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| − | == STIX instrument Scientific Objectives ==
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| − | STIX plays an important role in enabling Solar Orbiter to achieve two of its major
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| − | science goals: (1) determining the magnetic connection of Solar Orbiter back to the Sun
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| − | and (2) understanding the acceleration of electrons at the Sun and their transport into
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| − | interplanetary space. The X-ray measurements made with STIX determine the intensity,
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| − | spectrum, timing, and location of accelerated electrons near the Sun. Flare-accelerated
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| − | electrons escaping the Sun can then be tracked into the inner heliosphere through their type-
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| − | III radio emission observed by RPW (the Radio and Plasma Waves instrument), and by their
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| − | in situ detection by the Energetic Particle Detector (EPD) suite. In this way, STIX, together
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| − | with RPW and STEIN, provides direct tracing of the magnetic structure, field line length, and
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| − | connectivity and is able to magnetically link the heliospheric location observed in situ back to
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| − | regions at the Sun where the electrons are accelerated. STIX thus plays a key role in
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| − | connecting the Solar Orbiter in situ and remote sensing observations.
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| − | == Measurement principle ==
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| − | Observationally, STIX determines the location, intensity, spectrum and timing of transient X-
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| − | ray emission on the Sun at energy ranges that encompass bremsstrahlung emission from both
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| − | hot thermal plasmas and from energetic electrons. The properties of the electrons that
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| − | generated the X-rays can be inferred from their X-ray spectrum. The distinction between a
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| − | thermal plasma and non-thermal electron population is based on the shape of the X-ray
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| − | spectrum with the latter having a characteristic power law (or broken power law) profile and
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| − | the former providing a black body spectrum (corresponding to 106 to 108 K). The spectra are
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| − | very steep and so good spectral resolution is required for their interpretation. There is also an
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| − | Iron line complex at 6.7 keV which, if isolated, can be interpreted in terms of the thermal
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| − | electron population. Since a typical flare typically generates both thermal and nonthermal
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| − | emission, which often are not co-located (for example with locations at the top and footpoints
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| − | of magnetic loops respectively), both good spatial and good spectral resolution are required.
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| − | The observational objectives are achieved by imaging the Sun as a function of time and energy
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| − | with enough spatial, spectral and temporal resolution to match the sources of interest.
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| − | Comparing the resulting images at different energies yields the X-ray spectra of individual
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| − | features (e.g. footpoints or flaring loops). Comparing the images as a function of time reveals
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| − | the temporal behavior of the hot plasma and accelerated electrons. The data can also be
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| − | combined to yield spatially-integrated light curves and spectra. In all cases, the basic
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| − | observational datum is a single, photometrically-accurate image corresponding to a
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| − | well-defined time and energy interval.
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| − | Within Solar Orbiter constraints, focusing optics is not a feasible option for arcsecond-class
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| − | hard X-ray imaging. As a result STIX uses an indirect Fourier imaging technique based on X-
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| − | ray collimation. This is implemented through three mechanically separate modules:: X-ray
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| − | transparent windows; a passive imager containing front and rear grids; and a
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| − | Detector/Electronics Module (DEM) containing electronics and passively-cooled X-ray
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| − | detectors.
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| − | The Imager is comprised of 32 subcollimators, each of which consists of a pair of well-
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| − | separated X-ray opaque grids located in front of a corresponding CdTe X-ray detector in the
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| − | DEM. The X-ray transmission of each grid pair forms a large-scale Moire pattern on the
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| − | detector. The properties of these Moire patterns are very sensitive to the angular distribution
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| − | of the incident of the X-ray flux. Although individual CdTe detector pixels associated with each
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| − | subcollimator provide only ~2 mm spatial resolution, this is sufficient to characterize the Moire
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| − | pattern formed by its grids. As a result, high-angular resolution X-ray imaging information is
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| − | encoded into a set of large scale spatial distributions of counts in the detectors. These
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| − | distributions can be subsequently decoded on the ground to reconstruct an image of the X-
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| − | ray source.
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| − | For each detected X-ray, the detectors provide an output pulse proportional to its energy. By
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| − | reconstructing images using counts within specific energy intervals, the combined system
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| − | functions as a high-resolution X-ray imaging spectrometer. Relative pointing information is
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| − | provided by the spacecraft aspect system while an internal STIX aspect system intermittently
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| − | establishes the pointing offset of the Instrument Line of Sight (ILS) and the instrument Optical
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| − | Axis (used for absolute location of images) relative to the spacecraft aspect.
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| − | == Instrument Overview ==
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| − | The STIX instrument is made up of three mechanically-separate sections: the X-ray windows;
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| − | the imager with widely separated grids and aspect system; and the Detector/Electronics
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| − | Module containing CdTe detectors and electronics. See Figure 2-2 for a sketch and Figure
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| − | 2.3 for a functional block diagram of the STIX instrument.
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| − | The system consists of two X-ray
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| − | Windows, the Imager and DEM. DEM includes two parts: main array of detectors with
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| − | associated electronics (plus Attenuator and the drivers for it) and the IDPU/PSU block. The
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| − | detectors of Aspect System are located inside the Imager Module, separate from the DEM.
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