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NOAO Newsletter - GONG - March 2000 - Number 61


Global Oscillation Network Group

John Leibacher

The Global Oscillation Network Group (GONG) Project is a community-based activity to operate a six-site helioseismic observing network, to do the basic data reduction and provide the data and software tools to the community, and to coordinate analysis of the rich data set that is resulting. GONG data are available to any qualified investigator whose proposal has been accepted. Information on the status of the project, the scientific investigations, as well as access to the data, is available on our WWW server at http://www.gong.noao.edu.

Although the network hiccuped briefly through the passage into the new millennium, the Project embraces Y2K as we near the transition to the high-resolution GONG+ network. Components and subsystems of the 10242 camera systems are arriving daily, and the production and integration of the GONG+ systems is in full swing. The DMAC is gearing up as well, and is looking to the review of the GONG+ processing changes by the DMAC Users' Committee. Deployment teams are being identified and procedures readied. The first team should hit the road for California shortly.

Operations

The GONG telescope network performed well through the last quarter of 1999, despite several periods of downtime. Udaipur and El Teide suffered weather related shutdowns from the monsoon in India and the onset of winter storms in Tenerife, respectively. In addition, some scheduled downtime was incurred when preventive maintenance trips were made to Mauna Loa in October and Udaipur in December. Downtime due to hardware can be traced to tape-drive failures (somewhat routine) and an unreliable GPS receiver at Learmonth. The GPS problem required reprogramming several times before a spare receiver arrived on site.

A day of data was sacrificed at Udaipur and Mauna Loa when tests were run to improve the determination of the orientation of the camera rotator. The measurements were performed at the two sites, nearly simultaneously, to determine the consistency of the alignment method around the network in conjunction with the Mercury transit, 15 November 1999, which allowed us to independently verify the technique (see Data Algorithm Development). The GONG+ prototype instrument in Tucson also observed the event.

After a preventive maintenance visit to CTIO last Spring, an unusual variation appeared in the velocity data. A new power supply, which had been installed for the temperature-stabilized oven, was identified as the culprit and the old method of delivering power to the oven was rapidly restored. The power supply was not able to keep the Michelson interferometer sufficiently stable, thus producing the anomalous signal. However, the periodic variations in the data are of such low frequency that the effects on the p-mode results should be negligible.

Data Management and Analysis

During the past quarter, the DMAC produced month-long (36-day) velocity, time series, and power spectra for GONG months 40, 41, 42, and 43 (ending 990801), with respective fill factors of 0.83, 0.87, 0.89, and 0.83, and tables of mode frequencies, which were computed from the power spectra using the three-month-long time series centered at GONG months 36, 37, 38, 39, 40, and 41. The main development activity currently underway in the DMAC is related to the development and testing of the GONG+ camera and data system upgrade.

Data Algorithm Developments (and Some Science)

As mentioned in the previous Newsletter, we have compared the results of peakfitting a 108-day time series containing two different versions of Month 36 (with and without the rectangular pixel correction). Fortunately, a comparison of the inversions showed no significant differences, in contrast to the test performed with a 36-day time series. We conclude that the processing change has had no significant systematic effect on the fitted frequencies; therefore, we will not reprocess the first 35 months to include this change. Meanwhile, we now have a history of the solar cycle variations up through GM 42.

Inversions of the even-splitting coefficients are continuing. As reported in the last Newsletter, the images of the temporal variations in the solar structure are similar to those of the flows --- bands of enhanced sound speed are seen to migrate from high latitudes at solar minimum to lower latitudes as activity increases. However, it currently appears that the bands of increased sound speed are confined to shallow depths very close to the surface, in contrast to the torisonal oscillation pattern which extends down at least 60,000 km. There are also indications that the rotation rate at the bottom of the convection zone is periodically changing. With the large number of new results, a small informal workshop on the recent helioseismic observations of the solar cycle and their theoretical implications was held January 6-12 in Boulder.

In anticipation of the deployment of the GONG+ system, work continues to improve the angular registration between simultaneous images at different sites. Tests with artificial data indicate that we will be able to achieve a precision of better than 0.01 (0.7 pixel at the limb) in relative angular offset between simultaneous measurements. The verification of this measurement with actual multi-site data will be a very high priority during the deployment of GONG+. On the other hand, the accuracy of the absolute value of P (the position angle of the solar rotation axis) is also vital. For local helioseismology analyses such as ring diagrams, a 0.01 error in the P angle results in a maximum 2 m/s velocity error near the limb. While we are continuing to improve the analysis of the drift scans, which provide the zero point for P, the transit of Mercury, which occurred in November, provided a rare opportunity for an independent determination. Analysis of the data is still underway.

GONG+ Camera Development

Minor mechanical modification to the cameras and final characterization of the data continues. The GONG+ cameras are being equipped with mechanical mounts that permit adjustment of tip and tilt, such that the CCD imaging device will be positioned perpendicular to the optical axis of the instrument, helping to minimize geometric distortion across the image plane. Masks have been attached to the imaging devices to facilitate image correction as well. Extensive illumination testing is in progress, the results of which will provide gain adjustments that will optimally match camera response with the light level at each site.

Caption: The GONG+ cameras enjoy a last get-together with some friends, before heading off to their new homes around the world.

After completion of the Data Acquisition System acceptance tests and the camera acceptance tests, which are now in the last stages, imaging system components will be integrated into complete deployment systems for the final stages of characterization, testing, and burn-in. Despite continued camera/DAS related electronics problems, we still hope to deploy the first GONG+ system to Big Bear real soon.


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