SOHO Status Report - May 2004
24 May 2004 10:02Mission Status
The spacecraft status is nominal, with the High Gain Antenna (HGA) Z-axis in a fixed position.
From 21-23 April, SOHO went into Emergency Sun Reacquisition (ESR) four times. The first three ESRs were triggered by the Fine Sun Pointing Attitude Anomaly Detector (FSPAAD). It was determined that the FSPAAD now always triggers at the nominal pointing, suggesting an obstruction of the sensor unit as the likely cause.
Before returning to normal mode, the following measures were taken:
In case of an isolated hardware failure, the FPSS monitoring will trigger an ESR at a 2-degree off-point. In other failure scenarios, the CSPAAD will trigger an ESR at a 25-degree off-point.
The fourth ESR was due to an oversight in the implementation of the new FPSS monitoring. The spacecraft was back in Normal Mode on 27 April at 21:00 UT after a Station Keeping and Momentum Management manoevre to correct for disturbances from the four ESRs.
Saab Ericsson Space is working on a patch to the Central On-Board Software (COBS) to make more efficient use of the solid state recorder (SSR) during keyholes by selectively recording certain packets. This will allow recording of critical low-bandwith helioseismology and total solar irradiance data during long coverage holes.
Operations and ArchivingAll instruments are nominal and SOHO science operations are progressing smoothly.
The spring keyhole operations was carefully planned to secure continuous data from the GOLF and VIRGO instruments. This was successfully achieved by making full use of these instruments' delayed redundant data streams and by judicious use of the combined recording capacity of both the tape and solid state recorders. Overall keyhole operations went very smoothly. Links to the SOHO archives (including mirrors) are accessible at http://soho.estec.esa.nl/data/.
Polarization analysis of single-view LASCO images has resulted in the first three-dimensional reconstructions of coronal mass ejections (CMEs). The scattering point's distance from the plane-of-the-sky is determined by the ratio of polarized-to-unpolarized electron scattered emission. From the brightness ratio the 3-D structure, position and velocity could be computed for several CMEs. A paper is in press in Science magazine.
The SUMER team has presented a spectral atlas of solar coronal features (coronal holes, quiet corona, active corona, and flares), covering the wavelengths range from 670 to 1609Å and complementing the SUMER spectral atlas of solar disk features. It comprises 504 lines, 300 (60%) of which have been identified. The published line intensities provide a rich source for probing electron densities, electron temperatures, opacities and elemental abundances of the upper solar atmosphere. It also presents a powerful tool for the design of future instruments and the planning of future observations.
LASCO's observations of Comet Bradfield during 16-20 April were closely watched by a large world-wide audience.