm) has been mea-sured (Wilhelm et al., 1998).

3.2. Geometry and contrast

The HCO/S055 spectrometer on Skylab had alreadyprovided the main characteristics of the TR network(Reeves, 1976) during the descending phase of solar activitycycle 21. New results (Table 3) obtained by SOHO confirmthese main parameters. The network is seen from the chro-mosphere to the low corona where it diffuses over the cellareas. The width of the network wall has little variationin the 2 · 104 to 6 · 105 K temperature range (Reeves,

Table 2Quiet Sun geometry and contrast

104 K 105 K 106 K Reference

Area cell/total 54% 54% 54% 154% 50% 60% 2

Network emission 60% 70% 50% 1P60% 70% 70% 2

Network/cell contrast 5 7

Network full width at half-maximum (arcsec)10 10 10 112 10 15 315 15 20 2P7 P5 P11 5

17 6

Cell intensity ratio between quiet Sun and coronal hole1 1 1 1

1.5 4

(1) Reeves (1976); (2) Gallagher et al. (1998); (3) Patsourakos et al. (1999);(4) Lemaire et al. (1999); (5) Gontikakis et al. (2003); (6) Ravindra andVenkatchrishnam (2003); (7) Feldman et al. (1999).

1976; Gallagher et al., 1998; Patsourakos et al., 1999), inagreement with Gabriel (1976) model. The ratio betweencell and total area is either constant (Reeves, 1976) orhas a small variation (Patsourakos et al., 1999, Wordenet al., 1999) with temperature.

The network emission in a coronal hole is diminished inthe upper part of the TR. Some measurements done bySUMER show a strong coronal hole signature as low as105 K (average quiet Sun to coronal hole network ratioof 1.5, Lemaire et al., 1999), while there is no detectionin chromospheric observations (Bocchialini and Vial,1996).

3.3. Temperature, density and abundance

While the temperature reaches more than 106 K at thequiet Sun solar limb (Table 3), it is limited to around8 · 105 K over a coronal hole (David et al., 1998).

The determination of density and abundance in theTR is very difficult. The density in a quiet cell is higherthan the density in the network by a factor between 1.4and 1.9 at 1.6 · 105 K, while the densities are equivalentabove 3 · 105 K (Del Zanna and Bromage, 1999). Thedensity in quiet Sun is about 1.6 times higher than inan equatorial coronal hole (Del Zanna and Bromage,1999).

Elemental abundance variation were detected from Sky-lab (Noci et al., 1988; Feldman and Widing, 1993; Sheeley,1996) and from NRL/HRTS (Doschek et al., 1991) data.The abundance determination done by Del Zanna and Bro-mage (1999) seems to indicate a depletion of low FIP (FirstIonization Potential) ions in the quiet Sun and in coronalhole network, with an enrichment of high FIP ions in thecells.

It should be noted that in a dynamic atmosphere, ele-ments may have different behaviors and may not be entirely

1878 P. Lemaire / Advances in Space Research 39 (2007) 1876–1881

ionized. The greatest care must be taken in the interpreta-tion of the measured line intensities.

3.4. Dynamics

Dynamical properties are inherent to the TR. Theinstability of the chromospheric base (supergranular flowpattern over the photospheric granulation, local emer-gence and drift of magnetic elements, differential rota-tion, etc.) propagates through the TR. The networkpattern is in continuous interaction with moving mag-netic elements and is approximately replenished in oneday. This results in dynamics events, Doppler shiftsand line broadenings.

3.4.1. Doppler shifts

Systematic Doppler shifts in the TR were discovered bythe NRL/S082b spectrometer (Doschek et al., 1976) andconfirmed by OSO8 (Lites et al., 1976).

As shown in Fig. 1 there is a large dispersion of dataobtained by Brekke (1994), Achour et al. (1995), Hassleret al. (1991), Brekke et al. (1997), Chae et al. (1998c), Ter-iaca et al. (1999) and Peter and Judge (1999). The determi-nation of the shift is the result of some averaging overdispersed solar values and is sensitive to the absolute wave-length reference (data obtained by Chae et al. (1998c) andBrekke et al. (1997) above 5 · 105 K are hampered by awrong wavelength reference, and with revised values blueshifted upflows are obtained as suggested by Dammaschet al. (1999) and Teriaca et al. (1999)).

Although strong redshifts are predominant there isalways a local mixture of redshifts and blueshifts (Brynild-sen et al., 1998; Lemaire et al., 2002a,b). The evidence ofupflows near the quiet Sun network and the predominanceof upflows in coronal holes are important clues for theunderstanding of the fast and slow solar wind (Warrenet al., 1997; Hassler et al., 1999; Stucki et al., 1999).

Fig. 1. The Doppler shift distribution over the transition region temper-ature range.

3.4.2. Non-thermal velocities

The non-thermal velocities obtained from the width ofthe line profiles corrected from the instrumental profileand the thermal width are displayed in Fig. 2. The Dopplerwidth is a function of the kinetic temperature of movingions (Woods and Holzer, 1991), sometimes split into theelectron temperature and a non-thermal contribution (withthe maxwellian velocity distribution hypothesis).

The data (Dere and Mason, 1993; Chae et al., 1998; Ter-iaca et al., 1999; Peter, 2001) show a maximum of the non-thermal velocity near 3 · 105 K, at the same temperaturewhere the redshift is maximum.

Some data (Lemaire et al., 1999; Stucki et al., 1999) indi-cate an increase of the line broadening with line intensity(cell to network) and an enlargement of the line profilesin coronal hole. The center-to-limb line width in TR dis-plays a weak variation (Doyle et al., 2000).

3.4.3. Dynamical events

The jets and turbulent events in the TR were firstobserved by HRTS during rocket flights (Brueckener andBartoe, 1983). The rocket observations were confirmedby SpaceLab2 (Brueckener et al., 1986). During this flightthe statistics were sufficient to determine lifetimes and sizes.Further studies (Dere et al., 1989a,b, 1991; Porter andDere, 1991) have located the appearance of explosiveevents at the edge of the network, while the network brightpoints are seen above the network neutral lines Falconeret al. (1998). The explosive events (Table 4) may appearin bursts (Innes et al., 1997). Some observations have beendone to compare chromospheric and TR events (Chaeet al., 1998a,b). Some similarities between chromosphericspicules (Beckers, 1972; Suematsu et al., 1995) and blinkers(Harrison, 1997) exist. A lot of work has been done tounderstand the behavior of explosive events and blinkers(see references in Table 4). The macrospicule rotation hasbeen reported in Pike and Mason (1998).

Fig. 2. The non-thermal line broadening distribution over the transitionregion temperature range.

Table 4Transition region dynamical events

Blinker

Cell/network/EUV brightenings Explosive event

Location Merging magnetic fields? Edge of network?Size 22 km2 9 km2

Lifetime P5–110 min 30–200 sRepetition Unit event, 2–3 min BurstVelocity ±10–35 km s�1 ±50–200 km s�1

Non-thermal width 15–45 km s�1

Radiance increase About 1.8 Somerelation to Ha brightenings? In chromospheric emission80% above main polarity Border of cellmass density/filling Ne increase by factor 3coronal contribution coronal contribution 610%

Beckers (1972), Dere et al. (1991), Suematsu et al. (1995), Harrison (1997), Innes et al. (1997), Chae et al. (1998b), Wang et al. (1998), Chae et al. (2000),Teriaca et al. (2001), Marik and Erdelyi (2002), Teriaca et al. (2002), Bewsher et al. (2003), Harrison et al. (2003), Mardjarska and Doyle (2003), Peter andBrkovic (2003), Brooks et al. (2004), Doyle et al. (2004), Ning et al. (2004).

P. Lemaire / Advances in Space Research 39 (2007) 1876–1881 1879

4. Conclusions

The two XUV–EUV spectrometers on SOHO provide alarge amount of data, which have been used to greatlyimprove our knowledge of the processes acting in thedynamics of the quiet Sun transition region. In this frontierbetween the dense and cold chromosphere and the thin andvery hot corona, highly contrasted structures and phenom-ena are detected. It is also a place where the radiative con-tribution to heating ends and where the acceleration of thesolar wind begins. From the data acquired and new obser-vations with these spectrometers, we may expect to have abetter understanding of the main parameters of the transi-tion region.

In the future, given the high dynamics of the transitionregion, new instrumentation must provide more angular,spectral and temporal resolution. The combination ofobservations from the low chromosphere (with magneticfield information) to the corona is also required to makea good interpretation of the data.

Acknowledgments

SOHO is a mission of international cooperation betweenESA and NASA. The SUMER project is financially sup-ported by DLR, CNES, NASA and the ESA PRODEXprogramme (Swiss contribution). CDS was built and isoperated by a consortium led by the Rutherford AppletonLaboratory and including the Mullard Space Science Lab-oratory, the NASA Goddard Space Flight Center, OsloUniversity and the Max Planck Institute for Extraterres-trial Physics, Garching.

References

Achour, H., Brekke, P., Kjeldseth-Moe, O., Maltby, P. Observed redshiftsin the solar transition region above active and quiet regions. ApJ 453,945–952, 1995.

Bartoe, J.-D.F., Brueckner, G.E., Purcell, J.D., Tousey, R. Extremeultraviolet spectrograph ATM experiment S082B. Appl. Opt. 16, 879–886, 1977.

Beckers, J.M. Solar spicules. Ann. Rev. Astro. Astrophys. 10, 73–100, 1972.Bewsher, D., Parnell, C.E., Pike, C.D., Harrison, R.A. Dynamics of

blinkers. Solar Phys. 215, 217–237, 2003.Bocchialini, K., Vial, J.-C. High-chromosphere and low-transition region

network: a different organization in an equatorial coronal hole? SolarPhys. 168, 37–45, 1996.

Bonnet, R.M., Lemaire, P., Vial, J.-C., Artzner, G., Gouttebroze, P.,Jouchoux, A., Leibacher, J., Skumanich, A., Vidal-Madjar, A. TheLPSP instrument on OSO 8. II – In-flight performance and preliminaryresults. ApJ 221, 1032–1061, 1978.

Brekke, P. Observed redshifts of transition region/corona lines. Space Sci.Rev. 70, 97–102, 1994.

Brekke, P., Hassler, D.M., Wilhelm, K. Doppler shifts in the quiet-Suntransition region and corona observed with SUMER on SOHO. SolarPhys. 175, 349–374, 1997.

Brooks, D.H., Kurokawa, H., Kamio, S., Fludra, A., Ishi, T.T., Kitai, R.,Kozu, H., Ueno, S., Yoshimura, K. Short-duration active regionbrightenings observed in the extreme ultraviolet and Ha by the Solarand Heliographic Observatory Coronal Diagnostic Spectrometer andHida Domeless Solar Telescope. ApJ 602, 1051–1062, 2004.

Brueckener, G.E., Bartoe, J.-D.F. Observations of high-energy jets in thecorona above quiet Sun, the heating of the corona, and theacceleration of the solar wind. ApJ 272, 329–348, 1983.

Brueckener, G.E., Bartoe, J.-D.F., Cook, J.W., Dere, K.P., Socker, D.G.HRTS results from Spacelab 2. Adv. Space Res. 6 (8), 263–272, 1986.

Bruner Jr., E.C. The University of Colorado OSO8 spectrometerexperiment. I – Introduction and optical design consideration. SpaceSci. Instrum. 3, 369–387, 1977.

Brynildsen, N., Brekke, P., Fredvik, T., Haughan, S.V.H., Kjeldseth-Moe,O., Maltby, P., Harrison, R.A., Wilhelm, K. SOHO observations ofthe connection between line profile parameters in active and quietregions and the net red shift in EUV emission lines. Solar Phys. 181,23–50, 1998.

Chae, J., Schuhle, U., Lemaire, P. SUMER measurements of nonthermalmotions: constraints on coronal heating mechanisms. ApJ 505,957–973, 1998.

Chae, J., Wang, H., Lee, C.Y., Goode, P.R., Schuhle, U. Chromosphericupflow events associated with transition region explosive events. ApJ504, L123–L126, 1998a.

Chae, J., Wang, H., Lee, C.Y., Goode, P.R., Schuhle, U. Photosphericmagnetic field changes associated with transition region explosiveevents. ApJ 497, L109–L112, 1998b.

1880 P. Lemaire / Advances in Space Research 39 (2007) 1876–1881

Chae, J., Yun, H.S., Poland, A.I. Temperature dependence of ultravioletline average doppler shifts in the quiet Sun. ApJ Suppl. Ser. 114, 151–164, 1998c.

Chae, J., Wang, H., Goode, P.R., Fludra, A., Schuhle, U. Comparison oftransient network brightenings and explosive events in the solartransition region. ApJ 528, L119–L122, 2000.

Dammasch, I.E., Wilhelm, K., Curdt, W., Hassler, D.M. The Ne VIII(k770) resonance line: solar wavelengths determined by SUMER onSOHO. A&A 346, 285–294, 1999.

David, C., Gabriel, A.H., Bely-Dubau, F., Fludra, A., Lemaire, P.,Wilhelm, K. Measurement of the electron temperature gradient in asolar coronal hole. A&A 336, L90–L94, 1998.

Del Zanna, G., Bromage, B.J.I. The elephant’s trunk: spectroscopicdiagnostics applied to SOHO/CDS observations of the August 1996equatorial hole. JGR 104, 9753–9766, 1999.

Dere, K.P., Mason, H.E. Nonthermal velocities in the solar transitionzone observed with the high-resolution telescope and spectrograph.Solar Phys. 144, 217–241, 1993.

Dere, K.P., Bartoe, J.-D.F., Brueckener, G.E. Explosive events in the solartransition zone. Solar Phys. 123, 41–68, 1989a.

Dere, K.P., Bartoe, J.-D.F., Brueckener, G.E., Cook, J.W., Socker, D.G.,Ewing, J.W. UV observations of macrospicules at the solar limb. SolarPhys. 119, 55–63, 1989b.

Dere, K.P., Bartoe, J.-D.F., Brueckner, G.E., Ewing, J., Lund, P.Explosive events and magnetic reconnection in the solar atmosphere.JGR 96, 9399–9407, 1991.

Doschek, G.A., Feldman, U., Bohlin, J.D. Doppler wavelength shifts oftransition region lines measured in SKYLAB solar spectra. ApJ 205,L177–L180, 1976.

Doschek, G., Dere, K.P., Lund, P.A. Relative abundances in the lowersolar transition region. ApJ 381, 583–590, 1991.

Doyle, J.G., Teriaca, L., Banerjee, D. Solar transition region linebroadening: limb to limb measurements. A&A 356, 335–338,2000.

Doyle, J.G., Roussev, I.I., Madjarska, M.S. New insight into the blinkerphenomenon and the dynamics of the solar transition region. A&A418, L9–L12, 2004.

Falconer, D.A., Moore, R.L., Porter, J.G., Hathaway, D.H. Networkcoronal bright points: coronal heating concentrations found in thesolar magnetic network. ApJ 501, 386–396, 1998.

Feldman, U., Widing, K.G. Elemental abundances in the upper solaratmosphere of quiet and coronal hole regions Te is approximativelyequal to 4.3 · 105 K. ApJ 414, 381–388, 1993.

Feldman, U., Widing, K.G., Warren, H.P. Morphology of the quiet solarupper atmosphere in the 4 · 104

6 Te 6 1.4106 K temperature regime.ApJ 522, 1133–1147, 1999.

Gabriel, A. A magnetic model of the solar transition region. Phil. Trans.R. Soc. Lond. 281, 339–352, 1976.

Gallagher, P.T., Phillips, K.J.H., Harra-Murnion, L.K., Keenan, F.P.Properties of the quiet Sun EUV network. A&A 335, 733–745, 1998.

Gontikakis, C., Peter, H., Dara, H.C. Sizes of quiet Sun transition regionstructures. A&A 408, 743–753, 2003.

Harrison, R.A. EUV blinkers: the significance of variations in the extremesolar ultraviolet quiet Sun. Solar Phys. 175, 467–485, 1997.

Harrison, R.A., Sawyer, E.C., Carter, M.K., Cruise, A.M., Cutler,R.M., Fludra, A., Hayes, R.W., Kent, B.J., Lang, J., Parker, D.J.,Paynes, J., Pike, C.D., Peskett, S.C., Richards, A.G., Culhane, J.L.,Norman, K., Breeveld, A.A., Breeveld, E.R., Al Janabi, K.F.,McCalden, A.J., Parkinson, J.H., Self, D.G., Thomas, P.D., Poland,A.I., Thomas, R.J., Thompson, W.T., Kjeldseth-Moe, O., Brekke,P., Karud, J., Maltby, P., Aschenbach, B., Brauninger, H., Kuhne,M., Hollandt, J., Siegmund, O.H.W., Huber, M.C.E., Gabriel, A.H.,Mason, H.E., Bromage, B.J.I. The Coronal Diagnostic Spectrometerfor the Solar and Heliospheric Observatory. Solar Phys. 162, 233–290, 1995.

Harrison, R.A., Harra, L.K., Brkovic, A., Parnell, C.E. A study of theunification of quiet-Sun transient-event phenomena. A&A 409, 755–764, 2003.

Hassler, D.M., Rottman, G.S., Orrall, F.Q. Systematic radial flows in thechromosphere, transition region, and corona of the quiet Sun. ApJ372, 710–718, 1991.

Hassler, D.M., Dammasch, I.E., Lemaire, P., Brekke, P., Curdt, W.,Mason, H.E., Vial, J.-C., Wilhelm, K. Solar wind outflow andthe chromospheric magnetic network. Science 283, 810–812, 1999.

Innes, D.E. Coordinated observations of the quiet Sun transition regionusing SUMER spectra, TRACE images and MDI magnetograms.A&A 378, 1067–1077, 2001.

Innes, D.E., Brekke, P., Germerott, D., Wilhelm, K. Bursts of explosiveevents in the solar network. Solar Phys. 175, 341–348, 1997.

Lemaire, P., Bocchialini, K., Aletti, V., Hassler, D., Wilhelm, K. Searchfor signatures of a coronal hole in transition region lines near diskcenter. Space Sci. Rev. 87, 249–252, 1999.

Lemaire, P., Artzner, G., Vial, J.-C., Curdt, W., Schuhle, U., Wilhelm, K.Transition region quiet Sun velocity field evolution. Adv. Space Res.30 (3), 487–490, 2002a.

Lemaire, P., Vial, J.-C., Curdt, W., Schuhle, Wilhelm, K. Quiet-Sunchromospheric network evolution in Magnetic Coupling of the SolarAtmosphere, in: Sawya-Lacoste, H. (Ed.), Noordwijk, ESA SP-505,pp. 477–480, 2002b.

Lites, B.W., Bruner Jr., E.C., Chipman, E.G., Shine, R.A., Rottman, G.J.,White, O.R., Athay, R.G. Preliminary results from the Orbiting SolarObservatory 8 – Persistent velocity fields in the chromosphere andtransition region. ApJ 210, L111–L113, 1976.

Mardjarska, M.S., Doyle, J.G. Simultaneous observations of solartransition region blinkers and explosive events by SUMER, CDSand BBSO. Are blinkers, explosive events and spicules the samephenomenon? A&A 403, 731–741, 2003.

Marik, D., Erdelyi, R. What is the real nature of blinkers? A&A 393, L73–L76, 2002.

Miller, M.S., Caruso, A.J., Woodgate, B.E., Sterk, A.A. Ultravioletspectrometer and polarimeter for the solar maximum mission. Appl.Opt. 20, 3805–3814, 1981.

Ning, Z., Innes, D.E., Solanki, S.K. Line profile characteristics of solarexplosive event bursts. A&A 419, 1141–1148, 2004.

Noci, G., Spadaro, D., Zappala, R.A., Zuccarello, F. Elementalabundances in different solar regions from EUV observations. A&A198, 311–321, 1988.

Patsourakos, S., Vial, J.-C., Gabriel, A., Bellamine, N. Transition-region network boundaries in the quiet Sun: width variation withtemperature as observed with CDS on SOHO. ApJ 522, 540–546,1999.

Peter, H. On the nature of the transition region from the chromosphere tothe corona of the Sun. A&A 374, 1108–1120, 2001.

Peter, H., Brkovic, A. Explosive events and transition region blinkers:time variability of non-gaussian quiet Sun EUV spectra. A&A 403,287–295, 2003.

Peter, H., Judge, P.G. On the doppler shifts of solar ultraviolet emissionlines. A&A 522, 1148–1166, 1999.

Pike, C.D., Mason, H.E. Rotating transition region features observed withthe SOHO Coronal Diagnostic Spectrometer. Solar Phys. 182, 333–348, 1998.

Porter, J.G., Dere, K.P. The magnetic network location of explosiveevents observed in the solar transition region. ApJ 370, 775–778,1991.

Ravindra, B., Venkatchrishnam, P. Structure and evolution of thetransition region network observed in the He II k304. Solar Phys.215, 239–259, 2003.

Reeves, E.M. The EUV chromospheric network in the quiet Sun. SolarPhys. 46, 53–72, 1976.

Reeves, E.M., Huber, M.C.E., Timothy, J.G. Extreme UV spectroheli-ometer on the Apollo Telescope Mount. Appl. Opt. 16, 837–848, 1977.

Sheeley Jr., N.R. Elemental abundance variations in the solar atmosphere.ApJ 469, 423–428, 1996.

Stucki, K., Solanki, S.K., Ruedi, I., Stenflo, J.O., Brkovic, A., Schuhle,U., Wilhelm, K., Huber, M.C.E. Coronal hole properties observedwith SUMER. Space Sci. Rev. 87, 315–318, 1999.

P. Lemaire / Advances in Space Research 39 (2007) 1876–1881 1881

Suematsu, Y., Wang, H., Zirin, H. High resolution observation of diskspicules. I. Evolution and kinematics of spicules in the enhancednetwork. ApJ 450, 411–421, 1995.

Teriaca, L., Banerjee, D., Doyle, J.G. SUMER observations of dopplershift in the quiet Sun and in an active region. A&A 349, 636–648, 1999.

Teriaca, L., Madjarska, M.S., Doyle, J.G. Electron density variationsduring ultraviolet transient events. Solar Phys. 200, 91–114, 2001.

Teriaca, L., Madjarska, M.S., Doyle, J.G. Transition region explosiveevents: do they have a coronal counterpart? A&A 392, 309–317, 2002.

Wang, H., Johannesson, A., Stage, M., Lee, C., Zirin, H. Study of Ha jetson the quiet Sun. Solar Phys. 178, 55–69, 1998.

Warren, H.P., Mariska, J.T., Wilhelm, K. Observations of doppler shiftsin a solar polar coronal hole. ApJ 490, L187–L190, 1997.

Wilhelm, K., Curdt, W., Marsch, E., Schuhle, U., Lemaire, P., Gabriel,A.H., Vial, J.-C., Grewing, M., Huber, M.C.E., Jordan, S.D., Poland,A.I., Thomas, R.J., Kuhne, M., Timothy, J.G., Hassler, D.M.,

Siegmund, O.H.W. SUMER – Solar Ultraviolet of Emitted Radiation.Solar Phys. 162, 189–231, 1995.

Wilhelm, K., Lemaire, P., Dammasch, I.E., Hollandt, J., Schuhle, U.,Curdt, W., Kucera, T., Hassler, D.M., Huber, M.C.E. Solar irradi-ances and radiances of UV and EUV lines during the minimum ofsunspot activity in 1996. A&A 334, 685–702, 1998.

Wilhelm, K., Dwivedi, B.N., Marsch, E., Feldman, U. Observations of theSun at vacuum-ultraviolet wavelengths from space. Part I: conceptsand instrumentation. Space Sci. Rev. 111, 415–480, 2004.

Woods, D.T., Holzer, T.E. The effects of mass flow on the temperatureand abundance structure of the solar transition region. ApJ 375, 800–817, 1991.

Worden, J., Woods, T.N., Neupert, W.M., Delaboudiniere, J.-P. Evolu-tion of chromospheric structures: how chromospheric structurescontribute to the solar He II 30.4 nanometer irradiance and variability.ApJ 511, 965–975, 1999.