The solar atmosphere (photosphere, chromosphere, transition region and corona) is permeated by magnetic fields, with strengths going from only a few gauss to thousands of gauss. The magnetic field controls the solar activity, the clearest manifestation of which is the 11 year sunspot cycle, as well as the ejection of particles and magnetized plasma from the outer solar atmosphere (upper chromosphere, transition region and corona). The explosive events caused by the magnetic activity in the outer solar atmosphere drive the near-Earth space weather, which impacts our life on Earth.
We need to “measure” the magnetic field in the outer solar atmosphere for understanding how the upper chromosphere and the million-degree corona are sustained, how the solar wind and the acceleration of particles is driven, and how the eruptive phenomena that produce the near-Earth space weather are activated. However, our empirical knowledge on solar magnetic fields is basically restricted to the Sun’s visible surface (the photosphere). In practice, we have remained blind to the main driver of solar activity, namely the magnetic field in the outer solar atmosphere.
The information about the magnetic field that permeates the solar atmosphere is encoded in the polarization of the spectral line radiation emitted by the atoms and molecules of the solar atmospheric plasma. POLMAG has achieved a breakthrough in the development and application of polarized radiation diagnostic methods for exploring the magnetic fields of the solar chromosphere, transition region, and corona via the interpretation of the light polarization produced by optically polarized atoms and the Hanle and Zeeman effects in ultraviolet, visible, and near-infrared spectral lines.
To that end, we combined expertise on atomic physics, the quantum theory of radiation, high-precision spectropolarimetry and plasma diagnostic techniques, advanced methods in numerical radiative transfer, and the confrontation of spectropolarimetric observations with spectral synthesis in increasingly realistic three-dimensional models of the solar atmosphere.
Remarkably, our theoretical investigations on the polarization of the solar UV spectrum motivated two suborbital space experiments, which we carried out in 2019 and 2021 in collaboration with NASA and Japan. These CLASP2 and CLASP2.1 missions provided unprecedented spectropolarimetric data of quiet and active regions of the solar atmosphere in the near-UV spectral region of the Mg II h & k lines, and the interpretation of the data via the application of our plasma diagnostic techniques has allowed us to map solar magnetic fields from the photosphere to the base of the solar corona.
