View source for The Spectrometer/Telescope for Imaging X-rays (STIX)

=== 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.