Researchers from IRSOL and IAC solve twenty-year-old paradox in solar physics

Storage/images/document/ba24cdf47fa96e963cc6c53f3355dcd6.jpeg - In 1998, the journal Nature published a seminal letter concluding that a mysterious signal, which had been recently discovered analysing the polarization of sunlight, implies that the solar chromosphere (a very important layer of the solar atmosphere) is practically unmagnetised, in sharp contradiction with common wisdom. This paradox motivated laboratory experiments and theoretical investigations, which instead of providing a solution, raised new issues and even led some scientists to question the quantum theory of matter-radiation interaction. Today, researchers at the Istituto Ricerche Solari (IRSOL) in Locarno-Monti (affiliated to USI Università della Svizzera italiana), and the Instituto de Astrofísica de Canarias (IAC) in Tenerife, have found the solution to such intriguing paradox, opening up a new window for exploring the elusive magnetic fields of the solar chromosphere in the present new era of large-aperture solar telescopes. Their findings are now published in Physical Review Letters, the prestigious scientific journal of the American Physical Society. Twenty-five years ago, an enigmatic signal was discovered while analysing the polarization of sunlight with a new instrument, the Zurich Imaging Polarimeter (ZIMPOL), developed at ETH Zurich and later installed at IRSOL. This mysterious linear polarization signal, produced by scattering processes, appears at the wavelength of a neutral sodium line (the so-called D1 line) where, according to quantum mechanics, no such scattering polarization should be present.