Unprecedented map of the Sun’s magnetic field created by the CLASP2 space experiment

Every day, space telescopes provide spectacular images of solar activity. However, their instruments are blind to its main engine: the magnetic field in the outer layers of the solar atmosphere, where the explosive events that occasionally affect the Earth occur. The extraordinary observations of the polarization of the Sun’s ultraviolet light made by the CLASP2 mission have made it possible to map the magnetic field in the entire solar atmosphere, from the photosphere to the base of the extremely hot corona. This survey, published today in the journal Scientific advances, was carried out by the international team responsible for this suborbital experiment, which includes several scientists from the POLMAG group of the Instituto de Astrofísica de Canarias (IAC).

The chromosphere is a very important region of the solar atmosphere spanning a few thousand kilometers between the relatively thin and cool photosphere (with temperatures of a few thousand degrees) and the warm and extended corona (with temperatures above one million degrees). Although the temperature of the chromosphere is about a hundred times lower than that of the corona, the chromosphere has a much higher density and therefore much more energy is needed to maintain it. In addition, the mechanical energy required to heat the corona must pass through the chromosphere, making it a crucial interface region for solving many key problems in solar and stellar physics. One of the current scientific challenges is to understand the origin of the violent activity of the solar atmosphere, which on certain occasions disrupts the Earth’s magnetosphere with serious consequences for our current technological world.

“It is impossible to understand the solar atmosphere if we cannot determine the magnetic fields of the chromosphere, especially in its outer layers where the plasma the temperature is in the order of ten thousand degrees and the magnetic forces dominate the structure and dynamics of the plasma, ”explains Javier Trujillo Bueno, CSIC professor at the IAC and principal scientist of the POLMAG group of the IAC. Theoretical research carried out by this group, funded by an advanced grant from the European Research Council, indicated that this goal could be achieved by observing the polarization that various physical mechanisms produce in the radiation emitted by neutral hydrogen atoms and ionized magnesium chromosphere.

Because the Earth’s atmosphere strongly absorbs solar ultraviolet radiation, it must be observed at altitudes above 100 kilometers. An international consortium has been created for this purpose, led by the NASA Marshall Space Flight Center (NASA / MSFC), the National Astronomical Observatory of Japan (NAOJ), the French Institute of Space Astrophysics (IAS) and the Spanish Institute of Astrofísica de Canarias (IAC). This international team has designed a series of space experiments that have been selected through competitive calls under NASA’s Sounding Rocket program. These space experiments are known as CLASP, the “Chromospheric Lyman-Alpha Spectro-Polarimeter” (CLASP1, launched September 3, 2015) and the “Chromospheric LAyer Spectro-Polarimeter” (CLASP2, launched April 11, 2019). Both experiments were a great success, which NASA recognized by awarding the “Group Achievement Honor Award” to the international team.

The research paper recently published in the prestigious journal Scientific advances is based on a small part of the unpublished data acquired by CLASP2. The team analyzed the intensity and circular polarization of ultraviolet radiation emitted by an active region of the solar atmosphere in the spectral domain containing the h & k lines of Mg II (ionized magnesium) around 2800 Å (see figure 1 ). In this spectral region, there are also two spectral lines produced by atoms of Mn I (neutral manganese).

The circular polarization observed by CLASP2 results from a physical mechanism known as the Zeeman effect, by which radiation emitted by atoms in the presence of a magnetic field is polarized. “The circular polarization signals of the magnesium (Mg II) lines are sensitive to magnetic fields in the middle and outer regions of the solar chromosphere, while the circular polarization of the manganese (Mn I) lines respond to the deepest magnetic fields. regions of the chromosphere ”, explains Tanausú del Pino Alemán, one of the scientists of the POLMAG group and the international team.

While CLASP2 was carrying out its observations, the Hinode the space telescope was simultaneously pointing at the same active region on the solar disk. “This made it possible to obtain information on the magnetic field in the photosphere thanks to the polarization observed in the spectral lines in neutral iron (Fe I) of the visible domain of the spectrum”, notes Andrés Asensio Ramos, another researcher of the IAC who participated in the project. The team also performed simultaneous observations with the IRIS Space Telescope, measuring the intensity of ultraviolet radiation with higher spatial resolution (IRIS was not designed to measure polarization).

The team’s investigation, led by Dr Ryohko Ishikawa (NAOJ) and Dr Javier Trujillo Bueno (IAC), made it possible to map for the first time the magnetic field in the active region observed by CLASP2 throughout its atmosphere, of the photosphere at the base of the crown (see figure 2). “This mapping of the magnetic field at different heights of the solar atmosphere is of great scientific interest, because it will help us to decipher the magnetic coupling between the different regions of the solar atmosphere”, comments Ernest Alsina Ballester, researcher of the international team which has just joined the IAC after its first post-doctorate in Switzerland.

The results obtained confirm and prove that, in these regions of the solar atmosphere, the lines of force of the magnetic field expand and fill the entire chromosphere before reaching the base of the corona. Another important result of this investigation is that the strength of the magnetic field in the outer layers of the chromosphere is strongly correlated with the intensity of the radiation at the center of the magnesium lines and with the electron pressure in the same layers, revealing the origin magnetic heating. in the outer regions of the solar atmosphere.

The CLASP1 and CLASP2 space experiments represent an important step in astrophysics, providing the first observations of the relatively weak polarization signals produced by various physical mechanisms in the spectral lines of the solar ultraviolet spectrum. Such observations have dramatically confirmed previous theoretical predictions, thereby validating the quantum theory of the generation and transfer of polarized radiation that these scientists apply in their research on the magnetic field in the solar chromosphere.

The international team has just received the good news that NASA has selected its recent proposal to conduct a new space experiment next year, which will allow them to map the magnetic field over larger regions of the solar disk. “Of course, systematic observations of the intensity and polarization of solar ultraviolet radiation will require a space telescope equipped with instruments like those of the CLASP, because the few minutes of observation allowed by a suborbital flight experiment are not sufficient. », Specifies Javier Trujillo Bueno. The team is convinced that, thanks to what CLASP1 and CLASP2 have achieved, these space telescopes will soon become a reality and that the physical interpretation of their spectropolarimetric observations will allow a better understanding of the magnetic activity in the outer layers of the Sun and other stars.

Reference: “Mapping of the solar magnetic fields of the photosphere at the base of the crown” by Ryohko Ishikawa, Javier Trujillo Bueno, Tanausú del Pino Alemán, Takenori J. Okamoto, David E. McKenzie, Frédéric Auchère, Ryouhei Kano, Donguk Song, Masaki Yoshida, Laurel A. Rachmeler, Ken Kobayashi, Hirohisa Hara, Masahito Kubo, Noriyuki Narukage, Taro Sakao, Toshifumi Shimizu, Yoshinori Suematsu, Christian Bethge, Bart De Pontieu, Alberto Sainz Dalda, Genevieve D. Vigil, Amy Winebarger, Ernest Al , Luca Belluzzi, Jiri Stepan, Andrés Asensio Ramos, Mats Carlsson and Jorrit Leenaarts, February 19, 2021, Scientific advances.
DOI: 10.1126 / sciadv.abe8406

Principal investigators of the CLASP2 space experiment:

• David McKenzie (NASA / MSFC, United States)
• Ryohko Ishikawa (NAOJ, Japan)
• Frédéric Auchère (IAS, France)
• Javier Trujillo Bueno (IAC, Spain)

IAC scientists participating in CLASP2:

• Ernest Alsina Ballester (IAC)
• Andrés Asensio Ramos (IAC)
• Tanausú del Pino Alemán (IAC)
• Javier Trujillo Bueno (IAC)

CLASP2 is an international collaboration led by the Marshall Space Flight Center of NASA (United States), the National Astronomical Observatory of Japan (Tokyo, Japan), the Instituto de Astrofísica de Canarias (IAC, Tenerife, Spain) and the ‘Institute of Space Astrophysics (IAS, France). Additional members are Istituto Ricerche Solari Locarno (Switzerland), Astronomical Institute of the Academy of Sciences of the Czech Republic, Lockheed Martin Solar & Astrophysics Laboratory (USA), Stockholm University (Sweden) and the Rosseland Center for Solar Physics (Norway).

The participation of the IAC in CLASP2 is funded by the European Research Council (CER) within the framework of the European Union’s Horizon 2020 research and innovation program (Advanced Grant Agreement n ° 742265).