Franziska Zeuner and Michele Bianda (IRSOL Locarno) -- Measuring the Hanle modified scattering polarization in the solar photosphere

Measuring the Hanle modified scattering polarization in the solar photosphere

The solar photosphere, the visible surface of the Sun, hosts a complex and dynamic magnetic structure that plays a crucial role in shaping solar activity across scales. Understanding these magnetic fields, particularly their small-scale, turbulent components, is essential for unraveling key processes in solar physics. The Sr I 4607 Å line, one of the most strongly polarizing lines formed in the photosphere, provides a unique window into these fields through scattering polarization. This scattering polarization, modified by the Hanle effect in the presence of magnetic fields, provides a powerful tool for probing the Sun's turbulent magnetic field. Unlike established techniques based on the Zeeman effect, Hanle effect measurements are robust against signal cancellation, even in the presence of small-scale, unresolved magnetic structures. However, these measurements are inherently challenging due to the extremely low polarization signals, typically below 1% in linear polarization, necessitating instruments with exceptional sensitivity.

In this seminar, I will discuss the principles and challenges of Hanle effect measurements in the Sr I line, highlighting the unique insights they provide into the photospheric magnetic field. I will present results from observations made using state-of-the-art instruments at various observatories, including those in Switzerland, the Canary Islands, and Hawaii. Finally, I will outline exciting prospects for future advancements in this field, leveraging the capabilities of current and next-generation solar telescopes to quantify the turbulent magnetic field component and explore its variations over the solar cycle.