Solar Orbiter STIX

The Solar Orbiter mission of the European Space Agency (ESA; launch in February 2020) will fly as close to the Sun as 0.28 Astronomical Units, i.e. closer than the orbit of the planet Mercury. Solar Orbiter is ESAs flagship mission in heliophysics and will revolutionize our understanding of the heliosphere, a huge plasma bubble created by our Sun and embedding our solar system. The most energetic outbursts from the Sun, solar flares and coronal mass ejections, are the main drivers for severe disturbances of our space weather at Earth. Due to the increasing reliance on sensitive and space-based technology, our modern society becomes also increasingly vulnerable to space weather effects. The main scientific question addressed by the Solar Orbiter mission is to investigate how the Sun creates and drives our heliosphere. The Solar Orbiter payload consists of ten different instruments which make combined remote-sensing and in-situ observations, including close-up views of the Sun and its poles. Solar Orbiter will provide us with new insight about the generation of the magnetic field of the Sun, the solar wind, the heliospheric magnetic field, solar energetic particles as well as flares and coronal mass ejections. The Spectrometer Telescope for Imaging X-rays (STIX) on-board Solar Orbiter will provide spectra and images of the Sun at hard X-ray wavelengths. Hard X-ray observations provide two main diagnostics: on flare-accelerated energetic electrons that interact with ambient protons to produce non- thermal hard X-ray bremsstrahlung emission, and on the thermal bremsstrahlung of the hottest solar plasma that is heated to temperatures of 10-50 million degrees during flares. In addition, STIX will be most sensitive to the smallest flares from the Sun, so-called microflares. The core science objectives of this project are: a) to study microflares and their importance to the heating of the million-degree corona, b) to perform combined measurements of flare-accelerated electrons at the Sun observed by STIX and in-situ measured escaping electrons by Solar Orbiter and NASAs unprecedented Parker Solar Probe that will approach the Sun as close as about 9 solar radii, and c) to perform stereoscopic observations of hard X-ray flares to study beaming in the accelerated electron distribution through directivity measurements. Combining STIX with other Solar Orbiter instruments and the close-to-Sun in-situ observations of NASAs Parker Solar Probe (which will approach the Sun as close as 9 solar radii) will give us unprecedented insight into the acceleration, transport and escape of high-energy particles from our Sun.

Solar Orbiter, launched in February 2020, is currently the most important solar and heliospheric mission of the European Space Agency ESA. The Solar Orbiter spacecraft has 10 scientific instruments on board that combine in-situ measurements of the interplanetary plasma with imaging instruments for remote sensing of the Sun's atmosphere in an unprecedented combination to better understand how the Sun generates high-energy eruptions and particles, and how these propagate in our heliosphere. One of these instruments is STIX, an X-ray telescope which was co-developed by the University of Graz. STIX measures the signature of high-energy electrons that are accelerated to fractions of the speed of light during flares from the sun. The STIX observations include time series, images and spectra in the hard X-ray range. In addition, observations from STIX can be used to determine the temperature and density of the solar plasma that is strongly heated during solar flares (up to 10-50 million degrees). Among the key results of this project are a better understanding of so-called microflares, i.e. particularly small but very frequent bursts of radiation. It was shown that microflares can produce particularly hard (high-energy) particle spectra if the associated magnetic field is anchored directly in a sunspot. In addition, the coronal magnetic field in flare eruptions was modeled and, in combination with the STIX X-ray data and observations at extreme ultraviolet wavelengths, the underlying magnetic reconnection processes that lead to the vast energy release were determined. A better physical understanding of these processes that lead to flares and coronal mass ejections on the Sun is key to improving methods for predicting disturbances of our "space weather" near the Earth.