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, A.Fludra@rl.ac.uk
Contents:
Close Association of an EUV Sunspot Plume with
Depressions in the Sunspot Radio Emission
J. W. Brosius and S. M. White (ApJ 601, p. 546, 2004 Jan 20)
We obtained coordinated observations of the large sunspot in NOAA
Region 8539 on 1999 May 9 and 13 with the Very Large Array and three
instruments (CDS, EIT, MDI) aboard the Solar and Heliospheric
Observatory satellite. The EUV observations reveal a plume in the
sunspot umbra on both observing dates. The plume appears brightest in
emission lines formed at temperatures between 1.6 x 10^5 and 5.0 x 10^5
K. Radio emission from the sunspot umbra is dominated by thermal
gyroemission from the plume, which accounts for radio brightness
temperatures <1 x 10^6 K in the umbra on both dates, as well as umbral
brightness temperature depressions in the 4.535 and 8.065 GHz
observations on May 13. A compact 14.665 GHz source persists near the
umbra/penumbra boundary during our observing period, indicating a
long-lived, compact flux tube with coronal magnetic field strength of
at least 1748 G. It occurs in a portion of the sunspot that appears
very dark in EUV emission.
Signature of Oscillations in Coronal Bright Points
Ugarte-Urra I., Doyle J.G., Madjarska M.S., O'Shea E. (A&A, in press,
available at
http://star.arm.ac.uk/preprints/ ).
A detailed study of two consecutive bright points observed simultaneously with
the Coronal Diagnostic Spectrometer (CDS), the Extreme ultraviolet Imaging
Telescope (EIT) and the Michelson Doppler Imager (MDI) onboard the Solar and
Heliospheric Observatory (SOHO) is presented. The analysis of the evolution
of the photospheric magnetic features and their coronal counterpart shows
that there is a linear dependence between the EIT Fe XIII 195 A flux and
the total magnetic flux of the photospheric bipolarity. The appearance of the
coronal emission is associated with the emergence of new magnetic flux and
the disappearance of coronal emission is associated with the cancellation of
one of the polarities. In one of the cases the disappearance takes place ~3-4
hours before the full cancellation of the weakest polarity.
The spectral data obtained with CDS show that one of the bright points
experienced short time variations in the flux on a time scale of 420-650
seconds, correlated in the transition region lines (O V 629.73 A and O III
599.60 A) and also the He I 584.34 A line. The coronal line (Mg IX 368.07 A)
undergoes changes as well, but on a longer scale. The wavelet analysis of the
temporal series reveals that many of these events appear in a random fashion
and sometimes after periods of quietness. However, we have found two cases of
an oscillatory behaviour. A sub-section of the O V temporal series of the
second bright point shows a damped oscillation of five cycles peaking in the
wavelet spectrum at 546 seconds, but showing in the latter few cycles a
lengthening of that period. The period compares well with that detected in
the S VI 933.40 A oscillations seen in another bright point observed with the
Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer,
which has a period of 491 seconds. The derived electron density in the
transition region was 3 x10^10 cm^{-3} with some small variability, while the
coronal electron density was 5 x10^8 cm^{-3}
Transit of Venus
Peter Young
On June 8 this year a transit of Venus across the face of the Sun will
occur for the first time since 1882. Unfortunately, the transit will not
actually be visible at SOHO, as Venus crosses the southern hemisphere of
the Sun, while SOHO will be north of the ecliptic in its orbit around the
L1 Lagrange point. Venus will miss the solar limb by 158 arcsec. However,
it may still be possible to see Venus against the backdrop of the Sun's
corona and so the CDS team will perform observations of Venus at this
time.
Some technical details about the (not quite) Venus transit as seen from
SOHO are:
Time of closest approach: 13:30:17 UTC
Minimum separation*: 157.98 arcsec
Apparent size of Venus at SOHO: 59.5 arcsec
* minimum distance between the limbs of Venus and the Sun
The transit as seen from Earth lasts from around 05:13 to 11:25 UT at
Earth (times vary slightly according to location), and so the time of
closest approach as seen from SOHO lies outside this period.
CDS will perform observations to look for the dark disk of Venus against the
background corona for a period of time around the moment of closest
approach.
If you have suggestions for the observing sequence, or would like to be
kept in touch about plans for the transit, please contact Peter Young at
p.r.young@rl.ac.uk.
An IAU Colloquim (#196) entitled 'Transits of Venus: New Views of the
Solar System and Galaxy' is being held at the University of Central
Lancashire in Preston, England, during the week of the transit (7-11
June). For more information, please visit:
http://www.transit-of-venus.org.uk/conference/
Call for Active Region and Quiet Sun Studies
Andrzej Fludra
The monthly averaged sunspot number is now hovering between 50 and 60,
compared to over 120 at the maximum of Cycle 23. The number of active
regions has decreased accordingly. Therefore, we remind all CDS observers
to prepare and submit all your outstanding
active region studies before we enter the solar minimum.
Have a look at the solar cycle prediction:
http://science.msfc.nasa.gov/ssl/pad/solar/predict.htm
and try to judge for yourselves how much time and how many opportunities
for active region observations there are left before active regions disappear.
Consequently, the quiet sun periods become more frequent, and we
sometimes have unexpected opportunities to run tens of hours of quiet
sun studies. If you anticipate making future quiet sun observations,
we recommend to submit them as soon as possible.
Weekly Operations Update
Andrzej Fludra
Soon, Peter Young will be taking on the administration of weekly
science operations and sending the weekly planning notes. Please copy
to Peter (P.R.Young@rl.ac.uk) all your requests for creating new
studies, study modifications, scheduling of
observations, and booking your planning weeks at RAL. We
appreciate your giving us several weeks notice to fulfill your requests.
When your observation is already in progress, any requests for
last-minute changes, selection of targets, etc., should be sent directly
to the current planner, and copied to Peter as well.
Planning from RAL
Andrzej Fludra
We intend to increase the number of planning weeks from RAL's CDS
management facility. We expect that many UK and European
Ph.D. students and post-docs may want to take advantage of RAL's
proximity and a lower cost of travel, and will set up a regular
schedule of visits to RAL, spending part of their time as CDS planners.
On-site help will be provided by
the RAL staff (Peter Young, Jeff Payne, Dave Pike and Andrzej), and
we will endeavour to make your visit productive and comfortable.
Please contact us to arrange a visit.
Pointing Offsets
Dave Pike
Because of readout problems with the slit movement encoder, there are
now restrictions on the movement of the GIS slits when rastering in the
North-South direction.
However, the planning software has been updated so that the movements of
the slit mechanism required to circumvent this problem are automatically
calculated according to the raster parameters and users should not see any
effect or need to take any specific action. GIS rasters can once again be
planned as normal.
If accurate feature location is required in a GIS raster, planners should
continue to take note of the pointing offsets between NIS and GIS images
as detailed in the CDS user guide:
../software/uguide/uguide.shtml
SOHO Keyholes - What's it all about?
Stein V. H. Haugan
As most of you probably have heard by now, the Z-axis (azimuth)
mechanism of SOHO's High Gain Antenna (HGA) is no longer working in
the nominal operational mode. It can still be moved by using the prime
and redundant motor windings simultaneously, doubling the torque, but
the root cause of the problem has not been identified. To avoid
getting the antenna stuck in an unfavourable position, it has been
parked in a position that maximises the time when the signal is strong
enough for normal use (i.e. normal telemetry reception on 26-metre DSN
stations).
This so-called "sweet spot" of the HGA covers the outer parts of one
half of the halo orbit. By turning the spacecraft upside down for the
other half of the orbit, the time per orbit with normal telemetry
reception is doubled. As a result of the parked position, the HGA is
useless when SOHO is in the central portions of the halo orbit. These
periods are dubbed "keyholes", using standard antenna terminology (for
more on this, including figures, see
this Hot Shot).
A SOHO keyhole can be divided into three distinct periods based on the
signal strength at Earth: At first, the 26-metre DSN stations can no
longer lock on the HGA signal. The 34-metre stations can still hang
on, though, due to the larger dish and (more so) the lower noise
levels of their receivers. This period is called a 26-metre keyhole.
As the signal strength decreases further, the 34-metre stations also
lose lock on the HGA signal, and SOHO switches to the omnidirectional
Low Gain Antenna (LGA), signifying the start of what is called the
34-metre keyhole. At some point during this period, SOHO will roll 180
degrees to reorient the antenna, and the process runs in reverse, with
another 26-metre keyhole at the end.
During the 34-metre keyhole, when SOHO is using the LGA, 34-metre
stations cannot lock on the normal high rate telemetry. But they can
receive medium rate telemetry - meaning "real-time data only" since no
recorder dumps can be done. A 70-metre DSN station, however, is able
to get high rate data even on the LGA. With unrestricted access to
70-metre stations, SOHO could have tossed the HGA entirely without any
operational impact!
Alas, there are others who claim the time on 70-metre stations as
well! Most "deep space" missions have a real requirement for either
70-metre or 34-metre stations, and SOHO does not compete favourably
for time with most of them. Due to severe competition with the Mars
missions and Stardust, the winter keyhole saw fairly severe data
losses (61%), as will the spring 2004 keyhole. Unfortunately, things
will not get much better in the future, and we will just have to live
with data dropouts and fewer (and shorter) opportunities to send
commands to the instruments.
The largest impact is of course on the global helioseismology
experiments: GOLF, VIRGO, and MDI's structure program. We are
therefore pursuing a software patch that will allow the Solid State
Recorder (SSR) to record telemetry from these three instruments only.
This will stretch the SSR's capacity far beyond the 11 hours it can
record in the normal configuration, hopefully eliminating all data
gaps for these experiments.
And what is the bottom line for CDS and its users? First of all, we
can almost "guarantee" significant data losses during the 34-metre
keyholes. For the 26-metre keyhole periods, the situation is much
better, since 34-metre stations are not quite as difficult to get as
the 70-metre stations. As a very general statement, we should see few
if any data losses in the 26-metre keyholes, but we cannot give any
guarantees. Some keyholes will be worse than others, and we will try
to keep you informed as early as possible (through the monthly SPWG
meetings and through links in the Daily Minutes). In general, we
should have a reasonably firm idea of the data losses about 1 or 2
months ahead of each keyhole. It is then up to the CDS PI to
prioritise the time left for observations.
In some ways the keyholes introduce more long-term predictability for
planning: EIT bakeouts will take place during 34-metre keyholes, and
so will the routine station keeping and momentum management
manoeuvres. There will be less MDI high rate data during 26-metre
keyholes, and almost no MDI high rate data during the 34-metre
keyholes. In other words, look out for the keyholes when planning
future campaigns!
For dates of future keyholes, follow
this link.
Between the keyhole periods, CDS planners will not notice much
difference, but keep in mind that we're upside down for half the time!
Although pointing selection etc. is not affected thanks to updated
ground software, bear in mind that raster scanning may not be
happening right to left as you would normally expect! The raster data
will also be upside down during these periods, of course.
