• current CDS science topics
• developments in CDS data analysis
• instrumental matters
• operational issues

• CDS Science Session at NAM/UKSP 2003
• CDS Users Meeting at NAM/UKSP 2003
• Paper Abstract - J. Brosius
• Paper Abstract - V. Andretta et al.
• Paper Abstract - T. Kucera et al.
• Paper Abstract - G. Del Zanna et al.
• Paper Abstract - A. Fludra and J. Ireland
• Book Announcement - The Radiometric Calibration of SOHO
• CDS NIS wavelength calibration
• Programming in IDL - Sockets
• Report on the Coronal Loops Workshop

We present EUV light curves, Doppler shifts, and line broadening measurements for a flaring solar active region obtained with CDS under conditions of (1) comprehensive temporal coverage (including the quiescent preflare, impulsive, and gradual decline phases), (2) high time resolution (9.83 s), and (3) narrow field of view (4 arcsec by 20 arcsec). The four strong lines of O III at 599.587 A, O V at 629.732 A, Mg X at 624.937 A, and Fe XIX at 592.225 A are analyzed, and provide diagnostics of plasma dynamics for log T between 4.9 and 6.9. Wavelengths and widths measured during the preflare and late decline phases provide standards against which Doppler shifts and excess line broadening are measured during the impulsive and early decline phases. The entire profile of all four lines is blueshifted early during the impulsive rise of the flare, but only the O III, O V, and Mg X lines subsequently exhibit multiple components and downflows. These downflows provide evidence of ``warm rain" due to cooling coronal flare plasma following chromospheric evaporation during the impulsive phase. O III and O V exhibit a pronounced precursor brightening during which the Fe XIX emission emerges above the noise; this, combined with the fact that the O III and O V intensities begin their impulsive rise earlier than do those of Mg X and Fe XIX, is consistent with the transport of coronal flare energy to the chromosphere by nonthermal particle beams.


The EUV helium spectrum in the quiet Sun: a by-product of coronal emission?

V. Andretta, G. Del Zanna, and S. D. Jordan, A&A, in press (2003)

In this paper we test one of the mechanisms proposed to explain the intensities and other observed properties of the solar helium spectrum, and in particular of its Extreme-Ultraviolet (EUV) resonance lines. The so-called Photoionisation-Recombination (P-R) mechanism involves photoionisation of helium atoms and ions by EUV coronal radiation, followed by recombination cascades. We present calibrated measurements of EUV flux obtained with the two CDS spectrometers on board SOHO, in quiescent solar regions. We were able to obtain an essentially complete estimate of the total photoionising flux in the wavelength range below 504 A (the photoionisation threshold for HeI, as well as simultaneous measurements with the same instruments of the intensities of the strongest EUV HeI and HeII lines: HeI 584, HeI 537, and HeII 304. We find that there are not enough EUV ionising photons to account for the observed helium line intensities. More specifically, we conclude that HeII intensities cannot be explained by the P-R mechanism alone. Our results, however, leave open the possibility that the HeI spectrum could be formed by the P-R mechanism, with the HeII 304 line as a significant photoionisating source.


Prominence Motions Observed at High Cadences in Temperatures from 10,000 to 250,000 K

T.A. Kucera, M. Tovar, B. De Pontieu, Sol. Phys., in press (2003)

We report here for the first time observations of prominence velocities over a wide range of temperatures and with a high time cadence. Our study of ultraviolet movies of prominences reveals that multithermal features with speeds of 5-70 km/s perpendicular to the line of sight are common in the prominences which showed traceable motions. These speeds are noticeably higher than the typical speeds of 5-20 km/s observed in H-alpha data from "quiet" prominences and are more typical of "activated" prominences in which speeds of up to 40 km/s have been reported. The observations were performed using five separate datasets taken by the Solar and Heliospheric Observatory's Coronal Diagnostic Spectrometer (SOHO/CDS) in its wide slit overlappogram mode in lines from He I, O V, and Mg IX and a separate prominence observation taken with both the Transition Region and Coronal Explorer (TRACE) in its 1216 and 1600 Å bands and in H-alpha by the Swedish Vacuum Solar Telescope (SVST) at La Palma. The movies were taken with cadences > 1 image per minute and were made simultaneously or near-simultaneously in spectral lines formed at two or more temperatures. We traced motion that lasted for 3 to 20 minutes and went distances up to 10^5 km. Most, but not all, of these were chiefly horizontal. In many cases we were able to observe the same motions over temperature ranges from 20,000 to 250,000 K or 10,000 to 100,000 K. Observations are compared with model predictions.


Spectroscopic characteristics of polar plumes

G. Del Zanna, B.J.I. Bromage and H.E. Mason, A&A, in press (2003)

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 x 10^5 K. At these temperatures, plumes have averaged densities Ne = 1.2 x 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 A' and `Fe XV 284 A' filters are not isothermal, and in coronal holes and plumes lower-temperature lines dominate the EIT signal.


Inversion of the Intensity-Magnetic Field Relationship in Solar Active Regions

A. Fludra and J. Ireland, A&A, in press (2003)

We discuss the relationship between the EUV spectral line intensities and the photospheric magnetic flux in solar active regions. Since the histograms of the magnetic flux density in active region plages can be approximated by an exponential function, the equation describing how the observed total line intensity integrated over an active region area arises from the magnetic field, can be approximated by a Laplace integral. We use this property to solve an inverse problem and derive a function relating the line intensity from individual loops to the photospheric magnetic flux density at their footpoints. We propose a simple model in which the intensity of a coronal line Fe XVI 360.8 A in an individual coronal loop is proportional to the footpoint magnetic flux density to the power of 'delta' and explore how well the value of 'delta' is constrained by the observations. Using EUV spectra from the Coronal Diagnostic Spectrometer (CDS) on SOHO and magnetograms from SOHO Michelson Doppler Imager for 26 active regions without sunspots, we find that the value of 'delta' depends on the magnetic flux density threshold used to define active region magnetic area. When even the weakest fields are included, we obtain 'delta' = 1.3, where 1.0 < 'delta' < 1.6 with 90% confidence. This result can be used to provide constraints on coronal heating models.


The Radiometric Calibration of SOHO
A. Pauluhn, M.C.E. Huber and R. von Steiger (Eds.), ISSI Scientific Report SR-002, ESA Publications Division, Noordwijk, The Netherlands, 2002

The radiometric calibration of spectrometric telescopes assures that the observed spectral radiance (or irradiance) is measured on a scale that is defined by the radiometric standards realised and used in terrestrial laboratories. All SOHO instruments therefore have been calibrated by use of source and detector standards that are traceable to the primary radiometric standards. During several Joint Observing Programmes (JOPs) the performance of individual instruments has been monitored and in-flight comparisons have been made.
In two workshops held at the International Space Science Institute in Bern the individual instrument calibrations were discussed and reconciled. The outcome of the workshops and related work, to which all instrument groups contributed, is summarised in the book "The Radiometric Calibration of SOHO".
The book has been published within the ISSI Scientific Report Series, and is available (40 Euros) via ESA Publications Division, contact Mr. Frits de Zwaan, email: Frits.de.Zwaan@esa.int Tel.: (+31)71 565 3405, Fax: (+31)71 565 5433


CDS NIS wavelength calibration - Dave Pike and Jeff Payne

Users often ask about the accuracy of the CDS NIS wavelength calibration - the conversion of detector pixel positions to wavelengths. The data analysis software provides the routine PIX2WAVE which performs a conversion. However, we have always insisted that this can only be taken as a guide and should never be used for calculating absolute velocities. Even though the CDS optical bench is temperature-controlled to try and minimize the temperature effects on the wavelength calibration found early on in the mission, significant changes in the wavelength zero-point can still occur.

Some of these are illustrated below where we plot the average OV velocity of raster #5 from the daily synoptic program. Raster #5 was chosen as previous analysis has shown that any short-term temperature variations have settled down by then. The plot shows that the wavelength zero-point can change abruptly over a range of 30 km/s. At 630A this corresponds only to about 0.5 pixels.

All of the major changes are associated with significant spacecraft and/or CDS events, as labelled. In addition, during the longest 'unbroken' series of data (mid-2000 to early-2002), there is a hint of a much smaller effect which mimics some of the annual temperature variations seen in the CDS structure.