Hanle rotation signatures in Sr I 4607 Å Franziska Zeuner 1, Luca Belluzzi 1,2,3, Nuno Guerreiro1, Renzo Ramelli 1 and Michele Bianda1,3 1 IRSOL Istituto Ricerche Solari “Aldo e Cele Daccò”, Università della Svizzera Italiana (USI), CH-6605 Locarno-Monti, Switzerland 2 Leibniz-Institut für Sonnenphysik (KIS), D-79104 Freiburg i.Br., Germany 3 Euler Institute, Università della Svizzera Italiana (USI), CH-6900 Lugano, Switzerland Scattering polarization and Hanle rotation ●The limb of the Sun is linearly polarized due to scattering. ●Magnetic fields modify scattering polarization via the Hanle effect. ●One signature of the Hanle effect is a rotation of the polarization plane (e.g., observe Stokes U). ●One of the strongest linear polarization signals in the photosphere are found in the Sr I 4607 Å line. ●Hanle rotation signatures have been observed in many other lines, but they have not been detected yet in Sr I. ● observe the linear polarization in the Sr I 4607 Å line close to the limb, where zero Stokes U is expected ● We demonstrated the existence of Hanle rotation signatures in Sr I from weak longitudinal magnetic fields clearly for the first time. ●We used a new measurement technique for achieving high absolute accuracy. ● These signatures have an invaluable diagnostic potential. Contact: franziska.zeuner@irsol.usi.ch Link to Zeuner+ 2022, A&A 662, A46 Method U Q maximum zero Observation High-precision spectropolarimetry at the IRSOL observatory, Locarno: ● using a high-resolution spectrograph, slit perpendicular to the limb, +Q parallel to limb ● and ZIMPOL (fast modulating to suppress seeing- induced cross-talk) ● in combination with a new slow modulation technique (to achieve high zero level accuracy, see Zeuner et al. 2022 (SPIE Proc.)) Academic example of the linear polarization (Stokes Q and U) of the Sun’s limb from pure scattering. Polarization profiles at the limb (Stokes I and Q), and at one position with some magnetic activity (indicated by orange arrows in the previous figure showing the observed Stokes images). Profiles with suff- icient signal-to-noise are fitted with a Gaussian or the first derivative of a Gaussian (St- okes Q and V, respectively.) Low signal-to-noise profiles are spectrally averaged over a few pixel (orange shaded ar- eas). Sr I Stokes U profiles are clearly single lobed. Data reduction ● Applying ZIMPOL standard data reduction (dark frame and polarimetric calibration). ● Demodulating the slow modulation states to correct Stokes Q and U from systematic errors. ● Removing the mean of the Stokes V image (not corrected by slow modulation). ● Fitting the Stokes Q profiles in Sr I, Stokes V in Sr I and Fe I, and averaging Stokes U in a small spectral region within the core of both lines to asign each limb distance a polarization amplitude. Positive and negative Stokes U signals are visible in Sr I. Analysis limb Observed full St- okes images for 70 min integr- ation time at the limb, scattering polarization (St- okes Q) most prominently visi- ble in the Sr I line, not in the neighbouring Fe I line. Longitdinal Zeeman induced Stokes V is stro- nger in Fe I be- cause of its high- er Landé factor. The Stokes U amplitude is correlated with the Stokes V amplitude in Sr I. Therefore, cross-talk from Stokes Q to U can be ruled out as a source for Stokes U in Sr I. Such a correlation is not demonstrated in Fe I, therefore any cross- talk from Stokes V can be excluded as a source for Stokes U in Sr I. The transversal Zeeman effect can be excluded due to the single lobed shape of the Stokes U profiles. However, the correlation shows that longitudinal magnetic fields are a possible source for Stokes U and Stokes V. Correlation of the linear polarization amplitde in U (normalized to the maximum) with the cir- cular polarization ampli- tude V (normalized to the maximum), and fit- ted linear regressions. Conclusions