To study X-rays, you have to leave Earth’s atmosphere. For a fraction of the cost of an orbiting telescope, astrophysicists can use short-range rockets that carry instruments for observations lasting just a few minutes. The instrument’s second flight FOXSI has just delivered its results and it shows solar phenomena that astronomers have been trying to observe for years: solar micro-eruptions.
The imager FOXSI-2 (Solar X-ray imager with focusing optics) was launched from the missile base of white sands (New Mexico) on December 11, 2014. It flew for only 15 minutes, for 6 minutes of actual data acquisition. That was enough to highlight these very small solar flares that have the peculiarity of producing temperatures so high (up to 10 million degrees) that they emit X-rays. These flares are too small to be detected from Earth.
The American-Japanese team behind this study appeared on Astronomy of natureled by JAXA solar physicist Shin-nosuke Ishikawa, has succeeded for the first time in observing areas of the Sun that appear calm, in the wavelengths of “hard” X-rays (energies up to 9 keV).
These observations are crucial to understanding the problem of heating the solar corona. The solar corona is in fact at a very high temperature, several million degrees, without us understanding the mechanism at play, while the surface of the Sun is 1000 times less hot.
One possible hypothesis is the presence of a continuous source of small flares in the solar atmosphere, so small that they cannot be observed directly. But, if taken in large numbers, they could explain the significant heating of the corona.
These micro-eruptions would give rise to the appearance of pockets of plasma superheated enough to emit hard X-rays, which are difficult to detect on Earth or even in orbit, in particular because of the difficulty of focusing this type of radiation.
The FOXSI-2 imager uses a technique called “direct focusing,” which allows it to precisely determine the point of origin on the Sun’s surface of the detected X-ray photon. In just 6 minutes of data collection, FOXSI-2 achieved better sensitivity than the best satellite that would have used indirect imaging.
To draw conclusions about the evidence that nano-eruptions produced the observed X-rays, the researchers also used data from the observatory Hinodethe result of a NASA/JAXA collaboration. This allowed them to conclude with certainty that the incriminated regions of the Sun did not show large flares at the time of the FOXSI-2 observations. They could therefore only be micro- or nano-eruptions.
Astronomers can’t yet say how much thermal energy these nano-eruptions of superheated plasma actually inject into the corona. To fully account for the coronal heating problem, these nano-eruptions would have to happen continuously across the entire surface of the Sun, the researchers say.
To learn more, the American-Japanese team has already built a third, further improved version of the FOXSI imager, which is scheduled to be launched on a mini-rocket in the summer of 2018.
The first very encouraging results have also convinced those responsible for the program Little explorers from NASA to fund a concept study for a possible satellite in the next decade.
Source
Detection of nanoflare-heated plasma in the solar corona by the FOXSI-2 sounding rocket
Shin-nosuke Ishikawa, Lindsay Glesener, Säm Krucker, Steven Christe, Juan Camilo Buitrago-Casas, Noriyuki Narukage & Juliana Vievering
Astronomy of Nature (October 9, 2017)
Illustrations
1) X-ray image of the Sun, the areas imaged by FOXSI are circled in blue (JAXA/NASA/Hinode/FOXSI)
2) The FOXSI team in front of the rocket Black Brant IX used for suborbital flight of FOXSI-2 instrument (NASA)