ANGEO Annales Geophysicae ANGEO 1432-0576 Copernicus GmbH Göttingen, Germany 10.5194/angeo-26-2955-2008 The redshifted footpoints of coronal loops Dammasch I. E. 1 Curdt W. 2 Dwivedi B. N. 2 3 Parenti S. 1 Royal Observatory of Belgium, Brussels, Belgium Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany Department of Applied Physics, Banaras Hindu University, Varanasi, India 15 10 2008 26 10 2955 2959 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.ann-geophys.net/26/2955/2008/angeo-26-2955-2008.html The full text article is available as a PDF file from http://www.ann-geophys.net/26/2955/2008/angeo-26-2955-2008.pdf

The physics of coronal loops holds the key to understanding coronal heating and the flow of mass and energy in the region. However, the energy source, structure maintenance and mass balance in coronal loops are not yet fully understood. Observations of blue- and redshifted emissions have repeatedly been used in the construction of loop models. But observations and interpretations of line shifts have been widely debated. Here we present detailed SUMER observations, which clearly show a steady downflow in both footpoints of coronal loops observed at transition region (TR) and lower corona temperatures. We also show and quantify a correlation existing between this Doppler shift and the spectral radiance. Our results indicate a strong correlation which holds from the chromosphere to the lower corona. We suggest that the downflow in the footpoints may be a common phenomenon on all scales, which could explain, why on a statistical basis bright pixels tend to be more redshifted. We conclude by presenting interpretation of such results and their implications in the light of a viable coronal loop model. The observation of steady downflow in redshifted footpoints seems to be in conflict with impulsive heating.

 References Antiochos,~S K.: A dynamic model for the solar transition region, Astrophys. J., 280, 416–422, 1984. Antiochos,~S K.: Mass flows in coronal loops, Space Sci. Rev., 70, 143–148, 1994. Brekke, ~P.: Observed redshifts in O~V and downflows in the solar transition region, AAstrophys. J., 408, 735–743, 1993. Brekke,~P., Hassler,~D M., and Wilhelm,~K.: Doppler Shifts in the Quiet-Sun Transition Region and Corona Observed with SUMER on SOHO, Sol. Phys., 175, 349–374, 1997. Brynildsen,~N., Brekke,~P., Fredvik,~T., Haugan, S V H., Kjeldseth-Moe,~O., Maltby,~P., Harrison,~R A., and Wilhelm,~K.: SOHO Observations of the Connection Between Line Profile Parameters in Active and Quiet Regions and the Net Red Shift in EUV Emission Lines, Sol. Phys., 181, 23–50, 1998. Budnik,~F., Schröder,~K.-P., Wilhelm,~K., and Glassmeier,~K.-H.: Observational evidence for coronal mass injection by "evaporation" of spicular plasma, Astron. Astrophys., 334, L77–L80, 1998. Chae,~J., Yun,~H S., and Poland,~A I.,: Temperature Dependence of Ultraviolet Line Average Doppler Shifts in the Quiet Sun, Astrophys. J. Suppl. S., 114, 151–164, 1998. Curdt,~W., Brekke,~P., Feldman,~U., Wilhelm,~K., Dwivedi,~B N., Schühle,~U., and Lemaire,~P., Astron. Astrophys., 375, 591, 2001. Dammasch,~I E., and Wilhelm,~K.: SUMER observations of solar transition region structures and dynamics, Adv. Space Res., 30, 495–500, 2002. Doschek,~G A., Bohlin,~J D., and Feldman,~U.: Doppler wavelength shifts of transition zone lines measured in SKYLAB solar spectra, Astrophys. J., 205, L177–L180, 1976. Doschek,~G A., Mariska,~J T., and Akiyama,~S.: The Relationship of the Chromosphere to the Lower Solar Transition Region, Astrophys. J., 609, 1153–1163, 2004. Doschek,~G A.: Doppler Shift Correlations in the Solar Transition Region, Space Sci. Rev., 649, 515–528, 2006. Feldman,~U., Dammasch,~I E., and Wilhelm,~K.: On the Unresolved Fine Structures of the Solar Upper Atmosphere. IV. The Interface with the Chromosphere, Space Sci. Rev., 558, 423–427, 2001. Hansteen,~V.: A new interpretation of the redshift observed in optically thin transition region lines, Astrophys. J., 402, 741–755, 1993. Hansteen,~V., Maltby,~P., Malagoli,~A.: Are the Redshifts Observed in Transition Region Lines Caused by Magnetic Reconnection?, in: Magnetic Reconnection in the Solar Atmosphere, edited by: Bentley, R D. and Mariska, J T., ASP conf. series, 111, 116–121, 1997. Kjeldseth-Moe,~O.,: The solar transition region, in: Dynamic Sun, edited by: Dwivedi, B N., Cambridge University Press, 196–216, 2003. Klimchuk,~J A.: On Solving the Coronal Heating Problem, Sol. Phys., 234, 41–77, 2006. McClymont,~A N.: Can cool loop flows explain "transition region" redshifts?, Astrophys. J., 347, L47–50, 1989. McIntosh,~S W. and Poland,~A I.: Detailed SUMER Observations of Coronal Loop Footpoint Dynamics, Astrophys. J., 604, 449–454, 2004. Müller,~D A N., Peter,~H., and Hansteen,~V H.: Dynamics of solar coronal loops. II. Catastrophic cooling and high-speed downflows, Astrophys. J. , 424, 289–300, 2004. Teriaca,~L., Wiegelmann,~T., Lagg,~A., Solanki,~S K., Curdt,~W., and Sekii,~T.: Loop morphology and flows and their relation with the magnetic field, in S Matthews, J Davis, & K Shibata (eds.), Announcing first results from Hinode, Dublin 20-24 Aug 2007, ASP conf. series, in press, 2007. Tsiropoula,~G. and Tziotziou,~K.: The role of chromospheric mottles in the mass balance and heating of the solar atmosphere, Astrophys. J. , 424, 279–288, 2004.