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Issue 8: 20 January 2003
Latest News

Welcome to the CDS Newsletter. The goal of this Newsletter is to inform the CDS user community of
  • current CDS science topics
  • developments in CDS data analysis
  • instrumental matters
  • operational issues
We invite your contributions on CDS-related matters: data analysis, science results, instrument calibration, software and questions on these topics. Your responses will influence the content of future issues. Please send newsletter inputs and correspondence to the editor, fludra@cdso8.nascom.nasa.gov

This Month's Topics:

CDS Science Session at NAM/UKSP 2003

We are holding a special 'CDS Science' session at the joint National Astronomy Meeting & UK Solar Physics Meeting in Dublin in April 2003, to present latest results obtained with the Coronal Diagnostic Spectrometer on SOHO. The session will take place on Thursday, 10 April, 16:00 - 17:30.
The 'CDS Science' session will consist of one review presentation and four contributed talks, and posters. Everyone is welcome. Oral and poster contributions are invited. Results based on the CDS data are preferred for presentation in this session.
Participants should register for the NAM/UKSP meeting at:
http://star.arm.ac.uk/nam2003/
Paper title and abstract can be entered on the NAM registration page. Titles and abstracts should also be e-mailed to Andrzej Fludra, A.Fludra@rl.ac.uk The selection of oral contributions is at the discretion of organizers.


CDS Users Meeting at NAM/UKSP 2003

On the last day of the NAM/UKSP meeting, Friday, 11 April, there will be a CDS Users Meeting, devoted to the discussion of practical issues relevant to the analysis of past data and planning of new observations with the Coronal Diagnostic Spectrometer. Each of the three sessions (09:30-11:00, 11:30-13:00, 14:30-16:00) will consist of a brief introduction and an open discussion. The session themes are: 'Active Region Studies', 'Dynamic Events', and 'Analysis of Large Data Sets'. Here is the list of suggested topics and questions we would like to address:

Session I. Active Regions
- density diagnostics (what diagnostics are best; blends and other line fitting problems [pre-, and post-June 1998]; can densities be measured along loops)
- identifying loops (how to identify loops at different temperatures? how to co-align loops from NIS1 and NIS2?)

Session II. Dynamic events
- how are CDS line profiles used to study dynamic events (line broadening, line shifts; what analysis tricks are there)?
- what periodic behaviour has been seen in CDS data?

Session III. Large data-sets, automatic data processing
- what large data-sets (defined as a consistent set of data obtained regularly over a period of several months or more) are available; what was their original purpose; what is the progress in their analysis.
- what techniques have people developed for automatically processing large quantities of CDS data? (particularly line profile fitting)
- what data events (flares, blinkers, etc.) and parameters could be automatically extracted from CDS data to form a useful catalogue for grid work?

To give us an idea of the number of participants, please send an e-mail to Andrzej at fludra@cdso8.nascom.nasa.gov. Also indicate if you have issues you would like discussed in these sessions or contributions you would like to make. Contributions from participants are encouraged.

Please mark the CDS sessions in your calendars. We look forward to seeing you in Dublin.
- Andrzej Fludra & Peter Young


Chromospheric Evaporation and Warm Rain During a Solar Flare Observed in High Time Resolution with the Coronal Diagnostic Spectrometer Aboard SOHO

Jeff Brosius, ApJ 586, in press (April 1, 2003)

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 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.




Programming in IDL - Sockets

Dominic Zarro has developed a set of useful IDL routines to support accessing remote data via TCP/IP sockets. Documentation for these routines can be found at:
http://orpheus.nascom.nasa.gov/~zarro/idl/sockets.html


Report on the Coronal Loops Workshop, MEDOC, Institut d'Astrophysique Spatiale in Orsay, November 13-15, 2002
- Helen Mason


Last November we held a very lively and enjoyable workshop on Active Regions Loops. There were around 40 participants, lots of young folk and some more mature ones! Everyone had a chance to talk, to discuss and at times to engage in somewhat heated debate.

Recent observational and theoretical work has revealed that coronal loops are in many respects more mysterious than ever. We tried to pin down the plasma properties and heating of these fundamentally important structures. We focused on the long-lived loops, that appear to change slowly in relation to the radiative and conductive cooling times. However it soon became evident that defining these was non-trivial. We had animated discussions on the field-aligned and cross-field thermal properties and in particular the diagnostic techniques used with TRACE, EIT, CDS and YOHKOH data. Among the questions addressed was whether loops are isothermal structures or collections of multi-thermal strands. Finally, we considered the possible heating mechanisms - is the heating steady or episodic, is it localized, e.g., at the loop top or footpoints.

Several collaborations and future projects were nurtured. In particular, from the CDS perspective, the need was identified for more joint CDS and TRACE observations to combine the strengths of imaging and spectroscopy. The campaign JOP146 was led last week by Jonathan Critain and colleagues from the TRACE team, together with assorted members of the CDS team. We think we've got some good data and would encourage others to carry out more joint TRACE/CDS observations.

Our thanks go to colleagues at MEDOC who hosted the workshop. We thought that it would be very useful to have a follow up meeting, and especially good if this could be in Italy!

The Scientific Organizing Committee was Jim Klimchuk (Chair), Karine Bocchialini, Helen Mason and Giovanni Peres





From the CDS Operations Management Team in the Space Science & Technology Department at CCLRC Rutherford Appleton Laboratory
Site maintained by John Rainnie.
Last revised on Monday (20/Jan/2003) at 16:50.