 COronal Solar Magnetism Observatory |
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Circular polarization: breakthrough measurementsIn 1969, Harvey published work with the coronagraph at Climax, Colorado, in which he attempted to measure the circular polarization of the green coronal line. His pioneering efforts revealed the extreme difficulty of measuring the weak circular polarization signal under the presence of much stronger intensity and linear signals, containing noise from a variety of sources. It was not for almost three decades that Kuhn (1995) re-vitalized the study of circularly polarized magnetic dipole lines, recognizing the advantages of the infrared spectral region when observed with de-classified 2D IR detectors. He derived an upper limit of 40G from the circularly polarized data of the Fe XIII infrared lines. This value is not too far from values typically expected at low heights in the corona.Lin et al. (2000) used a very sensitive IR polarimeter at the Evans Coronagraph on Sacramento Peak to provide the first reliable measurement of the Stokes V profile of Fe XIII 1074.7 nm, and hence of the line of sight coronal field strength. Lin et al. (2004) progressed by making measurements using a new fiber-fed spectrograph at a new, 50cm reflecting coronagraph (SOLARC) on Haleakala, Hawaii. They were able to detect fields with strengths as weak as 4G +/-1G, in about 1 hour using a 20 arcsecond resolution. |
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