Scientists at the University of Applied Sciences and Arts Northwestern Switzerland (FHNW) have spent around ten years building the Spectrometer / Telescope for Imaging X-rays (STIX). Since 10 February, the research instrument is travelling to the Sun. It will provide accurate measurements of the solar atmosphere and the solar wind and will also cover the polar regions of the Sun that cannot be observed from Earth.
24 March 2020 is the day Marina Battaglia and her research colleagues are looking forward to. For many people, it is an ordinary Tuesday in spring, but for the FHNW scientist, it is the day that decides whether to be or not. On March 24, the X-ray telescope STIX will be switched on for the first time during its three-and-a-half-year journey to the planned orbit of the Sun. Then it will become clear whether the ten years of preparation have really been worthwhile: Will STIX work as planned? Will the telescope be able to record data and transmit them to Earth? And if all this succeeds: Do these data perhaps already show events from the atmosphere (corona) of the Sun?
On the way into solar orbit
So far, everything is running like clockwork in the space mission. On 10 February at 5:03 am (Swiss time), an Atlas V launch vehicle took the 'Solar Orbiter' spacecraft from the US spaceport at Cape Canaveral into space. Marina Battaglia and her colleagues watched the launch in a livestream at the FHNW site in Windisch. "The launch happened all of a sudden and the thing was already up there," says Marina Battaglia, who holds a doctorate in physics from ETH Zurich and is now a research scientist at FHNW. First the ignition of the second rocket stage, then the separation of the space probe, establishing radio contact, extending the solar sails - everything went according to plan. Since then the Solar Orbiter is on its way to the Sun. In November 2021, the space probe is scheduled to reach its destination: an strong elliptic orbit 45 million kilometres from our central star at its nearest point, which is about a quarter of the distance between the Earth and the Sun and lies within the orbit of Mercury.
Marina Battaglia is now standing in the rooms of the 'Institute for Data Science' on the second floor of the new FHNW building right next to Brugg station. The scientist is looking into a showcase in which a copy of the X-ray telescope STIX on a scale of 1 to 1 is set up. STIX is one of ten experiments that are travelling to the Sun with the Solar Orbiter. Each experiment carries out different measurements. But all instruments pursue one overriding goal: to understand the so-called solar flares and the solar wind, i.e. the enormous streams of neutral and charged particles (photons, electrons, protons and also heavy ions) that the Sun emits into space in the course of massive ejections - accelerated by a powerful magnetic field.
Using X-rays to observe electrons
The X-ray telescope STIX was built to measure the intensity, time and direction of photons in the x-ray range from 4 to 150 keV. These photons come from electrons strongly accelerated and decelerated in the corona (Bremsstrahlung). "We conduct particle physics in the corona of the Sun to understand the solar flares by using X-rays to study where the electrons are accelerated and how they propagate," says Marina Battaglia about her field of research.
The X-ray telescope STIX is therefore dedicated to the big open questions in solar physics. When you then see the telescope in the FHNW display case, you are almost surprised how small it is: the actual detector for measuring the X-rays, including the electronics for control and data transmission, is only as big as a shoebox. In addition, there are two apertures at a short distance through which the X-rays fall onto the detectors. Each aperture contains 32 gratings measuring only a few square centimetres. These gratings can be compared to the lens system of a camera: thanks to them, STIX can indirectly photograph the source of the X-rays and thus obtain information about the position and size of the X-ray source. Experts know the process as 'Fourier imaging'.
STIX follows RHESSI
Like other experiments in particle physics, STIX builds on earlier experiments. The predecessor of STIX was RHESSI (short for: Reuven Ramaty High Energy Solar Spectroscopic Imager). This NASA space telescope was used for 16 years to observe solar X-rays before it was decommissioned in 2018. "The Paul Scherrer Institute was heavily involved in RHESSI, so RHESSI was also a Swiss mission," says FHNW Professor Säm Krucker. The astrophysicist had already accompanied the RHESSI mission and is the father of STIX: Together with a 12-member research group from the 'Institute for Data Science' and the 'Institute for Product and Production Engineering' (Prof. Hans-Peter Gröbelbauer) of the FHNW as well as Swiss industry and international partners, he designed and built the STIX components, which were later tested at the Paul Scherrer Institute (Villigen/AG) and at the University of Bern, before they were brought to the Airbus plant in Stevenage (GB) in 2017 and installed in the 'Solar Orbiter'.
STIX and the further experiments on the Solar Orbiter are expected to record data from the Sun's corona for at least seven years. In addition to a better understanding of solar flares and the solar wind, the scientific space mission may also provide answers to one of the biggest unsolved questions in solar physics, the 'Coronal Heating Problem'. This expression describes the so far unexplained observational fact, that the corona of the Sun has a temperature of about one million degrees Celsius, while the surface is only 6000 degrees hot. "One possible explanation could be that the large number of tiny 'solar flares' in the corona act like powerful heaters, heating the atmosphere of the Sun to these extremely high temperatures. The conversion of magnetic energy is the origin of the heating, but we don't yet know which physical process makes this in the most efficient way," says Säm Krucker. "It would be a huge scientific success if this hypothesis could be confirmed by the STIX mission."
Further information: https://stix.i4ds.net and https://sci.esa.int/web/solar-orbiter/home
Author: Benedikt Vogel
Solar Orbiter is a mission led by the European Space Agency (ESA) to observe our closest star, the Sun. The launch from Cape Canaveral, USA, is now foreseen on the morning of 10 February (CET). The Spectrometer / Telescope for Imaging X-rays (STIX) is one of the 10 instruments on board and was developed in Switzerland by the "Fachhochschule Nordwestschweiz" (FHNW).Image: ESA