Issue 3: 09th November 2001
A Note from the Editor and Contents
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This Month's Topics:
Polarisation Measurements with CDS - Roger Thomas
An attempt is planned to validate our understanding of basic emission mechanisms in the solar corona during a special roll maneuver of the SOHO spacecraft on 14-15 November, by making use of the polarization sensitivity of CDS.
Intensity gradients up to 0.1Rsun above the solar limb will be measured before, during, and after the roll maneuver, in emission lines that include He I 584 A, He II 304 A, O IV 555 A, O V 630 A, Mg IX 368 A, Mg X 610 and 625 A, and Si XI 303 A, formed at temperatures that span the range from 4.1 < log(Tmax) < 6.2. Since the entire sequence will take nearly two and a half days to complete, observations are planned for the off-limb corona above one of the solar poles, whichever is brightest at the time, to minimize effects of temporal variations.
Prior studies have shown that, 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 Ne^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, however, 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 Ne 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. Theoretical calculations indicate that the expected polarization strength of resonantly scattered EUV radiation at these heights averaged over the line of sight should be on the order of 20 percent, but this prediction has never before been tested by direct measurement.
The CDS normal-incidence spectrometer has a polarization sensitivity of about 50 percent at EUV wavelengths because it is fed by sectors of a telescope with two grazing-incidence reflections. Therefore, response variations near 10 percent as a function of roll angle are expected from heights at which resonance scattering should dominate. The SOHO maneuver will allow observations at roll angles of 0, -90, then 0 again, each for up to 16 hours in duration, the length of time needed to get reasonable signals from the faint coronal emission of cooler EUV lines at sufficient height above the limb. Detection of the expected intensity gradients and polarization signatures would provide a unique and compelling confirmation of the two principal emission mechanisms thought to operate in the solar corona.
From the CDS Operations Management Team in the Space Science & Technology Department at CCLRC Rutherford Appleton Laboratory
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Last revised on Thursday ( 8/Nov/2001) at 18:36.