FOXSI: NASA sounding rocket for solar X-rays

Discovering faint X-ray sources in the hottest plasmas of the corona

The Focusing Optics X-ray Solar Imager (FOXSI) is a NASA sounding rocket to study the hard X-rays coming from the hottest plasmas in the corona — five to ten million degrees Kelvin, about 10 times hotter than the superheated corona itself —. Thanks to FOXSI’s cutting-edge technology to measure hard X-rays with high sensitivity, combining FOXSI data and measurements from Hinode (a Japanese spacecraft that observes the Sun in soft X-rays), we have found that there is one hundred times less of the 10 million degrees plasma in active regions than we have deduced by using data from other previous instruments alone. This result may have important effects on the way we currently understand tiny solar flares.

Images of the Sun in X-rays provide us with information on the hottest parts of the solar atmosphere: those with temperatures up to millions of Kelvin. Faint X-ray sources, like multiple tiny flares, are a key phenomenon in open questions in the solar physics like “the coronal heating problem”. They are difficult to observe. Instruments with a high sensitivity in X-rays are needed to improve our understanding of the processes that occur in the solar corona.

The Focusing Optics X-ray Solar Imager (FOXSI) is a sounding rocket payload that has flown twice from the White Sands Missile Range in New Mexico as part of the NASA “Low-Cost- Access-to-Space” (LCAS) program. FOXSI is a telescope that observes the Sun in X-rays by direct- focusing (bending X-rays and collecting them on a focal plane). Thanks to its superior sensitivity and dynamic range, FOXSI performs cleaner observations of the Sun in X-rays than previous instruments.

The reflection of X-rays differs from that of visible light. For X-rays the index of refraction is slightly less than unity, so the angles of incidence must be kept very small in order to obtain appreciable reflectivity. FOXSI uses several sets of nested mirrors molded according to an appropriate geometry (Wolter-I) to reflect X-rays at grazing angles.

FOXSI’s cutting edge technology for observing the Sun in X-rays allowed us to perform “imaging spectroscopy” of faint X-ray sources in the solar atmosphere. This is, we can make X-ray images of, for example, tiny flares constraining the energies of the emission source.

FOXSI observations of non-flaring active regions, combined with Hinode/XRT measurements, suggest that the amount of plasma at temperatures beyond 7 million Kelvin is less than we estimated with previous instruments in at least a factor of one hundred. This result may have profound implications in the way we understand the physics of small solar flares and their connection  with how does the corona as a whole get hot.

The FOXSI payload is currently scheduled to fly a third time in  summer, 2018 from White Sands, New Mexico. More observations of FOXSI would bring new discoveries on how the hottest parts of the solar corona behave.