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Session 1 - Chromosphere, Corona, Flares.
Display session, Friday, June 27
Ballroom B, Chair: Charles Kankelborg

[1.16] EUV Intensity Gradients and Polarization above the Limb as Observed by SERTS and by SOHO

R. J. Thomas (NASA - Goddard Space Flight Center)

Intensity gradients up to 0.4 R_ødot above the solar limb were measured in a number of EUV lines at different times by the Solar EUV Rocket Telescope and Spectrograph (SERTS), and by the CDS and SUMER instruments aboard the SOHO spacecraft mission. Observed emission lines include He I 584ÅHe II 304ÅN V 1239 amp; 1243ÅO IV 555ÅO V 630ÅMg IX 368ÅMg X 610 amp; 625ÅSi II 1265Åand Si XI 303Åformed at temperatures that evenly cover the range from 4.1 \le log(T_max) \le 6.2. Observations were made of the off-limb corona above various solar features, such as fairly active equatorial regions and the relatively cool coronal holes at both north and south poles.

Near the disk, measured intensities of all lines fall off exponentially at different rates that can be used to determine the density scale-heights of the emitting plasma, since this emission is dominated by collisional excitation with an N_e^2-dependence. Assuming hydrostatic equilibrium, the intensity gradient for each line can then be converted into a `scale-height temperature', which is found to be closely related to the ionization temperature of each line over the wide range of lines and solar conditions observed. This result implies that the large-scale corona is remarkably uniform, even though clearly displaying a great deal of structure and non-uniformity on smaller spatial scales.

Beyond a certain distance, intensity gradients of the cooler lines switch over to a flatter exponential slope, suggesting that this radiation is dominated by resonance scattering which varies as N_e to the first power. Such radiation should also be linearly polarized in the plane containing the line-of-sight and the solar center, a signature that would strongly confirm this interpretation. An attempt to measure polarization in EUV emission above the limb was recently made using the SOHO/CDS normal-incidence spectrometer, which has a polarization sensitivity of about 50% at these wavelengths.

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