Front Page
 Scientific Ops
  » Latest
  » Science
  » Calibration
  » Operations
  » Past Issues
Issue 5: 12th February 2002
A Note from the Editor and Contents

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
The Newsletter is updated monthly. 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:

This Month's Topics:

Models for solar magnetic loops: Comparison with SOHO-CDS observations on the solar disk

A. Brkovic, E. Landi, M. Landini, I. Ruedi and S.K. Solanki, 2002, Astron. Astrophys., in press.

The present work describes a detailed comparison between SOHO-CDS observations of active region loops with a static, isobaric loop model developed assuming a temperature-independent heating function in the energy balance equation and a variable loop cross-section. The loop model is described in Landini & Landi (2001 - Paper I).

Observations of an active region recorded by CDS have been analyzed. Additional data from the EIT and MDI instruments on board the SOHO satellite, and broad band soft X-rays images from the Yohkoh satellite, have been used to complement the CDS dataset. CDS monochromatic images from lines at different temperatures have been co-aligned with EIT, MDI and Yohkoh images and a loop structure has been identified. Two other loop structures are visible but their footpoints are not clearly identified, and have not been analyzed.

Electron density, temperature and pressure along the selected loop structure have been measured by means of line ratio techniques. These quantities have been used to test the assumption of constant pressure adopted in the theoretical model, and to compare their values with its predictions. The loop filling factor has also been estimated from the CDS data after assumptions on the loop geometry have been made.

Comparison with CDS data has shown that a classical model is NOT able to reproduce the observations; despite the large uncertainties, mainly given by the limited CDS spatial resolution, indications suggest that agreement occurs only if an ``ad hoc'' isothermal region is added on top of the loop and a large conductive flux at the base is assumed.

Suggestions for improvements of theoretical loop models and further studies with the EIS instrument on Solar-B, due for launch in 2005, are given.

Transition Region Blinkers: I Quiet-Sun Properties

D. Bewsher, C.E. Parnell, R.A. Harrison, 2002, Solar Physics, in press.

An automated method of identifying transition region blinkers is presented. The distribution and general properties of blinkers identified in the quiet Sun are discussed. The blinkers are seen most clearly in the O V (629A) transition region emission line, but they also have strong signatures in O IV (554A) and the chromospheric line, He I (584A). The strongest O V blinkers can also be identified in O III (599A). No significant signatures are found for the blinkers in the coronal lines Mg IX (368A) and Mg X (624A).

A few hundred O V blinkers are analysed. Their global frequency is between 1 s^-1 and 20 s^-1 depending on how significant the peaks of the blinkers are. They have a typical area of 3 x 10^7 km^2, a typical lifetime of 16 minutes and a typical intensity enhancement factor of 1.8. We find ratios of the oxygen lines to be flat confirming the result that blinkers are not temperature events, but are density enhancements or due to increases in filling factor. Blinkers are found to occur preferentially over regions of enhanced chromospheric or transition region emission such as network boundaries, however, it is not clear that they appear below the brightest coronal regions. A rough analysis of the magnetic fragments show that blinkers preferentially occur above regions where there are large or strong magnetic fragments with 75% occurring in regions where one polarity dominates.

Transition Region Blinkers: II Active-Region Properties

C.E. Parnell, D. Bewsher, R.A. Harrison, 2002, Solar Physics, in press.

The distribution and general properties of events identified in an active region that have the same characteristics as quiet-Sun blinkers are discussed and named 'active-region' blinkers. The events are identified using an automated scheme 'BLinker Identification Program (BLIP)' which was designed for and tested on quiet-Sun blinkers. Like quiet-Sun blinkers, the active-region events are most easily identified in the 629 A emission line from O V although evidence for them is also found in other extreme UV lines emitted from He I, O III and O IV. Unlike quiet-Sun blinkers, however, they may also have coronal signatures in the lines Mg IX and Mg X. Their properties are very similar to those of quiet-Sun blinkers with mean lifetimes of 16-19 minutes, mean areas of 2.4-4.3 x 10^7 km^2 and mean intensity enhancement factors of 1.8-3.3. Their global frequency of 7-28 s^-1 is about 42%-700% higher than that for quiet-Sun blinkers. The blinkers discussed here are found above both active-region (plage) magnetic fields, as well as above the umbra and penumbra of a sunspot.

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 (11/Feb/2002) at 21:23.