First RPW measurements
The instrument "listens" to the radio signals produced by the energetic electrons coming from the Sun. The Sun is an efficient particle accelerator. In particular, during solar flares, the Sun produces electron beams that can be injected into the interplanetary medium and which can be detected directly in-situ by the EPD instrument on board the Solar Orbiter, if the magnetic connection is favourable, or by the radio emission they produce in the interplanetary medium. The RPW (Radio & Plasma Waves) instrument on Solar Orbiter can thus listen to the radio emissions produced by energetic electrons (in particular the so-called type III bursts).
The figure below shows one of the very first Type III burst observed by RPW on May 6, 2020. At that time, Solar Orbiter was already located at half the Sun-Earth distance. The radio burst starts around 09:33 UT, when the energetic electrons are still in the Sun’s outer atmosphere and stops around 09:47 UT, when the electrons have travelled about 40 solar radii.
Observing the Sun in X-rays with the STIX spectro-imager
STIX detects solar X-ray emissions produced mainly during flares. Although solar activity is still low, the STIX team had the chance to detect a solar flare, which, although weak, allowed testing all aspects of STIX’s functionality.
The figure below shows the X-ray spectrum detected during the flare and illustrates how it varies over time during the flare. The red curve shows the signal coming from hot loops which are coronal magnetic structures filled with gas that is raised to 11 million degrees during the flare. The blue curve shows the signal produced by the energetic electrons accelerated during the flare. These energetic electrons heat the lower layers of the corona which provide the material that then fills the coronal loops.
Direct (focusing) imaging is difficult in the hard X-ray range, requires telescopes and was not suitable for a mission such as Solar Orbiter. STIX’s imaging system is therefore an indirect system based on the measurement of 30 complex visibilities in the Fourier plane. Images are then built from visibilities after data transmission to Earth.
Obtaining a first correct image requests significant scientific and mathematical effort as well as an excellent instrumental calibration of the different visibilities. The effort is still ongoing. Once the image reconstruction process has been validated, STIX X-ray images can be made automatically.