Yohkoh, a solar X-ray satellite of ISAS, was launched on 30 August
1991 and has been observing the solar corona and solar flares for
more than ten years. Data obtained from a total of four scientific
instruments, including an X-ray telescope (SXT; Soft X-ray Telescope)
and a spectrometer (BCS; Bragg Crystal Spectrometer) which were
jointly developed by U.S.-Japan and U.K.-U.S.-Japan international
teams respectively, are on board the satellite. All the data have
been made available for solar physics studies to researchers worldwide,
and used for real-time monitoring of solar activity, both domestic and
abroad, for the sake of space weather studies of the Earth environment.

On 15 December 2001, at around 3:50 pm (time in JST, hereafter), the
operators at the Kagoshima Space Center (KSC) noticed that the power
supply to the scientific instruments had been shut down, and henceforth
scientific observations were suspended.

ISAS immediately started to take necessary measures and to investigate
the cause of this trouble. The investigation so far has revealed that
at around 8:00 am, the satellite power system detected a low battery
voltage, which triggered a UVC (Under Voltage Control) automatically,
and shut down the power system to the scientific instruments.

The above situation was initiated in relation to the annular eclipse
observed over the Pacific Ocean at around 7:00 am, the same day.
Yohkoh experienced an unexpected satellite-body rotation, when the
satellite attitude control switched from the normal Sun-acquisition
mode to the stand-by mode, as the orbit crossed the annular-eclipse
belt. As a result, the total power generation started to decrease
and the battery was discharged, as the solar panels could not accept
stable and sufficient solar light.

During ten contacts at KSC so far since the beginning of the UVC, the
operation has been successful in minimizing the power consumption of
the satellite by shutting down the power lines of the other instruments
and heaters. The orbital average of the satellite power balance is
achieved, and no more decrease of the battery voltage is expected at
the moment. However, the satellite is still spinning and has not been
controlled yet as of 18 December 2001 and scientific observations have
been suspended.

The measures to retain the normal control of the satellite are
continuing. However, many steps will be required for recharging the
battery, re-establishing attitude control, and so forth. Therefore,
it may take some time before scientific observations can resume.