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Issue 11:     5 February 2004
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:
http://solar.bnsc.rl.ac.uk/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.





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 Thursday (25/Nov/2004) at 10:59.