Difference between revisions of "The Spectrometer/Telescope for Imaging X-rays (STIX)"

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

Latest revision as of 13:27, 22 June 2021

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