Eit Observations of the Extreme Ultraviolet Sun |
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Authors: | D Moses F Clette J-P Delaboudinière G E Artzner M Bougnet J Brunaud C Carabetian A H Gabriel J F Hochedez F Millier X Y Song B Au K P Dere R A Howard R Kreplin D J Michels J M Defise C Jamar P Rochus J P Chauvineau J P Marioge R C Catura J R Lemen L Shing R A Stern J B Gurman W M Neupert J Newmark B Thompson A Maucherat F Portier-Fozzani D Berghmans P Cugnon E L Van Dessel J R Gabryl |
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Institution: | (1) Naval Research Laboratory, Washington, DC, 20375, U.S.A;(2) Observatoire Royal de Belgique, Brussels, Belgium;(3) Institut d'Astrophysique Spatiale, Université Paris XI, 91405, Orsay Cedex, France;(4) Centre Spatial de Liège, B4031 Liège, Belgium;(5) Institut d'Optique Théorique et Appliquée, 91403 Orsay, France;(6) Lockheed Palo Alto Research Laboratory, Palo Alto, CA, 94304, U.S.A;(7) NASA/Goddard Space Flight Center, Greenbelt, MD, U.S.A;(8) Present address: SEL/NOAA, 325 Broadway, Boulder, CO 80303, USA;(9) Present address: USA;(10) Laboratoire d'Astronomie Spatiale, Marseille, France;(11) Observatoire Royal de Belgique, B-1180 Brussels, Belgium |
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Abstract: | The Extreme Ultraviolet Imaging Telescope (EIT) on board the SOHO spacecraft has been operational since 2 January 1996. EIT
observes the Sun over a 45 x 45 arc min field of view in four emission line groups: Feix, x, Fexii, Fexv, and Heii. A post-launch
determination of the instrument flatfield, the instrument scattering function, and the instrument aging were necessary for
the reduction and analysis of the data. The observed structures and their evolution in each of the four EUV bandpasses are
characteristic of the peak emission temperature of the line(s) chosen for that bandpass. Reports on the initial results of
a variety of analysis projects demonstrate the range of investigations now underway: EIT provides new observations of the
corona in the temperature range of 1 to 2 MK. Temperature studies of the large-scale coronal features extend previous coronagraph
work with low-noise temperature maps. Temperatures of radial, extended, plume-like structures in both the polar coronal hole
and in a low latitude decaying active region were found to be cooler than the surrounding material. Active region loops were
investigated in detail and found to be isothermal for the low loops but hottest at the loop tops for the large loops.
Variability of solar EUV structures, as observed in the EIT time sequences, is pervasive and leads to a re-evaluation of the
meaning of the term ‘quiet Sun’. Intensity fluctuations in a high cadence sequence of coronal and chromospheric images correspond
to a Kolmogorov turbulence spectrum. This can be interpreted in terms of a mixed stochastic or periodic driving of the transition
region and the base of the corona. No signature of the photospheric and chromospheric waves is found in spatially averaged
power spectra, indicating that these waves do not propagate to the upper atmosphere or are channeled through narrow local
magnetic structures covering a small fraction of the solar surface. Polar coronal hole observing campaigns have identified
an outflow process with the discovery of transient Fexii jets. Coronal mass ejection observing campaigns have identified the
beginning of a CME in an Fexii sequence with a near simultaneous filament eruption (seen in absorption), formation of a coronal
void and the initiation of a bright outward-moving shell as well as the coronal manifestation of a ‘Moreton wave’.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1004902913117 |
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