There are generally two Data Processor (DP) modes when Yohkoh is taking scientific data, FLARE mode and QUIET mode. Yohkoh uses the WBS HXS and SXS instruments to monitor solar activity and when a threshold is passed the spacecraft (S/C) enters FLARE mode. There are times when Yohkoh is in FLARE mode but there is no flare.
There are three different telemetry rates, but only two are used when observing the sun. HIGH rate is 32 Kbits/s (a major frame every 2 s) and MEDIUM is 4 Kbits/s (a major frame every 16 s). When first entering FLARE mode, the DP goes into HIGH rate. After 10 minutes, it will go into MEDIUM rate if the intensity of the flare has subsided, but is not below a minimum threshold.
Instrument: Bent Crystal Spectrometers
Spectral lines: Fe XXVI (1.76 - 1.81 A) (Chan 1)
Fe XXV (1.83 - 1.90 A) (Chan 2)
Ca XIX (3.16 - 3.19 A) (Chan 3)
S XV (5.01 - 5.11 A) (Chan 4)
Spectral resolution (l/dl): 3000 - 8000
Angular resolution: Full disk
Best time resolution: 0.125 sec
Typical time resolution: 3.0 sec in FLARE/HI
Instrument: Fourier Synthesis Telescope
Energy bands: (15 - 100 keV, 4 channels)
Low 14 - 23 keV
Medium-1 23 - 33 keV
Medium-2 33 - 53 keV
High 53 - 93 keV
Angular resolution: ~5 arcsec
Effective area: 1.5 cm2 avg. x 64 elements
Field of View: 35 x 35 arcmin
Best time resolution: 0.5 sec
Typical time resolution: 0.5 sec in FLARE/HI
Instrument: Glancing incidence mirror/CCD sensor
Co-aligned optical telescope using same CCD
Wavelength ranges: 2.5-46 A no analysis filter (or Noback)
2.5-36 A 1265 A Al
2.4-32 A 2930 A Al, 2070 A Mg, 562 A Mn, 190 A C
2.4-23 A 2.52 micron Mg
2.4-13 A 11.6 micron Al
2.3-10 A 119 micron Be
4600-4800 A Wide band optical filter
4290-4320 A Narrow band optical filter
Spectral discrimination: Filters
Angular resolution: 3 arcsec
Field of View: 42 x 42 arcmin
Best time resolution: 0.5 sec
Typical time resolution: 2.0 sec in FLARE, 8.0 sec in QUIET
After the failure of an entrance filter in Nov-92, the narrow band, wide band optical filters, and Noback X-ray filter became unusable.
SXT uses sequence tables to define the order that the images will be taken. There are thirteen `slots' which are available to set the parameters. For each slot a separate filter, resolution and observing region can be specified. The observing region selection is from a table of nine entries. Each entry specifies a location and the size of the output image in pixels (this means that the field of view is different depending on the pixel resolution used).
The sequence is started from the beginning whenever the ``SXT CNT AUTO'' command sequence is issued from the ground or by programmed control, or if the telemetry rate or mode changes. Therefore, the sequence does not automatically begin at the top at the beginning of a new orbit, for example.
LOOP 1 (n1=infinity)
Img 1-1 --------------------+
LOOP 2 (n2= ) |
Img 2-1 ------------+ |
Img 2-2 | |
LOOP 3 (n3= ) | |
Img 3-1 ----+ | |
Img 3-2 | | |
Img 3-3 | | |
Img 3-4 ----+ ---+ |
LOOP 4 (n4= ) |
Img 4-1 ------------+ |
Img 4-2 | |
LOOP 5 (n5= ) | |
Img 5-1 ----+ | |
Img 5-2 | | |
Img 5-3 | | |
Img 5-4 ----+ ---+ ---+
SXT has automatic exposure control (AEC) available for partial frame images. The algorithm works on a per observing region basis and runs separately for each ``slot'' shown in the sequence above (image 3-3 exposure control is independent of image 5-3, even if they are for the identical filter and location). An upper and lower intensity, or Data Number (DN), threshold is set and the number of pixels above and below that threshold is set. An allowable number of pixels above and below is also set, and if the image exceeds this set level, then the exposure duration is shortened or lengthed, depending on whether it is over exposed or under exposed. See the ``Red Book'' article for more details on the exposure control.
The automatic region selection (ARS) allows two different selection methods. ARS-1 takes an exposure of the sun and locates the brightest points and selects the brightest four locations. Normally the sequence table simply uses the single brightest active region. ARS-2 allows the operator to specify a sub-region of the sun, and the tracking algorithm will point to the brightest region within that sub-region. ARS-2 is normally used to force the pointing to stay on a fixed region, and to correct for solar rotation. See the ``Red Book'' article for more details on the region selection.
Instruments:
Soft X-ray Spectrometer (SXS)
Hard X-ray Spectrometer (HXS)
Gamma-ray Spectrometer (GRS)
Radiation Belt Monitor (RBM)
SXS: Gas Proportional Counter (3-30 keV (nominal), 128 channels)
HXS: NaI scintillation counter (20-657 keV, 32 channels)
GRS: BGO scintillation counter (0.3-100 MeV, 144 channels)
RBM-SC: RBM NaI Scintillation Detector (5-300 keV)
RBM-SD: RBM Si Detector (>20 keV)
Angular resolution: Full disk
Best time resolution: 0.125 sec
Most of the sub-instruments have pulse height (PH) data and pulse count (PC) data. PH is essentially a spectrum with counts as a function of energy, and PC is the sum of all counts for a given energy range.
The structure for the WBS data has numerous tags. Some of the data tags and types of data available are listed below.
SXS has two detectors with a 2-channel PC and 128-channel PH for each detector. The energies listed below are nominal values.
sxs_pc11 - SXS-1 Detector, chan 1 (3-15 keV)
sxs_pc12 - SXS-1 Detector, chan 2 (15-40 keV)
sxs_pc21 - SXS-2 Detector, chan 1 (3-15 keV)
sxs_pc22 - SXS-2 Detector, chan 2 (15-40 keV)
sxs_ph1 - SXS-1 Detector, 128-channel PH (3-30 keV)
sxs_ph2 - SXS-2 Detector, 128-channel PH (3-30 keV*)
HXS has a 2-channel PC and a 32-channel PH.
hxs_pc1 - HXS Detector, chan 1
(20-60 keV for 1-Oct-91 to 9-Jun-92)
(25-75 keV for 9-Jun-92 to present)
hxs_pc2 - HXS Detector, chan 2
(65 - 657 keV for 1-Oct-91 to 9-Jun-92)
(75 - 830 keV for 9-Jun-92 to present)
hxs_ph - HXS Detector, 32-channel PH
(20 - 657 keV for 1-Oct-91 to 9-Jun-92)
(25 - 830 keV for 9-Jun-92 to present)
GRS has two detectors with a 6-channel PC and a 128-chan PH (16-channel for the high words) for each detector.
grs_pc11 - GRS-1 Detector, chan 1 (0.27 - 1.04 MeV)
grs_pc12 - GRS-1 Detector, chan 2 (1.04 - 5.47 MeV)
grs_pc13 - GRS-1 Detector, chan 3 (5.47 - 9.3 MeV)
grs_pc14 - GRS-1 Detector, chan 4 (9.3 - 13.1 MeV)
grs_pc15 - GRS-1 Detector, chan 5 (8 - 30 MeV)
grs_pc16 - GRS-1 Detector, chan 6 (30 - 100 MeV)
grs_phl1 - GRS-1 Detector, 128-chan (0.3 - 13.6 MeV)
grs_phh1 - GRS-1 Detector, 16-chan (8 - 100 MeV)
grs_pc21 - GRS-2 Detector, chan 1 (0.3 - 1.24 MeV)
grs_pc22 - GRS-2 Detector, chan 2 (1.24 - 5.66 MeV)
grs_pc23 - GRS-2 Detector, chan 3 (5.66 - 9.37 MeV)
grs_pc24 - GRS-2 Detector, chan 4 (9.37 - 13.6 MeV)
grs_pc25 - GRS-2 Detector, chan 5 (8 - 30 MeV)
grs_pc26 - GRS-2 Detector, chan 6 (30 - 100 MeV)
grs_phl2 - GRS-2 Detector, 128-chan (0.3 - 13.6 MeV)
grs_phh2 - GRS-2 Detector, 16-chan (8 - 100 MeV)
RBM has two detectors. RBM-SC has 2-channel PC and 32-channel PH, and RBM-SD has 1-channel PC.
rbm_sc_pc1 - NaI scintillation detector (5-60 keV)
rbm_sc_pc2 - NaI scintillation detector (60-300 keV)
rbm_sd_pc - Si detector (>20 keV)
rbm_sc_ph - RBM-SC Detector, 32-chan (5-300 keV)
The attitude control system uses momentum wheels, magnetic torquers, and control-moment gyros as the actuators. As the attitude sensors, two sun sensors and a star tracker, as well as geomagnetic sensors, are available for determining the spacecraft pointing relative to the direction of the sun and to the ecliptic plane, respectfully. An inertial reference unit comprising four gyros detects changes of attitude with time.