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Academy of Sciences of the Czech Republic |
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Academy of Sciences of the Czech Republic |
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Solar Broad-Band Hard X-Ray Spectrometer Was Launched on March 12, 2000 onboard the MTI Satellite. Details Follow:
| PRINCIPAL INVESTIGATOR: Frantisek Farnik
1. INTRODUCTION The HXRS experiment is a joint endeavour by the Astronomical Institute
of the Academy of Sciences of the Czech Republic and the Space Environment
Center (SEC) of the National Oceanic and Atmospheric Administration (NOAA).
The spectrometer was developed and manufactured in Prague by the private
firm Space Devices Ltd. The manufacturer has a long association with the
Astronomical Institute in the development of astrophysical and geophysical
instrumentation for space applications including participation in many
Russian Space Agency missions in Earth orbit and planetary exploration.
In the present endeavour to test the HXRS in a space environment SEC has
had the lead role in obtaining a space flight through the Department of
Defense Space Test Program (STP) in time to exploit the rise and peak activity
period of the new solar cycle in progress since May 1997. The HXRS was
selected by STP to fly onboard the MTI satellite which was launched on
March 12, 2000 and has an expected three year operational lifetime.
2. SCIENTIFIC AND OPERATIONAL OBJECTIVES The mission has three primary objectives: a) to collect high time resolution solar hard X-ray data for flare research; b) to evaluate the efficacy of this type of instrument to predict interplanetary proton events and, c) to test the effectiveness of new shielding methods applied to this instrument to mitigate the effects of ambient high energy electrons – a combination of magnetic deflection shielding and organic plastic moderators which will enable this type of instrument to make long term solar hard X-ray observations at geostationary orbit. Ad a) Flare research: Hard X-rays can provide essential data concerning fundamental processes that take place at the core of a flare during the primary energy release phase. This information is required for the understanding of how energy is acquired from the surrounding medium, where it is located, how it is distributed and how the mechanism operates by which particles are energized during a flare. These studies are intended to address the following questions:
A statistical analysis by Kiplinger (1995) showed a strong empirical association between spectrally hardening hard X-ray flares (SHHX) and solar energetic proton events (SEP). Garcia (1994), and Garcia and Kiplinger (1996) also reported on this association and described some of the other attributes of this peculiar type of solar flare. SHHX flares are recognized as a special flare class because of a characteristic and distinctive hard X-ray signature as well as other commonly observed features such as smooth, long-duration light curves in both soft and hard X-ray emission. The most exploitable attribute of SHHX flares for SEP prediction is the manner in which the hard X-ray spectra evolve with time during the flare: the great majority of hard X-ray flares which are characteristically impulsive, typically exhibit spectra that evolve in the pattern ‘soft-hard-soft’; in contrast, SHHX flares which may commence with typically impulsive features will eventually exhibit a spectral pattern ‘soft-hard-harder’. It is primarily this distinctive behavior of SHHX flares that allows this type of flare to be discriminated from all other flare types with very high degree of accuracy. Statistically, the method has demonstrated an ability to predict a SEP event at a 96% success rate and to correctly predict that a SEP will not occur (i.e., method will not cause a false alarm) with a 99% success rate, see the paper by Kiplinger, 1995. Ad c) Ambient energetic electron shielding: An important adjunct of the HXRS experiment is to demonstrate that the
combination of active magnetic deflection shielding and passive organic
plastic shielding will permit hard solar X-ray measurements to be made
in the presence of relatively high fluxes of ambient MeV electrons. It
is anticipated that a hard X-ray spectrometer of similar design will be
used to monitor the Sun from geostationary orbit where the spectrometer
will have to cope with very high fluences of charged particles at the outer
edge of the proton radiation belt. To test this proposed method of electron
shielding two identical scintillation detectors – one unshielded and the
other one shielded (by a strong permanent magnet that
3. INSTRUMENT DESCRIPTION HXRS is designed to measure hard X-ray emission of the Sun in eight energy bands with high time resolution (up to 200 ms). Its principal block diagram is shown in Figure 1. Two identical scintillation detector are mounted side by side (it can be seen in the outside photograph, Figure 2). The shielding is provided by a strong permanent magnet and a thick layer of plastic material. Each detector consists of a Nal crystal (25 mm of diameter, 4.5 mm thick) coupled with a Hamamatsu photomultiplier. Each detector will be automatically calibrated in-flight by means of a Am241 radioactive source on a movable arm. All in-flight operations – such as calibration, house-keeping, data handling and other minor operations will be controlled by an on-board 16-bit microprocessor. Figure 1 shows the principle of the instrument
function: Incident X-ray photons create optical pulses in the crystals;
the optical pulses are transformed into electrical pulses which are then
amplified by photomultipliers . These signals are further amplified electronically
and fed to pulse height discriminators. Because the height of each electrical
pulse is proportional to the incident photon energy , the output pulse
train containing an assortment of pulse magnitudes can be sorted according
to energy and binned into appropriate energy bands by the amplitude discriminator.
The number of pulses in each energy band counted per observation time interval
is registered and that count rate sent to the telemetry processor. The
energy band limits as well as maximal count rates of each identical detector
are shown in Table 2.
The on-board computer automatically maintains a constant energy calibration
throughout the entire mission by adjusting the photomultiplier’s high voltage.
It also controls the flow of data processing and prepares the on-board
processed data for down link telemetry. The microprocessor is capable of
solving minor defects and detecting system failures – as, for example,
a fatal disruption of either of the two detectors. The external view of
the instrument is shown in Figure 2.
4. OPERATIONAL CONCEPT The HXRS will observe the Sun continuously during satellite daytime except for brief periods of telemetry download and when the host MTI experiment is in its own observation mode. Tests of the electron shielding system will be conducted during the mission in daylight or in darkness when flying in radiation belts until the shielding efficiency is fully evaluated, or these tests will continue. Operational modes: The instrument has four principal operating modes. Each principal mode has a number of sub-modes, as described below.
There are two sub-modes in the BDN observing mode. During quiet Sun periods, i.e. weak hard X-ray emission (Quiet Sub-Mode), the HXRS exposure time is 1 second; during solar flares (Flare Sub-Mode) the exposure time is 0.2 second. Quiet Sub-Mode changes into Flare Sub-Mode automatically when the total count rate in energy bands #2-#7 is above 100 pulses per second. During times when the satellite moves into the Van Allen radiation belts or when a strong flare appears – the HXRS automatically switches into the BDS observing mode. In the BDS mode the photomultiplier high voltage (HV) is reduced such that the lowest energy band limit becomes 20 keV (instead 13.2 keV in BDN mode) i.e., the lowest energy band is temporarily eliminated. If the HV decrease in this sub-mode is till not sufficient to maintain the count rate within prescribed limits another HV step decrease is commanded automatically eliminating the 20-30 keV band, putting the HXRS into the BDS second sub-mode. In the most critical condition when the second HV step decrease is still insufficient the HV can be further reduced automatically until the count rate is brought within prescribed limits. In the Technological Mode a full information record on the instrument state is formatted into a technological block. The HXRS is put into the Technological Mode approximately 15 minutes after instrument ‘turn-on’; then every 12 hours provided the instrument is in the ”Quiet Sub-Mode”. Each detector is calibrated independently but not simultaneously with the other. The calibration mode has three phases and each phase has three steps. The three phases correspond to the three BDS sub-modes. The three steps are: BEF, DUR and AFT. During the step (BEFore calibration) the Am241 radioactive source is placed in front of the detector and the count rate is registered under the ‘old’ band limits. During the DUR step (DURing calibration) the HV is tuned such that the Am241 peak output (59.5 keV) is centered over the two midrange bands sensitive to this energy. Finally, during the AFT (AFTter calibration) step the calibration is checked by recording the Am241output for each detector, provided that the HXRS is in the ”Quiet Sub-Mode”; if it is in the ”Flare Sub-Mode” calibration is postponed until the next ”Quiet Sub-Mode”. 5. MTI SATELLITE The Multispectral Thermal Imager satellite is 3-axis stabilized and spends most of its time in a stand-by mode (when the host MTI experiment is not observing and the satellite is not down linking to the Sandia receiving station). In the stand-by mode the spacecraft attitude is Sun pointing, i.e., the solar panels are oriented to the Sun with an approximate pointing accuracy of 1 degree. This operating arrangement is ideal for the HXRS observing since no additional pointing mechanism is required. The principal spacecraft attributes are as follows: 610 kg total spacecraft
mass; 575 Watts maximum power consumption; 2048 bit/s uplink capacity;
and 8 Mbit/s downlink capacity. Launch by a Taurus LV took place on March
12, 2000 placing the MTI into a 3 year, 555 km circular, sun-synchronous
orbit. Figure 3 shows the MTI satellite ready to
be integrated with the rocket.
Figure 4: MTI on the top of Taurus rocket at the Vandenberg Air Force Base Figure 5: The Czech team in front of the TAURUS Rocket Figure 6 and Figure 7: The TAURUS Rocket launch during night Figure 8: Artistic view of the MTI satellite
in space
ACKNOWLEDGEMENT The authors would like to acknowledge financial support of the HXRS
project by two grants from the U.S.-Czechoslovak Science and Technology
Program (grants No. 92021 and 95056), by one grant from the Grant Academy
of the Academy of Science of the Czech Republic (No. 303108) and the support
from EOARD (No. SPC-98-4020) as well as from NASA JSC SRAG. The project
was also supported by the Astronomical Institute of the Academy of Sciences
of the Czech Republic.
REFERENCES Garcia, H.A., 1995 ApJ 420, 422-432 Garcia, H.A. and Kiplinger, A.L., 1996, Solar Drivers of Interplanetary and Terrestrial Disturbances, ASP, Conference Series, Vol. 95, Eds. K.S. Balasubramaniam, S.L.Keil and R.N. Smartt Kiplinger, A.L., 1995 ApJ 453, 973-986
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