Relative intensity calibration of CDS-GIS detectors on SOHO using a plasma diagnostic technique

Bromage

Landi

et al. 2001

A&A

Self Cite

Abstract. An in-flight calibration study of the Coronal Diagnostic Spectrometer (CDS) instrument on board SOHO (Solar and Heliospheric Observatory) is presented. The relative intensity calibration of CDS is a fundamental requirement for deriving important physical parameters of the solar transition region and corona from the observations. This comprehensive study provides the first complete in-flight relative calibration of all nine CDS channels, first and second order. This has been achieved with the use of a spectroscopic calibration method, mainly based on the comparison between observed line ratios and theoretical predictions provided by the CHIANTI atomic database. The calibration method has been applied to a large number of observations (on-disc, off-limb, quiet sun, active region), to enable the use of a wide range of spectral lines from low to high temperatures of formation. The results are compared to the pre-launch calibration and other post-launch studies. Significant differences with the ground calibration results are found, while there is good agreement with the post-launch studies, based on rocket flights. It is also shown that the relative calibration has not significantly changed over a long period of time, thus confirming the excellent stability of the CDS instrument.

“…A few corrections were applied to the GIS data, as described in Landi et al (1999a). These corrections are also small, and do not usually exceed 1%.…”

Section: Discussionmentioning

confidence: 99%

Section: Background To the Cds Calibrationmentioning

confidence: 99%

Section: Background To the Cds Calibrationmentioning

confidence: 99%

Section: Observations and Data Analysismentioning

confidence: 99%

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Solar EUV spectroscopic observations with SOHO/CDS

Bromage

Landi

et al. 2001

A&A

Self Cite

“…These include de-biasing, flat-fielding, corrections for the burn-in of a few lines, nonlinear corrections, cosmic ray removal and correction for the tilt of the spectra (details may be found in Del Zanna et al 2001a). A few minor corrections were applied to the GIS data, as described in Landi et al (1999). Among the various detector effects, ghosting is one which can have a major impact on the validity of the derived intensity of an emission line.…”

Section: The Cds Instrument and Data Analysismentioning

confidence: 99%

Spectroscopic characteristics of polar plumes

Bromage

Mason

2003

A&A

Self Cite

Abstract. Extreme ultraviolet observations of plumes in polar coronal holes are presented and their spectroscopic signatures discussed. The study focuses on the base of plumes seen on the disk of the Sun with the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) satellite. Spectroscopic diagnostic techniques are applied to characterise the plumes in terms of density, temperature, emission measure and element abundance. Attention is drawn to the particular limitations of some of the techniques when applied to plume structures. In particular, we revisit the Widing & Feldman (1992) findings of a plume having a large first ionization potential (FIP) effect of 10, showing that instead the Skylab data are consistent with no FIP effect. We present for the first time CDS-GIS (grazing incidence spectrometer) observations of a plume. These observations have been used to confirm the results obtained from normal incidence (NIS) observations. We find that polar plumes exhibit the same characteristics as the Elephant's Trunk equatorial plume. The most striking characteristic of the plume bases is that they are near-isothermal with a peak emission measure at transition region temperatures 8 × 10 5 K. At these temperatures, plumes have averaged densities N e 1.2 × 10 9 cm −3 , about twice the value of the surrounding coronal hole region. Element abundances in the plumes are found to be close to photospheric, with the exception of neon which appears to be depleted by 0.2 dex relative to oxygen. The absence of a significant FIP effect in plumes is consistent with fast solar wind plasma, although it is not sufficient to prove a link between the two. Finally, we present a comparison between GIS spectra and the SOHO EIT (EUV Imaging Telescope) broad-band images, showing that temperatures derived from the EIT ratio technique are largely overestimated, for plumes and coronal holes. This is partly due to the fact that the so called "Fe XII 195 Å" and "Fe XV 284 Å" filters are not isothermal, and in coronal holes and plumes lower-temperature lines dominate the EIT signal.

The In-Flight Performance of the SOHO/CDS Grazing Incidence Spectrometer

Kuin

2007

Sol Phys

Self Cite

We present the characteristics, operations history, performance, and calibration of the Grazing Incidence Spectrometer (GIS) of the Coronal Diagnostic Spectrometer onboard SOHO. The GIS sensitivity has been monitored in a direct manner by examining the quiet Sun count rates during 1996 -2006, nearly a whole solar cycle of observations. Overall, the instrument, with its grazing-incidence optics and microchannel plates, has performed exceptionally well. For most spectral regions, changes in the instrument sensitivity have been very small over a 10-year period. The trends in sensitivities support the use of the radiometric calibration of Del Zanna et al. (Astron. Astrophys. 379, 708, 2001) throughout the mission. The verification of the detector performance over such a long period allows us to point out the spectral lines that can reliably be used for scientific analysis.