Issue 5: 12th February 2002
A Note from the Editor and Contents
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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,
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
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
From the CDS Operations Management Team in the Space Science & Technology Department at
CCLRC Rutherford Appleton Laboratory
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Last revised on Monday (11/Feb/2002) at 21:23.